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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