parylene?

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Macko, Joe
@ IEC
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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