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August 2003

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Subject:
From:
Charles Caswell <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Mon, 11 Aug 2003 11:50:40 -0500
Content-Type:
text/plain
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text/plain (107 lines)
I have no experience with underfill and have only coated a few boards. A labor intensive method used by one of my customers is to mix a batch of acrylic CC with extra thinner, squirt it under the BGA with a syringe. Cure then seal the edges with silicone. Then remove the excess CC before spraying. 

-----Original Message-----
From: Collins Graham [mailto:[log in to unmask]]
Sent: Monday, August 11, 2003 11:17 AM
To: [log in to unmask]
Subject: Re: [TN] conformal coating of BGAs


Hi Joe!
Well, as Joyce observed, Parylene will coat everything, so that's one
(expensive) way. Another way is to underfill the BGA before coating,
with an underfill material. This is what we are currently doing.

I do question the ability of anyone to truly meet that requirement
though, when using a conventional spray technique. For example, if you
have vias under components (which is pretty much standard these days)
then you will have exposed circuitry unless you are using a dry film
solder mask.

regards,

Graham Collins
Process Engineer,
Northrop Grumman Canada Corporation
Halifax
(902) 873-2000 ext 6215

>>> [log in to unmask] 08/08/03 03:04PM >>>
Tech Netters,

I am looking for some input on a special issue related to the
conformal
coating of boards with BGAs. According to IPC-A-610C, section 9.1.2,
voids
in the conformal coating material can not expose circuitry, bridge
lands or
adjacent conductors. In order words, all exposed circuitry/conductors
are
to be conformally coated.

BGAs have conductors/solder balls located "under" the package unlike
leaded
parts where the leads are exposed and easily covered with conformal
coating
material. Conformal coating may partially wick under the BGA covering
some
of the conductors/balls but not all the way. Some of the BGAs also
stand off
the board a measurable distance which the conformal coat material may
not
bridge resulting in an uncoated gap between the BGA and the pwb. Some
would
interpret this as leaving exposed conductors (under the BGA).

I would like to know how other manufacturers are dealing with this
issue.
Ideally, one would seal the perimeter of the BGA with conformal coat
thereby
not exposing conductors but conformal coat material will not bridge
gaps
greater than x mils. Look forward to your response. thanks

joe



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