Tin plating on Aluminum -- is intermediate layer of copper needed?

A. On your cyanide copper solution you should be using a "strike" formulation which is high in cyanide and low in copper metal which is the opposite of a plate solution. Also you should have live entry into the cyanide copper solution.

"The Surface
Treatment &
Finishing of
Aluminium and
Its Alloys"
by Wernick, Pinner
& Sheasby

(note: this book is two volumes)

on eBay or

AbeBooks

or Amazon

(affil links)

A. We do a lot of tin on aluminium. What you need is an electroless nickel strike prior to plating with copper, although the copper should not be necessary. We use a zincate process followed by electroless nickel strike (your local suppliers that you have mentioned would have a suitable process) then either straight to the tin or better still a coat of conventional bright nickel). We find this works fine.

Regards

A. The principal and safer method for plating aluminum is to use an intermediate layer of electroless nickel instead of copper. If you have no possibility of electroless plating, you must use the system zincate + copper. It is important to enter the bath "live" and to start with high current density (3 Amp/dm2) and slow it down to 2 Amp/dm2. Remember that each type of aluminum alloy has different pretreatments.

A. (Rinsing omitted):
1. Clean in a non-silicated cleaner STAY OUT OF ALKALI ETCHING! Alkali etching does the opposite of what you want to do. You want to decrease the area of exposed silicon and INCREASE the area of bare aluminum. By alkali etching you are doing the opposite, as the alkali removes aluminum and leaves the silicon.
2. 100% Nitric Acid + 2 pounds per gallon ammonium bifluoride, immerse until the part TURNS FROTHY WHITE ALL OVER, remove, do not stop to look, hurriedly rinse two times and then enter
3. Dilute zincate
4. 4 oz/gal Baking Soda (Sodium Bicarbonate) FORGET THE COPPER CYANIDE
5. ALKALINE ELECTROLESS NICKEL from one of the top three EN suppliers, 90 °F, 5 minutes, the Johnny-come-latelies are not good enough.
6. Do anything you want to it now.

A. You cannot electroplate directly onto aluminum because it is so active that it forms a passive skin instantly. The normal procedure is to immersion plate zinc onto the aluminum via a zincating bath, and then proceed with alkaline copper and tin plating, or alkaline electroless nickel and tin plating. As an alternative, Atotech offers a proprietary sequence for plating aluminum busbar called Alstan, which uses stannate instead of zincate, and then direct tin plating without the copper or electroless nickel.

A. Hello Mr Charu,

The copper is required to protect the zincate as the zincate may strip off in the acidic tin bath. Yes you may consider it a necessary step.

Best,

A. Hello Mr. Charu,

You can try Nickel in place of copper after zincate. The nickel will give brightness to the tin layer.

A. I don't know, Mr. Ramesh. Alkaline copper plating then nickel plating is okay. But nickel plating (in my experience) strips off the zincate.

A. Hi,

You can use either copper or nickel, both will dissolve the zincate. I believe only alkali EN nickel will be plated over zincate. If you use copper plating over aluminum you must have the right pH range.

Regards,

A. I beg to differ with Ted, I have successfully used a zincate treatment on aluminium alloys and plated nickel onto it. Furthermore, it has withstood excessive thermal cycling as an engine bearing without failing. In my opinion, you can use nickel or copper with zincate.

My error, Trevor. Thanks! I'm only familiar with using either alkaline copper plating or alkaline electroless nickel as the first layer on top of zincate, and I have watched zincate dissolve in an acid copper tank, but just because I hadn't seen nickel properly plated directly on zincate certainly doesn't mean it's not doable. I'll try to keep learning :-)

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

A. The common process of tin plating on aluminum is mild caustic treatment - zincating - copper plating and finally tin plating.

A. Sorry Martin, I don't know whether having copper between the aluminum and the tin is beneficial to RF reduction or not. But we appended your inquiry to a thread which indicates that you can go with an all tin approach on aluminum, or a copper and then tin approach, or an electroless nickel on the zincate layer before the tin plating. Good luck.

Regards,

A. Hi Praveen. There are possibly many causes, but the first two that come to mind are cyanide bleed-out from the cyanide copper step, or sulfides forming because of a sulfide rich atmosphere. With some more facts, and hopefully some pictures, we can probably land on a pretty good solution. Good luck.

Regards,

A. Let's "keep it simple", and just use stannate instead of zincate, then go right on top with bright tin.

That's thinking right, Robert. Thanks. You made me recall visiting a large GE aluminum bus bar factory using Atotech's Alstan process. Although I think Alstan usually had a bronze plate between the immersion tin and the electrolytic tin.

Regards,

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

A. I'm not sure whether or not this will meet your requirements, but aluminum pistons were often immersion tin plated directly in a mildly alkaline bath containing polyphosphates. If this process is good enough for your parts, then it would be much simpler than a multi-step process that starts with zincating.

Thanks for the reminder, Lyle. Now I remember seeing that at the Ford engine plant in Cleveland many years ago.

Luck & Regards,

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

A. The only inorganic tin that is black is SnO. Since the melting point of pure tin is 446 °F There is a high probability it is the oxide of tin.

