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Electroless nickel/electroless palladium/immersion gold (ENEPIG) is sometimes referred to as the universal finish, because of the versatility of its applications. It is a multifunctional surface finish, applicable to soldering and wire bonding (gold, aluminum, copper and palladium clad copper). In addition, it is also suitable as the mating surface for soft membrane and steel dome contacts, low insertion force (LIF) and zero insertion force (ZIF) edge connectors, and for press-fit applications. ENEPIG is formed by the sequential deposition of electroless Ni (120–240 µin) followed by 2–12 µin of electroless Pd with an immersion gold flash (1–2 µin) on top.
Electroless Process: This chemical process promotes sustained deposition of a metal or metal alloy onto the PWB surface through an oxidation-reduction chemical reaction, without the application of an external electrical potential. Reducing agents, such as sodium hypophosphite or sodium formate, react at catalytic surfaces to release electrons, which immediately reduce the positively charged metal ions (e.g., nickel ions in ENIG and ENEPIG and palladium ions in ENEPIG), promoting their deposition onto the PWB.
This type of reaction is described as autocatalytic, as the deposition process will continue even after the substrate is completely covered by a continuous layer of the plated deposit. The deposit thickness will therefore continue to rise in the presence of source metal ions and a reducing agent, until the board is removed from the plating bath. The thickness of plated deposits will vary depending on the bath temperature, chemical parameters (such as solution pH) and the amount of time spent in the plating bath.
Immersion Process: This chemical process uses a chemical displacement reaction to deposit a layer of a second metal onto a base metal surface. In this reaction, the base metal dissolves, releasing the electrons that reduce the positively charged ions of the second metal present in solution. Driven by the electrochemical potential difference, the metal ions in solution (e.g., gold ions in ENIG or ENEPIG process) are deposited onto the surface of the board, simultaneously displacing ions of the surface metal into solution.
Editor's Note: This article originally appeared in the May 2015 issue of The PCB Magazine.