Handheld X-ray analyzers can help electronics assemblers keep hazardous substances out of their products.
July 1, 2006, forever changed the manufacturing landscape. As of that date, manufacturers must comply fully with the European Union’s Restriction of Hazardous Substances (ROHS) law, which bans products containing excessive amounts of certain toxic substances. Specifically, the product’s materials, parts or subassemblies cannot contain more than 1,000 milligrams per kilogram each of mercury, lead, hexavalent chromium, polybrominated diphenyls (PBD) and polybrominated diphenyl ethers (PBDE), nor can they contain more than 100 milligrams per kilogram of cadmium.
Another EU law, the Waste Electrical and Electronic Equipment (WEEE) directive, became effective in August 2005. It orders manufacturers to collect and recycle products that enter the market after Aug. 13, 2005, or face a penalty equal to 2 percent of the revenue generated by the sale of the product.
Of course, for these regulations to be effective, OEMs must have some way of knowing that the parts and materials going into their products do not contain illegal levels of hazardous substances. Testing 100 percent of incoming and outgoing product is simply not feasible. Many manufacturers rely on suppliers to certify that their parts and materials do, in fact, comply with ROHS regulations. Others take matters into their own hands. For example, Sony Corp. (Tokyo) deploys an army of auditors to visit suppliers’ factories for random checks. Other companies do both-trusting some suppliers to deliver “green” components, but testing parts from other suppliers that are considered “high-risk.” Ultimately, the only sure way for OEMs to comply with ROHS regulations is through an in-house testing program.
Unfortunately, testing becomes more difficult-and more critical-as an assembly moves from raw materials to a finished product. Analyzing a raw material is relatively simple. There’s an ample supply, and it’s in a form that’s easy to test. Testing a finished product is more daunting. Be it a computer monitor or cell phone, the sheer diversity of component materials, shapes and sizes makes the task nearly impossible to perform quickly and inexpensively.
The difficulty is compounded by the requirement that the tested object be “homogeneous.” That is, the object “cannot be mechanically disjointed into different materials.” In practice, this translates into inspecting every single component and even individual segments of a component, such as a semiconductor package.
Fortunately, there is a tool that can analyze the makeup of finished products without completely disas-sembling them-a handheld X-ray fluorescence (XRF) spectrometer. These devices have been used for years to analyze metal alloys and detect metallic contaminants in soil. Now, due to ROHS and WEEE regulations, these tools are increasingly being used by manufacturers in a wide range of industries.
Handheld XRF spectrometers are fast, accurate, repeatable and simple to use. They can analyze any material-solid, liquid or powder. Unlike some analytical instruments, handheld XRF spectrometers determine all the elements in a sample simultaneously, rather than sequentially. They can determine the concentration of an element in a sample from a fraction of a percent to 100 percent. Little or no sample preparation is required. Analytical algorithms are robust, accommodating samples of various sizes and shapes. They are less expensive than their laboratory counterparts, they do not require consumables, and their maintenance costs are negligible.
XRF spectrometry is nondestructive. The sample is not altered, defaced or destroyed, even after repeated analyses. This is useful for situations involving litigation, which require the original sample to be preserved.
Analysis With X-raysXRF analysis is based on the phenomenon of fluorescence: When a material absorbs high-energy radiation, it immediately re-emits low-energy radiation. When gamma rays or high-energy X-rays strike an atom of the sample material, the atom may eject one of its low-energy, inner-shell electrons. That vacancy is instantly filled by a high-energy electron from an outer shell. The energy difference between the two shells is released in the form of X-ray radiation. This radiation is called a “characteristic X-ray,” because its energy is unique to the emitting element. By measuring the energy and intensity of the characteristic X-rays emitted by the sample, we can obtain qualitative and quantitative information about the elements contained in it.
An XRF spectrometer has five main components: a radiation source, an X-ray detector, a multichannel analyzer, analytical software, and some means of presenting samples consistently for analysis. During measurement, the spectrometer acquires an X-ray spectrum from the sample. That spectrum is then converted into qualitative and quantitative information on all the elements in the sample.
Each spectrum is displayed as a histogram of the intensity with which X-rays of a particular energy arrive at the detector. An increased intensity (as read on the vertical axis) at a particular energy region (as read on the horizontal axis) means that X-rays at that energy are more frequently generated in a sample than X-rays at other energies. The regions of increased intensity-the peaks-correspond to the characteristic X-rays of an element. For example, if bromine is present in a material, the XRF spectrum will show a peak at an energy of 11.907 kiloelectronvolts. The peak amplitude represents the quantity of the element in the sample.
Fortunately, engineers need not know the characteristic X-ray energy of every element. Spectrometers have software that provides immediately useful results, rather than just X-ray spectra.
