Damage produced in model solder (Sn-37Pb) joints during thermomechanical cycling

The microstructure of solder plays a key role in the reliability of electronic packages. In this study, the cyclic shear deformation experienced by Sn-37Pb solder joints was simulated by thermomechanically cycling model joints between 30degreesC and 125degreesC, and the nature of the damage investigated. Most of the developed shear strain was accommodated by the solder adjacent to the interface with the intermetallic layer, and its severity diminished exponentially with distance from the interface. Shear bands formed at this location and within the shear bands, significant microstructural coarsening occurred together with crack initiation on the outer free surface of the solder. Subsequent cycling produced multiple cracking, fragmentation, and macroscopic decohesion, progressing toward the interior of the solder. Secondary cracks initiated from the primary cracks and propagated along colony boundaries in the surface layers of the solder perpendicular to the shear direction. In the interior of the solder, well away from the interface with the intermetallic layer, a limited amount of coarsening occurred. Apart from smoothing of undulations, the intermetallic layer was unaffected.