Authored By:
Hemant M. Shah Ph.D., Kevin Byrd, Satyajit Walwadkar
Patrick Nardi, Pubudu Goonetilleke
Intel Corporation
OR, USA
Summary
Hybrid low temperature solder (LTS) joints are created when a component with tin-silver-copper (SAC) solder balls is joined to the printed circuit board with a tin-bismuth based LTS paste. As the industry increases data collection with LTS pastes, several unique phenomena have been noted in hybrid solder joints. A defect mode commonly referred to “ball drift” or “ball shift” has been documented as occurring during thermal cycle testing. As well, very different microstructure development has been seen after isothermal again of hybrid joints using high vs. low silver SAC balls.
This paper will review a model to illustrate the source of “ball drift” and will report empirical results from testing designed to confirm the mechanism and highlight a possible mitigation method. Additionally, mechanical strength results will report for high and low silver solder joints assembled with LTS pastes of varying bismuth concentrations and aged for a range of times and different temperatures. Conclusions will be provided for both the “ball drift” mechanism as well as the mechanical performance of isothermally aged hybrid solder joints.
Conclusions
This study demonstrated that mode CTE is not a definitive variable for ball shift phenomenon. Key interesting observation we found was that while given the metallurgical properties of LTS and SAC system, ball shift can be observed in LPDDR and WLCSP packages, however, a mitigation strategy of adhesives application can be employed to over the ball shift phenomenon.
Initially Published in the SMTA Proceedings
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