Eutectic Sn-Bi as an alternative to Pb-free solders

This paper reports on several issues that relate to 58Bi-42Sn as a replacement for 63Sn-37Pb in surface mount electronic assembly applications. Most no-clean fluxes currently used in surface mount electronic industry are developed for 63Sn-37Pb. They activate at temperatures above 150°C; therefore they are not suitable for 58Bi-42Sn which melts at 139°C. The newly developed low temperature fluxes were evaluated by wetting balance test and spreading test. Mechanical properties were examined in terms of shear strength, steady state creep rate, and isothermal fatigue resistance as a function of temperature, and compared with 63Sn-37Pb. The pros and cons of using 58Bi-42Sn are discussed. A small addition of Au or Ag into 58Bi-42Sn significantly improved the isothermal resistance and the strength of solder joint on Alloy 42. However, a small amount of Pb dissolution into eutectic Bi-Sn solder could destroy the long term reliability of the solder joint when thermal cycling between 0°C and 100°C, but only little effect for thermal cycling between 20°C and 75°C