XRF analyzers are fast becoming the industry standard for the non-destructive RoHS screening of components and solder. With the various XRF systems out there (both handheld and bench top) how is it that the LeadTracer-RoHS™ can make an accurate measurement within seconds? Wouldn't a longer measurement time (>60 seconds or so) be more effective with respect to accuracy?
You’re absolutely correct about the coming of age of the XRF tool “industry”. These tools are fast becoming a “must-have” and not just a “that’s interesting” response from customers who are trying to sort out the RoHS directive in terms of capital equipment purchases.
The ability to eject an electron from an atom is a function of the “energy” of any XRF device. Since higher energy allows deeper penetration then, K-shell (higher energy) characteristic X-rays of Pb will not be stopped by other elements such as Sb (antimony), Sn (tin), Br (bromine), Au (gold), etc. that are present in most electronic components. This will eliminate any false negative possibility.
Furthermore, ability to create higher energy characteristic x-rays (K-shell) leads to a cleaner spectrum which is not as crowed as low energy spectrum for the same component is. Therefore, analysis can be done using clear peaks that are not overlapped eliminating false indications in a much shorter time.
Below is a spectrum created by an x-ray tube excitation that is only capable of exciting L-shell characteristic x-rays of elements present in this capacitor.
The presence of Sn (tin) in this sample totally masks the presence of Pb resulting in a false negative indication. The same component’s elemental analysis using our method clearly shows that lead is present. The reason is that our method excites the K-shell x-rays and this radiation will penetrate through Sn and will be recorded as Pb.
Another example of an extreme case is shown below. In this sample, Cr/Cu (chromium/copper) plating totally blocked all the Pb L-shell x-rays resulting in a false negative indication.
No doubt, as stated, longer measurement time results in a better precision. However, the other XRF systems’ longer measurement time is due to their analysis algorithm not actual irradiation for higher count rates that would lead to a better precision. The extra time is required for those systems’ mathematical function to perform de-convolution of overlapping peaks in the spectrum since several elements’ characteristic x-ray lines fall on top of each other at lower energies such as Pb, Hg, Au, Br, Se, etc. Therefore, these tube-based XRF systems have to use elaborate mathematical models that require “multi-mode” programs, which consume time for performing the calculations necessary to analyze the spectrum.
The LeadTracer-RoHS™ has only one measurement mode, regardless of target sample. In Standard Test Mode, the operator sets the desired level of detection. In this mode, the unit measures until one or all of the desired elements are statistically quantified as present or absent. The measurement time in this mode depends on the concentration of the elements in the target material. If there is a significant amount of an analyzed element such as lead (Pb), the unit will determine this in seconds. If the concentration is low such as only a few hundred ppm, the unit extends the measurement time until a statistically meaningful result is achieved. This can take a couple of minutes in some cases. Remember the operator can choose a set time or allow the unit to choose the time based on concentration of the elements in each sample.
The LeadTracer-RoHS is the only XRF system on the market, which is specifically designed for the RoHS application. Besides the operational features such as laser guided sample locator, variable beam area capability (spot size as small as 1.5mm), bar-code reader, etc.
The main distinguishing feature of our system is our approach in using the K-shell x-rays of Pb, Hg, and Cd for analysis. The high-energy K-shell x-ray technique addresses the difficulties being experienced by other XRF systems that use L-shell x-rays induced from a x-ray tube. Electronic components and materials are complex, contain many elements, and are not homogenous therefore; the K-shell x-ray approach is more suitable for the Screening Process than he L-shell method for RoHS application.
The other advantage of a K-shell XRF system is the ability to analyze the entire depth of a component for material composition used in its construction. This capability allows a user to test for counterfeit components that do not contain the desired material such as Au and Bi in QFP’s and other expensive parts. The counterfeit parts are becoming a major issue especially for automotive, military and avionics applications that require specific material composition for extreme environmental conditions.
I suggest any customer who is interested in an XRF tool to present the same component to all XRF manufacturers that they might be considering and ask for actual spectrum of the resulting analysis for that component. I believe, after observing these spectra, they will see the advantage of the higher energy method that the RMD tool utilizes.
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