Gold Plating

Article From: Products Finishing, from Technic Inc.

Posted on: 7/1/1997

A brief history and current explanation of the process...

Why are there so many different gold baths? Fad and fashion, for the most part. Gold electroplating was originally developed with decoration in mind, and decoration is subject to the whims of those who specify its use.

In the past 148 years, fads and fashions have changed so much that the original Elkington patent has evolved into about 300 different modifications in the United States and about 100 in Europe.

Fashion, being highly changeable, demands only a fraction of the colors and shades at any given time. But most of the baths are asked for in any given 20- or 30-year period.

With the development of the electronics industry in the 1940's and 1950's, platers found a demand for different deposits than had been required for decorative plating. Electronics manufacturers wanted the physical properties of the gold deposits to be modified. They were interested in the conductivity, contact resistance, corrosion resistance, electrical as well as physical wear resistance, and the hardness and purity of the deposits. This has led to the development of over 200 more gold and gold-alloy baths.

The development of hundreds of different gold baths has been paralleled by only about 40 different nickel baths and an equal number of copper baths. The reason for this difference is the price of the metal. The economic laws of gold plating are: use the smallest amount of gold to provide the thickness desired; and plate as fast as possible from a minimum amount of solution, to lower the investment in gold plating facilities.

As the price of an ounce of gold has changed from $20.57 to $35.00 to $850.00 and back to $470.00 (at this writing), the above three economic laws have resulted in great chemical and operating changes in gold plating. The day when a plater could have a single gold solution in the corner of his shop and do any job, competitively, and at a profit, is gone.

Nevertheless, gold plating to meet today's requirements is neither impossible nor overly complicated. At any given time, only certain colors and tints are called for by those who specify decorative gold electroplates, and the electronics industry requires only certain physical properties. Thus only a small number of bath types are really needed at any given time.

In the following sections--Jewelry and Electronic--the golds most widely used now will be described.

Jewelry

Economics and the Trade Practice Rules of the Federal Trade Commission determine how gold is used on jewelry and as decorative finishes. Most costume jewelry that is marked "Gold Flash," or left unmarked, is plated with two to four microinches of gold, over bright nickel. In any season, only 10 to 30 shades at most are in demand in the jewelry-plating centers. Solutions that will economically produce this year's shades are listed in Table I.

TABLE I—Representative Gold "Flash" Baths

Hamiltons
Common Names* English
or 24K
Hard English
or 18K
Yellow
Hamilton
Green
Hamilton
Pink
Hamilton
White Vermillion
or 14K
Rose Green Barrel
Flash
Gold as Potassium
Gold Cyanide
(oz [Troy]/gal)
.2 .2 .15 .15 .15 .05 .2 .5 .25 .1
Free Potassium
Cyanide (oz/gal)
1 1 1 .25 .25 2 0 .5 1 1
Dipotassium
Phosphate (oz/gal)
2-4 2-4 2-4 2-4 2-4 2-4 4-6 -- 2-4 8-12
Sodium Hydroxide
(oz/gal)
-- -- -- -- -- -- -- 2 -- --
Sodium Carbonate
(oz/gal)
-- -- -- -- -- -- -- 4 -- --
Nickel as Potassium
Ni Cyanide (oz/gal)
-- .03-.2 .03 .03 .03 .12 -- -- -- .035
Copper as Potassium
Cu Cyanide (oz/gal)
-- -- .2 -- .04 -- .2-.3 -- -- --
Silver as Potassium
Ag Cyanide
-- -- -- 50 ppm -- -- -- -- 200ppm --
Temperature (F) 140-160 140-160 150-160 140-160 -- -- 150-160 150-180 130-160 130-140
Current Density (asf) 10-40 10-40 10-35 10-40 -- -- 10-40 20-50 10-20 7-10

*These are commonly used names.
The names do not accurately correspond to the colors—and change with time.

While most decorative deposits are two to four microinches thick, there is a growing demand for deposits with thicknesses of seven to about 30 microinches. These finishes are also plated over bright nickel, although sometimes over silver, and are marked "Gold Electroplated." Gold deposits over 100 microinches thick earn the designation of "Heavy Gold Electroplate." Both of these latter thickness ranges require the use of a higher gold concentration and a lower pH. Generally these plating solutions operate at a much lower temperature than is customary when using flash baths.

Typical baths are listed in Table II. Note: the "N" designation of color refers to a European color standard. Color samples are available, and they make matching and maintaining color much easier.

