2. Experiments

a metal ACF joint, high contact resistance, poor power handling capacity and reliability can be improved by using solder metallurgical ACF joint, due to wide electrical paths and stable metallurgical interconnection [9, 10]. In order to remove solder oxide layer and improve solder wettability, two methods, a thermal compression (TC) bonding combining a flux material [11] and an ultrasonic (US) bonding without flux materials [12], are used. According to previous results, the heating rates were raised rapidly to 400°C/s and the temperature of solder ACF joints reached above 250°C under US vibration. By adjusting various ultrasonic amplitudes of vibration (from 4 to 13 μm), the ACF temperature could be precisely

A comparison of the conventional Ni and Sn solder metallurgical anisotropic conductive films (ACFs) joints.

A perfect solder ACF joint morphology containing a Sn–3Ag–0.5Cu (SAC305) alloy has been optimized by using lower viscosity, faster curing speed, higher resin property based cationic epoxies with high elastic modulus on a 250°C bonding temperature for FOB assembly [13]. Low viscosity helps resin flow during bonding process [14] and faster curing speed indicates higher cross-linking density and mechanical property of polymer resins [15]. Compared with acrylic resin, imidazole resin and multifunctional epoxy enhanced imidazole resin, cationic epoxy resin has the highest elastic modulus when it is fully cured, therefore, few solder joint cracks are taken place at ACF joints after FOB assembly. Not only resin property is a basic issue, bonding time also plays an important role in solder joint morphologies, especially for cracks [16]. Since micron sized solder ACF joint is so tiny that Sn

controlled from 70 to 250°C.

Figure 1.

Lead Free Solders

Figure 2.

64

Google glass teardown and FOB interconnection.

#### 2.1 Test vehicles and materials

Test vehicles was shown in Figure 3. FR-4 printed circuit board (PCB) was 1-mm-thick and flexible printed circuit (FPC) board was made by polyimide with 50-μm-thick, and 500-μm-pitch Cu patterns with electroless nickel immersion gold (ENIG) finish were plated on test vehicles.

Three kinds of polymer resins were compared for the ACFs, acrylic resin, imidazole resin and cationic resin. These products were all bought from H&S company in South Korea. About 5 wt% 8-μm-diameter Ni particles, 0.2 μm silica fillers,

Figure 3. 500-μm-pitch printed circuit board (PCB) and flexible printed circuit (FPC) board.


#### Table 1.

The specification of solder ACFs.

30 wt% 25–32 μm diameters Sn-58Bi particles, and 2 wt% flux material were added in the pure resins and then proceeded the film coating process. After that, 50-filmthickness anisotropic conductive film was achieved. Table 1 gives the specifications of the added materials, such as weight percentages of the pure polymer resins and the calculated volume percentages of the total ACFs materials with additives.

Molten solder joint was like a water above its melting temperature and squeezed by bonding pressure, resulting into uniform solder joint morphology after bonding [18–20]. Thus, Ni particles were used to obtain uniform joint gap size and control solder joint morphology. Before coating process of ACF, polymer matrixes and conductive particles were stated in a solution. Silica fillers were gradually added as a function of weight percentage by hands [21–23]. Since the solution viscosity was be very high when adding the nano-size solid silica fillers, plenty of toluene would be added to decrease the solution viscosity and make sufficient dissolution between nano sized silica fillers and polymer matrixes. In addition, considering the agglomeration of nano size silica fillers, a magnetic stirring apparatus was carried out at 30° C for 0.5 hour and then a rolling mechanical stirring was performed at room temperature for 12 hours. As a result, a sufficient dissolution between nano size silica filler and polymer resin solution was achieved. Figure 7 shows the fresh ACFs were put on the PCB ENIG metal electrodes before bonding process.

process should be investigated. At here, in-situ temperature is precisely measured

Figure 6 shows another mechanism of local heat generation by ultrasonic vibration at room temperature. Unlike TC bonding, US bonding was applied at room temperature and bonding pressure can be controlled as a function of time, which means the bonding pressure can be maintained through the whole bonding procedure until ACF joint cooling to room temperature [25]. In this way, ACF adhesives and solder joint were protected under bonding pressure during cooling process, to reduce the influence of heated resin to molten solder joints. During the US bonding, polymer resin can also be cured by the spontaneous ultrasonic vibration at room temperature

environment. Compared with TC bonding, the joint temperature was slowly

Mechanism of local heat generation by ultrasonic vibration at room temperature.

by a K-type thermocouple every 0.1 seconds [25].

