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July 2004

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Subject:
From:
"COOKE, ROBERT W. (JSC-NX) (WGI)" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Mon, 12 Jul 2004 09:25:28 -0500
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text/plain
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text/plain (110 lines)
Most conformal coating chemistries are compatible with SMT, with the
possible exception of rigid resins such as acrylic (AR) and epoxy (ER).
Resilient materials such as silicone (SR) and urethane (UR) work very well.
For ultimate protection, you may consider Paraxylene (XY) - but only as a
last resort. Paraxylene requires a very specialized application process
($$$$$).

The key is to provide the printed wiring assembly with effective coverage
and protection for the expected operating environment, without adversely
affecting component strain relief or thermal dissipation. For conformal
coating, more is not necessarily better, and thin coatings are better than
thick.  All conformal coatings present some mechanical load to the
component's leads and solder joints, and have a thermal "blanketing effect"
by reducing the assembly's ability to dissipate heat. Because of the
increased thermal density of SMT, anything that impedes the ability to shed
heat must be understood and taken into consideration of the overall design.
Obviously, you would not want to use a coating design that would cause a
device (or board) to overheat and fail.

Also, you and your customer must decide how much pain you are willing to
endure if rework is desired. Silicone and urethane is easily removed and
replaced for rework, paraxylene is extremely difficult to cut through and
boards coated with paraxylene are generally considered "unrepairable".

Bob Cooke

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Douglas O. Pauls
Sent: Monday, July 12, 2004 7:43 AM
To: [log in to unmask]
Subject: Re: [TN] conformal coating on SMD

Yes they can.

There are dozens of varieties and chemistries of coatings.  I suggest that
you get a copy of the IPC Conformal Coating Handbook for further education.

Doug Pauls




                      Murulidhara
                      <rpcblab_bgp@ITIL        To:       [log in to unmask]
                      TD.CO.IN>                cc:
                      Sent by: TechNet         Subject:  [TN] conformal
coating on SMD
                      <[log in to unmask]>


                      07/12/2004 03:59
                      AM
                      Please respond to
                      "TechNet E-Mail
                      Forum."; Please
                      respond to
                      Murulidhara







Dear Technetters,

I want know whether PCB assemblies with surface mount components can be
conformal coated.
If so, what type of conformal coating compound for SMD components?

Thabks in advance

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