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With the selection of Pb-free solder alloy replacements for SnPb coming into focus, much of the recent discussion about the Pb-free transition has been about demonstrating compliance. The European WEEE and RoHS directives define specific amounts of toxic materials permitted for use in electronics equipment. Conforming to the legislation may require quantifying the amount of the 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 increased interest in elemental analysis techniques such as x-ray fluorescence (XRF). This non-intrusive, nondestructive measurement method has been used in electronics manufacturing 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.

Unlike the more common transmission x-ray imaging, XRF 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 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 (Figure 1). 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.

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 the EU's banned substances. It is by no means limited to this function, however. A traditional use for XRF is the measurement of plating thickness. XRF can efficiently and precisely measure surface finish thicknesses. It can also be used for process control, incoming material quality control and qualification of components, assemblies and processes. Other uses include measurement of metal films using plating, deposition, sputtering and ion plating.

One side effect of the transition to Pb-free products has been unexpected changes to component surface finishes. 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 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. % Pb in Sn-finished components. Studies have shown that Sn-based finishes with 3% Pb or more have a significantly reduced risk of forming tin whiskers as compared to pure tin finishes. Thus DoD contractors and other companies exempt from the EU directives require at least 3% Pb finishes from component suppliers. XRF can screen incoming product finishes for Pb 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.

As the Pb-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 are market leaders for SnPb replacement. While each has its own advantages and disadvantages, none will perform properly if the thickness of the plating is not controlled. With the exception of OSP finished boards, the plating thickness of incoming lots of bare boards can be recorded prior to the first run build using XRF.

Wave solder contamination is another issue associated with the Pb-free conversion. Solder pots that contain high-Sn alloys such as SnAg, SnAgCu and SnNiCu 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 Pb and Pb-free solders. The elemental analysis capability of XRF is suited for analyzing solid samples of wave solder alloys. Similarly, elemental analysis of through-hole and SMT solder joints can be achieved. This is useful when investigating the possibility of excessive gold in a solder joint (which could result in gold embrittlement).

One advantage XRF has over other elemental analysis techniques is that it is nondestructive. The relatively low energy x-rays do not damage electronics components, circuit boards or assemblies, and components can be used immediately after testing. If static-sensitive devices are to be tested using XRF, implement special grounding precautions to ensure ESD protection.

Unlike other elemental analysis techniques, no additional sample preparation is required to examine test samples. For example, 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 techniques. Analysis can be completed in 30 sec., 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 analysis of a sample using XRF is typically less than half the cost of analysis using SEM and EDS.

Most XRF analyzers 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 Sn-based surface finishes are typically supplied components with Pb content of 10, 15, 37 or 40%. Rarely are components finished with 97% Sn and 3% Pb finishes. A standardless reading of tin and Pb over copper may yield as low as 0.25% error; therefore, a standardless reading that returns 5% Pb with 0.25% error has sufficient precision to ensure that the risk of whisker formation is reduced. The precision of the standardless measurement is dependent on the quality of the equipment, consistency of the measurement parameters and the material being analyzed. For years, environmental agencies have used XRF to detect Pb in soil down to the parts per million (ppm) level. Using XRF to detect ppm levels of Pb 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 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 substance 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 Pb in the terminal metallization underneath the surface finish exhibited a false positive Pb reading in what was a pure Sn finish. XRF is also ineffective at measuring lighter weight elements, so organic contamination analysis is not possible.

The recent European directives have led to a surge in interest in XRF. XRF is a quick, 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.