Hi Earl, Werner and all Sorrysorrysorry because caused technet shy. I've use the expression engelnotes as synonym for all responses, not only of some experts. Ok, I would also like to hear of similar problems. Bernhard ---------- Von: Enter your name here[SMTP:[log in to unmask]] Gesendet: Samstag, 28. August 1999 00:30 An: TechNet E-Mail Forum.; Wanner Bernhard Betreff: Re: [TN] crushed prepreg integrity Jeez, I was just getting warmed up with an answer until you referenced the Engelmaster. Now, I've become technet shy. Anyway, I yeild to a position of one so much knowledgeable about matters laminataion/relamination. Love you Werner, but this might be about what the poor "downcast" recluse is complaining. How can we lesser beings compete? It ain't possible, yet you ask the question - Steve. Still, I can't help but learn and respect. Earl Moon ----- Original Message ----- From: Wanner Bernhard <[log in to unmask]> To: <[log in to unmask]> Sent: Friday, August 27, 1999 11:35 AM Subject: [TN] crushed prepreg integrity > Hi all > > I have to analyze failures of a subassemblies of our subcontractor. As cause > of problem I suppose cracks/voids in the prepreg (of a16-layer pcb), caused > either by poor siloxane-treatment between glass and epoxide - or - because > crushing of prepreg integrity by microlocal mechanical stress during the pcb > press operation. As cause of this stress I assume following: During the > press operation the prepreg conforms to the bounds formed by the laminate > and their conductors at both sides. Between the walls/shoulders ot two > buried vias the prepreg will be pressed, the resin will flow away, but the > glass will be squeezed. I think glass fibres squeezing together can resulte > to resin recession. > > But let me tell you some details: The failing subassemblies I've to analyze, > contains transformer windings which are implemented as a 16-layer pcb. The > winding shows dielectric breakdown between different winding layers (and > sometimes between different windings on the same layer). This breakdown > happens after some hours run-in at 120°C and 500VDC (500VDC as test voltage, > failures also at lower tension at at AC) .Before them, all pcb has passed > successful a hi-voltage test at ambient temperature. > The diel. strength for the closest distance of 65mm prepreg should be around > 2.5kV, or a little bit less because IPC-2221, § 4.1.2, not 500V! > I tend to see any loss of the electric strenght of a laminate/prepreg as > function of the temperature, the hi-temp aging, and material defects. > The occuring pcb temperature of 120°C seems in accordance with IPC-2222, § > 4.3. So I'm looking for material defects, especially cracks: if cracks span > a part of > the 65mm (prepreg), the electric strenght will decrease, because the > electric strenght of a cracks (like air) amounts 1kV/mm (instead of 40kV/mm. > > > Hope to get some engelnotes. > > Bernhard > > ############################################################## > 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. > If you need assistance - contact Gayatri Sardeshpande at [log in to unmask] or > 847-509-9700 ext.5365 > ############################################################## ############################################################## 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. If you need assistance - contact Gayatri Sardeshpande at [log in to unmask] or 847-509-9700 ext.5365 ##############################################################