If you get dewetting from plastic components, then they are dirty, or 
     you are using the wrong coating. 
     
     This can pose a reliability issue. An extreme example is a fin pitch J 
     lead. If the coating dewets the body, are the leads covered adjacent 
     to the body. Will the dewetting prohibit coverage of the solder joint 
     under the component body?
     
     I have tested and qualified coatings. This was found on some, but not 
     all coatings, and usually upon one specific part or supplier. I 
     disqualified all coatings that exhibited this problem from the matrix, 
     and implemented a coating that does not dewet from them.
     
     It is acceptable to leave plastic bodies uncoated using masking or a 
     Nordson, since plastic is generally inert relative to tin/lead, but 
     don't allow dewetted coating. It can result in catastrophic failure.
     
     Aric Parr
     [log in to unmask]


______________________________ Reply Separator _________________________________
Subject: FWD: pwb-cca : conformal coating
Author:  [log in to unmask] at internet
Date:    10/23/96 7:01 AM


Technet,
     
I'm forwarding this from a colleague.
     
Cindy Kemp
Lockheed Martin
Orlando, FL
*******************************************************************************
     
From: [log in to unmask]
Date: Tue, 22 Oct 96 16:35:22 EDT
Message-Id: <[log in to unmask]> 
To: [log in to unmask]
Subject: pwb-cca : conformal coating 
Sender: [log in to unmask]
Precedence: bulk
     
hi group;
     
We are finding that in designs using conformal coating (UR) over plastic parts 
that the cc is dewetting on the parts.
     
Question - is this seen as a problem ?
     
I am proposing that a note should be added to our designs as follows:
     
	CONFORMAL COAT DEWETTING ON PLASTIC ENCAPSULATED MICROCIRCUIT 
	COMPONENT BODY IS ACCEPTABLE.
     
If there is a better way please let me here from you.
     
What is the real would doing ?
     
     
			thanx; Skip Greb
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