Hi Ramon
Using a Silicone as an underfill is a really bad idea.
Keep-in-mind the following much of which was a posting from Brian Ellis:
The TCE of silicones are huge at ~300-350ppm/°C whereas solder is only ~16
ppm/°C
They have a unique property that however soft they feel, they are rock-hard
when subjected to mechanical shock (hence super-bouncing balls and bouncing
putty)
At only slightly elevated temperatures there would be tension on a
BGA-ball-pad combination, that would increase by orders of magnitude should
it be subjected to a small shock at the same time
They are 10 - 20 times more hygroscopic than other coating types. Single
component silicones usually require the presence of free hydroxyl radicals
to crosslink, that means that they will not cure in a perfectly dry
atmosphere. In this sense, before curing, they are hygroscopic. After
curing, the hygroscopicity is reduced, but they remain porous (otherwise,
the trapped humidity would not have escaped).
I hope this helps
--
Regards Graham Naisbitt
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On 20/7/05 14:00, "Dehoyos, Ramon" <[log in to unmask]> wrote:
> The one coating material that works well as an underfill is SR.
> In a previous life we used to coat most of our boards with SR coating
> and we made sure there were no bubbles since some of the boards were
> used in airplanes. A bubble under the BGA or QFP was a potential
> disaster due to the pressure difference as the airplane gained altitude.
> One factor that may help in avoiding bridging the under the component
> gap is the viscosity of the coating. Spraying with under 20 second Zhan
> cup #2 coating viscosity may do the trick.
> Ramon
>
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Fox, Ian
> Sent: Wednesday, July 20, 2005 7:13 AM
> To: [log in to unmask]
> Subject: Re: [TN] Coating under BGA's
>
> 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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