Correct. Think of your electronic assembly inside of a plastic bag. You vacuum seal the bag. No matter how much vacuum you apply, there are air pockets. Immersion of the sealed bag with the board into water or other gas/liquid for a short term results in no gas/liquid making direct contact with the assembly inside, assuming there are no holes in the bag. However, some moisture can penetrate through the bag over time when exposed to air. Upon reaching equilibrium, which all matter tends to do, there is an equal amount of moisture in the small air pockets inside of the bag as there is in the air around the bag. With temperature changes, there can be some small amount of condensation. Due to the inability of the condensation to evaporate quickly through the bag, the moisture is now trapped in liquid form inside the bag, but in very small amounts. Without the bag, any liquids make direct catastrophical contact. With the bag, there is no direct catastrophical contact, but there can be small amounts of moisture, over time. Conformal coating is the equivalent of the bag. The idea here is that you are preventing the catastrophical contact, and attempting to prevent as much as possible the amount of condensation that can be formed from available moisture inside of the bag. The smaller the amount of moisture inside the bag, the greater the temperature extremes that must be seen to create significant amounts of condensed liquids. Significant = enough to cause electrical performance issues. Significant threshhold for a power supply or switching terminal is much higher than for a high-impedance high-frequency RF transmitter/receiver, as an example. Sorry if this is kind of simplistic. I am just trying to portray the big picture. The primary reason for cleanliness prior to cc is to promote the adhesive seal of the cc with the surfaces to be coated. But a secondary reason why cleanliness is critical for conformally coated assemblies: the presence of flux entrapped with condensation and a power source is an incubator for all kinds of unhealthy life forms. And it is also why conformally coated assemblies should never be totally immersed in liquids, as part of the process. The bag usually has some small holes, and as a result some liquid will be absorbed and trapped inside. Conformal coated assemblies are (usually) housed in a box or cabinet that is sealed from the elements to prevent any direct immersion. -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Douglas O. Pauls Sent: Wednesday, December 06, 2006 7:00 AM To: [log in to unmask] Subject: Re: [TN] conformal coatings as water proofing What you are hearing is that no organic conformal coating makes an electronic assembly absolutely bulletproof against the environment. All of them pass water vapor, and sometimes other gasses, to varying extents. It impedes the flow of moisture but does not completely eliminate moisture. Important distinction. Doug Pauls Bill Kasprzak <[log in to unmask] OM> To Sent by: TechNet [log in to unmask] <[log in to unmask]> cc Subject 12/06/2006 06:56 Re: [TN] conformal coatings as AM water proofing Please respond to TechNet E-Mail Forum <[log in to unmask]> ; Please respond to Bill Kasprzak <[log in to unmask] OM> Good morning, I am surprised that there are such opposing views on whether conformal coating is effective as a moisture barrier. My impression has always been that the primary purpose for coating boards was to act as a moisture and corrosion barrier. It has dielectric strength as a secondary property. So, I am I hearing from Brian that conformal coating is useless as a moisture barrier on PC Boards? Bill Kasprzak Moog Inc Process Engineer, Electronic Assembly Brian Ellis <[log in to unmask]> Sent by: TechNet <[log in to unmask]> 12/06/2006 01:31 AM Please respond to TechNet E-Mail Forum <[log in to unmask]>; Please respond to Brian Ellis <[log in to unmask]> To [log in to unmask] cc Subject Re: [TN] conformal coatings as water proofing I disagree with your disagreement. CC CANNOT protect against atmospheric moisture as they are all as porous as a sieve. The best they can do is to delay the onset of undesirable effects. CCs SHOULD cover everything except contacts on 2- or 3-D assemblies and, as such, protect against the effects of condensation. They do not allow the passage of water in liquid phase, only in vapour phase. Brian Edward Mines wrote: > Doug Pauls wrote than conformal coatings protect against condensation. > I disagree. > > Conformal coatings protect against atmospheric moisture. It is giant leap of faith that many engineers make that they protect against condensation. When I left the industry 2 years ago there was no reference to condensation in IPC-CC-830 or MIL-I-46058 or UL746. > > Conformal coatings do not always completely cover all conducting > items on 3 dimensional boards. > > Doug's employer is not the only one that uses a thicker layer of conformal coating or similar material to protect against moisture. IPC-CC-830 (and predecessor MIL-I-46058) specify a maximum coating thickness for conformal coating. I am sure there are reasons for this. By putting on more coating (a thicker layer) the assembler is out of compliance with IPC & MIL > conformal coating specs. > > If industry wants to use a material to protect PC boards from liquid water that material should be called a different name (I called the one we sold at HumiSeal a selective encapsulant). Tests should be developed to insure that that material does the job for which it was intended; a QPL should be established. > > IPC, MIL & UL tests for conformal coating all include accelerated aging and thermal cycling tests for conformal coatings within a specified thickness range. Would qualified coatings pass those tests in thicknesses outside those ranges? Can the user be certain that conformal coatings cure properly in thicknesses outside those ranges? Might overly thick coatings crack if subjected to those tests? Or in the field? If a board fails because the conformal coating was used incorrectly is the board manufacturer negligent? I am aware of one instance where this last issue was seriously addressed in the automotive industry. > > Are the tests in MIL-I-46058 (and grandfathered into IPC-CC-830) realistic? Are they overkill? > Is there a reason for the thickness limitation? > > At HumiSeal I heard of several instances each year where customers tried to put more conformal coating on a board than specified by MIL-I-46058 and recommended by HumiSeal (it seems like more would be better). I know for a fact that some conformal coatings do not cure properly above a certain thickness. > > > Edward Mines > > > > > > > > --------------------------------- > > --------------------------------------------------- > 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 847-615-7100 ext.2815 > ----------------------------------------------------- > Need a quick answer? Get one in minutes from people who know. Ask your question on Yahoo! Answers. > --------------------------------------------------- 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 847-615-7100 ext.2815 ----------------------------------------------------- --------------------------------------------------- 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 847-615-7100 ext.2815 ----------------------------------------------------- --------------------------------------------------- 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 847-615-7100 ext.2815 ----------------------------------------------------- --------------------------------------------------- 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 847-615-7100 ext.2815 -----------------------------------------------------