With the selection of lead-free solder alloy replacements for SnPb (tin-lead) coming into focus, much of the recent discussion about the lead-free transition has been about demonstrating compliance with the legislation that initiated the conversion. The European WEE and RoHS directives define specific amounts of toxic materials allowed for use in electronic equipment. These include lead (Pb), cadmium (Cd), mercury (Hg), hexavalent chromium (Cr6+), and multiple forms of bromine (Br). Conforming to the legislation may require quantifying the amount of these banned substances in solder joints, components, connectors, board materials, and the completed assembly.

The need to efficiently identify banned substances in plastic, ceramic, and metallic materials has led to an increase in interest in elemental analysis techniques such as X-ray fluorescence (XRF). This non-intrusive and non-destructive measurement method has been in use in electronic manufacturing facilities for years, primarily to measure the composition and thickness of surface platings and coatings. Recent strides in XRF technology have improved the equipment’s efficiency, precision, and cost of ownership, making elemental analysis by X-ray fluorescence a popular tool for detecting the presence of banned metals.

X-ray fluorescence defined
Unlike the more common transmission X-ray imaging, X-ray fluorescence analyzers are used strictly for obtaining elemental information. XRF analyzers do not use X-rays for imaging. XRF uses the X-rays from an excitation source (either an X-ray tube or radioactive source) to produce an incident beam. As the X-rays contact the sample surface, they are either absorbed by or dispersed through the surface atoms. A photoelectric effect occurs when the X-rays are absorbed into the atoms. The X-ray transfers its energy to the inner shell electrons of each atom, thereby ejecting the electrons and creating a vacancy in the atom’s inner electron shell. Electrons from the atom’s outer shell then stabilize the atom by filling in the inner shell. This electron movement results in energy differences that produce X-rays.

XRF detectors absorb these X-rays and measure their energies. Each individual element produces X-rays with a unique set of energies. XRF spectrometers use X-ray detector elements (Si-PIN, Si(Li), or Ge) to create a spectrograph that shows all the elements detected. This technique is similar to energy dispersive spectroscopy (EDS), an elemental identification technique commonly coupled with scanning electron microscopy (SEM). Unlike EDS, however, XRF can be performed in normal atmospheres and does not require an expensive vacuum pump system.

X-ray fluorescence applications
Much like transmission X-rays, XRF has an equal number of production and analytical applications. XRF can be used to precisely identify and quantify the presence of all of the European Union’s (EU) banned substances. However, its use is by no means limited to this function. One of the more traditional uses for XRF is the measurement of plating thickness. XRF has a proven track record of efficient and precise measurement of surface finish thicknesses. It can also be used for process control, incoming material quality control, and qualification of components, assemblies, and processes. Additional uses include measurement of metal films using plating, deposition, sputtering, and ion plating, etc.

One side effect of the transition to lead-free products has been unexpected changes to component surface finishes. More and more frequently, manufacturers will alter component finishes without notification. In some cases, changes in component surface finish composition were identified from one lot to the next. Such variations can result in changes in wetting performance, storage life, process efficiency, and solder joint reliability. Technical assistance from the manufacturers and distributors to identify the surface finish on a lot-by-lot basis can be difficult to obtain. Quite often, data sheets do not include surface finish information. Using XRF to screen incoming lots of components for surface composition can prevent the detrimental effects of unknown changes to the surface finish.

Some DoD and NASA contractors are using XRF to mitigate tin whisker risk by mandating at least 3 wt. % lead in their tin-finished components. Studies have shown that tin-based finishes with approximately three percent lead or more have a significantly reduced risk of forming tin whiskers compared to pure tin finishes. This has prompted DoD contractors and other companies that are traditionally exempt from the EU directives to require at least 3% lead finishes from their component suppliers. XRF has been used to screen their incoming product finishes for lead levels. In the case of one contractor, the EMPF laboratory evaluated every metallic surface on the assembly, including discrete components, ICs, transistors, screws, connectors, nuts, bolts, and eyelets.

