3.4 T/C reliability

Figure 16 shows the strain amount decrease of acrylic based Sn-58Bi solder ACFs as a function of temperature by adding 0, 5 and 10 wt% of 0.2 μm silica fillers. Both the amount of the thermal expansion and the elastic strain of polymer resin were reduced by adding silica filler. It was known that reducing the thermal expansion strain and the elastic strain as a function of temperatures are good for the joint reliability in T/C test and the good solder joint formation, respectively [35]. Storage modulus of acrylic based Sn-58Bi solder ACFs with addition of 0, 5, and 10 wt% 0.2 μm silica was increased from 0.4 to 0.9 and 1.3 Mpa in Figure 17,

Temperature 200°C 150°C 100°C 50°C 30°C Modulus 0.4 MPa 0.44 MPa 0.53 MPa 5.2 MPa 34 MPa

Effects of delaying ultrasonic horn lift-up times (0, 1.2, 5.4, 30.2, and 60.3 seconds) on the Sn-58Bi solder

Strain of acrylic based Sn-58Bi solder ACFs with added silica fillers as a function of temperatures.

respectively.

Lead Free Solders

Table 3.

Figure 15.

Figure 16.

74

ACFs joint morphologies.

Storage modulus of acrylic adhesive during cooling process.

Figure 18 were listing the solder joint morphologies by adding 0, 5 and 10 wt% 0.2 μm sized silica filler of acrylic based Sn-58Bi solder ACFs joints before and after T/C reliability for 1000 cycles. Referring to the enhancement of elastic modulus by adding several silica fillers in Figure 17(b), solder joint cracks were completely removed for acrylic based Sn-58Bi solder ACF joints at 200°C TC bonding condition with several bonding pressures.

For acrylic based Sn-58Bi solder ACFs without added silica fillers, total joint failure occurred at the interface between Sn-58Bi solder joints and Cu metal electrode after 1000 cycles T/C reliability test, because initial solder joint cracks have already existed before the T/C reliability test. Although elastic modulus was increased double by adding 5 wt% 0.2 μm silica fillers, a small solder crack still remained and propagated at Sn-58Bi solder joints resulting in unstable joint contact resistance during T/C reliability. Excellent solder joint morphology was obtained after the 1000 cycles T/C reliability, because initial solder joint crack was perfectly

Figure 17.

Elastic modulus of acrylic based Sn-58Bi solder ACFs by adding 0, 5, and 10 wt% 0.2 μm silica fillers at (a) 0–80 MPa and (b) 0–10 MPa ranges.


#### Figure 18.

Solder joint morphologies of acrylic based Sn-58Bi solder ACFs added by 0, 5, and 10 wt% 0.2 μm silica filler before and after 1000 cycles reliability test.

removed at Sn-58Bi solder ACF joints by adding 10 wt% 0.2 μm silica fillers. Figure 19 shows the 1000 cycles T/C reliability results in terms of contact resistance, and stable joint contact resistance was achieved by adding 10 wt% 0.2 μm

In this paper, we investigated the resin properties and bonding parameters on the solder joint morphologies of Sn-Bi58 ACF joints. As a result, we found storage modulus of resin adhesives was the determined factor for solder joint cracks and regardless of bonding pressures. We thought cracks at solder joints happened, probably due to the high elasticity of polymer resin. Apart from that, two suggestions were listed to solve the solder joint cracks by increasing the resin storage modulus. The 1st one was to remain the hot-bar until cooling to its Tg, but this method will excessively consume solder and lead to brittle IMC at interfaces in TC bonding, while it is ok to US bonding. The other method was to add silica fillers in polymer resin to increase its thermos-mechanical property and reduce the polymer rebound when bonding process was finished. For ultrasonic bonding, storage modulus above 5 MPa of was at least needed to prevent solder joint cracks. On the contrast, 70 seconds for maintaining bonding pressure was too long. More than 1.38 MPa storage modulus at 200°C was needed for a crack-free Sn-58Bi solder joint

The authors thank National Natural Science Foundation of China (Grant

silica fillers due to crack-free solder joint.

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

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

morphology for a conventional TC bonding.

51805115) for research funding support.

The authors declare no conflict of interest.

Acknowledgements

Conflict of interest

77

4. Conclusion(s)

Figure 19.

Solder joint contact resistances of acrylic based Sn-58Bi solder ACFs up to 1000 cycles T/C reliability as a function of (a) 0, (b) 5, and (c) 10 wt% 0.2 μm silica fillers addition.

removed at Sn-58Bi solder ACF joints by adding 10 wt% 0.2 μm silica fillers. Figure 19 shows the 1000 cycles T/C reliability results in terms of contact resistance, and stable joint contact resistance was achieved by adding 10 wt% 0.2 μm silica fillers due to crack-free solder joint.
