XRF measurement of residual materials in electronics.

The aim of this study was to assess the suitability of using X-ray fluorescence (XRF) systems for screening electronics parts in two applications - compliance with RoHS regulations, and tin whisker mitigation. Fifteen XRF systems were evaluated using typical electronic components and assemblies, ranging from contaminated plastic components, through bulk solder alloys, to solder joints and solder-terminated components. The results indicate that while PIN (semiconductor diode) and SiLi detector based systems are suitable for RoHS compliance measurements in plastics and solders, proportional counter based systems are not. XRF systems using PIN or SiLi detectors generally proved efficient at distinguishing between non-compliant components (containing typically 2000+ppm of restricted substances) and compliant components (typically 1000ppm cadmium. Below this level, however, additional techniques may again be required to provide discrimination. The lower RoHS limit for cadmium of 100ppm did result in a number of false detections for this element. Proportional counter based systems were capable of registering the presence of RoHS-banned elements at levels >3% e.g. such as found in some plastics. Below this level, however, their ability to detect any banned substances was questionable, and their use for such applications is not recommended. For tin whisker mitigation applications, lead levels in excess of 4% are required for solder samples. All the systems successfully detected lead at or above this level, providing the sample size was large enough to fill the measurement window. Indeed, all systems proved capable of detecting/measuring lead levels above 1% in solder. It is important to remember that the use of these instruments for both the applications studied (RoHS compliance, or tin whisker mitigation) requires the operators to have both a thorough knowledge of the instrument themselves, and a good understanding of the structure and materials involved in the test samples These are required to prevent incorrect interpretation of the data provided i.e. incorrect indications of RoHS compliance, or of tin whisker mitigation (e.g. for samples having lead in base materials beneath a lead-free metalisation). Recommendations on instrument practice to obtain meaningful, repeatable results are given.