Fellow technetters: I would appreciate any help in the following subject.... We are experiencing temperature cycle (-37C to 55C) test failures on mixed technology (thru-hole one side, SMT other side) PWB assemblies coated with Polyurethane per Mil-I-46058. The same assemblies pass static temperature testing at the low, ambient and high points of the temperature range. Uncoated assemblies will pass the temperature cycle test. It is suspected that small bubbles in the conformal coating (within allowable size per specification criteria) are accumulating condensation during temperature transitions, causing bridging between conductive surfaces on the SMT components. The surface mount components that we are using are attached to the board with a small dab of adhesive in the middle of the component. Then, after the components are wave soldered to the board, two very small gaps exist under the components between the adhesive and the solder connections. The air bubbles that we see in the conformal coat are next to these gaps, leading us to believe that the air under the components escapes during the coating cure thereby forming the air bubbles. Some of these bubbles are large enough to bridge between the two solder connections. It is suspected that these bubbles are accumulating moisture which causes leakage currents between the conductive surfaces on the SM components. How do we get the air out from under the components without causing bubbles in the conformal coat? Is it acceptable to have conformal coat flow under the SM components? > Bill > > Bill Bromley > Lead Designer > Lucas Aerospace PSA > MailStop D960 > 777 Lena Dr. > Aurora OH 44202-8025 USA > voice: 330-995-1000 ex 3092 > email: [log in to unmask] >