Processing and Characterization of NiTi Shape Memory Alloy Particle Reinforced Sn-In Solders

摘要

In the typical packaging of a printed circuit board, the tiny-yet- critical solder joints provide both electrical connection and mechanical support for the silicon chips and their substrate. These solders are subjected to serve thermo-mechanical strains during usage and the most common failure arise from thermo-mechanical fatigue (thermal cycling). This is due to the mismatch in the coefficient of thermal expansion between the chip and the packaging substrate. In previous work, it was proposed that reinforcement of solder by NiTi shape memory alloy particles to form smart composite solder reduces the inelastic strain of the solder and hence, may enhance the low cycle fatigue life of the solder. In this thesis, a new fabrication process for incorporating NiTi particles (10 vol.% NiTi) into Sn-In solder (80Sn-20In) using liquid phase sintering has been developed. The microstructures of the solders were characterized. The behavior of the solder joints during thermomechanical cycling was also characterized and the results showed that the shear stress induced in the composite solder joint is significantly reduced relative to that in the monolithic solder joint due to the generation of a back- stress associated with the B19'--> B2 phase transformation of the NiTi particles during the heating part of the cycle. This causes an appreciable reduction of the total inelastic strain range during cycling.