The correlation between the intermetallic compounds growth pattern and the addition in the solder matrix

The additives Ni and Co were selected to modify the micro structure of Sn3.5Ag lead-free solder. The soldering was performed between the solders and Cu substrate at 250degC. The additives were detected to participate in the interface reaction. And the non-homogeneous micro structure was identified due to the discontinuity of the additive concentration at the entire IMC region. The outer area of IMC region was probed higher concentration of Ni and Co than those of at the inner area. At a short soldering time, the IMC morphology at the outer area shows the faceted and elongated shape, while at the inner area the nearly rounded IMC morphology was observed. The cuboid or hexagonal prism morphology was suggested for the IMCs at the outer area. After the prolonged soldering time, this phenomenon tends to be more remarkable. For the Sn3.5Ag0.1Co/Cu reaction couple, the ripening behavior at the inner area of IMC seems more severe than that for Sn3.5Ag0.1Ni/Cu couple. In the meantime, at the outer area of IMC region, the abnormal grain growth (AGG) was found for both of the reaction couples. It is interesting that for the Sn3.5Ag0.1Co/Cu reaction couple, a large amount of tiny size IMC structure was generated and attached on the surface of greater size IMCs. This implies that the nucleation can be enhanced significantly and result in a higher growth rate of the overall IMC