Material Restrictions and Reporting: Raytheon Prepares for the Future
Emerging material restriction and reporting requirements include the Registration, Evaluation, Authorization and Restriction of Chemicals regulation, known as “REACH”; the Restriction of Hazardous Substances, or “RoHS” regulations; rare earth materials/minerals supply constraints; the Department of Defense restriction of hexavalent chromium; and conflict minerals reporting requirements.
A common approach for reducing risks is to replace the regulated or constrained materials with alternatives. However, the replacement of legacy materials that meet product performance requirements is not a trivial matter, and it often requires extensive research, engineering development and qualification. Complex risk trade-offs have to be made among performance, compliance costs and supply continuity risk. Material substitutions in legacy applications must be carefully controlled, assessed for impact and communicated accordingly.
Raytheon has established an integrated and cross-functional Global Substances Program to address the risks to our products and programs. Through this program, Raytheon ensures alignment with solutions to address materials restrictions issues across the company, as well as with the U.S. and European aerospace and defense industry, government stakeholders and academia.
One area being addressed through the Global Substances Program is the development of technologies to improve product reliability and reduce performance risk associated with the use of lead (Pb)-free materials. Another is the development of suitable replacements for hexavalent chromium in corrosion-resistant coatings.
Reducing the Risk of Pb-free Electronics
The global movement toward Pb-free electronics has completely transformed the commercial sector. The aerospace and defense industry relies heavily on Pb-free components borrowed from the commercial sector, many of which use pure tin finishes that are susceptible to the spontaneous growth of metallic filaments called tin whiskers. Tin whiskers can induce failures in electronic assemblies. Commercial off-the-shelf products and the increased volume of aerospace industry products that Raytheon relies on are now being assembled with Pb-free solder.
Raytheon is a lead participant in several industry working groups and consortia focused on obtaining performance/reliability data to understand the impacts of using such materials, as well as generating strategies and methodologies to address the risks, which include:
- Tin whiskers (from Pb-free finishes) compromising functional performance (Figure 1).
- Performance/reliability degradation of Pb-free interconnections; i.e., some Pb-free materials are inferior to Pb materials in tolerating thermal cycling, vibration and mechanical shock (Figure 2).
One approach to risk mitigation is to replace the tin finish with a tin-lead finish. This can be accomplished through either of two methods:
- 1) Self-mitigation – Cover the tin with lead-rich solder. This is normally accomplished during the surface mount solder reflow attachment with eutectic tin-lead solder.1
- 2) Re-dipping – Dip component leads in liquid tin-lead solder to a specific dip height criterion.2
Figure 3 illustrates a metric developed by Raytheon to evaluate the self-mitigation process. Raytheon has also collaborated with the University of Maryland Center for Advanced Life-Cycle Engineering (CALCE) and other industry partners in performing an Office of Naval Research-sponsored study to qualify a robotic process for the solder dipping of parts.
Another approach to risk mitigation is to conformally coat tin-plated parts for retention and tin whisker bridging prevention.
The conformal coat acts as a barrier to prevent a tin whisker from causing an arc or short circuit. The coating is designed to “stretch” over a growing whisker to form a tent-shaped barrier. Raytheon is partnering with CALCE and other industry participants to model the relationship between the physical properties of coatings and whisker retention (Figure 4).
Raytheon’s use of advanced computational materials engineering (CME) techniques to describe the basic process of whisker formation at the atomic level has led to the development of a new theory of whisker growth and inhibition. This new theory has been welcomed by CALCE members because it unites many observations that other theories cannot reconcile. These theories are leading to a better understanding of the role of lead content and microstructure on whisker inhibition.
Raytheon is a contributor to the NASA-DoD Lead-Free Electronics Project, launched in 2006, to build on the results from the 2005 Joint Council on Aging Aircraft/Joint Group on Pollution Prevention (JCAA/JG-PP) Lead-Free Solder Project. The program includes the evaluation of test vehicles under vibration, mechanical shock, thermal cycling, vibration/thermal cycle combinations and other stringent conditions. Raytheon’s role within the group is to perform the combined environment test to determine the operation and endurance limits of the solder alloys by subjecting the test vehicles to accelerated environments.
As a member of the Aerospace Industries Association (AIA) Pb-free Electronics Risk Mitigation (PERM) consortium, Raytheon personnel have taken lead roles in several teams chartered with generating standards to support the development of risk mitigation approaches for aerospace/ defense companies affected by the integration of Pb-free materials in their global supply chain.
Addressing the Risks Posed by the Restriction of Hexavalent Chromium
Hexavalent chromium compounds are particularly effective in protecting a variety of metals from corrosion. Because of this, they have been widely used in military and aerospace equipment for decades. Unfortunately, these compounds have been identified as carcinogens and are subject to increasing restrictions on their use and disposal. Raytheon has reduced its dependence on these compounds without compromising product lifecycle performance through the evaluation and implementation of existing alternatives, and through the development of new materials technologies.
Work has been done across Raytheon to evaluate and qualify non-chromium-bearing paint primers in a variety of applications. The testing of paint systems for general use on aluminum and ferrous surfaces was completed years ago, and alternatives are already in place. Several major manufacturing sites have eliminated all uses of hexavalent-chromium-bearing paints and primers. Testing on special applications, like fastener sealing, has also been completed, and alternatives for these will soon be implemented. As a result of these measures, significant reductions in the use of hexavalent chromium have been achieved. Unfortunately, there remains a limited number of challenging applications where no suitable alternatives currently exist. One such application involves the adhesive bonding of complex assemblies for use in undersea sensors. These applications have been identified as focus areas for further research and development.
Raytheon has performed an extensive evaluation of a safe trivalent chromium alternative for the widely used hexavalent chromium conversion coatings on aluminum. Trivalent alternatives for corrosion protection on aluminum are now being implemented in applications where qualification has been achieved.
Additional testing is currently in progress by the NASA/DoD Technology Evaluation for Environmental Risk Mitigation (TEERM) consortium, of which Raytheon is an active member. It is anticipated that current research will lead to an expansion of qualified surface applications, further reducing the use of hexavalent chromium conversion coatings.
The evaluation of impacts due to material restrictions, and the development and execution of measures to avoid related risks are on-going. Given the magnitude of the problems faced, Raytheon’s collaboration with academia, industry and the government through the company’s Global Substances Program has proven to be effective in developing the right solutions and mitigations to safeguard our environment and address the needs of our customers.
1T. Hester, presentation to the Space Parts Working Group meeting, April 2011.
2Final report of the TMTI Robotic Hot Solder Dip project, Office of Naval Research, 2006.
Dave Pinsky, Tony Rafanelli, Tim Sheehan
Contributors: Cynthia Garcia, Bill Rollins