Ok Ok Bernhard, Then lets go back to the beginning. What are you trying to protect and from what? Maybe you can use a Conformal coating, back fill with silica sand and top off the result with a thin potting material. Last saw this in Ford Lansdale in 1979. Horses for courses. Give me a response with what your problem is and we'll go forward from there - is that a lively discussion basis? Regards, Graham Naisbitt [log in to unmask] WEB: http://www.concoat.co.uk Concoat Ltd Alasan House, Albany Park CAMBERLEY GU15 2PL UK Tel: +44 (0) 1276 691100 Fax: +44 (0) 1276 691227 ----- Original Message ----- From: Wanner Bernhard <[log in to unmask]> To: <[log in to unmask]> Sent: Friday, July 02, 1999 7:32 AM Subject: [TN] WG: [TN] Embedment, potting - reliability, thermal cycling > Hi Werner and other interested technetters, > > Thanks for your responses. The products in question are small > modules, potted with a two-component epoxide resin. Such resins are much > harder than most silicones (rtv etc.). Worldwide there are tons and megatons > of small devices, often small DC/DC-converters, which are potted with (more > or less) a hard potting compound. Apparently there is no worldwide epidemic > of failures because of cracks. > > I am personally convinced that a rigrid potting mass kills (SMT-) > system reliability. I estimate that there are many cracked solder joints > which don't provoke an electrical failure, because often cracked solder > joints make contact most of the time like a touch-button. Come on potting > and lacquering (and MIL) guys honestly, am I the only one with this opinion? > > To qualify a potted SMT-assembly I need to define an effective test. > Because the mechanical modulus of elasticity will increase dramatically at > low temperatures (-25°C...-40°C), simply cycles at low temperature would > provoke the failure mechanism occuring under real conditions. And this would > exactly the opposite to use the acceleration factors described in IPC-D-279. > > > Can anyone describe to me used/effective thermal cycling parameters > for potted SMT-assemblies? > > I hope this mail promotes a lively discussion of this subject. > Bernhard > > > ---------- > Von: Werner Engelmaier[SMTP:[log in to unmask]] > Gesendet: Freitag, 25. Juni 1999 02:15 > An: [log in to unmask] > Betreff: Re: [TN] Embedment, potting > > Hi Bernhard, > To my knowledge, an I am the chair of the IPC Reliability > Committee, there is > no document that addresses the reliability of > embedded/potted electronics, > either THT or SMT. Certainly, the recommendations made in > IPC-SM-785 for > accelerated testing are useful in your situation; you just > will not be able > to use the acceleration factors. > The possible problems posed by potting compounds have to do > with their CTE, > their cure temperature [which sets the initial stress > conditions (you want > compressive on the solder joints and a cure temperature > higher than the > highest operating temperature)], and the operational > temperature excursions > (severity and number). These parameters will determine what > accelerated test > conditions should be used and if there is an acceleration > factor that can be > applied. > > Werner Engelmaier > Engelmaier Associates, L.C. > Electronic Packaging, Interconnection and Reliability > Consulting > 7 Jasmine Run > Ormond Beach, FL 32174 USA > Phone: 904-437-8747, Fax: 904-437-8737 > E-mail: [log in to unmask], Website: www.engelmaier.com > > > > ---------- > Von: Wanner Bernhard > Gesendet: Donnerstag, 24. Juni 1999 16:27 > An: [log in to unmask] > Betreff: Embedment, potting > > Hi Technetters > > I have to consider and to take decision about the > reliability of embeded (potted) assembled PCB's (THT and SMT). To evaluate > the solder joint (and componenet) reliability I intend to perform thermal > cycles (no load). IPC-SM-785 defines such cycling tests, but it seems > Conformal Coating has been only with secondary importance and Embeding > /Potting is not mentioned. > > Can anyone tell me if there is an IPC or other > document handling the reliability of embedded/potted electronics (especially > after thermal cycling/lifetime)? > > At the good old MIL-world the Requirement 47 of > MIL-STD-454K (a old version, I know) adresses some MIL-Std's (MIL-I-16923, > MIL-M-24041, MIL-I-81550 etc.) for this purpose. Now I'm looking after > equivalentes at the IPC-world (or J-STD) > > Bernhard > [log in to unmask] > > > >