At 3:16 PM -0400 9/19/96, Barry Allen <[log in to unmask]> wrote:

>I also will be grateful for some conformal coating advice
>on an issue related to the current thread.
>
>We have a product that goes into aircraft with a
>combination of digital and RF circuitry.  We are arguing
>about conformal coating -- there are several RF parts
>affecting about half of the board surface that cannot be
>coated for performance and serviceability reasons.
>
>Based on this, we want to drop the conformal coating
>because only half the board can be protected anyway.
>The "other" camp argues that a half-coated board is better
>than nothing.  The cost to mask the board is significant.
>
>Anyone deal with this issue before?  The main environmental
>problems for this product are fungus and moisture.

Hi Barry,
I didn't see your note until today.  Let me say right off that under
certain operating conditions, I'm from the camp that says a partially
conformal coated (cc) board doesn't buy you much.  That said, I will tell
we just completed testing, as piece parts, circuit card assemblies which
were conformal coated on the component side only.  Our cc is urethane based
and spray applied. The cards in question contained a mixture of older
mil-spec thru-hole components and newer SM devices.

Cards went through fungus testing, no problem.  Blowing sand & dust, no
issues.  Salt fog, no problems attributable to the partial cc.  We also
have test history showing single sided cc card pass system level humidity
testing (biased for a relatively short period of time) without significant
problems due to the partial cc.  These were all Mil-Std-810E test methods.

However, the one area in which we have experienced real world failures,
from surface resistivity/contamination, is difficult to test for.  I have
analyzed several failures of field equipment which was directly
attributable to the lack of cc protecting board surfaces.  Most of the
equipment used fans for cooling using unfiltered or coarsely filtered air -
capable of depositing dirt, sand, insects, fibers, exhaust, salt air, etc
on the exposed board surfaces.   Just add humid air and the resistive paths
appear.  On old time materials engineer told me once that one of the
purposes of a conformal coat is to "preserve the as fabricated cleanliness
of the board".  Without cc, these board got "dirty" and failed.

>From what you describe, an airborne application and sensitive RF circuitry,
crys out for a protective coating of some sort.  If the circuitry is
sensitive to the application of the cc, how sensitive will it be when
enough particles from the atmosphere accumulate on it, from jet exhaust,
airplane food, etc?  Altitude reduces dielectric withstanding so any
contamination may cause a worse problem for you airborne than on the
ground.  Is this board part of the avionics or the galley?  What are the
implications of failure?

You need to do the trade-offs and decide what you can live with.  Good luck.
-Gregg

_________________________________________________________
Gregg Klawson                        tel: +1.508.880.1822
GTE Government Systems Corp          fax: +1.508.880.4316
Taunton, Massachusetts USA         email:  [log in to unmask]


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