Failure mechanism and microstructural evolution of Pb-free solder alloys in thermal cycling tests: Effect of solder composition and Sn grain morphology

An improved understanding of how materials and processing affect solder joint microstructure provides insight into how to enhance the fatigue performance of Pb free solder joints. Our previous results showed that differences in solder volume can alter the Sn grain morphology of a near eutectic, SnAgCu Pb-free solder joint and change the lifetime in thermal cycling test. Solder composition and PCB surface finish were also shown to influence Pb free solder joint microstructure and the lifetimes of package interconnects in an ATC test. However, the effects of those parameters on failure mechanisms, particularly recrystallization behavior and crack propagation, are not well understood. Thus, in the current work, Pb-free solder joints were assembled with a range of materials, and the microstructure and reliability of those solder joints were characterized and correlated with the different fabrication parameters. In each case, the failure mechanism was examined. Solder joints of various sizes were assembled onto test boards with Cu-OSP, ENIG or ENEPIG PCB surface finishes using commercial solders alloys with different microalloying elements, such as Ni and Mn. For comparison, Sn3Ag0.5Cu (SAC 305) and eutectic SnPb alloys were included in the sample set. Accelerated thermal cycling testing was performed on the assembled vehicles. Microstructural analysis was performed on as-reflowed, on partially cycled samples, and on samples after electrical failure, to understand the initial microstructure and its evolution during the test. Thus, selected samples were taken out of the thermal cycling chamber at specific intervals of characteristic life (i.e., 20% and 50% of their characteristic life) for microstructural analysis. Polarized light microscopy and electron backscatter diffraction (EBSD) techniques were used to assess the Sn grain morphology of solder joints. The effect of solder composition, Sn grain morphology and PCB surface finish on solder joint microstructure and lifetime was evaluated. The effect of microstructure on the recrystallization behavior and on crack initiation, and thus the failure mechanism of joints in thermal cycling test is reported.