Microscopic Experimental Investigation on Shear Failure of Solder Joints

A microscopic investigation has been made on the shearing of one leaded and two lead-free solders by using an in situ SEM method. A shear lap joint specimen is designed and fabricated to accommodate a thin layer of solder alloy between copper strips. A non-contact method that measures strains in a very narrow area in the solder was applied. A laser grid was also used on the copper strip for measuring the back-face strain. Simultaneously micrographs at various stages were also taken. Where in situ measurements and micrographs are recorded they can reveal the continual development of damage and fracture mechanisms consistent with observations generated by low-cycle fatigue loading. This means that the shear test can be used as an alternative test to fatigue loading tests. By comparison, two lead-free solder specimens showed much smaller elongation to failure than the leaded solder, although all specimens showed similar sequence of events leading to final failure, including the boundary layer fracture phenomenon. The back-face strain indicator for the formation of a macro crack is due to the shifting of high stress concentration area from the joint-edge region to outside the joint region as revealed by a damage-coupled finite element procedure. The procedure also provides an estimate on the critical back-face strain.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/42763/1/10704_2004_Article_2311.pd

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 30°C and 125°C, 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