Glenn Robertson and Stephen Schoppe
Process Sciences, Inc.

RoHS and other legislative initiatives for the elimination of Lead and other materials in electronic products continue to create confusion. Equipment manufacturers are uncertain to what extent they need to verify compliance. A paperwork trail from component and board suppliers will help provide the necessary documentation of compliance, but what amount of analytical testing will be required for validation?

The answer to this question depends on a number of questions, such as:

• What is your budget for analytical test services?
• What is you level of risk tolerance?
• What is your confidence level in your suppliers to deliver parts that are not mixed, mislabeled, or otherwise suspect?

RoHS regulations require every “homogeneous material” to contain less than 0.1% by weight of lead and other banned materials (0.01% for Cadmium). Testing the entire assembly at one time will not assure compliance; testing 100% of the components will cause your CFO to resign. What testing to perform, and how often is a crucial question that needs to be answered to address RoHS compliance concerns.

The number of analytical techniques to quantify the concentration of RoHS-restricted elements is limited. Atomic Absorption (AA) and Inductively Coupled Plasma (ICP) will quantify these elements down to sub-100 ppm levels. Both methods are similar in that they require an acid digestion to put the elements into solution. The solution is then analyzed by the machine, which produces characteristic radiation that traverses the media. Absorption or emission of the radiation (spectral lines) reveals the presence of the different elemental constituents.

The restrictions on Hexavalent Chromium and on many of the Bromine compounds used to reduce flammability will also require special analytical processes. Some analysis processes can identify only the presence of Chromium and Bromine. Further specialized testing is required to determine the concentration of Hexavalent Chromium and specific banned Bromine compounds.

As you have probably guessed by this point, these techniques are not economically feasible for the enormous quantity of components that populate most electronic products. They will be used primarily for validating lead-free solder alloys and for confirmation testing of specific suspect components.

There is a bright side to this story. X-Ray Fluorescence (XRF) is rapidly gaining popularity as a “screening” test technique. XRF has long been used in the electronics industry as a quick, non-destructive means of determining coating thickness for component leads and solder pads.

XRF works by bombarding the test object with X-rays. When an X-Ray is absorbed by an atom, an electron may be ejected, leaving a vacancy. When another electron moves into the vacancy, a “characteristic” X-Ray photon is generated. The X-Ray is detected and analyzed by the instrument. The energy (or wavelength) of the X-Ray is characteristic of the element present, and the number of X-Rays detected will be proportional to the element’s concentration.

Virtually all XRF systems today use a compact sealed tube with a collimator or capillary to produce the X-Ray beam. Proprietary software is provided to aid with data analysis and storage. Two categories of XRF systems are available. Portable hand held types are relatively inexpensive, and are useful for field-testing of sheet metal, plastics, and bulk metal samples. Desktop-style systems are more sophisticated and can provide significantly better sensitivity and accuracy.

As you might suspect, analysis by XRF has some significant limitations. This technique detects elements only and cannot identify specific compounds or properties such as the valence state for Chromium. Accuracy of the analysis is heavily dependent on system calibration using Certified Reference Materials (“standards”) that must be specific to the application. Availability and cost of the necessary CRMs are still open questions.

XRF results are also affected by a number of factors. Probably the most significant of these is interference resulting from overlapping energy peaks from the various detected elements. Matrix effects such as scattering of the fluorescent X-Rays will also affect repeatability and accuracy.

One particular shortcoming of XRF with regard to RoHS compliance testing is in its ability to detect elements at very low concentration. Many claims are made regarding detection limits, and this topic is the subject of considerable confusion. The “true” minimum detectable concentration for a given element is dependent on a multitude of factors and can vary greatly. Optimum system setup and operation as well as careful interpretation of the data are essential for best results.

Unfortunately, realistic XRF detection limits will generally be too high to guarantee RoHS compliance. What to do when the test report states simply “not detected?” Fortunately, in most cases where RoHS-banned materials are present, they are added at levels that are easily detected by a well-maintained desktop-style XRF system and an experienced operator. On the other hand, if risk tolerance is low, or there is reason to suspect that a low level of banned materials may be present, then further (and more costly) testing may be justified. Reliability of supply chain documentation and supplier history must also be considered. In any case, the experience and expertise provided by your analytical services supplier can help with your decision.

Other specialized tests are available for Chromium and Bromine compounds. Techniques developed by the EPA and other organizations may be used for measurement of Hexavalent Chrome. These tests often use High Performance Liquid Chromatography (HPLC) or Gas Chromatography / Mass Spectrometry (GC/MS). Evaluation of Bromine content requires identification of the particular organic compounds present. GC/MS is most commonly used, but HPLC and Fourier Transform InfraRed (FTIR) may also be considered. In many cases accepted standardized procedures and CRMs are still needed, and there is much work still to be done here.

In conclusion, ROHS testing is a complex topic. Accepted standard test procedures and CRMs are still needed. It is essential to know your analytical lab and the tests that they use. Partnering with a lab such as Process Sciences for services such as product testing and vendor surveys can be an important part of your strategy for compliance.

Glenn Robertson (glennr@process-sciences.com) is a Senior Process Engineer and Stephen Schoppe (sms@process-sciences.com) is Technical Sales Manager with Process Sciences, Inc. in Leander, Texas.