# Development of conductive inks with bio-based matrix for Printed Electronics and 3D Structural Electronics.

Thomas Guérin¹,², Vincent Semet¹, Tony Gerges¹,
Philippe Lombard¹, Bruno Allard¹, Jean-Yves
Charmeau¹, Guy Masse², Michel Cabrera¹

¹Univ Lyon, Université Claude Bernard Lyon 1, Ecole Centrale de
Lyon INSA-Lyon, CNRS, Laboratoire Ampère, F-69622, Villeurbanne,
France,

²Encres Dubuit, 77290, Mitry-Mory, France

Encres
DUBUIT
INNOVATIVE INKS

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**We consider a conductive ink consisting of a charge of silver and an organic matrix (binder + solvent).**

**Questions:**

⇒ How to formulate a biobased organic matrix ?

⇒ How to develop an ink for Printed Electronics (EI) and In-Mold Electronics (IME) ?

## Introduction and context

In the context of electronics, a conductive ink is formulated to be capable of being printed on flexible polymer supports for EI, or even thermoformed and overmolded for IME, with satisfactory electrical conductivity and adhesion.

⇒ Among all the printing techniques, we will retain **screen printing**.

⇒ **Thermoforming** is a process for shaping a thermoplastic polymer substrate film by heating and deformation with 3D tooling.

⇒ **Overmolding** is the injection of a molten thermoplastic polymer onto a previously printed substrate.

A solvent and four biosourced binders were selected. The influence of thermoforming on conductive tracks is studied using aluminum tools. The four bio-based matrix formulations are compared, as well as a non-bio-based commercial ink dedicated to IME. The electrical resistance is studied as a function of the stretching of the thermoformed tracks.

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(1)⇒ Aluminum thermoforming tooling, with 6 radius of curvature, R=5 mm up to R=30 mm and a conical connector.

(2)⇒Simulation (T-SIM software) of the thickness reduction of the polymer substrate during thermoforming (Polycarbonate substrate, initial thickness 375 µm, Covestro Makrolon).

(3)⇒Thermoformed PC film with 3D tooling. Critical stretch areas are highlighted in red. Beforehand, a grid was printed on the 2D film.

## 1-Inks formula with different binders

### Formulation (%wd) :

⇒ 60 % silver content.

⇒ 4 % to 10 % of biobased binder (variable content to adapt the rheology).

⇒ 30 % to 36 % of biobased solvent (in link with binder content).

Mixing with planetary system (Hauschild Speedmixer DAC400).

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## 2-Measurement of electric resistivity

After screen printing of tracks, with conductive ink.

### Resistivity formula :

$$ (eq.1) \qquad \rho = \frac{R.S}{L} $$

Where :

ρ = resistivity in (μΩ.cm)

R = resistance (Ω)

s = cross-section area (μm²)

L = distance between probes (mm)

(1)⇒ Measurement of the resistance R of a conductive track with a 4 probes head.

(2)⇒Determination of track section(s) with a confocal polychromatic probe.

(3)⇒Distance between fixed internal probes (1.27 mm) (L).

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## 3-Characterization at the thermoforming stage

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(A)
Manual printing by
screen-printing & curing
(130°C/30')

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(B)

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confocal
measurement

s = section (μm²)

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(D)

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2 probes method for
local measurement

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(step 1)

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(step 2)

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4 probes

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(C)

R = resistance (Ω)
L = length (mm)

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credit: Ossila

## 4-Results with different binders

### Methodology

A)⇒ **The inks are printed** with a polyester screen printing screen with a mesh size of 90 threads per cm, with threads with a diameter of 40 µm, on a polycarbonate substrate (thickness 375 µm). The screen allows the printing of a network of 49 tracks 250 mm long, with a grid every 5 mm.

B)⇒ After printing, **the inks are cured** in a forced air oven at 130°C for 30 min.

C)⇒ Then, **the resistance is measured** each 5 mm segments, all along the tracks, with a 2 probes method.

