Circuits Assembly - February 2008 - (Page 32)

RoHS Compliance lower lead levels present than were actually the case. One proportional area. Two systems also falsely indicated the terminations contained counter-based system recorded significantly higher lead (34%) and the noncompliant levels of cadmium. Generally, it is recommended that remaining system (proportional counter-based) did not quantify the when testing resistor component terminations for RoHS compliance, amount of lead present. When known RoHS-compliant chip resistors the resistor should be examined from the reverse side, with the resistive (Pb-free terminations but with lead in the passivation of the resistive element facing away from the instrument detector. element) were tested on the reel, four of nine systems incorrectly indiCare must also be exercised when testing components with mulcated the resistors were not RoHS-compliant (up to tiple layers. A spacer tested during the project was 2.4% lead). Thus when RoHS compliance testing of RoHS-compliant because its surface comprised Snchip resistors, removal of components from the reel plating on brass containing lead at <4% (brass has is recommended. an exemption in the legislation). However, when SOIC components in sticks or tubes were also evaluated, 12 of 14 systems indicated the compostudied. Four of the 10 quantitative systems could nent was noncompliant or recorded lead peaks in not identify noncompliant components. All four were the spectra. This sample illustrates operators need portable, with larger spot sizes. As discussed with the a sound understanding of the materials involved in SOIC solder joints, these systems’ measurement winthe samples to ensure sensible analyses. Challenging dows were relatively large compared to the composamples may require destructive testing by removing nent termination size, and thus, surrounding materithe coating and then testing separately. als attenuated the lead signal. The other systems, with Figure 4. Larger spot sizes can Finally, a Cr-passivated, Zn-coated screw was generally smaller measurement windows, did not dilute RoHS contaminant levels tested (Figure 5). Chemical tests proved inconclusive in smaller solder joints. record accurate values for the lead in the terminations for CrVI, but chromium was clearly present on the with values between 1.5 and 21% for a termination surface. The quantitative test instruments recorded with 36% lead. For compliant components in tubes, levels of chromium varying from 0.77 to 1.8%. four of the 10 systems evaluated indicated lead was Clearly these XRF systems can detect the presence present above 0.1%, or recorded lead peaks in the relof chromium, but because of the lack of speciation, evant spectra, suggesting an incorrect RoHS noncomare unable to confirm RoHS compliance. The three pliance. Thus for RoHS compliance testing, removing proportional counter systems failed to detect any the components from the stick or tube would be chromium. required. For sensible analysis in systems with larger Conclusions measurement windows, several component terminaXRF systems offer a viable method of screening for tions may be required to fill the window. RoHS compliance. Compared to chemical analysis, When a sample of Sn62 (36% lead) solder paste in an unopened pot was tested, all 10 systems evaluthese systems offer lower unit cost, lower running ated indicated the paste was not RoHS-compliant costs and faster results. Smaller sample sizes are also or contained lead peaks in the relevant spectra. But possible. Use of these systems, however, requires a the measured values were, with one exception, well semi-skilled operator with a sound understanding of below the true value (in two cases as low as 1%). For the technique/equipment capabilities and the likely a paste contaminated with lead at a lower level (~1%) composition of materials involved in component in an unopened pot, four of the 10 systems did not assembly. correctly identify the paste as noncompliant. When Nondestructive testing is the norm, but some an unopened Pb-free (0.040% lead) solder paste was samples may need separation into constituent parts, examined, three systems falsely indicated the paste was or removal from placement packaging, for meaningnoncompliant. In view of the degree of uncertainty Figure 5. Typical Sn-plated ful testing. ■ with these solder paste measurements, it is recom- spacer with RoHS-compliant lead mended a sample sufficient to fill the measurement in brass. Acknowledgments window of the test instrument be removed from the pot for testing. The NPL would like to thank the following project partners, without Screening resistors. Even when removed from their reels, the analysis whose help the project would not have been possible: Alcatel Alenia Space Italia; EADS Astrium; of some chip resistors still presented problems. Some components with Fischer Instrumentation (GB) Ltd.; MBDA (UK) Ltd.; Oxford Instruments Analytical; Research in pure tin terminations were found to contain lead, the signal clearly Motion; RMD Instruments; Roentgenanalytik; Rolls Royce Marine; RS Components Ltd.; Thermo emanating from the passivation of the resistive element. When the Fisher Scientific Niton Analyzers; Tin Technology. component was tested with the resistive element toward the detector, The work was carried out as part of a project in the Materials Processing Metrology all 12 systems under evaluation falsely indicated the component was Programme of the UK Department for Innovation, Universities and Skills. RoHS-noncompliant, or recorded lead peaks in the spectra. The lead values varied between 0.14 and 30%. The higher levels of lead were Martin Wickham is a consultant and Dr. Christopher Hunt is group leader, Electronics Intercongenerally associated with those systems that used a larger measurement nection Group, at the National Physical Laboratory (npl.co.uk); martin.wickham@ntlworld.com. 32 Circuits Assembly FEBRUARY 2008 circuitsassembly.com http://www.npl.co.uk http://circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - February 2008

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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