Coating thickness must be specified accurately to ensure that components can be assembled properly. The following information will help you to create a drawing call-up for the electroless nickel plating process.
Typical Thickness Ranges
Typical thickness of general purpose deposits would be in the range of 10µm up to 50µm and can achieve a tolerance of ±2µm to 5µm on external surfaces under ideal bath conditions. However, for very tight tolerance, (±1µm to 2µm) this can only be achieved on very simple geometry and under strict process control. Also cost and time will increase exponentially with reduction in tolerance.
How Geometry Affects Build-Up
Internal surfaces such as bores receive less coating than external surfaces such as flats. This is due to the restricted solution exchange. Deep narrow bores can expect to receive 20 to 30% less coating than the external surface on similar areas of the part. Sharp edges can expect to receive slightly more coating than the surrounding surface. The critical measurement points on the part should be nominated and the minimum thickness specified for these areas. A single nominal thickness for the entire surface is not sufficient.
Phosphorus Content and Measurement
The Phosphorus content of an electroless nickel coating can affect its magnetic properties. High Phosphorus coatings (over about 10% P) are non-magnetic and thus can only be measured using non-magnetic gauges, such as eddy current gauges. Low Phosphorus coatings (below about 10% P) are magnetic and thus can be measured using magnetic induction gauges on a steel substrate.
How Thickness Is Measured
Magnetic induction gauges can be used to measure low-phosphorus coatings deposited onto steel. Eddy-current gauges are generally more suited for non-magnetic coatings deposited onto non-magnetic substrates such as aluminium. However, X-ray fluorescence offers the advantage of a precise measurement at a specific point on a part without physically touching the coating. The cross-section of a coating can also be analysed using metallography; this is the definitive method of measurement but the sample is destroyed in the process. Relevant techniques and standards on coating thickness measurement methods are published by the National Physical Laboratory. There is more on Electroless Nickel Coating at //www.poeton.co.uk/surface-treatments/plating/electroless-nickel-plating.
Writing a Usable Drawing Callout
The key elements for a callout are to state whether a dimension is pre- or post-plate, a thickness range, a Phosphorus content for the coating (a high Phosphorus coating is non-magnetic), and a point on the part to measure thickness. A drawing callout for an electroless nickel coating could read: “12 to 18 µm EN, 8% P min, measured at datum face A.”
A clear specification will save arguments and rework.