Thanks for your input. In regards to your questions (below):
1. When you say the boards are too dirty, is there some number (less than 5
ug/sq. in.) that is considered clean? The assembly house runs the finished
boards through a continuous process system that cleans the boards in
successively cleaner stations. The last one is "clean" DI water.
2. The RTVS 27 is a 2-part system. I don't think there is significant out
gassing.
3. Do you think the RTV is fracturing internally due to shocks?
4. Are there particular environmental elements in "behind the tire" road
spray that would attack RTVS 27?
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: David Douthit [mailto:[log in to unmask]]
Sent: Wednesday, March 06, 2002 9:57 AM
To: TechNet E-Mail Forum.; Carl VanWormer
Subject: Re: [TN] AUTOMOTIVE PACKAGING/SEALING REALIBILITY
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
>
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