Adhesives: Science, Technology, and Applications

#### **Chapter 7**

## Introductory Chapter: The Importance of Adhesives in the World

*António B. Pereira and Alexandre Luiz Pereira*

#### **1. Introduction**

An adhesive is a natural or synthetic product that can hold pieces together, usually by surface connection. These materials can be metals, composites, ceramics, etc., and combinations between ones. Its use has been going on for many years, but from the year 40, the technology of adhesives evolved considerably [1]. It is due to the use of synthetic polymers in the adhesives.

These polymers easily adhere to most materials and can transmit stress considerably. There are several types of adhesives, each of which is naturally more suitable for each application.

The range is extensive, from epoxy, and polyurethane, to polyimide, with one or more components. They can be applied, e.g., in the form of a paste, liquid, film, or pellets. There are hot melt adhesives, reactive hot melt, thermosetting, thermoplastic, pressure sensitive, and contact. The applications can be of the structural kind, in cases where high mechanical strength is usually required, but there are also applications for other purposes, such as silicone sealing. The adhesives may also contain additives in your composition, such as metal nanoparticles (e.g., copper, nickel, or silver), water, oil, etc., to improve their properties and increase their durability.

#### **2. Applications of the adhesives**

Aeronautical applications have been one of the main motivations for the development of adhesive technology with the use of adhesively bonded joints. However, today, other areas are also gaining from adhesive bonded technology. They are the automotive, naval, sports industries, and so on. Reducing the weight of an automotive vehicle generates fuel savings, an increase in speed, and a decrease in pollutant levels, for this, lighter materials such as aluminum, composites, and plastics have been used in their projects. The binding of these materials by traditional methods (bolts, rivets, welds, brazing, and other interference connections) is difficult to make, hence the preference for adhesive bonded [2]. Composites and adhesives develop together because bonding with adhesive is better in this case. It is possible to find in the literature several works made of composites reinforced with vegetable fibers and their adhesively bonded [3]. Another example in the naval area is the use of composites or metal-composites glued to repair pipelines, it is due to corrosion resistance and low

weight, also the bonding in the composite with adhesive is better [4]. In the industry in general, it has become a common practice to repair pipelines using adhesive joints [5]. In the sports area, equipment such as bicycles, helmets, rackets, etc., that use lighter materials such as plastics and composites in their projects, also use this bonding technology through adhesively bonded joints [1]. Adhesives are used in almost all consumer products. You can bond almost anything, from rock (civil construction, decorative items), and metals to plastics, including natural materials such as sisal fibers. In a general way, the use of adhesives ranges from simple pens to much more sophisticated pieces, like some components of a spaceship.

Adhesive bonded and their projects for adhesively bonded joints are areas that need knowledge of various sciences and technologies: namely, physics, chemistry, mechanics, the study of surfaces, types of polymers for adhesives, the mechanical design of adhesive joints, as well as knowledge of economics [1]. Thus, the study of adhesives is a multidisciplinary area of great growth and technological importance today.

#### **3. Some advantages and limitations of the adhesives**

There are several advantages to using adhesively bonded joints, some are uniform stress distribution in the bonded area, vibration dampening, joining on surfaces of different materials, (e.g., vegetable-fiber-reinforced polymer matrix composites bonded with metals [6]), allows joining surfaces with irregular geometries, may be more economically viable [1].

Adhesive applications still have limitations (compared with traditional mechanical methods), therefore, the importance of the study and development of this area. In an adhesive bonded, stresses such as cleavage and peel on adhesive bonded must be avoided, shear stresses are preferable, avoid geometries that present localized stresses, a careful preparation of the surfaces to be bonded (cleaning and degreasing with solvents, abrasion, etc.) [1]. **Figure 1** shows the types of stresses that should be avoided in adhesively bonded joints.

**Figure 2** shows a single-lap adhesive joint working in shear stresses. This is a better condition, as the stresses are parallel to the adhesive bonded and are also better distributed.

Mechanical strength in structural metal applications still has limitations. Really, for example, the bonding of two stainless steel pieces is very compromised being done with adhesives. Just think that steel has an ultimate strength of 600 MPa, while the adhesive resists, at most 10% of that, i.e., 60 MPa. In metal of the thickness thin, these limitations are less.

**Figure 1.** *Stresses that should be avoided in adhesively bonded joints: (1) cleavage, and (2) peel.*

*Introductory Chapter: The Importance of Adhesives in the World DOI: http://dx.doi.org/10.5772/intechopen.108295*

**Figure 2.** *Single-lap adhesive joint working in shear stresses.*

### **Author details**

António B. Pereira1 \* and Alexandre Luiz Pereira2

1 TEMA – Centre for Mechanical Technology and Automation, Department of Mechanical Engineering, University of Aveiro, Aveiro, Portugal

2 Federal Center of Technological Education in Rio de Janeiro (CEFET/RJ), Brazil

\*Address all correspondence to: abastos@ua.pt

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### **References**

[1] Lucas F, de Magalhães AG, de Moura MFSF. Juntas Adesivas Estruturais. Portugal: Publindústria Edições Técnicas; 2007. Available from: https://www.booki.pt/loja/prod/juntasadesivas-estruturais/9789728953218/

[2] Banea MD, da Silva Lucas FM. Adhesively bonded joints in composite materials: An overview. Proceedings of the Institution of Mechanical Engineers, Part L: Journal of Materials: Design and Applications. 2009;**223**(1):1-18

[3] De Queiroz HFM, Banea MD, Cavalcanti DKK. Adhesively bonded joints of jute, glass and hybrid jute/glass fibre-reinforced polymer composites for automotive industry. Applied Adhesion Science. 2021;**9**(1):1-14

[4] de Barros S et al. Experimental analysis of metal-composite repair of floating offshore units (FPSO). The Journal of Adhesion. 2017;**93**(1-2):147-158

[5] Zugliani PA et al. Bonded composite repair of metallic pipeline using energy release rate method. Journal of Adhesion Science and Technology. 2019;**33**(19):2141-2156

[6] De Queiroz HFM, Banea MD, Cavalcanti DKK. Experimental analysis of adhesively bonded joints in syntheticand natural fibre-reinforced polymer composites. Journal of Composite Materials. 2020;**54**(9):1245-1255

#### **Chapter 8**

## Mechanical Strength of Adhesively Bonded Metals

*António B. Pereira and Alexandre Luiz Pereira*

#### **Abstract**

Adhesive joints are nowadays widely used in fields ranging from packaging to aeronautics. Nevertheless, the absence of accurate failure criteria remains an important obstacle that often prevents the use of adhesive joints in structural applications. The main objective of this work is to be an introduction to the subject, and it was for this to evaluate the factors that most influence the strength of overlap adhesive joints.

**Keywords:** adhesive joints, overlap adhesive joints, strength of adhesive joints

#### **1. Introduction**

Glued joints currently have a wide range of applications, from the packaging industry to the demanding aeronautical industry. The characteristics of the so-called structural polymeric adhesives allow the increasing use of primary adhesive joints, that is, connections whose performance is critical for the integrity of the structure in which they are inserted. Among the main advantages of glued joints, we can mention:


Glued joints are particularly interesting for joining advanced high-strength materials, such as polymer matrix composites. Alternative riveted and bolted connections are much less efficient than metallic materials due to the low ductility and poor crush strength of composites.

Adhesive joints, however, have several limitations:


Adhesive joints load can be ordered in three main ways, namely (1) shear; (2) tensile; and (3) cleavage (**Figure 2**).

A fundamental principle in the design of bonded connections is that the adhesive should preferentially transmit shear forces. Cleavage loads are highly harmful. Tensile demands are also to be avoided, as unavoidable misalignments cause cleavage efforts.

#### **Figure 1.**

*The most common types of glued joints: (1) single-lap; (2) double-lap; (3) stair; (4) ramp.*

*Mechanical Strength of Adhesively Bonded Metals DOI: http://dx.doi.org/10.5772/intechopen.108872*

**Figure 2.** *Fundamental loading ways: (1) shear; (2) tensile; (3) cleavage.*

It should be noted, however, that, in the overlap joints, there are always localized cleavage stresses.