The in-situ temperature profile of solder ACFs joint by a TC bonding.

2.2.2 Ultrasonic bonding

Figure 4.

Figure 5.

Figure 6.

67

Heat conduction mechanism in a TC bonding.

A Review: Solder Joint Cracks at Sn-Bi58 Solder ACFs Joints

DOI: http://dx.doi.org/10.5772/intechopen.83298

#### 2.2 Bonding methods

#### 2.2.1 Thermo-compression bonding

According to previous results [17], the bonding parameters (peak temperature, time, and pressure) were optimized as 200°C 10 seconds and 1 MPa. Figure 4 gives a heat conduction mechanism from a hot bar to ACF joint on a FOB application. Thermal setting adhesives were cured due to the heat conduction from the high temperature of hot bar. Figure 5 shows a clear temperature profile according to previous result, where temperature was quickly up to Sn-58Bi solder melting point (139°C) and then gradually reached the 200°C peak temperature at 4 seconds. Afterwards, solder ACF joint temperature was remained at 200°C from the 4th to 10th second, and then solder ACF joint entered cooling procedure without any pressure protect.

However, it showed solder joint temperature reached 139°C in the cooling process until 11th second, which means from 10th to 11th second solder ACF joint freely cooled and was not protected with pressure. It is well known that molten solder joint is very weak and its morphology is easy to be destroyed by polymer property, for example, polymer will be rebound on the moment of TC bonding finish and hot bar releasing from test vehicles [24]. Therefore, the mismatch between resin property and molten Sn-58Bi solder property under the cooling

A Review: Solder Joint Cracks at Sn-Bi58 Solder ACFs Joints DOI: http://dx.doi.org/10.5772/intechopen.83298

#### Figure 4.

30 wt% 25–32 μm diameters Sn-58Bi particles, and 2 wt% flux material were added in the pure resins and then proceeded the film coating process. After that, 50-filmthickness anisotropic conductive film was achieved. Table 1 gives the specifications of the added materials, such as weight percentages of the pure polymer resins and the calculated volume percentages of the total ACFs materials with additives.

Silica filler (2.65 g/cm3 )

ACF 1 30% 1 5% 0% 6.25% 92.75% 1% 0% ACF 2 30% 1 5% 5% 6.1% 89.7% 0.97% 3.23% ACF 3 30% 1 5% 10% 5.9% 86.9% 0.93% 6.27%

Weight percentage Calculated volume percentage

Sn-58Bi solder (8.56 g/cm<sup>3</sup> )

Polymer resin (1.25 g/cm<sup>3</sup> )

Ni particles (8.9 g/cm3 )

Silica filler (2.65 g/cm<sup>3</sup> )

Molten solder joint was like a water above its melting temperature and squeezed by bonding pressure, resulting into uniform solder joint morphology after bonding [18–20]. Thus, Ni particles were used to obtain uniform joint gap size and control solder joint morphology. Before coating process of ACF, polymer matrixes and conductive particles were stated in a solution. Silica fillers were gradually added as a function of weight percentage by hands [21–23]. Since the solution viscosity was be very high when adding the nano-size solid silica fillers, plenty of toluene would be added to decrease the solution viscosity and make sufficient dissolution between nano sized silica fillers and polymer matrixes. In addition, considering the agglomeration of nano size silica fillers, a magnetic stirring apparatus was carried out at 30° C for 0.5 hour and then a rolling mechanical stirring was performed at room temperature for 12 hours. As a result, a sufficient dissolution between nano size silica filler and polymer resin solution was achieved. Figure 7 shows the fresh ACFs

were put on the PCB ENIG metal electrodes before bonding process.

entered cooling procedure without any pressure protect.

