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PCB-Aug2015

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40 The PCB Magazine • August 2015 by Stan Heltzel euroPean SPace agency Abstract Latent short circuit failures have been ob- served during testing of PCBs for power dis- tribution of spacecraft of the European Space Agency. Root cause analysis indicates that for- eign fibers may have contaminated the PCB laminate. These fibers can provide a pathway for electromigration if they bridge the clear- ance between nets of different potential in the presence of humidity attracted by the hygro- scopic laminate resin. PCB manufacturers re- port poor yield caused by contamination em- bedded in laminate. Inspections show that fiber contamination is present on prepreg and etched innerlayers. Further fiber contamination may be attracted in the manufacturing environ- ment due to static charging. The requirements for cleanliness that are specified for final PCBs are orders of magnitude more stringent than those specified for base materials. This paper describes inspections performed on base mate- rials, manufacturing processes and final PCBs. It describes test methods that detect reduced insulation caused by contamination and elec- tromigration. Moreover, a proposal is presented specifying tightened requirements for a new class of base materials for the manufacture of high-reliability PCBs. I. Introduction Latent short circuit failures have been ob- served in PCBs during testing of power distribu- tion units of spacecraft for the European Space Agency (ESA). Root cause analysis has been con- ducted under review of non-conformance re- view boards (NRB). Printed circuit board assem- blies failed after prolonged functional testing in ambient laboratory environment or after ther- mal vacuum cycles. Due to the large amount of damage caused by the electrical overstress, it was not possible to obtain direct evidence of the Latent Short Circuit Failure in High-Rel PCBs due to Cleanliness of PCB Processes and Base Materials FeAture

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