Edward,

One of the most interesting issues concerning coatings is there is no
test method to determine if the coating is "protecting" the circuitry
once the product is in the field.
This leads to a "Catch-22" situation:   1. There are very few
reported equipment problems due to coating issues.
                                                                   2. Thus there is no "problem" requiring action concerning
coating reliability.
                                                                   3. Thus no tests are created to verify coating abilities
in the field.
                                                                   4. With out test procedures there
are very few reported equipment problems reported due to coating issues.

As equipment is exposed to "harsh" environments (temperature,
humidity/moisture, and contamination) in the field there are
increasing reports of "intermittent" and/or
No-Trouble-Found. My personal experience has been that nearly half of
this incidents are due to coating deterioration or coverage problems.
"Black light" visual inspection is not capable of "spotting"
microscopic voids or problems under components.

As long as the OEM can produce a product that functions tell the end
of the warranty period and/or meets contractual reliability
requirements (again the lack of field tests
to determine if the coating is working covers the reliability issue.
No-Fault-founds are not counted) there is no issue. The hole in this
can be found in certain high reliability hardware requirements for
the military, aircraft, medical, and other forms of transportation.
This can and does lead to cannon fodder for lawyers!

This is one of the reasons the IPC-CC-830 coating handbook was created.

BTW a simple test to determine if the coating has covered components
properly is to roll a moist Q-Tip over the components while the
system is operating.
If it quits there is a problem (i have found that management takes a
very dim view of this little procedure!).

David A. Douthit
Manager
LoCan LLC


On Dec 7, 2006, at 10:06 AM, Edward Mines wrote:

> As good as dipping is for applying conformal coating most conformal
> coating users spray it or use a Nordson select coat. Conformal
> coatings have a nasty habit of wicking up into places they
> shouldn't be (like connectors). Masking is time consuming and ill
> defined.
>
>   Spraying will not cover the back sides of the flat leads of ball
> grid arrays. In a perfect world the coating would provide a
> complete envelope  over these leads, bridging the gaps between the
> leads. This doesn't always happen, particularly in the corners and
> particularly when  a tall component blocks spray from one side. In
> this instance the complete envelope (or bag without a hole) is
> wishful thinking. Sometimes bridging of the coating between leads
> is iffy.
>
>   Liquid conformal coatings have a tendency to run off sharp bends
> in flat wire leads (we called those bends knees). These "knees" do
> not fluoresce when coated boards are inspected with a black light
> in a darkened room.
>
>   Doug Pauls' employer (and I'm sure some other companies)
> recognized the short comings of traditional conformal coatings for
> this application and developed their own techniques.
>
>   I have seen many instances where smaller companies try to develop
> similar solutions "on the fly" with unexpected and undesired
> consequences.
>
>   Edward Mines
>
>
>
>
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