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Reliability and fitness for purpose testing of flexible

and wearable electronics

Dr Adam Lewis

[email protected]

Electronic and Magnetic Materials 20/03/2018

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Performance and Lifetime of Printed SemiconductorsPresented by Dr Sebastian Wood

Tuesday 15 May 2018 (14:30 hrs UK time)

Printed semiconducting materials have great potential for a range of electronic applications, particularly

where large active areas or flexible/stretchable devices are required. This class of materials is expanding

rapidly, with specific interest currently in organic semiconductors and hybrid organic-inorganic lead-halide

perovskites. As these materials begin to see commercial uptake, their short operational lifetime has become a

critical limitation. NPL has developed a suite of tools for monitoring and understanding the degradation

mechanisms affecting these devices in order to guide their ongoing development

In preparation for the event and to ensure you are equipped to gain the maximum benefit, please read our

simple Webinar Guidelines

The majority of webinars run for between 60–90 minutes, with a Q&A session. The webinars are limited to 100

delegates/companies. A copy of each of the slides presented and links to NPL reports will be provided after

the webinar

Book your place online at http://www.npl.co.uk/science-technology/electronics-interconnection/webinars/

Reliability and fitness for purpose testing of flexible

and wearable electronics

Dr Adam Lewis

[email protected]

Electronic and Magnetic Materials 20/03/2018

https://www.bobwillis.co.uk/wp-content/uploads/2017/01/WebinarAttendance2017.pdf

http://www.npl.co.uk/science-technology/electronics-interconnection/webinars/


Outline

▪ Brief introduction to NPL

▪ What do we mean by ‘flexibles’ and ‘wearables’?

• Examples

• Material sets and components

▪ Failure modes

▪ Harsh environments

▪ Test methods and standards

▪ Metrology for large area electronics

▪ CORNET project

▪ Conclusions

5

Brief Introduction

National Physical

Laboratory

▪ World leading national measurement

institute founded in 1900

▪ Over 600+ specialists in

measurement science

▪ State-of-the-art standards facilities

Electronic and Magnetic

Materials Group

▪ Printed electronics, sensors and

metrology for wearables

▪ Embedded electronics

▪ High temperature interconnects

▪ Tin whisker mitigation

▪ Coating/SIR/Condensation testing

▪ Electronics recycling

▪ Organic photovoltaics

▪ Magnetic materials characterisation

6


What do we mean by ‘flexible’ and

‘wearables’?

▪ Merging of textiles or flexible

substrates and electronics

• This could include

implantable devices

▪ Not a conventional PCB

made into a discrete

wearable device, e.g.

external heartrate monitor,

fitness watch

• These can be tested using

conventional test methods

Wearables/

Flexibles

Flexible substrates

Textiles and fabrics

Flexible interconnects

Hybrid flexi-ridged

Protective coatings

7

Wearable electronics

▪ Fully printed electronics

• Very limited functionality

• Smart labels/greeting cards

▪ Hybrid electronics

• Conventional packages,

conductive adhesive, printed

tracking

• Tactile feedback monitoring

– example

8

Sensor: printed or discrete (+ conductive adhesive)

Interconnection: printed/woven strand

Data processing/communication: PCB + conductive adhesive


Examples of Printed, Flexible and

Wearable Electronics

9https://www.oled-info.com/new-audi-a8-sports-oled-rearlights

http://www.heliatek.com/en/applications/pilot-projects

https://flisom.com/products/

https://www.digitaltrends.com/cars/toyota-prius-prime-solar-roof/

Automotive

Aerospace

Energy

Examples of Printed, Flexible and

Wearable Electronics

10https://www.labelandnarrowweb.com/issues/2016-08-01/view_narrow-web-europe/french-labels-rising-gently/

https://cartamundi.com/en/press/cartamundi-imec-holst-centre-win-best-product-award-printed-electronics/

https://iecetech.org/issue/2017-01/New-edition-of-Standard-for-OLED-displays

https://www.yaabot.com/31512/smart-clothing-health-care/

Health monitoringConsumer electronicsSmart packaging


Materials sets

▪ Substrates

PET , PEN, Textiles (woven, printed) …

▪ Adhesives (epoxy, acrylics)

mostly silver based

▪ Some solders

Low temp (SnBismth)

▪ Printed active devices (not typically moisture tolerant)

11

Low temperature

Failure modes: conductive

adhesive/tracking failure

▪ Increase in W of interconnect

Conductive particle oxidation

Ag oxides conducting

Polymer matrix swelling (moisture)

Breaking contact of conductive

particles

Bond-line failure

Resin to filler adhesion failure

Polymer relaxation (above Tg)

Component

PCB

Oxides Interfaces

Conduction paths

12


Failure modes: conductive

adhesive/tracking failure

▪ Reduction in mechanical strength

• adhesive/component interface adhesion

• adhesive/substrate interface adhesion

• bulk material failure

▪ Component or substrate interconnect

failure

• Fracture or delamination

• Can be caused by bending/flexing

13

Failure modes: metal migration

▪ Dendritic growth from metals (typically

silver) under bias and in the presence of

moisture, causing low surface insulation

resistance

• Metal fillers are encapsulated in resin and

thus not in direct contact with moisture

• NPL has encountered issues with surface

insulation resistance testing of Ag

tracking materials

• Lifetime improved by printing dielectric

over the top

http://nepp.nasa.gov/whisker./dendrite/Ag_dendrite-02.jpg

14


Harsh environments for

wearables

▪ Temperature

Often high temperatures are used as an acceleration factor for

conventional electronics – generally not suitable for wearables due

to low melting points etc.

