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Silver Epoxy Filled Via Analysis for High Reliability RF MCM Microelectronics
Keywords: Thin Film Vias, RF MCM, Silver Filled Via
The use of thin film ceramic substrates containing laser drilled and electroplated vias attached to a ground plane with electrically conductive epoxy is a common design approach used for RF MCM’s products. This construction technique requires the vias be filled or plugged with an adhesive material to prevent the electrically conductive epoxy from flowing up through the via and onto the top surface of the thin film substrate metallization pattern during the installation of the substrate into the housing. After completing the assembly of the RF path, the path is “tuned” to optimize output power based on the assembled “as built” configuration. Performance characteristics are tested and reviewed for “in family” results during the processing of high reliability products used in space applications. An indication of degradation in RF performance was identified prior to hermetically sealing the modules during “pre-seal” screening (temperature and mechanical extremes). The data suggested that the output power from the modules was “out of family” by comparison to the expected result. Utilization of a fishbone diagram resulted in several potential root cause investigations that were launched in parallel to repair the RF power anomaly. The “out of family” conditions were corrected and the degraded ground path root cause was verified. A finite element analysis of the electroplated ground via was developed, mechanical and thermal testing was conducted to repeat the failure mechanism. At the conclusion of the investigation, it was determined the via thin film metallization failed under load caused by the CTE of the silver epoxy used to fill the via. The failure rate was found to exceed the allowable reliability criteria. An alternate material for via plugging was selected and tested to validate that the manufacturing solution met the product life expectations.
Joseph J. Kreuzpaintner, Technologist
Harris Corporation
Palm Bay, FL

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