Problems with tin plating over copper or brass

A. You cannot plate tin directly on brass, you need a barrier like a copper deposit or nickel deposit between the brass and the tin.

A. Please can you ask if the suppliers are using acid or alkali tin plating solutions?

A. Mr. Allen,

Use Tin/Lead to resolve your soldering and appearance issues.

Regards,

A. Try asking your supplier about "Dull Tin". We use it and it passes solderability. You still have to have a copper or nickel barrier but it works very well.

A. If you already have a copper underlayer, the recommendation for DULL or Matte tin should solve your problem.

A. Everyone is giving good advice. Always remember there is more than one correct answer to a question. The more plating experts you get, the more answers. One reason you may be experiencing problems with your present process cycle is that you say you are applying a copper flash. I assume this is from an alkaline bath. You need to plate sufficient thickness to overcome the amount of the copper plate which will be absorbed (not the technically correct phrase but descriptive). Try doubling or tripling the thickness of the copper flash. Also check your copper plating chemistry to insure that you are not depositing excessive organic compounds.

Good Luck,

A. If you are using bright tin plating and having blistering try adjusting time and temperature in the reflow process and make sure your reflow material is fresh. Use to more then twice.
If that doesn't work, switch bright tin to matte tin plating.
Or try copper flash, nickel, then matte tin plating.
Definitely use matte tin, stay away from bright.

A. The bubbling could be outgassing from organics in the tin OR the copper plate. Check with the supplier that they are using a pure matte copper and a pure matte tin. Also, the copper should be 100 microinches (2.5 microns) thick. If you are using a nickel barrier, the industry standard for the barrier thickness is 50 microinches (1.25 microns).

This is because the CuTin intermetallic grows at a much faster rate than NiTin intermetallic. The CuTin intermetallic is detrimental to reflow and solderability, whereas the NiSn intermetallic is not.

I worked at MacDermid Inc as a research chemist and tech service engineer for 11 years; my area of expertise was tin and tin lead plating over connectors. If you can switch to nickel, it will improve the lifespan of your parts and minimize whisker growth on connectors.

I hope this info is helpful.

A.
1. if the etching process is with current, you may try to setting the current to 0.
2. set up acid active process with sulfuric acid after Etching process

A. Hello Vincent,
The 3 major causes for carbon to be present in the tin deposit is storage conditions, and excessive organic additives in the bath and bath impurities. Because you are using a Matte Tin Bath, you can pretty much rule out the organic brighteners. Are these parts stored at room temperature right after tin plating, up until the point of soldering? What about humidity and the general atmosphere where the tin plated parts are kept?

Here is a test you can run... Plate the parts in tin normally. Be sure you have good rinsing and clean drying techniques. Wrap parts in sulfur free paper, or just a plastic bag and store in a room temperature environment. Wait the usual time before soldering. If you still have the problem, the tin bath should have a full analysis including metallic impurities. It would also benefit you to put a few carbon filters in your filter system for a few hours apiece in case you are dragging in organics during your plating cycle. Good Luck.

A. Hi Faouzi. I'm not sure if you're saying that the plating solution goes bad after a few days and starts making bad parts, or whether you are saying that parts which looked okay coming out of the plating tank start turning bad in storage a few days later. It would make a big difference in my answer. I'm also not sure of the scale of these photos; a measuring tape would help. But it looks to me like you have a huge "particulates" problem, with loose fines attaching to the parts and getting occluded into the plating as extreme roughness.

Do you have anode bags on the anodes and continuous filtration in the plating bath?

We appended your inquiry to a previous thread which suggests the need for a barrier layer of copper or nickel before the tin plating, but I don't think that's the main problem here :-)

I, too, would like to hear from tin plating experts though because I find this issue very strange! Although, as I say, it looks like particulates to me, they seem to be exclusively in low current density areas. Why? Maybe I'm wrong and it's a reaction between very thin plating and the brass substrate? Those are mighty rough castings.

Regards,

A. Faouzi,

It's nothing else but you are stripping Zn and Cu from the brass from which impurities increase daily. You move ahead and it's getting plated in low current density areas. Initially few hours or days you must be getting good results.

As suggested by Mr. Ted, you need barrier layer before entering Acid Tin bath.

A. Hello Faouzi,
The 2 previous responses revealed some good points. In troubleshooting plating problems I have always employed the process of elimination to get to the root of the problem.
1) As Ted mentioned you want to rule out particulate matter. Check the filter tubes to see if they contain insoluble matter. Anode bags must be used, and replaced when they don't drain the solution at at good rate. Pure tin anodes should be used, and they are supplied with a certification of purity.
2) Make sure you have adequate solution agitation to ensure the tin ions are being replenished at the cathode (parts).
3) In addition to a routine bath analysis, have the tin bath checked for metallic impurities such as zinc, copper and lead (if plating leaded brass). If you don't have the means to do this, send a sample out to a lab or supplier of your tin bath.
4) Be sure you are maintaining additions of your bath additives, whatever they may be.
5) Make sure your rinse tanks prior to the tin bath are clean and not loaded with metallics.
This may seem a lot to verify, but if a malfunction in the process is found, you will know how to correct the problem if it re-occurs. Good Luck!

