Hazardous Material Reduction in Printed Circuit Boards

For many years, lead-tin solder was the industry standard for creating electrical bonds between components. However, as a result of research on the dangerous of hazardous materials utilized in electronics manufacturing, governments across the world have taken action to reduce industry reliance on environmentally unfriendly processes and materials. Perhaps the most sweeping change occurred in 2006 with the European Union Restriction of Hazardous Substance directive. This banned numerous hazardous metals and chemicals used in electronics manufacturing. As a result, engineers had to find new techniques to manufacture goods or face substantial fines. Additionally, consumers have indicated a willingness to pay up to 10% more for “green” goods.



Hazardous materials in electronics constitute a substantial cost to both process and dispose of. Lead, mercury, hexavalent chromium, polybrominated biphenyls, and polybrominated diphenyl ethers are all common materials which must be reduced whenever possible in order to comply both with EPA mandates, EU directives, and consumer demand. Additionally, reduction of hazardous material usage reduces disposal costs significantly. There is reduced need to certify and test operators on hazardous materials, reduced waste treatment and monitoring, as well as simplification of recycling programs.

Changing materials may result in unexpected operation of PCBs. Design engineers must be cognizant of electromagnetic shielding changes from removal of materials such as hexavalent chrome coating from electroplated shielding sheaths. PCB surface finish changes may result in the necessity of re-tuning the PCB to prevent extra capacitance coupling or inductance changes from shifting the electromagnetic susceptibility of the board in both AC and DC systems.

Table of Operating Conditions
Table of Operating Conditions

As PCBs typically involve high clock speeds, these high frequency signals are susceptible to transfer function changes as a result of resistance changes from changing connector materials. Trace interconnect material changes may also result in race condition formation due to increased propagation delays as a result of increased filtering of the leading edge of signal pulses. Additionally, changing connect materials may result in impedance mismatches with external sources. This will result in undesirable signal filtering through the connection terminal. Design engineers must ensure that all material changes come with a subsequent impedance matching evaluation. Another unexpected change as a result of removing hazardous materials from a PCB is charge coupling. Some control circuits make use of magnetic field coupling to provide feedback for control circuitry. By altering the magnetic permeability of the feature, this coupling may be lost resulting Similar results may happen with electric field coupling as well.

Implementation of lead-free soldering is a topic of great interest to design engineers seeking to expand into ROHS-compliant markets while also seeking to reduce waste streams and hazardous constituent monitoring. Unfortunately, viscoplastic variances between lead-free solders results in a high potential for material failure. Viscoplastic behavior is of greatest concern during periods of moderate stress over long periods of time. As a result, design engineers face particular challenges in this material substitution. An additional complication is that thermal cycling of solder joints results in increased strain introduction to the joint. Design engineers must consider operating environments and select lead-free solder based on the environment and expected thermal cycling experienced by the product.

References:

Cohen, A., 2015, Prototype to Product: A Practical Guide for Getting to Market, O’Reilly, Boston, USA.

Shina, S., 2008, Green Electronics Design and Manufacturing: Implementing Lead-Free and RoHS-Compliant Global Products, McGraw-Hill, New York, USA.

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