Fatigue and Cyclic Deformation Behavior of High Temperature Solder Alloys

High temperature solders are essential not only for some real high temperature electronic packaging applications, such as high power semiconductors, but also for multistage manufacturing processes where different components are assembled by using different solders at the appropriate processing stage. While a lot of attention has been given to the most commonly used Sn-Pb eutectic solder, very little research has been done on high temperature solder systems. In this paper, fatigue and cyclic deformation behavior of two lead-free high temperature solder alloys, 95Sn-5Ag and 99Sn-1.0Cu, a high lead alloy 97.5Pb-1.5Ag-1.0Sn, and an Ag-modified eutectic alloy 62.5Sn-36.1Pb-1.4Ag, were investigated. Characterization of fatigue behavior of the four high temperature solder alloys used in high power semiconductor packages is important to develop a capability to predict the service life of solder joints in a high power thermal cycling environment. Room temperature and high temperature (100 °C and 150 °C) fatigue tests (with cyclic strain amplitude up to 6.0%) for the four solders were conducted, with the fatigue lives ranging from a few cycles to more than 100,000 cycles. Among the alloys studied, 62.5Sn-36.1Pb-1.4Ag (the modified Sn-Pb eutectic alloy) has the lowest fatigue resistance in term of low cycle fatigue life (strain controlled). The high lead alloy, 97.5Pb-1.5Ag-1.0Sn, has the highest strain fatigue resistance in the large strain region (Δε > 2.0%). At lower strain regime (Δε < 1.0%), the high tin alloys, 95Sn-5Ag and 99Sn-1.0Cu, have better fatigue resistance compared with other systems. Temperature has a significant effect on alloys 95Sn-5Ag and 99Sn-1.0Cu, but has a negligible effect on the Ag modified Sn-Pb eutectic alloy 62.5Sn-36.1Pb-1.4Ag and 97.5Pb-1.5Ag-1.0Sn. The cyclic deformation of alloy 95Sn-5Ag was simulated by using constitutive relations built upon a 2-cell model, which covers both primary and secondary creep. This model provides a good estimate of the peak stresses (the minimum stress and maximum stress in each cycle); it agrees with experimental results when the applied cyclic strain is small and/or the applied strain rate is very low.