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determining phosphorus content in en plating using xrf spectroscopy continues provide direct measurement of the phosphorus content. Energy-dispersive X-ray spectrometry (EDX) utilizing electrons for excitation in electron probe microanalysis (EPMA) or charged particles in particle induced X-ray emission (PIXE) have been analytical techniques used for determining phosphorous content for a long time. While the first method is integrated into many electron microscopes, the latter requires an accelerator. High vacuum is required in either case. In XRF, an incident X-ray beam is used as the excitation source. This is typically an X-ray tube. For all three described excitation methods, the resulting fluorescent signal is interpreted in the energy dispersive X-ray spectrometer. XRF is well established in process control instrumentation, especially in the electroplating industry and has been used for decades to determine both coating thicknesses and coating compositions[2]. However, it is impossible to determine the thickness of a nickel/phosphorus coating using X-ray fluorescence without knowing the phosphorus content. The phosphorus content in the nickel changes the coating density and attenuates the other fluoresced components used in the measurement process. The phosphorus concentration of a nickel/phosphorus alloy coating has been obtained indirectly, by measuring the attenuation of base material fluorescence as described in section 2.2. This method, without directly measuring the P-K signal, was described in 1989[3] and is integrated in some instrument manufacturers' application software. Reliable direct measurement of the P-K radiation has been limited in conventional, air path XRF instruments by detector technology (i.e., proportional counter tubes or Peltier-cooled Si-PIN diodes). The low-energy P-K radiation can either not be detected or insufficiently detected. Nonetheless, this widely used technique has great advantages; it requires no vacuum and operation of the instruments is simple enough that it can be used on the plating floor. Now with the recently available silicon drift detectors (SDDs) direct measurement of P-K radiation in air is possible and, therefore, extends the application to base materials other than Fe, such as Al or plastics. The following discusses this in greater detail. 40 The PCB Magazine • January 2014 2. Direct and Indirect Determination of the Phosphorus Content 2.1The Coating Model In the coating model depicted below (Figure 1), the nickel/phosphorus coating is viewed as a plane parallel alloy coating that contains only the elements nickel and phosphorus with a homogeneous element distribution. Typically, organic and/or metallic stabilisers (i.e., lead) in trace concentration ranges can be neglected. If lead-free stabilisers are present in concentrations that affect the XRF analysis, they can be taken into account as an additional alloy element or elements. The radiation components relevant for XRF are Ni-K (7.5 keV and 8.3 keV) and P-K (2.0 keV), as well as the fluoresced components of the substrate material, which in Figure 1 is iron (6.4 keV and 7.1 keV). 2.2 Indirect Phosphorus Measurement The indirect determination of the phosphorus content[3] uses only the easily measurable radiation components of nickel and the substrate material iron. This approached is used with proportional counter tube based instruments, because a proportional counter tube is not able to detect the P-K radiation. Figure 2 shows related model calculations for the conditions of a Fischerscope X-ray XULM proportional coun- Figure 1: Coating model and schematic of the fluorescence excitation.