An experimental approach to characterize rate-dependent failure envelopes and failure site transitions in surface mount assemblies

This paper presents an experimental approach to identify the fatigue failure envelopes for solder damage in Printed Wiring Assemblies (PWAs) subjected to dynamic loading. An empirical, rate-dependent, power-law durability model, motivated by mechanistic considerations, is proposed to characterize the failure envelopes in terms of PWA flexural strain and strain rate, as damage metrics. It is further shown that there are critical combinations of these damage parameters, beyond which the failure site changes from the solder to other locations on the PWA. A case study, using a simple PWA specimen containing a single area array component, is presented to demonstrate the proposed approach. Under certain loading conditions, the failure site changes from the bulk solder to the Cu-trace/FR4 interface. The proposed durability model is shown to successfully describe the solder damage failure envelopes. The concept of a “Failure Map” is shown to provide a suitable framework to quantify the durability of the solder interconnect, determine its failure envelope, and identify the failure transition zone of the specimen. The applicability of the proposed technique for comparing the durability of different packaging styles and for developing design guidelines for PWAs subjected to dynamic loading (for example: drop) is discussed.