The growth of intermetallic compounds at Sn-Ag-Cu solder/Cu and Sn-Ag-Cu solder/Ni interfaces and the associated evolution of the solder microstructure

The evolution of intermetallics at and near SnAgCu/Cu and SnAgCu/Ni interfa ces was examined, and compared to the behavior near PbSn/metal and Sn/ meta l interfaces. Two different solder compositions were considered, Sn93.6Ag4. 7Cu1.7 and Sn95.5Ag3.5Cu1.0 (Sn91.8Ag5.1Cu3.1 and Sn94.35Ag3.8Cu1.85 in ato mic percent). In both cases, phase formation and growth at interfaces with Cu were very similar to those commonly observed for eutectic SnPb solder. H owever, the evolution of intermetallics at SnAgCu/Ni interfaces proved much more complex. The presence of the Cu in the solder dramatically altered th e phase selectivity at the solder/Ni interface and affected the growth kine tics of intermetallics. As long as sufficient Cu was available, it would co mbine with Ni and Sn to form (Cu,Ni)(6)Sn-5, which grew instead of the Ni3S n4 usually observed in PbSn/Ni and Sn/Ni diffusion couples. This growing ph ase would, however, eventually consume essentially all of the available Cu in the solder. Because the mechanical properties of Sn-Ag-Cu alloys depend upon the Cu content, this consumption can be expected to alter the mechanic al properties of these Pb-free solder joints. After depletion of the Cu fro m the solder, further annealing then gradually transformed the (Cu,Ni)(6)Sn -5 phase into a (Ni,Cu)(3)Sn-4 phase.