Accelerators

Complexing agents reduce the speed of deposition and can cause the plating rate to become uneconomically slow. To overcome this, organic additives, called accelerators or exultants, are often added to the plating solution in small amounts. Accelerators are thought to function by loosening the bond between hydrogen and phosphorous atoms in the hypophosphite molecule, allowing it to be more easily removed and absorbed onto the catalytic surface. Accelerators activate the hypophosphite ion and speed the reaction shown in Eq 1 (Ref 2, 3). In hypophosphite-reduced solutions, succinic acid is the accelerator most frequently used. Other carbonic acids, soluble fluorides, and some solvents, however, have also been used (Ref 2). The effect of succinate additions upon deposition rate is illustrated in Fig. 3 (Ref 3).

Fig. 3 Effect of succinate additions on the plating rate of an electroless nickel solution. Solutions contain 16 g/L (2.1 oz/gal) nickel chloride (NiCl2) and 24 g/L (3.2 oz/gal) sodium hypophosphite (NaH2PO2). 5 g/L (0.7 oz/gal) ammonium hydroxide (NH4OH) and 1 mg/L (4 mg/gal) lead at 5 pH and 95 °C (205 °F).

Fig. 3 Effect of succinate additions on the plating rate of an electroless nickel solution. Solutions contain 16 g/L (2.1 oz/gal) nickel chloride (NiCl2) and 24 g/L (3.2 oz/gal) sodium hypophosphite (NaH2PO2). 5 g/L (0.7 oz/gal) ammonium hydroxide (NH4OH) and 1 mg/L (4 mg/gal) lead at 5 pH and 95 °C (205 °F).

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