Tin Plating Problems

"Trouble in Your Tank: Handbook for Solving plating Problems"
by Larry Durney

on Amazon

(currently UTL
on AbeBooks & eBay)

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A. Hi Josafat

Fretting corrosion is certainly a very real problem with tin plating; thread 5106 illustrates terminals which turned completely black from fretting corrosion ...

Still, if parts got spots within 24 hours, it seems unlikely to me that it has do to fretting from tissue paper. I'd find at least 3 explanations more likely:

• 1. The plating is probably too thin; on the current page Lyle Kirman suggests more than doubling it.
• 2. Sulfur in the packaging is always a suspect when there is blackening, as expounded on by James Watts on thread 5106 -- and the stains seem to match the texture of the packaging, giving credence to this theory.
• 3. Tin must be properly rinsed as explained by Robert Probert on the current page.

Although you can try one thing at a time, you'll never fix it that way if there are two things wrong :-(
-- which is a good example of why this advice from Lawrence Durney in "Trouble in Your Tank" ⇨
is priceless: 'Follow the Letter of the Law' -- i.e, increase the plating thickness so it is more robust, rinse the parts properly, and carefully choose the correct packaging material. Do them all, not just one of them, and only after the problem is solved should you, one at a time, try to simplify or economize :-)

Luck & Regards,


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

A. Hello Jay,
"Curret's" is apparently a typo or a bad autocorrect. Please tell us what you meant.
Also please let us know what kind of tin plating process you are running -- stannate, acid, MSA -- and whether you are rack, barrel, or continuous plating, and what (if any) post treatments you are doing. The readers are probably willing to help but usually don't want to spend time introducing a dozen ifs-ands-&- buts that are not applicable to your situation.

Tin plating is highly susceptible to "fretting corrosion" and can therefore get black spots from handling when nothing is wrong with the plating. Thanks!
Luck & Regards,


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

A. Aletta,

Sounds like you having a lot of difficulties. Let me try to help you with your questions one by one.

1. Pending the type work you have your choices are to rack plate with rack to connect the work on to, barrel plate using steel media for electrical contact, or get electroless or immersion plating bath. However, electroless/immersion tin baths require very specific conditions to operate and are not widely used and may not get the plating thickness that you want.

2.Your different in plating thickness is caused by top part of your strip is a high current density area compare to the bottom. This means the top part wants to plate faster. Try increasing the distance between the anode basket and your work. This will reduce your thickness difference. A thick copper under plate and lowering your plating current may help in this area also.

3. Can't help here.

4. If you are using copper as a strike to help your adhesion, leave this step in the process.

"Soldering in Electronics"
by R.J.Klein Wassink

on AbeBooks

or eBay or

Amazon

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5. Temperature control is important pending on the bath. A high operating temperature will improve your plating rate with baths that are alkaline based. However, acid tin baths do not like too high operating temperature and prefer to be operated at room temperature.

6. There is a lot information published on tin whiskers. A good book on the subject is "Soldering in Electronics" by R.J. Klein Wassink ⇨

Tin whiskers growth occurs only with pure tin under certain conditions such as long term storage with high humidity. Tin-lead alloys, bright tin, and I think reflowed tin is not prone to whisker growth.

7. The best method to set-up your plating parameters is current and time. Set your current per the surface area of your work. For example, 2.5 to 4.0 amps/dm2. A few trials you can determine a plating rate vs current and time. Note there are other variables that will affect your plating rate such as solution metal concentration, temperature, acid or caustic concentration, bath contamination, and so on. Also note the fast plating rate is not always the best. I seen too many people who use the maximum plus current to get the work out. In the end, they generate poor quality work, rework, and scrap.

8. What you may have is tarnishing of your work. Be sure you have good rinsing and drying after plating. The more rinsing the better.You could try an alcohol dip before drying to remove water on or in the plating surface.

I hope this helps and good luck.

A. Hi,

Top and bottom will have different thicknesses. Lowering the Current can improve the variance. Or using shields might help. Cathode part nearer to anode basket tends to get higher thickness. Shielding some part of cathode with non-conductive material will reduce thickness.

