For the past several years, the electronics industry has scrambled to move its component products to RoHS (Restrictions on Hazardous Substances) compliance. RoHS will ban five toxic materials, one being lead, in electronics for any products offered for sale within the European Union. China, California, and other jurisdictions are following suit. Companies are now concerned with becoming compliant by the RoHS legislation deadline of July 1, 2006. The defense industry, while technically exempt from RoHS requirements, is feeling the pinch as commercial component makers work to meet the deadline. The defense industry must often use electronic components built by these non-exempt suppliers for the commercial world. Even electronic hardware that has dual commercial as well as military use, as in COTS (Commercial Off The Shelf) hardware, is specifically NOT exempt from the lead-free RoHS legislation.

The GEIA Industry Standard for Mitigating the effects of Tin Whiskers in Aerospace and High Performance Electronics Systems is expected to be issued in 2006 as one of four standards and guidelines that are designed to provide assistance to the defense and avionics industries who are trying to deal with and accommodate the lead-free electronics mandate for high performance and military markets. Having been in development for over 12 months, the document is currently in draft form, and undergoing final approvals by the members of the GEIA (Government Electronics and Information Technology Association) LEAP (Lead-free Electronic Products) committee chartered with its development. The standard will be issued jointly by ANSI (American National Standards Institute) and GEIA and is expected to be adopted by the military and commercial aerospace industry.

The GEIA-STD-0005-2 tin whisker mitigation standard first documents the background and scope by citing the European Union legislation known as RoHS. The imminent banning of SnPb solders, which have been used routinely in electronics for some 60 years, requires electronic manufacturers to switch to lead-free solders and lead-free component finishes.

This restriction of lead use has started the transition of many piece part and board suppliers from tin-lead to lead-free finishes such as pure electroplated tin. However, the pure tin finishes being applied to electrical component contacts and circuit boards are susceptible to the spontaneous growth of single crystal, electrically conductive "tin whiskers," (see Fig. 4-1.) which can cause electrical failures by shorting between the closely spaced components and other tin plated parts in lead-free electronic assemblies.

Tin whisker growth is a particularly difficult failure mechanism which has become a prime source of risk with the switch to pure tin plating for lead-free component leads and circuit boards. There are two factor of tin surface finish that pose a much more severe risk for the high-reliability DoD applications than for the commercial electronics world.

The first factor is the tendency of tin whiskers to grow for extremely long periods of time, long past the obsolescence timeframe (and replacement) of commercial electronics. Figure 4-2 is an electron microscopic picture of a circuit board surface having immersion tin plating after 3 months of room temperature storage onsite at the EMPF. Note the small, 10 to 20 micron tin whiskers that are present. These whiskers are too small to be a serious risk, even to fine pitch circuitry. Figure 4-3 shows the same sample after 30 months of storage (perhaps a long time for a commercial application, but a short time for a DoD application) showing some tin whiskers that have grown to over 100 microns, long enough to electrically short closely spaced modern electronic circuits.

The second factor is the growth rate of these whiskers. Their growth rate, and artificial methods that could be used to accelerate this rate, are unpredictable. Therefore, the only way to assure whiskers do not grow to excessive length is to observe the whiskers for that length of time. While for commercial applications, this might be practical, for military applications this would be unacceptable.

Finding ways to deal with these issues prior to the manufacture of specialized military assemblies is going to be the best way to prevent sustainment issues in the future.

The GEIA-STD-005-2 Tin Whisker Mitigation document specifies the steps that need to be taken to assure that the correct whisker risk level is applied to a given program, and once applied, actions taken to mitigate the risks pertaining to the assigned level. The whisker risk levels are defined in the document as follows:

Level I. No restrictions on tin finish use.

Level II. Tin finish is allowed under some circumstances.

Level IIA. Use of tin finish without explicit controls is acceptable under most circumstances but the likelihood of whiskers and methods used to estimate their impact and mitigation strategies will be documented. Tin finish may be prohibited in some specific circumstances called out in contractual documents.

Level IIB. Tin finishes may be used but only with customer approved and specified control measures. These tin finish approvals may be blanket approvals for multiple components and applications within the system. Tin finish may be prohibited in some specific circumstances called out in contractual documents.

Level IIC. Restricted use of tin finish. Tin finish is prohibited unless an exception is made. Specific instruction on use of tin finish and required control measures to be provided and reviewed on a case-by-case basis.

Level III. Use of tin finish is prohibited and measures must be taken to verify compliance.

The document continues delineating the whisker risk mitigation techniques that correspond to each level that can be assigned to a program.

More details on the state of research and the difficulties of currently providing specific, quantitative assessments are provided in the appendices. Appendix A provides guidance on selecting control levels and performing risk assessments. Appendix B describes mechanisms of formation, properties, and potential deleterious effects of tin whiskers. Appendix C provides some background on various tin whisker mitigation methods.

This standard, when issued, will be available for reference in proposals, requests for proposals, work statements, contracts, and other documents. It may be used as a stand-alone standard or as part of compliance with GEIA-STD-0005-1, "Performance Standard for High-Reliability Electronic Systems Containing Pb-free Solder." In combination with the other three GEIA documents cited, a complete guide to the DoD and other high performance electronics program accommodation of the worldwide commercial lead-free movement is anticipated.

The EMPF has been working with the DoD and members of the electronics manufacturing industry for the past few years in an attempt to mitigate the problems surrounding tin whiskers. Because tin whiskers present a latent threat in terms of reliability to military and avionics hardware, the EMPF will continue to direct its efforts toward preventative measures with the idea of improving long-term reliability of these crucial types of electronics.

For more information on the EMPF’s work in the field of lead-free electronics, visit the EMPF website at www.empf.org.