Crack Propagation in Solder Joints During Thermal-Mechanical Cycling

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Differential expansion induced creep-fatigue resulting from temperature cycling is an important cause of solder joint failure. The deterioration of solder joint integrity typically involves a sequential development of local stressing, microcracks, crack initiation and crack propagation, ultimately resulting in electrical open-circuiting by total joint separation from the PWB footprint. To better understand the failure process, a series of combined analytical and experimental investigations have been performed on flat-pack parts with gull-wing leads ranging in height from 9 to 56 mils. JPL’s special purpose nonlinear finite element computer program has been modified to dynamically simulate the crack propagation process. Solder creep properties, including the effect of grain growth, are also incorporated into the model. Depending upon the system geometry and materials properties, the combination of crack development and grain growth may have either a positive or negative effect on the rate of cracking of the solder elements. Consequently, the crack propagation process may accelerate or decelerate as cycles progress. Sensitivity studies are used to highlight the individual effects of crack propagation and grain growth and compare the failure prediction results with those obtained using conventional finite element techniques. The analytical results are corroborated using visual observations obtained using SEM photography of test samples exposed to the thermal-cycle environment used in the analysis.