Gold Plating on Copper: Diffusion barrier needed?

A. Two specific suggestions:

1. Go to the library and seek out a good electroplating reference, like the Metal Finishing Guidebook, Modern Electroplating, or Electroplating Engineering Handbook [on AbeBooks or eBay or Amazon] . Some of the answers to your questions depend on the gold bath formulation.

2. It is usually bad practice to plate gold directly on copper, as the two metals will form a solid solution. Generally, a nickel barrier is plated between the copper and gold.

A. Hi Sam,
To answer some of your questions:

1) I'm a metric man and don't want to have to think in those units so I'll leave this to someone else :-)

2) No one uses solid gold as anodes. just use some platinized titanium, these won't dissolve nor (obviously) replenish the gold being plated out of the solution so this is added back as a gold salt. The ratio is a AREA ratio not a mass ratio. usually 2 to 1 or higher.

3) Masking lacquers and tapes are common ask the supplier that sold you the gold solution for something.

4) The cotton is used to prevent solid chunks of anode falling off and plating as roughness on your part, but if you use the anode I mentioned above that doesn't dissolve and this is not an issue.

5) Did you know that gold over copper will tarnish! The copper diffuses into the gold to create an alloy and can appear spotty. Usually a thick barrier layer (sometimes of nickel plating) is applied between the two.

good luck and beware of the cyanide

A. Hi Sam.

1. If the area of your component is 0.7" inches^2, and the desired plating thickness is 0.001", then you want to deposit 0.0007 inches^3 of gold. Let's switch to metric at this point and call it 0.0115 cm^3. Gold weighs 19.3 g/cm^3, so you want to deposit 0.222 grams of gold.

If your gold bath is based on gold potassium cyanide, K⁺Au⁺(CN)^-₂, then the oxidation state of the gold is Au⁺¹. This means if you pump one electron from the anode to the cathode with your power supply, one ion of gold will come out of solution and become an atom of gold on the cathode. Cutting through some of the conversion factors, Faraday's Law of Electrolysis says that one Faraday (96,485 coulombs or ampere-seconds) will deposit one gram atomic weight. Since the atomic weight of gold is 197, one Faraday deposits 197 grams. So to deposit 0.222 grams requires 0.01127 Faradays or 108.7 ampere-seconds.

In practical gold plating, though, not all 100% of the applied electricity deposits gold; some (maybe 10%) is wasted to the hydrolysis of water into H and OH. So, say it will actually require about 108.7/0.9 or 120.8 ampere-seconds to deposit your gold. Continuing with some empirical issues, you'll find that your current density for satisfactory plating will be limited to about 4 amperes/ft^2, which is 0.0278 amperes/inch^2, or 0.0194 amperes on your piece.

Thus to apply 108.7 ampere-seconds at a rate of 0.0194 amperes will take about 5600 seconds -- roughly an hour and a half.

2. As previously mentioned by Peter, the anode to cathode surface area should be about 2:1, which means your anode area should be about 1.5 inch^2

3. The easiest approach to very simple masking is "plater's tape". This is very similar to electrician's black vinyl tape except a bit thicker and with extra care given that the materials used are compatible with plating.

4. Peter has given one explanation of the cotton, but I'm thinking it is possible that you were reading about "brush plating". In that approach, rather than immersing your part and the anode into a small vat of solution, the anode is equipped with a sock-like cloth cover which is soaked with the plating solution and brushed onto the cathode. I don't think that's what you are looking for though.

Please note that this theoretical discussion should not be read to imply that working with poisonous cyanide solutions is safe (it isn't!), or that the plating is easy. Typically, parts should be sent to specialty plating shops to have the plating applied to them. Good luck.

Regards,

A. You won't get galvanic corrosion, but the copper will form a solid solution with the gold. Normal practice is to plate nickel over the copper to prevent this, .0003" should be sufficient.

A. You do need an interlayer of nickel plating, but not as a guard against galvanic corrosion, but rather to prevent the gold from diffusing into the copper.

The 10 microns sounds more reasonable than the 30, but 10 microns is still very heavy for gold plating in an application like this. Perhaps the recommended range was 10 to 300 microinches rather than micrometers? Then I'd say 10 microinches is very thin and acceptable only where very good testing and process control is possible, whereas 300 microinches is probably overkill, and a realistic range for your experiments would probably be 30-100 microinches.