Joint failure can occur in three ways:


One of the main causes of adhesive breakage is inadequate surface preparation [2]. The specific action of the preparation normally consists of:


The procedures naturally depend on the materials to be connected and are often the subject of standards, which are particularly well established for several metal alloys [3, 4]. The first stage of the preparation is cleaning the surfaces, especially in terms of degreasing, using solvents, detergent solutions, trichloroethane vapor (toxic), ultrasound, etc. The surface roughness can be increased by applying fine abrasive paper or by shot blasting, after which it is necessary to remove the loose particles. In the case of metals, it is recommended to carry out a chemical attack with appropriate solutions, or even electrochemical treatments, as is the case of anodizing Al alloys. The application of primers favors the durability of the connection.

Several studies have already been presented on the effect of surface preparation on failure mode and on the strength of bonded joints [2, 5]. The conclusions, however, do not always go in the same direction, either in terms of failure modes or in terms of the classification of surface treatments. In [5] it is considered that the interfacial rupture is due to deficient bonding procedures, namely inadequate preparation or contamination of the surfaces. However, in [6], where a vast amount of experimental results of glued Al joints were reviewed, there were cases of interfacial ruptures even when sophisticated treatments were used. Interfacial ruptures even seem to be quite frequent after more or less prolonged exposure to environments of relatively high temperature and humidity. In a large-scale study carried out in Japan [7], frequent

adhesive failures were observed in joints with steel adherents. However, according to [5], there are cases of apparent interfacial rupture in which more sophisticated analysis methods allow us to verify the presence of a very thin adhesive layer on the fracture surfaces.

Another factor to be controlled is the thickness of the adhesive layer, for which there is an optimal range, generally between 0.1 and 0.3 mm [8]. The strength of the joint decreases markedly with the thickness of the adhesive layer above certain values, due to the greater probability of the existence of defects. On the other hand, thicknesses that are too thin considerably increase the risk of failure of the adhesive layer. Thickness control can be done through the clamping devices used in the gluing operation. In other cases, small glass spheres can be added to the adhesive that guarantee a certain thickness. The use of adhesives in the form of films allows better control of the thickness of the joint, although with generally higher costs.

Finally, the proper choice of adhesive is critical to joint performance. Structural adhesives are normally thermosetting polymers, as thermoplastics are more susceptible to creep and property degradation from environmental exposure. The most common types of adhesives are epoxides, polyurethanes, modified acrylics, and cyanoacrylates. Epoxy adhesives are the most used, given their good chemical resistance and good creep behavior. There is a great variety of formulations, which are relatively fragile based on, but which become very ductile with the addition of rubber or thermoplastic particles. Curing generally takes place at temperatures between 20 and 120°C, so heating means may be required. Polyurethane adhesives cure by reaction with ambient humidity, have excellent toughness and moderate cost. Resistance to environmental exposure and creep are the main limitations, which are shared by acrylic adhesives, said to be modified, as they are derived from thermoplastic formulations. These, however, have good cleavage strength, moderate cost, and are less demanding in surface preparation. Cyanoacrylates cure quickly and have good cleavage strength, but bond durability is relatively low.

#### **2. Characterization of adhesives**

The characterization of the behavior of the adhesives is somewhat delicate. In fact, the most common tests of adhesive joints do not directly provide the mechanical properties of the adhesives, having mainly a comparative or quality control value. Cleavage assays are clearly in this category. **Figure 3(1)** and **(2)** represent the two most common specimens, specified by ASTM D 1876 [9] and ASTM D 3762 [10], respectively. In the first case, the force necessary to progressively break the joint is

**Figure 3.** *Adhesive joint cleavage tests: (1) ASTM D1876; (2) ASTM D3762.*

*Mechanical Strength of Adhesively Bonded Metals DOI: http://dx.doi.org/10.5772/intechopen.108872*

measured, while in the second test, the advance of the crack in the joint relative to the position of the wedge is normally measured. These tests only allow comparing adhesives and/or surface preparation techniques, as well as evaluating the effect of environmental exposure.

The shear test of simple overlap joints is also widely publicized at ASTM D 1002 [11] standard for metals (**Figure 4**).

The overlap length L is determined in such a way that there is no yielding of the adherents before the joint breaks, since it is intended to measure the ultimate stress at the average shear of the adhesive. Once again, this test has only comparative value, as it does not allow measuring the true shear strength of the adhesive. In fact, the distribution of the shear stress along L is not uniform (**Figure 5**). On the other hand, the eccentricity of the load causes bending of the adherents (**Figure 6**) and cleavage stresses at the ends of the bond.

The tests that allow obtaining the mechanical properties of the adhesives are more complex. The shear strength can be obtained from the so-called "thick tack" test (ASTM D 5656) [12]. It is again a simple overlap joint with 9.5 mm thick adherents to minimize bending deformations and cleavage stresses. The overlap length L is proportionately small (9.5 mm), so that the shear stress distribution is approximately uniform. The use of a strain gauge also makes it possible to obtain the shear modulus of the adhesive, Ga.

**Figure 4.** *ASTM D 1002-10 test sample.*

#### **Figure 5.**

*Distribution of shear stresses in a lap joint.*

**Figure 6.** *Bending effect on a simple lap joint.*

#### **3. Strength of overlap adhesive joints**

There are important difficulties in the design of glued joints. In stress analysis, there is a singularity in the adherent/adhesive interface that makes it difficult to use the stresses obtained with Finite Element (FE) models. Therefore, simplified analyses are normally used, which, despite the inevitable limitations, are still recommended by design codes [13]. These analyses apply mainly to joints with adherents in tensile mode. **Figure 7** shows the case for single-lap joint in shear.

The best-known analysis is that of Goland-Reissner [14], which takes into account the effect of bending in the simple lap joint, but which is clearly unrealistic in assuming linear elastic behavior for the adhesive. Instead, the Hart-Smith analysis [15] considers the plasticization of the adhesive through an elasto-perfectly plastic approach.

In either case, the fundamental dimensioning parameter is the overlap length L. This must be sufficient to prevent failure due to cleavage stresses and that the average shear stress is too high, promoting excessive creep deformations. However, beyond a certain value, there is no advantage in increasing L, as it penalizes the joint in terms of weight without any gains in joint strength. At this stage, the difficulty lies in the absence of a sufficiently stringent failure criterion. Hart-Smith [15] found that, in the short term, joints can reach breaking loads close to the smallest of the following values:

$$P\_1 = \sqrt{2\tau\_p t\_a \left(\frac{\mathcal{V}\_e}{2} + \mathcal{V}\_p\right) E\_i t\_i \left(\mathbf{1} + \frac{E\_i t\_i}{E\_o t\_o}\right)}\tag{1}$$

$$P\_2 = \sqrt{2\tau\_p t\_a \left(\frac{\mathcal{V}\_e}{2} + \mathcal{V}\_p\right) E\_o t\_o \left(\mathbf{1} + \frac{E\_o t\_o}{E\_i t\_i}\right)}\tag{2}$$

However, given the uncertainties, the design philosophy is mainly aimed at guaranteeing the joint's durability and creep resistance. Hart-Smith [15] suggests that the plastic zones at the ends of the joint be dimensioned to fully support the applied load, while the inner elastic zone is reserved to give the joint resistance to fatigue and creep.

Another type of approach to the problem of predicting the rupture of bonded joints consists of the application of fracture mechanic. The most well-known fracture tests are: the "Double Cantilever Beam" (DCB), mode I (**Figure 8**) [16], and the "End Notched Flexure" (ENF), mode II (**Figure 9**) [17].

The aforementioned tests allowed to obtain a failure criterion expressed as a function of the critical rates of energy release GIc and GIIc, as well as the percentage

**Figure 7.** *Single-lap joint.*

*Mechanical Strength of Adhesively Bonded Metals DOI: http://dx.doi.org/10.5772/intechopen.108872*

#### **Figure 8.** *DCB test [16].*

#### **Figure 9.** *ENF test [17].*

of solicitation modes. This criterion was then applied to predict the failure of overlapping joints.

### **4. Conclusions**

From the literature review carried out, it is evident that there are still many aspects to be clarified in relation to the structural performance of adhesive joints. We highlight three key issues here:


### **Funding**

The present work was done and funded under the scope of projects UIDB/00481/2020 and UIDP/00481/2020—FCT—Fundação para a Ciencia e a Tecnologia; and CENTRO-01-0145-FEDER-022083—Centro Portugal Regional Operational Programme (Centro2020), under the PORTUGAL 2020 Partnership Agreement, through the European Regional Development Fund.

