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February 2002

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Subject:
From:
Gaby Bogdan <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 28 Feb 2002 20:51:55 +0200
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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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