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A soon-to-be-released GEIA standard covers strategies for high-rel electronics.

For the past several months, industry has scrambled to bring its components into compliance with the RoHS directive. The emergency drill has been mostly about whether companies can become compliant by the legislated deadline of July 1, 2006. The defense industry, while technically exempt from RoHS requirements, is feeling the pinch as commercial component makers scramble to meet the deadline. Reason: Defense contractors often must rely on components built by suppliers to the (nonexempt) commercial world. Even hardware for dual (commercial and military) use, such as commercial off-the-shelf (COTS) hardware, is specifically not exempt from RoHS legislation.

The GEIA Industry Standard, planned for release this year, is one of four standards and guidelines designed to assist the defense and avionics companies that are trying to manage Pb-free electronics for high performance and military markets. In the pipeline for more than 12 months, the document is undergoing final approvals by members of the committee chartered with its development, the Government Electronics and Information Technology Association Lead-free in Aerospace Program committee. The standard will be issued jointly by the American National Standards Institute and GEIA and is expected to be adopted by the military and commercial aerospace industry.

Last month we discussed the GEIA-LEAP Program Managers Handbook. This month, we look at the standard that covers tin whisker mitigation, GEIA-STD-0005-2. As this document notes, restriction of lead use has pushed many suppliers from SnPb finishes to Pb-free alternatives such as pure electroplated tin. However, 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," which can cause electrical failures by shorting between the closely spaced components and other Sn-plated parts such as RF shields or common hardware (Figure 1).

FIGURE 1. Tin whiskers growing from Sn-plated connector pins and causing electrical shorts between pins after 10 years in service. (Photo courtesy of NASA)

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

• Level I. No restrictions on tin finish use.

• Level II. Tin finish is permitted 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 is 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. 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 delineates whisker risk mitigation techniques that correspond to each level that can be assigned to a program.

Details on the state of research and the current difficulties of providing specific, quantitative assessments are provided in the appendices. In the appendices, guidance on selecting control levels; performing risk assessments; mechanisms of formation, properties and potential deleterious effects of tin whiskers; and background on various tin whisker mitigation methods are detailed.

The American Competitiveness Institute (aciusa.org) is a scientific research corporation dedicated to the advancement of electronics manufacturing processes and materials for the Department of Defense and industry. This column appears monthly.