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44 The PCB Magazine • May 2015 FEaturE Electroless nickel/electroless palladium/im- mersion gold (ENEPIG) is sometimes referred to as the universal finish, because of the versa- tility of its applications. It is a multifunctional surface finish, applicable to soldering and wire bonding (gold, aluminum, copper and palladi- um 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 electro- less Pd with an immersion gold flash (1–2 μin) on top. Chemical Definitions Electroless Process: This chemical process promotes sustained deposition of a metal or metal alloy onto the PWB surface through an by George Milad UyeMURa inTeRnaTional CoRPoRaTion ENEPIG: The Plating Process oxidation-reduction chemical reaction, without the application of an external electrical poten- tial. Reducing agents, such as sodium hypo- phosphite 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 autocat- alytic, 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 reduc- ing 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 sur- face. In this reaction, the base metal dissolves,

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