Hello Phil

You are completely correct that the thickness of the coating can be critical
and factors like CTE are crucial. 

The general problem is the coatings are meant to be applied homogeneously
and thinly. This can be pretty difficult since it is liquid unless you are
applying parylene! Normally, the problems occur when supplying too much
coating to a circuit board at once that can then lead to all sorts of flow
processes taking place like capillary effects. This can then give large
variation of thickness of the coating across the circuit and in places you
don't want it to be too thick. 

If in doubt more is not better. The key is to apply thin layers and resist
the temptation to pour the coating on!

In terms of research I believe there was some a long while ago out of the
Swedish Institute on thermal cycling which showed an improvement in the
results with coatings applied. But, I would have assumed the coating was
applied thinly. 

Sorry if I can't be more help on this.


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Phil Bavaro
Sent: 25 October 2013 20:45
To: [log in to unmask]
Subject: [TN] Exceeding Maximum thickness of urethane conformal coating

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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