Customer says our tin plating has RoHS-incompatible lead. Where from?

A. There may be a number of reasons (some of them real and just, some not) for a customer to use the RoHS theme against you. Check for yourself Sergio. Hire a good level certified lab to check your plating. There must be good labs in your home town or in Mexico City. If your product passes but still they insist, get a second test from a reputable lab in their own country. If it doesn't, search the origin and do your job.
G. Marrufo-Monterrey, Mexico

A. Sergio,

First check that the incoming water to your process tanks is lead free (very old water pipes may be made of lead, newer old pipes have lead soldered joints if copper)If all your plumbing is plastic do not discount this source since water lines outside your building may be at fault.

You should also find out what method you customer used to "discover" the problem. Is it an approved method? Is the test being ran by someone properly trained?

Do you send out periodic samples to an outside lab or conduct any tests of your own? If not perhaps you should to see if you get the same result as your client before you begin a major investigation of your process

A. The tin 'A' anodes may be the problem.
ASTM B339 -00, 'Standard Specification for Pig Tin' lists three grades of tin. Compositions below, omitting elements other than tin & lead:

Grade 'A': 99.85% tin (min), 0.05% Pb (max).
Grade 'A' for the Manufacture of Tinplate: 99.85% Sn (min), 0.010 Pb (max).
Ultra Pure Grade: 99.95% Sn, 0.001% Pb (max).
Possibly, these ASTM grades correspond to the A, AA and AAA grades from your supplier; ask for certificates of analysis for each.

E51-67 (1978), 'Method for Spectrographic Analysis of Tin Alloys by the Powder Technique (Withdrawn 1984)' can be used for analysis [Probably, spark source atomic emission spectrometry since uses solid samples and is mentioned by NIST].

The US National Institute of Standards (NIST) sells 'Standard Reference Material (SRM) 1727 Anode Tin' with certified Pb content. SRM 1727 is a 30x30x30 mm cube of tin containing 33.26 ± 0.33 ppm Pb. Costs $601 but valid until 1/1/2028 expiration date. Small samples can be chipped off for spark source atomic emission spectrometry or dissolved in acid for other spectroscopy methods.
http://www.nist.gov

The first supplier below that I checked, Grant Mfg & Alloying, has 'Mamore' and 'Minsur' Grade "A" tin anode compositions with Pb contents well below 0.01%.

A. From a recipe for 90Sn-10Pb plating (bath: 75 g/L Sn, 10 g/L Pb; anode: 90Sn-10Pb), tin may slightly preferentially plate. Starting with pure tin plating salts and a tin (0.01%Pb) anode, I'd expect the lead content of dissolved metals in the bath to reach about ~0.012% Pb, after which the composition plating out will match the anode. Thus, at steady state, the plating will contain ~0.01% Pb, well below the RoHS 0.1%Pb limit.

My tin plating experience is rather limited; experts please feel free to comment.

A. Ask for the LEAD REQUIREMENTS and the PROOFS that you customer had performed.
Otherwise Lead analysis can be performed with AAS ⇦ this on eBay or Amazon [affil links] or better with Electron Microprobe analysis.

A. Another thing to check is the material you are plating on.
Brass (And conform copper, copper that is formed from the billet cold instead of warm), contain a slight amount of lead on the surface.
This is normally deposited as the brass or copper is pushed from the billet through the die during manufacturing.
The lead is contained in a lubricant that used to help this process.

We discovered this to be a problem during our cleaning of brass in preparation for tin and silver plating.
The dry acid we used was causing a lead sulphate to form on the surface of the part causing bad adhesion.
We ended up using the dry acid with another chemical to bump up the fluoride in the solution.

Anyway, you can see that it may also be the copper that is not conforming to RoHS.

(P.S. our suppliers carried out an SEM scan for free, you should also try this if you do not take up Mr. Thompson's offer).

A. As a manufacturer of the bulk of tin anodes in the US I thought I'd add my two cents. For 95% of the tin produced in the world, the lead content will range from 30 - 350 parts per million. This is approx. 99.95% pure tin. If you want high purity tin anodes (99.99% tin) you can get the lead down to around 15 ppm. That's the best you'll do for anodes. 99.999 tin (laboratory grade) is made with lead down to single digit ppm, but is not realistic from a cost or availability standpoint for electroplating in any decent quantity.
There should always be a lead value on your certificate of analysis for tin anodes.

A. Hi Richard. Probably not. There seems to be no requirement in that spec, which was cancelled long before RoHS was written, for coatings to be lead-free. In fact it warns repeatedly of the usual need for lead. Further, are you sure the brass itself is compliant (lead is frequently added to brass for machinability)?

Regards,

A. Hi Bala. We appended your inquiry to an earlier thread on the subject with lots of food for thought. Also, while I am not familiar with the "sulfuric acid treatment" of MSA baths, please note that contamination of sulfuric acid with lead is widely reported.

Regards,