A commonly asked question across a wide number of electronic manufacturing market segments right now is, “Does the component or assembly meet the RoHS/WEEE limits?” Those asking these questions are referring to banned or restricted materials according to RoHS (Restriction of certain Hazardous Substances)/WEEE (Waste from Electrical and Electronic Equipment) legislation. These substances and their limits include lead (Pb), mercury (Hg), hexavalent chromium (Cr VI), polybrominated biphenyls (PBB), and polybrominated diphenly ethers (PBE). Limits for these materials are 1000 ppm. For Cadmium (Cd), the limit is 100 ppm.

X-Ray fluorescence is one of several test methods available to ensure RoHS/WEEE compliance. The major advantage of the X-Ray fluorescence technique is that it is non-destructive. Other advantages of X-Ray fluorescence include no sample preparation and no lengthy operator training of the test method. Since there are several types of X-Ray fluorescence instruments, it is important to know the capabilities and differences between units. Knowing the capabilities is important because the screening of components could negatively affect sales volume and/or result in legal consequences.

Bench top X-ray fluorescence instruments (Figure 9-1) have been used by PCB manufacturers for decades. They are also commonly found in incoming inspection or quality control departments that test printed circuit boards. The radiation of an X-ray tube excites the sample to emit X-ray fluorescence radiation, which is characteristic for each element. The detector registers the energy spectrum. The elements contained in the sample can be identified through the characteristic energies of the peaks of the spectrum and the concentrations of the elements. Coating thicknesses are determined by the intensity of their radiation portions. A proportional counter tube or a semiconductor detector delivers the measuring signal.

The first factor to consider when selecting an X-ray fluorescence instrument for RoHS/WEEE screening is whether the instrument can physically measure the sample. This includes the spot size of the measurement as well as the dimension of the part. Hand held X-ray instruments are limited in the spot size that they are able to measure. This presents a huge challenge when testing items such as chip capacitors. If a spot size is too large, the result could be a false positive, when in reality it is not compliant. This occurs when a mean value is formed over a large area.

The use of the appropriate collimator is significant. Both the FISCHERSCOPE ®XAN and the FISCHERSCOPE® XDAL offer four collimators that are software controlled with motor driven adjustment. The collimator is essentially a pin-hole that delivers the size and position of the measurement spot. The approximate size of a hand held collimator is 2 mm. The smallest collimator size of the FISCHERSCOPE®XDAL is .1 mm. The difference in collimator size determines whether a reading can be taken on small solder joints.

When selecting the X-Ray fluorescence instrument, the user should consider the size of the measurement chamber. Specifically, will the instrument accommodate the sample without having to alter or move the part? The FISCHERSCOPE® XAN has a large chamber on which the operator manually positions the sample part on the measuring stage. Measurements are from the bottom up. The FISCHERSCOPE® XDAL measures from the top down. It has a slotted design to allow for sample parts that would not otherwise fit in the measurement chamber (Note: if the sample is larger than the measurement stage, the sample can extend to both the right and left of the hardware).

Equally important to the size of the measurement chamber is the time that it takes to get a reading. In the case of bench top systems, there is no elaborate sample preparation. The parts are simply placed in the measurement chamber. The FISCHERSCOPE® XDAL offers an auto focus feature in addition to the visual focus tool that is available with the FISCHERSCOPE ® XAN. Measuring times are dependent on the respective sample and specified detection limit ranging from 50 to 200 seconds. With a hand held system, it would be extremely tiresome to hold a unit steady even for this time period and it makes accurate positioning difficult leading to a high error rate. Furthermore, portable units have the potential danger of exposing operators or others to stray radiation.

Currently, suppliers are transitioning from lead to lead-free components. This results in mixed lots in which some components with the same part number contain lead while others are lead-free. They are shipped without being identified. Furthermore, some industries such as Aerospace and Military are exempt from the RoHS/WEEE legislation, meaning that there will always be the risk that lead components will be present. As a result, much testing needs to be conducted as quickly and easily as possible.

Factors that influence the correct reading include the difference between bulk analysis and coating thickness capabilities. Hand held instruments use a bulk analysis technique. PC boards, however, are coated; therefore, the ability to analyze each layer separately is critical. Portable instruments do not analyze each layer separately, possibly resulting in erroneous readings. The FISCHERSCOPE® XAN and FISCHERSCOPE® XDAL distinguish between various layers and can identify many elements such as Pb, Bi, and Ag. In the example of MLCC, the Pb and Ag may be present both in the solder as well as in the paste/frit material. Not only can it be identified, the FISCHERSCOPE can identify the same element and different layers on the same component.

Not all bench top X-ray fluorescence instruments offer the same capabilities. Some use different detectors, and the fluorescence radiations of the coating components as well as the substrate material are registered by the detector. They form the foundation for the evaluation in their pulse height distribution or spectrum. The spectrum accomplishes the “conversion” of the measurement signals into thickness and concentration values. They also reflect the uncertainties due to the overlapping peaks and count statistics. Overlapping of the peaks reduces measurement sensitivity. Bench top XRF instruments using a proportional counter do not have the same sensitivity to meet the demands of RoHS/WEEE as semiconductor detectors.

Instruments with high resolution detectors (semiconductor detectors) are much more likely to separate peaks where even very small concentrations can be determined reliably. Small concentrations are a challenge when meeting the demands of RoHS/WEEE.

The final factor to consider is ease of use of the software. Fischer Win FTM® 6 is an advanced X-ray fluorescence fundamental parameter software program. As many as 24 parameters can be evaluated at a time. As more substances are added to the list of banned substances, having many parameters available will add to the benefit of the unit. The Fischer Win FTM® 6 is user-friendly, including automatic product search capability. This feature is crucial for components that may have been mislabeled or switched.

There are multiple types of X-ray fluorescence instruments available. Some have been designed for specific applications and are now being marketed for applications such as RoHS/WEEE testing. The user should be aware of the differences between bulk analysis X-ray fluorescence and those X-Ray fluorescence instruments that measure layer thickness as well as provide material analysis. The difference could determine whether a component passes or fails RoHS/WEEE compliance. Likewise, an informed decision-maker will consider the practical aspects of a portable device versus a system that will measure small spot sizes and scan automatically with a programmable stage. These features, combined with automatic product recognition and visual documentation, save time and reduce error rates. Finally, the user should be aware of the capabilities of proportional counter detectors and semiconductor detectors. The resolution of the semiconductor detectors allows for sampling of parts with small concentrations that are typical of RoHS/WEEE applications.

For more information related to this article, or to schedule a demonstration of the Fisherscope equipment located at the EMPF, contact Ken Friedman, 610-362-1200 x 279 or via email at kfriedman@aciusa.org.