Circuits Assembly - February 2008 - (Page 26)

RoHS Compliance XRF Equipment as a RoHS Screening Tool By Martin Wickham and Dr. Christopher Hunt A study of 15 systems finds most – but not all – offer a viable solution. he requirement to comply with Europe’s RoHS regulations has driven adoption of a range of new materials in electronics components. A company failing to comply with RoHS can be fined. Hence, to ensure only RoHScompliant materials are used, the industry has turned to energy-dispersive x-ray fluorescence (XRF) for incoming goods inspection. However, the technical capabilities of the related instruments are not well understood by the electronics manufacturing community. A jointly funded industry/DTI collaborative project, led by the National Physical Laboratory, has been undertaken to determine the suitability of these techniques for determining the presence and levels of any restricted substances in typical electronics components. The project focused on an inter-comparison of different XRF equipment and test sites in a matrix experiment. XRF is used throughout a range of industries for fast, nondestructive materials elemental analysis. Samples are bombarded with high-energy x-rays, and some of the x-rays are absorbed by the atoms of the sample. If the captured x-ray is of sufficient energy, an electron will be ejected from an inner shell of the atom, creating vacancies. To stabilize the atom, electrons from the outer shells fall to the inner shells, giving off an x-ray whose energy is the difference between the two binding energies of the inner and outer electron shells. As T each element has a unique combination of electron shell energy levels, the spectrum of emitted x-rays is characteristic of the elements contained in the sample. The peak intensities of the emitted x-rays provide information about the concentration of the elements present. The incident x-rays are usually provided by one of two alternative sources: an x-ray tube or a radioactive isotope. The x-ray tube is inert until activated by the operator, while the isotope source needs shielding to prevent operator exposure. Examples of both types were included in this study. The emitted x-rays were analyzed using three types of detectors: SiLi detectors. These have the best resolution, but require liquid nitrogen cooling to maintain stability. Si-PIN photodiodes (formed from p-type/intrinsic/ n-type semiconductor). These can be cooled using Peltier devices and are less expensive, but lack the resolution of SiLi detectors. These detectors form the majority of the systems evaluated here. Proportional counters. These use photo-ionization of gases within the counters to detect x-rays. They are the least expensive of the options explored in this study and are widely used in PCB fabrication for plating thickness measurements. Initially, a range of samples known to contain RoHS-banned substances was assembled by NPL. For each sample type, one sister specimen (i.e., from the same batch) was chemically analyzed to determine its composition and the levels of any banned substances. A master sample from each batch was then given to each partner for “blind” 26 Circuits Assembly FEBRUARY 2008 circuitsassembly.com http://circuitsassembly.com

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Circuits Assembly - February 2008 Contents Caveat Lector Letters Industry News Market Watch Global Sourcing Better Manufacturing Maximizing Lean Copper As a Viable Solution for IC Packaging Embedded Active Components for High-Rel Products Cover Story: XRF Equipment As a RoHS Screening Tool Tech Tips Selective Soldering Test and Inspection Process Doctor Pb-Free Lessons Learned Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - February 2008

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