Coating thickness measurement of thin gold and palladium coatings on printed circuit boards using X‐ray fluorescence

Abstract

PurposeThe purpose of this paper is to compare the performance of X‐ray fluorescence (XRF) instruments with different detector systems (proportional counter, positive intrinsic negative and Si drift detectors) for measuring thin Au and Pd coatings on printed circuit boards and to investigate different ways of background treatment. It also aims to provide and certify suitable reference materials which are similar to samples used in production.Design/methodology/approachXRF measurements were performed with different instruments and detector types. The quantification of the reference materials is based on XRF, gravimetric analysis and Rutherford backscattering (RBS).FindingsThe well‐established X‐ray instrumentation for coating thickness measurement, with proportional counter detectors, are not very suitable for measuring thin ( < approx. 100 nm) coatings of gold and palladium due to the poor energy resolution of the proportional counter‐tubes. Systems with semiconductor detectors achieve results that are more reliable with a significantly higher accuracy. A correct background treatment is especially important for very thin coatings. The composition of the base material has to be taken into account by the software evaluation algorithm for each measurement. A global base subtraction performed prior to the measurement can achieve better repeatability, but can also lead to incorrect absolute values.Research limitations/implicationsIf small measuring spots (e.g. 150 μm) have to be realized with semiconductor detector systems, special X‐ray optics (polycapillaries) have to be used to obtain an intensity comparable to that offered by proportional counter devices. This will be the subject of a further publication.Originality/valueThe paper provides an overall review and results for different types of instruments (detectors) and compares different background treatments. Suitable reference materials have been developed for precise and traceable measurements. Their quantification is based on gravimetric analysis and RBS. The standard‐free energy dispersive X‐ray fluorescence (ED‐X‐ray fluorescence analysis (XRFA)) was used for interpolation of the gravimetric data for thin coatings. For the region below 100 nm, measurement uncertainties of less than 1 nm can be achieved.