X-Ray Fluorescence (XRF) is a fast, convenient, and non-destructive method used for material analysis, elemental composition, and plating thickness testing for all types of materials. This measurement method is recognized in several industry standards including the B568-98 ASTM standard. XRF requires no use of chemicals and sample preparation is not necessary. XRF provides an accurate thickness measurement in as little as 15 seconds. This happens by measuring the fluorescent (secondary) X-Ray emitted from a sample after it has been excited by a primary X-Ray source. Since all of the elements present on the sample have special characteristics that are unique, this allows the XRF to be able to identify each element and makes it an excellent source for qualitative and quantitative analysis of material composition and thickness.
Electro-Spec utilizes XRF technology in its inspection processes to determine plating deposit thicknesses, as well as composition of the plating. Using XRF, up to three layers of a deposit can be routinely measured. Limitations of this method are dependent on the substrate. For example, measuring Au over Ni over Cu alloy substrates is done relatively easily. Conversely, measuring “like” or similar metals to the substrate like Cu over brass is difficult because of the similar elemental composition of the deposit over the base material.
Plating thicknesses can be measured in a wide range from 5-500µ” depending on the number of layers and their density. Units of measure typically are in micro-inches and microns. Thicknesses are reported with mean, range, standard deviation, and Cp/Cpk values if applicable. An accuracy of 95% within a computed confidence interval is possible. Electro-Spec has the ability to perform GR&R studies as well as in-house cross-sectioning correlation studies with other thickness testing equipment and XRF machines. Electro-Spec XRF capabilities also include alloy composition analysis on plating and un-plated substrates.
Technology/Process/Equipment
Electro-Spec utilizes Fischerscope XRF technology for inspection of deposit thicknesses and material identification. During this process, a heated cathode emits electrons in the X-ray tube. These electrons are accelerated by a high voltage supply. The energy of the electrons is then converted to electromagnetic radiation. A large percentage of this is X-ray radiation. This constitutes the primary X-ray beam. The radiation then passes through the optical imaging device known as the collimator. The collimator is what allows the electromagnetic radiation to focus on the measurement area of the substrate. The atoms in the measurement area of the substrate then emit their own x-rays. A radiation detector uses a proportional counter to convert the x-ray count into data that will be used to compute a spectrum of intensity. The amount of fluorescent x-rays generated is proportional to the thickness of the deposit.
Samples are staged inside the unit and the specific program file is selected by the operator. Once the proper program file is selected and the samples are in the proper position relative to collimator size, shape, and part geometry – measurement begins.
Upon completion of measurement, the unit produces a thickness and composition report that is used for analysis and disposition of samples. Statistical data is also captured for process control and continuous improvement.
How Does Electro-Spec Ensure XRF Accuracy?
- Issue – XRF accuracy with similar metal compositions between the base substrate and subsequent plating layers
- Standards calibration and base material corrections are part of our process
- Electro-Spec has the ability to check multiple layers of plating over multi-layer substrates, composite substrates, & alloy substrates. A typical example would be Au over Ni over brass substrate. Au over Ni over Cu strike over Cu substrate however, potentially confounds the Cu strike with any Cu alloy substrate. Other limitations of this method include total number of plating layers, and the thickness of each layer.
- Alternative methods for thickness verification (dissimilar substrate, coupons or even cross section) are utilized by Electro-Spec if necessary
- Issue – The right size collimator on the XRF Machine
- Understanding the importance/significance of collimator size as it relates to part geometry
- The collimator of the XRF directs the primary x-ray beam to the material under test. Collimator’s vary in size and shape and are dependent on the geometry and size of the test sample.
- Standard protocol by Electro-Spec inspection personnel is to ensure that the right size collimator is used for XRF verification to improve the accuracy of measurement
- Micro-miniature parts should utilize a small collimator to ensure accuracy and prevent interference. Conversely, a larger part should use a larger collimator to provide more appropriate measurements for the larger surface area.
- Issue – Interference of the X-ray beam that distorts the calculated value of the measurement
- Orientation of the parts in the x-ray chamber and beam interference are critical to ensure the most accurate results
- Parts must be oriented in such a manner to allow the beam to reach the detector unobstructed. This makes the orientation and position of the part inside the XRF critical to achieve accurate results.
- Best practices are such that the surface being tested should be flat if circumstances allow. Test sample surfaces that are concave, convex, knurled, or threaded, present a unique challenge that should be avoided when possible.
Why X-Ray Fluorescence (XRF)?
One of the most important and critical tools for plating conformance and specification adherence is a fluorescence x-ray machine. Critical attributes of the coating thicknesses in relation to customer specification requirements can be ascertained very easily with no destruction or disturbance to the samples being analyzed. Whether being used specifically for thickness correlations or compositional analysis of the coating, it is extremely important that technicians understand how the XRF works. As we have outlined in this paper, there are many characteristics that go into analyzing a plated part and not understanding those characteristics can potentially produce misleading and incorrect results. As is the case with any apparatus meant for qualitative and quantitative analysis and testing, a good understanding of the equipment followed by having the proper controls and process in place is critical to the certification of the results.