Damage Mechanisms And Acceleration Factors For No-Pb LGA, TSOP, And QFN Type Assemblies In Thermal Cycling
Authors: Luke Wentlent, Liang Yin, Michael Meilunas, Babak Arfaei, and Peter Borgesen Company: Systems Science & Industrial Engineering, Binghamton University and Universal Instruments Corporation Date Published: 10/16/2011
Abstract: Despite the prevalence of reliability tests throughout the electronics packaging industry, only a relatively small number of them are used to help predict the actual in-service life for an electronics assembly. Much more often ‘engineering testing’ is essentially focused on comparisons to previous performance or between different designs, materials, or process parameters. Empirical testing can, however, often be misleading. For example, Ball Grid Array (BGA) assemblies on a specific product are found to have longer lives than Thin Small-Outline Packages (TSOP) assemblies in accelerated thermal cycling. In order for this result to be meaningful a vital question is then: Are the acceleration factors the same and, if not, how do they differ? Significant knowledge has been gathered about the performance of lead-free solder joints in thermal cycling. A systematic mechanistic understanding has been established in the case of typical BGA joints and we are starting to develop guidelines for comparisons of accelerated test results [1-4]. Nevertheless, a serious concern is that very different solder joint configurations (e.g., those with very different shapes and/or ratios of solder volume-to-pad areas) may lead to different solder microstructures and, therefore, different acceleration factors. We have addressed this concern for Land Grid Array (LGA), TSOP, and Quad Flat No-Lead (QFN) assemblies with SAC 305 solder joints by a combined study of the initial post-reflow solder microstructure, its subsequent evolution, and the associated type and propagation of damage. Based on a discussion of the results, judgments were made as to whether acceleration factors to long term service conditions are likely to be similar to or different from those for BGA assemblies.