Interfacial Reaction and Mechanical Characterization of Eutectic Sn--Zn/ENIG Solder Joints during Reflow and Aging

The interfacial reactions and joint reliabilities between Sn–9mass%Zn solder and an electroless nickel-immersion gold (ENIG) plated Cu substrate were investigated during reflow and isothermal aging at temperatures between 343 and 423K for aging times of up to 2400 h. After reflowing and aging, the intermetallic compound (IMC) formed at the interface was found to be AuZn3. No Ni-containing reaction products, such as Ni–Zn, Ni–Sn and Ni3P, were observed to form at the interface. The interfacial microstructure and shear strength remained nearly unchanged, irrespective of the reflow time, and the fractures occurred in the solder matrix. In the ball shear tests conducted after aging treatment, the shear strength of the samples decreased during the initial 100 h of aging and then remained constant with prolonged aging. The interfacial reaction between the Sn–Zn solder and the Ni–P plating layer was found to be suppressed by the formation of a layer-type Au–Zn IMC layer, resulting in the desirable interfacial reaction. Compared to the fast interfacial reaction between the Sn–9Zn solder and Cu substrate, the Sn–9Zn/ ENIG solder joint was considered to have a superior joint reliability.