▪ Humidity/moisture

▪ Chemical (sweat, detergent, shampoo …)

▪ Mechanical (stretch, bend, flex, …)

Typical daily

usage.

Washing

machine.

A combination of chemical contamination + moisture + bias voltage will

lead to electrochemical failures protective coatings15

Protection

▪ Coatings – conventional coatings, nano-coatings, etc.

• Most testing is done on new coatings

• Knowledge gap – how good are coatings that have

been in the field for x years?

• Evaluation methods:

• Repeat SIR/mechanical/other tests as performed on

new coating

• Chemical analysis – FTIR and other spectroscopic

analysis

16


TEST METHODS

17

Test methods for wearables

▪ Peel test

▪ Shear test

▪ Bend/flex testing

▪ Stretch rig

▪ Pneumatic

adhesion tester

▪ Washing machine

trials

Standards

IPC-9204: Guideline on Flexibility and Stretchability Testing for Printed Electronics

IEC TC 119 – Printed Electronics

IEC TC 124 – Wearable Electronics Devices and Technologies 18


Accelerated ageing regimes

▪ Damp heat ageing Causes joint resistance rises due

to oxidation of PCB, component

and conductive particle surfaces

Bond-line failures due to loss of

adhesion.

▪ Thermal cycling For low TCE components, during

thermal cycling, the mismatch to

PCB causes strain in joints,

resulting in crack initiation usually

along joint interfaces, causing

joint resistance increases and

failure.

19

SOIC/Material A

40C

85C

125C

40C

/93%

RH

85C

/85%

RH

125C

/93%

RH

-20C

/+80C

-55C

/+125C

0

20

40

60

80

100

% F

ailu

res(

>100%

Resis

tan

ce In

cre

ase)

Peel/tape test, shear test

▪ Peel/Tape TestTape adhered to printed ink on

surface and peeled off

Gives information about

adhesion strength

▪ Test shear strengthProbe moved across surface

Force require to shear

component measured

20


Pneumatic adhesion tester

▪ Test to measure bonding of

tracking to substrate

▪ Attachment of 1cm stud to

printed ink

▪ Pressurised collar used to

“lift” stub from surface

▪ Issues with finding adhesive

for stub

21

Printed track

area

Stub

Adhesive

Substrate

SIR testing of wearables

▪ SIR testing involves

bias, moisture and

contaminants (flux)

▪ should this

contaminant be:

• synthetic sweat

• detergents

• shampoo

▪ AutoSIR

22


Failure modes: cracking due to

bending

▪ Stress caused by bending

can cause cracks to from

across tracks:

• increased resistance

• open circuits.

23

Fabric, flexible and stretchable

sensors: stretch test

24

Rig designed to stretch

fabric and measure

resistance throughout



sensors: stretch test

25


sensors: strain test

26

Rig designed to apply

pressure to fabrics and

measure resistance

throughout


On-going stretch and flex testing

▪ Modified Instron Electropuls E3000

with electrical contacts

1. Measure resistance as function of

tensile extension (can be

monotonic or cyclically)

2. Novel bend test where clamp is

able to move to give control of

bend location and apply pressure

over bend rather than at clamp

location27

http://www.renewableenergyfocus.com/view/44227/developing-the-next-generation-of-flexible-solar-panels/

METROLOGY FOR LARGE

AREA ELECTRONICS

28


Metrology during printing

Large Area Metrology

▪ Batch checking

▪ Optical inspection

• Track width, length, thickness

• Defects

▪ Functional analysis

• Sheet resistivity

• Contact (e.g. bed of nails)

• Non-contact

• PV efficiency

29

Printing technologies

Wide Range

▪ Screen printing

▪ Roll-to-roll processes

▪ Flexographic printing

▪ Gravure printing

▪ Offset lithography

▪ Inkjet printing

▪ Aerosol printing

Roar R Søndergaard et al. Roll-to-roll fabrication of polymer solar cells Mater. Today Jan-Feb 2012 Vol. 15 No. 1-2

(DOI: 10.1016/S1369-7021(12)70019-6)

