Role of shape-memory alloy reinforcements on strain evolution in lead-free solder joints

Abstract

Microelectronic solder joints are exposed to aggressive thermomechanical cycling (TMC) during service, resulting in strain localization near solder/bond-pad interfaces, which eventually leads to low-cycle fatigue (LCF) failure of the joint. In order to mitigate these strain concentrations, a “smart solder” reinforced with a martensitic NiTi-based shape-memory alloy (SMA) has been proposed before. In the present work, the role of NiTi particles on strain evolution in composite solders was studied using a combination of experimental and numerical means. Finite element modeling showed that NiTi pariculate reinforcements can reduce inelastic strain levels in the solder via shape recovery associated with the B19′ → B2 transformation. In situ TMC studies in the scanning electron microscope (SEM), in conjunction with strain analysis via digital image correlation (DIC), showed evidence of reverse deformation in the solder commensurate with the NiTi phase transformation, demonstrating the conceptual viability of the smart solder approach. The SEM-DIC experiments also suggested that the presence of particulates mitigates shear localization, which is commonly observed in monolithic solder joints close to joint/bond-pad interfaces. Finally, TMC experiments on monolithic solder and NiTi/solder single-fiber composite joints highlighted the beneficial effect of shape-memory transformation in reducing inelastic strain range of solders.