How high quality plating on plastics is done, with problems and solutions

Consider for a moment the situation of a nickel and chrome plated automotive grill. In this case you have a layer of metal (relatively low expansion with temperature change) deposited onto plastic (very high coefficient of expansion). The car may sit out in an Alaskan winter at temperatures way below zero, travel to Death Valley with temperatures of 130 °F, and sun so strong that you'll burn yourself if you touch the grill. In between, the car may go to car washes with scrubbing brushes, or suffer exposure to corrosive road salt, flying gravel, incidental contact with steam cleaner ⇦ this on eBay or Amazon [affil links] , etc. For the metal plating to adhere without bubbling, blistering, chipping, peeling, corroding, or wearing through in the face of decades of this abuse requires fantastic adhesion (courtesy of ABS plastic which can be etched for terrific tooth), very robust and usually proprietary pretreatment and metallization cycles (consisting of steps like etching, activation, acceleration, and electroless nickel plating), and very high quality electroplating (a sequence of copper plating, semi-bright nickel plating, bright nickel plating, and chrome plating).

ABS is an engineering plastic with a number of outstanding properties (see wikipedia.org/wiki/Acrylonitrile_butadiene_styrene), but the property that makes it attractive for plating onto is that, because it is comprised of three different monomers, it is possible to expose it to an etching chemical (chromic-sulfuric acid) which will dissolve the butadiene portion, leaving a spongey, toothy surface which the electroplated metal can really lock into.

1). ABS is first etched in chromic-sulfuric acid to dissolve the butadiene.
2). After neutralizing, the plastic parts are "catalyzed", i.e., a catalyst, tiny seeds of palladium, is formed on (and in the sponginess) of the plastic via a series of steps. Generally the parts are sequentially dipped into a proprietary solution of palladium chloride and a proprietary solution of tin chloride, and the tin reduces the palladium ions to metal, and then the excess tin is washed away. The terms "activation" and "acceleration" are sometimes associated with these steps, but it must be continually emphasized that these pretreatment cycles are invariably proprietary: most cycles use an acid pre-dip before activation to prolong their life and stabilize them; some pretreatment cycles may not require 'acceleration' (or at least don't use that word), etc. So some people like to simply use the word 'catalyzation' to cover whatever steps and process sequences are involved in getting the palladium seeds in place and ready to go.
3). Next comes the autocatalytic Electroless nickel process. The parts are put into this solution, which is sort of "spring-loaded"; it contains reducing agents that are triggered both by the palladium seeds and by the deposited nickel, so that the palladium triggers the nickel to start depositing, and the deposited nickel triggers it to keep going, depositing a uniform layer of nickel. It keeps going until the parts are removed.

The central idea in this metallization approach is that metal ions permeate right down to the spongey plastic and electrochemically grow into metal from there. There is no layer of conductive paint which could limit the adhesion of the plating to the plastic. While a conductive paint layer between the plastic and the plating might prove perfectly acceptable for many applications, it's not adequate for exterior automotive brightwork.

Parts which are intended for exterior automotive use are copper plated after the electroless nickel process, then duplex (double) nickel plated, maybe triplex nickel plated, and finally chrome plated.
The copper is not for corrosion resistance, but to offer a relatively a soft layer between the electroless nickel and the electrolytic nickel plating to help absorb the stresses caused by the different thermal expansion coefficients of metal and plastic. The two or more separate layers of nickel plating offer maximum corrosion resistance as explained in the previously mentioned FAQ, "Introduction to Chrome Plating). And finally the chrome plating. Some automotive specifications may require some additional intermediate steps, but they always include the copper, semi-bright nickel, bright nickel, and chrome plating steps.

A. Since you cannot provide any technical details of the process, I can only speculate on your problems. Pin pricks in electroless plating can be due to many causes. Firstly the ABS may be the wrong grade or be moulded under the wrong conditions. The holes could also be due to inadequate cleaning of the moldings prior to electroless plating. This could be due to the presence of mould release agent or other surface contamination. The pin pricks could also be due to the non-release of the hydrogen bubbles generated during the electroless nickel process - this could be due to the bath being out of specification or inadequate agitation. Failure during thermal cycling can be due to inadequate bonding between the ABS and electroless nickel, or bond failure between the numerous metal-metal interfaces, or quite simply the temperature range being too great. Remember that the expansion coefficients for ABS and metals are very different. One tip, - when I have done electroless PoP, I have tended to use slightly compressive nickel, as this seems to assist adhesion in thermal cycling. There are numerous books on the metallisation of plastics, some of which are listed in the "must have" bit of this website, so have a look and see what you can get.

