The effect of Pb contamination on the solidification behavior of Sn-Bi solders

This paper presents experimental results and theoretical calculations to evaluate the effects of Pb contamination on the solidification behavior of Sn - x Bi alloys (x = 5, 10, and 58 mass %). The pasty (mushy) range, the type of solidification path, and the fraction of the ternary eutectic are described. The experimental results are obtained from thermal analysis and quantitative metallography, and the solidification calculations are performed using Lever and Scheil assumptions. The experimental, results agree with the Scheil calculations. The freezing range of Pb contaminated Sn-Bi solders is greatly increased due to the formation of a ternary eutectic reaction at (95.3 +/- 0.5)degrees C. This increase is a likely cause of porosity in contaminated solder joints. The results provide an example of an analysis method for use in solder alloys in general.