Phil,
I will go along with what Lee said.  CTE is one major consideration for 
thick coatings.  Another would be residual stresses from shrinkage when 
the coating dries and cures.  High residual stress can mean cracking and 
flaking of the coating in the end use environment if thermal cycling is 
expected.   Another is coating adhesion is you try to apply a thick 
coating all in one pass.  As most coatings skin over and cure/dry from the 
outside in, the last surface to cure is the the one at the 
coating/substrate interface.  In a very thick coating, that may not 
properly cure for a LONG time, and if you go into HASS or some other kind 
of cyclical thermal stress, you can get coating delamination.

Doug Pauls



From:   Phil Bavaro <[log in to unmask]>
To:     <[log in to unmask]>
Date:   10/25/2013 02:48 PM
Subject:        [TN] Exceeding Maximum thickness of urethane conformal 
coating
Sent by:        TechNet <[log in to unmask]>



I was reviewing a process specification regarding Type UR conformal 
coating and it contained a sentence which struck a nerve.

This particular specification stated that the thickness shall be 
.003"+/-.002" and .010" maximum if two coatings are applied.

I always thought there was a good reason why the coating should not exceed 
.005" such as CTE mismatch which might cause cracking of the coating or 
perhaps worse, component damage in thermal cycling.  But I haven't found 
any papers to that effect yet.

Naturally, any comments/experiences from this forum will be appreciated, 
especially if I am completely off based.

Phillip Bavaro
Senior Manufacturing Engineer, Manufacturing Engineering
L-3 Telemetry-West
9020 Balboa Ave. | San Diego, CA 92123
858-694-7874 | 858-740-8260 mobile
[log in to unmask]<mailto:[log in to unmask]> | 
L-3com.com/TW




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