Investigating and Mitigating Gold Wire Stitch Breakage Failures in Tire Pressure Monitoring Sensor

The Tire Pressure Monitoring Sensor (TPMS) is subjected to a protective gel coating process on the pressure sensor, accelerometer and Microcontroller Unit (MCU) die to safeguard against environmental factors. These factors include moisture, humidity and foreign materials that could potentially cause corrosion or reliability issues. In particular, an additional gel layer is applied to the pressure sensor’s surface, followed by a subsequent coating on all three dies. During the qualification phase, the removal of the extra gel layer resulted in a failure after 1300 Temperature Cycling (TC) cycles. This failure was attributed to broken stitch wire bonding between the pressure sensor and the MCU dies. Scanning Electron Microscope (SEM) analysis of the qualification samples, in comparison to a baseline pinpointed the capillary dimensions as the primary factor responsible for the failure. Simulations and empirical matching processes revealed a critical dimension that impacts bonding, leading to the broken stitch wire. Furthermore, wire bond testing highlighted suboptimal settings as a secondary contributing factor. By integrating findings from the defective capillary and the suboptimal wire bond settings, the team was able to replicate the broken stitch wire failure. Corrective measures implemented during the qualification rebuild not only allowed the device to endure 1300 cycles of TC but also to pass additional reliability tests