Substrate and Conductor Materials Towards More Sustainable Electronics

Conductive Materials Beyond State-of-the-art

Although substrates constitute bulk electronic circuit waste, the components, electrical leads, and soldering materials also contain scarce and/or environmentally harmful materials. While copper is traditionally used as the lead material in FR4-based PCBs, silver represents the state of the art in printed and flexible electronics due to excellent printing ink properties and general environmental stability, retaining these qualities even in particulate form and low-temperature processing. However, silver is well known as an anti-bacterial material and thus harmful to the natural environment, requiring recovery and recycling of the material, which are both cost and energy-intensive processing procedures.¹⁰ Furthermore, silver is becoming a scarce resource and increasingly expensive to extract from natural sources. Consequently, various methods to reclaim silver are in development.²¹

Copper tracks in standard PCBs are made by etching, resulting in significant waste material that renders them unsuitable as ecological substrates. However, printable copper-based materials have been developed and are increasingly attractive as silver alternatives when careful steps are taken to reduce copper oxidation.^22,23 The sintering process is one of the key shortcomings of copper-based inks. However, converting resources completely to bulk copper can help the material achieve electrical conductivity analogous to printed silver. Because this process is conducted at low temperatures, it qualifies as an ecological substrate and flexible material. Current inks can be sintered at temperatures as low as 130–140 °C.²⁴

It has been shown that the impact of switching from silver to copper (Cat. No. 901889) in printed antenna board manufacturing reduced the global warming potential by 28%. When implemented on a large scale, switching from silver to copper would bring significant reduction potential to printed electronics.¹⁹ Mixing silver with more renewable materials such as graphene can also reduce silver usage.²⁵ Table 2 illustrates that silver is unsustainable as a conductor material as the material supply is already threatened.²⁶ Copper supply is currently maintained, but the availability is limited from conventional sources. However, carbon in this context (not as carbon-based fuel) is abundant, and no supply issues are anticipated.

Carbon-based conductors are ideal from an environmental and ecological perspective, provided they demonstrate sufficient conductivity for the intended application. They can be produced and applied without using unsustainable chemicals, high energy processing, or water consumption. From a material property perspective, graphene (Cat. No. 900561), graphene nanoribbons (Cat. Nos. 922714, 922676), and carbon nanotubes (Cat. Nos. 901019, 698849, 755125) show superior electrical conductivity compared to metals including copper and silver (~10⁶ S/cm). Still, in application, interfaces between high conductivity domains limit conductivity, resulting in a practical conductivity of 1–0.1 % of the metal conductivity.²⁷ Carbon conductivity can be increased with doping, adding processing steps and cost. However, doped graphene fibers can almost reach the level of Al/Cu/Ag wires.

In general, printing ink production can be conducted using either solvent or water-based formulations. The desired application determines the suitability of water-based ink, as it usually is not compatible with hydro-phobic substrates, and the adhesion of water-based printed features is typically lower than that of solvents. Water-based ink generally requires higher drying temperatures and longer drying times, increasing environmental impact. That said, solvent-based inks emit toxic Volatile Organic Compounds (VOCs), and their effect on sustainability will depend entirely on the ability of the production facility to manage emissions.²⁸ When using printed silver nanoparticles, the contribution of conductors is typically the primary factor in environmental consequences, while the influence of other chemicals is much lower.⁶

Aluminum is a low-cost, abundant alternative to both copper and silver. While not directly environmentally hazardous, it is energy intensive to produce and recycle. Aluminum oxidizes very easily, but when applied in thick layers, its native surface oxide protects the lower levels from further oxidation. Therefore, aluminum is best applied in a thick layer as particle-based systems are impractical due to oxidation. Patterned layers of aluminum can be applied by foil transfer via various methods, the most common being thermal foil transfer.²⁹

Conclusion

Circular design enabling reusability and recyclability of electronics is critically important alongside waste stream management and material recovery. The work towards sustainable substrates is well underway and increasing effort is being directed towards more sustainable conductors. Optimistically, the first novel material recovery based on biodegradable substrates has been reported.³⁰ We anticipate these advances will be joined by renewable active components via bioelectronics in addition to the further exploitation of these materials’ intrinsic properties.