Role of anisotropic behaviour of Sn on thermomechanical fatigue and fracture of Sn-based solder joints under thermal excursions

Solder joints experience thermomechanical fatigue (TMF) as a consequence of thermal stresses that arise from coefficient of thermal expansion (CTE) mismatches between various entities present in the joint under thermal excursions. Sn present in solder joints made with alloys containing significant amounts of Sn, exists in a body centred tetragonal (BCT) structure, under normally realized thermal excursion regimes encountered during service. BCT Sn exhibits significant anisotropic behaviour in its physical and mechanical properties as a consequence of its highly unusual c/a ratio of about 0.5. Such severe anisotropy causes significant stresses at the Sn grain boundaries present within the solder joints during thermal excursions, resulting in damage accumulation within the solder. Stresses resulting from this anisotropy can be much larger than those that can arise from CTE mismatches between entities such as solder/substrate, solder/intermetallics etc. Damage accumulation under TMF progresses in the severely constrained region of the solder/substrate interface, and causes the initiation and propagation of the catastrophic crack. This crack propagates within the solder in a region very close to the solder/substrate interface and results in the TMF failure of the joint.