ROLE OF TIN CONTENT IN THE WETTING OF CU AND AU BY TIN-BISMUTH SOLDERS

This study describes tests in which solder composition, substrate metallization, temperature, and dwell time were combined in a factorially designed experiment to determine the effect of those factors on solder spread area. Measure of spread area, reflowed solder shape, solder microstructure, and solder and interface chemistry were taken in order to provide insight about the wetting mechanism(s). The reactivity of Au vs Cu metallization with solder was found to be a major factor in increasing spread area. The role of increasing tin content is to increase spread and spread rate. A similar effect is seen by increasing temperature. Time allowed for spread is a minor contributor to the spread area. Segregation of the tin and bismuth solder components during the wetting process was observed which indicated the role of bismuth as a carrier species. Analysis of variance methods based on the statistically designed experiments1a, 1b were used to show how to generate a model which estimates the spread area as a function of the tested factors.