IEC 63251:2023

IEC 63251 ®
Edition 1.0 2023-11
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Test method for mechanical properties of flexible opto-electric circuit boards
under thermal stress
Méthode d’essai des propriétés mécaniques des circuits optoélectriques
souples sous contrainte thermique

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IEC 63251 ®
Edition 1.0 2023-11
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
colour
inside
Test method for mechanical properties of flexible opto-electric circuit boards

under thermal stress
Méthode d’essai des propriétés mécaniques des circuits optoélectriques

souples sous contrainte thermique

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 31.180  ISBN 978-2-8322-7750-8

– 2 – IEC 63251:2023 © IEC 2023
CONTENTS
FOREWORD . 4
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 6
4 Test method . 7
4.1 General . 7
4.2 Test sample . 7
4.3 Test process . 8
4.3.1 General description of the test . 8
4.3.2 Preconditioning . 9
4.3.3 Test . 9
4.3.4 Recovery . 9
4.3.5 Final measurements . 10
5 Report . 10
Annex A (informative)  Example of optical bending loss test results with general glass
optic fibres . 11
Annex B (informative) Example of preparation method of O-E circuit test samples
(optic fibre type) . 12
B.1 General . 12
B.2 Manufacturing processes of the FOECBs with optic fibres (POF, GOF) . 12
B.3 Manufacturing processes of the FOECBs with optical polymer waveguides . 13
B.4 Characteristics of the optic fibres . 14
Annex C (informative) Example of reflow assembly simulation test results . 15
C.1 General . 15
C.2 Results of reflow assembly simulation test for a LED chip mounted FOECB
with GOF . 15
C.3 Results of reflow assembly simulation test for a transparent FOECB with
GOF for display applications . 16
C.4 Results of reflow assembly simulation test for a polyimide (PI) based FOECB
with GOF . 16
C.5 Results of reflow assembly simulation test for a polymer-based FOECB . 17
Annex D (informative) Example of thermal shock endurance test results . 18
D.1 General . 18
D.2 Results of thermal shock endurance test for an FOECB with GOF . 18
Annex E (informative) Example of humidity storage test results . 19
E.1 General . 19
E.2 Results of humidity storage test for an FOECB with GOF . 19
E.3 Results of humidity storage test for an FOECB with POF . 19
Bibliography . 21

Figure 1 – Schematic diagram of FOECB (top view) . 7
Figure 2 – Schematic diagrams of the FOECB test samples of fibre type . 8
Figure 3 – Schematic diagram of the FOECB test samples of fibre type . 8
Figure A.1 – Bending loss test setup . 11
Figure A.2 – Optical loss versus bending diameter . 11
Figure B.1 – Arrayed structure of the FOECB test samples formed on one sheet . 12

Figure B.2 – Fabrication of the optic circuits with optic fibres . 13
Figure B.3 – Fabrication of the optic circuits with optic polymer waveguide via the
photo-etching method . 13
Figure C.1 – LED chip mounted FOECB. 15
Figure C.2 – Appearance of a LED chip mounted FOECB after the reflow assembly

simulation test . 15
Figure C.3 – Appearance of a transparent FOECB with GOFs after the reflow assembly
simulation test . 16
Figure C.4 – Appearance of a PI based FOECB with GOF after the reflow assembly
simulation test . 16
Figure C.5 – Appearance of a polymer-based FOECB after the reflow assembly

simulation test . 17
Figure D.1 – Appearance of an FOECB with GOF after the thermal shock test . 18
Figure E.1 – Appearance of an FOECB with GOF after the humidity storage test . 19
Figure E.2 – Appearance of an FOECB with POF after the humidity storage test . 20

Table 1 – Thermal endurance test class for FOECB . 9

– 4 – IEC 63251:2023 © IEC 2023
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
TEST METHOD FOR MECHANICAL PROPERTIES OF FLEXIBLE
OPTO-ELECTRIC CIRCUIT BOARDS UNDER THERMAL STRESS

FOREWORD
1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising
all national electrotechnical committees (IEC National Committees). The object of IEC is to promote international
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shall not be held responsible for identifying any or all such patent rights.
IEC 63251 has been prepared by IEC technical committee 91: Electronics assembly technology.
It is an International Standard.
The text of this International Standard is based on the following documents:
Draft Report on voting
91/1898/FDIS 91/1914/RVD
Full information on the voting for its approval can be found in the report on voting indicated in
the above table.
The language used for the development of this International Standard is English.

