Effect of isothermal aging on the interfacial reactions between Sn–0.4Cu solder and Cu substrate with or without ENIG plating layer

The interfacial reaction between the Sn–0.4 wt.% Cu solder and two different kinds of substrates (Cu and ENIG (electroless nickel-immersion gold)) during isothermal aging at 150 °C were studied. In case of the Cu substrate, two IMCs (intermetallic compounds) of Cu6Sn5 and Cu3Sn formed at the interface after aging for 250 h. After aging at 150 °C for 1000 h, crack occurred along the interface between the Cu3Sn IMC and Cu substrate. In case of the ENIG substrate, the solder/electroless Ni–P interface exhibited a duplex structure of (Cu,Ni)6Sn5 and (Ni,Cu)3Sn4 after aging at 150 °C for 24 h. Many discontinuous (Cu,Ni)6Sn5 IMCs formed on (Ni,Cu)3Sn4 IMC layer. The presence of P in the electroless Ni–P layer caused complicated interfacial reactions such as formation of a P-rich Ni (Ni3P) and Ni–Sn–P layers. The growth of the Cu–Sn IMC layer formed on the Cu substrate was much faster than that of the Cu–Ni–Sn IMC ((Cu,Ni)6Sn5 and (Ni,Cu)3Sn4) layer formed on the ENIG substrate. In addition, the formation of the Cu–Ni–Sn IMC layer retards the consumption of the Ni from the electroless Ni–P layer. Results of this study showed that the ENIG plating layer acts as a good diffusion barrier for the Sn–0.4Cu solder.