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

Shuye Zhang,Tiesong Lin, Peng He and Kyung-Wook Paik

### Abstract

In this chapter, solder joint cracks at Sn-Bi58 solder ACF joints were investigated in conventional thermal compression bonding and ultrasonic bonding. It was found that resin storage modulus is the crucial for solder joint morphology regardless of bonding pressures. At high temperature, polymer resin tends to rebound above Tg and break the molten solder morphology. We proposed two useful methods to keep off solder joints cracks during bonding process. One is to remain bonding pressure until room temperature, the other is to use fillers to increase resin thermal mechanical property. The thermal cycling reliability was significantly enhanced when solder joint morphology was modified using 10 wt% 0.2 μm SiO2 fillers in acrylic based Sn-Bi58 solder ACF joints.

Keywords: solder cracks, ACF assembly, flex-on-board assembly, high reliability

## 1. Introduction

In 2013, Google Company had launched Google Glass, which the first-generation of wearable electronics in the history of humans [1]. Apart from the limited packaging sizes, high-density packaging technologies are demand for chips, passive components, and printed circuit boards. Several functions such as cameras, global positioning system (GPS), wireless communication, touch screen, FM radio, Audio, are also featured in Google Glass [2]. Generally, socket-type connectors have been used to connect between a flexible printed circuit (FPC) module and the main board of Google Glass [3], on the purpose of electrical interconnect. Flex-on-board (FOB) is one type of flip-chip technologies, to assembly printed circuits board (PCB) and FPC using anisotropic conductive films (ACFs) [4].

Aiming at replacing the socket-connectors, FOB assembly is attracting more and more attention, due to a lower thickness (about 50 μm) and a higher fine-pitch capability (under 100 μm) [5]. So Google started to use FOB in mother board assembly to partly take place of connectors, as shown in Figure 1. ACFs are usually to be as the interconnection materials to assembly FOB, consisting of thermosetting polymer adhesive matrix and conductive balls [6]. Adhesives will be cured by temperature and functional group will be cross-linked, resulting into the mechanical connection to PCB board and metal pad surfaces [7]. Current flows through an ACF joint formed by a physical contact between electrodes and conductive balls (such as Au/Ni metal balls or Au/Ni coated polymer balls) [8].

Figure 2 shows an Au/Ni metal ACF joint and a Sn based metallurgical ACF joint in a cross-section view using a scanning electron microscope. Compared with

elements quickly diffused into Au/Ni metal surface to form brittle intermetallic compound (IMC), cracks were formed by the residual Bi phase and the newly formed IMC phases. As a result, 10 seconds bonding time was optimized to avoid

At optimized 10 seconds bonding time, depending on their solder melting temperatures, SAC305 (221°C) and Sn-58Bi (139°C) solder ACFs are bonded at 250 and 200°C joint temperatures, respectively. For those resins with high elastic modulus (such as imidazole resin, multifunctional epoxy enhanced imidazole resin, and cationic epoxy resin), solder cracks were rarely found at ACF joints after TC assembly in FOB interconnection. On the contrary, for these resins with low elastic modulus (such as acrylic resin), solder joint crack was a critical issue after FOB assembly [17]. Although bonding temperature was suggested not higher than 220°C for acrylic ACF resins to avoid thermal decomposition and solder joint cracks, solder joint cracks were even observed at low modulus based acrylic resin joints at

Although polymer rebound of the cured acrylic resin had been measured as approximate 1–3% dimension change of polymer resin, when the bonding pressure was released at 200°C bonding temperature [17]. In this chapter, we aimed at finding out the obvious inner factor of polymer resins to determine Sn-58Bi solder cracks after FOB assembly at acrylic ACF joints, rather than a perfect solder joint morphology using other ACF resins. After that, two available throughout methods were discussed to increase acrylic resin elastic modulus to solve solder joint crack after FOB assembly. Moreover, the consequent solder joint morphologies were observed, compared and analyzed. The significance of this research is to guide ultra-low elastic modulus ACF resin assembly to form reliable solder joints for low

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

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,

over-diffusion behavior of Sn elements at micron solder ACF joints.

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

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

melting solder materials and electronic device packaging.

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

200°C bonding temperatures.

2. Experiments

Figure 3.

65

2.1 Test vehicles and materials

(ENIG) finish were plated on test vehicles.

Figure 1. Google glass teardown and FOB interconnection.

#### Figure 2.

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

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 controlled from 70 to 250°C.

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

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

elements quickly diffused into Au/Ni metal surface to form brittle intermetallic compound (IMC), cracks were formed by the residual Bi phase and the newly formed IMC phases. As a result, 10 seconds bonding time was optimized to avoid over-diffusion behavior of Sn elements at micron solder ACF joints.

At optimized 10 seconds bonding time, depending on their solder melting temperatures, SAC305 (221°C) and Sn-58Bi (139°C) solder ACFs are bonded at 250 and 200°C joint temperatures, respectively. For those resins with high elastic modulus (such as imidazole resin, multifunctional epoxy enhanced imidazole resin, and cationic epoxy resin), solder cracks were rarely found at ACF joints after TC assembly in FOB interconnection. On the contrary, for these resins with low elastic modulus (such as acrylic resin), solder joint crack was a critical issue after FOB assembly [17]. Although bonding temperature was suggested not higher than 220°C for acrylic ACF resins to avoid thermal decomposition and solder joint cracks, solder joint cracks were even observed at low modulus based acrylic resin joints at 200°C bonding temperatures.

Although polymer rebound of the cured acrylic resin had been measured as approximate 1–3% dimension change of polymer resin, when the bonding pressure was released at 200°C bonding temperature [17]. In this chapter, we aimed at finding out the obvious inner factor of polymer resins to determine Sn-58Bi solder cracks after FOB assembly at acrylic ACF joints, rather than a perfect solder joint morphology using other ACF resins. After that, two available throughout methods were discussed to increase acrylic resin elastic modulus to solve solder joint crack after FOB assembly. Moreover, the consequent solder joint morphologies were observed, compared and analyzed. The significance of this research is to guide ultra-low elastic modulus ACF resin assembly to form reliable solder joints for low melting solder materials and electronic device packaging.
