Stretchable electronics - Semieuropesemieurope.omnibooksonline.com/2016/semicon_europa... · •...

Stretchable electronics Constraints and possibilities for direct printing on thermoplastic polyurethanes

From flexible printed electronics to stretchable

Eric Rubingh, Corné Rentrop, Piet Bouten

© Holst Centre

Holst Centre in a nutshell

Who we are

• Open innovation research institute

• Founded in 2005 by imec (1300 fte, BE)

and TNO (4500 fte, NL)

• Located at the High Tech Campus

in Eindhoven, The Netherlands

• Own staff 310 researchers

What we do

• Developing technologies for printed electronics

and wireless autonomous microsystems

• In close collaboration with leading

industrial partners along the value chain

Materials Analysis

Electronic measurement

Thin Film clean room

OLED Device Processing

Reliability lab

Photonics cleanroom

Electronic Prototyping

Equipment Engineering

Life Sciences

EMC lab

Holst Centre R2R lab

Holst Centre Offices

< 3

© Holst Centre

Holst Centre: Technologies targeted

Hybrid printed electronics such as wearable health monitoring and pressure sensor systems

Low power sensor systems for

monitoring food, air, water quality

Low-cost solar cells, CIGS, Thin film,

Perovskites, OPV

Flexible OLED lighting and

displays

< 4

© Holst Centre

Holst Centre: Technologies targeted

Hybrid printed electronics such as wearable health monitoring and pressure sensor systems

Low power sensor systems for

monitoring food, air, water quality

Low-cost solar cells, CIGS, Thin film,

Perovskites, OPV

Flexible OLED lighting and

displays

Combine into wearables

< 5

© Holst Centre < 6

An explosion of wearable tech

© Holst Centre < 7

Clothing as wearable tech platform

Smart clothing• We all wear it and it covers most of our body• Monitor vital signs, location, …• Electronics should be conformable, stretchable, washable, …

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Wearable electronics, from flexible to stretchable

Version 1Lamination of printed

flexible structures

Version 2Lamination of printed stretchable structures

Version 3Direct printed

stretchable structures

• Printing functional structures with meanders on films

• Assembly of components

• Laser structuring of meanders

• Textile lamination on TPU

• Using standard printed electronics materials

• Printing of functional structures on TPU


• Textile lamination

• Using special stretchable materials

• Printing of functional structures on thin films


• Textile lamination with TPU


< 8

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flexible structures

















< 9

© Holst Centre < 10

• Typical polymers: polyesters (PEN/PET), polycarbonate (PC), polystyrene (PS)

• Typical thicknesses: 25 – 2000 um

Version 1: Lamination of printed flexible structuresStart from thin plastic films


Multi-layer circuit patterns

• Done by printing alternating layers of conductive and isolating inks

• Printing up to 5 circuit layers demonstrated

Physical activity monitoring patch

Version 1: Lamination of printed flexible structuresPrinting of complex multi-layer structures


WiFi tag on film, 5 stacked circuit layers

Component integration

• Several technologies available for reliable integration of components on PE foils

• Based on ACA, ICA or special soldering technologies

Version 1: Lamination of printed flexible structuresComponent integration

© Holst Centre 13

Smart textiles• Accelerometer• Heart rate monitoring • Location• Not Stretchable

Version 1: Lamination of printed flexible structuresTextile lamination with TPU

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flexible structures

















< 14

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Meander approach

• Make local spring structures (‘meanders’) in the printed electronics film

• Structures are stretchable through opening/out-of-plane bending

Less strain on printed structures/lines

The result

• A stretchable device made from a flexible printed electronics film

Version 2: Meander approach for stretchable PE

< 15

© Holst Centre

start from polyester film

print electronic structures

assemble compo-nents

laser structure meander

Encapsulate in TPU film Washable

Laminate on fabric

Version 2: Meander approach for stretchable PE

TPU

TPUPEN

• Out of plane deformation of meanders allows stretchability

• Encapsulation into polyurethane enables washability

< 16


… or PCB’s that are stretchable

Reliability of meander approach + component integration

• Flex testing (10.000x @ 2 mm radius) and stretch testing (1000x @ 20% strain)

• Washability: 25 washing cycles at 50oC successfully completed

• Main failure cause: mechanical damage (drum rotation)

Version 2: Meander approach for stretchable PEReliability of meander approach


Wearables: Health patch

• Printed electrical sensors

• Enables ECG, EMG and breathing analysis

• Worn as a patch on the body for several days

• Using stretch technologies for improved comfort

Version 2: Meander approach for stretchable PEHealth Patch


Smart Garment:

• Printed electrical sensors

• Enables ECG, EMG etc.

