Wettability of Sn-Bi and Sn-Ag-Cu lead-free solder pastes on electroplated Co-P films

The wettability of the molten lead-free solder on the substrate is an important property for the lead-free solder joints in electronic products. In this paper, we investigated the wettability of lead-free solder pastes on electroplated Co-P films. We measured the contact angles of one kind of Sn-Bi and two kinds of Sn-Ag-Cu (SAC305 and SAC387) solder pastes on the Co-P films with nanocrystalline, amorphous and mixed structures and studied the dependence of the contact angles of the molten solders on the microstructure of the Co-P film. The experimental data showed that in general the amorphous Co-P film with a large phosphorous content was favorable for all three solder pastes in terms of wettability. SAC305 had a contact angle less than 10° on the amorphous Co-P film and Sn-Bi had a contact angle of about 20° on it. Although SAC387 spread out completely on the amorphous and mixed Co-P films, Sn in SAC387 diffused quickly into the Co-P films and even penetrated the films. In addition, we observed the cross-sections of the solder joints, studied the interfacial reaction between the solders and the Co-P films by scanning electron microscopy and explained the variation of the contact angles of the solders on different Co-P films.