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Subject:	Re: acid tin plating question
From:	R Sedlak <[log in to unmask]>
Reply To:	TechNet E-Mail Forum <[log in to unmask]>, R Sedlak <[log in to unmask]>
Date:	Fri, 29 Feb 2008 08:58:52 -0800
Content-Type:	text/plain
Parts/Attachments:	text/plain (56 lines)

Adam:

You are coming across the big issue with plating tin from either sulfate, or methane sulfonate plating solutions. Have you ever seen one of these baths brand new, freshly made? They are clear. Over time, the Stannous Tin is oxidized to Stannic (partly) which is insoluble, and forms the milky appearance you see in most Tin baths. This Tin oxide co-deposits with the Tin metal on the board. You don't notice if Stannic oxide is present in small amounts. The Stannic oxide is soluble in strong alkali (photoresist stripper) and dissolves, leaving you with pits in the Tin metal.

There are 2-3 ways to deal with this:
Filter your Tin plating bath, to eliminate STannic oxide
Prevent/slow reoccurance by covering Tin bath when not in use, and insure Anode area is kept high (when anode area gets low, Tin tends to go directly to Stannic oxide, instead of the preferred Stannous.

Consider the wild concept of going way back, and installing a Tin/Lead Fluoborate bath, but don't put in the Lead. The Fluoborate bath dissolves the Stannic oxide, so this problem never occurs.

Rudy Sedlak

Adam Seychell <[log in to unmask]> wrote: Hi all.
I'm trying to solve a problem of pitting around the holes after
ammoniacal etching. I suspect my problem is insufficient deposits of
electroplated tin resist around the holes, which is making it more
susceptible to attack by the etchant. The reason I think its a tin
problem is because prior to etching, the tin deposits appear solid and
smooth when viewed under a microscope. However after etching there is
evidence of pitting around many holes.
The problem seems to go away with excessive plating, about 8um or more.
Normally I've not had a problem at only 2~3um.

Also is it normal to have a small amount of white foaming hydrogen gas
above the PCB while tin plating ? The tin bath is an acid stannous
sulfate type with brighteners, and has always been a milky yellow
appearance.

Adam Seychell

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