Tin lead plating problems

? Hi LeAnna. So that we don't misunderstand you, I assume that one part, or some small percentage of the parts exhibit this problem, not all of the parts?

Also, what do you mean by "after cure" -- what is this "curing" operation? Thanks.

Regards,

Q. The entire lot of parts exhibit this condition. The curing is not related to the plating application. The parts leave the plater and return to the filter OEM and they temperature cycle the parts.

A. Hi again Leanna.

Leanna, my first guess, is that this area is metallurgically different, perhaps just not as clean and active. I would suggest asking the plating shop to manually scrub a few parts with pumice and tampico brush, and see what they look like after plating and curing.

You might have some difficulty telling the plating shop that the lot is rejected unless you have some agreed upon visual standards. They might well feel that an unpolished part, full of nicks and scratches, with a dull tin-lead plating, is hardly a decorative part to be judged by variations in the darkness of the deposit :-)

Regards,

Q. The discoloration can be scrubbed off. But at the leading edge their is a ridge left over. The parts are very rejectable as we can not put pure tin parts into space. The specification is greater than 3% Pb. So, back to my question, can the difference in thickness, end-to-end, be attributed to "current density" in a barrel plated process?

A. Hi again. Yes, certainly you can reject parts that don't meet your specifications.

Thanks for the graphic. I'd better let another reader get "back to your question regarding difference in thickness end-to-end".

Regards,

Surely in almost all alloy plating, particularly in a barrel, the ends will get different current density from the middles and that will skew the alloy. This will be aggravated by the speed of the barrel rotation, faster is better for consistency.

Another problem here, maybe not related to alloy variation, but surely related to over all quality variation. You mentioned the part as "tubular". In that case, merely rotating in all the rinse tanks is not enough to clear the tubes of contamination - you must go in/out/in/out/completely in/completely out to flush out the inside of the tubes.

 Hi cousin Kati. I'm not sure exactly what you meant to say with "inhabit". Hopefully another reader will be more insightful than me.

Based on your several postings here, I'm guessing that English is your second language, and that you're encountering a little difficulty expressing yourself -- but that's actually a reason to be wordy rather than to be brief. Because nobody cares much if we misspell a word or if our sentence construction doesn't match the way they would write it, but we do care when we can't figure out the question :-)

You might mean "inhibit", i.e., that the antimony slowed down or stopped the reaction. To which I would have to say, where is the antimony coming from, how did it get there? Is it in the substrate which you are trying to plate with tin-lead? But you might be asking how antimony got into the plating solution in some unknown way, inhabiting it, in which case me asking you how it got there would be silly :-)

Please spend a few paragraphs trying to explain the problem in detail. Thanks!

Regards,

A. Hi again.

I've tried off & on many times to learn Spanish, but didn't have the stick-to-it-iveness to achieve it. Thank heaven for people like you with the ambition to learn more than one language, or humanity would be divided up into isolated non-communicating islands. In the end, the word "inhabit" may be a good pun, because you mean both that it has come into the deposit somehow and you feel that it is inhibiting the deposition of lead.

Where the antimony is coming from is hard to determine except by examining the raw materials. Apparently silver-antimony alloys are not uncommon, so it could be coming from the substrate. But apparently antimony is sometimes added in small amounts to tin-lead alloy baths, and could be building up, or perhaps you just never looked for it before?

Although I know almost nothing about this subject, Brenner's "Electrodeposition of Alloys" gives some possible hints. For example, he notes that ingredients like peptone have almost no effect on the antimony content, but do drive the tin way up which means the lead has to go way down. My interpretation is that, although the source of the antimony is still an unknown, it may not in fact be responsible for the suppression in lead content.

Hopefully an experienced tin-lead plater will chime in.

Regards,

 Ted thank you again for your help! If I discover the antimony source or find the reason for the low percentage of lead, I will write here.
Kati M.

A. I'm not sure where your antimony is coming from, but antimony will plate out preferentially to tin and lead. In fact, antimony may also immersion plate on the anodes and the tin-lead deposit. It also acts as a brightener in tin-lead plating. So, because it plates out preferentially, a one-time contamination should disappear fairly quickly, and if it persists, then it is probably not a one-time thing.