Roles of Cu in Pb-free solders jointed with electroless Ni(P) plating

In this study, the interfacial reaction of Sn-3.5Ag and Sn-3.0Ag-0.5Cu solders jointed with electroless Ni(P) substrate was investigated. Thick Ni3P and Ni2SnP layers formed in the Sn-3.5Ag joints because of the formation of Ni-Sn compound. In contrast, with the Cu addition in the solder, the interfacial (Cu,Ni)6Sn5 suppressed the growth of P-rich layers. However, after multiple reflows, the phase transformation occurred in the Sn-Ag-Cu joints, indicating a potential risk for crack to propagate through the dual IMC structure. To evaluate the bonding strength of solder joint, the high speed impact test was utilized to find out the weakest position among these two joints. By the comparison of the bonding energy and the X-ray fracture surface mapping between these two joints, both thick P-rich layers and dual IMC structure deteriorate the metallurgical bonding of solder joint. Eventually, the elemental cross-interaction should be considered in the final board assembled solder joints. The continuous Cu migration from the Cu metallization of board side inhibited the phase transformation on the electroless Ni(P) substrate, which might ensure the superior reliability of Cu-bearing solder joint. The roles of Cu on the interfacial reaction and mechanical strength would be addressed in the Sn-Ag-Cu/Ni(P) joints. Finally, the detailed correlation between microstructure variation and fracture failure as well as the possible mechanism was investigated and discussed.