Mixing conformal coat (Acrylic and silicone) are scary business, unless
you have done good evaluation study about the combination....
jk
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of
>Charles Caswell
>Sent: Monday, August 11, 2003 12:51 PM
>To: [log in to unmask]
>Subject: Re: [TN] conformal coating of BGAs
>
>
>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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