Voltage can be calculated using Faraday's Law.

Rinsing your product using deionised water before drying using hot air will reduce discoloration.

A. Hi Navaletchumy. Please try to give us some starting points :-)

Are you doing alkaline tin plating or bright acid tin? Could the dullness/blackness be fretting corrosion from prolonged tumbling? Have you done any Hull Cell testing? Do you have any photos to show us what you mean by 'dull or black'. Is this a problem which just occurred for the first time, or does it come and go, or is this a new plating line which has not generated proper parts yet.

Most plating texts will introduce you to troubleshooting the process. The Canning Handbook [on eBay, Amazon, AbeBooks affil links] says dull deposit (in Bright Acid Tin) bath may be due to excess chloride, lack of brightener, temperature too high, sulfuric acid concentration too low, anodes passive, current density too low, metal concentration too high, or suspended solids. Good luck.

Regards,

A. Hello Kristoff, you didn't mention if the bath contained organic brightening agents. A high level of organics in MSA tin plating will cause internal stress of the deposit. This is true with both brightened and matte baths.

A. Hi Vasan. I'm sure the readers will be happy to try to help but it's still a bit vague. We don't know if this is a one-time hobbyist problem or a long-running factory problem, or whether it's acid, alkaline, or MSA zinc; or what the substrate is; or anything about your process cycle; or the quality of your rinsing. Please give us a paragraph or two of introduction & detail, and send pics for posting here to mooney@finishing.com if you can. Thanks!

Regards,

A. Hi Pankaj. It might be best if you describe the process sequences that you do on steel and brass substrates separately because we quickly get into confusion when they are merged...

For example: why are you copper plating the steel before tin plating, and what copper plating process are you using (cyanide, pyrophosphate, proprietary alkaline)? It sounds like it might be introducing more problems than it prevents :-)

What are these components? Are they highly polished before plating? If not, 3-4 microns strikes me as rather thin.

Regards,

A. Hi Pankaj. I certainly haven't seen all possible workable processes, but I've never heard of electroplating onto brass without an activating dip in mild acid, and I've noy heard of electroplating either steelwithout alkaline cleaning first. It surprises me that your plating has been exhibiting satisfactory adhesion.

As for the blackness developing in a week however, I would strongly suggest sulfur ⇦ this on eBay or Amazon [affil links] in the packaging or the environment and especially because the tin plating over the copper is very thin. Thread 5106 is specifically about tin plating turning black and should offer further food for thought.

Regards,

A. I routinely plated both acid and alkaline tin on steel and brass and bronze parts at a thickness of 0.0002-0.0004" [5-10 µm] with good shelf life and no darkening. I think you need a thicker tin plate.

LK

A. Just a thought. The tin thickness is very low and tin has a propensity to migrate into the base substrate to form an intermetallic compound if the base is copper or iron/steel. In the case of steel, the intermetallic compound is a grey colour and can be easily seen in tin cans, especially when the can has been opened and left exposed to the air for a day or so. If the contents are acid (say pineapples) you will see a dull grey ring around the meniscus - this is due to the tin being dissolved and exposing the intermetallic compound. I note that this enquiry is from the hotter climes and wonder if the thin layer of tin is diffusing into the base...

A. All of the above two reports PLUS, all tin plating solutions do not "rinse" off, you must go back into a mild cleaner to remove the tin plating solution, then rinse again, then it will not turn black in two weeks.

A. Hi Ananda. Sorry but I'm not really following this description. Is one plate plated at a time, or are all 8 plates put into the tank before plating? Acid or alkaline plating? A pic to mooney@finishing.com for posting here would sure help :-)

Luck & Regards,

A. AFTER YOU RESPOND TO MR. MOONEY'S QUESTIONS, THEN, IF ALKALINE TIN, BE AWARE THAT THE SOLUTION IS EXTREMELY SENSITIVE TO COPPER CONTAMINATION. A SEPARATE TIN "STRIKE" WOULD PAY FOR ITSELF BY PROLONGING THE MAIN PLATING SOLUTION.