Electroless plating is a chemical plating technique used to deposit a layer of metal plating without using an outside electrical energy source to charge the plating bath. Unlike electroplating where metal is deposited on objects by reduction precipitation using a DC current, this method uses chemical reduction agents to plate and therefore does not require DC power sources, cathodes, or anodes. Only a thin layer can be deposited by displacement plating or immersion plating where the plated metal (workpiece) is dissolved into a plating solution and the metal in the solution is deposited on the workpiece surface in return. The electroless plating generally refers to an autocatalytic process where the metal is deposited only on the workpiece surfaces with catalytic actions. This method allows you to deposit a thick layer.
In the case of electroless plating using hypophosphite as the reduction agent, Nickel-Phosphorus (Ni-P) alloy plating is obtained, and Nickel-Boron (Ni-B) alloy plating if boron compounds are used. There are Nickel, Cobalt, Palladium, Copper, Silver, and Gold for electroless platings, though the ones that are put to practical industrial uses are Nickel, Copper, and Gold platings.
Electroless Nickel Plating
Nickel-Phosphorus (2~15%) alloy plating which is a representative of the electroless platings is valued high and utilized in various application fields due to its excellent mechanical, electrical, and physical properties with its ability to uniformly coat complex shaped components. The properties are shown in the table below.
Many characteristics can be expected for the Electroless Ni-P Alloy plate coating. The differences in the characteristics are based on the plating baths and conditions, but the differences can be made clear by classifying by the P (Phosphorus) contents in the coatings as shown in the following table.
The low P type has a hard coating and good alkali resistance. Since it has good adhesion to special material such as ITO (Indium Tin oxide), polyimides, and glass, therefore used on electronic components, valves, and composite plating. The medium P type has been in long use, and this type often represents Electroless Nickel Plating when spoken of. With a stable bath, it is characterized by fast precipitation, good adhesion, and good corrosion resistance. The high P type is known for being used as base plating of hard disk substrates due to being non-magnetic. It is also used as the resistive material for ceramic resistors due to its low-temperature coefficient of electrical resistance, as well as for acid resistant components.
There are other electroless nickel platings with boron hydride compounds (Normally DMAB Dimethyl-amino-boron). Ni-B alloy platings do not easily form oxidation films on coating surfaces thus do not discolor when heat treated, has good solder adhesion, has very low electrical conductivity compared to Ni-P alloy plating. But since the baths are unstable, difficult to manage well and expensive, they are typically used on rather special purposes such as semiconductors and electronic components.
Types of Electroless Plating
Gold: Gold-boron alloy plating is used for tiny electrical components.
Silver: Since early times, this method has been used as a silver mirror reaction in various fields. Examples of this plating are the conductive treatment for giving an electroforming mother die the electrical conduction property and a mirror finish performed on the inner surface of thermos bottles.
Chrome: This displacement plating from trivalent chromium is not designed for depositing a thick layer.
Cobalt Alloy: This metal works as magnetic plating and is used for memory apparatus. This type of plating includes cobalt-iron-phosphorus alloy, cobalt-tungsten-phosphorus alloy, cobalt-nickel-manganese alloy and more. Electroless plating composed of only cobalt is also available and used for high-density memory apparatus.
Tin: This method has long been used as immersion tin. This is also classified as displacement plating and is not designed for forming a thick layer. It is used for preventing rust temporarily or improving lubrication and solderability.
Copper: There are two methods available. With the first method, a layer of up to 1 µm can be deposited under normal temperature. In the latter method, a layer of between 20 and 30 µm can be deposited in a high-temperature bath. The first method is used for through-hole plating on printed circuit boards or conductive treatment for general plastic plating. The latter is used for circuit formation of printed circuit boards.
Palladium: This low-cost material with excellent electrical properties is used for electrical contact points and connectors as an alternative of gold plating.
One of the characteristics of electroless nickel plating is that the hardness increases by applying heat treatment after plating. The hardness of Hv500 to 550 right after plating becomes around Hv800 and 1000 by treating with heat at 400℃. Although electroless nickel plating can be applied to most of the metals, plastics, and ceramics, the plating will not be deposited on particular metals, including tin, lead, zinc, cadmium, or antimony. This is because the electroless nickel plating inhibits the catalytic action at the metal deposition. In addition to nickel-phosphorus plating, nickel-boron (1% of boron) alloy is also used for electroless nickel plating. This material is more expensive, but it is the preferred choice for electrical components because of its remarkable solderability and heat resistance performance.
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