Surface mount assembly failure statistics and failure free time

This paper documents improved practices to analyze Surface Mount (SM) attachment failure statistics. These include the use of a failure free time metric obtained from three parameter Weibull analysis of solder joint fatigue data. Compared to two parameter Weibull and lognormal distributions, the three parameter Weibull consistently gives a better fit of early wear out failures across a large test database. The failure free time represents the minimum amount of time required for cracks to initiate and propagate through the weakest solder joints of a population. The failure free metric defines a warranty period during which thermo-mechanical fatigue failures of solder joints are not expected. The Comprehensive Surface Mount Reliability (CSMR) model has been extended by correlating failure free times scaled for the solder crack area to cyclic inelastic strain energy. The three parameter Weibull treatment of SM failure data provides more accurate reliability projections, potentially qualifying component assemblies that would be rated marginal or unacceptable based on more conservative two parameter Weibull or log-normal analysis.<
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