Bondability and challenges of Cu ultra-fine-wire bonding

摘要

Cu wire bonding has matured much over the years with improvements made not only to the Cu bonding process and optimization methodologies from bonder manufacturers, but also on capillaries and Cu wire itself. It is also part of the roadmap of many assembly houses to include ultra-fine-pitch(UFP) Cu wire applications. In this paper, various Cu wire types including coated Cu going down to as fine as 0.5mil diameter size is evaluated and compared in terms of Free-Air-Ball(FAB), 2nd bond performance in a customized QFP leadframe, 1st bond performance in a customized BGA substrate and extreme looping conditions at high and ultra-low loop are studied too. Challenges faced in Cu ultra-fine-wire(UFW) application will also be discussed. Improvement process through wire bonding parameters and making use of wire characteristics will also be covered. As wire diameter gets smaller, energy that is required to melt wire to form FAB also gets lower. Range of energy input in term of EFO Current and Firing Time also get smaller, and might be more sensitive towards noises from surrounding. Shorter Firing Time might also means less time to form concentric sphere before freezing. EFO firing approach gets more critical in this sense. Unlike gold wire that is malleable, copper wire is harder and hence does not deformed as easily. Limited by smaller capillary tip in UFP, 2nd bond contact area at UFW bonding also gets smaller. 2nd bond approach with segmented bonding was used to enhance the 2nd bond contact in this evaluation. UFP application with bond pad pitch down to as small as 30um has been reported in gold wire, however due to Cu FAB's harder nature, presence of Al splash in 1st bond posed a greater challenge in achieving this pitch with similar wire size made with Cu. Ease of bondability of CuPd has created a tendency to run CuPd wire as plug and playable wire to existing bare Cu wires. However difference observed in bonding response of CuPd from bare Cu means that slight fine tunin- in looping such as the kink location might still be required.