According to previous results [17], the bonding parameters (peak temperature, time, and pressure) were optimized as 200°C 10 seconds and 1 MPa. Figure 4 gives a heat conduction mechanism from a hot bar to ACF joint on a FOB application. Thermal setting adhesives were cured due to the heat conduction from the high temperature of hot bar. Figure 5 shows a clear temperature profile according to previous result, where temperature was quickly up to Sn-58Bi solder melting point (139°C) and then gradually reached the 200°C peak temperature at 4 seconds. Afterwards, solder ACF joint temperature was remained at 200°C from the 4th to 10th second, and then solder ACF joint

However, it showed solder joint temperature reached 139°C in the cooling process until 11th second, which means from 10th to 11th second solder ACF joint freely cooled and was not protected with pressure. It is well known that molten solder joint is very weak and its morphology is easy to be destroyed by polymer property, for example, polymer will be rebound on the moment of TC bonding finish and hot bar releasing from test vehicles [24]. Therefore, the mismatch between resin property and molten Sn-58Bi solder property under the cooling

2.2 Bonding methods

66

Solder ACFs

Lead Free Solders

Table 1.

Sn-58Bi solder (8.56 g/cm<sup>3</sup> )

The specification of solder ACFs.

Polymer resin (1.25 g/cm3 )

Ni particle (8.9 g/cm3 )

2.2.1 Thermo-compression bonding

Heat conduction mechanism in a TC bonding.

Figure 5. The in-situ temperature profile of solder ACFs joint by a TC bonding.

process should be investigated. At here, in-situ temperature is precisely measured by a K-type thermocouple every 0.1 seconds [25].

#### 2.2.2 Ultrasonic bonding

Figure 6 shows another mechanism of local heat generation by ultrasonic vibration at room temperature. Unlike TC bonding, US bonding was applied at room temperature and bonding pressure can be controlled as a function of time, which means the bonding pressure can be maintained through the whole bonding procedure until ACF joint cooling to room temperature [25]. In this way, ACF adhesives and solder joint were protected under bonding pressure during cooling process, to reduce the influence of heated resin to molten solder joints. During the US bonding, polymer resin can also be cured by the spontaneous ultrasonic vibration at room temperature environment. Compared with TC bonding, the joint temperature was slowly

Figure 6. Mechanism of local heat generation by ultrasonic vibration at room temperature.

by the thickness of 50 μm, the length of 10 mm, and the width of 2.5 mm, and it was cured at the oven environment of 150°C for 3 hours. In DMA environment, resin film was tested under a 0.1 Hz load with a 10 mN dynamic stress, and the static tensile force was prepared by 50 mN. In addition, the DMA environment was heated by 5°C/ minutes until 200°C from room temperature. The elastic property of polymer resin in this study was considered in z direction, because the polymer rebound tool place vertically, resulting into the ACF dimension change and solder joint cracks. As Eq. (1) was shown in the following, elastic strain is mathematically determined by the ratio of length change (ΔL) over the initial ACF length (L). The larger changed length of polymer resin by DMA force indicates the larger polymer rebound when bonding tool was disappeared. In addition, the coefficient of thermal expansion (CTE) of adhe-

After bonding process, ACFs joint was formed and its resistance was detected by a contact method, which is called 4-point-probe kelvin method. Referring to Ohm's Law, we have learnt that resistance is mathematically named by the ratio of electrical voltage above its corresponding current. As a constant current was applied through all over the circuit, the resistance of the specific part is easy to know unless

Actually, a delta mode in KI 6220 nano-voltmeter has been widely used to measure the microscale ohms from the American Keithley company. The constant current was designed and perfectly applied in Cu lines at PCB and FPC substrates, as a result, solder ACFs joints were performed by current, voltage drop by can be measured by KI 6220 m, which was shown as the grown overlapped part in Figure 8. For accurately measuring the contact resistance of solder ACF joint, one measurement is repeated by 10 times. In details, Cu pad areas was 0.3 mm<sup>2</sup> and 40

A scanning electron microscope (SEM) in this paper was utilized to apparently observe the changing of solder ACFs joint morphologies before and after sever cracks. In order to obviously compare Sn and Bi elements in solder joint morphologies, a backscattered electron mode was carried out in SEM environment. For the typical solder morphologies, such as solder joint heights, shapes, and cracks, we

To characterize cracks at solder joint morphologies on the effects of electrical performance, a thermal cycling reliability from �45 to 125°C was tested until 1000 cycles. TC bonded Sn-Bi58 solder ACFs joints with 0, 5, and 10 wt% of 0.2 μm

performed at SEM images and evaluated them at certain conditions.