This document was drafted in accordance with ISO/IEC Directives, Part 2, and developed in
accordance with ISO/IEC Directives, Part 1 and ISO/IEC Directives, IEC Supplement, available
at www.iec.ch/members_experts/refdocs. The main document types developed by IEC are
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The committee has decided that the contents of this document will remain unchanged until the
stability date indicated on the IEC website under webstore.iec.ch in the data related to the
specific document. At this date, the document will be
• reconfirmed,
• withdrawn, or
• revised.
IMPORTANT – The "colour inside" logo on the cover page of this document 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.

– 6 – IEC 63251:2023 © IEC 2023
TEST METHOD FOR MECHANICAL PROPERTIES OF FLEXIBLE
OPTO-ELECTRIC CIRCUIT BOARDS UNDER THERMAL STRESS

1 Scope
This International Standard defines the thermal endurance test methods for reliability
assessment of flexible opto-electric circuit boards. The purpose of this document is to
accommodate the uniform thermal characteristics required by the flexible opto-electric circuit in
high temperature environments such as automobiles. In particular, this document specifies a
test method to inspect the occurrence of colour exchange, deformation and delamination of
flexible opto-electric circuit boards under thermal stress.
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 60068-2-2, Environmental testing – Part 2-2: Tests – Test B: Dry heat
IEC 60068-2-14, Environmental testing – Part 2-14: Tests – Test N: Change of temperature
IEC 60068-2-78, Environmental testing – Part 2-78: Tests – Test Cab: Damp heat, steady state
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following
addresses:
• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp
3.1
optic circuit
circuit that is composed of optical waveguides and can transmit optical signals
3.2
glass optic fibre
GOF
optic fibre made of glass material
3.3
polymer optic fibre
POF
optic fibre made of polymer material
3.4
opto-electric circuit
O-E circuit
circuit that contains both optic circuit and electric circuit

3.5
flexible opto-electric circuit board
FOECB
flexible circuit board that contains both optic and electric circuits integrated into a flexible sheet

NOTE Figure 1 shows an example of the top view of a flexible opto-electric circuit board.
Figure 1 – Schematic diagram of FOECB (top view)
4 Test method
4.1 General
The FOECBs shall include optical and electrical circuits on a same board. Since the materials
used for the O-E circuits have different thermal properties from those used for the conventional
electric circuits, the requirements for the thermal performance tests will be different between
the two types of circuits. Thus, the thermal properties of the materials, such as glass optic fibre
(hereafter referred as GOF), polymer optic fibre (hereafter referred as POF), polymer film, etc.,
shall be identified in advance for applications to optic circuits in the given applications.
4.2 Test sample
The test samples of the FOECBs consist of electric and optic film circuits. The optic circuit shall
be positioned at the central part of the entire FOECB. An electric circuit shall be positiond at
the periphery part of the optic circuit in a symmetrical structure. The symmetrical structure shall
have superior characteristics in terms of size stability from the point of view of design and
reliability of the FOECBs.
The test samples shall be divided into two groups: a parallel array structure for multichannel
photonic circuit test (Figure 2a)) and a round loop structure for long-distance photonic circuit
test (Figure 2b)). If the test sample contains a fibre optic circuit, it shall have a protruded
structure with a length of l at one side, and this structure shall allow an easy connection to
another fibre with fusion splicing (or with another optical connection means). The protruded
length l shall have a sufficiently long length over 10 cm for easy connections with other fibres
(for an example, via the fibre fusion splicing). In the case of the round loop structure, the closed
side of the test sample shall have a structure of continuous fibre bending with a bending
diameter larger than 6 mm (Figure 2b)). The bending diameter l of larger than 6 mm for optic
fibre should be maintained to minimize the optical bending loss.
An example of the test results from measurement of the optical bending loss of an FOECB with
the general optic glass fibre as a function of the bending radius is shown in Annex A.
The size of the test samples can be determined from the user’s requirements.

