Effect of Cu concentration on the reactions between Sn-Ag-Cu solders and Ni

The reaction between the Sn-Ag-Cu solders and Ni at 250degreesC for 10 min and 25 h was studied. Nine different Sn-Ag-Cu solders, with the Ag concentration fixed at 3.9 wt.% and Cu concentrations varied between 0.0-3.0 wt.%, were used. When the reaction time was 10 min, the reactions strongly depended on the Cu concentration. At low-Cu concentrations ( less than or equal to 0.2 wt.%), only a continuous (Ni1-xCux)(3)Sn-4 layer formed at the interface. When the Cu concentration increased to 0.4 wt.%, a continuous (Ni1-x Cu-x)(3)Sn-4)3Sn, layer and a small amount of discontinuous (Cu1-yNiy)(6)Sn-5 particles formed at the interface. When the Cu concentration increased to 0.5 wt.%, the amount of (Cu1-yNiy)(6)Sn-5 increased and (Cu1-yNiy)(6)Sn-5 became a continuous layer. Beneath this (Cu1-yNiy)(6)Sn-5 layer was a very thin but continuous layer of (Ni1-xCux)(3)Sn-4. At higher Cu concentrations (0.6-3.0 wt.%), (Ni1-xCux)(3)Sn-4 disappeared, and only (Cu1-yNiy)(6)Sn-5 was present. The reactions at 25 h also depended strongly on the Cu concentration, proving that the strong concentration dependence was not a transient phenomenon limited to a short reaction time. The findings of this study were rationalized using the Cu-Ni-Sn isotherm. This study shows that precise control over the Cu concentration in solders is needed to produce consistent results.