Thermomechanical and Fatigue Behavior of High-Temperature Lead and Lead-Free Solder Joints

Abstract

Thermal fatigue tests were performed on six solder alloys, i.e., 90Pb-10Sn, 97Sn-2Cu-0.8Sb-0.2Ag, 97Sn-3Cu, 95.5Sn-4Cu-0.5Ag, 95Bi-5Sb, and 63Sn-37Pb to study their stress/strain hysteresis response and corresponding thermal fatigue behavior. The first five solder alloys, with melting temperature ranging from 250 to 310°C, are regarded as high temperature solders compared to solders with lower melting point, e.g., 63Sn-37Pb (TM = 183°C). A double-beam joint specimen has been developed for this test. The specimen is made by bonding an A12O3 beam and an Al 2024-T4 beam together at their ends with solder. A method based on the measured stress relaxation data at dwell time has been developed to determine the steady-state creep parameters of the solder. Simplified viscoplastic constitutive equations employing the measured creep properties were assumed for the solder and implemented in a finite element program to evaluate their validity in modeling thermal cyclic behavior. During thermal fatigue, the evolution of the stress/strain hysteresis loop was measured, and the fracture surface was examined with SEM to identify the dominant failure mechanism. The results show that some Sn-Cu alloys have a much better thermal fatigue resistance than the high lead one and can be considered as lead-free alternatives. In addition, the eutectic solder also exhibits a surprisingly longer fatigue life than 90Pb-10Sn solder, which implies that using melting temperature as a guideline for solder selection against operation temperature may need to be modified.