Semiconductor devices - Flexible and stretchable semiconductor devices - Part 4: Fatigue evaluation for flexible conductive thin film on the substrate for flexible semiconductor devices

IEC 62951-4:2019 specifies an evaluation method of the bending fatigue properties of conductive thin film and flexible substrate for the application at flexible semiconductor devices. The films include any films deposited or bonded onto a non-conductive flexible substrate such as thin metal film, transparent conducting electrode, and thin silicon film used for flexible semiconductor devices. The electrical and mechanical behaviours of films on the substrate are evaluated. The fatigue test methods include dynamic bending fatigue test and static bending fatigue test.

Dispositifs à semiconducteurs - Dispositifs à semiconducteurs souples et extensibles - Partie 4: Evaluation de la fatigue pour les couches minces conductrices souples sur les substrats pour dispositifs à semiconducteurs souples

L’IEC 62951-4:2019 spécifie une méthode d’évaluation des propriétés de fatigue en flexion des couches souples conductrices et des substrats souples pour l'application à des dispositifs semiconducteurs souples. Les couches comprennent toute couche déposée ou collée sur un substrat souple non conducteur telle qu’une couche métallique mince, une électrode conductrice transparente et une couche de silicium mince utilisée pour des dispositifs à semiconducteurs souples. L'évaluation porte sur les comportements électriques et mécaniques des couches sur les substrats. Les méthodes d’essai de fatigue comprennent l’essai de fatigue en flexion dynamique et l’essai de fatigue en flexion statique.

General Information

Status
Published
Publication Date
26-Feb-2019
Technical Committee
Current Stage
PPUB - Publication issued
Completion Date
27-Feb-2019
Ref Project

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IEC 62951-4
Edition 1.0 2019-02
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Semiconductor devices – Flexible and stretchable semiconductor devices –

Part 4: Fatigue evaluation for flexible conductive thin film on the substrate for

flexible semiconductor devices
Dispositifs à semiconducteurs – Dispositifs à semiconducteurs souples et
extensibles –
Partie 4: Evaluation de la fatigue pour les couches minces conductrices souples
sur les substrats pour dispositifs à semiconducteurs souples
IEC 62951-4:2019-02(en-fr)
---------------------- Page: 1 ----------------------
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IEC 62951-4
Edition 1.0 2019-02
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Semiconductor devices – Flexible and stretchable semiconductor devices –

Part 4: Fatigue evaluation for flexible conductive thin film on the substrate for

flexible semiconductor devices
Dispositifs à semiconducteurs – Dispositifs à semiconducteurs souples et
extensibles –
Partie 4: Evaluation de la fatigue pour les couches minces conductrices souples
sur les substrats pour dispositifs à semiconducteurs souples
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 31.080.99 ISBN 978-2-8322-6610-6

Warning! Make sure that you obtained this publication from an authorized distributor.

Attention! Veuillez vous assurer que vous avez obtenu cette publication via un distributeur agréé.

® Registered trademark of the International Electrotechnical Commission
Marque déposée de la Commission Electrotechnique Internationale
---------------------- Page: 3 ----------------------
– 2 – IEC 62951-4:2019 © IEC 2019
CONTENTS

FOREWORD ........................................................................................................................... 3

1 Scope .............................................................................................................................. 5

2 Normative references ...................................................................................................... 5

3 Terms and definitions ...................................................................................................... 5

4 Test piece ....................................................................................................................... 6

4.1 Design of test piece ................................................................................................ 6

4.2 Preparation of a test piece ...................................................................................... 7

4.3 Measurement of dimensions ................................................................................... 7

4.4 Storage prior to testing ........................................................................................... 7

5 Testing method and test apparatus .................................................................................. 7

5.1 General ................................................................................................................... 7

5.2 Test apparatus ........................................................................................................ 7

5.3 Method of gripping .................................................................................................. 7

5.4 Bending test ............................................................................................................ 8

5.5 Dynamic bending fatigue test .................................................................................. 8

5.6 Static bending fatigue test ....................................................................................... 8

5.7 Bending fatigue test of flexible substrate ................................................................. 8

5.8 Speed of bending fatigue test ................................................................................. 8

6 Test ................................................................................................................................. 8

6.1 Test procedure ........................................................................................................ 8

6.2 Failure criterion (test termination) ........................................................................... 9

6.3 Test environments .................................................................................................. 9

6.4 Recorded data ........................................................................................................ 9

