The joint strength and the joint interfaces between electroless Ni-P plating and Sn-Ag solders ,as replacement solders for the eutectic Sn-37 Pb, were investigated. The joint strength between Cu plated by electroless Ni-P and Sn-3.5 Ag and Sn-3.5 Ag-(8, 9, 10) In decreased after annealing at 373 K, 398 K and 423 K. The effect of Zn addition to Sn-3.5 Ag and Sn-3.5 Ag-8 In on the joint strength and thickness of reaction layers was investigated. The joint strength of as-joined interfaces between Sn-3.5 Ag and Sn-3.5 Ag-8 In and electroless Ni-P plating increased by 1 % Zn addition and decrease of the joint strength was inhibited by 1 % Zn addition after annealing. The joint interface between Sn-Ag solders and electroless Ni-P plating consists of Nix-Sny and P-rich reaction layers. It was found that the decrease of the joint strength relates to the growth of these reaction layers. The growth of these reaction layers is inhibited by 1 % Zn addition.
