Versatile TIM Solution with Chain Network Solder Composite

Abstract

A novel epoxy-based SAC solder paste TIM system has been developed with the use of non-volatile epoxy flux. A Cu filler was added to the solder paste, with the Cu volume % of metal ranged from 17 to 60 volume % of metal. Formation of semicontinuous high-melting Cu chain network was achieved, with Cu particles bridged by the CuSn IMC. This chain network, at sufficient concentration, serves as a skeleton and maintains the shape of the sandwiched solder paste layer, thus preventing further spread out at subsequent SMT reflow process, and also allowing formation of the TIM joint, even in the absence of solderable metallization on flip-chip and packaging housing. This chain network hampered the flow of liquid solder, and thus restrained the expansion of outgassing, and consequently resulted in low voiding. Existence of crevices was attributed to excessive oxide brought in by Cu particles and appeared to increase with increasing Cu filler content. The presence of ductile solder within the TIM joint promises high resistance against brittle cracking under stress. The Cu content could be further optimized between 17 and 60 volume % of metal to (1) avoid flux bleeding, (2) maintain good epoxy adhesion between TIM phase and parts, and (3) to retain ductile solder phase. The 20°C thermal conductivity achieved was 6.1 W/mK and could be up to about 13 W/mK with further epoxy flux optimization.