A case study in ultrasonic cleaning damage

A single lot of 32 SAW (Surface Acoustic Wave) filters, exposed to an undocumented, post-seal, ultrasonic cleaning process, resulted in latent damage that was not discovered until late in the build cycle of a space qualified frequency synthesizer. The integrity of space qualified hardware requires the mandatory identification of root cause for all failures. The 35th frequency synthesizer in the hardware build in a multi-satellite program was subjected to acceptance level vibration testing and was observed to have a spectrum breakup. The synthesizer performed normally before and after the vibration. The failure was isolated to the fracture of a 1 mil diameter gold wire in a SAW filter. The wire, although fractured, was in contact with the ball-bond and maintained good electrical contact. SEM (Scanning Electron Microscope) examination showed evidence of significant high cycle fatigue on the fractured wire and varying amounts of fatigue (none to major) on other wires in the same filter. An investigation team prepared an Ishikawa (fishbone) diagram and eight plausible hypotheses were identified for possible root cause. This unique investigation was compounded because the filter was built eight years earlier-the investigation required significant deductive reasoning to reconstruct the facts. A total of 31 filters (14 lot acceptance test samples and 17 flight filters) from eight lot-date-codes (LDC) were examined in the SEM; testing was used to simulate several of the failure scenarios, the build history was reconstructed and inter/intra lot evaluations were performed. The analysis of the information led to a clear probable root cause and eliminated the reachback concern to other lots on other flights. It was concluded that a single lot of 32 filters was exposed to an undocumented, post-seal, ultrasonic cleaning.