Carl,
1. There is no "magic" number. The only way to find out is to test the product in chamber
Accurate testing prior to fielding this product would have prevented all of this. The sooner you
start testing the sooner these problems end. Designing or correcting problems using "WAGs"
is very expensive.
2. Okay
3. Don't know and it doesn't matter. RTV is about as elastic as aluminum during vibration or mechanical
shock.
It will tend to delaminate due to cleanliness issues and moisture ingress.
4. Yes, but there is not enough space to go through them and the synergistic reactions which occur.
I will send you a copy of my paper "Avionics System, Reliability, and Harsh Environments"
Even though it says "Avionics" it centers on Tropospheric chemistry and effects on electronic
circuitry.
As Phil pointed out use a parylene coating (.001 in thick), which requires a very clean circuit, and
seal that unit up better.
Your management must determine how much correcting this issue is costing and compare it to a
well managed testing program integrated into the design process.
If they fail to grasp this concept you should probably update your resume!
David A. Douthit
Manager
LoCan LLC
Carl VanWormer wrote:
> 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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