0.1 mm steel or other rolled metal is better than 100 km or more of silicone or epoxy!!!!! Water does not pass through polymers but water vapour does; in fact, they absorb them! Many years ago (in the 1970s) I did an experiment mentioned in Ch. 30 (protective coatings) of a well-known book. I constructed a Plexiglass box with two compartments with a 20 mm thick separator with a 100 mm diam hole. I cast 5 mm thick samples of different coating materials (silicone, epoxy, polyurethane) by building them up over several layers and clamped them over the whole with an O-ring to ensure a seal. I maintained a 95% 40°C atmosphere on one side and a <5% RH on t'other side, with the help of a phosphorus pentoxide desiccant. After 10 days, to ensure equilibrium, I weighed the desiccant then again after 5 more days. The silicone allowed about 1 g of water vapour to pass per day, the epoxy about 0.022 g and the polyurethane about 0.046 g. I did not do a control with a metal disc, but I'm sure it would have yielded 0.00 g/day. Note that if I had filled the "wet" compartment with liquid water, the result would also have been as near 0.00 g as dammit (surface tension is the secret!). Brian Robert Kondner wrote: > Brian and Blair, > > From what I hear a sealed metal box will keep out water. A layer of steel > or glass does not allow water to pass. > > Is this not the case for a "Thick" layer of silicone or epoxy? > > What does "Thick" mean for different materials. How think does a silicone > adhesive need to be to equal the water penetration resistance of 1/16 (or > 1/8) inch of steel? > > How do folks measure how water (or even any ionic material) passes through > a substance? > > Thanks, > Bob Kondner > > > > > > -----Original Message----- > From: TechNet [mailto:[log in to unmask]] On Behalf Of Blair Hogg > Sent: Thursday, March 05, 2009 8:38 AM > To: [log in to unmask] > Subject: Re: [TN] Comformal Coating vs Potting > > Bob, > > Most conformal coatings can and will pass moisture through to the assembly > in varying degrees, Thus, if there are any contaminants on the assembly > prior to coating that can activate with the addition of moisture the coating > will only slow down the effect. Also, if the environment in which your > module will be placed is humid for a significant portion of the time, it > will not have the opportunity to dry out, and its performance may be > affected. > > Blair Hogg > > On Wed, 4 Mar 2009 10:50:38 -0500, Robert Kondner > <[log in to unmask]> wrote: > >> Hi, >> >> I have been playing with some conformal coating material and I have a >> couple of general questions. >> >> 1. Coatings get VERY thin around sharp corners. Using a number of coats >> seems like the only way to build up any serious thickness. >> >> 2. Potting seems like another name for "Very Thick Conformal Coating". >> >> I am looking as an application where a sensitive electronic module will be >> placed in a wet humid environment. It is a sensor going into the cooling >> tower of a HVAC system. >> >> After reading about conformal coating and playing with some sprays and >> brush on material I think thick potting is the only option to keep moisture >> out of circuits. We need at least 20 Meg ohm between circuits spaced 10 > mils >> on FR4 material. >> >> Can anyone verify I am on the correct track and can they point me towards >> selecting the proper materials and process? >> >> Thanks, >> >> Bob Kondner > > --------------------------------------------------- > Technet Mail List provided as a service by IPC using LISTSERV 15.0 > 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 > 847-615-7100 ext.2815 > ----------------------------------------------------- > > --------------------------------------------------- > Technet Mail List provided as a service by IPC using LISTSERV 15.0 > 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 847-615-7100 ext.2815 > ----------------------------------------------------- > --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 15.0 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 847-615-7100 ext.2815 -----------------------------------------------------