Development of a novel adaptive lead-free solder containing reinforcements displaying the shape-memory effect

Abstract

Microelectronic solder joints are typically exposed to aggressive thermomechanical cycling (TMC) conditions during service. During TMC, strain localization occurs near solder/bond pad interfaces, where large, inelastic-shear strains accumulate, eventually causing low-cycle fatigue (LCF) failure of the joint. In this study, a novel methodology to mitigate the effects of strain localization within the joint is discussed, wherein the solder alloy is reinforced with a martensitic NiTi-based, shape-memory alloy (SMA). In this scheme, the SMA reinforcement deforms in shear concurrently with the solder during TMC and, subsequently, undergoes martensite-to-austenite (M → A) transformation, placing the solder matrix next to the reinforcements in reverse shear. This is purported to reduce inelastic-strain localization within the solder and, thus, enhance joint life. In this paper, we present results of thermal-mechanical loading experiments conducted on a monolithic 95.5Sn-3.8Ag-0.7Cu solder, a Cu/Cu6Sn5 particle-reinforced solder, and NiTi-solder, single-fiber composites (SFCs) to elucidate the impact of the shape-memory effect on the overall joint behavior. It is demonstrated that during TMC, the phase transformations occurring in NiTi can significantly reduce the inelastic-strain range to which a joint is subjected (by ∼25% in the present experiments). Finally, we report on the successful fabrication of a composite solder paste from which adaptive solders with a uniform distribution of about 5 vol.% of NiTi particulates may be produced.