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Report: XRF a Viable RoHS, Tin Whisker Screener |
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Written by Chelsey Drysdale
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Friday, 30 November 2007 |
TEDDINGTON, UK – A National Physical Laboratory investigation into the suitability of XRF systems for screening electronics parts found PIN (semiconductor diode) and SiLi detector-based systems suitable for RoHS compliance measurements in plastics and solders, but proportional counter based systems are not.
All systems successfully detected lead at or above 1%, the group also revealed, easily beating the 4% threshold for determining tin whisker mitigation.
The group tested 15 systems with a range of 40 typical components and assemblies. Eleven different systems were evaluated at 12 different sites. Systems based on PIN, SiLi and proportional counter detectors were included. Eleven systems were benchtop instruments; four were portable. Forty different samples were included in the study, ranging from contaminated plastic components through bulk solder alloys, to solder joints and solder-terminated components, says NPL.
XRF systems using PIN or SiLi detectors generally proved efficient at distinguishing between noncompliant components (containing typically 2000+ ppm of restricted substances) and compliant components (typically <500 ppm of restricted substances). For levels between 500 ppm and 2000 ppm, the use of additional techniques may be required to provide discrimination, NPL says.
The PIN or SiLi detectors also proved efficient at distinguishing compliant and non-compliant systems containing >1000 ppm cadmium. Below this level, however, additional techniques may again be required to provide discrimination. The lower RoHS limit for cadmium of 100 ppm did result in a number of false detections for this element, the company reports.
Proportional counter-based systems were capable of registering the presence of RoHS-banned elements at levels >3%; e.g., such as found in some plastics. Below this level, however, their ability to detect any banned substances was questionable, and use for such applications is not recommended.
For tin whisker mitigation applications, lead levels in excess of 4% are required for solder samples. All systems successfully detected lead at or above this level, provided the sample size was large enough to fill the measurement window. Indeed, all systems proved capable of detecting/measuring lead levels above 1% in solder, according to NPL.
The use of these instruments for both the applications studied (RoHS compliance or tin whisker mitigation) requires operators have both a thorough knowledge of the instrument themselves, and a good understanding of the structure and materials involved in the test samples. These are required to prevent incorrect interpretation of the data provided. Recommendations on instrument practice to obtain meaningful, repeatable results are given.
In conclusion, XRF systems offer a viable method of screening for RoHS compliance and tin whisker mitigation, says NPL. Compared to chemical analysis, these systems offer lower unit and operating cost and faster results. Smaller sample sizes are also possible. However, use of these systems does require a semi-skilled operator.
The work was carried out as part of a project in the materials processing metrology program of the UK Department of Trade and Industry.
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