### **Conflict of interest**

The authors declare no potential conflicts of interest concerning the research, authorship, and publication of this article.

### **Author details**

António B. Pereira1 \* and Alexandre Luiz Pereira2

1 Department of Mechanical Engineering, TEMA—Centre for Mechanical Technology and Automation, University of Aveiro, Campus de Santiago, Aveiro, Portugal

2 Federal Center of Technological Education in Rio de Janeiro (CEFET/RJ), Brazil

\*Address all correspondence to: abastos@ua.pt

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Mechanical Strength of Adhesively Bonded Metals DOI: http://dx.doi.org/10.5772/intechopen.108872*

#### **References**

[1] Arumugaprabu V et al. Polymer-Based Composites: Design, Manufacturing, and Applications. CRC Press; 2021

[2] Wegman R et al. Surface Preparation Techniques for Adhesive Bonding. Elsevier Inc; 2013

[3] ASTM-D3933-98. Standard Guide for Preparation of Aluminum Surfaces for Structural Adhesives Bonding (Phosphoric Acid Anodizing). West Conshohocken, USA: ASTM International; 2017

[4] ASTM D2651-01, Standard Guide for Preparation of Metal Surfaces for Adhesive Bonding. West Conshohocken, USA: ASTM International; 2016.

[5] Davis M et al. Principles and practices of adhesive bonded structural joints and repairs. International Journal of Adhesion & Adhesives. 1999;**19**:91-105

[6] Rudawska A. Surface Treatment in Bonding Technology. NY, USA: Elsevier Inc; 2019

[7] Ikegami K et al. Benchmark tests on adhesive strengths in butt, single and double lap joints and double cantilever beams. International Journal of Adhesion & Adhesives. 1996;**16**:219-226

[8] D Bahadori, A., Essentials of Coating, Painting, and Lining for the Oil, Gas and Petrochemical Industries. NY, USA: Elsevier Inc.; 2015

[9] ASTM D1876-08(2015)e1. Standard Test Method for Peel Resistance of Adhesives (T-Peel Test). West Conshohocken, USA: ASTM International; 2016

[10] ASTM D3762-03. Standard Test Method for Adhesive-Bonded Surface Durability of Aluminum (Wedge Test). West Conshohocken, USA: ASTM International; 2010

[11] ASTM D1002-10. Standard Test Method for Apparent Shear Strength of Single-Lap-Joint Adhesively Bonded Metal Specimens by Tension Loading (Metal-to-Metal). West Conshohocken, USA: ASTM International; 2019

[12] ASTM D5656-10. Standard Test Method for Thick-Adherend Metal Lap-Shear Joints for Determination of the Stress-Strain Behavior of Adhesives in Shear by Tension Loading. West Conshohocken, USA: ASTM International; 2017

[13] Clarke J. Structural Design of Polymer Composites. Boca Raton, Florida, USA: CRC Press; 2019

[14] Goland M, Reissner E. Stresses in cemented joints. Journal of Applied Mechanics. 1944;**66**:A17-A27

[15] Hart-Smith LJ. Rating and comparing structural adhesives: A new method. In: Engineered Materials Handbook. Vol. 3: Adhesives and Sealants. USA: ASM International; 1987

[16] Samborski S. Mode I Interlaminar fracture of glass/epoxy unidirectional laminates. Part I: Experimental studies. Materials. 2019;**12**(10):1607

[17] Pereira A. Development of a delamination fatigue testing machine for composite materials. Machines. 2019;**7**:27

#### **Chapter 9**

## The Effect of Different Adhesive Types on Failure Load

*Bahadir Birecikli*

#### **Abstract**

In this study, bonding joint with double zigzag type geometry was used. There are four types of overlap angles in the bonding geometry: 30°, 45°, 60° and 75°, respectively. Composite materials that are made of glass fiber have been used in this adhesive bonding geometry. These materials were produced by using prepreg (preimpregnated) technique and [0°/90°] of orientation angle. Thickness of composite material is 3 mm. Ductile and brittle-type adhesives were used for the bonding joint. DP460 adhesive type shows ductile material properties while ATLAC580 adhesive type shows brittle material properties. The effect of adhesive type on the failure load was investigated experimentally. Test results demonstrated that failure load values were higher in the ductile-type adhesive.

**Keywords:** adhesives, composite material, failure load, bonding geometry, tensile test

#### **1. Introduction**

Industrial adhesives are a joining method used as an alternative to mechanical joining methods such as bolts, rivets, welding, and soldering. They have found a suitable area for development, as the bonding process is carried out below the melting temperatures of the joined parts. In addition, industrial adhesives do not create stress concentrations that occur in welding, soldering, and other connection types. The use of industrial adhesives, which are used as an alternative to existing bonding methods, is rapidly increasing. There are many applications joining with adhesive, especially in the aerospace, aviation, and automotive industries.

In this study, a mechanical analysis of the bonding joint geometry was realized using different types of adhesives.

The use of adhesive, which is a more efficient joining method, has become common instead of traditional joining methods [1]. An adhesive is defined by ASTM (Standard test method for strength properties of adhesives in shear by tension loading) as "a substance that can hold materials together by surface contact" [2]. In another definition of adhesive, it is a polymeric material that can hold surfaces together and prevents separation when applied to surfaces [3]. Neto et al. [4] carried out an experimental study on the bonding joint of composite materials. They used two different adhesives, brittle and ductile type in a single lap joint with different lap lengths between 10 and 80 mm. It was seen that failure load increased with growing

overlap length in the ductile-type adhesive joint. It was determined that 30 mm overlap length failure load occurred in the brittle-type bonding joint. Sawa et al. [5] analyzed the single lap joint formed by different types of adhesives subjected to tensile loads. They showed that the material thickness and the modulus of elasticity have an extremely large influence on the stress distributions in bonding area. Guess et al. [6] studied analytically and experimentally the strength of adhesively bonded single lap joints using two different adhesives. Apalak et al. [7] made analysis on corner joints. Adhesive is considered as a linear elastic material, and its effects on the bonding stresses created in corner joints were investigated.

Pinto et al. [8] experimentally investigated the mechanical behavior of single lap joint under tensile load by using two different adhesives, rigid and flexible. They stated that there was an insignificant decrease in the strength of the joint in the flexible adhesive type, but there was an increase in the bond strength when rigid adhesive was used. Ozel et al. [9] carried out an experimental study on a single lap joint under bending load using two different types of adhesives. They demonstrated that thickness of material has an extremely major effect on bonding joint performance. Wu et al. [10] applied a method they developed on single lap joints formed using different adhesives of different thicknesses. Temiz [11] stress analysis was performed using a flexible and rigid adhesive on single lap joint. It has been shown that the use of flexible adhesive reduces stress concentrations and increases the strength on the bonding joint. Kline [12] studied the effect of thickness on stress distribution in bonding layer. The alteration of stresses along the thickness is considered linear. He investigated the effect of parameters on the stress distribution in the bonding layer. Dean and Duncan [13] prepared bulk samples with thicknesses varying between 0.5 and 4.0 mm and examined whether the mechanical properties of the adhesive change with thickness. They used four different types of structural adhesives. One and two component epoxies, two-component polyurethane, and two-component acrylic adhesives were tested on samples with different thicknesses. According to the tensile test results, they determined that the material properties did not change with the sample thickness.

The goal of this study is to experimentally analyzed the effects of Vinylester Atlac 580 brittle-type adhesive produced by Huntsman Company and DP460 ductile-type adhesive produced by 3 M Company on failure load.

#### **2. Material and method**

DP460 is the center formed by epoxy and accelerator in a 2:1 volume ratio. It is used in a facility from metal, ceramics, and glass.

ATLAC 580 is a low-viscosity epoxy-based vinylester resin that is heat-resistant and flexible. It can also be used for wrapping and spraying in fabrication methods. It is resistant to acid and salt solutions. It is cured with accelerator and hardener mixture.

Curing conditions of brittle type of ATLAC580 and ductile type of DP460 adhesives used in the experiment are given in **Table 1**.

The mechanical properties and stress-strain diagram of DP 460 adhesive are taken from Akpınar's [14] doctoral thesis. Also, the mechanical properties and stress-strain diagram of ATLAC 580 adhesive are taken from Adin's [15] doctoral thesis as given in **Figures 1** and **2**.


