Assessing Potential Impact of Tin Whiskers on Rail Transportation Safety

Abstract

This paper identifies and explores possible safety implications of RoHS (Restriction of Hazardous Substances) compliance on railway equipment, with the purpose of presenting this topic for further consideration in North American industry standards, e.g. AREMA (American Railway Engineering and Maintenance-of-Way Association) guidelines. Of specific interest is the topic of tin whiskers, which are known to grow in electric circuits that are Lead free. Tin whiskers have the ability to bridge the gap between two vital signals in a circuit, creating the possibility for a potentially hazardous scenario to occur in a safety-critical system. As rail transportation technologies become progressively more dependent on safety critical electronics that are RoHS compliant, the subject of tin whiskers merits its due consideration in the industry. There are verified incidents of tin whiskers having adverse impact in the automotive and aerospace industries; examples are provided in the paper. If precautionary measures are not respected, it is possible that a short circuit resulting from tin whiskers may lead to an undesirable incident in the rail industry also. Research has shown that there are proven techniques which can inhibit tin whisker growth to an acceptable level. Mitigating measures and their effectiveness in deterring whisker growth are discussed in this paper. Awareness is essential in prevention of a hazard. Railroads and Transit agencies that recognize hazards posed by metal whisker growth will be in a better position to evaluate the safety of RoHS compliant products provided by system integrators, who, in turn, must account for the issues presented in this paper in their internal quality processes. In due course, the safety implications of RoHS transitioning should be considered in an industry-wide guideline such as AREMA, laying the foundation for future RoHS compliant railway equipment that is safeguarded against safety concerns from tin whisker growth.