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August 2003

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Subject:
From:
"Dehoyos, Ramon" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Mon, 11 Aug 2003 10:09:41 -0700
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        Graham:
                What underfill material do you use and how do you apply it?
        Regards,
        Ramon

> -----Original Message-----
> From: Collins Graham [SMTP:[log in to unmask]]
> Sent: Monday, August 11, 2003 12:17 PM
> 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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