Intermetallic formation of SN solder on CU-XNI for a harsh environment composite solder paste

Continued improvement of our Cu-Ni/SN100C composite solder paste as a Pb-free alternative to high-Pb solder seeks to improve the reliability of printed circuit board (PCB) solder joints, even for extremely high temperatures, possibly up to 400°C. The designed paste is predicted to be inexpensive and able to withstand elevated temperatures while still maintaining a standard commercial processing temperature around 250°C. The paste performs using liquidphase diffusion bonding (LPDB) to form the room-temperature stable intermetallic compound (Cu, Ni)₆Sn₅. The nickel addition to this high-temperature phase increases joint ductility by suppressing the brittle transformation of the IMC upon cooling. Updates to the research explore the maximum possible diffusion based on Cu-Ni powder composition and the use of a Sn-alloy as the liquid-half of the solid/liquid diffusion couple within the paste. Results suggest this could be an excellent “drop-in” replacement for solders soon to be eliminated by RoHS. The Iowa State Research Foundation and Nihon-Superior, Inc. Ltd., through Ames Lab (DE-AC02-07CH11358) are gratefully acknowledged.