Challenges of wirebonding with polyimide flexible printed circuit board (FPCB)

摘要

Wire bonding is still popular and leading interconnect technologies due to its reliability, reputation for versatility and performance. Increased demand for disposal medical sensors, miniaturization for portable electronic equipment and specialty parts which are small in dimension and light weight area are at utmost importance, which lead to the chip-on-flex technology to become more attractive technology for above application. Thermo-sonic Aluminum (Al) wedge-wedge bond is preferred [1] compare to Gold (Au) bonding in most commercial chip-on-flex applications for chip to substrate interconnection due softening polymers at flexible substrate when heating at 180°C~200°C. This softening of polymers during heating absorbs the ultrasonic energy which caused failure in ball wire bond formation on the pad/wedge, hence reducing the wire bonding yield. In this paper, we shall discuss the challenges face during Au ball bonding using polyimide flexible printed circuit board (FPCB), which has Copper as the single conductor layer and a polyimide layer. The SoC die consist of the temperature sensor, a MEMs pressure sensor, a power management unit (PMU) with an oxygen sensor together with the IC interface (Signal readout interface). The bonding temperature selection, to minimize ultrasonic energy absorption by the FPCB, the Au wire type choice used for this for successful ball bonding/wedge and the wirebonding optimization parameters involved will also be evaluated and covered. A successfully wirebonding onto the polyimide FPCB with ball shear and wire pull measurement that meet the specification will also be shown and discussed.