Eco-Friendly Printed Circuit Boards: Present and Future Manufacturability
A December 16, 2019 study by the Potsdam Institute for Climate Impact Research provided insight about how human interactions with one part of the earth’s systems can impact another area. The research presented in the study speaks about nine planetary boundaries or critical systems that regulate the planet. While this topic may seem obvious, the study shows that a dense and very complex network of interactions exist and that those interactions cascade--and amplify in a non-linear fashion--from one system to the next.
While the negative implications of the study capture our attention, positive implications also exist. Johan Rockstrom, Director of the Potsdam Institute, describes the positive factors in this way:
“If we reduce our pressure on one planetary boundary, this will in many cases also lessen the pressure on other planetary boundaries. Sustainable solutions amplify their effects--this can be a real win-win.”
Accomplishing the “win-win” requires a high-level, integrated understanding of the dynamics that exist in those systems.
Our Electronic Industry Impacts the Environment
Unfortunately, the electronics industry contributes to the imbalance that affects earth’s systems. Traditional PCB manufacturing relies on energy intensive and high-emission processes that involve copper, epoxy resin, glass fiber, and water. After a product lifecycle ends, PCBs become a waste product. More than 40 million tons of global electronic waste current exist and that number continues to annually increase by 3-5%. Two million tons of printed circuit boards are part of that total.
The improper handling of the waste circuit boards further exacerbates environmental problems as well as threats to human health. Separating metals from PCBs through incineration releases toxic gases. Using acids to remove metals creates large expanses of acidified waste water.
Reversing the impact of our industry on the earth’s critical systems occurs through the same innovation that has propelled the electronics industry to more profitability. Innovation in producing environmentally friendly printed circuit boards ranges from additive processes that use inkjet and laser printing to the limited production of fully biodegradable boards. Although some processes currently only work for limited run PCB production, the introduction of the technologies offers promise for the future. The innovation continues with methods that promise to recycle 90 - 98% of the copper from waste boards and with the use of environmentally friendly etchants for existing subtractive processes.
Additive Processes Promise Sustainability
Additive PCB printing processes using 3D printed electronics, conformal electronics, aerosol, inkjet and laserjet printing, and direct wire production take us completely away from the multistep subtractive processes that continue to dominate PCB manufacturing. In contrast to subtractive processes that remove and waste unneeded materials, additive manufacturing only applies needed material.
With additive processes, a design team can build schematics and layouts in PCB design software, ensure that the work adheres to design rules and error checks, manipulate the design in ECAD and MCAD environments, and then export the finished design to a printer for printing traces on the substrate. Additive processes do not require etchants or photomasks and work well for complex designs on thin substrates.
While we haven’t learned how to plant wind turbines, there are healthier alternatives for circuits.
Manufacturers use 100% solid conductive inks and toners loaded with charged particles that do not contain volatile organic compounds (VOCs) or have a need for etch resists. Conductive inks may consist of silver nanoparticles that can print circuits to plastic, fabric, and paper substrates. Given the precise nature of the printing, little or no material waste occurs as the processes build live circuitry.
3D printing for electronic circuits uses a base material and conductive materials for applying the circuitry. By combining these materials, a manufacturer can 3D print the complete electronic circuit including the board, traces, and components as a single, continuous part. This technique gives the advantage of printing PCBs that have different shapes or designs and that can match product requirements. In addition, 3D printing for circuits allows a design team to customize a printed circuit according to customer needs.
Additive manufacturing can occur as either a single-build process or as a post-production process that builds electronic circuits separate from producing the entire device. A single-build process produces both the internal electronic circuitry and the external case as a single assembly. Because of this advantage, manufacturers can produce nanometer-sized integrated circuits that substantially reduce product footprints. While the post-production process requires another assembly step, integrating an electronic circuit into a product allows for additional checks.
Are Paper PCBs Part of the Future?
During 2014, researchers produced the first prototype paper-based multilayer printed circuit board. Combined with additive processes that print conductive materials to paper and fabrics, the introduction of a paper PCB (P-PCB) could offer significant environmental benefits. The research team focused on the mechanical strength of the single-, double-, and three-layer paper made from cellulose nanofibers and used universal testing procedures to verify that paper offered the tensile strength needed for supporting electronic components.
Along with testing the mechanical properties of paper, the research team also tested the capability of paper to accept inter-layer connections. The team produced multilayer P-PCBs with five and ten layers of printed circuits with filled via connects. Adhesion tests showed that the conductive patterns constructed from silver particles retained strength and bonded well to the paper substrate. Other tests showed that the flexibility of the P-PCBs did not harm the conductivity of the patterns and that the paper PCBs had lower lifecycle costs than FRP-based boards.
Research teams have also developed printed circuit boards made from natural cellulose fibers extracted from agricultural wastes and coproducts. In contrast to non-biodegradable glass fiber and epoxy boards, the biocomposite boards do not contain chemicals. Biocomposite boards offer a dielectric constant that varies between two and 36 and do not exhibit problems when exposed to high humidities or high temperatures.
Partners in the United Kingdom-based ReUse (Reusable, Unzippable, Sustainable Electronics Project) introduced PCBs constructed from a series of unzippable polymeric layers that make up a thermoplastic substrate. Synthetic biodegradable polymers consisting of polylactide, polycaprolactone, and polyglycolide offer the advantages of predictable physical properties and adaptability through chemical engineering.
Testing of the prototypes demonstrated that the boards can withstand prolonged thermal cycling and damp heat stressing. When the boards reach the end of lifecycle, the layers of ink, adhesive, and polymers separate after immersion in hot water and become available for reuse. The unzippable polymeric layers work for rigid, flex, and rigid-flex circuit designs.
Biodegradable Electronics Have Arrived
The work accomplished with biodegradable PCBs has introduced polymers can function as insulators or conductors. Because biodegradable substrates used in PCBs and electronic devices cannot withstand subtractive processes subject the materials to etching and chemical washes, additive process that use transfer printing can build traces or components onto a substrate.
Biodegradable dielectrics used for PCBs and semiconductor components occur through the placement of high-dielectric constant fillers into a degradable polymer matrix. Researchers have discovered that plant-based fibers provide desirable dielectric properties. As an example, cotton fibers have a dielectric constant of 17 for a frequency range of 60 - 1000 hertz. Synthetic polymers such as polyglycerol sebacate offer the dielectric constants required for capacitive sensors.
Although work with biodegradable semiconductors remains in its infancy, ongoing research into charge transport of semiconducting polymers may lead to biodegradable components. Along with the research into biodegradable semiconductors, teams have also doped polymers into a conducting state. Testing has shown that those polymers can work as device interconnects and contact points.
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