Improvement of the second bond strength in copper wire bonding on pre-plated leadframe

Due to the low cost and high mechanical and electrical properties of the copper wire as well as the environmental friendly pre-plated frame (PPF), copper wire bonding on PPF has been considered the most promising candidate to replace the gold wire bonding in actual volume production in the coming years. However, the nature of the copper, such as high hardness, sensitivity to oxidation, work hardening effect etc., coupled with the low adhesion between the copper wire and the PPF surface are the main causes to the low and unstable bonding strength, especially at the second bond. The proposed solution is to introduce a heat treatment after the PPF copper wire bonding. Two kinds of unsealed PPF copper wire bonding samples prepared under different parameters were submitted to a vacuum heat treatment at 200 °C and 10-3 Pa for 10 min. It was found that the average wire pull strengths of both samples increased. The main wire pull break modes changed after vacuum heat treatment, i.e., the second bond lift-off disappeared and the heel breaks decreased after the heat treatment. The results indicate that the heat treatment enhances the interfacial adhesion of copper wire bonding on PPF, and increases the tensile strength of the second bonds.