**Table 1.**

*Curing conditions of adhesives.*

**Figure 1.** *Stress-strain diagram of DP 460 ductile adhesive*

The mechanical properties of ATLAC580 and DP460 adhesives used in the experiment are given in **Table 2**.

The high strength of adhesive bonding joint depends on surface preparation methods. The samples must be cleaned from foreign materials such as oil, dirt, and dust that will prevent adhesion. The surfaces were first washed with pure water and then wiped with microfiber cloths. Then, the surfaces to be bonded with pure alcohol were washed and kept on hold until the alcohol exactly evaporated from the surfaces.

Consistency of the adhesive thickness on the surface is possible with the use of a well-designed mold. For this, the thickness of the adhesive was kept constant at 0.20 mm so that the sample length is kept constant, it was placed in a certain mold.

#### **2.1 Experimental study**

The composite plates used in the experiments were cut in CNC milling device in accordance with ASTM standards and in desired geometric dimensions. The length of each test specimen is 250 mm. About 25 mm jaw margin is left for the specimens to be fixed to the tensile test device.

#### **Figure 2.**

*Stress-strain diagram of ATLAC580 brittle adhesive.*


#### **Table 2.**

*Mechanical properties of ductile and brittle adhesive.*

Glass fiber composite materials were used in the experiment. Glass fiberreinforced composite materials are produced as prepreg (pre-resin-impregnated wet fiber). Composite materials were prepared for testing with a sample thickness of 3 mm and fiber orientation [0°/90°] and are given in **Figure 3**.

In the experimental study, four different types of adhesive joints with overlap angles of 30°, 45°, 60° and 75° were used as seen in the **Figure 3**. The bonding length of each angle was kept constant at 60 mm.

#### **Figure 3.** *Composite samples with four different overlap angles.*

#### *The Effect of Different Adhesive Types on Failure Load DOI: http://dx.doi.org/10.5772/intechopen.107335*

The Shimadzu AG-X model tensile test device was used in the experiment (**Figure 4**). The device has an integrated extensometer and with a capacity of 100 kN.

The test device was calibrated before it was started. The test device was given a preload of 0.10 Mpa. Experimental tests 1 mm/min carried out at pulling speed as seen in the **Figure 4**. The experiment was terminated after the samples were completely detached from the bonding area.

#### **2.2 Experimental results**

Tensile tests were performed at four different overlap angles. Composite materials are produced in [0/90°] fiber orientation and 3 mm sample thick. Failure load values of each sample were determined by experiment. For the precision of these values, three samples of the same bonding type were produced, and the test was repeated. Results in graphs are the average of three test specimens.

Failure load and displacement graphs for DP460 (ductile) and ATLAC580 (brittle) adhesives are shown in **Figures 5–8**.

**Figure 4.** *Tensile test device and composite sample.*

**Figure 5.** *Failure load-displacement graph for ductile and brittle adhesive at 30° angle.*

#### **Figure 6.**

*Failure load-displacement graph for ductile and brittle adhesive at 45° angle.*

#### **Figure 7.**

*Failure load-displacement graph for ductile and brittle adhesive at 60° angle.*

#### **Figure 8.**

*Failure load-displacement graph for ductile and brittle adhesive at 75° angle.*

Failure loads increased with increasing overlap angle in the same bonding area. Increasing the overlap angle increased the failure load by approximately 81%. The highest failure load value was observed at 75° overlap angle. Test results demonstrated that failure load values were higher in the ductile-type adhesive.

*The Effect of Different Adhesive Types on Failure Load DOI: http://dx.doi.org/10.5772/intechopen.107335*

#### **Figure 9.**

*Failure load-displacement graph for all overlap angles for ductile adhesive.*

#### **Figure 10.**

*Failure load-displacement graph for all overlap angles for brittle adhesive.*

Failure load and displacement graph for entire overlap angles for DP460 (ductile) adhesive is shown in **Figure 9**.

Failure load and displacement graph for all overlap angles for ATLAC580 (brittle) adhesive is shown in **Figure 10**.

The failure load values of the ductile-type adhesive were bigger than the failure load values of the brittle-type adhesive. Modulus of elasticity of the ductile-type adhesive is 2077.10 MPa, while the modulus of elasticity of the brittle-type adhesive is 442.46 MPa.

As a result of the experimental study, it has emerged that the adhesive type has a significant effect on the failure load.

When the whole graphs of both adhesive types were examined, more displacement was obtained in the ductile-type adhesive, while almost half of this amount of displacement was obtained in the brittle-type adhesive.

When the bonding surfaces were examined, cohesion damage was observed in the ductile type of adhesive, and adhesion damage was observed in the brittle type of adhesive.

#### **3. Conclusions**

In this study, glass fiber composite materials were used at different overlap angles and with different adhesive types by using a single lap joint exposed to tensile loads.

As a result of the experiment, it has been revealed that the failure load increases with the increase of the overlap angle value in the bonding joint geometry.

Cohesion damage was occurred in the ductile type of adhesive, and adhesion damage was occurred in the brittle type of adhesive.

In addition, it can be said that the ductile type of adhesive increases the strength of the bonding joint.

### **Author details**

Bahadir Birecikli Vocational School of Technical Sciences, Batman University, Batman, Turkey

\*Address all correspondence to: bahadirbirecikli@gmail.com

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### **References**

[1] Kaya F. Ana Hatlarıyla Yapıştırıcılar. Istanbul: Birsen Yayınevi; 2004

[2] Apalak M, Günes R, Fidancı L. Geometrically non-linear thermal stress analysis of an adhesively bonded tubular single lap joint. Finite Elements in Analysis and Design. 2003;**39**:155-174

[3] Adams R, Wake W. Structural Adhesive Joint in Engineering. London: Elsevier Scinece Publisher; 1984

[4] Neto J, Campilho R, da Silva L. Parametric study of adhesive joints with composites. Journal of Adhesion Science and Technology. 2012;**37**:96-101

[5] Sawa T, Liu J, Nakano K, Tanaka J. A two dimensional stress analysis of single lap adhesive joints of dissimilar adherents subjected to tensile loads. Journal of Adhesion Science and Technology. 2000;**14**(1):43-66

[6] Guess T, Allred R, Gerstle F. Comparison of lap shear test specimens. Journal of Testing and Evaluation. 1977;**5**(2):84-95

[7] Apalak M, Davies R. Analysis and design of adhesively bonded corner joints. International Journal of Adhesion and Adhesives. 1993;**13**(4):219-235

[8] Pinto AMG, Campilho RDSG, Mendes IR, Baptista APM. Numerical and experimental analysis of balanced and unbalanced adhesive single lap joints between aluminum adherends. The Journal of Adhesion. 2004;**90**:89-103

[9] Ozel A, Kadioglu F, Sen S, Sadeler R. Finite element analysis of adhesive joints in four point bending load. The Journal of Adhesion. 2003;**79**(7):683-697

[10] Wu G, Crocombe AD. Simplified finite element modeling of structural adhesive joints. Computers and Structures. 1996;**61**(2):385-391

[11] Temiz S. Application of Bi-Adhesive in double-strap joints subjected to bending moment. Journal of Adhesion Science and Technology. 2006;**20**(14):1547-1560

[12] Kline RA. Stress analysis of adhesively bonded joints. In: Proceeding of the International Symposium on Adhesive Joints. Kansas City; 1982. pp. 587-610

[13] Dean GD, Duncan BC, Tensile behavior of bulk specimens of adhesives, NPL Report DMM (B). 1995; UK

[14] Akpınar S. Yapıştırıcıyla Birleştirilmiş T-Bağlantılarda Üç Boyutlu Gerilme Analizi. [PhD thesis]. Erzurum: Erzurum Üniversitesi Fen Bilimleri Enstitüsü; 2012

[15] Adin H. Yapıştırıcı ile Birleştirilmiş Ters Z Tipi Kompozit Malzeme Bağlantılarının Mekanik Analizi. [PhD thesis]. Elazığ: Fırat Üniversitesi Fen Bilimleri Enstitüsü; 2007

#### **Chapter 10**

## High-Performance Reworkable Underfill Adhesives Based on Dicyclopentadiene Epoxy Thermoset

*Laxmisha M. Sridhar and Timothy M. Champagne*

#### **Abstract**

A highly reliable and reworkable underfill adhesive based on thermoset epoxy resin possessing thermally reversible dicyclopentadiene (DCPD) moiety is described. The adhesive can be cured rapidly at moderate temperatures resulting in high Tg cured network, which gives high reliability to the bonded semiconductor components. The inherent thermal reversibility of DCPD moiety causes network breakdown at high temperatures enabling easy removal of defective semiconductor chips. A discernible trend between loading level of the thermally reversible epoxy resin and high-temperature die shear strength was observed. Using this novel adhesive system, both high reliability and reworkability can be achieved concurrently, which is normally not possible with other thermoset adhesive systems. The epoxy resin used in the study was scaled up to multi-kg quantities demonstrating industrial applicability of the approach.

