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Flipchip Reliability Improvements for an Automotive Sensor Application
Keywords: Flipchip, Reliability, Underfill
During the launch and subsequent validation testing of a small diameter automotive pressure sensor that utilized a flipchip on flex electronics module, we experienced lower than expected reliability levels after extended thermal shock exposure. A subsequent failure analysis indicated cracked copper traces in the flipchip region of the flex circuit. FEA analysis confirmed that the copper trace stresses were in excess of the copper yield strength. Using the knowledge gleaned from the FEA, we were able to reduce the trace stresses by optimizing the trace layout. By changing the flipchip underfill, further trace stress reductions resulted in improved circuit reliability. Analytical results were confirmed by running confirmation tests on actual hardware. Trace changes alone resulted in a 3-4x improvement in thermal shock reliability, while trace changes coupled with underfill changes made a 4-5x improvement in thermal shock reliability. The paper highlights the improvement process and the test to failure approach. Keywords; Flipchip, Flex circuit, Underfill, Reliability, Test to Failure, FEA.
Gifford Plume, Design Engineer
Sensata Technologoes
Attleboro, MA

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