Authored By:
David M. Lee and Lesly A. Pinol, PhD
Johns Hopkins Applied Physics Laboratory Laurel, MD USA
Summary
The physical mechanisms behind tin whisker formation in pure tin (Sn) films continue to elude the microelectronics industry. Despite modest advances in whisker mitigation techniques (i.e., barrier metal underlayers, substrate/film annealing, etc.) and encapsulation, ways to fundamentally prevent whiskers from forming remain unknown. It has been said that tin plating thicknesses of <0.5 um or >20um are "whisker inhibiting."
In the case of the former claim, it may be argued that as film grains approach an equiaxed proportion (i.e., the average columnar grain height is roughly equivalent to the average grain diameter), stress compensation is no longer preferential to the normal direction with respect to the plated film. Grain morphology has often been pointed to in the literature as a likely factor in Sn whisker formation due to the fact that SnPb, which does not whisker, has equiaxed grains while pure Sn exhibits columnar grain growth, which is only equiaxed when the film thickness matches the average grain diameter.
Our work examines the effect of adding grain refiners during tin electroplating, with particular focus on the 'as-deposited' film morphology and the associated incidence of whiskering. We have included polycrystalline Sn 'control samples' in our study, and as an extension of our previous work, we have compared the structure and whiskering incidence of nanotexturized Sn on both polycrystalline and nanocrystalline Cu underlayers.
Conclusions
We have successfully demonstrated the ability to modify the grain size, shape, and texture of tin through pulse plate deposition and the addition of a grain refining additive. Sample cross-sections, pre- and post-etching, may indicate an increased intermetallic compound volume in samples with grain refined tin on nanocrystalline copper. Preliminary imaging suggests that nanocopper underlayers prevent whiskers from forming in polycrystlline tin, while polycrystalline copper underlayers do not - an inversion of our previous finding.
Our intent is to conduct ambient aging of all 28 samples over the coming year, and to perform follow-on microscopy analysis to evaluate any differences in whiskering propensity with respect to the degree of nanotexturization and/or surface smoothing.
Lead-free solder is a reality in today's microelectronics industry, and the unpredictable nature of tin whiskers poses a significant challenge to determining microelectronics reliability. Further work is needed (and planned) in determining whether modifying the intrinsic grain morphology of plated filsm can help to redistribute the as-plated and evolutionary stresses and either retard or prevent tin whisker growth from occurring.
Initially Published in the IPC Proceedings
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