Stephen,

Conformal coatings will not prevent water vapor from permeating your board.
If you need to bake your boards for 8 hours for them to work you should
test them after coating by
500 to 1000 hours exposure to high humidity (>85%). See ISO 9223 Time of
Wetness chart to
correlate field exposure times to testing time.

David A. Douthit
Manager
LoCan LLC

Shoda, Stephen J (US SSA) wrote:

>Doug,
>We are only a medium sized mid-west corn growing state, so I'm certain the ambient moisture failure mode is now ruled out!
>
>I working on a development product which failed humidity qualification test. Upon return testing, the board was baked out for numerous hours. As it dried out it failed less electrical tests, but it took over 8 hours of baking.
>
>I want to make sure the modules are very dry prior to conformal coating the retest samples.
>
>
>Steve
>
>
>
>-----Original Message-----
>From: [log in to unmask] [mailto:[log in to unmask]]
>Sent: Monday, October 18, 2004 4:03 PM
>To: [log in to unmask]; Shoda, Stephen J (US SSA)
>Subject: Re: [TN] Moisture on PWB and Conformal Coating
>
>
>Has anyone experienced poor conformal coat wetting due to moisture on the
>pwb surface?
>**Yes, er um, well, I have *heard* it happens other places, not here in the
>town of Perfect of course......
>
>**Non Smart Ass Answer (NSAA): it depends largely on what kind of conformal
>coating you are using.  Water-based, not a problem.  Solvent based, could
>be a problem.  You might also want to check on the amount of moisture that
>is in the coating itself.  Ahem, I have *heard* that if you are feeding
>from a 55 gallon drum in the summer states in large mid-west corn growing
>states, you can have ambient moisture condensing in the barrel, combining
>with the coating.  The coating properties and processing characteristics
>can be compromised, and may require longer bakes to cure the coating.  May
>also lead to dewetting and have nothing to do with the assembly.
>
>Could moisture embedded in the pwb be problematic?
>**Could be, but I doubt it.  Two hours at 250F should be enough to bake
>everything bone dry.
>
>In our process, we batch clean our boards which includes multiple deionized
>water rinse cycles.  The product is then baked at 250°F for 2 hours.  We
>have a new solder mask material which coats well prior to our processing.
>The 2 hour bake at 250°F has always proved adequate. Is anyone using a
>longer bake time?
>**I think you are overbaking.  We dry ours at 100C for 60 minutes and it
>works fine.  Your parts establish equilibrium with ambient humidity in the
>first 15 minutes anyway.
>
>What is the substantiation for establishing an extended time?
>**Whatever works and works consistently.  There are no IPC specs that I am
>aware of on the topic.
>
>Doug Pauls
>
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