A. Gold can be plated directly over copper but the copper will migrate into the gold layer over time. Copper is usually plated with nickel before being plated with gold and gold can be plated to almost any thickness from only a few micro-inches up to whatever thickness you are likely to want.

A. You'll have to do a recall on every inch of those billions of feet, Mr. Tetorka :-)

Only kidding, of course; thank you for sharing some very practical knowledge with us. We have a paper on Nickel Diffusion Barriers in our on-line library [www.finishing.com/library/stransky/nidifbar.html] that you might find interesting. Is it possible that the reason for the gold plating was solderability or wire bonding, or something which is done early in the life cycle such that longer term diffusion is of no consequence?

A. One reason for eliminating Ni underplate is Ni magnetic properties. It will mess up in the areas where magnetic field measured or sensed.

A. Hi Adam,

Solder non-wetting on PPF leadframe, among others things like soldering temperature, solder paste/flux quality, etc, could be due to poor plated Pd/Au purity and/or morphology. Metal source like palladium solution and gold salt must be of high purity while proprietary additive/replenisher should be analysed and added according to chemical supplier recommendation.

For IMC separation, usually C & O could be detected by EDX to certain extent which is common unless C and/or O is abnormal high. The separation could be due to inadequate intermetallic layer formation, improper soldering temperature profile, nickel oxidation prior Pd/Au plating, etc.

Regards,
David

A. Joel,
What is the maximum temperature? This information is crucial to the calculation.

A. Hi Sravan. The first thing to realize if you are new to this is that few people (except Ph.D. students for a thesis) attempt to formulate a gold plating bath from raw chemicals; most just purchase proprietary formulations that have been developed over the decades.

The next thing to realize is that there are many different general types of gold plating solutions for electrolytic plating: alkaline cyanide baths, old-fashioned ferrocyanide, neutral cyanide, acid cyanides, citrates, sulphites; hard and soft golds; pure and alloyed golds; bright and matte golds depending on the relative importance of appearance, hardness, freedom from porosity, wear, solderability, tarnish resistance, etc.

If you can introduce yourself (hobbyist, high school student, post-doc) and what it is that you wish to gold plate, and why, I think our readers would be able to distill it down to some basic instructions of what you would need to do to apply gold electroplating to a properly prepared substrate. Thanks.

Regards,

A. Hi Justin
You are not clear if the part itself has to have conductivity or are you designing a contact.
In the first case it is unlikely that the plating will make any difference as the part carries the current.
If the part is a contact, you have many more questions to ask.
Is it a sliding contact with many insertions or a plug and forget? What is the contact pressure etc. Voltage, current etc ?
The service atmosphere is important because the limiting factor is not diffusion but porosity in the gold plating allowing corrosion.
In the long gone days when gold was $35 per oz. specs regularly called for 5 microns minimum of acid hard gold. Top military specs are now half that or less over about 5 microns of nickel. If your application really is critical you also need to specify porosity testing of the finished part.
If you are not asking these questions, I cannot believe that your application is truly critical by aerospace/military/ medical standards.

A. Hi Nigel

First; no oxides of copper are green

Looking at the SEM scan, the high carbon and oxygen content indicates an organic compound. Pure copper is never found in spring contacts in switches. Beryllium copper alloy is common which explains the silicon. Both Beryllium and aluminium are difficult to detect by SEM and that peak could be either. Aluminium is a non-starter as a barrier layer and the minor elements are typical background contamination so we are left with a green organic copper compound.

I once was presented with a huge mainframe computer with every solder joint green (forty shades) and thousands of them!

My money goes on soldering flux residue.

Shorting by water seems probable and water would remain trapped in a small switch long after it had evaporated on an open circuit board.

A. Hi Nigel

copper carbonate would not explain the very high sem peak for carbon.
The fluxes used in electronics are either based on colophony resin or in current production organic acids such as lactic, stearic acid [on eBay or Amazon] or citric. They work by dissolving oxides of copper and the salts produced are blue / green. The residues are non conductive but are usually removed for aesthetic reasons and to avoid the possibility of future corrosion.
So, though I believe the deposit may be flux residue, I would not expect it to cause a short circuit. Water is much more likely to cause a short but may not be connected with the blue deposit, although some corrosion could be present. However, switch contacts are usually gold plated to avoid corrosion so there could be two unrelated observations.
Two things to try.
Flux residues should dissolve easily in isopropanol.
Check if the short still exists when the switch is completely dry.
One other thought; was this switch specified for 24v at whatever current?