Development of a New Wire Bonding Technology on Integrated Circuit Devices

In IC packaging, copper wire is generally regarded as a competent successor to gold wire due to many advantages in mechanical characteristics and cost efficiency. However, its known disadvantages restrict it to low-end integrated circuits as well. This paper discusses the development of a new wire bonding technology — copper ball on gold bump (COG) bonding in current wire bonders with both 1 mil copper and gold wires. It covers material and tool selection, wire bonding process window development, electrical characterization and reliability studies. The material and tool selection includes copper wire, experimental chip, capillary and wire bonder. Process window development focuses on two crucial stages, copper free air ball (FAB) formation and bonding process window development for both gold bump and copper ball bonded on Au bump. DOE approach is introduced into the relative process developments. The experimental studies reveal that flow rate of shielding gas is a key factor to obtain the qualified FAB formation and process window optimization for gold bump and copper ball on gold bump is more crucial to develop a successful COG bonding. Wire pull test, ball shear test and crater test are involved in the output measurement based on the criteria of JEDEC during the process window development. Package integrity and reliability performance with the COG bonding are fully assessed by performing electrical characterization and package internal delamination detection on the evaluated samples which have respectively undergone three reliability tests, High Temperature Baking, Temperature Cycle and Autoclave. The study result finally indicates that the COG bonding is a practicable approach to achieving copper wire application to high-end integrated circuits.