Guest Editorial -For Plateworld.com                                                 

 Don Baudrand, Don Baudrand Consulting,   e-mail:donwb@tscnet.com

 

Doís and Doníts of Industrial Silver Plating

Why silver? Silver has the highest electrical conductivity of all metals. Thus its use for electronic devices such as Packaging of integrated circuits, electrical contacts, lead frames, waveguides silver to silver bonding, and replacement for gold without loss of performance and at much lower cost. (Silver surface protection or encapsulation to prevent corrosion of the silver surface is required for some applications.) Other unique properties lead to other applications such as for bearing surface for high load applications. The anti-biotic and antiseptic properties make silver coatings idea l for surgical tools. Also for underwear such as socks that have silvered threads can protect feet form infections and keep feet from having a bad odor. Silver containing gloves and complete armed forces uniforms is being tried, successfully so far.

Most silver plating solutions are alkaline cyanide based. These plating solutions are relatively simple and easy to maintain. The major negative is that they contain cyanide. Unusual precautions must be practiced. Exposure of the plating or silver strike to any acid releases hydrocyanic acid gas that has the ability to penetrate human skin causing poisoning, and when breathed, could be deadly. It is fast acting. If breathed fast first aid must be administered, away from the gas containing area. The gas can penetrate clothing. There are alkaline non-cyanide formulas available form chemical suppliers.

With good precautions, cyanide based plating solutions performs very well and are relatively easy to maintain. When silver plating onto copper, or other reactive metals, a silver strike is required to achieve good adhesion. Without a strike, an immersion (replacement) deposit occurs that has poor if any adhesion to many basis metals, particularly copper. The silver strike is very important to assure adhesion. Close attention to the free cyanide and silver metal content amounts are necessary to keep the best adhesion and efficiency. Silver too high or the cyanide too low can result in poor adhesion. Many platers have the current on and connected to items to be plated for entrance into the strike solution.

A silver strike is usually a formula such as below:

Potassium silver cyanide             0.5 to 2.5 oz/gal (3.5-5 g/L)
Potassium cyanide                       10-13 oz/gal (80-100g/L)
Potassium carbonate                    2 oz/gal. (15 g/L)
Current density                            5 to 10 A/sq.ft. (0.5-1 A/dm sq.)

Stainless steel anodes are preferred in the silver strike to prevent buildup of silver. It is important to monitor the silver content. High silver in the strike results in poor adhesion. Careful monitoring is required to maintain the all the constituents in range.

The formula for silver strike is very important in that adhesion of silver to copper, nickel and other metals depend on the silver strike.

Formulas for cyanide silver and non-cyanide plating solutions can be found in The Metal Finishing Guide book and Vol. 5 of the ASM Metals Handbook, Surface Engineering, also plating chemical suppliers, who are often the best source of information.

There are a number of additives that are used to brighten the deposit and/or harden the silver deposit. Examples are antimony, bismuth, selenium and several organic compounds. It is best to consult with your supplier, since new and often better addition agents are developed and made available.

The recommended anode is pure silver. The anode to cathode areas should be maintained at one to one. Changes in anode area vs. cathode area can cause gain of silver or loss of silver in the solution depending which is one is not in ratio resulting in poor silver deposits. Imbalance of brighteners and/or hardening agents will not work properly.

Chemical suppliers have troubleshooting literature and charts, available on request.

Doís

Be very careful handling cyanide containing chemicals such as silver cyanide, potassium cyanide

Keep acids far away for cyanide containing materials. The release of hydrogen cyanide gas is deadly.

Keep the free cyanide within the recommended range to maintain a constant plating rate and current efficiency.

Keep the silver strike in close chemical balance to assure good adhesion.

Keep all constituents within close parameters by frequent analysis.

Enter parts with the current on. Particularly copper items. Copper is not easily removed from the solution and can cause discoloration of the silver deposit.

Doníts

Let carbonate concentration become too high. Possible roughness, loss of efficiency results.

Allow copper contamination. See above. Copper causes off color, and is not easily removed, often leading to removing the solution and reclaiming the silver and starting over with a new solution.

Allow any particulate matter to enter the solution. Filtration would be required and reclaiming the silver from the cartridge.

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