XRF at AEPMaintaining plating thickness as specified by a buyer and applicable standards is critical for the quality and longevity of the end product. In order to be sure that a component’s thickness is correct and consistent, tight controls must be maintained during the entire plating process. A key function to achieving this is accurate plating thickness measurement. The collected data helps determine if the job is running as it should, or if adjustments need to be made to achieve the stated thickness requirements.

There are various methods by which plating thicknesses can be measured. For measuring zinc on steel, Eddy Current may be a good choice as long as the base material is electrically conductive but not magnetizable such as aluminum or copper. Magnetic Induction is suitable for measuring zinc or chrome, but the base material must be magnetizable such as iron or steel.

When considering gold plating, the most prominent means of checking thickness are Beta Backscatter, X-Ray Fluorescence (XRF) and Mechanical Cross Sectioning. Beta Backscatter and XRF are non-destructive tests, while Cross Sectioning is a destructive test.

Beta Backscatter Method
Beta Backscatter Method

Beta Backscatter

Beta Backscatter units were introduced over 40 years ago primarily for measuring precious metal plating being used in the electronics industry. They are not difficult to use but require the use of standards to obtain accurate results. The unit functions when electrons or beta particles penetrate the surface of a plated part and interact with the gold layer and are counted as “backscattered” to a detector or Geiger-Muller tube. For accurate readings, the density difference must be at least 20% between the plated layer and the substrate.

X-Ray Fluorescense

XRF technology has been available for about 30 years. It can be used to measure plating thickness and elemental composition. XRF also uses standards, software is available that provides the same function. A power supply and XRF tube create a beam whose size can be defined depending on part size by a collimator. The beam penetrates the plating. Decaying atoms or fluorescence that is indicative of a particular element is detected by a proportional counter or silicone drift detector and the thickness is calculated. Using software, measurements can be downloaded to statistical spreadsheets.

Mechanical Cross Sectioning

Mechanical Cross Sectioning is a process used to check thickness as well as potential plating defects. The skill of the technician performing the process is crucial to getting precise results. Since gold is a soft metal, parts to be measured are first over plated with a harder finish, such as nickel, to prevent smearing. The parts are then positioned in a housing that allows a resin (hot or cold) to be added. Once the resin has cured, the parts are firmly secured in a mold that resembles a small hockey puck. The sample is now ready for lapping, a process by which a surface is rubbed with an abrasive-like sandpaper. This is typically done with a lapping machine. It takes time to get a sufficiently polished surface. Once polished the puck is swabbed with an etch solution. It is then rinsed, dried and ready for microscopic analysis.

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Back Scatter Image Source: Continuous In-Line Chromium Coating Thickness Measurement Methodologies: An Investigation of Current and Potential Technology - Scientific Figure on ResearchGate.