X-ray fluorescence spectroscopy enables RoHS action-level analysis

The usually non-destructive measurement procedure is a popular choice for compliance assurance.

BY JIM BOGERT

Although no one analytical instrument technique is capable of completely satisfying all validation requirements of the Restrictions on Hazardous Substances (RoHS) directive, x-ray fluorescence (XRF) spectroscopy is the most versatile, capable, and generally accepted method of analyzing all the restricted elements.

XRF analysis for restricted elements compliance usually requires no sample preparation and, in most cases, is nondestructive. This means that measured product can actually be shipped.

FIGURE 1. This block diagram shows a typical benchtop energy-dispersive x-ray fluorescence system configured for RoHS applications.Click here to enlarge image

The two instrumental approaches to XRF analysis are energy-dispersive XRF (EDXRF) and wavelength-dispersive XRF (WDXRF). The type most widely adopted for RoHS analysis is EDXRF, because of its ability to inspect samples of widely varying shapes and sizes with minimal operator training.

Of the EXRF instruments commercially available, handheld XRF and benchtop micro-beam XRF systems have proven the most applicable.

FIGURE 2. The peak-to-background response of a silicon PIN detector with digital noise pulse processing is required to obtain the sensitivity for trace element analysis demanded by the RoHS directive.Click here to enlarge image

Because RoHS-level analysis (1,000 ppm Pb; 1,000 ppm Hg; 1,000 ppm Cr+6; 1,000 ppm Br as PBB and PBDE; and 100 ppm Cd) is trace analysis from an EDXRF perspective, EDXRF detection limits are ppm levels. (XRF provides total elemental content; it does not differentiate between Cr+3 and CR+6, or the molecular state of Br.)

Measurement essentials

Equipment used for RoHS validation measure two key hardware features: a solid-state detector, and incident x-ray beam filtration to provide the necessary peak-to-background (signal-to-noise) response (see Figure 3).

Both hand-held and benchtop units may offer these features, so how do you decide which to choose? As is usually the case, it depends on your samples. In some cases, you may want to consider both types.

FIGURE 3. Primary beam filters are used to condition the spectral output of x-ray fluorescence tools. The spectral output is optimized for the specific RoHS application that is being measured. RoHS XRF-capable machines have a palette of programmable primary beam filters that enable optimization of the beam for specific RoHS elements.Click here to enlarge image

Here are three important considerations when choosing hand-held and/or benchtop micro-beam EDXRF for RoHS analysis:

• Analysis area. The analysis area of most hand-held units is approximately 7 mm (0.3 inches) in diameter. If the components or parts to be analyzed are smaller than this, quantitative analysis with a hand-held unit may not be possible; it may indicate a RoHS element at action levels, but it cannot validate compliance. Benchtop units with selectable collimation and magnified video viewing for confirmation of analysis area can perform the analysis and validate compliance (see Figures 1 and 2).
• Thickness and composition software. In many electronic applications, such as tin and solder finishes, a material is a coating or layered system. In these cases, quantitative analysis requires knowledge of the layered system and the thickness of the layer or layers. This requires thickness and composition software, which is not available on most hand-held units. Therefore, if you are going to purchase a hand-held machine for RoHS analysis of coatings, you must have film thickness software capability on your machine; otherwise, accuracy will be suspect. Film thickness software is generally available on benchtop units.
• Large area scans. If the part to be analyzed is non-homogeneous in the X and Y dimensions (the obvious electronic example would be a circuit board), then the entire part needs to be evaluated. Benchtop units with beam collimation and X-Y-Z stage automation often provide a scanning (element mapping) mode, which can be used to identify potentially non-compliant components. Once a component or suspect area has been identified, magnified video viewing with the appropriate collimation enables quantitative analysis of the specific component or area.

Tools of choice

Hand-held XRF tools offer convenient (portable) RoHS screening and quantitative analysis when the sample or analysis area is larger than the incident beam (>38.55 mm2 circular area), and infinitely thick to the analyte x-ray emission, unless the hand-held tool is equipped with film thickness software.

Hand-held x-ray fluorescence tools (left) provide the versatility of portability and are excellent screening tools, while benchtop machines (above) deliver much smaller analysis areas and excel at sample measurement and assembled-part compliance testing.Click here to enlarge image

Benchtop EDXRF systems, however, are necessary for quantitative analysis when the part or component area is less than 38.55 mm2 (7 mm diameter) and/or the analysis spot is a coating (finish) or layered structure. Systems with X-Y-Z stage automation provide convenient screening and analysis of large, non-homogeneous samples or sample areas.

JIM BOGERT is national sales manager at Matrix Technologies Inc. (www.matrixtechnologies.com). He holds a bachelor’s degree in biology and chemistry from Valparaiso University and has served the analysis and measurement field for more than 30 years.