30


Non-contact electrical resistivity

measurements

Measurement principle

▪ Measurement of

coil inductance

▪ Simple model

Coil design

𝐹𝑆𝑅 =1

2𝜋 𝐿𝐶

𝑍 ≅ 𝑗𝜔𝐿 + 𝑅 𝑍 ≅1

𝑗𝜔𝐶

ω: angular frequency

δ: skin depth

µ: magnetic permeability

σ: conductivity

f: frequency

𝛿 ≈1

𝜋𝑓𝜇0𝜇𝑟𝜎

31


measurements

Samples Typical sensor response

32A. Lewis et al. 2017 Flex. Print. Electron. 2 044001

https://doi.org/10.1088/2058-8585/aa9875

https://doi.org/10.1088/2058-8585/aa9875


Effect of frequency on measured

response

Skin-depth theory Sensors over frequency range

33


measurements

In-situ roll-to-roll metrology

34


Multiscale Modelling and Characterization to Optimize the

Manufacturing Processes of Organic Electronics Materials and

Devices (CORNET)

This project has received funding from the European Union’s HORIZON 2020 research and

innovation programme under Grant Agreement No 760949.

CORNET consortium

Part no. Participant Organisation Name Short Name Country Nature

1 Aristotle University of Thessaloniki AUTh Greece HE

2 University of Surrey USUR UK HE

3 University of Ioannina UOI Greece HE

4 Centre National de la Recherche Scientifique CNRS France RES

5 Fluxim Fluxim Switzerland SME

6 Aixtron AIXTRON Germany IND

7 National Physical Laboratory NPL UK RES

8 Organic Electronic Technologies P.C. OET Greece SME

9 Centro Riserche Fiat CRF Italy RES

10 Granta Design Granta UK SME

11 Hellenic Organic & Printed Electronics Association HOPE-A Greece OTH

This project has received funding from the European Union’s HORIZON 2020 research and

innovation programme under Grant Agreement No 760949.


CORNET objectives1. Develop an Effective Open Innovation Environment (OIE) Connecting World-class Industrial, Academic & Research experts in Manufacturing, Multiscale Characterization & Modelling, for Optimization of OE Materials, Materials’ Behaviour & Process Optimization and for Reliable Database, Citable Protocols & Contribution to Standards (TRL4)

2. Multiscale Characterization & Modelling to Optimize OE Materials’ & Devices’ Fabricationand Validation of Materials’ Models for Faster Development Cycle and Time-to-market. (TRL4)

3. Optimization of the Fabrication of OPV, PPV & OLED Devices by R2R Printing and OVPDManufacturing Processes (TRL5)

4. Efficient Large scale Fabrication of Tailored (OPV, PPV, OLED) Nano-devices by R2R Printingand OVPD Processes and Demonstration to Industrial Applications (TRL6)

This project has received funding from the European Union’s HORIZON 2020 research and innovation programme

under Grant Agreement No 760949.

This project has received funding from the European Union’s HORIZON 2020 research and innovation programme

under Grant Agreement No 760949.

http://www.cornet-project.eu

CORNET website


Conclusions

▪ Materials used are typically not suitable to high

temperature and hence we cannot use significantly higher

temperatures to accelerate failure mechanisms

▪ Harsh environments for wearable electronics are different

to conventional electronics

introduces new failure mechanisms

particularly harsh with regards in electrochemical and

mechanical aspects

▪ New test methods need to be developed. These will need

to be combinatorial to mimic realistic end usage.

testing of protective coatings is important for reliability39

Questions

The National Physical Laboratory is operated by NPL Management Ltd, a wholly-

owned company of the Department for Business, Energy and Industrial Strategy

(BEIS).

Adam Lewis

[email protected]

40

▪ Acknowledgements:Martin Wickham, Kate Clayton,

Ling Zou, Laura Kent, Fernando

Castro, Tony Samano, Owen

Thomas


Performance and Lifetime of Printed SemiconductorsPresented by Dr Sebastian Wood

Tuesday 15 May 2018 (14:30 hrs UK time)

Printed semiconducting materials have great potential for a range of electronic applications, particularly

where large active areas or flexible/stretchable devices are required. This class of materials is expanding

rapidly, with specific interest currently in organic semiconductors and hybrid organic-inorganic lead-halide

perovskites. As these materials begin to see commercial uptake, their short operational lifetime has become a

critical limitation. NPL has developed a suite of tools for monitoring and understanding the degradation

mechanisms affecting these devices in order to guide their ongoing development

In preparation for the event and to ensure you are equipped to gain the maximum benefit, please read our

simple Webinar Guidelines

The majority of webinars run for between 60–90 minutes, with a Q&A session. The webinars are limited to 100

delegates/companies. A copy of each of the slides presented and links to NPL reports will be provided after

the webinar

Book your place online at http://www.npl.co.uk/science-technology/electronics-interconnection/webinars/

https://www.bobwillis.co.uk/wp-content/uploads/2017/01/WebinarAttendance2017.pdf

http://www.npl.co.uk/science-technology/electronics-interconnection/webinars/

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