Wave solder contamination is another issue associated with the lead-free conversion. Solder pots that contain high-tin alloys such as tin-silver, tin-silver-copper (SAC), and tin-nickel-copper must be monitored for solder pot corrosion. This corrosion can occur due to the solder pot lining composition, copper dissolution from the board and components, or cross-contamination of lead and lead-free
solders. The elemental analysis capability of XRF is well-suited for analyzing solid samples of wave solder alloys. Similarly, elemental analysis of through-hole and SMT solder joints can be achieved. This is particularly useful when investigating the possibility of excessive gold in a solder joint (which could result in gold embrittlement).

As the lead-free transition initiates the demise of SnPb board finishes, it simultaneously promotes interest in alternative board surface finishes. Electroless nickel immersion gold, immersion tin, immersion silver, and organic solderability preservative (OSP) have been the market leaders in terms of a SnPb replacement. While each has its own set of advantages and disadvantages, none of these surface finishes will perform properly if the thickness of the plating is not well controlled. With the exception of OSP finished boards, the plating thickness of incoming lots of bare circuit boards can be recorded prior to the first run build using XRF.

XRF vs. other elemental analysis techniques
XRF has many advantages over other elemental analysis techniques. One main advantage is that it is non-destructive. The relatively low energy X-rays have no damaging effects on electronic components, circuit boards, or assemblies, and components can be used immediately after testing. If static sensitive devices are to be tested using XRF, special grounding precautions can be implemented to ensure electrostatic discharge (ESD) protection.

Unlike other elemental analysis techniques, no additional sample preparation is required to examine the test samples. For example, energy dispersive spectroscopy (EDS) often requires applying a conductive coating to the samples as part of SEM preparation. Auger Electron Spectroscopy (AES) and Atomic Absorption (AA), two highly precise elemental analysis techniques, require dissolution of the samples into a solvent.

XRF also has a speed advantage over other elemental
analysis techniques. Analysis can be completed in as little as 30 seconds, making XRF practical for the time-sensitive manufacturing environment. The short analysis time reduces the overall cost of analysis, particularly if samples are sent to an outside laboratory. Cost for elemental analysis of a sample using the X-ray fluorescence
technique is typically less than half the cost of analysis using SEM and EDS.

Most XRF analyzers will create precise quantitative elemental measurements without the use of expensive standards. For plating thickness measurement and general elemental composition analysis, standardless measurements are sufficient. For instance, DoD manufacturers requiring 3% Pb in their tin-based surface finishes are typically supplied components with 10, 15, 37, or 40% lead. Rarely are components finished with 97% tin and 3% lead finishes. A standardless reading of tin and lead over copper may yield as low as 0.25% error; therefore, a standardless reading that returns 5% lead with 0.25% error has sufficient precision to ensure that the risk of whisker formation is reduced. The precision of the standardless measurement is highly dependent on the quality of the equipment, the consistency of the measurement parameters, and the material being analyzed. For years, many environmental agencies have been using XRF to detect lead in soil down to the parts per million (PPM) level. Using XRF to detect PPM levels of lead in electronic materials such as solder, ceramics, and plastic encapsulates is best performed with the help of a certified calibration standard.

If excessive levels of chromium are detected by XRF, additional analysis will be required to confirm whether the chromium is in the hexavalent form banned by the directives. XRF cannot account for the form in which the substance is present, and the directives set limits specifically for the hexavalent form of chromium (Cr6+). Distinguishing the banned substances polybrominated biphenyl (PBB) from polybrominated diphenyl ether (PBDE) presents the same challenge.

As with any other analysis technique, knowledge of the sample can increase the precision of the result. XRF operators are at a severe disadvantage when attempting to gather quantitative data from an unknown system with multiple layers. XRF is known as a surface technique, but the XRF incident X-rays can penetrate through multiple layers. This can lead to imprecise, exaggerated results. The EMPF laboratory has witnessed cases where lead in the terminal metallization underneath the surface finish exhibited a false positive lead (Pb) reading in what was a pure tin finish. XRF is also ineffective at measuring lighter weight elements, so using this technique for organic contamination analysis is not possible.

Conclusion
There has been a recent surge in interest in XRF caused by the European WEE and RoHS legislation. XRF has been recognized as a quick and efficient tool for ensuring compliance to the legislation. However, for precise levels of Pb, Hg, Cr6+, and other banned substances, the use of standards and additional analysis techniques may be necessary.