Circuits Assembly - February 2008 - (Page 28)

RoHS Compliance evaluation using their own XRF system. At each test site, therefore, the same sample was tested for each sample type. When all the XRF trials had been completed, the actual samples used were chemically analyzed using mass spectroscopy and SEM electron-dispersive x-ray analysis to confirm their elemental compositions. A range of nearly 40 samples was used in the evaluation process, of which 22 contained RoHS-prohibited substances (Figure 1). The latter included Pb/Br/Cd/Hg in plastics (plasticizers/ pigments) and lead in solders (bulk, joints and coatings). Fifteen RoHS-compliant samples were included to help evaluate the performance of the XRF systems. Each sample was presented to the XRF systems three times, with the sample being removed from the equipment between tests. The XRF system parameters such as incident x-ray energy, analysis area (spot size) and emitted x-ray acquisition times, were set by the end-users/suppliers consistent with their own best practices. In total, 15 systems (11 different instruments from six manufacturers) were tested: 11 benchtop instruments (seven Si-PIN, one SiLi and three proportional counters) and four less expensive, portable machines (all Si-PIN). All employed x-ray tubes as the source for the incident x-rays, except one system, which used a Co57 source. RoHS Screening of Plastic Components In choosing plastic samples for these tests, a range of typical electronics components was evaluated before the final selection was made. During this procedure, it was noted that noncompliant components did not typically contain lead, mercury, bromine or chromium at levels around the RoHS limit of 1000 ppm. Rather, while typical values for noncompliant components were usually above 0.25% (2500 ppm), those for compliant components were typically less than 0.05% (500 ppm). The Si-PIN or SiLi detector-based systems were capable of identifying the noncompliant components containing lead, mercury or cadmium. Eight typical RoHS-noncompliant plastics from electronic components were examined, and all 12 systems correctly indicated the samples were noncompliant. Three typical components containing bromine or chromium were also all correctly identified as containing these elements, hence requiring alternative tests for speciation of the hexavalent chromium and bromine compounds (PBB/PBDE/other). Three RoHS-compliant components were tested, and all 12 PIN/SiLi systems indicated compliance for lead and mercury. Results regarding false negatives (i.e., incorrect indications of noncompliance) were also encouraging (Table 1). Table 1. PIN/SiLi System False Negative Results Element No. of No. of Systems Giving False Negatives False Negatives Tests Pb Hg Br Cr Cd 73 135 125 137 111 0 0 0 2 7 0 0 0 1 5 Figure 1. Examples of samples tested using XRF. % False Negatives 0 0 0 1.5% 6% In the case of cadmium, both benchtop and portable XRF systems were able to distinguish noncompliant systems for levels above 1000 ppm Cd. All the false indications of noncompliance occurred when the level was <260 ppm Cd. The three proportional counter-based systems did not perform well for testing plastics. Although not specifically designed for testing for RoHS compliance in plastics, all the systems incorrectly indicated the presence of RoHS-banned elements. Moreover, quantitative results were unavailable or inaccurate. It is recommended proportional counter systems not be used for RoHS compliance measurements. How the sample is presented to the test equipment is important. When a cable strain relief was tested while still attached to its accompanying cable (Figure 2), the lead level recorded was reduced by the effect of the material within the strain relief itself. In this instance, the lower results were noncompliant, but should this example have had lead content closer to the RoHS limit, this modification could have led the sample to be incorrectly marked as compliant. Where recorded contaminant levels are close to the RoHS limit, it is recommended the plastic be tested in isolation. RoHS Screening of Bulk Solder Alloys For larger samples, large analysis spot sizes can be an advantage. Sampling over a larger area can reduce any effects of segregation within the sample and give a more meaningful analysis. For the lead-contaminated, circuitsassembly.com 28 Circuits Assembly FEBRUARY 2008 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

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.