**Keywords:** epoxy, dicyclopentadienedicarboxylic acid diglycidyl ester, thermoset, reworkable, underfill adhesive, thermally reversible

#### **1. Introduction**

The popularity of handheld display devices (HHDDs) has made their demand increase dramatically in recent years. Manufacturing throughput has consequently been challenged to meet the growing demand. One area that is particularly troublesome for manufacturers is the treatment and handling of defective semiconductor chips on a circuit board. For instance, during the manufacture of a circuit board subassembly, a multitude of semiconductor devices are electrically connected to the circuit board in chip scale packages ("CSPs"), ball grid arrays ("BGAs"), land grid arrays ("LGAs"), and the like [1]. The board may then be tested to evaluate function and sometimes the board fails. In such cases, it is desirable to identify the semiconductor device that caused the failure, remove it from the board, and reuse the board with the remaining functioning semiconductor devices. This would save cost for the manufacturer of HHDDs.

**Figure 1.**

*A cross-sectional representation of Si die bonded to a circuit board through solder balls without an underfill adhesive.*

Ordinarily, semiconductor devices (also called as chips) are connected to electrical conductors such as Cu pads on printed circuit boards (PCBs) by solder connection as shown schematically in **Figure 1A**. The coefficient of thermal expansion (CTE) of the Si die and the solder balls is significantly different. When the resulting subassembly in a HHDD is exposed to mechanical shocks such as vibration, distortion, drop (**Figure 1C**), or rapid temperature change (thermal shock, for example, when a device is left in a car during winter or summer), the reliability of the solder connection between the circuit board and the chip often becomes suspect. There are other modes of failure that can occur in a subassembly such as electrical shorting or stress cracks in solder balls (B & D, respectively, **Figure 1**).

Underfill adhesives are widely used to improve the overall thermal shock resistance, mechanical and electrical reliability of the assembly [2]. After a chip is mounted on a circuit board, the space between the chip and the PCB is filled with an underfill adhesive resin. Once the adhesive is cured, the stress is uniformly distributed throughout the bondline rather than just at the contact point between the solder ball and the chip. This enhances the overall thermal and mechanical shock properties and thus the reliability of the assembly [3, 4]. The adhesive formulation is typically a low-viscosity liquid (<1000cPs), which penetrates the gap between the chip and the PCB by capillary action when dispensed. The circuit board can also be slightly heated to about 50°C to lower the viscosity further and to accelerate the capillary fill process. In a high-throughput assembly, the dispensing and the fill process is completed in less than a minute, which requires low viscosity in the underfill resin. The adhesive resin is typically cured in the temperature range 120–130°C in less than 10 minutes. A picture of two CSPs with cured underfill adhesive is shown in **Figure 2** (adhesive is shown in black color indicated with arrows).

The underfill adhesives are typically thermosetting resin compositions that form cross-linked networks when cured. With conventional thermoset adhesives, it is difficult to remove the chip without damaging the subassembly in the event of a

*High-Performance Reworkable Underfill Adhesives Based on Dicyclopentadiene Epoxy Thermoset DOI: http://dx.doi.org/10.5772/intechopen.107334*

 **Figure 2.**

 *Two CSPs with cured underfill adhesive filling the gap between the chip and the PCB (indicated with arrows).* 

failure of a semiconductor chip on the circuit board. Several approaches have been published in the literature on reworkable, reversible, or degradable thermoset networks used in a number of adhesive applications [ 5 – 9 ]. Reworkability or removability is also a valuable attribute of adhesives used for electronic packaging applications including underfill adhesives [ 10 ]. It is highly desirable for an underfill adhesive to provide good electrical reliability, mechanical and thermal shock resistance, while allowing for the semiconductor chips to be easily separated in a defective assembly without causing damage to the circuit board. Several chemistry approaches have been explored to make the underfill adhesive reworkable at high temperature [ 11 , 12 ]. For good reliability (mechanical and thermal shock resistance), high modulus and Tg are essential for underfill adhesives, which allow them to have lower CTE at the service temperature of the HHDDs or during T-cycle tests (hence reduced mismatch in CTEs between bonded substrates) since the CTE of thermosets increases rapidly above the Tg. However, with conventional thermosets, it is difficult to achieve both high reliability and reworkability without a built-in rework chemistry mechanism. The results shown below demonstrate that by using a carefully designed thermoset resin system, both high reliability and reworkability can be achieved concurrently.

#### **2. Results and discussion**

 The concept of highly reliable and reworkable thermoset underfill adhesive system is shown schematically in **Figure 3** . The adhesive needs to be a highly cross-linked system (high modulus and Tg) for high reliability at the service temperature of the

 **Figure 3.**  *Highly reliable and reworkable thermoset adhesive concept.*  HHDD or during thermal cycle (T-cycle) tests. The thermoset adhesive needs to undergo network break down at rework temperatures (typically around 220°C for this application) for easy removability of the faulty semiconductor chip. The design needs to be such that the rework temperature is sufficiently high so that there is no network breakdown occurring during adhesive curing or thermal cycling reliability tests. Typically for T-cycle tests, the bonded components are subjected to a temperature ramp from −40°C to 80°C (thermal shock) with a 15-minute hold time at each temperature. For good reliability performance, 1000 T-cycles without electrical failure are required, and hence, the need for a high-performance underfill adhesive system.

#### **2.1 Resin system design**

For the resin system design, the built-in reworkable or reversible chemistry was carefully chosen so that the cross-linked polymer network breakdown takes place at sufficiently higher temperature than that used for adhesive curing. The dicyclopentadiene backbone was chosen because of its high Tg and relatively high retro-Diels-Alder temperature. Based on previous literature report on activation energy required for retro reaction [13], we sought to incorporate electron withdrawing ester groups on dicyclopentadiene double bonds such that the network breakdown occurs at the correct temperature range for good reworkability. Thiele's acid **1** (**Figure 4**) was chosen as starting material for the resin design. The synthesis of this acid was first reported more than a century ago [14], and design of thermally reversible polymeric system based on alkylation of potassium salt of this acid with dihalides has been reported [15]. A cyclic diester of this diacid, also known as "mendomer," has been made and used in thermally remendable composite design [16]. The temperature used in this study for remendability was above 150°C, which is consistent with the expected retro reaction temperature of dicyclopentadiene-based systems. Prior to our discovery, the diglycidyl ester **2** and its use in reworkable underfill adhesive system have not been reported [11].

For the proof-of-concept study, the diacid **1** was made following a literature process by reaction of sodium cyclopentadienide (supplied as a THF solution, Boulder Scientific Company) with dry ice followed by acidification [17]. The diglycidyl diester **2** was synthesized by reacting the potassium salt of **1** with epibromohydrin in DMSO as solvent in 60–70% yield [11]. The literature process for making diacid **1** resulted in several isomers with the isomer represented by structure **1** being the major component. Since diacid **1** was a mixture of isomers, synthesis of **2** also resulted in a mixture of isomers. Both **1** and **2** were made in multi-kg scale in a production plant demonstrating industrial scalability. However, significant process change was necessary for the scale-up of **1** in the plant. The diacid **1** has been obtained as a single isomer before by a multistep process, but the synthesis used a different synthetic route [18]. During the process development for **1**, a proprietary isomerization process was developed to

**Figure 4.** *Synthesis of diglycidyl ester* **2** *from diacid* **1***.*

#### *High-Performance Reworkable Underfill Adhesives Based on Dicyclopentadiene Epoxy Thermoset DOI: http://dx.doi.org/10.5772/intechopen.107334*

convert mixture of isomers into mainly one isomer represented by the structure **1**. This allowed for better characterization of the diacid by 1 H NMR and other analytical techniques. The 1 H NMR of **1** thus obtained (**Figure 5**) matches closely with that reported in the literature [18]. For better structural confirmation, the diglycidyl ester **2** was also made starting from single isomer of **1**. While trace isomerization was observed during the synthesis, the 1 H NMR of **2** matches well with the assigned structure and shows predominantly one isomer (**Figure 5**). During formulation and testing, no significant dependence of isomer ratio in **2** on reliability or reworkability performance was seen. This is likely because the reactive epoxy functionality is remote from the adduct forming double bond carbon centers (sterics) to be consequential for polymerization reactivity. Also, all of the reported main isomers of **1** have the two carboxylic acid groups on the norbornene and cyclopentene rings of the DCPD unit, and thus, they are not expected to significantly affect the network breakdown [19].

