Aluminum-Copper Ribbon Interconnects for Power Devices

Robust bonding of bilayer aluminum-copper ribbons was developed. Bond lifetime improvements in power cycling of a factor 4 or greater compared to aluminum ribbons were experimentally demonstrated. The copper portion of these bilayer materials, its properties, and the interface with the aluminum layer make processing these ribbons more challenging than processing aluminum ribbons. A review of the equipment functions, particularly the bond head and the ultrasonic transducer; the development of appropriate consumables, particularly the bond tool and cutter; and more robust bond process profiles were required to minimize the likelihood of die damage. This approach enables the bonding yields required in high-volume production while simultaneously achieving the desired bond reliability improvement in power cycling. Methods were developed to study and control the bond formation stages and to explore the die damage limits in the various bond profile phases. As part of the bond process development, a power cycling station was constructed to submit bonds to accelerated stress testing. The information collected from power cycling was linked to the bonds' properties after bonding. From this testing, the characteristics of improved bond process sequences and of bond tool designs were deduced. Ribbons with different aluminum-to-copper thickness ratios were tested in terms of reliability under power cycling and process yield. This cause and effect approach enabled a significant yield loss reduction while maintaining the bond reliability improvement over aluminum ribbon close to the factor expected based on the ribbon material properties.