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March 2002

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Subject:
From:
Earl Moon <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 6 Mar 2002 20:53:00 -0500
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text/plain
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text/plain (215 lines)
Dave,

You are a cookin'.

Love it,

MoonMan

----- Original Message -----
From: "David Douthit" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, March 06, 2002 12:56 PM
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
> >
> > -----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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