High Melting Lead-Free Mixed BiAgX Solder Paste System

Although lead-free soldering has been main stream in the industry since 2006 with the replacement of the eutectic SnPb system by the SnAgCu system, the development of drop-in lead-free alternatives for high melting high lead solder alloys is still far from mature. The BiAg alloy exhibits acceptable bulk strength, but very poor ductility and wetting. Therefore, it is not acceptable as an option. In this current work, a mixed-powder BiAgX solder paste system has been developed as a viable alternative high temperature lead-free solder. The metal powder in the paste is composed of a high-melting first alloy powder as a majority and the additive powder as a minority. The additive contains a reactive element to react with various metallization surface finishes. The additive will melt and react on the parts before or together with the melting of the majority solder. The reactive element in the additive is designed to be converted completely into IMCs during the reflow process, resulting in a high-melting solder joint. In the mixed-powder paste system, a melting temperature above 260°C was verified by both DSC and TMA data. The mixed-powder solders show significantly improved wetting compared to Bi/11Ag. The voiding and TCT performance are comparable with high-lead solders. The IMC layer thickness of the mixed-powder system is insensitive toward thermal aging at 175°C while the high-lead solders show a considerable increase. The fine and well-dispersed Ag particles in the joint, together with the controlled IMC thickness, are attributed for the reliability improvement.