The handheld analyzer represents the state of the art in XRF spectrometry. This ergonomic, environmentally sealed, battery-operated device weighs just 1.5 kilograms. The radiation source is a miniature, low-power X-ray tube. The end-window, transmission anode tube operates at a maximum of 40 kilovolts of high voltage and 35 microamperes of current. A high-resolution, silicon PIN diode detector, with an en-ergy resolution better than 240 electronvolts, registers the characteristic X-rays from the sample. A rechargeable lithium-ion battery powers the instrument for 10 hours. The operator communicates with the analyzer via a touch-screen LCD. The front end of the analyzer is configured as a flat plate with a small, rectangular window (10 by 20 millimeters) sealed with polyimide foil. This plate provides a repeatable means of presenting the sample for analysis.
To analyze a solid object, the operator presses the nose of the instrument against the object and pulls the trigger. Results of each measurement are continuously updated and displayed on the LCD. They are also stored permanently in the instrument’s memory. From there, the data can be uploaded to a PC for report generation or archiving. The analyzer can hold more than 6,000 readings, complete with X-ray spectra. The analyzer is calibrated to detect any element assemblers might be interested in. It does not require calibration.
To analyze bulk samples, such as powders, fluids or pellets, the instrument can be put into a special stand that accepts sample cups filled with the material. Irregular or large objects, such as circuit boards, can be positioned directly over the nose of the analyzer. The analyzer is equipped with a Bluetooth wireless link to a PC, so it can send data directly to a PC or be controlled from a PC-resident program.
The latest version of the analyzer has a 50-kilovolt X-ray tube, rather than a 40-kilovolt one. This helps the device detect and measure critical elements, such as cadmium. The analyzer also has a collimator, which can be set, through the controller, to either 3 or 8 millimeters in diameter. A collimator filters a stream of X-rays so that only those traveling parallel to a specified direction are allowed through. The collimator allows the operator to focus the X-rays on a small spot, which is useful for analyzing small electronic parts.
Analytical PerformanceThe performance of an XRF analyzer is judged by its accuracy, precision of measurement, and limits of detection. These parameters depend on what is being analyzed, the complexity of its chemical makeup, and measurement time.
In addition, while XRF analysis can determine how much of an element is present in a sample, it cannot specify the chemical state of the element. For example, XRF can detect chromium, but it cannot distinguish hexavalent chromium from trivalent chromium. Therefore, if XRF determines that the concentration of chromium in a sample is above the permissible level of 0.1 percent, further testing will be mandatory with a method specific to hexavalent chromium. However, if XRF reports a chromium level less than 0.1 percent, further testing is unnecessary. Even if the sample contains hexavalent chromium, its concentration would certainly be less than 0.1 percent.
Plastics. When analyzing plastics with XRF, engineers must pay attention to the thickness of the sample. Most plastics are weak absorbers of X-rays. As a result, a sample that is too thin could yield inaccurate results. The thicker the object is, the better the results will be. If the plastic part is not at least 5 millimeters thick, the elemental concentrations reported by the analyzer could be off by more than 10 percent. However, if the thickness of the part is known, the analysis software can correct for this error.
When analyzing small objects, such as pellets or foils, we recommend placing them in a sample cup in an amount sufficient to create a layer of minimum thickness.
In addition, each time the analyzer is turned on, and then periodically during a measurement session, engineers should analyze a reference sample and record the results. This is to verify the performance of the instrument. The reference sample should always be analyzed under the same conditions. The reference sample should be a well-characterized sample of solid polymer with elemental concentrations similar to the applicable threshold concentrations. (The latter requirement applies to analysis of other materials as well.)
Handheld XRF spectrometers can be used to quickly screen circuit boards and other plastic parts for the presence of brominated flame retardants. According to the ROHS regulations, plastic parts cannot contain more than 1,000 milligrams of brominated flame retardant per kilogram. Thus, if the bromine concentration is less than 350 milligrams per kilogram, the part complies with the regulation, since few brominated flame retardants contain less than 50 percent bromine.
Lead-free solders. Analysis of metal alloys has been a flagship application for handheld XRF spectrometers. These devices can identify the composition of steel alloys, titanium alloys, copper alloys and precious metal alloys in seconds-with a precision ranging from 0.01 percent to 0.5 percent.
This ability is a boon to electronics assemblers. To comply with ROHS regulations, solder alloys cannot contain more than 0.1 percent of lead. With a 60-second test of the standard lead-free alloy (96.5 percent tin, 3 percent silver, 0.5 percent copper), a handheld XRF spectrometer can detect lead at a concentration as small as 0.015 percent.
Small parts and solder joints. Determining the elemental composition of components on a circuit board is the ultimate analytical challenge for electronics assemblers, especially if the analysis must be nondestructive. Not only are the components nonhomogeneous by design, but they are also very small and mounted very close to each other.
With an aperture of 1 to 2 square centimeters, a typical handheld XRF spectrometer is excellent for analyzing objects as small as 1 by 0.5 centimeter, but it would have difficulty distinguishing individual components on a crowded circuit board. To facilitate the task, the volume of the analyzed portion of the object must be reduced to about 10 cubic millimeters. Such volume corresponds approximately to a component 2 by 2 by 2 millimeters in size-or an average surface-mount capacitor or resistor.
