For more than 60 years the electronics industry has relied on tin-lead solder as the primary bond between electronic devices. Its reliability and strength are well-known. Electronics systems built with tin-lead solder have performed in such demanding environments as missile launches, deep space explorations and high performance aircraft missions; and have also withstood shock and vibration onboard ships from high sea states and rapid firing of large bore guns.
But the ever increasing movement to eliminate materials deemed hazardous to the environment has forced the electronics industry to adopt solders free of lead. The introduction of lead-free solder (LFS) into electronics manufacturing presents profound implications regarding the reliability of military electronics systems.
The following discussion addresses the transition from tin-lead solder and its impact on the cost and reliability of existing and future Navy systems.
Concerns for e-waste
The European Union Reduction of Hazardous Substances (RoHS) Directive, which went into effect July 1, 2006, specifies maximum concentration values on new electronics products manufactured with solders containing lead. The RoHS Directive aims to reduce pollution and prevent its damaging effects to the environment; and prevent human health problems due to occupational and post-disposal exposure to these harmful substances.
In addition to lead (Pb), the other restricted substances are: cadmium (Cd), mercury (Hg), hexavalent chromium (Cr (VI)), polybrominated biphenyls (PBBs) and polybrominated diphenyl ethers (PBDEs).
The Waste Electrical and Electronic Equipment (WEEE) Directive is the European Community directive 2002/96/EC on waste electrical and electronic equipment which, together with the RoHS Directive 2002/9595/EC, form the foundation of European law setting collection, recycling and recovery targets for all types of electrical products.
The RoHS directive places the responsibility of compliance on the "producer" of the equipment. Individual EU member states are responsible for enforcement and manufacturers self-declare compliance to the directive.
Restrictions on solders that contained lead were first enacted in the United States and other countries more than 15 years ago for use in metal pipes containing potable water and metal cans that contained food products. Lead is highly toxic and remains stable over time so there are obvious health reasons why solder alloys containing lead are prohibited in these instances.
In an effort to be green, a number of nations have opted to eliminate potentially hazardous materials from electronics, including tin-lead solder. China enacted Article 11, legislation that bans lead and other hazardous substances from electronics in March 2007.
Japan enacted JIS C 095950 (J-Moss or Japan RoHS) in 2006. In the United States, the California legislature passed the Electronic Waste Recycling Act (SB 20/50) in January 2007, which mandates the removal of lead and other hazardous materials from new consumer electronics by 2008.
Who is affected?
Electronics manufacturers that do not comply with the RoHS Directive are not able to sell products in Europe that contain tin-lead solder and may be liable for significant financial penalties for each violation. Other repercussions for non-compliance include lost profits and loss of consumer confidence.
Transition to lead-free solder has been expensive and costs may continue to climb as more manufacturers move to LFS. This is because manufacturers must use alternative materials, retool assembly lines and use different coatings for parts.
There have been negative reports on product quality and reliability resulting from the lead-free restrictions, in addition to the high cost of compliance, especially to small business.
Exemptions to the maximum allowed concentrations of restricted materials do exist for cases where technology does not yet allow for substitutions, or where alternatives may have a worse impact on human health and the environment. Some exemptions include lead in steel and aluminum alloys, lead in some types of solder, and medical, aerospace and military applications.
These products are often used in mission-critical applications where their failure can be potentially catastrophic.
But while electronics for these types of systems received exclusions from the RoHS Directive and other legislation, the reality is that the global consumer marketplace, not the defense or aerospace industries, drives the electronics industry.
What is at stake?
The Department of Defense (DoD) is faced with the prospect that many of its suppliers have already or will change from traditional tin-lead based electronics to lead-free ones. DoD, even though exempt from the legislation, will have to acquire electronic components and sub-assemblies that will be available from the supply line only in the lead-free version. Performance of lead-free electronic devices in harsh military environments may not be as reliable as legacy electronic devices.
Electronic failures could occur with the use of lead-free solder as a result of spontaneously formed “tin whiskers” and from the mixing of tin-lead and lead-free components and solders during manufacturing, rework and repair.
Tin whiskers refers to needle-like crystalline structures of tin (Sn) that form and grow on surfaces that use pure or nearly-pure tin as a final finish. Tin whiskers get their name from the thin strands of tin that resemble whiskers. Tin whiskers can cause problems in the external pins or leads of semiconductor devices, since they can bridge two adjacent leads together and form an electrical short.
The short will be temporary if the resulting current flow is enough to fuse open the whisker. Otherwise, the short will be stable and can result in equipment failures.
