Microstructure evolution and interfacial reaction of Co-P/SAC305/Co-P solder joints at high current density

The interfacial reaction of Co-P/SAC305/Co-P solder at current density of 1.0 × 10
⁴
A/cm
²
was studied. In Co-P/SAC305/Co-P solder joints, Sn-3.0wt.%Ag-0.5%Cu (SAC305) BGA solder ball as a connection between Co-P UBM and Co-P UBM. Ultrasonic assisted electro-deposition process was used to manufacture the Co-P UBM. The P (phosphorus) concentrations were 5%. Conducted EM test on the solder joints after soldering with the DC stabilized power supply, the solder joints were tested at current density of 1.0×10
⁴
A/cm
²
. The interface intermetallic compound (IMC) of Co-P/SAC/Co-P solder joint was mainly CoSn3. After 200h of EM test, the average IMC thickness at the cathode of the Co-P/SAC/Co-P solder joint increased from 2.10 µm to 5.12 µm, while at the anode increased from 2.15 µm to 12.05 µm. During the EM process, the cathode dissolution phenomenon was found in Co-P/SAC/Co-P solder joints. With the increase of electromigration (EM) time for Co-P/SAC/Co-P solder joints, the IMC morphology at the anode became more regular and the surface smoothness improved. Due to the presence of Cu atoms in the solder, a local (Cu, Co)
6
Sn
5
compound was formed at the anode. Therefore, this provides an important theoretical basis for the resistance of EM performance of the new Co-P UBM.