D)⇒ **The resistivity is determined by (eq.1).**

E)⇒ **The thermoforming is made** at 160 °C, under vacuum of -1 bar, during 30 sec, with tools.

F)⇒ Once again, the **resistance is determined all along the tracks**. Warped/stretched plastic parts will be more resistive, because the ink will also be distorted.

## Results

<table><thead><tr><th rowspan="2">Inks</th><th colspan="4">Before thermoforming</th><th>After thermoforming</th></tr><tr><th>Resistance R (Ω)</th><th>Cross-section s (μm²)</th><th>Thickness of tracks (μm)</th><th>Resistivity (μΩ.cm)</th><th>Cracked tracks /49</th></tr></thead><tbody><tr><td>L1</td><td>0.094 ± 0.002</td><td>4353 ± 208</td><td>9.84 ± 0.34</td><td>32.5 ± 1.0</td><td>12 ± 2</td></tr><tr><td>L2</td><td>0.080 ± 0.001</td><td>3925 ± 520</td><td>10.35 ± 0.75</td><td>21.8 ± 3.3</td><td>15 ± 2</td></tr><tr><td>L3</td><td>0.182 ± 0.009</td><td>3844 ± 360</td><td>9.07 ± 0.67</td><td>60.2 ± 5.0</td><td>3 ± 2</td></tr><tr><td>L4</td><td>0.123 ± 0.003</td><td>4832 ± 330</td><td>9.56 ± 0.87</td><td>45.6 ± 3.1</td><td>12 ± 2</td></tr><tr><td>Commercial ink</td><td>0.107 ± 0.002</td><td>4953 ± 305</td><td>11.32 ± 1.52</td><td>43.4 ± 2.3</td><td>4 ± 3</td></tr></tbody></table>

⇒ L2 ink offers the best electrical resistivity, however it is not suitable for thermoforming (large number of printed lines cut during thermoforming). This formulation is retained for use in 2D flexible printed electronics (EI).

⇒ L3 ink has a some higher resistivity than other formulations, but remains within an acceptable range. It has very good thermoforming performance. We retain this solution as the formulation for the IME.

⇒ L1 and L4 inks offer intermediate performance, not relevant in EI or IME.

### 3D experimental characterization of line resistances after thermoforming
(average resistance of 5 mm segments)

Ink L3

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Cracked tracks after
stretching, at the level of
maximum electrical resistance.
(Hirox RH2000)

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Commercial ink

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Cracked tracks after
stretching, at the level of
maximum electrical resistance.
(Hirox RH2000)

### Results interpretation :

⇒ The areas of stretching and deformation constraints are found between the base plane and the start of the 3D tooling.

⇒ Commercial ink has lower peak resistances than L3 ink, to be weighted with the track thickness and loading rate of the commercial ink.

⇒ The L3 ink, which is suitable for the IME process, has overall performances which are in accordance with the state of the art, comparable with the reference commercial ink.

⇒ Overmolding tests with polycarbonate, as part of the ESS, were successfully carried out with L3 ink.

## Conclusions & outlooks

⇒ These conductive inks partly based on green materials allow to offer a **more responsible** alternative to traditional conductive inks for EI and IME, with performance in line with the state of the art in the field.

⇒ L2 ink is chosen as the **solution for EI**, and L3 ink for IME.

⇒ In perspective, the chemical **recycling** of these inks is envisaged to reuse the silver and the binder in new inks.

⇒ **Improving the performance** of inks in conductivity and thermoforming is considered with the evolution of the formulation.

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UNIVERSITÉ
DE LYON

Université Claude Bernard

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UB

Lyon 1

INSA

INSTITUT NATIONAL
DES SCIENCES
APPLIQUÉES
LYON

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Encres
DUBUIT
INNOVATIVE INKS

ART
ASSOCIATION NATIONALE
RECHERCHE TECHNOLOGIE
N° 2020/0979

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SGE

2023

Symposium de
Génie Électrique