A. "Trouble Shooting Decorative Electroplating Installations part 4: Plating on Non Conductors" by N.V. Mandich and D.W. Baudrand, Plating and Surface Finishing magazine September 2001 Journal of the American Electroplaters Society (AESF) covers the questions you are asking. This should be available from a Library.

A. We find the most common cause of "pinskip" to be airborne chrome from the chrome tanks if sufficient wetter is not present. There should be a thick foam blanket upon the chrome bath and exhaust system setup to draw in the air.

A. Hi Muhammad. We've now added an Introduction section to this page which should answer your questions.

Hunt through the university and town libraries and see if you can find a copy of "Standards & Guidelines for Electroplated Plastics". This will describe all of the typical methods for etching, sensitizing, and activating before electroless nickel plating, etc. Also, keep alert for current technical articles as the pretreatment of plastics is evolving to try to make it more economical, ecological, and applicable to additional types of plastics. Good luck.

A. Hi Saad. I think the best reference is the book I just mentioned. People take months or years writing a book, with the expectation of being compensated for all that effort, so it is always more comprehensive, more authoritative, and more tutorial than any web page or forum article.

But if you want to try to piece it all together, you can start with this page as a framework of what the overall process is about, then hunt for additional detail on each of the steps. Best of luck!

Regards,

A. Hi Noman. The complete process is etch > activate > accelerate > electroless nickel plate > acid copper plate > semi-bright nickel plate > bright nickel plate > gold plate, with rinsing between each step.

But each of these steps makes a good chapter in a plating text, and almost all plating shops use proprietary plating solutions rather than generic home-brew. So your quest for "complete process" details is probably beyond what you can receive. You can probably do it, but it will be a development effort of months rather than a few minutes of reading the forum. If possible, contact a major supplier of plating processes as they will have all of the processes for you, although they will retain the fine details as trade secrets. Good luck.

Regards,

A. Good day Rameez.

There have been to date no replies, and I thought I would throw something at you. The only reason I can think of is if the copper is applied electrolytically, and the ASF is too high initially, you can "burn" through the metallization on the surface. As thickness builds, you can ramp up the ASF to the desired range. This may be why electroless nickel would be easier, in my opinion.
Hope this helps.

Regards,

Eric Bogner, Lab. Tech
Aerotek Mfg. Ltd. - Whitby, Ontario, Canada
December 16, 2015

A. Eric, my guess is that Rameez is referring to electroless copper vs. electroless nickel.

Hi Rameez. You feel that electroless nickel would have been easy; but you must do electroless copper, which is hard? Well ...
"The grass is always greener on the other side of the fence."

But seriously, yes, in the early days of electroless plating, electroless copper was sometimes used in decorative plating; these days to my knowledge it is only used on circuit boards where the conductivity of copper is required. It is apparently more difficult to control.

Regards,

A. Hi Rameez. How did you "make it conductive"? Are you sure that the 'burning' you created did not affect the metallization layer you had applied? Burning causes tramp contaminants to co-deposit which would not have been deposited at lower current density; it causes the evolution of excess hydrogen gas at the surface which might possibly affect the topography of the joint. But with no disrespect, I think you may be poorly framing the question because electroplating is in large measure an empirical science; so when things are done wrongly or poorly, and it consequently doesn't work, the question of "why" may be unanswerable, and the only meaningful answer may be "do it right instead" :-)

But if you want to continue the discussion anyway, we'll need a lot of data on the plastic, the metallization, the copper plating solution, etc.

Regards,

A. Hi. I'm still not hearing what method you used to make the ABS conductive.

At low current density there is probably no relationship between current density and adhesion. But as you get too high, copper ions cannot move from the solution, through the boundary layer, quickly enough to satisfy all the electrons accumulating on the cathode. They can't migrate fast enough. So those electrons start getting satisfied by electrolyzing the water of the solution into H2 and 2OH, which causes 'burning' (blackening, rough surfaces, and poor quality of deposited metal).

Unfortunately, as previously noted, there is a lot of empirical knowledge involved in the science of electroplating. So usually we cannot say from first principles what the satisfactory upper limit for current density is; rather we only know what works and what doesn't. Plating at greater than about 30-40 ASF usually starts getting problematical unless you can apply extreme agitation to thin the boundary layer. Good luck.

Regards,