Interface microstructures between Ni-P alloy plating and Sn–Ag–(Cu) lead-free solders

The interface microstructures of Sn-Ag and Sn-Ag-Cu solders with Au/Ni-6P plating were studied primarily using transmission electron microscopy. During soldering at 230°C, Au dissolved into molten solder, and double reaction layers of Ni₃Sn₄/η–Ni₃SnP formed between Sn-3.5Ag solder and Ni-6P layer. P content increases in the surface region of the Ni-6P layer due to the depletion of Ni diffused into molten solder, resulting in the formation of Ni₃P+Ni layer. For Sn-3.5Ag-0.7Cu solder, an η-(Ni,Cu)₃Sn₂ single layer, containing Cu of about 50 at.%, formed as a reaction layer.