Carl,
Some quick points:
1. The boards are too dirty (clean them with heated DI water at a minimum).
2. RTVS 27 - Is this a Room Temperature Vulcanization S??? potting compound?
If so be sure that the out gassing during the cure process does not contain acids.
3. RTV does not absorb sudden shocks and vibration. It is forgiving during thermal changes.
4. Your box leaks at numerous locations and RTV is very permeable to moisture and certain reactive
gases.
David A. Douthit
Manager
LoCan LLC
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?
>
> ---------------------------------------------------------------------------------
> Technet Mail List provided as a free service by IPC using LISTSERV 1.8d
> 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 delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL
> To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
> Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives
> Please visit IPC web site http://www.ipc.org/html/forum.htm 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 1.8d
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 delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives
Please visit IPC web site http://www.ipc.org/html/forum.htm for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
---------------------------------------------------------------------------------
|
