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RoHS Compliance – Inherent Risks and Mitigation Strategies

RoHS Compliant Compasses

Sparton RoHS (Restriction of Hazardous Substances) compliant digital compasses are fully compatible with systems that require high-reliability standards due to extreme-environment operating conditions.

The materials and chemicals used in the design, assembly, and manufacture of Sparton digital compasses do not exceed the thresholds set forth in the RoHS Directive 2002/95/EC for Lead, Cadmium, Mercury, Hexavalent Chromium, Polybrominated Biphenyls (PBB), and Polybrominated Diphenyl Ethers (PBDE) and have been specifically designed for both commercial and high-reliability applications.

High-Reliability Products and RoHS

High-reliability is a classification for products that require elevated reliability standards for their operation. Certain military, aerospace, and medical device markets are examples.

There are products within the high-reliability class that are technically exempt from RoHS compliance. However, these products are significantly impacted by the rest of the industry’s need to comply. Component manufacturers are switching to lead-free materials and processes, eventually discontinuing their lead-containing product lines. Lack of long-term reliability data with lead-free alloys has redirected the high-reliability market segment toward mitigation activities in order to control the risk of tin-whisker induced failures.

Tin Whiskers

Tin whiskers are electrically conductive, hair-like strands of single tin crystals that sometimes grow from surfaces where tin is used as a final finish. Tin is only one of several metals that are known to be capable of growing whiskers. Others include zinc, cadmium, indium, and antimony. Whisker sizes vary and some have grown to lengths of several millimeters and in a few cases up to 10 mm.

RoHS – Tin Whisker Risk Analysis and Mitigation

With the onset of the RoHS directive in 2006, many component manufacturers began phasing in tin plating of component leads without fully addressing the associated risks. The use of electroplated tin on component leads introduced a finite, long-term reliability risk through its propensity to grow tin whiskers. These metallic, highly conductive needles are only a few microns in diameter, but as mentioned previously, can grow to lengths of several millimeters and can cause electrical shorts between adjacent, fine-pitch conductor leads.

Since tin whiskers do not grow immediately, but often over a number of years, each segment of the electronics industry views the risk differently. The consumer electronics industry, with its shorter product life cycles, does not view tin whiskers as a major reliability risk. But, component users in the defense and aerospace electronics community, whose electronics are mission critical, view tin whiskers as a serious risk.

Current data from numerous studies point to different factors and causes, but there is no scientific consensus on tin whisker formation and growth fundamentals. It is known, however, that room temperature conditions are most conducive to tin whisker growth, and the most commonly cited driving force for whisker formation is a buildup of compressive stresses in the plated tin layer. These compressive stresses originate from inter-metallic growth, grain size, surface damage, and environmental stresses. Based on this understanding, an assortment of mitigation strategies can be implemented to prevent whiskers from forming. Strategies include nickel underplating, use of low stress tin finishes, solder dipping, and annealing. In the event whiskers do form, mitigation strategies such as conformal coating or the use of potting materials to prevent them from growing and causing potential shorts can be implemented.

NASA’s Goddard Space center has done a tremendous amount of research on tin whiskers and lists a considerable amount of background information on tin whisker problems and mitigation strategies on their website.

The mitigation strategy employed must be balanced against costs at the system-level. Different systems exhibit different levels of tolerance for failure. This tolerance depends upon the criticality of the application and the ability of the system to function in spite of the failure.

There are two organizations that have gained global acceptance through their work on tin-whisker risk mitigation strategies: GEIA (Government Electronics & Information Technology Association) and iNEMI (International Electronics Manufacturing Initiative.

GEIA – Government Electronics & Information Technology Association

The GEIA has sponsored a focus group which is led by a consortium of OEMs (Original Equipment Manufacturers) called the Lead-Free Electronics for Aerospace Programs (LEAP). LEAP has developed a set of guidance documents for the purpose of identifying, categorizing, and recommending risk mitigating strategies.

GEIA-STD-0005-2, a document which was developed by LEAP, established processes for documenting the mitigating steps taken to reduce the harmful effects of tin finishes in electronic systems. This standard is applicable to Aerospace and High Performance electronic applications which design and/or procure equipment that may contain Pb-free tin finishes.

iNEMI – International Electronics Manufacturing Initiative

The iNEMI has a Tin Whisker User Group that consists of eleven large manufacturers of high-reliability electronic assemblies. The goal of the User Group is to define methods and tests that minimize the probability of tin whisker induced failures. This result is achieved by a combination of known mitigation practices, process controls, and some level of testing. The iNEMI has several projects and initiatives related to lead-free electronics and RoHS compliance which can be viewed in further detail by visiting their website.

Tin Whisker Mitigation Efforts at Sparton

Although there are no guarantees against the formation of tin whiskers, Sparton has incorporated significant whisker mitigation strategies in the design and manufacture of our RoHS compliant digital compass options. Several strategies recommended by both the GEIA LEAP committee within GEIA-STD-0005-2 and the iNEMI Tin Whisker User Group have been implemented at Sparton.

  1. Implementation of an epoxy based potting compound that cures to a rock-hard physical insulation barrier that is used to protect against shorts and the environment.
  2. Instituted periodic lead-free component analysis for all parts on our digital compasses.
  3. Pad and pin geometry designs have been optimized so that solder coverage is complete and adequate.
  4. The surface finishes on PCB lands (copper) are designed to protect the base metal against oxidation that could result in poor solder joints during assembly operations. HASL (Hot Air Solder Leveled) tin-lead finishes have been the coating of choice for most of the last fifty years. To comply with legislation and provide the mitigation needed for high-reliability products, Sparton has selected ENIG (Electroless Nickel Immersion Gold) for finishing. With recent developments regarding RoHS, ENIG has been growing as one of the best finishing methods for printed circuit boards. This material has an excellent history for contact connections, has excellent wetting capabilities and provides outstanding electrical testability. The alloy offers solderability for up to one year and has proven to be an excellent choice for reflow, keypad, wire bonding, and EMI shielding.
  5. For lead-free manufacturing certification, Sparton designs and manufactures all of its digital compasses and follows strict process to conform to RoHS requirements. Through Sparton’s dedication to becoming a leader in designing and manufacturing RoHS compliant products, we received the Lead-Free Process Capability Validation Certificate from CE Analytics, a third party lead-free certification group.
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