Growth behavior of IMCs layer of the Sn–35Bi–1Ag on Cu, Ni–P/Cu and Ni–Co–P/Cu substrates during aging

In this work, the interfacial reactions and IMC growth of the Sn–35Bi–1Ag on Cu, Ni–P/Cu and Ni–Co–P/Cu were studied during reflowing at 220 °C for 10 min and solid–state treatment at 150 °C with various aging times. For the solder joints of Sn–35Bi–1Ag/Cu, Cu₆Sn₅ IMC was formed at the interfacial layer after soldering, while Cu₃Sn appeared after aging treatment. In the case of the electroless Ni–P plating of Cu, the IMC formed at the interface during isothermal aging was mainly Ni₃Sn₄ and a small amount of P-rich Ni (Ni₃P) layer between the Ni₃Sn₄ IMC and the electroless Ni–P plating layer. In the case of the electroless Ni–Co–P plating of Cu, the IMC formed at the interface during isothermal aging was mainly (Ni, Co)₃Sn₄ and a small amount of P-rich Ni ((Ni, Co)₃P) layer between (Ni, Co)₃Sn₄ IMC and the electroless Ni–Co–P plating layer. The addition of Co atoms could effectively inhibit the IMC growth rate, which inducing a lower growth rate of (Ni, Co)₃Sn₄ than that of Ni₃Sn₄. The thickness of Ni₃P and (Ni, Co)₃P layer reached about 2.31 µm and 1.25 µm after aging for 360 h, respectively. Also, the growth kinetics of the Ni₃P and (Ni, Co)₃P layer was found to be a diffusion–controlled process, which followed a parabolic relationship in the thickness increase. During the aging, the consumption of the electroless Ni–Co–P plating layer was greatly reduced compared with that of the electroless Ni–P plating layer. Moreover, there were no voids observed in the electroless Ni–Co–P plating layer while some defects formed in that of the electroless Ni–P plating layer. The electroless Ni–Co–P plating layer will be a good diffusion barrier for the lead-free soldering.