Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 219 of 707

Semiconductors Printed circuits Connectors Table V. Electronic Gold Plating Solutions Alkaline Cyanide Class D — Class E Table VI. Alkaline Cyanide Baths Gold as potassium gold cyanide (g/L) Silver as potassium silver cyanide (g/L) Dipotassium phosphate (g/L) Potassium cyanide (g/L) pH Temperature (° Anodes F) Anode/cathode ratio Agitation Current density (A/ft2 Rack Barrel Current efficiency (%) Time to plate 0.0001 in. Replenishment ) Matte 8-20 — 22-45 15-30 12 120-160 Stainless steel 1:1 Moderate to vigorous 3-5 1-2 90-95 8 min at 5 A/ft2 1 oz gold/4 A-hr Bright 8-20 0.3-0.6 — 60-100 12 60-80 Stainless steel 1:1-5:1 None to moderate 3-8 1-2 90-100 7 min at 6 A/ft2 1 oz gold/4 A-hr cipitates); noncyanide (usually sulfite), pH 9 to 10. Table V lists the baths that are pri- marily 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 sur- face, 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. ALKALINE CYANIDE BATHS (GROUP 1, CLASS D) Table VI lists typical alkaline cyanide baths that are still used. Note that for matte deposits, the higher the temperature the better the deposit and the higher the speed of plating; however, temperatures over 150° F result in a rapid breakdown of the free cyanide to 160° 218 and a buildup of cyanide breakdown impurities. The alkaline cyanide baths are particularly sensitive to organic impurities, both those introduced by drag-in and by the absence of general cleanliness, as well as those caused by cyanide breakdown. To maintain a deposit that has a good appearance and is structurally sound it is necessary to carbon treat and filter the solution period- ically. The grade of carbon used must be pure enough not to introduce more impurities than it removes. Constant filtration through a filter packed with carbon is accepted practice but is not as efficient in removing impurities as a batch treat- ment. If the solution is quite contaminated before treatment it is important to save the used carbon and the used filter cartridge for refining to recover any gold lost in the treatment. The best method to carbon treat a solution is as follows: (1) Heat the solution to 150 F. (2) Transfer the hot solution to an auxiliary tank. (3) Add 1/8 to 1/4 oz carbon Neutral Cyanide Class D — — Acid Cyanide — Class E Class E Noncyanide — — —

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