SMTA International Conference Proceedings

Effect Of Sn Grain Morphology On Failure Mechanism And Reliability Of Lead-Free Solder Joints In Thermal Cycling Tests

Authors: Babak Arfaei, Ph.D. and Martin Anselm, Ph.D.; Shantanu Joshi, Sam Mahin-Shirazi, Peter Borgesen, Ph.D. and Eric Cotts, Ph.D.; James Wilcox, Ph.D.; and Richard Coyle, Ph.D.
Company: Universal Instruments Corporation, Binghamton University, IBM Corporation, Alcatel-Lucent, Ltd.
Date Published: 10/13/2013   Conference: SMTA International

Abstract: The failure mechanisms in SnAgCu solder joints subjected to either conventional (-40/125°C, 0/100°C) or mild (20/80°C) accelerated thermal cycling (ATC) tests were examined. The precipitate and Sn grain morphologies of LGAs, BGAs, CSPs and QFN were characterized, and correlations with the failure mechanisms in these SnAgCu solder joints were sought. Both beach ball and interlaced Sn grain morphologies were observed in these components, and recrystallization and intergranular crack growth were observed in both cases. For larger as-reflowed SnAgCu (SAC) ball grid array (BGA) solder joints that showed beach ball structures, strain enhanced coarsening close to the crack area was also observed. Smaller samples that exhibited interlaced structures displayed relatively long fatigue lives. While these samples were completely recrystallized by the time they were cycled to 25% of their projected life, crack initiation was delayed for their joints. The recrystallized Sn grains grew, with cracks ultimately propagating intergranularly along those recrystallized Sn grains. Isothermal pre-aging for 10 days at 125°C did not significantly change the reliability behavior of these solder joints (Sn grain growth was characterized for such anneals).

Key Words: 

Accelerated Thermal Cycling, Pre-aging, Sn Grain Morphology, Interlaced twinning, EBSD, Recrystallization

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