– 8 – IEC 63251:2023 © IEC 2023

a) Parallel array structure (type 1) b) Round loop structure (type 2)

Key
1 Optic circuit
2 Electric circuit
Figure 2 – Schematic diagrams of the FOECB test samples of fibre type
For each test group a minimum of three samples having the same physical conditions shall be
prepared in the same fabrication processes.
Details of the preparation for the FOECB test samples (optic fibre type) are shown in Annex B.
4.3 Test process
4.3.1 General description of the test
In order for the FOECBs to be used in harsh environments, the thermal properties of the FOECB
test sample shall be specified with four severe test conditions of reflow assembly simulation,
thermal shock endurance, high temperature endurance and humidity storage.
It is recommended to use different test samples to perform each test.
A flow chart for each test should be as follows. The test process should be started with the
loading of the test sample into the test chamber, and proceed in the order of preconditioning,
test, recovery, final measurement, etc (see Figure 3).

Figure 3 – Schematic diagram of the FOECB test samples of fibre type
The thermal endurance test conditions of the test sample shall be classified as two grades
(class 1, class 2) according to the application area (see Table 1).

Table 1 – Thermal endurance test class for FOECB
Classify Temperature cycling High temperature exposure Humidity Storage
Class 1 (harsh area) 5 cycles (−55 °C to +125 °C) Duration 1 000 h at 150 °C Duration 56 days at 85 °C,
85 % RH
Class 2 (room area) 5 cycles (−40 °C to +82 °C) Duration 1 000 h at 82 °C Duration 12 h at 85 °C, 85 %
RH
4.3.2 Preconditioning
The test sample shall be at the ambient temperature of the laboratory, +25 °C ± 5 K. If required
by the relevant specification, the test sample shall be brought into the operating conditions.
4.3.3 Test
4.3.3.1 Reflow assembly simulation test
The reflow assembly simulation test conditions (260 °C, 20 s to 30 s, 2 cycles to 3 cycles) for
the FOECB test samples follow the detailed guidelines given in IPC-TM-650 2.6.27.
4.3.3.2 Thermal shock endurance test
The thermal shock endurance test conditions for the FOECB test samples shall follow the
detailed guidelines (test Na) given in IEC 60068-2-14.
The preferred number of test cycles shall be five with 30 min at extreme temperature of the
technical data sheet, unless otherwise specified in the relevant specification.
4.3.3.3 High temperature endurance test
The high temperature endurance test conditions for the FOECB test samples shall follow the
detailed guidelines given in IEC 60068-2-2.
The preferred number of test cycles shall be 1 000 h at extreme temperature of the technical
data sheet, unless otherwise specified in the relevant specification.
4.3.3.4 Humidity storage test
The humidity storage test conditions for the FOECB test samples shall follow the detailed
guidelines given in IEC 60068-2-78.
The preferred test durations are 12 h, 16 h, 24 h, 2 d, 4 d, 10 d, 21 d or 56 d at humidity storage
condition (85 °C, 85 %RH).
4.3.4 Recovery
At the end of the test cycle, the test samples shall remain in the standard atmospheric conditions
for testing for a period adequate for the attainment of temperature stability.
The duration of recovery shall be 24 h ± 2 h at room temperature conditions.