• Lamination into shirt using TPU

• Using stretch technologies for reduction of motion artifacts

Version 2: Meander approach for stretchable PESmart garment

© Holst Centre



flexible structures

















< 22

© Holst Centre

Thermoplastic polyurethane:

• Foldable and stretchable

• One step lamination onto textile

• TPU used extensively in garments and graphical printing industry

Version 3: Direct printing of electronics on TPU< 23

© Holst Centre

Thermoplastic polyurethanes (TPU)

• Stretchable

• Robust

• Solvent resistant

• Washable

• Adjustable mechanical properties

• Hardness, young’s module, stress /strain characteristics

But also:

Creep /Hysteresis

Thermal shrinkage

Time dependency

Strain hardening

Residual strain

*Desmophan - Covestro

< 24

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Mechanical testing of TPU’s and printed structures

• Test Set-up

Instron Tensile Tester

Influence stress-strain on resistance

Monitor Load, displacement and resistance

• Evaluate TPU substrates

Different types of TPU

Polyester or polyether based

Different thicknesses and E-moduli

Characteristics

Compatibility with printing process

Thermal shrinkage, creep, residual strain

• Evaluate stretchable inks and pastes

Types of materials

Conductive pastes, dielectrics, sensor materials, adhesives

Characteristics

Stretchability, adhesion, cohesion

Conductivity under/after strain

Component assembly and die shear testing

< 25

© Holst Centre

Typical test regimes

• Linear strain

Stretching until failure

Report absolute and relative resistance (ΔR/R [%])

• Progressive strain

Deformation: 2% - 5% - 10% - 15% - 20%

Hysteresis effect

Report residual strain

Report absolute and relative resistance (ΔR/R [%])

• Cyclic strain tests

Repeated deformation (20% - relaxation)

100 times and up

Report maximum and minimum stress/resistance

Mechanical testing of TPU’s and printed structures

< 26

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Mechanical testing: Linear strain

• Stress/resistance under line strain

Resistance is increasing exponentially

Conductivity in line fully lost at 70% strain

Workable resistance until 30% strain

Start

End

< 27

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Mechanical testing: Progressive strain

• Progressive strain curves as a function of used TPU

Residual strain varies per type

But residual strain is not the only important parameter!

Residual strain 2.1%Residual strain 5%

Residual strain 1.3%

< 28

© Holst Centre


• Influence progressive strain on line resistance

Resistance increases with increasing strain

Max ΔR/R at 20% strain: 275%

Resistance in relaxed state after testing important

ΔR/R after progressive strain test: 100%

Partly caused by residual strain in TPU

Partly caused by reorientation of particles in structure

No visible defects/cracks in line

0

1

2

3

4

5

0

50

100

150

200

250

300

0 5 10 15 20

Str

ess

[M

Pa]

ΔR

/R

[%

]

Strain [%]

Bright field: 50x

Bright field: 500x

Dark field: 200x

Visual inspectionafter test

< 29

© Holst Centre

Paste A: Time dependency resolved by over-coating

Paste C: High conductivity, no stretchability

Paste B: High stretchability with increased stiffness

Paste A: Time dependency observed in relaxation curve

< 30

• Stress-strain-resistance curves as a function of used silver paste

Several commercial silver pastes tested


Paste D: High conductivity, high residual strain

Silver paste characteristics

Printability

Adhesion and cohesion

Stretchability

Conductivity

Initial

Max strain

After strain

Component assembly

Die shear strength



• Stress-strain-resistance curves as a function TPU substrate

Using silver paste B

TPU with high residual strain TPU with low residual strain

Observations

Resistance curves on both TPU substrates very similar

Minimal resistance for low residual strain TPU slightly lower

Maximal resistance at strain for high residual strain TPU lower

© Holst Centre

Mechanical testing: Cyclic strain test

• Stress-strain-resistance curves as a function of cycles

Using silver paste B

Resistance at min and max strain increases

After ~70 cycles resistance @ min strain stabilizes

After ~150 cycles resistance @ max strain stabilizes

Stress [MPa] @ 20% strain (high)

Stress [MPa] @ 7% strain (low)

Resistance [Ω] @ 20% strain

Resistance [Ω] @ 7% strain

< 32


Health patch on TPU

• Monitor vital signs

• 3 layer prints

• Sufficient registration

• Stretchable

Demonstrator direct printing on TPU


Demonstrator direct printing on TPU

Flex smell platform

• Sensor platform on TPU

• Temperature and capacitive sensors

• NFC enabled, app assisted


Other applicationsLight in clothingDesigner jacket

Solar shirt

Display in clothing

Pressure sensor shoe-inlay

© Holst Centre

Conclusions and work in progress

• Large variety of TPU substrates available

Stress-strain behavior and residual strain important

Several other parameters also important

Thermal stability, printability, lamination quality and washability

Cyclic tests typically reveal stability after ~70 cycles

Change of TPU changes stress/strain behavior and performance of pastes

• Large difference between stretchable pastes

Conductivity

Stretchability

Compatibility with TPU

• Possible to create functional demonstrators

• Work in progress

Improved component integration in stretchable electronics

Washability of devices with larger components

Other substrate types like Silicone

< 36

Stretchable electronics - Semieuropesemieurope.omnibooksonline.com/2016/semicon_europa... · •...

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