Both **1** and **2** were analyzed for weight loss using thermogravimetric analysis (TGA). The onset of weight loss began around 160–170°C for both compounds consistent with the expected retro reaction temperature of the dicylopentadiene units. While the TGA weight loss progressed up to a temperature of about 250°C for both resins, it reached a plateau for **2** above this temperature (**Figure 6A**). It is likely that onset of homopolymerization of epoxy functionality in **2** at higher temperature stabilizes the weight loss while no such effect is possible for diacid **1**.

To investigate if facile uncatalyzed homopolymerization of epoxy functionality in **2** takes place above 200°C, a differential scanning colorimetry (DSC) thermogram

**Figure 5.**

**1** *H NMR spectra of* **1** *(CDCl3 with two drops of DMSO d6) and* **2** *(CDCl3).* **<sup>1</sup>** *H NMR spectra were run on Varian 300 MHz instrument.*

#### **Figure 6.**

*A) TGA thermograms for* **1** & **2** *(Tests performed using Discovery TGA 55 instrument at a ramp rate of 10°C per minute). B) DSC thermogram for neat resin* **2** *at a ramp rate of 10°C per minute (DSC was run using Q-100 DSC from TA Instruments).*

was run using neat resin without any added epoxy hardeners at a scan rate of 10°C/ minute. The DSC thermogram shows an exotherm with a peak position at 272.86°C (**Figure 6B**). The magnitude of the exotherm indeed confirms facile homopolymerization of the epoxy group taking place above 200°C without the need for added hardeners. The onset of exotherm is around the same temperature as the weight loss was seen stabilizing for resin **2** in the TGA (**Figure 6A**).

#### **2.2 Development of underfill formulations**

Epoxy resin **2** was formulated in several epoxy-only and epoxy-acrylic underfill formulations (**Table 1**). Typical epoxy-only underfill formulations contain bisphenol-A epoxy resin and bisphenol-F epoxy resins, which contribute to high Tg and modulus of the cured adhesives. The use of reactive diluents such as 4-tert-butylphenyl glycidyl ether is essential to lower viscosity to below 1000cPs for better capillary flow. The epoxy formulations also contain a blend of epoxy-imidazole adduct hardeners to balance cure rate and work life. Several epoxy-only formulations were developed (F1, F2, F3, and F4) by using varying levels of the key epoxy resin **2**. An epoxy-acrylic formulation F5 was also developed consisting of high Tg acrylic cross-linker such as tricyclodecane dimethanol diacrylate (saturated DCPD backbone), a high Tg acrylic diluent such as isobornyl methacrylate, and hybrid resins such as glycidyl methacrylate, which presumably links the epoxy and acrylic networks to form interpenetrating type networks (IPN). A radical initiator along with a combination of epoxy hardeners was also used for the curing of epoxy-acrylic hybrid formulation F5. **Table 1** shows several epoxy-only and epoxy-acrylic hybrid formulations (F1–F5) where the amount of the key reworkable resin **2** was varied to study its impact on properties such as Tg, modulus, adhesion, and reworkability. In formula F5, 5% of **2** was used for direct comparison with the corresponding epoxy-only formula F3. Formula F5 also contained 5% of commercially available cyanate ester bisphenol E cyanate ester. Cyanate esters have been known to co-cure well with epoxy resins, and they help lower the viscosity and improve the Tg and modulus of cured networks [20]. The formulations also contain additives such as carbon black color, dispersants, inhibitors, and silane adhesion promoters.

For good mechanical and thermal shock resistance, the underfill adhesive needs to exhibit a stable storage modulus in the service temperature and T-cycle test


#### *High-Performance Reworkable Underfill Adhesives Based on Dicyclopentadiene Epoxy Thermoset DOI: http://dx.doi.org/10.5772/intechopen.107334*

#### **Table 1.**

*Epoxy-only and epoxy-acrylic underfill formulations containing key reworkable resin* **2***.*

temperature range (−40 to 85°C). Since underfill adhesives distribute stress along the bondline rather than at the point of contact between the die and solder ball, the mechanical reliability improvement by their use has been well established (1, 2). Higher Tg and modulus in the adhesive further improve the mechanical reliability due to lower CTE mismatch below the Tg of the cured adhesive. At rework temperatures (typically > solder melting temperature), a low modulus adhesive would facilitate easy removal of defective chips. The storage modulus vs temperature plots for cured samples of formulations F-1 to F-4 were run using dynamic mechanical analysis (DMA) and compared with controls (**Figure 7**). During a typical application process with low volume applied, the adhesives were cured at 130°C for 10 minutes. For material property testing, the same temperature conditions were used but with longer cure times to ensure full cure. Two control formulations, Loctite Eccobond UF 3800 (epoxy-acrylate hybrid underfill) and Loctite Eccobond E-1216M (epoxy-only

**Figure 7.** *Storage modulus vs temperature plots for formulas F1, F2, F3, F4, and controls UF3800 and E1216M. ASTM D5023 method was used for DMA analysis.*

underfill), were used for the initial screening work. UF 3800 has inferior reliability because of its lower Tg, and it shows rapid decrease in storage modulus beginning around 70°C (**Figure 7**), which is well within the temperature range used for T-cycle reliability tests (**−**40°C to 85°C). This formula however has very good reworkability as it displays low modulus at the rework temperature (220°C). In contrast, formula E1216M exhibits stable modulus in the temperature range −55 to 120°C (good reliability), but its high temperature modulus is higher than UF3800, which is consistent with its poorer reworkability. It is important to note that the good reworkability seen with UF3800 formula (discussed later in the chapter) is a manifestation of relatively low modulus at higher temperatures. This formula does not contain any resin with built-in chemistry that causes network breakdown at higher temperatures. All the new epoxy-only formulations (F1, F2, F3, and F4) show a storage modulus profile similar to the highly reliable E1216M in the temperature range −55°C to 85°C (**Figure 7**). As expected at high temperatures, these formulas show storage moduli much lower than the best reworkable control UF3800, which indicates superior reworkability. The drop in storage moduli at higher temperatures corresponds well with the relative level of epoxy resin **2** present in these formulas. F-1 with the highest level (10%) shows the lowest high temperature modulus while F-4 with lowest level (2%) shows the highest modulus. The magnitude of modulus decreases seen at higher temperature with increased loading of **2** is consistent with expected higher network breakdown caused by the retro Diels-Alder reaction.

The DMA storage modulus plot for formula F-4 with the lowest level of resin **2** (2%) was also compared with three benchmark underfill formulas, UF3810, UF3808, and UF3800. None of these benchmarks have a built-in chemistry feature, which

#### *High-Performance Reworkable Underfill Adhesives Based on Dicyclopentadiene Epoxy Thermoset DOI: http://dx.doi.org/10.5772/intechopen.107334*

causes network breakdown at higher temperatures. As discussed previously, UF 3800 shows good reworkability at high temperature and has inferior reliability among the controls. In contrast, UF3808 has highest reliability and lower reworkability. UF3810 has an intermediate balance of both reliability and reworkability. The reliability and reworkability performance of these benchmark formulas have been well established by their use in commercial products. The mechanical reliability (shock, vibration, and drop resistance) performance of UF3808 and UF3810 correlates well with their higher Tg and modulus profiles shown in **Figure 8**. The T-cycle reliability performance of these three formulas also correlates well with their storage modulus profiles and is discussed separately later in this chapter. As compared with the benchmark formulas, F4 shows stable storage modulus over a wider temperature range (**Figure 8B**), which suggests superior mechanical and T-cycle reliability. A closer examination revealed that in the temperature range 130–180°C, F4 exhibited sharper modulus decrease than benchmarks. This phenomenon cannot be explained entirely by expected modulus drop typically seen above the Tg of the cured networks and is strongly indicative of network breakdown occurring in this temperature range. A similar modulus trend was also seen with epoxy-acrylic hybrid formula F5 (**Figure 8A**). It appears that the impact of resin **2** loading in F5 (5%) on storage modulus at higher temperature is lower as compared with the corresponding epoxy-only formula F3, which contains the same amount of **2** (**Figure 7**). The likely reason for this is discussed separately in the following section.

