Many thanks for your comments Graham. Parylene was in the back of my mind.
Up 'till recently (intro of a PVA spray machine) we've always dipped and
manually sprayed so PGA's have been adequately coated underneath. We
probably therefore don't have the precedent you do. I didn't want to revert
to an underfill unless I absolutely had to. Bridging between the underside
of the BGA and board is exactly what I want to avoid as the modulus of the
standard acrylic increases greatly below about 10 degC.

Regards
Ian

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Graham Collins
Sent: 20 July 2005 11:49
To: [log in to unmask]
Subject: Re: [TN] Coating under BGA's


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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