Tin whiskers have induced shorts on circuit cards and other electronic assemblies causing numerous failures in commercial, DoD and NASA systems. Many of DoD’s and NASA’s tin whisker issues were traced back to pure tin-plated parts mistakenly installed in these systems. Other metals such as zinc, cadmium, indium and antimony also exhibit this whisker-growing phenomenon.
The lead content of the tin-lead solder alloy and component termination finish have helped mitigate tin whiskers for many years. The ramifications of lead-free solder use on the reliability of military electronics are complex and at present there are far more questions than answers.
The potential for problems is compounded by the introduction over the last few years of many different LFS al-loys, each with different physical properties that can impact circuit card and elec¬tronic assemblies design, manufacturing and repair processes.
More questions than answers
As previously mentioned, prior to the RoHS Directive, the standard solder used for more than 60 years in the manufacture and repair of commercial and DoD equipment and systems was tin-lead based solder alloy. Two tin-lead alloys are generally regarded as acceptable for electronics: the 63percent tin/37 percent lead eutectic alloy and the 60 percent tin/40 percent lead alloy.
The 63/37 eutectic alloy has a distinct melting point of 361 F and transitions directly from solid to liquid with no plastic state. A eutectic alloy, therefore, has a sharp melting point, and a non-eutectic alloy exhibits a plastic melting range. The rapid solid to liquid transition is a desirable characteristic for solder used for electronics manufacturing and repair and contributes to highly reliable solder joints.
The legacy tin-lead solder has helped Defense systems withstand many years of service life that includes strenuous shock and vibration, temperature extremes, high humidity, high g-force and other en¬vironmental and operational challenges.
MIL-HDBK 217F, the handbook of Reliability Prediction of Electronic Equipment, is the DoD standard for the reliability prediction of electronic components. It is cited in many Defense and aerospace contracts as the standard for performance thresholds, but it does not provide reliability data for components or assemblies that use lead-free solder.
Further, component parts of military electronic system circuit cards, integrated circuits and other electronic components, assembly hardware and laminate materials have historically been designed and certified to withstand the relatively low processing temperature of tin-lead solder.
Most of the new LFS alloys have much higher melting and processing temperatures and, in some cases, these temperatures will exceed the endurance temperatures of the circuit card and component parts.
The most common lead-free solder in use is a tin, silver, copper (SnAgCu) alloy known in the electronics industry as SAC.
SAC has a melting range of between 423F to 428F and contains approximately 95percent tin and 3percent silver with the remainder composed of copper.
Prior to manufacturing, the effects of higher temperatures on lead-free solder must be evaluated for possible changes to the design of electronic components, circuit card material, production and repair processes.
The higher melting temperatures of lead-free solder have resulted in the requirement to retool many circuit card and electronic assembly production lines, and higher temperature requirements have also resulted in increased energy consumption.
In addition to the overall increase in production costs, the cost of lead-free solder is significantly more than legacy tin-lead solder, especially for a LFS alloy that contains silver or indium.
The primary questions Navy and De¬fense Department acquisition personnel need to address regarding the implications of lead-free solder are:
• Do the physical characteristics of lead-free solders offer solder joints equivalent in strength and reliability to the predictable legacy tin-lead joints?
• What is the impact on life cycle costs and logistics?
• If lead-free components are surface fin¬ished with pure electroplated tin, which is the preferred surface finish to replace tin-lead, will the resulting electronics fail because of short circuits caused by tin whisker growth?
• Will the higher assembly temperatures, which must be used to assemble electronics with the high melting point lead-free solders, damage electronic functionality of the components of the assembly, thereby causing premature failure?
These reliability issues are generally not a concern for consumer electronics which usually have a shorter service life and are designed for normal household or business use.
But DoD requirements are much more stringent. What actions are necessary to address the risks associated with the transition to lead-free solder and to eliminate or mitigate problems with existing and future DoD systems and equipment that the transition may create?
Compliance in the short-term
Lead-free solder compliance may adversely affect the performance of electronic components, such as integrated circuits, transistors and diodes, which are used in the production of new systems and repair of fielded systems in DoD programs.
Some manufacturers of electronic components have migrated to a LFS termination finish and are not identifying their new lead-free components with new part numbers or special markings. This could result in electronic circuitry in new and repaired systems that contain a combination of both tin-lead and LFS components.
The primary concern with LFS components is the growth of tin whiskers and their potential to create short-circuits. But, in some cases, the mix of tin-lead and LFS termination finishes may be incompatible with the solder alloy resulting in reduced reliability leading to premature circuit card failure.
