Guest Editorial -For Plateworld.com                                                 

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

 

Doí s and Doníts of Industrial gold plating

Gold metal has unique properties that make it the best metal to use for many semi-conductor, electrical contacts, and printed circuits. Pure gold has good current carrying capacity, corrosion projection, and will not corrode in most environments found in nature. In electronic applications, gold is used for corrosion protection, low contact resistance, contacts and its ability to form a low temperature eutectic alloy with silicon that is useful for semi-conductors (Chips). The addition of nickel or cobalt to the gold solutions produces deposits that are hard, wear resistant and protective suitable for plug-in connectors, or sliding contacts. Nickel and cobalt hardened gold deposits are not suitable for jewelry or anything that comes in contact to people skin due to allergic reaction by some people.

We are all aware of the extraordinary high price for gold. People who electroplate gold and gold alloys know the importance of conserving gold as much as possible. The use of double drag-out rinses is now common. Some platers use fog spray over the tank for a few seconds before exiting to the drag-out tank. A bump and/or shake of the racks over the gold tank before exiting is used by some platers. Careful analysis of the gold plating solution on a regular time intervals to maintain constant plating rate is used.

Gold plating solutions that produce pure gold are usually plated from potassium gold cyanide solutions with no additives. The pure gold deposits are ductile, low stress and corrosion resistant, and can be heated to 500C (932F). Pure gold deposits are also suitable for wire bonding, contact applications, such as" wafer bumping," microwave manufacturing and electrical conductivity applications. Porosity free 99.9% is required for thermal bonding. Hard gold, containing nickel, cobalt, or other elements for special purpose applications are suitable for sliding contacts, plug-in connections and repeated contact applications.

The "hard gold" plating solutions produce a polymer as part of the surface structure that contributes as an aid to the sliding and plug-in applications, but can interfere with contacts where light pressure applications are used. Hard gold formulas usually contain a small amount of nickel, cobalt, iron or less often other hardening agents. Some organic addition agents for special purposes are available.

Another gold formulation is based on sulfite compounds instead of cyanide. These solutions are less hazardous. They can produce thick, smooth, bright ductile deposits with low stress. The sulfite formulations show better throwing power than cyanide containing formulas. They are non-toxic and plate at pH 9 to 10. Newer formulations are stable to pH 4.5.


Other formulations include thiosulfite, mixed sulfite-thiosulfate, nitrosulfite complex, hydantoin ligands, etc.

Agitation of gold solutions is important. It helps bring the supply of gold to the cathode and increases efficiency. However high agitation can alter the metal distribution because deposition is a diffusion process (Helmholtz double layer) high agitation favors thicker deposits in the high current density areas.

Doís

Be aware that most gold plating solutions contain cyanides. Use caution in handling the plating solutions. Keep the gold solution away from acid. Hydrogen deadly cyanide gas can be evolved. Sulfite gold solutions do not contain cyanide but if exposed to acids can cause hydrogen sulfide gas that smells like rotten eggs, and is toxic.

Everything possible to conserve gold solutions. Keep all gold containing materials safe locked away. Keep a secure inventory.

Keep gold solution in the tanks by using delayed transfer, bumping the racks of gold parts over the gold plating solution and/or pour DI water over the rack of parts or use a fog spray over the parts lifted above the gold solution tank before transferring to the drag-out rinses.

Select the best gold formula for the application: hard gold for plug in contacts, soft gold for wire bonding and high electrical conductivity.

Use Sulfite gold solutions for high throwing power and high purity deposits.

Use moderate solution agitation.

Doníts

Leave the gold plated items long enough to dry, or partially dry. Rinsing becomes more difficult.

Leave gold plated items unattended nor not locked in a safe place.

Add acid to a cyanide gold solution, release of hydrogen cyanide (a deadly gas) will take place.) Neglect minor pH adjustments that can be done using dilute acid following methods in the operating "Data Sheet" from your supplier of gold plating solutions.

Over agitate, this leads to poor thickness distribution, favoring high CD areas.

Reference: Todd A. Green, University of Newcastle "Gold Electroplating For Microelectronic, Optoelectronic and Microsystem Applications

Military Specification MIL-G-454204, Gold bulletin 2007.40/2.

Peter Wilkinson Understanding Gold Plating, Engelhard ltd Cinderford Gloucestershire, United Kingdom

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