Failure Analysis of Aluminum Wire Bonds in Automotive Pressure Sensors in Thermal Shock Environments

Wire bonding remains one of the most widely adopted interconnection techniques in the field of electronic packaging. At present, the most effective way to ensure a long life and high reliability of wire bonds is to improve the bonding quality. In this study, both experiments and finite element analysis (FEA) were employed to develop a fundamental understanding of wire bond degradation. Sensors with two protective silicone gels were loaded with the same thermal shock at a temperature ranging from $- 40^{\circ }\text{C}$ to 125°C, and the switching time was shorter than 10 s. The number of thermal shock cycles for the aluminum wire covered with transparent silicone reached 1200, but the maximum number of cycles for the other wire only reached 454. The experimental results indicated that the chosen transparent silicone performed better than did the black silicone originally selected, which was also verified by the simulation results. In addition, bond pull and shear tests were performed. The results revealed no degradation of either the Ag-Al or Ni-Al bonding joints under thermal loading. In summary, the root cause of failure was found to be improper protection silicone application, which, as often ignored in analysis, accelerated thermal fatigue of the aluminum wires. An explanation of the observed trend and a recommended aluminum wire bonding method were also provided.