Bill: Don't know about bubbles accumulating moisture - I'm not clear whether your problem goes away in hot or after you bake out the assembly. One thought: we've had problems in the past with type UR coats over thick film chip resistors - thermal cycling cracks the glassivation on the top of the component, the crack propagates into the resistive element, resistance changes. Same problem with adhesive if the parts are mounted glass-side down. Obviously, these failures are non-reversible. They are easy to spot under magnification, or measure the component by probing through the coating. Joe Kane Lockheed Martin Control Systems > ---------- > From: BROMLEY, Bill[SMTP:[log in to unmask]] > Reply To: TechNet Mail Forum.;BROMLEY, Bill > Sent: Monday, November 03, 1997 3:13 PM > To: [log in to unmask] > Subject: ASSY: SMT/Temp. Cycling Test Failures revisited > Importance: High > > 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] > > ############################################################## TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c ############################################################## To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TECHNET <your full name> To unsubscribe: SIGNOFF TECHNET ############################################################## Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information. For the technical support contact Dmitriy Sklyar at [log in to unmask] or 847-509-9700 ext.311 ##############################################################