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Determining Phosphorus Content in EN Plating Using XRF Spectroscopy
February 4, 2014 |Estimated reading time: 2 minutes
Phosphorus, the concentration of which significantly influences the mechanical and magnetic properties of the coating, is incorporated during electroless or chemical nickel deposition. For this reason, measurement of the phosphorus content has been an important issue ever since electroless nickel deposition methods were introduced. As an alternative to the established wet-chemical methods, during which the coating is dissolved and therefore destroyed, non-destructive and simpler methods are desired. XRF provides such a non-destructive test method, where utilisation of the characteristic emissions of P-K radiation can 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. 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 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.Read the full article here.Editor's Note: This article originally appeared in the January 2014 issue of The PCB Magazine.