**Table 2** shows comparison of Tg and storage modulus numbers (MPa) at different temperatures (−75°C, 25°C, 85°C, 125°C, and 220°C) for formulas F1–F5 along with those for controls. The relative level of epoxy resin **2** in formulas F1–F5 did not appear to affect the Tg of cured networks significantly as measured by DMA. Formulas F2, F3, F4, and F5 show similar Tg as the highly reliable UF3808 control. In addition, they also exhibit relatively stable modulus numbers in the temperature range −75°C to 125°C similar to or better than UF3808 indicating good reliability performance. The storage modulus numbers for F1–F5 formulas at 220°C correlate well with the relative loading of resin **2,** as discussed previously. It is interesting to note that UF3800, which has proven reworkability in commercial HHDD products, has a higher modulus number at 220°C than formulas F1–F5. These results strongly correlate with superior reworkability for F1–F5 formulas arising from adhesive network breakdown caused by the retro Diels-Alder reaction. The reworkability tests performed on these formulas (discussed later) confirm this further.

#### **Figure 8.**

*Comparison of storage modulus of formulas F4 and F5 with control formulas. ASTM D5023 method was used for the DMA analysis.*


#### **Table 2.**

*Storage modulus numbers at different temperatures and Tgs for formulas F1–F5 and those for controls. ASTM D5023 method was used for the DMA analysis.*

#### **2.3 Die shear adhesion properties**

An important property of underfill adhesives is good die shear adhesion at room temperature (25°C) and the peak temperature experienced during an assembly reflow process (260°C). In contrast, reworkable underfill adhesives need to exhibit high die shear adhesion at room temperature for high reliability performance while that measured at higher temperature needs to be low enough for easy removal of faulty chip at the rework temperature (220°C), yet high enough to prevent any solder extrusion during a reflow step (< 1–2 min @ 260°C). The die shear tests were performed on Dage 4000 instrument from Dage Precision Industries following MIL-STD-883 2019.9 die shear method. The 25°C die shear tests were completed using 3 mm2 size SiN dies on bismaleimide-triazine (BT) substrate. Adhesion at 260°C using the same 3 mm2 size dies was too low among all underfills for direct comparison. Therefore, 260°C die shear tests were performed using 7.6 mm<sup>2</sup> size dies for better response and comparison of underfill adhesion. The die shear adhesion of reworkable formulas F1–F5 was measured and compared with controls.

**Figure 9** shows the comparison of die shear strengths at room temperature. Formulas F1–F5 show similar or superior die shear strength than the highly reliable underfill benchmark UF3808 while another control formula UF3800 shows the lowest strength among formulas tested. There appears to be no discernible trend between loading level of resin **2** and die shear adhesion at room temperature**.** However, results from the die shear tests performed at 260°C discussed next show a clear trend, which further confirms network breakdown happening at higher temperature in formulas F1–F5.

The die shear tests performed at 260°C further corroborate the storage modulus results shown in **Figures 7** and **8**. The 260°C die shear strengths for formulas F1–F5 and their comparison with benchmarks are shown in **Figure 10**. The shear strengths correlate well with the amount of reworkable resin **2** used in the experimental formulations. F1, which uses the highest (10%), shows the lowest while F4, which contains the lowest amount (2%), shows highest die shear values. These results further confirm that the network breakdown arising from the retro Diels-Alder reaction of the DCPD moiety takes place at high temperature resulting in lower bond strengths in the

*High-Performance Reworkable Underfill Adhesives Based on Dicyclopentadiene Epoxy Thermoset DOI: http://dx.doi.org/10.5772/intechopen.107334*

#### **Figure 9.**

*Die shear strength comparison of F1–F5 with benchmark formulas (UF3800, UF3808, and UF3810) at 25°C. MIL-STD-883 2019.9 die shear standard was used for the die shear tests using 3 mm2 size dies on BT substrate. The tests were performed at a load of 0.5 kg and speed of 13.77 mil/sec at a height of 11 mil (settings on Dage 4000).*

**Figure 10.**

*Die shear strength comparison at 260°C for formulas F1–F5 and benchmark underfill formulas following MIL-STD-883 2019.9 die shear standard. The tests were performed using 7.6 mm2 size dies on BT substrate at a load level of 0.5 kg and a head speed of 13.77 mil/s at a height of 11 mil.*

adhesive. The results are also consistent with the storage modulus results discussed previously for these formulas. The only slight deviation appears to be the hybrid epoxy-acrylic formula F5, which shows higher 260°C die shear adhesion as compared

**Figure 11.** *DSC thermogram for neat resin* **2** *in the presence of 10 wt% of tert-butyl peroxy-2-ethylhexanoate. DSC thermogram was run using Q-100 DSC from TA Instruments at a scan rate of 10°C/minute.*

with the corresponding epoxy-only formula F3 both of which have the same amount (5%) of epoxy resin **2**. While the formulation components are different, the higher 260°C die shear value seen with formula F5 is likely resulting from relatively lower network breakdown occurring at this temperature. Presumably, the norbornene double bond in **2** (more strained as compared with cyclopentene) undergoes some radical copolymerization with acrylic components in F5 effectively lowering the level of thermally reversible adducts in the cured network. To validate this, a DSC thermogram of neat resin **2** was run in the presence of 10% tert-butyl peroxy-2-ethylhexanoate radical initiator. The resulting thermogram (**Figure 11**) showed an exotherm with a peak at 132.96°C that can be ascribed to the radical homopolymerization of the norbornene double bond while no such peak was seen in **Figure 6B** discussed previously. While the magnitude of this exotherm is relatively small (116 J/g), it is likely that the norbornene double bond possessing an electron-withdrawing ester group (as present in structure **2**) would exhibit significantly higher reactivity in radical copolymerization with other acrylic components. Radical copolymerization reactivity of norbornene double bond connected to electron withdrawing ester group has been reported before [21].

#### **2.4 Reworkability study**

Select experimental underfill formulas F1 (contains 10% of **2**), F4 (2% of **2**), and F5 (5% of **2**, epoxy-acrylic) were tested for reworkability and compared with controls. An underfill board array containing 0.4 mm CSP (chip scale package or simply called chip, die or semiconductor or component) test board (ID# ACEM 94V0 1612, nomenclature 1502009) was used for the study. Sample pictures of the (**A**)

#### *High-Performance Reworkable Underfill Adhesives Based on Dicyclopentadiene Epoxy Thermoset DOI: http://dx.doi.org/10.5772/intechopen.107334*

unpopulated test board, (**B**) populated and underfilled board, and (**C**) zoom-in of a single underfilled die assembly unit are shown in **Figure 12**. When a faulty chip is identified after assembly in such a board with a multitude of semiconductors, the chip needs to be removed, residues cleaned up, and the chip assembly process is repeated so that board can be reused again. **Figure 13** illustrates the whole rework process. (**A**) A typical rework station used for this study, (**B**) chip removal tool with a focused heating element, (**C**) the scavenger nozzle for removing glue and solder residues after chip removal, (**D**) diagram of the scavenger nozzle illustrating the heating and vacuum suction to remove residues. A typical rework process consists of localized heating of the circuit board on a hot plate (heated to around 180–200°C), and the faulty die on top is heated using a hot air nozzle (typically around 220°C, picture **B**, **Figure 13**) for a few seconds. The die is then removed using suction tool optionally with additional force using a metal tool. The adhesive and solder residues are then cleaned using suction nozzle (picture **C**, **Figure 13**) to make the board reusable again. When a good reworkable adhesive is used, the whole rework process is typically completed in under 2 minutes.

**Table 3** shows qualitative evaluation guideline and score card for the board rework process. The rating of the rework process considers a multitude of factors such as ease of die removal, amount of underfill left on the board after cleaning, number of pads and traces damaged on the board, total cleaning time, and solder mask damage, each with its own weightage (total adds up to 1). The ease of rework is rated on a scale of 1–10 where a rating of 1 indicates poor reworkability and 10 best reworkability.

