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Stain marks found on matte tin surface

Quickstart:
Traditionally there were two types of tin plating baths, an acid based bath (tin sulfate) which produced bright tin plating, and an alkaline bath (potassium or sodium stannate) which produced matte tin plating. Thus, bright tin implied that the bath was acid, and matte tin implied that the bath was alkaline.
Today there is a third class of baths (MSA), plus other variations, but most readers will infer the use of the alkaline stannate bath if the plating is called matte.

Q. Hello, I am doing connector manufacturing. Recently, we found stain marks on plated matte tin surface which cannot be removed using water or alcohol.

61855-1e 61855-1f

61855-1c 61855-1a 61855-1d 61855-1b

We have replaced the post-dip, rinsing water, half-madeup the tin bath, carbon treated the nickel bath, but the problem persists. Kindly advise, please. Thank you.

CSKoo
- Singapore
July 3, 2025

A. Hi CS,

Is the inspection done immediately after plating, with no possibility of the components touching each other or anything else? Tin plating is known for 'fretting corrosion'; some years ago this forum dealt with a problem of fretting corrosion so bad that the plating turned completely black :-)

If these components are barrel plated, the marks probably should be expected.

61855-1g

Luck & Regards,

ted_yosem
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

Q. Dear Ted, the stain spot was found after plating and not all pins show this defect. We do replace the new air piping for air wipe, but the problem persists. Is it possible due to De-ionised water quality or tin plating bath itself? The components never touch each other.

CSKoo [returning]
- Singapore

A. Hi again.

Please explain this "air wipe". What kind of blower or compressor is supplying the air?

How are the parts held for plating and air wiping -- is it a continuous belt of parts with the parts cut off after plating, or are they inserted into a bandolier? (I should be able to figure it out from the pics, I guess, but I can't).

Luck & Regards,

ted_yosem
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

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