Gaby
I've no experience in this. What kind of CSPs? What kind of flux? What
heating process?
I'll hazard a guess: if you flux the bottom side of a clean BGA or
similar with, say, a 10 - 15 % solids RMA (not paste), holding it in
place as you will, reflowing it and then underfilling it with a 30 - 50%
solids solution of the same RMA or, better, an R flux with the same
rosin type, you may have a fairly safe result, reliability-wise. I
haven't actually tried this but my theory is that the nasties remaining
from the soldering process (metal salts) will have plenty of time to
dissolve/diffuse in the underfill before the alcohol evaporates off and
be held harmlessly in the rosin matrix, away from the critical joint
area. However, if the chip is one which gets hot, it could ooze out in
service, providing a sticky, dust-trapping mess that could be cleaned
off subsequently only in Doug's Mountain Dew (he hasn't given us the
formulation, but I think it's about 85% isopropanol, 10% methanol and
water excip ad 100%). I know that many use a conventional underfill with
"no-clean" but I feel that this is just as hazardous as Doug and I have
already outlined for coating over "no-clean" without the same
qualification and process controls, possibly more so.
Hope this helps.
Brian
Gaby Bogdan wrote:
>
> Brian,
> I was following with great interest the subject. Right now, I am in the
> middle of trying tosolder CSP's with "Flux underfill", and I see
> similarities in the problems that could arise. Could you give me some hints
> for the reliability of the process?
> Gaby
> ----- Original Message -----
> From: "Brian Ellis" <[log in to unmask]>
> To: <[log in to unmask]>
> Sent: Thursday, February 28, 2002 9:10 AM
> Subject: Re: [TN] No No-clean?
>
> > OK, there are three issues at stake here.
> >
> > 1) The residues from "no-clean" products (and yes, there are residues -
> > always) may impede adhesion of the coating layer to the substrate
> >
> > 2) Many of the "no-clean" products rely on a slow sublimation of the
> > residues from the surface, requiring a few weeks to achieve optimal
> > performance. Sealing these in will prevent this from happening.
> >
> > 3) By definition, all soldering fluxes are ionic. Some may be slightly
> > hygroscopic, as may also the metal salts formed by the reaction between
> > the oxides and the flux acids. Some of the rheological additives to
> > pastes may leave slightly hygroscopic residues. This is an open
> > invitation to promote vesication, with a potentially very serious loss
> > of reliability. At the very least, a good "pressure cooker" test is a
> > must both in qualifying the process and at frequent intervals to check
> > that it stays on the rails.
> >
> > I am not saying that coating should never be applied over flux residues.
> > What I am saying is that anyone who does it without a really thorough
> > qualification and process control is being professionally irresponsible,
> > if the products must be reliable. After all, conformal coating is really
> > a high-rel process and "no-clean" chemistry started as a low-rel
> > process, creeping up to its present status of medium-rel. It is
> > therefore really a paradoxal conflict to try and combine the two, likely
> > to present the disadvantages of both processes with none, or few, of the
> > advantages.
> >
> > Brian
> >
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