Electroless Plating is the process of applying one or more layers of a metal to a substrate without the use of an external electrical charge.
It is also sometimes called autocatalytic because the metal being applied is in solution and adheres itself to the substrate (plastic or ceramic included) without the use of an outside power source, (electric current, for example in electroplating).
Electroless plating is also used to deposit a conductive surface on a non-conductive object to allow it to be electroplated due to the lack of electricity, electroless plating results in a completely uniform deposit, even on complex shapes, to improve wear and/or corrosion resistance, or to improve the solderability on selected materials.
The electroless nickel process provides a unique combination of high quality, rapid throughput and turn-round with superb performance due to its unique characteristics primarily achieved due to its lack of need for an electric current.
The coating thickness ranges from 2.5 to 150µm, but most components used for engineering purposes have a thickness of around 50 - 100µm.
The greater thickness is usually required when rough surfaces are encountered, the process hasn't been around as long as Electroplating but since being discovered in 1946 has become more popular each year.
Note that electroless plating differs from immersion plating in that desired thicknesses can be achieved as opposed to immersion plating where coverage with only nominal thickness is achieved.