Plating and Metal Finishing

Plating is the general name of surface-covering techniques in which a metal is coated onto a solid surface. Plating is indispensable for the manufacture of computers, mobile phones, and electronic devices as well as for surface treatment techniques. Moreover, it is a key technology for the development of new machines. It is also used for decorative purposes, typically to provide a silver or gold exterior. Thin-film deposition techniques have accomplished plating on scales as small as the width of an atom, so it is appropriate to call some plating applications nanotechnology.

There are several plating methods. For example, in one method, a solid surface is covered with a metal sheet and then plated by heating. Other plating techniques include vapor deposition under vacuum, sputtering, and methods using vacuum conditions or gas. Recently, however, only plating techniques using a liquid tend to be called "plating". Metallizing refers to the process of coating metal on non-metallic objects.

Electroplating

Electroplating is one of the plating techniques. One electroplating method grows a plating film by melting a positive electrode under an outer electric current source and a soluble positive electrode. Another method uses an insoluble metal, for example, platinum, titanium, stainless steel, or carbon, as the positive electrode, adding a metal for plating as a reagent and then depositing the metal from the reagent. The former method is useful for electrotyping, forming a thick plating film, and making copper sheets by continuous plating. The latter method is useful for plating insoluble metals and noble metals and making functional films by using alloys.

Electroless plating

Electroless plating, also known as chemical or auto-catalytic plating, it is a non-galvanic type of plating method that involves several simultaneous reactions in an aqueous solution, which occur without the use of external electrical power. The reaction is accomplished when hydrogen is released by a reducing agent, normally sodium hypophosphite, and oxidized thus producing a negative charge on the surface of the part. The most common electroless plating method is the electroless nickel plating.

Electroless nickel plating

Electroless nickel plating is achieved by forming a thin layer of nickel on other solid material such as metals and plastics. The thickness of the plating film is usually measured in micrometers. It is used for automobiles, computers (HDD), printers and other electronic devices uses this plating technique mainly to prevent the occurrence of oxidation (rust) and to increase the surface hardness of a particular component. Electroless nickel has several advantages versus electro nickel plating.

Basic control points in electroless nickel plating

1. Chemical concentration
2. pH of the solution
3. Contaminants in the solution
4. Temperature
5. Surface area of the substrate or item to be plated.

Electroless nickel plating vs. electro nickel plating

Advantages

1. Does not use electrical power.
2. Even coating on parts surface can be achieved.
3. No sophisticated jigs or rack is required.
4. There is flexibility in plating volume and thickness.
5. The process can plate recesses and blind holes with stable thickness.
6. Chemical replenishment can be monitored automatically.
7. Complex filtration method is not required.

Disadvantages

1. Lifespan of chemicals is limited.
2. Waste treatment cost is high due to the speedy chemical renewal.

Process flow

Before electroless nickel plating, material to be plated must be cleaned by a series of cleaning chemical such as bases and acids, this process is called the pre-treatment process. Failure to remove unwanted "soils" from part's surface would result to poor plating. Each pre-treatment chemical must be followed by water rinsing (nornally two to three times) to remove the chemical that adheres to the surface. Degreasing removes oils from surface; acid cleaning removes scales; activation is done with a weak acid that activates the surface. After the plating process, plated materials must be finshed with anti-oxidation or anti-tarnish chemical (trisodium phosphate, chromate etc) and pure water rinsing to prevent unwanted stains. After rinsing materials must be completely dried or sometimes baked to obtain the full hardness of the plating film.

Since electroless nickel plating is a non-toxic plating, this method is commonly used in medical devices. In our day to day life, we may encounter an item that was electroless nickel plated such as door knobs, kitchen utensils, bathroom fixtures, electrical/mechanical tools and office equipment.