Abstract: The phenomenon of printed circuit board (PCB) bonding finger surface discoloration was explained by analyzing the diffusion of copper atoms，that was, the copper atoms diffused from copper substrate to PCB bonding finger surface, then was oxidized. At the same time, the inherent driving force of copper atoms diffusion in the PCB bonding finger was analyzed. In addition, the diffusion flux and concentration distribution expressions of copper atoms in the PCB bonding finger were drawn by analyzing Fick's Law combined with actual conditions, which provided a theoretical basis for preventing diffusion of copper atoms in the PCB bonding finger.