Reliability Study of Low Silver Alloy Solder Pastes

Estimated reading time: 17 minutes

For the single-alloy QFP and QFN components, four different typical failure modes were observed. For SAC305 solder joints, cracking was primarily propagated along Cu6Sn5 and Ag3Sn intermetallic particles in the bulk of the solder (Figure 22). The single-alloy solder joints assembled with SAC0307, Material C and SnCuNi, in which far fewer intermetallic species were present, all showed evidence of ductile fracture at the grain boundaries between Sn dendrites, as shown in Figure 23. This was typically more severe than the cracking observed using SAC305 in the same components. Material D displayed a lesser degree of cracking than these three alloys. Cracking appeared to propagate at Sn grain boundaries (Figure 24). The SnBiAg solder joints displayed a different failure mode. Cracks were formed primarily as a result of grain boundary sliding (Figure 25). Some cracking through Sn grains also occurred as a result of the embrittlement caused by dissolution of bismuth in the tin matrix.

Figure 22: Typical cracking in QFP208 solder joint assembled using SAC305: ductile fracture propagating along grain boundaries of intermetallic species.

Figure 23: Cracking along Sn grain boundaries in QFP208 solder joint assembled using SAC0307.

Figure 24: Cracking along Sn grain boundaries in QFN88 solder joint assembled using Material D.

Figure 25: Cracking in QFN88 solder joint assembled using SnBiAg solder paste.

In the BGA solder joints, different effects were observed as a result of the mixing of the alloys. Cracking propagating along the grain boundaries of intermetallic species within the bulk of the solder joint were observed for the majority of components. However, the severity of this cracking was dependent on the solder paste alloy. More cracking appeared to occur when any of the four high-temperature low-silver alloys were used.

An increase in ductile fracture at the bottom of the solder joint is observed when using Material D solder. It is expected that as this alloy forms a more brittle solder joint than the others, a result of the Bi addition, that the upper part of the solder joint, in which SAC305 predominates, is more prone to plastic deformation than the lower part, which is likely to have a higher Bi content.

Many BGA solder joints assembled with SnBiAg solder paste remained inhomogeneous after thermal cycle test. Creep deformation of the BGA solder joints was visible. Grain boundary sliding occurred at the bottom of the solder ball, in the region composed primarily of Sn and Bi dendrites. It was observed that the SnBiAg solder joints cracked less than the low Ag high-temperature alloys for some of the BGA components and more for others. Cracking within the SAC305 area was less common than cracking within the SnBiAg area, the latter being more susceptible to deformation under strain. In several cases, cracking was visible at the interface between the SnBiAg and the SnAgCu regions of the BGA

Conclusions

There is no significant difference in the intermetallic layer thickness of the solder joints assembled with SAC305 and other alternative high temperature low-Ag, Pb-free alloy solder pastes (SAC0307, SnCuNi, Material C,Material D). In general, the IMC thickness of these materials slightly increased after thermal cycle test, but the change was negligible. The IMC thickness of the SnBiAg solder joints after reflow process was typically thinner than those of the high temperature lead-free alloys. The IMC layer of SnBiAg solder joints grew during the thermal cycle testing, to a similar IMC thickness to the other lead-free alloys. The thickness and composition of the intermetallic layers was not determined to affect the reliability of the solder joint during the thermal cycle testing.

The thermal reliability of alternative lead-free solder joint varied depending on the package types and the component size. In our study, this factor affected the solder joint’s thermal reliability more than the impact of alloy composition of the solder paste. The 2512 resistors was failed first as compared to other tested components. Complete failure and sever cracking were seen for most 2512 resistors after 3000 thermal cycles (0°C to 100°C). No complete failure was observed for small chip component such as 0603, 0402, 0201 components after the testing. Severe cracking and some failure were also observed for BGA196, BGA228, BGA97 and QFN88 after the thermal cycle testing. Minor cracking and no failure was seen for BGA1156, BGA64, QFN32, and QFP208 and QFP100 components. In general, solder joint assembled with SAC305 solder pastes still performed better than low Ag alloy solder pastes. Unexpectedly, low temperature SnBiAg solder joint performed well after thermal cycle test when it was the single alloy in the solder joint. When SAC 305 BGA reflowed with SnBiAg solder pastes, more defects and failures were seen. Further reliability study should be done for alternative lead-free alloy solder paste materials.

Acknowledgements

The authors would like to thanks Elissa McKay and Tu Tran at the company AEG labs for their help in the cross sections and failure analysis of this study.

References

1. J. Smetana, R. Coyle, P. Read, T. Koshmeider, D. Love, M. Kolenik and J. Nguyen, “ Thermal Cycling Reliability Screening of Multiple Pb-Free Solder Ball Alloys,” Proceeding of APEX conference, 2010.

2. G. Henshall, et al., “INEMI Pb-Free Alloy Characterization Project Report: Part I-Program Goals, Experimental Structure, Alloy Characterization and Test Protocols for Accelerated Thermal Cycling,” Proceeding of SMTAi, 2012.

3. K. Sweatman, et al., “iNEMI Pb-Free Alloy Characterization Project Report: Part III – Thermal Fatigue Results for Low Ag Alloys” Proceeding of SMTAi, 2012.

4. Jennifer Nguyen, et al., “Assembly Process Feasibility of Low/No Silver Alloy Solder Paste Materials” Proceeding of APEX conference, San Diego, 2013.

5. S. Sakuyama, T. Akamatsu, K. Uenishi, T. Sato, Trans. Japan Inst. Electron. Packaging V.2, 2009, pp. 98-103.

6. I. E. Anderson, B. A. Cook, J. L. Harringa, R. L. Terpstra, J. Electron. Mater. V.31, 2002, pp. 1166-1174.

7. H. Shimokawa, T. Soga, K. Serizawa, Mater. Trans. V.8, 2002, pp. 1808-1815.

Editor's Note: This paper has been published in the technical proceedings of IPC APEX EXPO.

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