Per-Erik An interesting idea. Briefly, you are proposing a more permeable coating so that humidity can leave, as well as enter. I suggest that if these conditions apply, then the best coating would be the most permeable. Probably 90% or more of electronic assemblies use such a coating. It is called air. It is extremely successful for the conditions under which such circuits are normally used. Conformal coating is usually used when air-coating is unsuitable, for whatever reason. Therefore, we need something as far removed from the conditions of air as possible, such as a theoretically impervious skin. The nearest we have to this in our armoury of weapons of mass construction is para-xylylene (Paralene/Parylene). This is near-ideal, but expensive and difficult to apply, so we usually look to other materials. I would suggest that acrylics are closest to this ideal, except that they have poor temperature and chemical resistances. So the choice, IMHO is either air, with maximum permeability, or a product with minimal permeability. Compromises between the two are probably second-best. Brian Tegehall Per-Erik wrote: > After having thought a little more on this, I will expound my answer. It > seems that all assume that high permeability is a bad thing but in my > opinion it is not that simple and a high permeability may even be > preferable. If you have adequate cleanliness on the assembly prior to > applying the coating, the moisture penetrating the coating will not be a > problem and the permeability will then not be an issue. It is when you > have to much hygroscopic and ionic contamination beneath the coating > that you may get problem with delamination and electrochemical > migration. Even if you use Parylene, quite a lot of moisture will be > picked up by hygroscopic contamination beneath it in a few days in a > humid environment. If you then turn the euipment on and the board is > heated due to power dissipation, it will take much longer time to dry up > the assembly if you have a coating with low water permeability. And it > is during this time period electrochemical migration will occur. > Therefore, a silicone coating may actually be better than Parylene in > this case. > > Per-Erik Tegehall > IVF > > -----Ursprungligt meddelande----- > Från: Tegehall Per-Erik > Skickat: den 5 september 2003 08:38 > Till: [log in to unmask] > Ämne: Re: [TN] Silicone conformal coating > > Steve, > > It depends on what you mean with good moisture resistance. All > conformal coatings absorbs moisture but the diffusion rates of water > in the coatings and the saturation level varies. Water molecules > have a high diffusion rate in silicone coatings but the saturation > level is lower than in most other coatings. Which is to be prefered? > It probably depend on the application. > > However, a general statement of good moisture resistance, I > interpret as that the coating properties are not degraded by moisture. > > Per-Erik Tegehall > IVF > > -----Ursprungligt meddelande----- > Från: Steve Gregory [mailto:[log in to unmask]] > Skickat: den 4 september 2003 21:35 > Till: [log in to unmask] > Ämne: [TN] Silicone conformal coating > > Hi All! > > I know when you look up properties for Silicone conformal > coating, it usually states that it has good moisture resistance, > but is it really that good? > > I've heard that it is somewhat permeable to moisture, that if > you were worried about moisture, and the assembly were going to > see the outside environment, you would be better off with an > acrylic, or urethane coating. > > Any thoughts? > > Thanks! > > -Steve Gregory- --------------------------------------------------- > Technet Mail List provided as a free service by IPC using > LISTSERV 1.8e > To unsubscribe, send a message to [log in to unmask] with > following text in the BODY (NOT the subject field): SIGNOFF Technet > To temporarily halt or (re-start) delivery of Technet send > e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) > To receive ONE mailing per day of all the posts: send e-mail to > [log in to unmask]: SET Technet Digest > Search the archives of previous posts at: > http://listserv.ipc.org/archives > Please visit IPC web site > http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional > information, or contact Keach Sasamori at [log in to unmask] or > 847-509-9700 ext.5315 > ----------------------------------------------------- > > --------------------------------------------------- > Technet Mail List provided as a free service by IPC using LISTSERV 1.8e > To unsubscribe, send a message to [log in to unmask] with following > text in the BODY (NOT the subject field): SIGNOFF Technet > To temporarily halt or (re-start) delivery of Technet send e-mail to > [log in to unmask]: SET Technet NOMAIL or (MAIL) > To receive ONE mailing per day of all the posts: send e-mail to > [log in to unmask]: SET Technet Digest > Search the archives of previous posts at: > http://listserv.ipc.org/archives > Please visit IPC web site > http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional > information, or contact Keach Sasamori at [log in to unmask] or > 847-509-9700 ext.5315 > ----------------------------------------------------- > > --------------------------------------------------- > Technet Mail List provided as a free service by IPC using LISTSERV 1.8e > To unsubscribe, send a message to [log in to unmask] with following text > in the BODY (NOT the subject field): SIGNOFF Technet > To temporarily halt or (re-start) delivery of Technet send e-mail to > [log in to unmask]: SET Technet NOMAIL or (MAIL) > To receive ONE mailing per day of all the posts: send e-mail to > [log in to unmask]: SET Technet Digest > Search the archives of previous posts at: http://listserv.ipc.org/archives > Please visit IPC web site > http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional > information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 > ext.5315 > ----------------------------------------------------- --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 1.8e To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 -----------------------------------------------------