For plastic packages, copper leadframes can provide many advantages such as good thermal and electrical performance, and cost effectiveness. Copper is, however, susceptible to popcorn cracking during the surface mounting process on the PCB (Printed Circuit Board) due to poor adhesion between the leadframe and EMC (Epoxy Molding Compound).The experiment results showed that the adhesion strength between copper leadframes and the EMC was affected by alloy composition, CuO/Cu20 ratios, oxide layer thickness, and oxidation conditions. The growth of the oxide layer for the Cr-Zr copper alloy was faster than for Ni-Si copper alloys, while the latter had lower adhesion strength than the former. The adhesion strength of the copper alloys depended on the CuO/Cu20 ratio rather than oxide thickness. The adhesion strength became the highest at a CuO/Cu20 range of 0.2 -0.9 regardless of alloy composition and oxide thickness. We found that this was caused by segregation effect of soluble elements at the oxiddleadframe interface.