A new robust thick-film conductor was required to make process-friendly, reliable 10 mil aluminum wirebonds in a multilayer circuit intended for use in severe automotive under-the-hood environments. Available conductor systems and processes were explored and were found to be inadequate to meet the manufacturing requirements. The failure mechanisms of such systems were evaluated and explained. Thickness dependent thermal mismatch stresses introduced in the firing process were shown to be the root cause of the identified process limitations. Based on the prevailing mechanisms, a thick-film supplier was requested to develop a conductor to suit the application and materials and process specifications were provided. The candidate materials supplied were evaluated and subjected to harsh environment testing: 500 hours at 150 degrees C, 500 hours at 85%RH/85 degrees C, and 500 cycles at -50 degrees C/+150 degrees C thermal cycles. The failure mechanism due to thermal expansion mismatches in the system was eliminated by the supplier by reformulating the ink chemistry. This will lead to process specifications which greatly improve the manufacturability and production yield. The tests conducted to identify the failure mechanisms are discussed in detail. The evaluation protocol and results are presented. A next-generation multilayer wirebond conductor material giving rise to a robust thick-film manufacturing process has been identified.


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