A Green Conformable Thermoformed Printed Circuit Board Sourced from Renewable Materials

Abstract

Printed circuit boards (PCBs) physically support and connect electronic components to the implementation of complex circuits. The most widespread insulating substrate that also acts as a mechanical support in PCBs is commercially known as FR4, and it is a glass-fiber-reinforced epoxy resin laminate. FR4 has exceptional dielectric, mechanical, and thermal properties. However, it was designed without considering sustainability and end-of-life aspects, heavily contributing to the accumulation of electronic waste in the environment. Thus, greener alternatives that can be reprocessed, reused, biodegraded, or composted at the end of their function are needed. This work presents the development and characterization of a PCB substrate based on poly(lactic acid) and cotton fabric, a compostable alternative to the conventional FR4. The substrate has been developed by compression molding, a process compatible with the polymer industry. We demonstrate that conductive silver ink can be additively printed on the substrate's surface, as its morphology and wettability are similar to those of FR4. For example, the compostable PCB's water contact angle is 72°, close to FR4's contact angle of 64°. The developed substrate can be thermoformed to curved surfaces at low temperatures while preserving the conductivity of the silver tracks. The green substrate has a dielectric constant comparable to that of the standard FR4, showing a value of 5.6 and 4.6 at 10 and 100 kHz, respectively, which is close to the constant value of 4.6 of FR4. The substrate is suitable for microdrilling, a fundamental process for integrating electronic components to the PCB. We implemented a proof-of-principle circuit to control the blinking of LEDs on top of the PCB, comprising resistors, capacitors, LEDs, and a dual in-line package circuit timer. The developed PCB substrate represents a sustainable alternative to standard FR4 and could contribute to the reduction of the overwhelming load of electronic waste in landfills.

Figure 1

(a) Scheme of the preparation of the green PCB substrate. The bare PLA pellets and cotton fabric (CF) and the relative cross-section SEM are shown on the left. The manufacturing process is exhibited in the central plot. An image of the resulting bio-based PCB substrate (PLA-CF) and its cross-section is displayed on the right. (b, c) Height profile of the standard FR4 substrate and the PLA-CF green substrate, respectively. The x- and y-axes units are micrometers.

Figure 2

(a) Tensile stress–strain curves of pure PLA and of the PLA reinforced with cotton fabric (PLA-CF). (b) Flexural bending test of the pure PLA, FR4, and PLA-CF. (c–e) Water, ethanol, and acetone contact angles of PLA-CF and FR4 as a reference.

Figure 3

(a) Resistivity of printed silver lines on top of FR4 and poly(lactic acid) cotton fabric (PLA-CF) green PCB substrate. (b) Optical image of the scratched surface on the inked FR4 and PLA-CF substrate with forces ranging from 1 to 3 N. (c) Change in the resistance R/R₀ of the printed silver lines on the PLA-CF substrate with bending. (d) Dielectric constant as a function of the frequency of the PLA-CF and FR4 substrates.

Figure 4

(a, b) Microdrilled holes, top and cross-section SEM views of a single hole, on FR4 and green PLA-CF substrates, respectively. (c) Circuit with dual-in-line and surface mount technology components. The circuit comprised a timer, LEDs, resistances, and capacitors. With both the FR4 and green PLA-CF substrate, the LEDs were lit up and blinking, applying a voltage of 4.5 V.