Benchmark underfill formulas UF3800, UF3808, and E1216M were compared with F1, F2, and F5 formulas for reworkability performance. A test board similar to that shown in **Figure 12** was used to bond the chips using benchmarks and the experimental formulas. **Table 4** shows total reworkability score for these formulas involving a multitude of factors discussed before. As expected, UF 3800 showed a high rating of 8 while the other two benchmark formulas UF3808 and E1216 formulas

#### **Figure 12.**

*Photographs of (***A***) unpopulated 0.4 mm BGA test board used for reworkability and reliability study; (***B***) test board array populated with 6 mm2 WLCSP (wafer level chip scale package) and underfilled; (***C***) zoom-in of a single, underfilled WLCSP on test board.*

#### **Figure 13.**

*Photographs of (A) a typical rework station; (B) chip removal tool with focused heating element; (C) the scavenger nozzle for removing glue and residues after component removal; (D) diagram of the scavenger nozzle illustrating the heating and vacuum suction.*

fared poorer for reworkability. The reworkability score for formulas F1, F4, and F5 was similar to or better than UF3800. The hybrid epoxy-acrylic formula F5 showed slightly inferior reworkability score than F1 and F2. The likely reason for relatively lower rework score for F5 as compared with F1 and F4 was discussed in the previous section and is suspected to be from the norbornene double bond copolymerization, which would result in partially (depending on extent of copolymerization) noncleavable networks.

**Figure 14** shows pictures of the test board after removal of the bonded die for the control formulas UF3800 and UF3808. The images on the left side show the substrate after removal of the die before rework and cleaning while those on the right show substrate board after cleaning. As expected, UF 3800 results in a clean board after die removal, rework, and cleaning. In contrast, UF 3808 causes damage to the board during die removal and leaves lots of adhesive residue even after cleaning. **Figure 15** shows images of the substrate board after die removal (on the left side) and after the rework process and cleaning (on the right) for formulas F1, F4, and F5. Consistent with the storage modulus profile and high-temperature die shear results discussed previously, all of these formulas enable easy rework process that is similar to or slightly better than reworkable UF3800 benchmark as evidenced by the clean substrate board obtained after the rework process. This result further demonstrates that resin **2** with built-in thermally reversible DCPD moiety enables good reworkability of underfill formulations.


**Table 3.** *Qualitative evaluation guideline and score card for board rework. Higher score indicates easier rework process.*

*High-Performance Reworkable Underfill Adhesives Based on Dicyclopentadiene Epoxy Thermoset DOI: http://dx.doi.org/10.5772/intechopen.107334*


*1 Highest score is 2. Higher number indicates lower amount of underfill left after die removal.*

*2 Highest score is 3, higher number indicates easier removal of die.*

*3 Highest score is 1, higher number indicates less damage to the pads on the board.*

*4 Highest score is 1, higher number indicates less damage to the traces on the board.*

*5 Highest score is 2, higher number indicates less time taken for cleaning.*

*6 Highest score is 1, higher number indicates less solder mask damage on the board.*

#### **Table 4.**

*Rework test scores for control formulas and F1, F4, and F5 formulas (higher total score indicates superior reworkability).*

#### **Figure 14.**

*Picture of substrates after removal of semiconductor before rework and cleaning for controls UF 3800 (top left) and UF3808 (bottom left). Picture of substrate board after underfill and solder residue removal for UF 3800 (top right) and UF3808 (bottom right). The marks indicated by the arrow are caused by removal of the adhesive that is not reworkable (UF3808).*

*High-Performance Reworkable Underfill Adhesives Based on Dicyclopentadiene Epoxy Thermoset DOI: http://dx.doi.org/10.5772/intechopen.107334*

#### **Figure 15.**

*Picture of substrates after removal of die before rework and cleaning for F1, F4, and F5 (on the left side). Picture of substrate board after cleaning (underfill and solder residue removal) for the same formulas (on the right side).*

#### **2.5 T-cycle reliability tests**

Since the HHDDs with bonded semiconductor components can be subjected to thermal shocks as described previously, the bonded components were tested for T-cycle reliability using an underfill board array containing 15 bonded semiconductor components similar to that shown in **Figure 12**. The reliability test was performed by using a 30-minute temperature ramp from −40°C to 85°C per cycle with a 15 min hold time at both temperatures (−40°C and 85°C). Air-to-air thermal cycling where the bonded parts cycle between an oven chamber and a freezer chamber in air was used for the study. The cycle is repeated several times, and the semiconductors on the board were tested for electrical failure. For good T-cycle reliability performance of HHDDs, no chip failure up to 1000 cycles is required.


*Fifteen bonded semiconductors were tested together for each formulation and the number of failures is indicated on the left side out of the total 15 components tested.*

#### **Table 5.**

*Air-to-air thermal cycling (−40°C* ← → *85°C, 30 min/cycle, 15 min hold).*

**Table 5** shows comparison of reliability performance of three benchmark formulas and the experimental underfill formulas F1, F4, and F5. The highly reworkable UF3800 formula started showing chip failures after 600 T-cycles (three failed chips) and complete failure of all the bonded chips after 2000 T-cycles (**Table 5**). In contrast, the highly reliable UF3808 and E1216 controls showed no failures up to 2000 T-cycles. As expected, none of the experimental formulas F1, F4, and F5 showed failures up to 2000 cycles indicating that the T-cycle reliability is similar to the highly reliable UF3808 and E1216M. The loading level of thermally reversible resin **2** did not appear to impact the reliability performance (compare F1 vs F4, **Table 5**). This result is consistent with reliability model established based on the storage modulus results discussed previously (**Figure 7**). As noted previously, the cured underfill adhesives also improve the mechanical reliability performance (shock, vibration, and drop) by distributing stress uniformly through the bondline. Higher Tg and modulus adhesives improve the reliability further similar to the reliability model shown before for T-cycle. Thus, the results discussed in this chapter demonstrate that using resin **2** with built-in reworkability feature, both high reliability and reworkability can be achieved concurrently with underfill formulations.

#### **3. Conclusions**

Synthesis of a new diglycidyl ester epoxy resin possessing dicyclopentadiene backbone was described. The resin was formulated in underfill adhesive formulations to provide cured adhesives with high Tg and modulus. Clear dependence of loading level of thermally reversible resin **2** on high-temperature die shear strength was seen. The formulated adhesives provided high mechanical and T-cycle reliability performance to the bonded semiconductor components consistent with the reliability model established based on storage modulus results. At high temperatures, the retro Diels-Alder reaction of the DCPD unit caused network breakdown enabling easy removability/reworkability of the bonded components. The network breakdown occurred at sufficiently above the cure temperature of the adhesive to not interfere during the cure of the adhesive. Thus, using this novel adhesive system, both high reliability and reworkability can be achieved simultaneously, which is generally not possible with

*High-Performance Reworkable Underfill Adhesives Based on Dicyclopentadiene Epoxy Thermoset DOI: http://dx.doi.org/10.5772/intechopen.107334*

other thermoset adhesive system. The key epoxy resin was scaled up to several kg's demonstrating industrial applicability of the high-performance reworkable adhesive technology.

#### **Conflict of interest**

The authors declare no conflicts of interest.

### **Author details**

Laxmisha M. Sridhar1 \* and Timothy M. Champagne2 \*

1 Henkel Corporation, Rocky Hill, CT, USA

2 Henkel Corporation, Irvine, CA, USA

\*Address all correspondence to: laxmishasridh@gmail.com and timothy.champagne@henkel.com

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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### *Edited by Longbiao Li, António B. Pereira and Alexandre Luiz Pereira*

Composite materials have become an optimal option for a range of modern, industrial, clinical, and sports applications, offering a material with the desired properties and a limitless choice of building and composite levels. They possess noteworthy physical, thermal, electrical, and mechanical properties, in addition to being lightweight and cost-effective. This book focuses on the next generation of fiber-reinforced composites and adhesives, including recent advances in their development as well as their applications in numerous fields.

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Next Generation Fiber-Reinforced Composites - New Insights

Next Generation Fiber-

Reinforced Composites

New Insights

*Edited by Longbiao Li, António B. Pereira* 

*and Alexandre Luiz Pereira*