Electroplating and electroforming involve certain chemical and physical processes that are always seen as part of every job, and as such can be planned out well in advance, presenting few surprises. Other physical and chemical processes are different, only occurring as result of unusual factors, e.g. odd object geometry (edges, corners, deep grooves; very tiny thru-holes, extremely thin plates with large surface area, etc.). Multi-metal electrical "batteries" may be established upon immersion in liquids. Such jobs may not always be easy to plan out. With time and investigation comes a "feel" for the processes involved. This needs to be communicated to others involved in a project, so that they may see why the plater thinks a certain approach will work or will fail.
To illustrate: consider a child's paper airplane. Flies nicely! Now imagine making such a plane 10,000 times scale. Would you expect it to fly? Probably not. But could you explain to others why? This is the situation that must be attended for one doing electroplating or electroforming jobs with unusual features.
Naturally, the primary users of electroplating and electroforming processes would include Industry. To maximize profit, most industry would avoid, as much as possible, undesirable factors such as those mentioned above. Unfortunately, the very nature of microwave electronics, as it is currently known, requires device design incorporating just such features, routinely. Since the Observatory is actively on the forefront of high-frequency microwave technology, and because NRAO is actively involved in education, it is appropriate to describe here for public benefit what NRAO has learned in its electroplating and electroforming endeavors. Immediately below is a list of some topics I will consider in this work page ...
In addition to a complex array of metals being used, some pieces possess many blind threaded holes, which trap the machining oils, polishing grits, cleaners, and traces of baths that are used to make them and finish them.
The parts are incorporated into small, high-frequency units for radio telescope applications, or other similar use. It has not been easy to develop procedures giving almost 100% yield and that are relatively cost-effective. Devices may even have to undergo repetitive temperature cycling, from room temperature to liquid helium temperatures over and over again.
At NRAO we formerly used the following, single solution, filtered:
The butyl cellosolve is both water soluble and dissolves in grease/oil, making an almost universal solvent for common solubles. The acetic acid mostly dissolves metallic oxides, carbonates, and such. It does little dissolving of the common metals. The sulfamic acid is a polishing agent, and the polytergent is an acid-stable surfactant. Occasional filtration of this liquid to get rid of particulate matter such as metal filings and polishing grit was the only routinely necessary conditioning for the cleaner. The cleaner works best with ultrasound. We used this cleaner for ordinary copper, OFHC copper, ETP copper, tellurium copper, selenium copper, beryllium copper, brass, aluminum, nickel, silver, solder, silver-solder, and gold.
Now we combine a larger portion of surfactant, acetic acid, and butyl cellosolve with little or no water. This provides greater solvent attack, and greater surfactancy. Then, we use a simple water rinse, followed by a several second dip in Oxy-Metal Industry's Actane 73 Cleaner, 20%. This brightens the metal, and prevents any immersion plating of one metal in an alloy back onto the metal surface. Superior pit-free surfaces result with this process. Lastly, a distilled water rinse, followed by electrically hot entry into the plating bath.
Some parts are cleaned and brightened first, if dirt is especially embedded, with a paste of Comet(tm) cleanser and concentrated dishwashing detergent. A tooth brush is used to rub the piece with this admixture. For aluminum oxide removal from aluminum (and surface activation), we use a 20% sodium hydroxide solution, followed by a thorough water rinse, followed by a 40% solution of concentrated nitric acid, followed by a thorough water rinse (until water sheets off with no beading up).
First, generally, a piece must be examined for geometry, cleanness, composition, etc. Then any necessary masking is done with electrical tape and/or vinyl-based coating. The vinyl is often applied over all edges of the tape in order to prevent unravelling and leaching under its surface and attacking underlying metal surfaces. The remaining exposed areas must have their surface area calculated or fairly accurately estimated. Whether this number will be used straight-out or adjusted in calculating current to be used, is determined on an individual basis. Next, the piece is thoroughly ultrasonically cleaned, just previous to plating, using one or more baths.
We use both electrolytic baths and the baths formerly referred to as electroless (and which for simplicity's sake, as well as mental imagery, I will use here). The former requires current supplied from a cell or power supply. Metal ions are reduced to metal atoms by electrons from the power source. Not to be totally ignored, the anions or negative ions and/or water from the bath are also affected. The electroless-bath plating is produced from electron changes, but from within the ingredients themselves. For instance, one cupric ion (+2) becomes one neutral copper atom (0) under the influence of a reducing agent such as formaldehyde. Each bath type has its advantages, and in fact, in some cases a job can only be done with one of the bath types.
