PHOTONIC CURING OF CONDUCTIVE SILVER
INKS AND GLUES FOR IN-MOLD ELECTRONICS
ON POLYCARBONATE AND BIO-BASED
BIODEGRADABLE POLYLACTIC ACID FILMS

Hugo Cauchy-Clerc, Tony Gerges, Philippe Lombard, Vincent Semet, Bruno

Allard, Michel Cabrera, Jean-Yves Charmeau

► To cite this version:

Hugo Cauchy-Clerc, Tony Gerges, Philippe Lombard, Vincent Semet, Bruno Allard, et al. PHOTONIC CURING OF CONDUCTIVE SILVER INKS AND GLUES FOR IN-MOLD ELECTRONICS ON POLYCARBONATE AND BIO-BASED BIODEGRADABLE POLYLACTIC ACID FILMS. LOPEC 2024, Mar 2024, Munich, Germany. hal-04498810

HAL Id: hal-04498810

https://hal.science/hal-04498810v1

Submitted on 11 Mar 2024

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Distributed under a Creative Commons Attribution 4.0 International License
# PHOTONIC CURING OF CONDUCTIVE SILVER INKS AND GLUES FOR IN-MOLD ELECTRONICS ON POLYCARBONATE AND BIO-BASED BIODEGRADABLE POLYLACTIC ACID FILMS

Hugo Cauchy-Clerc¹*, Tony Gerges², Philippe Lombard², Vincent Semet², Bruno Allard², Michel Cabrera²*, Jean-Yves Charmeau¹

¹ Université de Lyon, Université Claude Bernard Lyon 1, INSA Lyon, Ecole Centrale de Lyon, CNRS, Ingénierie des Matériaux Polymères IMP, UMR 5223

² Université de Lyon, Université Claude Bernard Lyon 1, INSA Lyon, Ecole Centrale de Lyon, CNRS, AMPERE, UMR 5005

*hugo.cauchy-clerc@insa-lyon.fr; michel.cabrera@insa-lyon.fr

## Projet ANR-22-CE05-0015-03 AMBRE : Antenne Multifonctionnelle Biosourcée pour la récupération d'Energie

## INTRODUCTION

<figure><img src="image_2.png" /></figure>

* In-Mold Electronics is a new manufacturing process allowing the production of electronic parts on a 3D polymer part by screen-printing the electronics directly on the surface of the polymer, using conductive silver-filled inks. It allows for production of lighter and simpler components compared to traditional electronics + housing assemblies.

<figure><img src="image_3.png" /></figure>

* This process a few drawbacks, notably that those inks have a higher resistivity than the bulk copper used in traditional PCB, and that during the manufacturing, they need to be thermally cured in ovens in order to evaporate the solvents they contain and improve their conductivity. This process is long and only usable on high-temperature resistant substrates such as polycarbonate.

<figure><img src="image_4.png" /></figure>

* In the AMBRE project, we want to use the IME process on a bio-based and biodegradable polymer substrate : Polylactic Acid (PLA). This polymer cannot stand the high curing temperatures needed for the IME process, thus we propose to use the Photonic Curing technology to cure the inks on this substrate.

## IN-MOLD ELECTRONICS

<figure><img src="image_5.png" /></figure>

## PHOTONIC CURING TO IMPROVE INK CONDUCTIVITY

<figure><img src="image_6.png" /></figure>

* Resistivity of the tracks depending on the type of curing:

<table><thead><tr><th rowspan="2">Oven Curing</th><th colspan="2">Configuration</th><th>Temperature</th><th>Duration</th><th colspan="2">source</th></tr><tr><th colspan="2">P0 PC</th><th>120°C</th><th>20 min</th><th colspan="2">Supplier data</th></tr><tr><th colspan="2">P0 PLA</th><th>55°C</th><th>2h30</th><th colspan="2">Previous work [1]</th></tr><tr><th rowspan="5">Photonic Curing</th><th colspan="2">Configuration</th><th>Frequency</th><th>Pulse duration</th><th>Number of pulses</th><th>Cycle Time</th></tr><tr><td>P1</td><td></td><td>1 Hz</td><td>1000 µs</td><td>20</td><td>20 s</td></tr><tr><td>P2</td><td></td><td>1 Hz</td><td>1000 µs</td><td>40</td><td>40 s</td></tr><tr><td>P3</td><td></td><td>10 Hz</td><td>1000 µs</td><td>40</td><td>4 s</td></tr><tr><td>P4</td><td></td><td>20 Hz</td><td>1000 µs</td><td>40</td><td>2 s</td></tr><tr><td>P5</td><td></td><td>30 Hz</td><td>1000 µs</td><td>40</td><td>1,33 s</td></tr></thead></table>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

Screen printed pattern for this study: 49 lines, L= 160mm, w = 1 mm

Photonic Curing allows a reduction of 67% of the track resistance on PC substrate, and 82% on PLA substrate, compared to regular oven curing.

