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|On the Life of Lead Free Solder Joints in Cycling with Varying Amplitudes|
|Keywords: lead-free solder, reliability, combined loading|
|Lead-free solder reliability in long term working environments may pose a huge risk for some industries. Efforts aimed at establishing a relationship between accelerated test results and life in service tend to focus on a single type and level of loading. However, real applications invariably involve cycling to different load amplitudes, often combined with substantial aging, i.e. they are far more complicated than reflected in a single load test. The often dramatic breakdown of current damage accumulation models when applied to lead free solder joints may make this critical as far as life assessment is concerned. It is certainly a cause of extreme concern with respect to the application of common ESS (Environmental Stress Screening) protocols. We have demonstrated the breakdown of current damage accumulation models in a variety of tests. In some cases preconditioning of assemblies at relatively low load amplitudes led to longer life in subsequent harsher cycling. In other cases mild preconditioning reduced the remaining life more than predicted by Miner's rule, while harsh preconditioning did much less damage than expected. Load controlled shear fatigue testing of individual solder joints showed preconditioning to shorten subsequent life to an order of magnitude less than predicted, and extrapolations of observed trends suggested that ESS may shorten the long term life of assemblies under some service conditions by more than two orders of magnitude! We ascribe the above trend to ongoing changes of the lead free solder properties during cycling. In thermal cycling this is associated with strain enhanced precipitate coarsening and eventual recrystallization. The evolution of properties during vibration or cyclic bending is correlated with the formation of slip bands in the Sn structure. A modified power law is proposed and validated experimentally.|
|Linlin Yang, Student
State University of New York at Binghamton