 

TABLE II--Typical Acid Gold Color Baths for Heavy Deposits

1N Color* 2N Color* 3N Color* Yellow
24K
Yellow
22K
Emerald
Green
Gold (Troy oz/gal) .5-1 .5-1 .5-1 .5-1 .5-1 .5-1
Conducting Salts
(oz/gal)
16 16 16 16 16 16
Nickel as Chelate
(oz/gal or ppm)
1-1.4 .5-.8 100-300ppm -- 200ppm 500ppm
Cobalt as Chelate -- -- -- 250ppm 1000ppm --
Brightener -- -- -- -- -- 100ppm
pH 4-4.2 4-4.2 4-4.2 4.4-4.8 4-4.5 4-4.5
Temperature(F) 120-140 100-120 90-100 80-90 90-100 100
Current Density
(asf)
10-20 10-20 10-20 10-20 10-20 10-20
Agitation necessary necessary necessary necessary necessary necessary

 

*European Color Standards

It cannot be stressed too much that color control is dependent on vigorous, and preferably automatic control of all the plating variables. The hobby plater, craftsman or occasional gold plater can get away with casual control. But for long-run production control in plating jewelry items, automatic controls must be used to regulate temperature, agitation and gold concentration. Ampere-hour meter control must be backed up with chemical-laboratory analysis, for not only the gold, but the alloys and electrolytes. As little as five pct deviation in gold concentration, a pH change of as little as 0.25, or a temperature change of only 3F will alter the color of a deposit enough to make it evident.

There are also decorative-color gold baths based on non-cyanide sulfite gold complexes. Besides the advantage of not using cyanide, these baths have very noticeable leveling and very good micro-throwing power. Baths of this type are listed in Table III.

 

TABLE III--Typical Sulfite Gold Baths
Flash
24K Green Pink Heavy Plating
Gold as Sulfite (oz/gal) .15-.25 .15-.25 .15-.25 1-1½
Conducting Salts
(Sulfite)(oz/gal)
12 12 12 6-10
Nickel as Ni Chelate
(oz/gal)
-- .15 .07 --
Copper as Cu Chelate -- -- .07 --
Cadmium as Cd Chelate -- 760ppm -- --
Brightener-often arsenic 20ppm 20ppm 20ppm 20ppm
Current Density (asf) 30-50 30-50 30-50 2-10
Temperature (F) 120-150 120-150 120-150 120-140
Time (seconds) 10-20 15-30 10-20 100 pct. efficient,
100 microinches,
3 asf, 12.5 min.

There are also numerous specialty gold finishes, such as those produced from baths that will plate directly on stainless steel. They are used for hypoallergenic jewelry and other items where the plater desires to decrease the galvanic potential produced when gold is plated over nickel.

Other specialty finishes fall into the general classification of "antique finishes" or "gold smuts." They have popular designations such as "Russian Antique," "Roman Gold," and "Coral Gold" (Table IV).

Electronic Gold Baths
Industrial electronic plating is diverse and specialized. Each industry segment has chosen its gold baths for maximum efficiency and minimum cost. Each has varied every facet of equipment and chemistry to minimize gold usage and still produce satisfactory parts. Baths have been developed to use as little gold per gallon as possible. Equipment has been designed to use tanks with the minimum amount of solution. And, equipment, chemistry and plating conditions have been designed to give the lowest variance of gold thickness from part to part and from point to point on each part. All this at the fastest possible plating speed.

TABLE IV—Antique Baths
Gold Green Gold
Smut
Gold as KAu(CN)2
(Troy oz/gal)
.75 .25
Sodium Hydroxide
(oz/gal)
2.0 --
Sodium Carbonate
(oz/gal)
4.0 --
Ammonium
Carbonate (oz/gal)
-- 5.0
Sodium Cyanide .5 2.0
Silver as KAg(CN)2 -- 1300 ppm
Temperature (F) 160-180 70-90
Current Density
(asf)
30-40 10

It does little good to have the fastest gold plating bath if gold-thickness variance has been neglected. In plating printed-circuit edge connectors, for instance, Xerox reported that using control immersion-tip plating, gold thicknesses varied from 30 to 110 microinches, with an average gold thickness of 60 microinches. By masking and using total immersion plating, the gold thickness varied from 30 to 60 microinches, with an average of 45 microinches. By using an automatic machine, designed for plating edge connectors, the platers found that gold thickness varied from 30 to 40 microinches. This gave an average of 35 microinches. It insured passing the minimum 30-microinch specification, but reduced gold consumption by 40 pct. With the cost of gold now at about $350 per troy ounce, it is well worth modifying the process to effect such savings (Fig. 1).

Just as savings can be realized by decreasing variance from finger to finger, the chemical makeup of the bath and the operating conditions used can significantly decrease the variance of gold from point to point on each finger or part.

This exposition is too short to go into all of the modifications and results; however, processes can be modified to save gold and you should be aware of all the ways to cut costs of gold plating.

To simplify an overall view of industrial or electronic plating, it is probably best to divide the subject into four segments: 1) soft, pure gold on semiconductors; 2) hard, bright gold on contacts and connectors; 3) hard, bright gold on printed-circuit tabs; and 4) soft, pure gold from special electrolytes that do not attack resists used in plating entire circuits.

The latter are used for "chip-on-board" applications. This is a fast-growing new application of gold, and the first one in many years that involves the use of more, not less, gold.

