In addition, remember that epoxies ALWAYS contain some ionics (oftem
sodium chloride) which can pump water vapour through the CC into it. In
fact CC or not, FR4 is slightly hygroscopic and will draw in water
vapour through the CC at almost the same rate as if the the CC were not
there. What, don't believe me? Then why do you bake the board before
soldering?

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
-----------------------------------------------------