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Tin Whiskers Inorganic Coatings Evaluation (TWICE)
David Hillman | Rockwell Collins, Inc.
Tin whiskers are thin filaments of tin that can grow from a tin surface and potentially cause electrical shorting or other failure modes. While the tin whisker phenomenon was recognized decades ago, it was not an active area of research after it was determined that a small amount of lead (~2-4%) in a surface finish greatly diminished the likelihood of tin whisker formation. The recent transition to lead-free solders and surface finishes has eliminated this mitigation approach and thus generated a need for other methods to inhibit the formation and growth of tin whiskers in high reliability electronics.
The objectives of the Tin Whiskers Inorganic Coating Evaluation (TWICE) program were to demonstrate the use of alkali silicate glass (ASG) coating to mitigate tin whiskers and to establish manufacturing processes to implement the technology into Department of Defense (DoD) applications. Phase 1 focused on establishing the viability of using ASG as a tin whisker mitigation method. Phase 2 leveraged the results from Phase 1 to improve the coatings developed and transition the application processes to a manufacturing-representative environment.
The TWICE program developed, applied, and evaluated the effectiveness of using ASG coatings to inhibit tin whisker initiation and growth. The ASG formulations were deposited with equipment and conditions that are typical of those used to apply conventional conformal coatings, such as acrylic materials. ASG materials are inorganic materials and therefore are much less permeable to moisture and oxygen, both of which can promote tin whisker growth. The relatively high strength and stiffness of ASG coatings compared to organic coatings allow them to better contain tin whiskers and alter their growth characteristics. Finally, because ASG coatings can be applied in very thin layers and are inorganic, they have less impact on the electrical performance of circuits to which they are applied, particularly those operating at high frequency such as Radio Frequency (RF).
The TWICE program investigated formulations of materials that included composites with nanoparticles. Design of Experiments (DOE) was used to identify process parameter combinations for controlling ASG coating properties. A number of coatings with different material formulations, nanoparticle filler, and thicknesses were applied to test substrates, test components specifically designed to exhibit tin whisker growth, and functional electronic components.
Phase 1 focused on material formulation and characterizing the effects of coatings on test samples. ASG coatings, as well as conventional coatings, were applied to samples both in a lab environment at Rockwell Collins and in a manufacturing environment at Plasma Ruggedized Solutions. These samples were subjected to an elevated humidity/temperature environment and subsequently inspected to characterize whisker growth over a range of exposure times. This testing revealed that some coating combinations inhibited tin whisker growth while other material combinations actually accelerated tin whisker growth. Locally high stresses within tin plating, due to residual stresses in the higher-stiffness ASG formulation, as well as unfilled cracks in the tin plating, led to above-average whisker growth. The most effective coatings for inhibiting tin whiskers were composites of more ductile ASG and smaller nanoparticles. Phase 1 also included testing at the University of Maryland to characterize the effect of coatings on inhibiting Metal Vapor Arc (MVA), which can be induced by tin whiskers. That work showed that ASG coatings inhibited MVA in a similar manner to conventional coatings.
Phase 2 extended the work of Phase 1 to update material formulations, apply coatings to representative test board assemblies, evaluate the effect of coatings on tin whisker growth, and conduct preliminary assessments of the system-level impact of using ASG coatings to mitigate tin whiskers. Coatings were again applied at both Rockwell Collins and Ruggedized Plasma Solutions. Inspection of coated assemblies revealed that coverage with ASG was better than with acrylic coating and two combinations of ASG coatings reduced the growth of tin whiskers relative to acrylic. Moreover, the ASG coatings disrupted tin whisker growth and reduced the risk of forming long tin whiskers that could lead to functional failures. Conventional acrylic coatings only slowed the growth of whiskers; it appeared that whiskers under this material will eventually penetrate through the coating and continue to pose a system risk.
The ASG coatings will not provide a ‘silver bullet’ that eliminates tin whiskers. However, they can inhibit the formation of tin whiskers to as great, or greater, an extent as conventional conformal coatings such as acrylic. Since the ASG coatings not only inhibit the formation of whiskers but also significantly affect their growth, they can potentially provide a more significant risk mitigation approach as they minimize the occurrence of whiskers long enough to cause electronic failure. In addition, the dielectric properties of ASG may make them more suitable for certain electronics, such as RF systems. Further work is needed to characterize the long-term growth of whiskers on functional hardware and the robustness of materials in harsh environments such as salt fog and vibration.