Carl
A silicone product would be the last choice for this application. As
mentioned in another post on coatings, the choice of product is a
compromise, but for this application I would choose a flexibilised
electrical grade 2- or more epoxy.
Remember, by definition, RTV polymerises by the absorption of moisture.
Brian
Carl VanWormer wrote:
>
> Thanks to all for the recent comments, questions, and suggestions. Here's a
> "quick" update of my recent status and findings:
>
> Our unit is mounted in an aluminum box (4 screws into the top) with some
> style of heavy duty circular connectors (with rubber gaskets), and I've seen
> no board corrosion around that area. The system is potted with Insulcast
> RTVS 27, after treating the connector sides, box mating surfaces and holes,
> and sides of 4 led (that protrude thru box holes) with the "proper" primer.
>
> Previous batches of boards have been built, cleaned, then stocked. After
> final programming (a J-leaded PROM, hand soldered with water soluble flux),
> they are washed under hot, running water with vigorous brushing. Recent
> testing (yesterday, thanks to QPL) indicated ionic contamination levels of 5
> ug. NaCl/sq.in, initially, and 15 ug. NaCl/sq.in after the PROM is added and
> "cleaned". This indicates that there is some bad stuff inside to start the
> process. This is like putting one bullet in your Colt 45 and playing
> Russian Roulette!
>
> Now we have 2 possibilities for the second half of the equation . . . either
> we are sealing enough moisture in the system to guarantee continued work for
> our repair tech, or water is getting in through our impervious barriers.
> Failure analysis shows the most common areas for the corrosion are in the
> area of the 4 LEDs that protrude through the box. I am suspicious of the
> quality of the adhesion to the "primered" LEDs, since they seem to pull
> clear of the potting compound leaving a smooth surface, not torn and ragged.
> Another area has been near the board (and box) edges, some of which have
> shown what appears to be oil incursion into potting fractures at the box top
> interface. I am wondering if vibration of the potting mass is causing
> fractures, even though it is hard to imagine when looking at the system.
> Has anyone seen anything like this with the Insulcast compound?
>
> Since I have to make some more units, I have cleaned a dozen (for a second
> time) and baked them overnight at 70 C, bagged them with desiccant to wait
> for potting. We are planning to flood the boards and box with primer,
> thinking that they may be more difficult to troubleshoot if they fail, but
> maybe they won't fail (at least at the present 30% rate). Any thoughts on
> this?
>
> Thanks for listening,
> Carl
>
> Carl Van Wormer
> Cipher Systems
> 1815 NW 169th Place, Suite 5010
> Beaverton, OR 97006
> Phone (503)-617-7447 Fax (503)-617-6550
>
> -----Original Message-----
> From: Roger Stoops [mailto:[log in to unmask]]
>
> Carl,
> Been readin' what everbody else is saying about potting and encapsulating,
> thought I would jump into the fray.
> We build equipment that is mounted on 'dozer blades, behind and under
> motorgraders, and such. Pretty dirty and nasty environment. We have found
> that, for consistent reliability, potting or coating a board (really squeaky
> clean to begin with mind you), mounted in a custom aluminum enclosure, with
> a sealing ring or o-ring under the lid, and using Mil-style circular
> connectors (with gasket seal behind connector) seems to cure these problems.
> Of course this may be more expensive than what your product can afford, but
> illustrates how we solved such a problem. Even a small, inexpensive
> off-the-shelf aluminum enclosure with a simple gasket system would go a long
> way to reducing your problem with external moisture.
> We would like to hear later about your efforts in solving your dilemma.
>
> Roger M. Stoops, CID [log in to unmask]
> TRIMBLE NAVIGATION LLC
> Engineering and Construction Division
> http://www.trimble.com
>
> -----Original Message-----
> From: Carl VanWormer [mailto:[log in to unmask]]
>
> We have a "potted" product that rides behind the wheels of a truck. All our
> best protection efforts have yielding significant failure rates. Returned
> units have had traces eaten off the board, with various colors of crud left
> over. The green is probably copper oxides. Some of the copper seems to
> have turned into a red powder. There is a lot of white crust around some of
> the lost traces and vias. We are guessing that the boards were not cleaned
> well enough before potting (Insulcast RTVS 27 thermally conductive potting
> compound). We believe the boards were produced with a water soluble flux,
> then washed, then a few hand-ads were installed with a "no-clean" flux. We
> think that either (1) moisture sealed inside the potted units may be
> "helping" the corrosion, or (2) moisture (with or without extra corrosive
> elements) is leaking it through poorly sealed boundaries of the potting
> compound. (we are using "primer" around the known "edges" of the units)
>
> I know that "Detroit" has solved this problem many times. Where can I find
> a "guide book" or a knowledgeable vendor to help me out of this board
> cleaning/potting mess?
>
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