Microstructure characterization and creep behavior of Pb-free Sn-rich solder alloys: Part II. Creep behavior of bulk solder and solder/copper joints

Microstructure plays a critical role in the mechanical behavior of Sn-rich solder alloys. A unified mechanistic understanding of creep in Sn-rich solder alloys, at various microstructural length scales, is missing. Part I of this study focused on microstructure characterization of Sn-rich solder alloys. Part II focuses on the creep behavior of bulk solder alloys and small solder joints comparable in size and geometry to those in electronic packages. Pure Sn, Sn-0.7Cu, Sn-3.5Ag, and Sn-3.9Ag-0.7Cu were used in these studies. At the bulk level, creep appears to be controlled by subgrain formation, whose size is controlled by Ag₃Sn orCu₆Sn₅ particles. At the smaller joint level, where the microstructures are much finer, creep in Ag-containing alloys was controlled by local climb and detachment along Ag₃Sn particles. The threshold stress for creep is inversely proportional to the Ag₃Sn interparticle spacing. At the joint level, the creep of Sn and Sn-Cu alloys was controlled by viscous flow at grain boundaries at low stress, and transition to dislocation climb at higher stress.