Microstructure and Reliability of Low Ag, Bi-Containing Solder Alloys

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Microstructure and Reliability of Low Ag, Bi-Containing Solder Alloys

This paper investigates a number of low (or no) Silver (Ag), Bi-containing Pb-free alloys for performance in accelerated thermal cycling (ATC).
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Authored By:

Eva Kosiba, Simin Bagheri, Polina Snugovsky, Ph.D.
Celestica Inc. Toronto, ON, Canada

Doug Perovic, Ph.D.
University of Toronto
Toronto, ON, Canada

Summary

Accelerated thermal cycling (ATC) was performed on a test vehicle which included various components, built with three low (or no) Ag, Bi-containing solders and compared to a baseline of SAC305 (Sn/Ag3%Cu 0.5%). Lead free SAC305 has become the default lead-free alloy for consumer electronic application use since the implementation of the RoHS legislation banning the use of Pb in solder.

SAC305 however does not perform as well as SnPb in some key areas which are of importance to consumer electronic applications; specifically drop/shock performance and survival under accelerate thermal cycling conditions. Efforts have been made in the consumer sector to improve upon these properties of SAC305 by, for example reducing the amount of Ag in order to improve drop/shock performance, however an alternate alloy is still required to meet all of the existing concerns.

It has been shown in the previous paper presented at last year's ICSR conference "Assembly Feasibility and Property Evaluation of Low Ag, Bi-Containing Solder Alloys" that these alloys show promise in drop/shock performance. This work shows that these same Bi-containing ternary and quaternary alloys also show promise in ATC. ATC of 3000 cycles was performed from 0 C to 100C. The results of the three Bi containing alloys were compared to SAC305. Additionally microstructural evaluation was performed at time zero and after ATC. Bi containing solders may prove to be adequate replacement for SAC305 in consumer electronic applications.

Conclusions

This screening experiment for selecting a Pb-free solder with a lower melting temperature then conventional SAC305 for consumer electronics is on going.

Observations and conclusions that can be made at this time are as follows:

All for Pb-free solders survived 0C-to-100C ATC in excess of 3000 cycles.
Even though there were no electrical failures at 3000 cycles ATC (i.e. all solder joints were electrically sound), cracks had initiated and were propagating through the solder joint.
These fatigue cracks form along the high strain region where recrystalization is occurring rapidly, induced by the stress that thermal cycling induces in materials with mismatched CTE

Initially Published in the SMTA Proceedings

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Microstructure and Reliability of Low Ag, Bi-Containing Solder Alloys

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