Effects of high temperature and high humidity on the reliability of copper/epoxy bond

Epoxy encapsulation is a promising technique to reduce the size of the power module. Its reliability is a key aspect. Many previous studies developed new epoxy materials or new surface treatment methods to improve reliability. However, reliability mechanisms are still not well understood. This study focused on the reliability of copper/epoxy bonds under high temperature and high humidity conditions because previous studies did not reach a unified conclusion on this issue. The copper/epoxy bonds are fabricated by compression molding. Then, these copper/epoxy bonds are placed in the 85 ℃/85 %RH reliability test for 1000 h. The bonding strength change was measured. Surprisingly, the bonding strength between copper and epoxy increased after the 85 ℃/85 %RH reliability test. The possible reason was investigated by observing the fracture surface and bonding interface in a nanoscale. In addition, molecular dynamics simulations were performed to better understand the experimental results. The results showed that the water absorbed in the epoxy caused complex physical and chemical changes at the copper/epoxy interface. The formation of a new cuprous oxidation layer with aligned crystal orientation is supposed to be the reason for the bonding strength increase. Our findings will provide a better understanding of copper/epoxy bonds.