<figure><img src="image_10.png" /></figure>

* Effect of repeated curing:

<figure><img src="image_11.png" /></figure>

Multiple Photonic Curing cycles allow to further reduce track resistance by 20%, while increasing the process time by only a few seconds.

<table><thead><tr><th>Configuration</th><th>Average track resistance</th><th>Comment</th></tr></thead><tbody><tr><td>P3</td><td>14.5 Ω</td><td rowspan="2">Reference</td></tr><tr><td>2x P3</td><td>12.5 Ω</td></tr><tr><td>3x P3</td><td>12.1 Ω</td><td></td></tr><tr><td>5x P3</td><td>11.8 Ω</td><td></td></tr><tr><td>10x P3</td><td>12 Ω</td><td>Damage to many tracks</td></tr></tbody></table>

## PHOTONIC CURING

<figure><img src="image_12.png" /><figcaption>Novacentrix PulseForge Invent Photonic Curing Machine</figcaption></figure>

* High-intensity light pulses (Xenon broad spectrum lamp)

* The light is mainly absorbed by the ink/solder paste

* Very short pulses to limit the heating of the polymer substrate (in the millisecond range, 1 µs increments)

* Depending on the parameters, ink annealing and SMD soldering may be done

<figure><img src="image_13.png" /><figcaption>Broad spectrum, can be limited by filters</figcaption></figure>

## PHOTONIC CURING TO IMPROVE GLUE BONDS

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /></figure>

a) Pattern used for this experiment

b) Pattern screen printed on a PC film with 1206 SMDs glued on

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /></figure>

* Effect on bond electrical resistance:

Mean bond resistance after treatment, log scale

<figure><img src="image_18.png" /></figure>

* Thermal curing parameters:

* 20 min @ 120°C for PC films (supplier recommended settings)

* 2h30 min @ 55°C for PLA films (based on previous work)

* Photonic curing :

* 450V, 1000µs, 10Hz, 40 pulses (P3)

<figure><img src="image_19.png" /></figure>

* Effect on bond mechanical strength:

<figure><img src="image_20.png" /><figcaption>XYZTec Shearing tester</figcaption></figure>

<caption>BEFORE</caption>

<figure><img src="image_21.png" /><figcaption>AFTER</figcaption></figure>

<figure><img src="image_22.png" /></figure>

* Fracture surface is measured with an optical microscope after the shear test

<figure><img src="image_23.png" /></figure>

Comparison between conventional oven curing and photonic curing for silver glue bond strength

<figure><img src="image_24.png" /></figure>

Photonic curing of conductive glues improves their conductivity by 86% (PC) to 95% (PLA) depending on the substrate, at the cost of a lowered mechanical strength.

## CONCLUSION

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* Photonic Curing substantially improves track and bond conductivity compared to regular oven curing.

<figure><img src="image_26.png" /></figure>

* It comes at the cost of slightly weaker bond strength: a structural adhesive is still needed to ensure mechanical strength of the whole part.

<figure><img src="image_27.png" /></figure>

* Photonic Curing also allows to make IME parts on low-temperature substrates like PLA, with close electrical performance compared to conventional substrates (PC).

<table><thead><tr><th></th><th>Oven</th><th>Photonic (P3)</th><th>Change (%)</th></tr></thead><tbody><tr><td>Ink Curing - resistance (PC)</td><td>28 Ω</td><td>10 Ω</td><td>-67%</td></tr><tr><td>Ink Curing - resistance (PLA)</td><td>83 Ω</td><td>14 Ω</td><td>-82%</td></tr><tr><td>Glue Curing - bond resistance (PC)</td><td>2.9 Ω</td><td>0.4 Ω</td><td>-86%</td></tr><tr><td>Glue Curing - bond resistance (PLA)</td><td>8.1 Ω</td><td>0.4 Ω</td><td>-95%</td></tr><tr><td>Glue Curing - bond strength (PC)</td><td>7.3 N/mm²</td><td>5.4 N/mm²</td><td>-26%</td></tr><tr><td>Glue Curing - bond strength (PLA)</td><td>6.0 N/mm²</td><td>3.8 N/mm²</td><td>-36%</td></tr></tbody></table>

<figure><img src="image_28.png" /></figure>

Plastronique
Convergence de l'électronique et de la plasturgie

<figure><img src="image_29.png" /></figure>

Ingénierie Matériaux Polymères

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cnrs

anr

agence nationale de la recherche

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UNIVERSITÉ DE LYON

INSA

INSTITUT NATIONAL DES SCIENCES APPLIQUÉES LYON

<figure><img src="image_32.png" /></figure>

ÉCOLE CENTRALE LYON

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Université Claude Bernard

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Lyon 1

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UNIVERSITÉ JEAN MONNET SAINT-ÉTIENNE

<footnote>[1] Goument, C., Gerges, T., Lombard, P. et al. In-Mold Electronics on Poly(Lactic Acid): towards a more sustainable mass production of plastronic devices. Int J Adv Manuf Technol 125, 2643-2660 (2023).</footnote>