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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