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August 2001

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Subject:
From:
peter blokhuis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 28 Aug 2001 14:00:53 -0700
Content-Type:
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text/plain (98 lines)
I would add to this that the rate of oxidation to
stannic tin is dependent on the temperature of the
solution, so we keep ours around 70F.

Peter Blokhuis



--- "<Rudy Sedlak>" <[log in to unmask]> wrote:
> Sean:
>
> The murky solution is due to Stannic Tin, and since
> Stannic is converted from
> Stannous, by oxidation, there really is only two
> ways it can happen.
>
> The first obvious one is by reaction with Oxygen in
> the air.  This is
> certainly a factor always, but if you are entraining
> a lot of air with the
> eductors, you could raise the rate of this occurring
> many times.
>
> The other way it can occur is by excessive (anode)
> current density..so if you
> are using bar anodes ask if they are being
> replenished as often as they
> should be.
>
> The next question to ask is, what is the effect of
> STannic Tin in the plating
> bath, and the answer, as in all of PCB's is "That
> depends"...
> The oxide dissolves a helluva lot faster in alkaline
> solutions than the
> metal, and this COULD cause/allow (apparent) etching
> of the Tin at a much
> faster rate, and thus could cause apparent
> overetching, or perhaps pitting of
> the Copper.
>
> And the other thing that can happen is that this
> Stannic Oxide in the deposit
> can dissolve into the photoresist stripper.  Some
> resist strippers lack the
> key ingredient in them to prevent immersion plating
> back on to the Copper,
> and thus you can wind up with copper partly unetched
> due to the presence of
> immersion Tin.  This is especially the case if you
> are attempting to strip in
> a tank, although, in this day of insane tight
> dimension specs, I have seen it
> cause problems in spray stripping as well.
>
> Filtration of Stannic is EXTREMELY difficult due to
> the extreme fine particle
> size (the brightener/leveler in the Tin plating is a
> surfactant that causes
> extreme small particles to form).
>
> Rudy Sedlak
> RD Chemical
>
>
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