This challenge can be overcome by collimating the X-ray beam that illuminates the sample. With a collimator, the beam can be focused on a circle approximately 3 millimeters in diameter. As a result, objects less than 3 millimeters wide can be analyzed as a whole, or sections of larger objects can be analyzed separately from each other. Collimation causes only small degradation of limits of detection, compared with a “broad-beam” instrument.
When using a collimator, the instrument is docked in a stand and controlled from a PC. A small, CCD color camera is installed inside the instrument snout, alongside a bright red LED. The camera and LED enable the operator to see on a PC screen exactly what is being analyzed. The LED shines a red circle, 3 millimeters in diameter, on the spot where the beam is focused. The operator moves the section to be measured into the circle and starts the analysis. The camera takes an image of the spot, and the computer stores the image along with the measurement results. Only objects inside that circle are analyzed.
Screening And Testing Strategy
Portability and nondestructive analysis are indisputable advantages of a handheld XRF spectrometer. These features make it a perfect screening tool.
By screening with XRF analysis, assemblers can determine whether more expensive and labor-intensive testing is required. For example, if we test a material with an XRF analyzer and find that lead content is 2,800 milligrams per kilogram, we can safely reject it as noncompliant without additional testing. Similarly, should the screening test detect a chromium content of 200 milligrams per kilogram, we can deem the material compliant. Without XRF screening, we would have to test the sample specifically for hexavalent chromium, only to learn after much more time and expense that the material is compliant.
If a sample is homogeneous, it can be analyzed with a portable XRF spectrometer. Based on the results, engineers can then decide if the sample meets ROHS requirements, or whether the results are inconclusive and further testing is necessary. To account for measurement error, acceptance threshold values for ROHS elements should be set lower than those set forth in the regulation: 100 parts per million for cadmium and 1,000 parts per million for the other five substances.
For example, if the lead concentration in a sample is 910 parts per million, and the one sigma error associated with the result is ±50 parts per million, we cannot decide if the lead content is below the regulatory threshold of 1,000 parts per million. At the two sigma level, the true lead concentration may be anywhere between 810 and 1,010 parts per million. At the three sigma level, the band of uncertainty stretches from 760 to 1,060 parts per million. To make a positive decision, we will need to repeat the test for lead with greater precision.
On the other hand, if the lead content is measured at, say, 750 parts per million, we would deem it compliant since even at the three sigma error level, it would not exceed the 1,000 parts per million threshold. Alternatively, a lead concentration of, say, 1,200 parts per million, would mean noncompliance.
For bromine, the threshold value is much lower-350 parts per million-because the threshold for brominated flame retardants refers to the total amount of the compound, and not just the bromine content. In addition, for chromium and bromine, there’s only one threshold value rather than a range of inconclusive readings. This is because the analyzer determines elemental rather then chemical composition of the sample. If the chromium content exceeds the threshold, it must be retested with a method specific for hexavalent chromium. Similarly, if the bromine concentration exceeds the threshold, a follow-up analysis is mandatory with a method specific for PBD or PBDE.
XRF analysis can be applied in almost every phase in the life cycle of plastic parts, from compounding the polymer, to collecting and separating used parts for recycling. The most beneficial time for XRF analysis is before materials and components are introduced into the manufacturing process for the final product, and after the product is fully assembled. These two points are the domain of ROHS.
Here is a list of the best ways to apply portable XRF analyzers:
* Incoming inspection of compounded plastics, such as granulate.
* Incoming inspection of components and subassemblies, such as cables, wires and enclosures.
* Screening plastics for brominated flame retardants and other elements, such as antimony and tin.
* Screening circuit boards for brominated flame retardants.
* Quantitative analysis of metals in plastics.
* Screening components for forbidden substances.
* Preliminary screening of finished products.
One plastics molder recently learned a lesson in just how important screening can be. The molder used a portable XRF analyzer to test an incoming shipment of polyvinyl chloride pellets. The analyzer was inserted into the bin of pellets and a 5-second test was run. The analyzer revealed that the pellets contained lead at a prohibitively high concentration of 8,600 parts per million. By preventing the material from entering production, this data saved the manufacturer from a very costly loss.
The advantage of portable XRF spectrometers cannot be overestimated. The ability to bring the “laboratory” to the object to be analyzed results in improvements on many fronts. First, the cost of testing is incomparably smaller. Additionally, on-site XRF analysis yields results in real-time. Consequently, more extensive testing will be performed, and many objects will be tested that otherwise might not be. More testing translates into better compliance.
Portability is not synonymous with inferior results. The portable XRF spectrometer analyzes samples without destroying them. If the sample is heterogeneous, the results will reflect that. However, if the sample is homogeneous or if it is prepared in the same way as a laboratory specimen, there is virtually no performance difference between the handheld instrument and the laboratory version.
Portable XRF spectrometry is not a panacea. As always, engineers should use the best possible tool to solve the problem at hand, rather than find a problem for the tool we happen to have. Portable XRF is such a tool.