Effects of Combined Thermal and Mechanical Loading on the Fatigue of Solder Joints

Currently in the electronics industry, Finite Element (FE) models are widely used as tools for reducing design costs. However, to have confidence in these models, they must closely match real world test results, including time to failure and failure mode. In the past, laboratory test data have been used to supplement, refine, and validate the FE models by means of a correlation between thermal variations and cycles to failure. In this work, mechanical loading is applied as well. Test coupons populated with components are tested in an apparatus that applies cyclic four point bending simultaneously with thermal cycles. This bending is regulated to a given surface strain measured by a thermally insensitive strain gauge mounted on the surface of the printed circuit board (PCB). In addition to the mechanical loading, the PCB is enclosed in a thermal chamber to provide synchronized cyclic thermal loading as well. Electrical continuity and resistance in the solder joints is monitored, and the specimens are tested until failure criteria, usually based on detection of an open circuit, are met.