Effects of Tin and Copper Nanotexturization on Tin Whisker Formation

摘要

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"[3,6]. 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[l], we have compared the structure and whiskering incidence of nanotexturized Sn on both polycrystalline and nanocrystalline Cu underlayers.