Process description: Autocatalytic plating
Back to Autocatalytic plating in metal industry
- Autocatalytic plating (catalytic chemically reduced coatings)
This is also known as electroless plating according to BS EN ISO 4527:2003, but the term should be avoided. The fundamental reaction requires the presence of a catalytic metal – the metal being deposited – that allows the reaction to proceed. The advantages of the system are:
- Provided there is sufficient agitation to maintain fresh undepleted solution in contact with all surfaces at all times, the deposit is uniform over the entire surface even if the shape is highly complex.
- Deposits are usually less porous then the same metal deposited electrolytically.
- Racking or fixing is greatly simplified.
- Non-conductors are coated (such as plastic).
- Deposits often have special chemical or physical properties since the deposit is an alloy of the metal and a compound formed from the reducing agent. For example, with hydrophosphite the deposit is an alloy of the metal and the metal phosphide, and with boron reducing agents the metal and the metal boride.
- Autocatalytic nickel on metals
Autocatalytic nickel electrolytes are based on nickel sulphate and nickel chloride (nickel 2 – 10 g/l). Sodium hydrophosphite (10 – 50 g/l) is the most often used reducing agent. Solutions also contain chelating agents (organic carboxylic acids 10 – 50 g/l) and buffers as sodium hydroxide and sodium carbonate. Cadmium may be present in some formulations as a brightener, 1-5 mg/l giving approximately 0.03% in the deposit. Lead may be used in some formulations as an alternative up to 3 mg/l. Modern formulations are available which avoid the use of both cadmium and lead, in line with the requirements of the End-of-Life Vehicle Directive.
Autocatalytically deposited nickel-alloy layers contain 2 – 15 % phosphorus. Their key properties are:
- Uniform thickness of deposit irrespective of the size and shape of items to be plated, provided that the plating solution can freely circulate around all surfaces of the components to be treated.
- deposit thickness can be controlled accurately,
- surface hardness as high as electroplated finishes and within the range 350-750 Knoop
- high resistance against wear and abrasion
- inherent high resistance to corrosion resistance
- good adhesion on base material
- special properties such as natural lubricity, easily soldered to, magnetic properties
Applications include (among may others):
- data storage devices as rigid memory discs
- components for chemical and oil and gas industry
- automotive, machine tool and electronics industries
- plastics molding tools
- Autocatalytic nickel coating for plastics
Similarly to autocatalytic copper, electroless nickel solutions are used for generating a first conductive metal layer on plastic surfaces prios to further electrolytic metal (copper, nickel) deposition.
Plastics require etching before depositon. Process solutions contain nickel sulphate or nickel chloride (nickel 2-5 g/l), reducing agents e.g. sodium hypophosphite (5-20 g/l), dimethylaminoborane (>10 g/l) and optionally, chelating compounds such as organic acids. Both weakly acidic (sulphuric acid at pH 3-6) and alkaline solutions (sodium hydroxide or ammonia hydroxide at pH 8-10) are in use.
An example of a process line plating on plastics (PCBs) using autocatalytic nickel is shown in Figure 2.7.
Autocatalytic copper on metals and plasticsAutocatalytic copper plating is till a key process in printed circuit boards and as well as for the metallisation of plastics. The main properties of the copper layers are uniform thickness, and fine crystalline, ductile layers with low internal stress. Copper layers are applied on small items as buttons, fashion jewellery as well as on plastic housings for electric shielding and printed circuit boards.
Plastics require etching before deposition. Copper deposition starts on metal nuclei such as palladium and continues automatically, thus providing an initial conductive layer. The deposition rate is 5-8 μm/h. The solution has a copper content of 2-5 g/l, with sodium hydroxide (15-20 g/l), chelating agents such as EDTA or similar (10-15 g/l) or tartrates (5-10 g/l) and reducing agents, e.g. formaldehyde (3-5 g/l).
The process solution lifetime is limited by the build-up of reaction products.
Source: BAT Surface Treatment of Metals and Plastic, Aug. 2006.