What particular metals do we plate here? Well, primarily two -- gold and copper. Then nickel of various types, silver, rarely platinum, rhodium, chromium, tin.
Now to the particular metals individually:
Copper electroless plating involves a source of copper. For this purpose, I manufacture my own cupric lactate. EDTA chelates (crab claws) the copper end of copper lactate molecules, creating a structure with just the right strength of copper bond (and other properties) to set the stage for plating. Addition of sodium hydroxide and formaldehyde initiates the plating.
Electroless copper uses no external power source -- just chemical instability that produces a shift of electrons, producing metal plate. This process would produce random particles and pieces of floating copper in the bath and produce no useful result were it not that accelerated deposition occurs at specific sites if one prepares the those sites in advance. For instance, take a quartz wheel or window used in coaxial work: the outer and inner rims are frosted. The top and bottom surfaces are smooth. Now immerse the quartz in a mildly acidified stannous chloride solution for a few minutes. Tin manages to attach to the glass. It strongly adheres exclusively at roughened locations, however. Now lightly rinse the piece and immerse in slightly acidified palladous chloride. Palladium either replaces or immersion plates the tin. Lightly rinse. Dip the piece in just mixed parts A and B of electroless copper. Reaction proceeds. Palladium acts as a catalyst to the reaction. Rinse the piece off when the reaction slows considerably, and you see copper only on the inside and outside rims of the quartz. Wrapping a wire around the rims and electroplating allows you to thicken the copper edges to the point where you can solder the quartz window into place!
Electrolytic Copper -- Cyanide Copper Flash Bath
Electrolytic copper baths are typically divided up into class according to use. Copper cyanide baths are typically used to flash other metals in preparation for some other purpose. For example, an aluminum form or mandrel might be zinc immersion-coated. It may be the goal to grow a quarter inch of copper on this mandrel. If the zincated mandrel were directly immersed in the typical acid-copper bath directly, the zincate would dissolve before the copper could grow over it, and since direct copper plating of aluminum is unsatisfactory, the part could be ruined. So a mildly alkaline plating of cyanide copper -- not so harsh to the zinc as the strongly acid bath--is used to plate the zinc before it dissolves off the aluminum, resulting in a satisfactory job. The piece can then be plated additionally in another bath. We use cyanide copper baths with OFHC copper anodes circularly arranged, and elevate the temperature to 34.5 degrees Celsius. Pieces go in electrically hot. It should be noted that slight agitation should be used here, since generated hydrogen gas assists in forming a bright, quality copper plate.
Electrolytic Copper -- Acid Copper Sulfate Bath
Next we have the Acid-Copper-Sulfate Bath. This bath contains cupric sulfate, sulfuric acid, perhaps formaldehyde, a tiny trace of chloride ion, and additional brighteners. In the bath used in the past, we added orthosphosphoric acid for deposit-smoothing properties over a broad spectrum of current densities. A proprietary bath is used, so it is hard to say what ALL the ingredients are.
This bath uses phosphorized-copper anodes of about one square foot surface area. They are triple-bagged to prevent sludge that forms on the anodes from finding its way into the bath, damaging or destroying resulting plating. The piece(s) to be plated become the cathode(s). Current densities must be in the 10-30 Amps-per-square-foot of surface area range for best results. That is for the cathode(s). The anodes should be in the 5-15 Asf range, probably. Agitation must be sufficient for the piece to avoid polarization due to a metal ion depletion region right next to the cathode(s), which would harm plating, perhaps leading to a breakdown of water and generation of nascent hydrogen at the piece(s) and nascent oxygen at the anodes.
There is no real value in discussing NRAO's use of electroless gold, here, because it is so seldom used, of proprietary origin, and, no doubt, adequately discussed elsewhere.
Non-Alloyed Cyanide Electrolytic Gold
We use a commercial bath of the above description that is a citrate-phosphate based gold bath at a pH of 5.75 and that has no "free" cyanide in it. Free cyanide is excess sodium or potassium cyanide, above what is needed to dissolve gold cyanide to make a sodium gold cyanide complex. It produces a soft, crystalline, and pure gold deposit, suitable for wire-bonding and thermal-conductivity purposes.
