A comparative study of ENIG and Cu OSP surface finishes on the mechanical reliability of Sn-3.0Ag-0.5Cu and Sn-36.8Pb-0.4Ag Solders

The mechanical reliability of surface finishes ; electroless Ni(P)/Au(ENIG), Cu OSP and electrolytic Ni/Au finishes, were evaluated with Sn-3.0Ag-0.5Cu (all in wt.% unless specified otherwise) and Sn-36.8Pb- 0.4Ag solder by lap shear and bending tests. Two substrates for test vehicle joined by solder have its own surface finish, respectively. In the case of Sn-3.0Ag- 0.5Cu solder, a combination of electrolytic Ni/Au(BGA side substrate) and ENIG(PCB side substrate) surface finishes showed the best performance. Sn-36.8Pb- 0.4Ag solder caused a lot of IMC spalling from the Ni(P) film and subsequent degradation of mechanical reliability in comparison to Sn-3.0Ag-0.5Cu solder which showed little spalling. In the case of Cu OSP, Kirkendall voids formed at the Cu₃Sn/Cu interface after thermal aging for both solders, which severely deteriorated the mechanical reliability. For the electrolytic Ni/Au metallization, formations of two layers of (Cu,Ni)₆Sn₅ and (Ni,Cu)₃Sn₄ IMCs at the solder joints during isothermal aging had adverse effects on the bending reliability