Effect of indium addition in Sn-rich solder on the dissolution of Cu metallization

An investigation has been carried out to study the dissolution of the Cu pad of the ball-grid-array (BGA) substrate into the molten Sn-9%In-3.5%Ag-0.5%Cu, Sn-3.5%Ag-0.5%Cu and Sn-0.7%Cu (wt.%) solder alloys. A fixed volume of BGA solder ball (760 μm dia) was used on a 13 μm thick Cu pad with a diameter of 650 μm. The dissolution measurement was carried out by measuring the change of Cu pad thickness as a function of time and temperature. Scanning electron microscopy was used to examine the microstructure of the solder joint and to measure the consumed thickness of Cu. The dissolution of Cu in Sn-3.5%Ag-0.5%Cu solder is higher than the other two lead-free solders. The presence of indium in the solder plays a major role in inhibiting the consumption of Cu in the soldering reaction. The intermetallic compounds (IMCs) formed at the Sn-9%In-3.5%Ag-0.5%Cu/Cu interface are determined as a scallop-shaped Cu6(Sn, In)5. Bulk of the Sn-9%In-3.5%Ag-0.5%Cu solder also contains Cu6(Sn, In)5 and Ag-In-Sn precipitates embedded in the Sn-rich matrix. It is also found that more Cu-containing Sn-0.7%Cu solder shows lower Cu consumption than Sn-3.5%Ag-0.5%Cu solder at the same heat treatment condition.