They only way to prevent this is to do it in a reducing atmosphere or a vacuum furnace.

A. It is possible that the aluminium is forming a eutectic alloy with the tin. I do not believe that Al-Sn forms a true intermetallic compound, but tin is "soluble" in aluminium and it has a eutectic with a melting point of 227 °C (441 °F). Alloys of metals have different colours to the pure metal constituents, so that could give the blue appearance, but it will not account for the blistering.
It is possible that as the tin layer is depleted by its migration into the aluminium, it is allowing the aluminium to oxidise and undermine the tin-aluminium interface - this could cause blistering.
I am of the view that a barrier layer is needed to prevent tin migration; this can be achieved by using a zincate process on the aluminium and then deposit a layer (~5 um) of either nickel or copper. The tin will form an intermetallic with both copper and nickel, but it should stop migration into the aluminium and subsequent broad failure.
I would also suggest that before spending too much time on trying to resolve the blistering, do some basic analyses and find out why the tin layer is blistering. You can get a lot of information from simple SEM studies, coupled with EDX.

A. Need more information related plating time and type of copper salt you are using. I think you should use lacquer-based anti-tarnish which protects tin plating from salty atmosphere. Such types of products are available in the market which are conductive also.

A. Hello Arjun,
If the Cu plating is also for conductivity purposes, I would deposit a minimum of .0008" or 8/10ths of one mil. Good Luck!

A. Hello Arjun,
I'm not a big fan of Cyanide baths, so if you have a non-cyanide alkaline Cu bath, use it as a strike, then go into the acid Cu plate to build thickness. Acid Cu has better leveling characteristics than Cyanide Cu and Pyrophosphate Cu baths. If you didn't need the Cu plate for finished part conductive purposes, you could skip the acid Cu plate.From what you stated in your question it sounds like you do need that step.

A. Hi Arjun. If there are no specifications which your process is not compliant with, I think it sounds fine :-)

Regards,

A. Hi Divahar. Immersion tin plating on aluminum is both possible and practical. Please see Robert Probert's and Lyle Kirman's replies above. I'm not familiar with electroless copper on aluminum, but hopefully another reader will be.

Please use several sentences to describe your situation as it's not clear whether you want electroless tin deposited on top of the electroless tin, or whether you are asking two different questions in your one sentence. And I'm also not clear if your question is restricted to autocatalytic plating or includes immersion plating. Thanks!

Regards,

A. Hi Al. Alstan ("all tin") was an M&T Chemicals (now Atotech) process. I don't know if they still offer it, but that would be a good place to start. The process includes an immersion plating of tin instead of zinc (stannate instead of zincate) and includes a top layer of electrolytic tin plating -- but I think there was usually an electrolytic bronze plate between the immersion stannate and the electrolytic tin plating.

Regards,

A. Hi again. According to some legal websites, but I don't know that their information is accurate and timely, the Alstan trademark is still held by Atotech, but the process is no longer offered in the USA. That would mean that you can't actually call it Alstan anyway. Google books has some short abstracts which seem to indicate that the Reynolds "Aluminum Finishes Process Manual" ⇦[this on eBay, AbeBooks or Amazon affil links] has good explanations of the many Alstan processes if you can get access to a copy (sorry, I don't have a copy).

Obviously, I don't know the dynamic between you and your customer, but I would suggest talking to them about this situation where they have specified an obsolete and unavailable, and largely unknown, process. Maybe they would be interested in updating their specs to something more modern and available. Good luck.

Regards,

"No-Ox-Id" Compound

on eBay or

Amazon

(affil links)

A. Hi Vimal. The simplest and least expensive alternative is bare copper. The DC from rectifiers in plating shops is usually bussed to the anode rods and cathode connections with plain copper, with the joints made up with anti-oxidation compound. But readers don't know if that will be satisfactory for your customers and your situation unless you tell us :-)
Bare aluminum, however, is rarely satisfactory.

The most feasible option for a small enterprise is usually to farm out any necessary electroplating services to a plating jobshop. Good luck.

Regards,

A. Hi Damien. Aluminum bus bar has been commercially plated with either copper or tin.

The copper plating process is probably the most common, and involves zincating followed by cyanide copper plating followed by bright acid copper plating.

The tin plating process involves stannating, probably followed by bronze plating, followed by tin plating and was offered by Atotech as their proprietary Alstan (all tin) process; the intermediate bronze plating is just a vague recollection, but the vendor can advise.

Although aluminum can be chromate conversion coated for conductivity, I don't think I'd trust it for bus bars; in the USA there was a disaster years ago (google "Beverly Hills Supper Club fire") that killed 165 people which was blamed on unplated aluminum wiring.

Regards,

A. Hi Jose. It's probably a very good idea, but when you intend to plate tin onto aluminum a simpler & better procedure might be a stannate immersion process, then directly to tin electroplating. Long ago when I was with M&T/Atotech it was called the Alstan process and was widely used as an alternative to the zincating, copper plating, and tinning of aluminum bus bar.

Luck & Regards,