Precious metal electroplating is commonly used in numerous electronic applications for the medical, aerospace and defense industries. When dealing with precious metals such as gold, rhodium and palladium, accuracy and precision are critical to controlling costs. Contact and connector parts must meet thickness requirements called out in MIL or ASTM specifications, and it’s imperative that minimum thicknesses are met without over-plating, which negatively affects profits.

What Causes Plating Thickness Consistency Issues?

Thickness variation is a significant challenge in the plating industry. This is usually due to a component’s geometry, which ranges from simple flat surfaces to complex, uneven surfaces. Features such as recesses, cavities and holes will have a lower amount of coating deposit compared to outside corners, edges, and flat areas. Underplating must be avoided since this leads to either poor performing parts, or those that don’t meet the specs. The ultimate goal is to achieve consistent thickness that meets the minimum requirements, and avoids overplating and its associated costs.  It therefor becomes critical to measure coating thickness quickly and accurately, and make process adjustments as required.

X-Ray Fluorescense to the Rescue!

Currently, the most popular means of verifying plating thickness is X-Ray Fluorescence Spectrometer (XRF). XRF is a fast, non-destructive method for measuring coating thickness and composition of plating deposits of a broad range of materials with high accuracy. One major advantage of XRF analysis is that thickness and composition of both single and multiple coatings can be measured simultaneously. Additionally, the equipment is easy to operate, and a measurement usually takes just a few seconds.

XRF for Plating Thickness
XRF Systems at AEP

XRF analysis is based on the phenomenon that, when atoms in a material sample are excited by the primary X-radiation, they generate fluorescent radiation, which is picked up by a detector (more on that in a bit). The wavelength or energy of those fluoresced emissions is characteristic of the elemental composition of the sample material. The number of emitted photons at those specific energies represents the number of atoms (mass) of the emitting element that is present in the material. Additionally, coating thickness can be determined either by the strength of the signal from the coating materials or by the attenuation of the base’s material radiation.

The detector takes on a crucial function in XRF analysis. It can detect if the atoms in the sample measured are excited by an X-ray beam to emit fluorescence radiation. The measurement software then evaluates the detected radiation. AEP utilizes two types of detectors in our 8 XRF units. The silicon PIN diode (Si-PIN diode) and the proportional counter tube.

The first detector on the market was the proportional counter tube. This is a counter tube filled with gas. The fluorescence radiation is detected by passing through the window and interacting with the gas. The Si-PIN diode has a higher energy resolution, which makes them perfect for the measurement of very thin single layer parts and parts with multiple plating layers.

Alternative Approaches for Thickness Measurement

Nevertheless, there are applications where the use of the proportional counter tube offers the optimal solution. It is the cost-effective entry point when it comes to routine measurements of known coating systems or alloys, however the proportional counter tube can lose accuracy, as it ages, when analyzing nickel over copper. One might think nickel and copper have very little in common, yet they are only one atomic number away from each other on the periodic table. So as the tube ages the width of the excitation curve expands and the two curves, nickel and copper can overlap. Many XRF systems have a measurement for this which should be periodically looked at, it is the Full Width Half Mass (FWHM). When these curves begin to overlap it is time to change the proportional counter tube. The high-resolution Si-PIN diode is not as susceptible to this condition.