Early transient creep of single crystal SnAgCu solder joints

Assessments of solder fatigue life require knowledge of inelastic deformation and damage properties. Previous work showed common constitutive relations for SnAgCu solder joints to be extremely misleading for the kind of joints formed in area array assemblies. At the relatively low stresses in most thermal cycling, room temperature creep rates varied linearly with the stress, rather than with the stress to a power of 4–5 or more as commonly assumed. This is characteristic of creep dominated by diffusion of individual atoms rather than by dislocation motion. The present effort extends this to higher temperatures and the much shorter loading times typical in accelerated thermal cycling testing. It also considers the mobility of dislocations of relevance to recrystallization and damage evolution. The temperature, coarsening of the Ag₃Sn precipitates, and the time of loading all affect the ‘cross-over’ stress below which inelastic deformation rates are dominated by diffusion. However, the evolution of stresses in cycling is shown to remain totally dominated by diffusion.