Contracting for legacy tin-lead solder in new DoD systems would require manufacturers to run two separate production lines. This would not be economically feasible because of the small percentage of product lines that are sold to Defense and aerospace industries. It would also reduce competition and has the potential to exacerbate an existing DoD problem: Diminishing Manufacturing Sources and Material Shortages (DMSMS).
Another potential issue for DoD systems integrators and manufacturers is the introduction of commercial-off-the-shelf products that contain lead-free solder into the DoD inventory. Many systems and electronic devices which have dual use in commercial and military applications already are being manufactured lead-free. This includes computers, monitors, test equipment, processors, and communications and navigation equipment.
The ability of these systems, which are built for a global market, to operate long term in harsh military environments is unknown. Some acquisition program offices may not even be aware that they have fielded COTS products that contain lead-free solder.
Where to find help
To address the issues associated with the transition to lead-free solder, there are ongoing joint industry and government studies.
December 2005 saw the kickoff of the initial meeting of a DoD Executive Lead Free Integrated Process Team (ELF-IPT). This IPT now includes membership from all the services, government agencies and private industry. The primary purpose of the ELF-IPT is to promote lead-free training; identify DoD-specific issues; coordinate service efforts; recommend policy and guidance; and identify and promote research efforts.
The eighth ELF-IPT meeting at Naval Surface Warfare Center, Crane Division, June 7, 2007, produced draft lead-free guidance and policy. The government co-chair will present this draft to the office of the Director of Defense Research and Engineering (DDR&E) for consideration in issuing departmentwide guidance. For information on the proposed guidance, go to www.leadfreedod.com.
The Naval Supply Systems Command funded a project, called the Logistics Impact of Lead Free Circuits and Components, led by NSWC Crane Division, to allow the Naval Sea Systems Command (NAVSEA) Miniature/Micro-miniature (2M) Electronic Repair Programs In-Service Engineering Agent to expand collaboration with LFS experts from industry and academia. Project results will assist systems commands and program executive offices in making informed decisions regarding the use of lead-free components.
Another DoD project, which began several years ago under the auspices of the DoD Joint Group on Pollution Prevention and was later turned over to the DoD Joint Council on Aging Aircraft, responds to the LFS affect on the electrical reliability of military and space electronics. Initial results from this project are available at www.acqp2.nasa.gov/.
The Environmental Protection Agency completed a life cycle assessment (LCA), which evaluated the environmental impact of eutectic tin-lead solder and several LFS alloys. The results of this LCA are available at www.epa.gov/.
The U.S. Air Force issued an Airworthiness Advisory (AA-05-01) in May 2005 in response to the transition to LFS alloys. This advisory applies to all its aircraft, manned and unmanned and those aircraft operated by the Air National Guard and Reserve. The advisory includes discussion, guidance, recommendations and references pertaining to lead-free solder. The Navy has not issued similar guidance to date.
NSWC Crane will consolidate many of these studies to help provide the military community with meaningful answers with respect to use of lead-free solder.
Tom Ingram is the program manager in NAVSEA 04 for the Miniature Microminiature Module Test Repair (2M/MTR) and Gold Disk Program.
A Look at RoHS and WEEE
RoHS takes its scope from the related WEEE Directive. Annex IA of the WEEE legislation (2002/96/ EC) lists 10 broad categories of equipment that fall within its scope — and consequently the scope of RoHS. Categories 8 and 9 are currently exempt from RoHS, but are not exempt from WEEE: large and small household appliances; IT and telecommunications equipment; consumer equipment; lighting equip¬ment (including light bulbs); electrical and electronic tools; toys, leisure and sports equipment; medical devices (with the exception of all implanted and infected products); monitoring and control instruments; and automatic dispensers.
Annex IB lists examples of products considered exempt from these broad categories. However, Annex IB is not all inclusive. For example, if your equipment isn’t listed, it doesn’t necessarily mean that it is outside the scope of RoHS. Category 8 “medical devices” is considered to include: medical devices (with the exception of all implanted and infected products); radiotherapy equipment; cardiology; dialysis; pulmonary ventilators; nuclear medicine; laboratory equipment for in-vitro diagnosis; analyzers; freezers; fertilization tests; and other appliances for detecting, preventing, monitoring, treating, alleviating illness, injury or disability.
In brief, RoHS does not apply to large-scale industrial tools, spare parts for the repair of equipment put on the market before July 1, 2006, and equipment associated exclusively with national security.