Abstract: The high temperature strorage reliability of Cu bonds formed with 20 μm copper wire by ultrasonic bonding process after encapsulation with epoxy molding compounds was investigated. SEM was used to analyze the interfacial microstructure and IMCs on the bonding interface after thermal aging. EDS was used to identify the IMCs compositions. A great amount of Cu/Al IMCs as well as micro-cracks and Kirkendall voids are found on the bonding interface after thermal aging with 9 days at 200℃ and with 9 hours at 250℃. With 16 hours aging at 250℃,the element Sb which is decomposed from the epoxy molding compounds started to react with Cu ball bonds to form Cu₃Sb. When the aging time is more than 49 hours at 250℃,Cu wire loop is broken and serious corrosion is found in Cu ball bond. Furthermore,Ag migration phenomenon occurs if the aging time is more than 24 hours at 250℃ or 4 hours at 300℃.