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|An Empirical Model for Lead Free Solder Joint Life in Thermal Cycling|
|Keywords: Empirical Model , Lead Free Reliability, Thermal Cycling|
|The most credible assessment of the life of an electronics product in long term service is through the extrapolation of the results of accelerated test results, but the credibility still hinges on the assumption of a model or expression for the ‘acceleration factor'. Current reliability models have all been shown to break down when applied to results for lead free solder joints in various thermal cycling tests , leaving us with little confidence in extrapolations to conditions for which no immediate experimental verification is possible. Given the absence of long term field data for currently relevant lead free solder alloys, design and process parameters real confidence can only be established through an understanding of the underlying damage and failure mechanisms. In the case of lead free solders this is more challenging than commonly recognized, although a systematic picture and understanding is emerging. Meanwhile, until a corresponding quantitative model can be developed practical reliability will have to rely on the best possible generalization of empirical and qualitatively understood trends. A parametric expression has been developed for the thermal fatigue life of lead free solder joints which provides for a prediction of acceleration factors based on 2-4 different thermal cycling tests. Step-by-step arguments will be presented for the form of the expression based on the generalization of experimentally observed trends and a requirement for simplicity. Results considered and accounted for cover a range of cycling temperature combinations (Tmax and Tmin) and dwell times, cyclic strains, solder alloys and volumes. The associated accelerated thermal cycling experiments required for extrapolation to life in service for a specific electronics assembly will be defined and discussed.|
|Younis Jaradat, Student