All you really want to know about XRF: XRF is an effective means for validating the presence of certain restricted elements.

"Well, you don't know what we can find ... You don't know what we can
see ..."
--Steppenwolf

One of the most daunting and frustrating aspects of RoHS and Pb-free has been the intermixing of SnPb and Pb-free materials and components. This has been and will continue to be a problem for both those seeking compliance and those who are deferred (read: currently exempt). For the latter, the great concern is backwards compatibility. While the diligent practitioner can take the steps necessary to avoid intermixing within their process, our eternal nem-eses--component suppliers--have proven themselves capable at buggering things up. Most of us have experienced horror stories of components that were labeled Pb-free but weren't, and vice-versa. For example, last year in one case of BGAs, despite the ASIC manufacturer's documentation and date code attesting to SnPb balls, it took the assembler (an "exemptee") 230[degrees]C to reflow them and 240[degrees]C to remove them; smells like Pb-free to me. And this was a mainstream device manufacturer (not a fly-by-night like ASICs-R-Us of Shenzhen). There have even been a number of incidents where the balls on a single BGA were mixed--SnPb and Pb-free. Zounds! Hey, humans are involved and mistakes are and will continue to be made. We have to deal with it.

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If you're attempting to comply with the RoHS directives (EU, China, Korea, California and others yet to follow), what exactly is expected with regard to trying to maintain a Pb-free product and avoid being victimized by less-than-perfect component vendors? At this point, with respect to EU RoHS, it appears that if you've done due diligence to ensure, as best you could, that components (and other materials) were Pb-free, you might be "OK." If you can produce a paper trail from your suppliers through your scrubbed BoM and into your process that you were, to the best of your knowledge, using Pb-free, you could essentially point the finger elsewhere. However, while you may avoid fines and other such penalties (lawyers are standing by!), you still will have to get the "contaminated" product out of the market stream: an expensive ordeal, no doubt. Remember, EU RoHS is self-declared compliancy; China and Korea are not.

If you want to avoid process hassles and not just be a "good do-bee," you really need to know what you're dealing with before it enters your line. Yet this also applies to RoHS-deferred assemblers. How, exactly, does one check incoming components (and PCBs)? After all, sending incoming component samples to a lab is impractical. Fortunately, there is a technology that permits nondestructive component and material screening in-house.

X-ray fluorescence (XRF) analysis is a well-established, nondestructive analytical technique for elemental analysis that has been around for more than 50 years. It has been used in a number of other industries including to detect lead in paint. It is now deployed in electronics assembly for screening incoming components.

XRF is initiated by ejecting an inner shell electron by an external force/energy that creates an electron vacancy. This vacancy is filled by the transition of an outer shell electron of the atom. The difference in the binding energies of the two electrons results in the creation of characteristic x-rays; this is known as x-ray fluorescence.

It is the absorbed radiation energy of the source by the atom that causes its rise to an excited state. The ejection of electrons and subsequent characteristic x-ray photons permits the atom to discharge the absorbed energy, fluoresce, and then return to its normal state. Since each atom has a unique energy pattern with its electrons having distinct quantum numbers, resulting characteristic x-rays are also unique with specific frequency and act as fingerprints of elements in the XRF analysis.

As is typical in this industry, a growing number of XRF manufacturers are getting into the act. It is important to note that XRF methodologies differ, and only a few are effective in determining the presence of RoHS-restricted elements. XRF analyzers used for RoHS applications range from portable to benchtop to large-scale laboratory systems. Many have unique capabilities due to their methodologies for their original intended functions and, hence, limitations for broad applications. As a result, only a few of the systems for PCB assembly are ideal; others are functionally limited. Particular attention must be paid to the XRF system's energy source. Buyers beware!

In their incoming, unassembled state, most components do not consist of single, homogeneous materials. The RoHS Directive requires that all "homogeneous materials" contained within an electronic product be compliant. This includes the plating on component leads and PCB pads. On one hand, XRF has proved effective in identifying restricted substances--especially lead but also mercury and cadmium--because of its ability to identify the spectral energy patterns of these elements. Unfortunately, lower energy, L-shell, x-ray energies for lead and mercury lie between 9.9-14.8 KeV. Many other elements commonly present in electronics materials such as arsenic, selenium, bromine, germanium and zinc produce characteristic x-ray spectral peaks in the same range. The sought-after elements may thus be masked, shadowed or otherwise misidentified. However, using higher (source) energy levels will induce fluorescence of the K-shell x-rays of the elements--in particular lead and mercury atoms. K-shell energies for lead and mercury are much higher--68.9-85 KeV--and thus suffer no interference from arsenic, selenium or bromine. An isotope-based XRF spectrometry for inducing higher energy K-shell x-ray is the most straightforward and effective method for component screening. K-shell x-rays having higher energies than L-shell are less likely to be absorbed by the specimen's matrix; this reduces the potential for false negatives. The bottom line: The most appropriate system for the PCB assembler is K-shell.

XRF is a powerful tool that can be effective in validating the absence or presence of certain RoHS-restricted elements. Note that XRF presents only an elemental analysis, and while it does not quantitatively measure compounds (and hence does not provide actual chemical composition), the qualitative data provided by XRF is a prerequisite for such measurements.

RoHS across the world--send lawyers, XRF guns and money. But remember, we're all in this together!

Special thanks to Sia Afshari and Jack Paster at RMD Instruments for sharing their knowledge, insights and expertise on XRF.

Phil Zarrow is president and SMT process consultant with ITM Consulting (itmconsulting.org); [e-mail]. He still bears the scars, physical and mental, of reflowing convection/IR ovens. His column appears bimonthly.

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