Effect of variation in the reflow profile on the microstructure of near eutectic SnAgCu alloys

The effects of changes in thermal history (changes in the reflow profile) on solder joint microstructure are examined. The effects of variations in the reflow temperature, and in the cooling rate from the melt, on the microstructure of near eutectic lead free SnAgCu solder joints were investigated. Changes in precipitate or Sn grain morphologies have previously been correlated with changes in the failure rates in these Pb free alloys. Thus correlations were sought between changes in reflow parameters and in both precipitate and Sn grain morphologies. Precise reflows were conducted in a differential scanning calorimeter and it was found that changes in microstructure were correlated with large changes in the reflow temperature, and that the precipitate microstructure was a strong function of the cooling rate. This effect was similar in magnitude to that observed in relatively large changes (1 to 3wt%) in the Ag concentration of these near eutectic SnAgCu alloys.