Silver Epoxy Filled Via Analysis for High Reliability RF MCM Microelectronics

摘要

The use of thin film ceramic substrates containing laser drilled and electroplated vias attached to arnground plane with electrically conductive epoxy is a common design approach used for RF MCM’srnproducts. This construction technique requires the vias be filled or plugged with an adhesive material tornprevent the electrically conductive epoxy from flowing up through the via and onto the top surface of thernthin film substrate metallization pattern during the installation of the substrate into the housing.rnAfter completing the assembly of the RF path, the path is “tuned” to optimize output power based on thernassembled “as built” configuration. Performance characteristics are tested and reviewed for “in family”rnresults during the processing of high reliability products used in space applications. An indication ofrndegradation in RF performance was identified prior to hermetically sealing the modules during “pre-seal”rnscreening (temperature and mechanical extremes). The data suggested that the output power from thernmodules was “out of family” by comparison to the expected result.rnUtilization of a fishbone diagram resulted in several potential root cause investigations that were launchedrnin parallel to repair the RF power anomaly. The “out of family” conditions were corrected and therndegraded ground path root cause was verified. A finite element analysis of the electroplated ground viarnwas developed, mechanical and thermal testing was conducted to repeat the failure mechanism. At thernconclusion of the investigation, it was determined the via thin film metallization failed under load causedrnby the CTE of the silver epoxy used to fill the via. The failure rate was found to exceed the allowablernreliability criteria.rnAn alternate material for via plugging was selected and tested to validate that the manufacturing solutionrnmet the product life expectations.