Non-Alloyed Sulfite-Based Electrolytic Gold
Our commercial bath that is sulfite-based uses sodium gold sulfite. There is a trace of arsenic brightener, although very little of the arsenic actually gets incorporated into the plating. It produces an amorphous gold that is relatively hard and very shiny. The plate is over 99% pure according to specs, and is suitable for corrosion resistance and aesthetic purposes. It is quite good for high-frequency electrical purposes. It resists "dogbone" growth. Still, when thermal properties must be of foremost consideration, or when compressibility is extremely important, the cyanide-gold is preferred. One more situation where cyanide-gold is preferred is in the plating of feedhorns and such, where deep narrow recesses are involved. These areas are low current-density areas, due to Reynolds shielding. BDT produces a dark, almost black deposit in these extremely shielded areas, especially where there is a prevalence of non-shielded surfaces. Cyanide gold produces no such dark deposit.
The NRAO intends to convert to a proprietary acidic phosphorous electroless nickel (EN) in the near future.
Woods-Type Stainless Steel Activation Nickel
Nickel chloride hexahydrate and concentrated hydrochloric acid are the ingredients in the bath, which is operated at room temperature at 100 Asf, typically for 3 minutes. We tend to use this bath infrequently now, since most of our stainless steel preparation is done by plating monoatomic hydrogen onto the steel, using 30% H2SO4, with the piece being cathodic at 125 Asf for 4 minutes, followed by immediate plating in the acid sulfate copper bath.
Nickel sulfate hexahydrate, combined with boric acid, sodium paraphenolsulfonate, and formaldehyde (pH adjusted to 4.0) is the bath, and is operated at 50 deg.C, 32-1/2 Asf, nickel anode, 7 minutes maximum. This is because the bath is not optimal, but sufficient for NRAO's sporadic use.
Sulfamate Electroforming Nickel
A commercial nickel electroforming bath is used and optimized by choosing conditions so that stresses are as low as possible. The nickel is soluble in hydrochloric acid, so 10% sodium hydroxide is used to dissolve out the aluminum mandrel.
Black Nickel is made from nickelous carbonate, Zinc, sulfuric acid, ammonium hydroxide, and sodium thiocyanate. The pH is adjusted to 5.75, and nickel anodes are used, at 1 or 2 Asf. The plate is black and antireflective. It has not recently been used.
Electroforms are usually made of nickel or copper. The manufacture of nickel electroforms probably greatly outclasses the manufacture of copper ones. Nevertheless, at NRAO copper electroforming is almost exclusively the variety of choice. Nickel may be stronger, but it is electrically contraindicated due to its lossy nature at high frequencies. Copper tends to be a little more uniform in its throwing properties. Hence wall thicknesses tend to be more uniform, and plating has an easier time of it in corners and grooves or wells with copper than with nickel.
In industry, corners, if used at all, may be beveled. Grooves are not usually deeper than they are wide (1:1 or less). This ratio is called "aspect ratio", and the 1:1 maximum ratio is due to difficulty in electroforming anything with a greater aspect ratio. Plate tends to be thin and often fragile in recesses if aspect ratio is very high. Agitation is important, and especially difficult for large electroforms, so often bubbled air is used for the purpose. Hence, worker health could come into play, since for copper, either sulfuric or pyrophosphoric acids are typically involved. Cost, when it is of overriding importance, requires minimal machining of resulting parts, and so perfection and uniformity of the outside surface may be sought and achieved by means of shields.
On the other hand, in an educational institution, especially one where state-of-the-art devices must be made and cost may not rule supreme, circumstances allow for different approaches -- in fact, often require them. For example, at NRAO, despite the literature NO! NO! to greater than 1:1 groove aspect ratios, NRAO has gone as far as 5.7:1. Corners and sharp edges are an engineering must for gigahertz electronic applications. In addition, where at most a few different methods may be required to construct a company's product line, dozens and dozens of seemingly endless different procedures must be used on a day-to-day basis at the Observatory.
In a nutshell, the main difference between electroplating and electroforming is how thick the coating is grown! Electroplating is primarily meant as a surface phenomenon, whereas electroforming is a thicker growth, perhaps for an actual structure formation. The chemistry is sometimes a little different. Generally electroforming metal must be laid smoothly and brightly so that growth in the long run is not dendritic --- fingers of metal reaching out. The result of electroforming is known as an electroform.
National Radio Astronomy Observatory
2015 Ivy Road, Suite 219
Charlottesville, VA 22903-1733
I like email responses to my page, but give you my address in such a way as to discourage spammers.
This Page was last modified on: 1/11/99