A strategy to improve the thermal fatigue resistance of solder joints based on amorphous Co-P coating to spontaneously build skeleton of CoSn3 laths

Abstract

Integrated circuit systems are always suffering the temperature cycling environment caused by frequent power on and off. Mismatch in thermal expansion coefficients among chip package components will cause severe cyclic stress to the solder joints. During thermal fatigue, β-Sn grains tended to generate thermal fatigue cracks at the grain boundaries. This work proposed a strategy of using amorphous Co-15 at.% P coating as the interface layer to inhibit the initiation and propagation of thermal fatigue cracks and improve the shear strength of solder joints by numerous endogenous CoSn3 laths. Mechanistically, based on the amorphous structure of the Co-P coating, Co atoms would massively and rapidly diffuse into the molten solder during the liquid-solid diffusion stage of reflow soldering, and the eutectic solidification together with β-Sn spontaneously precipitated numerous laths of CoSn3 intermetallic compounds (IMC), which were evenly distributed and became the skeleton of the solders. The skeleton-like CoSn3 laths with high elastic modulus were the reinforcing phase of Sn-based solder, but also refined the grains of β-Sn. The refinement of β-Sn grains and the strengthening of CoSn3 laths played an important role in inhibiting the initiation and propagation of thermal fatigue cracks. In addition, a discontinuous Co-Sn IMC layer was formed between the amorphous Co-P and the solder. The alternating appearance of CoSn3 and relatively soft β-Sn at the interface avoided the generation of cracks at the interface during temperature cycling.