Au–Ni plated Cu pads were reflow soldered by using lead free solder balls. The microstructure and strength of the as-reflowed solder joints were investigated. For solder joints using Cu-free Sn–Pb and Sn–Ag solder balls, a Ni₃Sn₄ reaction layer was formed on the boundary between solder and pads. On the other hand, a Cu–Sn based (Cu, Ni)₆Sn₅ reaction layer (η^′) was formed in solder joints using Cu-containing solder balls. The growth rate for an η^′ reaction layer during heat exposure at 423 K was much slower than that for a Ni₃Sn₄ reaction layer. This suppression of an η^′ reaction layer growth can be attributed to the fact that the Cu in solder balls was mostly removed during the formation of the η^′ layer. By ball shear test, cold bump pull and hot bump pull tests, mechanical properties of the obtained BGA joints were investigated. Fracture loads and crack propagation path changed by changing the mechanical tests, the BGA joints using Cu containing Sn–Ag–Cu solder or low P type Ni plating revealed better mechanical properties. We established the mismatch of the boundaries between reaction layers and the P-enriched Ni–P layer, which was caused by the chained voids formed due to the Kirkendall effect, led to low joint strength.