Parametric acceleration transforms for lead-free solder joint reliability under thermal cycling conditions

The conversion to Pb-free solders has already been implemented in the consumer market. However, the transition to Pb-free solders in the high performance, mission critical networking/server industry has not yet been fully implemented. A significant factor in the slow transition to Pb-free solders is the absence of industry accepted Pb-free acceleration transforms that can translate lab test results into field reliability life data. Without such industry accepted acceleration transforms, it is difficult to translate lab test data into reliable predictions of field life performance. In this study, a comprehensive set of test data have been generated over a wide spectrum of package types, solder joint metallurgies, joint size, pitch, die sizes, PCB thicknesses and thermal cycling conditions. The effect of aging on acceleration transforms was also included. The data has been analyzed to compare with existing Pb-free and SnPb acceleration transform models. The data helps explain contradictions in the existing literature, and offers more comprehensive acceleration transforms for Pb-free solders. Some publications indicate that the acceleration transforms for Pb-free solders are the same as SnPb acceleration transforms. Comprehensive results of this study also show that in specific conditions, Pb-free acceleration transforms could even be lower than SnPb acceleration transforms. In addition, the acceleration transforms for backward compatible (Pb-free solders attached with SnPb paste) assemblies are presented as a function of assembly peak temperature. Finally, Finite Element Analysis has also been performed to better understand the effects of the different package design variables on the experimental acceleration transforms. The results have been used to derive recommendations on what package design variables to consider in ensuring good long term Pb-free reliability.