Archive for Electroless Nickel

Apr
04

Electroless Nickel

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Reducing agent chemicals used to convert the metal ion to elemental form in electroless plating process. The Chemical reducing agents successfully utilized for electroless nickel plating include hypophosphite, hydrazine, borohydride, amine boranes, and certain derivatives of these.

Hypophosphite-based baths

Basic chemical reaction for deposition nickel using hypophosphite as reducing agent:

2H2PO2- + 2H2O + Ni2+ ————->   Ni + H2 + 2HPO32- + 4 H+

Hydride ions may also react with hydrogen ions, thereby reducing the efficiency of hypophosphite utilization.

The electroless nickel deposits are not pure nickel but contain about 3 – 15 % phosphorus, depending on bath composition and process conditions.

Borohydride-based baths

The reaction mechanism occurring with borohydrides  have been quoted as those in equations :

NaBH4 + 4NiCl2 + 8NaOH   ————>  4Ni + NaBO2 + 8NaCl + 6H2O

OR

2NaBH4 + 4NiCl2 + 6NaOH   ————>     2Ni2B + 8NaCl + H2 + 6H2O

Although these are only schematic representations since the details of the actual reactions are still unknown.

These reaction result in the deposition of an alloy containing 5 – 7 % boron at rate of 10 – 30 µm/h. This deposit has a hardness of 500-750 HV as-plated, and 1000-1250 HV after heating at 400oC for 1 hour.

Amine Borane-based baths

The overall deposition reaction can be represed by the following :

R2NH.BH3 + 3Ni2+ + 5OH- ——–>   3Ni + (R2NH2)+ + H3Bo3 + 2H2O

OR

4R2NH.BH3 + 6Ni2+ + 8OH- ———>  2Ni3B  + 4(R2NH2)+ + 2H3Bo3 + 3H2 + 2H2O

Alloys containing between 1 and 5% boron are deposited as a result of the reaction. The rate of deposition depends on the temperature and pH of the bath employed, but is usually between 0.1 and 0.15 µm/min, which is lower than in the much hotter borohydride baths, but quite adequate for the thin initial coatings required on plastics.

Hydrazine-based baths

The baths consist of nickel salt, a complexing agent such as tartrate, malonate or EDTA, and hydrazine as the reducing agent. The pH is adjusted to the desired level with sodium hydroxide.

The depositions produced from hydrazine solution contain 97 – 99.2% nickel. The rate of deposition is very depend on pH, with virtually no deposition below 10.3, but rate deposition over 12 µm/hr at pH 11.0. The as-plated deposit had a hardness of about 500 Knoop (300-g load) but was brittle. Heat treatment for 1 hr at up to 400oC had little effect on hardness. At higher temperature the hardness was decreased

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