Electroless Nickel Coating Thickness and Tolerances
Getting the thickness of plated parts right from the start is crucial, to avoid unnecessary work.
Choosing a Thickness Grade
The typical thickness range for Electroless Nickel Coating is given as light (5 – 13 µm), medium (13 – 25 µm) and heavy (25 – 75 µm). A light deposit is used to give corrosion protection to non-wearing surfaces. A medium deposit is used for general engineering applications. Heavy deposits are used for wear and/or abrasive applications. Choose the thickness grade required for the service conditions that the part will be subjected to.
Coverage on Recesses and Bores
Electroless Nickel coating is different from electroplated coatings in that it builds up by an autocatalytic chemical reaction rather than by an electrical current. This means that there is uniform build-up of coating on all surfaces within recesses, bores, slots and blind holes. The coating builds up at a constant rate on all wetted surfaces and thus is ideal for close tolerance internal features.
Calling Up Tolerances on a Drawing
So draw the coating as a finished dimension on the part drawing. It is common practice to dimension the part just prior to plating at the lower limit of the tolerance on all external surfaces. The plater then has the information needed to quote for the coating to the required thickness on all surfaces. State the deposit thickness as a finished dimension, with a bilateral tolerance, for example 25µm ± 5µm, and check with the plater what he can achieve.
Verifying the Finished Thickness
XRF (X-ray fluorescence) is the preferred non-destructive method to check the deposit thickness of flat surfaces and other accessible areas. On ferrous substrates magnetic gauges are much faster to use. For critical internal features a cross-section is made from a witness coupon, which has been plated in the same plating bath as the parts. This method is non-destructive but only possible for small areas.
When a Part Falls Outside Tolerance
Parts that plate heavily can sometimes be stripped and re-plated. Parts that plate lightly can sometimes be re-plated over the existing deposit. Both of these options carry additional time and cost, and must be agreed with your plater before the part is released for coating. This must be agreed in advance as parts that have been coated and then returned to the customer as being incorrect are not acceptable.
Getting thickness right on plated parts is quite simple once it is incorporated into a drawing at the very beginning of a part’s design.