7 Test report ....................................................................................................................... 9

Annex A (informative) Various bending fatigue testers ........................................................ 10

Bibliography .......................................................................................................................... 13

Figure A.1 – Bending fatigue tester using curved mandrel and roller ..................................... 10

Figure A.2 – Cyclic mandrel bending tester ........................................................................... 11

Figure A.3 – Collapsing radius bending fatigue tester ........................................................... 11

Figure A.4 – X-Y-θ bending fatigue test method .................................................................... 11

Figure A.5 – Schematic of the bending fatigue test ............................................................... 12

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IEC 62951-4:2019 © IEC 2019 – 3 –
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SEMICONDUCTOR DEVICES –
FLEXIBLE AND STRETCHABLE SEMICONDUCTOR DEVICES –
Part 4: Fatigue evaluation for flexible conductive thin film
on the substrate for flexible semiconductor devices
FOREWORD

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International Standard IEC 62951-4 has been prepared by IEC technical committee 47:

Semiconductor devices.
The text of this International Standard is based on the following documents:
FDIS Report on voting
47/2531/FDIS 47/2549/RVD

Full information on the voting for the approval of this International Standard can be found in

the report on voting indicated in the above table.

This document has been drafted in accordance with the ISO/IEC Directives, Part 2.

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– 4 – IEC 62951-4:2019 © IEC 2019

A list of all parts in the IEC 62951 series, published under the general title Semiconductor

devices – Flexible and stretchable semiconductor devices, can be found on the IEC website.

The committee has decided that the contents of this document will remain unchanged until the

stability date indicated on the IEC website under "http://webstore.iec.ch" in the data related to

the specific document. At this date, the document will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.

IMPORTANT – The 'colour inside' logo on the cover page of this publication indicates

that it contains colours which are considered to be useful for the correct

understanding of its contents. Users should therefore print this document using a

colour printer.
---------------------- Page: 6 ----------------------
IEC 62951-4:2019 © IEC 2019 – 5 –
SEMICONDUCTOR DEVICES –
FLEXIBLE AND STRETCHABLE SEMICONDUCTOR DEVICES –
Part 4: Fatigue evaluation for flexible conductive thin film
on the substrate for flexible semiconductor devices
1 Scope

This part of IEC 62951 specifies an evaluation method of the bending fatigue properties of

conductive thin film and flexible substrate for the application at flexible semiconductor devices.

The films include any films deposited or bonded onto a non-conductive flexible substrate such

as thin metal film, transparent conducting electrode, and thin silicon film used for flexible

semiconductor devices. The electrical and mechanical behaviours of films on the substrate

are evaluated. The fatigue test methods include dynamic bending fatigue test and static

bending fatigue test.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their

content constitutes requirements of this document. For dated references, only the edition

cited applies. For undated references, the latest edition of the referenced document (including

any amendments) applies.

IEC 62047-2:2006, Semiconductor devices – Micro-electromechanical devices – Part 2:

Tensile testing method of thin film materials
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following

addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
bending radius

radius of arc corresponding to the curvature of the central line between innermost and

outermost surfaces of flexible electronic devices during a bending test

[SOURCE: IEC 62715-1-1:2013, 2.5.1, modified – The words "a flexible display device" have

been replaced by "flexible electronic devices".]
3.2
critical bending radius
bending radius at which the failure of the flexible semiconductor devices occurs

Note 1 to entry: For the conductive films, the electrical resistance starts to exceed a predefined limit, and/or

fracture of the film or caused by delamination or initiation of the cracks occurs, or by damage of the substrate.

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– 6 – IEC 62951-4:2019 © IEC 2019
3.3
flexible substrate

substrate with flexibility onto which conductive thin films will be deposited, bonded or attached

[SOURCE: IEC 62951-1:2017, 3.1.4.]
3.4
outer bending test
test, where the test piece is bent into a convex shape (∩)
Note 1 to entry: Outer bending test induces tensile stress on the film.

Note 2 to entry: There are different names of outer bending test such as outward, face-out, or convex bending

test.
3.5
inner bending test
test, where the test piece is bent into a concave shape (∪)
Note 1 to entry: Inner bending test induces compressive stress on the film.

Note 2 to entry: There are different names of inner bending test such as inward, face-in, or concave bending test.

