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

This study describes tests in which solder composition, substrate meta llization, 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 mi crostructure, and solder and interface chemistry were taken in order t o provide insight about the wetting mechanism(s). The reactivity of Au vs Cu metallization with solder was found to be a major factor in inc reasing spread area. The role of increasing tin content is to increase spread and spread rate. A similar effect is seen by increasing temper ature. Time allowed for spread is a minor contributor to the spread ar ea. Segregation of the tin and bismuth solder components during the we tting process was observed which indicated the role of bismuth as a ca rrier 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.