The X-ray Fluorescence Spectrometer (XRF) is a non-destructive analysis technique that is able to provide product testing information in real time, making it easier for a product to proceed on to any subsequent processes. It works by irradiating the sample with high-energy X-rays or gamma rays. The inner orbital electrons of the constituent atoms of the sample are excited and become energized photoelectrons. This creates a hole in the inner orbital. Electrons from the outer shell will move to fill in these empty electron positions in the inner shell. The difference between energy levels causes X-ray emissions, resulting in X-ray fluorescence.
XRF is well-suited for element and chemical analysis, especially in the study of metals, glass, ceramics and building materials. It is also suitable for use in geochemical research, forensic science, electronic product quality control (such as ensuring compliance with the EU RoHS), archeology and other such fields, where it is already commonly used for non-destructive element analysis. The following real-life analysis cases will show you why this tool is so widely used in so many different fields.
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PCB Composition Analysis |
In order to make people’s lives more convenient, the development of electronic products is moving constantly towards the goal of being light, thin, short and small. In this process, the printed circuit board (PCB) can be considered the soul that supports electronic components. It not only makes electronics work more efficiently but also plays a key role in paving the way towards the future of 5G/6G, AR/VR, the metaverse and beyond.
Using XRF to determine the elements and their concentrations in specified areas of a PCB’s surface is an important step in the PCB manufacturing process. The copper surface of a PCB needs to undergo a surface treatment (such as nickel immersion gold, gold electroplating or tin spraying, etc.) to prevent copper oxidation. The figure below is an optical image of the copper foil on the PCB’s surface obtained via XRF (Figure 1 Left). In this manner, the concentrations of Ni, Cu, and Au can be measured (Table Below Figure 1). The tin element image was obtained by scanning a range of six solder balls (Figure 1 Middle). You can also see the Sn Kα energy spectrum of a single solder ball (Figure 1 Right).
Figure 1 Optical Image of a PCB (Left), XRF Mapping (Middle), XRF Spectrogram (Right) |
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Measurement of Metal Film Thickness |
The gold fingers of the PCB connect to the external connector through contact with the pins of the card slot. The surface of these gold fingers is often electroplated with nickel-gold to make them wear-resistant. Typically speaking, the thickness of the gold layer can be the cause of electrical conduction problems in electronic products. Figure 2 shows the thickness and average value of the surface Au layer and the lower NI layer respectively at five different positions on the gold finger of the PCB as obtained via XRF measurement (Table 2).
Figure 2 Optical Image of the PCB’s Gold Finger (Below) |
 Table 2 Thickness of Gold and Nickel Layers |
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RoHs Hazardous Substance Testing |
In view of the rise in environmental awareness, electronic products are now required to comply with the EU RoHS (Restriction of the use of Hazardous Substances) and ensure that the concentration of hazardous substances such as Cd, Pb, Hg, Cr6+, PBBs and PBDEs stay within certain specifications. XRF is recommended by the International Electrotechnical Commission (IEC) as a fast-screening instrument for RoHS testing.
The figure below shows the RoHS test results of the PCB and MEMS components respectively. The six harmful elements measured on the PCB are all within the specified values (Figure 3), so this component can be judged as qualified. As for the MEMS component, the amount of chromium measured in the metal outer shell significantly exceeds the acceptable limit (Figure 4), so it is unqualified.
Figure 3 The PCB’s ROHS Test Results |
Figure 4 The MEMS’s ROHS Test Results |
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Authentication of Antiques |
XRF does not destroy the sample being analyzed, so it is often used in archeological research. Take, for example, the analysis of Qinghua Porcelain. The Qinghua Porcelain manufacturing process uses cobalt ore (cobalt oxide) as a coloring pigment. After high-temperature firing, the porcelain presents a pattern. By analyzing the pattern locations on the porcelain using XRF (Figure 5), we can determine the composition of the iron and cobalt (Figure 6). As the ratio of the pigment composition evolved throughout the different dynasties, you can judge whether a piece was produced during the age of Qinghua Porcelain based on its element composition and concentration, thus establishing its authenticity as an antique.
Figure 5 Optical Image of Qinghua Porcelain |
 Figure 6 Mapping Diagram of Fe and Co Elements |
XRF analysis has a wide range of applications, and there are portable versions of the instruments used to conduct the analysis. This means it has the advantage of being able to provide rapid, on-site testing for expensive or difficult-to-handle samples. It can be used not only for product authentication but also for the materials specification inspections required by various regulations and more. In today’s rapidly evolving semiconductor industry, XRF technology enables the rapid inspection of IC internal defects as well as the analysis of metal films and coating thicknesses, making it an indispensable tool for the improving of product yields.
