Solid–Solid Reaction Between Sn-3Ag-0.5Cu Alloy 
and Au/Pd(P)/Ni(P) Metallization Pad with Various Pd(P) Thicknesses
The effect of Pd(P) thickness on the solid–solid reaction between Sn-3Ag-0.5Cu and Au/Pd(P)/Ni(P) at 180°C was investigated in this study. The reaction was conducted after reflow, thereby removing the Au/Pd finish before the solid-state reaction. The reaction products included (Cu,Ni)6Sn5, Ni2SnP, and Ni3P, and their growth strongly depended on the Pd(P) thickness, especially for the former phases [i.e., (Cu,Ni)6Sn5 and Ni2SnP]. As the Pd(P) thickness increased from 0 μm, to 0.1 μm, to 0.22 μm, the (Cu,Ni)6Sn5 exhibited a needle-like dense layer, chunk-like morphology, and discontinuous morphology, respectively. The alternative phase (Ni2SnP) behaved in a manner opposite to that of (Cu,Ni)6Sn5, growing with a discontinuous morphology to a dense layer with increasing Pd(P) thickness. However, this strong dependence disappeared when the solder joints were subsequently subjected to solid-state aging. The (Cu,Ni)6Sn5 and Ni2SnP both became layered structures for all cases examined. A high-speed ball shear (HSBS) test was conducted to quantify the mechanical response of the interfacial microstructures. The HSBS test results showed that any initial difference in shear strength caused by the various Pd(P) thicknesses could be reduced after the solid-state aging, which is consistent with the microstructural evolution observed. The mechanical strength of the solder joints was decreased due to the presence of a bi-intermetallic structure of (Cu,Ni)6Sn5/Ni2SnP at the interface. Detailed analysis of the growth of (Cu,Ni)6Sn5 and Ni2SnP is also provided.