Electronic Parts – Material Content Analysis With XRF

RoHSwell ViewSometimes, testing is the fastest, or the only, way to determine the material content of an electronic component. The fastest, most convenient test method is XRF. With a portable XRF unit you can easily determine the presence or absence of regulated substances in almost any sample.

Effective analysis takes a little bit of thought. XRF works by hitting a sample with low-level X-Rays and examining the X-Ray photons emitted by the irradiated atoms. The wavelength of the detected photons identifies the element, and the intensity relates to the concentration. For metal samples, penetration is only about 10 microns. For non-metals, the penetration is up to 1 centimeter. That means you can only analyze the surface of metallic materials. Plastics and other organic materials can be analyzed in bulk.

Portable XRF units are the most convenient for electronics manufacturers. You can carry the unit around easily and quickly screen parts or even assembly tools. It is a great way to test for contamination in work areas. Though XRF is quick, the most accurate results come from longer scans. The results are essentially a statistical sample. The more samples, the better the results. Regardless of what the marketing literature says, if you want dependable results, use scan times of one to three minutes.

The penetration limit is particularly important when trying to screen whole, intact electronic components. To get the best results, you need to know something about the internal geometry. Fortunately, most components are fairly similar within a type or package style. Before we get into component geometry, we need a place to test.

A good test platform is a simple, plastic cutting board. Use one that is at least 1 centimeter thick, or use two to get enough thickness. Unfinished and unstained wood is also acceptable, but could produce more background noise. To keep the surface uncontaminated, cover it with a clean sheet of white paper. Use the XRF analyzer to check that the platform is clean and free of contamination. Now let’s look at some component examples.

For a first test, put a tantalum chip capacitor on the platform, topside up, point the analyzer at the negative terminal and scan it. You will see the termination plating, probably Sn or Sn-Pb, possibly Br from a flame retardant in the body, and the metal used around the Ta body. What you are unlikely to see is any tantalum. That’s because the capacitor geometry shields the tantalum from the analyzer. To see the Ta, you need to scan the positive terminal end of the cap. That will expose the Ta wires where they are welded to the positive terminal. Scan the side edge of the capacitor and you might see silver, carbon, and manganese coating the Ta slug. If the cap won’t stand on edge, clear plastic tape works great to hold it in place. When I scanned one of the newer Niobium electrolytic capacitors, the Ni was revealed in an edge scan.

SOIC constructionNow put an IC with lead-free terminals on the platform, point the analyzer at the top of the chip package and shoot. Because the terminals are lead-free, you will not see Pb in the results. If the package contains a brominated flame-retardant, that will register. The smallest element detectable is sulphur at atomic number 16. Neither the Si chip nor Al traces will register, but gold bond wires will show up.

What you will not be able to detect is the die-attach material. If you flipped the chip over and scanned the bottom, the lead-frame would probably block you again. The only way to see the high-temperature Pb solder that might be used to attach the die is to scan it from the edge. And if the package is too wide the 1-centimeter penetration limit will still prevent you from getting a good scan. In this case, you will need to section one or more chips to expose the die-attach material. Just pot it in epoxy and grind it down to the level you want.

Wire and cable are very educational test subjects. Try a bit of power cord first. To make sense of this complex assembly, strip the cord down to component parts. Remove the outer jacket. Remove any padding material used between the individual wires. Strip the insulation from a single wire. Now you can scan each material separately. You will probably find Cl and Pb in the cord jacket. The most common material for this application is PVC, which contains Pb stabilizers. The fibrous padding will likely be clean, but it was a similar material that cost Sony millions of dollars due to Cd contamination. The wire insulation will probably look identical to the jacket: Cl and Pb. If the wire is bare copper, it will show Cu and possibly some Pb (up to 4% permitted by RoHS).

Scanning plastic parts is easier. Just point the XRF scanner at the part and shoot. I do like to scan both the exterior and interior sides of case parts. Sometimes the interior has a coating of another material. Don’t forget molded-in metal fasteners. Many fasteners have zinc chromate finishes which contain hexavalent chromium.

And speaking of fasteners, expect to find both hex-chrome and cadmium if you scan screws, bolts, nuts or just about any metal parts. Both elements are used extensively in anti-corrosion plating finishes.

XRF screening is a great way to speed up your initial search for compliant components. If you can’t find compliance data or certificates for a particular supplier, just use XRF. Keep good records of your tests and you can use them as part of your due diligence record. I would also incorporate XRF into an incoming inspection process to quickly spot non-compliant parts and keep them out of your inventory.

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