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February 2007

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Subject:
From:
"Gervascio, Thomas" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Gervascio, Thomas
Date:
Fri, 16 Feb 2007 10:08:02 -0500
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 The number of spray passes depends on what you want. Many times
reducing the number of passes requires you to put down more material and
if you take short cuts in drying, you may get imprefections in the
coating. Increasing the number of passes lets you put down less coating
and it is easier for the volatiles to flash off the surface. 

Also I have had problems with spider webbing. That is when the there is
so much thinner in the coating it all flashes off when spraying and the
resulting thicker material landing on the board doesn't coalese and
forms spider's web. Control of the mixing ratios is more critical

Also acrylic requires thinners so your facilities VOC emissions may go
up. Some newer lower VOC materials are entering the market but a lot of
people don't like them due to their odors.

Also got to worry about post coating cleaning. About anything except for
water will attack the acrylic surface- remember that is by design (for
reworkability)

Hope this helps
Tom

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of - Bogert
Sent: Thursday, February 15, 2007 7:24 PM
To: [log in to unmask]
Subject: [TN] Using Type AR Conformal Coating

February 15, 2007

Folks, we have some military OEM'S that are switching from Type UR
conformal coating to Type AR.  The main reason for the switch is because
it is hard to remove Type UR coating in event of a rework, and certain
chemicals sold by the coating manufacturers for UR coating removal are
highly conductive and have caused problems since the cleaning process
did not adequately remove them.  The other reason is we wanted a coating
that could be easily removed in event product failed in the field so we
could do ATE to quickly find the source of the problem.  Our product is
used in a military application but it is contained in a cabinet that is
dust tight.  We have some assemblies that are not coated at all with no
known problems.

Do any of you folks use Type AR for military product, and if so, what
are your experiences using the coating?  I have heard about problems
using alcohol for coating removal since it goes where you do not want it
to go.  I have also heard that if a spray process is used, it takes 4 or
more passes on each side of the board to achieve the minimum J-STD-001
thickness.  At least one source indicated using AR may cause some solder
joint reliability issues.

Any info you have would be a help.  Thanks.

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