Microstructure and interface reaction between Sn-3.5Ag solder and electroplated Ni layer on Cu substrate during high temperature exposure

The microstructure and interface reaction between Sn-3.5Ag solder and electroplated Ni layer on a Cu substrate was studied. The results show that the electroplated Ni layer can effectively hinder the inter-diffusion between Sn-3.5Ag solder and Cu substrate after soldering and aging. The intermetallic compound (IMC) formed at the interface is quite thin after soldering and nano-size Ag₃Sn particles are also observed on the surface. Under low temperature (70°C) exposure for 1000 h the IMC at the interface is Ni₃Sn₄. However, under high temperature (120°C, 170°C) exposure for 1000 h the IMC formed at the interface changes to (Cu_1-xNi_x)₃Sn₄ which implies that the Cu atoms from the Cu substrate diffuse into the Ni₃Sn₄ layer through the electroplated Ni layer. Coarse Ag₃Sn particles are also found on the surface of IMC after etching away the remaining solder at 170°C aging for 1000 h. The growth kinetic of IMC layer during aging is x=(kt)¹2/ which implies that the growth of IMC is controlled by a diffusion mechanism. The activation energy of the IMCs is 132.404 KJ/mol which is larger than that of Cu₆Sn₅ (58.95 KJ/mol). It means that the growth rate of the IMCs is much slower than that of Cu₆Sn₅ under low temperature aging while a bit faster under high temperature aging.