Effects of Pd layer thickness on mechanical properties and microstructures of Sn-3.0 Ag-0.5 Cu solder joints

In order to investigate effects of Pd layer thickness on the mechanical properties and microstructures of Sn-3.0 Ag-0.5 Cu solder joints, three different Pd layers, i.e., 0.05, 0.1, 0.15 µm, were employed on the electroless nickel immersion gold [ENIG, Ni-P(5 µm )/Au(0.08 µm)] surface finished flame retardant type 4 (FR4) printed circuit board (PCB) substrates. Our studies showed that the addition of Pd layer in ENIG treatment, i.e., ENEPIG, enhanced the bonding strength of solder joints by reducing the formation of intermetallic compounds (IMCs), (Cu, Ni)6Sn5 and (Ni, Cu)3Sn4. The different failure modes were also observed – the crack propagation occurred along the interface between P-rich Ni layer and Cu layer in 0.05 µm Pd specimens while specimens with 0.1 µm and 0.15 µm Pd layers showed the crack propagation proceeded mostly inside solders. In this study the effects of Pd layer thickness in ENEPIG surface finish on the bonding strength of solder joints were investigated by means of drop test and ball shear test, followed by observation of microstructures and interfaces using a scanning electron microscope (SEM) and an energy dispersive spectrometer (EDS).