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90 PCB007 MAGAZINE I SEPTEMBER 2020 Immersion Gold in ENIG The deposition of gold on the electroless nickel substrate is an immersion reaction. The presence of nickel metal in an electrolyte con- taining gold ions creates a spontaneous depo- sition reaction. The EMF of the cell comes up to + 1.75 v, and the difference between the half potentials of gold +1.5 v (reduced species) and nickel –0.25 v (oxidized species) can be ex- pressed as follows. ENIG reaction: E cell = E Au – E Ni E cell = + 1.5 v – (- 0.25 v) E cell = + 1.75 v Corrosion in ENIG Corrosion may occur when the electrons re- leased by the oxidation of nickel reduces the hydrogen ion present in solution releasing hy- drogen gas. The EMF of the cell is + 0.25 v and is the difference between the half potentials of hydrogen (0.00 v) and nickel (–0.25 v). Electroless nickel/hydrogen ion corrosion re- action: E cell = EH – E Ni E cell = 0.00 v – (- 0.25v) E cell = + 0.25 v The EMF driving force for this reaction is on- ly 14% of the EMF for the gold deposition re- action and would only occur if the availability of the gold ion is interfered with. An example would be localized areas (crevices) where gold is depleted. Nickel corrosion can be mitigated by: • Eliminating crevices in the nickel • Reducing the acidity of the electrolyte (reduced hydrogen ion availability) • Reducing the half potential of nickel by increasing its phosphorous content Immersion Gold in ENEPIG Electroless palladium/immersion gold reac- tion: E cell = E Au – E Pd E cell = + 1.5 v – (+ 0.98v) E cell = + 0.52 v In this galvanic cell, the gold is the reduced species, and the palladium is the oxidized spe- cies. Comparing the EMF of this cell to the EMF of the nickel/gold cell, it is clear that this reac- tion is less driven and would proceed slower than the deposition of gold on nickel. This cre- ates a problem if the underlying nickel is ac- cessible to the gold electrolyte. In this case, the gold would exchange with the nickel layer un- der the palladium and nickel corrosion would occur. For mitigation of nickel corrosion, refer to my previous column titled "Can 'Nickel Cor- rosion' Occur In ENEPIG?" Palladium Catalyst on Copper For an immersion palladium on copper reac- tion: E cell = E Pd – E Cu E cell = + 0.98 v – ( + 0.16v) E cell = + 0.72 v Immersion palladium on copper is an inte- gral part of electroless nickel (EN) deposition. For EN to initiate on the copper substrate, the copper surface must be catalyzed. The cata- lyst is immersion palladium. In this galvanic cell, the palladium is the reduced species, and the copper is the oxidized species. Palladium sits below gold and above copper in the EMF series. Immersion palladium on copper is a spontaneous reaction driven by + 0.72 v. A pal- ladium deposit is specific to the copper sub- strate and will not deposit on laminate or sol- der mask. The uniformity of the palladium catalyst lay- er is critical. The uniformity depends on the pre-treatment of the copper surface, which must be free of oxidation or any contaminants. An uneven palladium distribution would lead to uneven EN initiation, generating crevices in the EN deposit, which are potential sites for nickel corrosion. Immersion silver on copper: E cell = E Ag – E Cu E cell = + 0.8 v – ( + 0.34v) E cell = + 0.46 v