3.6
dynamic bending fatigue test

test designed for determining the bending properties of flexible semiconductor device to

withstand the repeated strain for a certain period of time
3.7
static bending fatigue test

test designed for determining the bending properties of flexible semiconductor device to

withstand the same strain for a certain period of time

Note 1 to entry: Test piece is bent at fixed bending radius for any length of time.

3.8
S-N curve
plot of stress (S) against the number of cycles to failure

[SOURCE: IEC 62047-12:2011, 3.3, modified – The expression "plot of stress or strain" has

been replaced by "plot of stress".]
3.9
ε-N curve
plot of strain (ε) against the number of cycles to failure
[SOURCE: IEC 62047-12:2011, 3.3, modified – The term has been modified and the
ε).]
expression "plot of stress or strain (S)" has been replaced by "plot of strain (
4 Test piece
4.1 Design of test piece

In order to minimize the influence of size, the test piece should have dimensions of the same

order as that of the objective device component as much as possible. The shape of a test

piece is based on 4.2 of IEC 62047-22:2014. Since the change in electrical resistance is

related to strain or stress, the electrical resistance shall be measured in a region of nearly

uniform strain. To measure electrical resistance, attach lead wires to the conductive thin film

of the test piece. For uniform strain distribution, the shape of the test piece is a rectangular

strip.
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IEC 62951-4:2019 © IEC 2019 – 7 –
4.2 Preparation of a test piece

The test piece shall be prepared using the same fabrication process as the real device

fabricated for flexible electronics and flexible semiconductor devices, because the mechanical

and electrical properties depend on the fabrication processes. Thin conductive film shall be

carefully prepared to prevent formation of cracks or flaws and delamination from the substrate.

4.3 Measurement of dimensions

The thickness and width of the conductive thin film and flexible substrate shall be accurately

measured respectively, because the dimensions are used to determine the mechanical and

electrical properties of test materials. Each test piece should be measured directly. Both the

thickness and width of the test piece shall be specified within the maximum error of ± 1% for

the thickness and ± 5% for the width. Thickness measurement shall be performed according

to Clause 4 and Annex C of IEC 62047-2:2006. There can be some combinations of thin film

and substrate where it is difficult to fulfil the tolerance of thickness measurement. In this case

the average and the standard deviation of the thickness measurement should be reported.

4.4 Storage prior to testing

Care should be taken on the storage environment, because for thin films and flexible

substrates there is a great risk to affect the electromechanical properties of the film by the

storage environment. For example, oxidation on the test piece surface will deteriorate the

electrical and mechanical properties of the test piece. If there is a longer duration between

final preparation and testing, particular care should be taken in storing the test pieces, and

the specimens should be examined by appropriate means to ensure that the surface has not

deteriorated during the storage period. If any deterioration is observed that was not present

after the specimens were prepared, testing shall not be performed.
5 Testing method and test apparatus
5.1 General

The test is performed by bending a test piece. The bending strain induced by the tensile or

compressive load shall be uniform in a pre-defined gauge section in the elastic region of the

substrate and the thin film. To measure the change in electrical resistance along with the

change in mechanical strain, carefully select the gauge section. The gauge section for

measuring mechanical strain shall be coincident with or scalable to that for measuring

electrical resistance.
5.2 Test apparatus

The cyclic bending test equipment includes the clamp to hold a bending test sample, the

moving part to shuttle, and control system which regulates cyclic bending number, moving

distance, and moving speed while testing. Specimen should be securely clamped with

gripping part during the test. Several cyclic bending test equipments are available and

described in Annex A. It is not necessary that a certain type of bending test equipment be

preferred. During the test, however, the test apparatus shall not cause any unintended

damage to the test piece such as scratches and other defects.
5.3 Method of gripping

It is recommended that the test piece be tightly clamped. However, this should be carefully

done in order to prevent local stress concentration in the clamped region. The stress

concentration in the clamped region could generate some cracks or any damage on the film,

resulting in failure of the film or substrate.
---------------------- Page: 9 ----------------------
– 8 – IEC 62951-4:2019 © IEC 2019
5.4 Bending test

The bending test of the test piece is recommended before the cyclic fatigue testing in order to

determine the testing conditions such as critical bending radius and prevent too early failure

of the test piece during the cyclic loading for the bending fatigue tests. For example, if the

critical bending radius of the test piece is found to be 5 mm from the bending test, the use of

a larger bending radius shall be recommended for the dynamic bending fatigue test.

