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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