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Glossary of Industry Terms

Gold Applications on PCBs
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There are a number of different combinations of gold types and processes that are used to place gold as a final finish on a printed circuit board.
Gold types
  • “Soft” gold – 99.99% pure – 24 carat gold.
  • “Hard” gold – Less than 99.9% pure – will contain 1 or more carats of silver or copper.
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    Processes
  • Electroplating - The process of electrically depositing precious or other metal of varying thickness onto the surface of another material such as base metal.
  • Electro less - The depositing of a metallic coating onto a surface without the use of an electric current, relying instead upon a controlled chemical reaction. In an "electro less" plating bath, a chemical that is already in the bath, which is called a reducing agent, donates the electrons. In an electro less copper bath the formaldehyde is the reducing agent.
  • Immersion - In an immersion-plating bath, the electrons are supplied by the base metal, and effectively the base metal is the reducing agent. As the base metal donates electrons to the metal being plated, the base metal itself goes into solution, which is why this is also called a replacement reaction. The key characteristics of the immersion plating bath is that it is self limiting, which means that once the base metal, copper or nickel, in the case of a PCB, is covered, the plating ceases. This can result in a very thin, dense, non-porous coating.
  • Flash – An extremely thin plating of metal using electroplating methods.
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    Applications
  • Electroplated hard gold is primarily used on areas like edge fingers or push pads contacts that will receive an extreme amount of wear or use. The thickness may vary but typically between 25 and 30 micro inches of gold are plated over 150 to 200 micro inches of nickel. This finish is not acceptable for wire bonding. Electroplated soft gold is done for wire bonding to the plated areas. This finish is used for both aluminum wedge and gold wedge & ball wire bonding.
  • Electro less Nickel Immersion Gold (ENIG) is the deposition of nickel followed by immersion gold. A typical finish is 3-5µm of electro less nickel under 0.03-0.05µm of gold. This surface will readily accept aluminum wedge and ball wire bonding, but is usually not robust enough for gold wire bonding.
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    Common Terminology
  • “Deep” Gold - General description used to indicate that full panels are placed in the plating tank– all processes would be deep gold except “tab plated” edge fingers. Not a good fabrication term.
  • “Body” Gold – Indicates that the full body of the board is plated gold. This may occur during either electro plating or immersion depending on the design. Again not a good fabrication term.
  • “Selective” Gold – Indicates that specific areas in the interior of the board would be plated. This does not include edge fingers. This term is an acceptable description for manufacturing.
  • “Tab plated” Gold – Actually describes the processing equipment that can be used for plating edge fingers. This is typically an electroplating operation and is not applicable for selective, or immersion processing.
  • “Flash Gold” – The term indicates a very thin application of electroplated gold on the board surface.
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