5.5 Dynamic bending fatigue test

This test is to provide a standard procedure for determining the cyclic bending fatigue

properties of flexible semiconductor devices to withstand the repeated strain for a certain

period of time. This test can be performed in an outer bending mode or inner bending mode

depending on the application of the films and devices.
5.6 Static bending fatigue test

This test is to provide a standard procedure for determining the bending fatigue properties of

flexible semiconductor devices to withstand the same strain for a certain period of time. Each

specimen is bent at fixed bending radius for a certain period of time. There are several types

of bending test equipments to measure the static bending fatigue property of the

semiconductor device. It is not necessary that a certain type of bending test method be

preferred. Mandrel or round bar with a certain radius can be used. In order to make sure that

the specimen is bent at a fixed bending radius, the specimen should be safely clamped with

pressing tool or securely adhere to the test equipment.
5.7 Bending fatigue test of flexible substrate

The bending fatigue properties of the conductive thin films or semiconductor devices

deposited on flexible substrates are influenced by the fatigue properties of flexible substrates.

Therefore, the dynamic or static bending fatigue testing of the substrate itself is

recommended prior to fatigue testing of the conductive thin films or semiconductor devices.

Any damages, deformation, or cracks of the substrate should be carefully monitored during

the bending fatigue test.
5.8 Speed of bending fatigue test

The frequency of the bending cycle will depend upon the testing environment, the type of

testing machine employed, and the stiffness of the test piece. Generally, the frequency should

be chosen properly depending on the application of flexible semiconductor devices. In

addition, the frequency shall not heat the test piece during the application of cyclic loading

due to the rapid dissipation of strain energy in the test piece. It is recommended that the

frequency of the cyclic motion should be within the range from around 0,1 Hz to 1 Hz. This

practice does not apply to fatigue testing of the viscoelastic films.
6 Test
6.1 Test procedure
The test procedure is as follows:

a) Place the test piece into the holder. The longitudinal direction of the test piece shall be

aligned with the actuating direction of the test apparatus, and the deviation angle shall be

less than 5°. In case of outer bending test, the sample is facing upward to apply tensile

stress, and in case of inner bending test, the sample is facing downward to apply the

compressive stress.

b) Measure the electrical properties such as electrical resistance of the test piece.

c) The test piece is bent with a decreasing bending radius by controlling the moving distance

from flat surface to a designated bending radius. The test is performed under a constant

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IEC 62951-4:2019 © IEC 2019 – 9 –

bending stroke speed depending on the material system of the test piece and the actual

usage condition of the customer. For the conductive films, change in electrical resistance

should be measured in situ during test.

d) Unload the test piece when electrical failure occurs in the test piece or fracture/damage of

the film or the substrate occurs.
6.2 Failure criterion (test termination)

Fatigue testing shall continue until the test piece is cracked or failed, or until a predetermined

number of cycles have been applied to the test piece depending on the application of flexible

semiconductor. The termination criterion (test piece fracture or predetermined number of

cycles) shall be described in the test report.
6.3 Test environments

It is recommended to perform a test under constant temperature and humidity. As the

environmental conditions such as temperature and humidity affect the electrical and

mechanical properties of thin films, the testing temperature and humidity shall be monitored

during testing. Fluctuations in temperature during the test shall be controlled to be less than

± 2°C. Flexible substrates made of certain polymeric materials can be sensitive to humidity;

thus, the change in relative humidity (RH) in the testing laboratory shall be controlled to be

less than ± 5 % RH for such materials. When the tests are completed, the environmental

conditions should be recorded.
6.4 Recorded data

The failure of the specimen shall be recorded. S-N curve is generally used for fatigue life

prediction. However, for the bending fatigue test of flexible semiconductor devices, ε (strain)-

N curve or critical bending radius-N curve can be more appropriate for the bending fatigue test.

7 Test report
The test report shall contain the following information.
a) Mandatory
1) Test piece material and substrate material
2) Test piece preparation procedures
3) Test piece dimensions and their measurement method
4) Description of the testing apparatus;
5) Fatigue test conditions
– bending radius
– speed of the test
– testing environments (temperature and relative humidity)
– bending mode (outer or inner bending)
6) Fatigue test result

– Fatigue life (S-N, or ε-N, and critical bending radius-N) curve. If the test piece is

not fractured during a predetermined number of cycles, the number of cycles and
the description “no failure” should be noted.
– Change of electrical resistance
b) Optional
1) Internal residual stress of the film
2) Surface roughness of test piece
3) Failure mechanism of the test piece
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– 10 – IE
...

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