The European Union’s Restriction Of certain Hazardous Substances (RoHS) directive requires finishers to provide lead content verification at Pb-free levels (RoHS action level of 0.1%, or 1000 ppm) for most products. Exempted applications, such as military and aerospace components, will have to provide verification of lead content at normal Pb-eutectic levels.
Many finishers supply to both exempt and no-exempt applications, and will need to verify compliance at both Pb-free and Pb-eutectic levels. The question is, what’s the best way to provide the required verification?
Sending samples to an outside laboratory for destructive testing is one alternative, as is doing such tests yourself. Another is X-ray fluorescence (XRF) spectrometry, a well-established non-destructive technique for analyzing material composition and the thickness and composition of plated films.
The basis of XRF analysis is excitation of atoms within the test material by photons from an X-ray source. The excited atoms fluoresce X-rays that are characteristic of the element’s atomic number, yielding elemental identification. Frequency of the emissions (intensity) yields quantitative mass thickness, which is converted to thickness for single-element layers, thickness and composition for alloy layers, or simply material composition when the layer is “infinitely thick” to the emissions of the fluoresced layer. XRF spectrometry can measure element concentrations from ppm levels to 100%.
In a demonstration of the technique’s sensitivity to Pb at both the RoHS action (1000 ppm) and eutectic levels, workers at Matrix Metrologies (Holbrook, NY), a supplier of XRF systems, analyzed three infinitely thick test materials—tin with 1,000 ppm Pb content, pure tin, and 90-10 Sn-Pb material with Pb certified at 9.9% by weight.
Analytical sensitivity is often expressed in terms of the Minimum Detection Limit (MDL). In X-ray spectrometry, this is defined as the amount of analyte that gives a net intensity equal to 3 times the standard counting error of the background intensity. Matrix lab workers set data acquisition time at two minutes to improve detection sensitivity, acquired spectra from both the Sn/1,000-ppm Pb standard and the pure tin standard, and calculated MDL at 91 ppm Pb.
Having demonstrated more than adequate sensitivity for RoHS compliance, the researchers then determined measurement error at 1,000 ppm Pb by repeating the measurement of the Sn/1,000-ppm Pb standard 50 times. Results showed a mean Pb value of 986 ppm ±45 ppm (±4.6%). Empirical repeatability is consistent with that predicted by Pb peak and background statistics of ±5.9% relative, or ±59 ppm at 1000 ppm Pb.
At eutectic Pb levels, compositions of lead solders are routinely measured with XRF. Counting statistics play a large role in analytical precision. When Matrix measured a 9.9% Pb eutectic under the same conditions on the same instrument, they achieved analytical repeatability of ±0.47% relative, or ±0.05% absolute at 9.9% Pb.