High Temperature Lead-Free Solder Joints Via Mixed Powder System

Lead free soldering has been the main stream of industry since 2006, with eutectic SnPb system phased out by SnAgCu system. However, development of lead-free alternatives for high melting high lead solder alloys is still struggling, thus hampering the completion of full lead-free conversion. BiAg alloy exhibits acceptable bulk strength but very poor ductility and wetting on copper, therefore is not acceptable as an option. In this work, a mixed powder BiAgX solder pastes 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 majority and the additive powder as 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, hence results in a high melting solder joint. In the mixed powder paste system studied here, a melting temperature above 260oC was verified by DSC and TMA data. The mixed powder solders show a significantly improved wetting comparing to BillAg. 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 175oC while the high lead ones do show a considerable increase.