I would be hesitant to underfill BGA's with a CC on boards with a
significant temp range. The CTE of the CC could be a problem. I'm also a
proponent of NO entraped air so I would use an underfill designed for this
application and then CC as normal (of course check compatability of the 2
materials).
Regards,
Bruce Misner
-----Original Message-----
From: Dehoyos, Ramon [mailto:[log in to unmask]]
Sent: Monday, August 11, 2003 10:10 AM
To: [log in to unmask]
Subject: Re: [TN] conformal coating of BGAs
Joe:
One way that can be done is to dam the BGAs with a silicon
bead around the part and squirt ( instead of spray) CC around the part a
given amount from two adjacent sides. The CC will flow and displace the air
under the part and conformal coat all the exposed areas. It is the same way
as if the board was immersed in CC. This can be done in a repetitive manner
with a programmable CC machine.
Regards,
Ramon
> -----Original Message-----
> From: Macko, Joe @ IEC [SMTP:[log in to unmask]]
> Sent: Friday, August 08, 2003 2:04 PM
> To: [log in to unmask]
> Subject: [TN] conformal coating of BGAs
>
> 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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