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determining phosphorus content in en plating using xrf spectroscopy continues Figure 6: Fluorescence spectra (log intensity scale) of a NiP/Fe sample (14.3 μm, 9.3 % P) for different excitation conditions: 50 kV, 10 μm Ni-Filter (blue) and 10 kV non-filtered (yellow). Fischerscope X-ray XDV-SDD, aperture Ø 1 mm, anode current in both cases = 1 mA. surement (multiple- excitation). In the process, multiple spectra are obtained for one measurement. For EN measurement, two excitations are sufficient: the first 50 kV "hard" excitation is used for the coating thickness measurement and the "soft" 10 kV excitation for the phosphorus content. 3. Measurement Uncertainty and Reference Samples Although standards-free measurements do deliver quite reasonable results with properly set-up instruments (Table 1), a calibration with reference samples and corresponding adjustment is essential for accurate routine measurement. For this reason, calibration standard sets consisting of several nickel/phosphorus samples with various phosphorus concentrations have been developed for the substrate mate- rials iron, aluminium, copper and for printed circuit boards. Traceability is established using ICP-OES analyses of three independent laboratories. Their good correlation to standards- free XRF values is presented in Figure 7. Deviations from the regression line have various causes. As a whole, they represent the systematic measurement uncertainty, which can be estimated at about 0.3% phosphorus. Thus, the total measurement uncertainty can be assessed to be about 0.5% phosphorus. 4. Measurement Examples 4.1 Nickel/Phosphorus on Aluminium— Thickness and Composition Table 2 shows the measurement results from an application for determining the thickness and composition of a nickel/phosphorus coating on any desired substrate material. January 2014 • The PCB Magazine 45