Strain ¼ ð Þ ΔL =L (1)

sives were also measured by a 50 mN tensile force.

A Review: Solder Joint Cracks at Sn-Bi58 Solder ACFs Joints

DOI: http://dx.doi.org/10.5772/intechopen.83298

2.5 Joint resistance and morphologies

the voltage is precisely measured [31].

channels were designed in PCB test vehicles.

Electrical design for a four-point-probe measurement.

2.6 Reliability evaluation

Figure 8.

69

Figure 7. The in-situ temperature and the designed lift-up time of bonding pressure in US bonding.

increased up to 200°C, however, resin would be fully cured and solder metallurgical joint could be obtained as well [26]. Figure 7 shows the in-situ ACFs joint temperature and the designed lift-up time of bonding pressure in US bonding. Referring to previous results, polymer rebound amount will be decreased as temperature decreased [17]. At here, we focus whether it is possible to prevent the solder joint damage at a lower temperature releasing bonding pressure during the cooling process.

#### 2.3 Differential scanning calorimetry (DSC)

Many studies have been reported the curing degree of acrylic ACF joints by TC or US bonding using DSC [14, 17, 26, 27]. It has been convinced that acrylic resin can be fully cured at 200°C 10 seconds condition and provide the best thermo-mechanical property after its full cure [28]. Considering there is an exothermic peak from 75 to 150°C caused by acrylic resin cure and a sharp endothermic peak from 135 to 142°C caused by Sn-58Bi solder melting, there might be interplay by two materials and unclear to demonstrate the Sn-58Bi solder melting in the temperature heating-up period. On the other hand, it is important to know the solidification temperature of Sn-58Bi alloy during the temperature cooling-down period. Because mechanical protection of Sn-58Bi alloy will be established when alloy is solidified [29, 30].

Therefore, only pure Sn-58Bi alloy was tested by DSC to obtain the melting and solidification temperature, respectively. In details, Sn-58Bi solder alloy was put by 20 mg weight. The heating rate was 20°C/minutes from 30 to 300°C, afterwards, the temperature was along with furnace cooling to room temperature at a nitrogen environment. The curing behavior of acrylic resin used in this study and curing degree after bonding process measured by a Fourier transform infrared spectroscopy can be found at here [14, 17, 26, 27].

#### 2.4 Thermomechanical analysis

A thermomechanical analyzer (DMA) was used to measure the storage modulus of the cured polymer resin as a function of temperature. The resin film was prepared A Review: Solder Joint Cracks at Sn-Bi58 Solder ACFs Joints DOI: http://dx.doi.org/10.5772/intechopen.83298

by the thickness of 50 μm, the length of 10 mm, and the width of 2.5 mm, and it was cured at the oven environment of 150°C for 3 hours. In DMA environment, resin film was tested under a 0.1 Hz load with a 10 mN dynamic stress, and the static tensile force was prepared by 50 mN. In addition, the DMA environment was heated by 5°C/ minutes until 200°C from room temperature. The elastic property of polymer resin in this study was considered in z direction, because the polymer rebound tool place vertically, resulting into the ACF dimension change and solder joint cracks. As Eq. (1) was shown in the following, elastic strain is mathematically determined by the ratio of length change (ΔL) over the initial ACF length (L). The larger changed length of polymer resin by DMA force indicates the larger polymer rebound when bonding tool was disappeared. In addition, the coefficient of thermal expansion (CTE) of adhesives were also measured by a 50 mN tensile force.

$$\text{Strain} = (\Delta \text{L}) / \text{L} \tag{1}$$

#### 2.5 Joint resistance and morphologies

After bonding process, ACFs joint was formed and its resistance was detected by a contact method, which is called 4-point-probe kelvin method. Referring to Ohm's Law, we have learnt that resistance is mathematically named by the ratio of electrical voltage above its corresponding current. As a constant current was applied through all over the circuit, the resistance of the specific part is easy to know unless the voltage is precisely measured [31].

