Degradation mechanism of CuPd wire in high temperature hermetically sealed electronics

There is a big push in the aerospace industry toward more electric aircraft (MEA)[1-2]. The progress is necessary to improve the overall power efficiency and reliability of the aircraft. Aircraft engine health monitoring (EHM) system is one the major effort from aircraft manufacturer toward MEA. This system employs arrays of strategically placed sensors that need to maintain high reliability and robustness under high temperature condition (i.e. 250°C). One key aspect of attaining robust EHM under harsh environment is having high temperature endurable electrical connection using wire bond couple with suitable barrier metallization. Gold (Au) and Aluminum (Al) wire are popular among high temperature electronics packaging solution and development [3-4]. Others employ conformal coating to shield Copper Palladium (CuPd) wires against oxidation from oxygen rich environment at extreme high temperature [5-6]. These are proven solutions for 225 to 250°C high temperature endurable electronics. CuPd wire has made many inroads for IC assembly packaging and this study was initiated to assess whether it could replace Au wire in 250°C high temperature endurable hermetic packages. An approach of using a hermetic (sealing in nitrogen) system in package design with alumina (Al2O3) substrate and Au thick film conductor technology and packaged with CuPd wire bond as interconnects has been investigated. The wires were bonded to the test chip using thermosonic wire bonding technology. The reliability test evaluation included Thermal Cycling (TC, -55°C to 225°C, 1000 cycles) and High Temperature Storage (HTS, 250°C, 2000 hours). Hermetic condition did not prevent CuPd wire pull strength from decreasing significantly after 500 hours of HTS and substantial wire resistance increase was observed. Hence, the work indicated that CuPd wire is not suitable as an alternative to Au or Al wire in high temperature hermetic environments due to the oxidative reaction even with low levels of oxygen.