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.