Actually, a delta mode in KI 6220 nano-voltmeter has been widely used to measure the microscale ohms from the American Keithley company. The constant current was designed and perfectly applied in Cu lines at PCB and FPC substrates, as a result, solder ACFs joints were performed by current, voltage drop by can be measured by KI 6220 m, which was shown as the grown overlapped part in Figure 8. For accurately measuring the contact resistance of solder ACF joint, one measurement is repeated by 10 times. In details, Cu pad areas was 0.3 mm<sup>2</sup> and 40 channels were designed in PCB test vehicles.

A scanning electron microscope (SEM) in this paper was utilized to apparently observe the changing of solder ACFs joint morphologies before and after sever cracks. In order to obviously compare Sn and Bi elements in solder joint morphologies, a backscattered electron mode was carried out in SEM environment. For the typical solder morphologies, such as solder joint heights, shapes, and cracks, we performed at SEM images and evaluated them at certain conditions.

#### 2.6 Reliability evaluation

To characterize cracks at solder joint morphologies on the effects of electrical performance, a thermal cycling reliability from �45 to 125°C was tested until 1000 cycles. TC bonded Sn-Bi58 solder ACFs joints with 0, 5, and 10 wt% of 0.2 μm

Figure 8. Electrical design for a four-point-probe measurement.

increased up to 200°C, however, resin would be fully cured and solder metallurgical joint could be obtained as well [26]. Figure 7 shows the in-situ ACFs joint temperature and the designed lift-up time of bonding pressure in US bonding. Referring to previous results, polymer rebound amount will be decreased as temperature decreased [17]. At here, we focus whether it is possible to prevent the solder joint damage at a lower temperature releasing bonding pressure during the cooling process.

The in-situ temperature and the designed lift-up time of bonding pressure in US bonding.

Many studies have been reported the curing degree of acrylic ACF joints by TC or US bonding using DSC [14, 17, 26, 27]. It has been convinced that acrylic resin can be fully cured at 200°C 10 seconds condition and provide the best thermo-mechanical property after its full cure [28]. Considering there is an exothermic peak from 75 to 150°C caused by acrylic resin cure and a sharp endothermic peak from 135 to 142°C caused by Sn-58Bi solder melting, there might be interplay by two materials and unclear to demonstrate the Sn-58Bi solder melting in the temperature heating-up period. On the other hand, it is important to know the solidification temperature of Sn-58Bi alloy during the temperature cooling-down period. Because mechanical pro-

Therefore, only pure Sn-58Bi alloy was tested by DSC to obtain the melting and solidification temperature, respectively. In details, Sn-58Bi solder alloy was put by 20 mg weight. The heating rate was 20°C/minutes from 30 to 300°C, afterwards, the temperature was along with furnace cooling to room temperature at a nitrogen environment. The curing behavior of acrylic resin used in this study and curing degree after bonding process measured by a Fourier transform infrared spectros-

A thermomechanical analyzer (DMA) was used to measure the storage modulus of the cured polymer resin as a function of temperature. The resin film was prepared

tection of Sn-58Bi alloy will be established when alloy is solidified [29, 30].

2.3 Differential scanning calorimetry (DSC)

Figure 7.

Lead Free Solders

copy can be found at here [14, 17, 26, 27].

2.4 Thermomechanical analysis

68

SiO2 fillers addition were compared in this thermal cycling test. What is more, the dwell time of -45°C was remained for 15 minutes and it rapidly increased to 125°C for 15 minutes dwelling time. Joint contact resistances in this thermal cycling test were designed to be recorded every 200 cycles. Until 1000 cycles, joint morphologies and each failure mode of Sn-Bi58 solder ACF joints with or without silica modified were compared through the observation of SEM images.