A. Hi, Faouzi! After long years of experience: Never do tin plating directly on brass ... if you do you will meet such problems after a few days.
You should have copper plating first before tin plating.
I hope your problem will soon meet the answer.
Regards.

A. Hi Suresh. Unfortunately I don't think you've actually provided any info yet from which people can use to help you try to troubleshoot. You are the plater? Send a picture of the part if you can; tell us what the tin plating process is; barrel plating vs. rack plating, etc. and we can perhaps get started. Good luck.

Regards,

A. Hi Narayan. Well, you can break some of the blisters and look at the underside of the blister and the surface they detached from, and tell us whether the nickel detached from copper, nickel delaminated from itself, or tin detached from the nickel -- and we can proceed from there. You can also send pictures of the blackening to mooney@finisihng.com, because cosmetic/aesthetic issues are difficult to cover with words alone. Good luck.

Regards,

A. Hi Sandip. You can review this page and maybe get some ideas regarding what your supplier might be doing wrong, but the chances of solving your supplier's problem for him this way are low. Usually your only realistic alternative is to try a different plating shop. But best of luck.

Regards,

A. Find out whether the brass is leaded. If so, dip in 30%/vol commercial fluoboric acid, then copper strike in copper fluoborate. This is because lead is insoluble in other mineral acids and cyanide copper will not stick to lead salts. The plumbing industry knows all about this.

A. Hi Rana. Yes, you can do tin-lead plating on brass. It's best and may be necessary to copper plate it first, before doing the tin-lead plating.

… but please introduce yourself! We don't know if you are a highly experienced tin-lead plater who just hasn't happened to try plating onto brass yet, or a young student wanting to do a science project for school. There are entire books on solder plating, so it is very hard to tell someone how to do it in a paragraph or two until they tell us their situation so we understand what they actually want to know. Thanks!

Regards,

A. Hello Marko, when you say you changed the bath, adding new components, I would assume it's a brand new tin bath, not a slide conversion attempt? Have you measured the thickness of the tin deposit? If you have had success in the past with plating directly on brass. The bath should undergo a full chemical analysis, and deposit thickness verified. Your tin bath supplier should become involved as well.

A. It is common to take the finished tin deposit back thru a mild soak cleaner to remove the tin solution. Otherwise the deposit turns gray/black in a few days.

A. Hi Yuvaraj. Your question is too abbreviated for me :-(

Are you saying that you have brass contacts that have been zinc plated, and copper contacts that have been tin plated? And these 'contacts' are for conduction of electricity? ... because I've personally never even heard of a zinc plated electrical contact, and even googling for that topic comes up empty. Maybe you have a zinc plated brass chassis and you trying to solder a grounding lug to it?

Please follow up with the full details of your situation, rather than an abstract proposition :-)

Thanks & Regards,

A. Hi Yuvaraj. Please just get a tin plated screw.

Regards,

A. Hi Sukhram. Sorry, I don't recognize "BS36" -- can you be a bit more explicit please? The plating should withstand the test, of course, and the fact that the base material was exposed tells us that the problem started early in the process -- the parts either not being clean, not being properly activated, or the nickel plating too thin or too poorly done.

Regards,

A. Tim, You raise some interesting points here. Firstly, since the substrate is old, it will be heavily oxidised, so I presume you are using a good flux before applying the solder - the oxide surface will be resistent to wetting by the solder, so it needs to be thoroughly and carefully prepared to make it wettable. Secondly, you are correct in that copper and tin readily forms intermetallics - under low temperatures only two are formed (Cu3Sn and Cu6Sn5), but at over about 400 °C other ones are formed (Cu10Sn3 and Cu41Sn11). Maybe heating the surface to over 400 °C is resulting in a tenacious oxide film that is preventing the solder wetting the surface. My first guess would be that you need to make sure your surface is clean and free of oxidation, because I would not expect IMCs to inhibit the surface wettability by the solder.
I am not sure if the following is relevant to you, but many years ago I worked on another tin based IMC - iron-tin intermetallics and these are commonly seen in tinplate. There is a range of them (FeSn2, FeSn, Fe3Sn2 and Fe2Sn3), but the only one found under normal conditions is FeSn2 - the others are thermodynamically unstable and will break down to FeSn2 and Fe. Furthermore, under conditions up to about 450 °C, the only initial IMC formed is FeSn2, but once the tin source is exhausted, the other compounds start to form as the tin diffuses deeper into the iron. It is hence possible to get all 4 Fe-Sn IMCs, but if they are left alone, they will all break down to FeSn2 and Fe.