Soft, Pure Gold
The semiconductor industry requires ultra-pure soft golds (60-85 Knoop) that can be soldered easily and die bonded. The baths used to deposit this gold must not only be extremely pure, but must be kept pure in production, to avoid rejects in subsequent manufacturing operations. You must constantly monitor the solutions to insure no buildup of base metals such as iron, nickel, copper and lead.

Occasionally formulators use organic brighteners, and, in some cases arsenic or other semi-metals, in parts per million quantities (Table V).

TABLE V—Electronic Pure Gold
Rack Barrel
Gold (Troy oz/gal) 1-2 1-2
Electrolyte (oz/gal) 16 25
Grain Refiner (ppm) 5-20 5-20
pH 5.8-6.2 5.8-6.2
Temperature (F) 140-160 120-140
Current Density
(asf)
1-20 .5-5

Hard, Bright Gold
Platers who gold plate contacts and connectors generally use bright acid gold formulations. These baths contain complexed cobalt or nickel in small quantities, to improve hardness and brightness of the deposit. Such gold electroplates will be 99.7 to 99.9 pct pure, and hardness can be 120 to 300 Knoop. The small amount of nickel or cobalt will interfere with die bonding, so these baths cannot be used for semiconductor plating. Hard, bright gold baths, if operated with good housekeeping and chemical control, have very long life—often three years and more.

Many of these formulations contain citric acid to maintain the mildly acid pH. An occasional problem in using these solutions is that the low concentration of citric acid in the dragout tank creates an excellent media for the growth of fungus. To avoid this growth, you should regularly filter the solutions in dragout tanks—and some-times the plating solutions as well. Occasionally you should treat both with special fungicide and other growth-limiting agents that do not affect the plated deposit or its characteristics.

The hard, bright gold baths are very suitable for high-speed automatic or continuous plating. Speed is limited only by the degree of agitation and the speed of replenishment of gold at the surface of the cathode (Table VI).

Speeds as high as 1000 asf are common, and occasionally platers have been able to plate at double this speed. However, plating at more moderate rates (600-800 asf) is easier to control and puts less strain on the solution chemistry. Plating speeds have been increased recently by increasing the cathode current efficiency of the solution instead of increasing asf.

Barrel plating of bright, hard, acid gold is now a very mature part of the industry. The new developments are increased cathode current efficiency and better distribution of gold thickness from part to part and from point to point. The saving in gold used in barrel plating is similar to the gold conservation described for printed circuits.

(Fig. 1).

Solutions used in plating bright, hard gold from an acid medium are described in Table VI.

Hard, Bright Gold on Printed-Circuit Tabs
Plating of printed-circuit edge connectors is a major segment of the gold-plating industry. The solutions used are also the acid hard, bright golds. The solutions used are variations of the one listed in the "Heavy" column of Table VI.

The method of plating the printed circuit cards will determine how bath compositions and operating conditions are varied. The best procedure is automatic in-line plating. It allows the lower range of gold concentrations to be employed. Low gold concentrations minimize the capital investment necessary and greatly reduce the gold dragout losses.

Edge connectors also may be plated by using controlled-depth immersion. When using this method it is necessary to use higher gold concentrations, to avoid thin deposits or flaking in the miniscus.

The modern trend in printed-circuit production is to use aqueous or semiaqueous resists. When these are encountered, it is necessary to either add special resist hardeners or to use different electrolytes to avoid removing the resist.

TABLE VI--Bright Electronic Golds
Heavy Barrel High-Speed
Gold (Troy oz/gal) .5-1.5 .5-1 1-3
Electrolyte (oz/gal) 12-16 15-20 20-30
Cobalt/Nickel (ppm) 500-1500 800-1500 600-1200
pH 3.8-4.5 3.8-4.5 4.5-5.0
Temperature (F) 95 95 120
Current Density (asf) 1-50 1-20 100-1000

Soft, Pure Gold, Special Electrolytes
The latest trend in printed-circuit production is to gold plate the entire circuit board. If chips or semiconductors are to be bonded directly to a gold-plated board, a pure, soft gold is needed. These bondable boards can be plated with one of the soft golds listed in Table V. However, if an aqueous or a semi-aqueous resist has been used, it is necessary to use the special electrolytes mentioned above.

Other boards that require gold plating on the entire surface will have to have components soldered on the surface—not die bonded. These can be plated with a modified cobalt- or nickel-brightened gold as listed in Table VI. If the resist is aqueous or semiaqueous, it may be necessary to change the electrolyte.

Immersion Plating

Immersion plating of gold is actually older than gold electroplating. It is practiced mostly in applying low-cost decorative finishes. Its usefulness is limited by the extreme thinness and by the lack of permanence of the deposit.

Electroless deposits of gold are, at present, possible from several baths. But they are mostly a curiosity. The baths are not dependable or easily reproducible.

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