Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 217 of 707

Table III. A Selection of Typical Acid Gold Color Baths for Thick Deposits Hamilton 22k Gold (g/L) Conducting salts (g/L) Nickel as nickel metal (g/L) Cobalt as cobalt metal (g/L) pH Temperature (° F) Current density (A/ft2 Agitation ) Yellow 4-8 120 0.2 1 4.0-4.5 90-100 10-20 Required 24k Yellow 4-8 120 — 0.5 4.4-4.8 80-90 10-20 Required 1N 4-8 120 7-10 — 4.0-4.2 120-140 10-20 Required 2N 4-8 120 4-6 — 4.0-4.2 100-120 10-20 3N 4-8 120 1-3 — 4.0-4.2 90-100 10-20 Required Required 2.Relieve the highlights on a deeply recessed piece or the flat surface on a fil- igreed piece by either hand rubbing with pumice and sodium bicarbonate or wheel relieving with a cotton buff, tampico brush, or a brass or nickel-silver wheel. Other methods are possible. 3.Flash gold or a gold alloy deposit on the imperfectly cleaned highlights. Typical formulations for antique gold baths are provided in Table II. The more the solutions in Table II are abused and the more the operator violates good plating practice and good cleanliness the better and more distinctive the finish will be. An expensive finish requiring double-racking, but a beautiful finish, is Russian antique. This may be produced by relieving the green-gold antique in Table II and then flashing over with the 24K or English gold. The old antique baths of the 1940s and 1950s that did not require double-racking or stringing are no longer practical because of the high price of gold. HEAVY DECORATIVE GOLD (CLASSES C-1 AND C-2) It is necessary to distinguish between the actual karat assay of a gold alloy electroplate and the apparent karat color of the plate. In general a decorative karat deposit will appear to be a much lower karat than it actually will assay. A 14K color deposit may actually assay 20 to 21K. The formulas in Table III will deposit karat colors but will actually assay a high- er karat. (In computing costs it is best to assume that the deposit is pure gold.) INDUSTRIAL/ELECTRONIC GOLD PLATING Gold is electroplated for many different electrical and electronic purposes; however, today the majority of gold plating is applied to three specific classes of components: semiconductors, printed/etched circuits, and contacts/connectors. The requirements for the deposit of each of these components and the methods of plating that are used are listed in Table IV. The gold plating solutions that are actually used by the electronic plater may be con- veniently classified by pH range: alkaline cyanide, pH >10; neutral cyanide, pH 6 to 9; acid cyanide, pH 3.5 to 5 (below pH 3.5 the gold cyanide is generally unstable and pre- Table IV. Industrial/Electronic Gold Plating Plating Method Knoop Purity Semiconductors Printed/etched circuits Contacts/connectors 216 99.95% 99.5-99.7% 99.5-99.7% Hardness No. Surface Rack 60-80 120-180 120-180 Matte Bright Bright Yes Yes Yes Barrel Continuous Yes No Yes Yes Yes Yes

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