IEC 62951-4:2019

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
• ICS: 31.080.99 - Other semiconductor devices
• ICS: 33.100 - Electromagnetic compatibility (EMC)
• Technical Committee: TC 47 - Semiconductor devices
• Drafting Committee: WG 6 - TC 47/WG 6
• Current Stage: PPUB - Publication issued
• Start Date: 27-Feb-2019
• Completion Date: 29-Mar-2019

Standards Content (Sample)

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

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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

– 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

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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patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
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.

– 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.
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.

– 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.
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.

– 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

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

– 10 – IEC 62951-4:2019 © IEC 2019
Annex A
(informative)
Various bending fatigue testers
There are several equipments for the bending fatigue test which were suggested by several
researchers. Bending fatigue equipment in Figure A.1 used a curved mandrel with a fixed
radius [3] . The mandrel produces the concave and convex shapes for a cyclic bending,
producing the concave and convex shapes. The rollers eliminate the vibration caused by the
movement of the balance weight, which ensures smooth bending. Fatigue equipment in
Figure A.2 also uses a mandrel and shaft which repeatedly rotates clockwise and counter
clockwise [5]. One end of the sample is firmly fastened to the mandrel by a metal plate
screwed on to the curved mandrel surface. The other end of the sample is clamped between a
pair of thin metal plates, which, in turn, are held in place by a low-tension spring. The
bending fatigue tester in Figure A.3 is known as a collapsing radius tester. The sample is
mounted between the plates in a curved shape of some relatively large radius and it is
subsequently squeezed between the plates. The bending radius is approximately the half of
the distance between the plates. In the X-Y-θ system [6] as shown Figure A.4, the test piece
is clamped in a flat position between a fixed plate and a co-planar plate mounted on top of a
stack of two linear and one rotary motorized actuator. These actuators control the spatial
coordinates of the specimen end in X-Y-θ space in such a way that the specimen end is
positioned on the coordinates of a circumference. The details on the test method can be found
in [6]. The angle θ is defined as the angle between the tangent to the test piece end and the
X-axis. The bending tester shown in Figure A.5 includes the test piece holder to place a test
piece, and control system in which separation distance is controlled by an actuator or motor.
Use of an optical microscope is recommended to observe the existence of cracks on surface
of the test piece during the test. The control system regulates the moving distance and
moving speed while testing. The calculation of the bending radius is described in [7].
Bending mandrel
Test vehicle
Roller
Balance weight
IEC
Figure A.1 – Bending fatigue tester using curved mandrel and roller
___________
Numbers in square brackets refer to the Bibliography.

Multimeter
ITO coated on flexible substrate
Sync signal
Direction signal Shaft Low tension
spring
Speed signal
Control
computer
Power supply
Stepper motor
IEC
Figure A.2 – Cyclic mandrel bending tester
IZO
SiO
x
AI O
2 3
Distance between plates
PET
Fixed
Moving
IEC
Figure A.3 – Collapsing radius bending fatigue tester
IEC
Figure A.4 – X-Y-θ bending fatigue test method
Radius
– 12 – IEC 62951-4:2019 © IEC 2019
Thin film
Holder
Substrate
Motor
Fixed jig
Moving jig
IEC
Figure A.5 – Schematic of the bending fatigue test

Bibliography
[1] IEC 62047-12:2011, Semiconductor devices – Micro-electromechanical devices –
Part 12: Bending fatigue testing method of thin film materials using resonant vibration
of MEMS structures
[2] IEC 62047-22:2014, Semiconductor devices – Micro-electromechanical devices –
Part 22: Electromechanical tensile test method for conductive thin films on flexible
substrates
[3] IEC 62715-1-1:2013, Flexible display devices – Part 1-1: Terminology
...