Illuminating Innovation: How to Print a Flexible PCB with Silver Ink on PET

Voltera's recent project showcases the successful prototyping of a flexible, multilayer LED roulette circuit using silver ink on a polyethylene terephthalate (PET) substrate. Traditionally fabricated on rigid materials like FR4, this innovative approach demonstrates the potential of flexible electronics in educational settings and beyond.

The redesigned circuit comprises three layers: a base conductive layer, a dielectric layer, and a top conductive layer. Each layer maintains flexibility after curing, ensuring the circuit's integrity even after encapsulation and repeated bending. When powered, the circuit emulates a roulette wheel, with LEDs lighting up in a rapid circular sequence before slowing down and stopping on a single LED.

This project underscores the educational value of converting rigid circuit designs into flexible ones, allowing students to explore diverse materials and optimize print settings through experimentation. It also demonstrates the potential of prototyping flexible PCBs to replace electronics that were considered rigid, achieving lightweight, compact, and resilient designs.

Project Overview

Purpose

The goal of this project was to prototype a flexible multilayer LED wheel roulette circuit that was traditionally considered rigid and validate the redesign of the circuit. 

Design 

This project involves a multilayer flexible PCB for the LED roulette circuit and a traditional control board that powers it. We adapted the design of an LED roulette circuit, originally developed for a 3” × 4” FR4 board by ITIZ, Voltera’s authorized reseller in Korea. This new version is printed on a coated biaxially oriented PET substrate, allowing the circuit to remain flexible — even when populated with components.

We divided the LED roulette circuit design into three layers:

• Base conductive layer
• Dielectric layer
• Top conductive layer

Figure 1: Original LED roulette circuit design by ITIZ

Figure 2: Voltera modified design of the LED roulette circuit

Desired outcome

We anticipated the circuit would function like a roulette wheel: each time the pulse generator is triggered, the LEDs on the flexible PCB light up in a rapid circular sequence before slowing down and stopping on a single LED.

Functionality

By printing a dielectric and crossover layer on top of the base conductive layer, we created a compact and lightweight LED roulette circuit without using jumpers. Each layer remained flexible after curing, and the circuit maintained its integrity even after encapsulation and repeated flexing. 

When connected to power, pressing the switch on the control board caused the LEDs on the flexible PCB to light up, replicating the behavior of a roulette wheel as intended.

Printing the control board

This board controls the pulse signals and powers the LED roulette wheel circuit. We used V-One to print the circuit and reflow the components, which included:

• NE555DR timer
• LED
• Switch
• 47 µF capacitor
• 0.1 µF capacitor
• 10 nF capacitor
• 100 kΩ resistor
• 330 Ω resistor

Figure 3: Schematic for the control board

Figure 4: The control board

Printing the flexible PCB

This flexible PCB receives signals from the control board and lights up the LEDs as directed. 

Base conductive layer

This layer consists of a roulette-shaped circuit (90 mm L × 70 mm W) that connects to the control board, as well as power and ground terminals, with designated gaps for dielectric pads.

Figure 5: Schematic for the base conductive layer

Figure 6: NOVA print settings, base conductive layer

Figure 7: Print result, base conductive layer

Dielectric layer

This layer consists of 29 dielectric pads that provide insulation for the top conductive layer. For better coverage, we printed two passes of dielectric ink.

Figure 8: Schematic for the dielectric layer

Figure 9: NOVA print settings, dielectric layer

Figure 10: Print result, dielectric layer

Top conductive layer

This layer consists of 29 fine crossover traces that connect the paths to the power terminal and control board. To address potential gaps over height changes, we decreased the trace width to 100 mm to enhance trace continuity.

Figure 11: Schematic for the top conductive layer

Figure 12: NOVA print settings, top conductive layer

Figure 13: Print result, bottom conductive layer

Post-processing the flexible PCB

Dispensing solder paste and reflowing the components

Once the circuit was cured, we dispensed solder paste using NOVA and populated the components before reflowing them in an oven.

Figure 14: NOVA print settings, solder paste

Figure 15: NOVA dispensing solder paste

Because the wires are not heat-resistant, we manually soldered them onto both the companion board and the flexible roulette circuit.

Encapsulating the components

To reinforce their connection to the substrate, we dispensed encapsulation resin using a syringe onto the components and cured the resin in a UV light box.

Figure 16: Encapsulated circuit being cured in a UV light box

Once cured, we manually soldered the wires of the control board and the wires of the flexible roulette circuit together. 

Figure 17: Soldering the wires together

Challenges and advice

Electrical shorts

We encountered issues with shorts due to multiple crossover lines on the dielectric layer. To address this, we recommend printing a second pass of the dielectric layer to achieve a thicker insulating layer that better supports additional layers printed on top.

Drastic height changes

A second pass of the dielectric layer can result in significant height differences. To handle this, it is advisable to set a low probe pitch to ensure a more accurate height map and print a thicker crossover layer for improved contact on the top layer.

Maintaining flexibility with components attached 

To ensure the circuit remained flexible after populating components, we opted for smaller, more flexible components. This, combined with the use of a flexible substrate, encapsulant, and inks, allowed the circuit to bend without damaging the connections.

Conclusion

This project highlights the educational value of converting rigid circuit designs into flexible ones, allowing students to explore diverse materials and optimize print settings through experimentation. It also demonstrates the potential of prototyping flexible PCBs to replace electronics that were considered rigid, achieving lightweight, compact, and resilient designs.

As flexible PCBs gain more popularity, hands-on experience with these technologies becomes essential for staying ahead in the rapidly evolving electronics industry. We invite you to explore our other projects as we continue to explore the possibilities of flexible PCBs and printed electronics.