Pronounced effects of Ni(P) thickness on the interfacial reaction and high impact resistance of the solder/Au/Pd(P)/Ni(P)/Cu reactive system

The effects of Ni(P) thickness, δNi(P), on the interfacial reaction and high impact resistance of Sn-3Ag-0.5Cu/Au/Pd(P)/Ni(P)/Cu microelectronic solder joints were investigated. During soldering, the surface layers of Au (0.1 μm) and Pd(P) (0.2 μm) were readily eliminated from the interface, and the Ni(P)/Cu structure underneath subsequently came into contact with the solder. When δNi(P) was 0 μm, a solder/Cu reaction occurred, and the intermetallic compounds (IMCs) Cu3Sn and Cu6Sn5 (dissolved with 1–3 at.% Pd) formed at the interface. When δNi(P) was sufficiently thick (i.e., 7 μm), the reaction between the components transformed the system into a solder/Ni(P) system with multilayer IMCs, which included (Cu,Ni)6Sn5, (Ni,Cu)3Sn4, Ni2SnP, and Ni3P. On the submicron scale (e.g., δNi(P) = 0.9 μm), the solder/Ni(P) and solder/Cu reactions can occur sequentially, indicating that a submicron-thick Ni(P) layer did not function as an efficient diffusion barrier between solder and Cu. With the aid of Cu–Ni–Sn phase diagram, diffusion path predictions can be made about the IMC translation in response of different δNi(P). High-speed ball shear testing showed that the impact resistance of the solder joints was significantly reduced with increasing δNi(P). These findings suggested that δNi(P) is an important factor in the interfacial microstructure and the resulting mechanical properties of solder joints and that the direct deposition of an Au/Pd(P) dual layer (i.e., δNi(P) = 0 μm) on top of the Cu pads can offer an appropriate solderability for the lead-free Sn–Ag–Cu alloy.