How Electroless Nickel Coating Covers Complex Shapes

How Electroless Nickel Coating Covers Complex Shapes

Electroless nickel coatings can cover complex shaped parts, whereas conventional electroplating is unable to coat recessed, bored and blind areas as these are outside the line of sight of the anode and so do not receive a coating.

No Line-of-Sight Problem

The key to the electroless nickel coating process is that the solution reacts with the surface of the part to form a coating. Since there is no anode in the solution throwing charge to exposed surfaces, all wetted surfaces of the part will reduce to form a coating of the same thickness. This means that all internal cavities of the part will be coated to the same depth as the external surfaces.

Internal Bores and Blind Holes

As the solution cannot penetrate a surface that is not in contact with the solution, the circulation of the solution within internal cavities must be adequate to ensure that solution reaches all surfaces within the cavity. In the case of blind holes, a drain or vent hole should be provided to allow solution to exit the hole and for fresh solution to enter the hole during immersion. Where such a hole is not possible, the bath may be agitated or the part racked at an angle to flood the cavity with solution and to remove any spent solution from the cavity.

Threads and Fine Features

Electroless Nickel Coating builds uniformly thick across all geometry features, including threads. As a result, threads do not develop a heavy bead at the thread crest. Typical deposit thickness for electroless nickel coatings is in the range of 5 to 25 microns depending on the specification and how it will be used. This controlled thickness is one of the factors that make electroless nickel coating so suitable for use as a finishing process, particularly where there are tight tolerance features and a coating thickness needs to be designed into as a finishing allowance.

Masking and Pre-Treatment

If some areas are to remain uncoated then these will need to be masked prior to immersion into the electroless nickel bath. The pre-treatment of parts prior to nickel plating (degaring, etching and activation) must also cover all areas intended to be nickel plated. If the plating is to adhere properly to the areas of recessed geometry then the surfaces must be wetted as with all electroless processes. There is more on Electroless Nickel Coating at https://www.poeton.co.uk/surface-treatments/plating/electroless-nickel-plating/.

Part Orientation in the Bath

Part orientation on the rack is also a critical factor as hydrogen gas is produced as the Electroless Nickel solution reacts. The gas can form bubbles which, if they are trapped against a surface, can leave local bare spots. By ensuring that parts are positioned on the rack so that the bubbles can escape upwards, and also checking that all rack contacts do not shade significant areas of the components, the most uniform coating can be achieved.

Getting the geometry right before coating is more important than after.