Quality Magazine

XRF Calibration Standards: Options for Coating Thickness Measurement

April 9, 2012
If an organization relies on accurate coating thickness measurements in its quality control process, then it is more than likely familiar with ED-XRF technology.



While recent advancements in XRF analyzers have broadened the technologies application reach, one of the most common uses remains the measurement of metal coating thicknesses. From basic single-layer measurement needs through complex coating systems that include multi-layer measurements, thin-films and plating solutions analysis, XRF will provide a non-destructive, fast and relatively inexpensive method to obtain highly accurate measurements.

When considering XRF analyzers for plating thickness applications, a potential user has many options to consider within the general technology. X-ray Fluorescence analyzers are now offered with three different styles of detection systems–proportional counter detectors, Pin Diode detectors (Si-PIN) or Silicon Drift Detection technology (SDD). Each of these has advantages depending on the type of application, desired speed of measurement and how thin or thick the measurements are. Additionally, machines are available that have XRF tubes from 4 watts up to 400 watts, and the list of options goes on and on. The best way to determine what is needed is to know the application and contact an XRF sales company. Tell them what materials are to be tested and the range of thickness values. Most organizations will be offered more than one XRF analyzer for consideration.

All XRF analyzers need to be recalibrated, or adjusted, periodically due to electronic component changes. When enough drift occurs, the instrument may not be able to accurately adjust accordingly to compensate for the differences. Utilizing quality reference standards is critical for periodically verifying the accuracy of the instrument. A daily verification of accuracy should be performed with reference standards before testing product. This is especially true for systems based on the empirical calibration method. A user will want to look for an accredited standards laboratory that specializes in standards for x-ray fluorescence technology and offers certified, traceable standards.

When it comes to purchasing XRF calibration standards for thickness analysis, know what type of format will be used as there are two styles to choose from: plated or foil. Both formats are commonly used throughout the industry and both provide reliable results. When determining the best fit, consideration must be made to the model of XRF analyzer the standards will be used on (including the collimator size(s) available for the application). The model matters because standards can be mounted on different platforms and software algorithms may vary from brand to brand, so the same calibration curve may require a different range and quantity of standards on different analyzers. In addition to the XRF model, the constituents of interest and target thicknesses will play a factor in the best approach to standards.



Foil Standards

This style of reference standard is the most common approach and is built with a foil of a desired material and thickness attached to a stainless steel foil holder. The foil is then temporarily secured on top of a plastic chip containing the desired base material (also known as the infinite). When a range of thicknesses is required, the user will have several foils of different thicknesses to span the desired range and will switch out the top layer when calibrating for the various thickness levels.

When multi-layer thickness measurements are the goal, a user will stack multiple foils onto the infinite. An example of this would be Au/Ni/Cu. A user would select a gold (Au) foil at the appropriate thickness, a nickel (Ni) thickness and the copper (Cu) base then stack them with the gold over the nickel over the copper.



Pros of Foil

  • Versatility: foil standards can be placed over any desired substrate, thereby increasing the range of applications.
  • Multi-Layer Flexibility: they are generally stacked on top of one another for a reliable method of multi-layer analysis.
  • Usable Curves: when using stacked foils for multi-layer analysis there are essentially three useful calibration curves, two single layer curves and one multi-layer curve. By nature, one of the single-layers is the first plated layer and can be used to monitor the plating in-process.



    Cons of Foil
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  • Fragile: while more versatile, the foils themselves are fragile and must be handled with care to avoid punctures. Generally speaking, foil standards are not ideal for everyday use or in environments where they might come into contact with a lot of dirt as they cannot be cleaned.

    Plated Standards

  • These are created by electroplating the desired material at a specific thickness onto a permanent base material. When a range of standards is required, the user will have standard pieces of several different thicknesses to span the desired calibration range (layers and base material in one piece for each thickness). So, if a user is measuring gold over nickel at 50u” to 300u”, they may have a three piece plated set of gold at 50u”, 150u” and 300u”, each over the base material of nickel. A “clean” infinite-only piece (nickel in this example) is also included to make it a four piece set.



    Plated Standard Pros

  • Durability: compared with foil pieces, plated standards tend to be more durable and tolerant of heavy handling. However, they are still prone to damage when the measured surface comes into contact with an object that scratches it. Even the user touching the surface can cause damage due to the dirt from the environment or oils on a user’s hands.
  • Extended Life: fortunately, if scratches or damages do occur on a plated standard, all is not lost. In most cases, the standard can be polished with a fine grit polish and remarked to reflect the new, lower value.



    Cons of Plated

  • No Versatility: with the coating permanently fixed onto a base material, a user will need to have a different plated standard for each thickness and base material combination they require.
  • Potential Variation: it is well documented that certain plated metals, such as lead, gold, silver and tin (to name a few), will migrate over time, thus potentially changing the thickness of the reference material. This change takes a long time but can easily be monitored by having reference standards certified yearly by an accredited laboratory.

    Regardless of the style of calibration standards used, regular certification is suggested. This will typically be a yearly service and is suggested even if it is not required to maintain an organization’s accreditation. Routine certification of standards is suggested because their conditions may change over time due to corrosion, inter-metallic migration, dirt and handling. This can cause a difference between actual coating thickness and the standard’s certified value and may lead to inaccurate measurements that can negatively affect an organization.

    While routine certification of an XRF analyzer is also suggested, it is often noted that the certification of reference standards is more critical to a system achieving sustained performance. Most accredited standards laboratories will have a tracking system in place that monitors certification due dates and contacts clients in advance to help ensure that a critical certification is not missed.



  • The Bottom Line When Measuring Coating Thickness with XRF Technology

    Most XRF instruments are only as accurate as the standards used to calibrate them. In order to control a process, it is an absolute must to have a way of verifying the machines accuracy. If a check-system is not in place now, then one needs to be added. Don’t take a chance on losing customers because a product is under plated and don’t lose money giving away over-plated material.