Ian,
I would agree with Graham has said.  Our experiences are about the same.
To Graham's 3 suggestions I would add a fourth.  You can possibly use an
elastomeric sealant around the outside of the BGA to seal off the inside.
This works well especially if you are going to conformally coat the
opposite side of the board for any access vias.

Doug Pauls




             Graham Collins
             <Graham.Collins@L
             -3COM.COM>                                                 To
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                                                                   Subject
             07/20/2005 05:49          Re: [TN] Coating under BGA's
             AM


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              Graham Collins
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Hi Ian
In our applications we have used one of three options depending on what
the product is and where it is going:
1) underfill the BGA with a purpose made material, not with conformal
coating.
2) parylene coat the assembly (which, by virtue of it being a vacuum
deposition process, will coat under the BGA)
3) spray coating on the board and accept that the BGA underside is
shadowed.

#3 isn't without precedent, we don't typically worry much about old
style boards with pin grid array components...

I would not underfill a BGA with a regular coating material.  This has
been discussed in the past on TechNet, one of the concerns is the
expansion with temperature putting stress on the joints.

Good luck!

regards,

Graham Collins
Process Engineer,
L-3 Communications Electronic Systems Inc.
Halifax
(902) 873-2000 ext 6215

>>> [log in to unmask] 07/19/05 01:23PM >>>
Fellow techies, a BGA related question. We're just starting to design
BGA's
into some new designs which are class 3 high rel assemblies and I am
asking
myself questions about the ability to adequately conformally coat under
the
packages. Our standard coating is a solvent based acrylic and we are
using a
456PBGA device and a 429CBGA device, both on 1.27mm pitch, as the
maximum
sizes so far and I'd be interested to hear what other class 3 builders
have
experienced when applying the coating.

Regards
Ian Fox
Goodrich ECS
Birmingham
UK

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