Metal Finishing Guide Book


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Table VII. Neutral Cyanide Solutions Rack or Barrel Plating Gold as potassium gold cyanide (g/L) 8-20 Monopotassium phosphate (g/L) 80 or Potassium citrate (g/L) 70 pH 6.0-8.0 Temperature (°F) 160 Agitation Desired Anodes Platinum-clad columbium Current density (A/ft2) 1-3 Current efficiency (%) 90 Time to plate 0.0001 in. 12 min Replenishment 1 oz gold/4 A-hr High-Speed Continuous Plating 15-30 — 90 4.5-5.5 120-160 Violent Platinum-clad columbium 100-400 95-98 10-20 sec 1 oz gold/4 A-hr Table VIII. Acid Cyanide Plating Solutions Barrel Plating, Matte Bath 1 Gold as potassium gold cyanide (g/L) 8 Citric acid (g/L) 60 Cobalt as cobalt metal (g/L) — or Nickel as nickel metal (g/L) pH 3.8-5.0 Temperature (°F) 120-140 Anodes Platinum clad Agitation Desirable Current density (A/ft2) 1-5 Current efficiency (%) Time to plate 0.0001 in. Replenishment Rack or Barrel Plating, Matte Bath 2 High-Speed Continuous Plating, Bright Bath 2 8 60 0.2-0.5 8-16 90 0.7 3.8-4.5 70-90 Platinum clad or stainless steel Desirable 5-20 30-40 10 min at 10 A/ft2 1 oz gold/12 A-hr 3.8-4.3 70-120 Platinum clad Violent 100-400 30-40 15 sec at 400 A/ft2 1 oz gold/12 A-hr 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 conveniently 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 precipitates); noncyanide (usually sulfite), pH 9 to 10. Table V lists the baths that are primarily used by the industry. Low-karat gold alloys [Group 2 or 3 (Class F)] have not found much application in the United States. The alloying metal generally affects the electrical properties of the gold adversely. As little as 1% of iron will increase the electrical resistance of gold over 1,000%, and similar amounts of other metals have less, but still unacceptable, effects on the conductivity of the gold deposit. Even amounts of alloy much less than 1% will inhibit or totally prevent good welding or die bonding of semiconductor chips to a gold surface. Duplex coatings of a low-karat gold base overplated with a high-karat gold surface, although acceptable in some applications from an electrical point of view, have tended to lose their economic advantages as good engineering and new design have required less total gold. 290

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