– 10 – IEC 63251:2023 © IEC 2023
4.3.5 Final measurements
When the thermal test for the test samples is completed, the following additional tests should
be carried out to check any changes in their characteristics:
– identification of changes in the mechanical properties;
– after the thermal endurance test for the test samples, any appearance change, such as
colour change, deformation, and delamination, of the test samples can be checked visually
first;
– the delamination can be checked by polishing the cross section of the test samples and by
performing microscopic examination on the polished sections;
– finally, the flexibility of the test samples should be checked according to the methods
described in the bending test (IEC 62496-3-1), the MIT test (ISO-5626), and the folding
flexible cable (IPC-TM-650 2.4.31).
Annex C shows an example of the simulation test results for the reflow assembly of an FOECB
test sample.
Annex D shows an example of the high temperature endurance test results for an FOECB test
sample.
Annex E shows an example of the humidity storage test results for an FOECB test sample.
5 Report
a) test sample . (size and number)
b) customer . (name and address)
c) test laboratory . (name and address and details of accreditation – if any)
d) test dates . (dates when test was run)
e) type of test . (reflow assembly simulation, themal shock endurance,
................................................................. high temperature endurance, humidity storage)
f) test standard, edition . (IEC 63251, edition used)
g) any variations/deviation from the sample . (change or not)
h) test sample description . (drawing, photo, quantity build status, etc.)
i) any variation from the defined test method . (change or not)
j) performance of test sample . (results of functional tests,
...................................................... optic & electric properties, mechanical properties etc.)
k) summary of test . (test summary)

Annex A
(informative)
Example of optical bending loss test results
with general glass optic fibres
The results of the optical bending loss measurement of an FOECB test sample with embedded
standard multi-mode fibres (as referred to in the IEC 60793-2 series) are described below (see
Figure A.1):
– optical loss increases with reduced bending diameter;
– bending diameter l shall have a folding range above 5 mm for the optical loss free (see
Figure A.2).
a) Schematic diagram for bending test b) Photograph of the experimental demonstration

Key
1 O-E circuit board
2 Optic fibre (MM)
Figure A.1 – Bending loss test setup

Figure A.2 – Optical loss versus bending diameter

– 12 – IEC 63251:2023 © IEC 2023
Annex B
(informative)
Example of preparation method of O-E circuit test samples
(optic fibre type)
B.1 General
The FOECB test samples are prepared in a sheet shape first. Then, several test samples are
formed on the sheet in an arrayed structure, each having the same size (see Figure B.1). Once
the test sample arrays are produced on one sheet during one batch process, all the test samples
are expected to have uniform mechanical structures and material characteristics. Therefore,
the samples can show the uniform measurement properties for the thermal endurance test.

a) Arrayed test sample b) Photograph picture of test sample

Figure B.1 – Arrayed structure of the FOECB test samples formed on one sheet

In general, the FOECB test samples can be fabricated with two types of optical waveguides,
optic fibre and optical polymer waveguide.
B.2 Manufacturing processes of the FOECBs with optic fibres (POF, GOF)

The FOECBs of the optic fibre-type are fabricated with a lamination process in which an optic
fibre is inserted into a polyimide (PI) film of an existing electric circuit structure (see
Figure B.2a)).
a) Lamination lay-up for optic fibre b) Photograph of micro-cross section of test sample

Figure B.2 – Fabrication of the optic circuits with optic fibres
Since the optic fibre is long, voids can occur between the adhesive and the film during the
manufacturing process of the optic circuits (see Figure B.2b)). It is necessary to have a
sufficient thickness of the adhesive to avoid occurrence of the voids. Otherwise, the voids can
have a large influence on the deterioration of thermal properties of the optic circuits.
B.3 Manufacturing processes of the FOECBs with optical polymer waveguides
Generally the FOECBs with the optical polymer waveguides can be fabricated with a photo
etching process, and have a structure whose optical core materials is surrounded by cladding
materials (see Figure B.3).
Figure B.3 – Fabrication of the optic circuits with optic polymer
waveguide via the photo-etching method
The polymer optical waveguides have the advantages of high flexibility and of PCB process
compatibility. However, they have the disadvantages of high material cost, low thermal
endurance, relatively short length, and high optical loss.

– 14 – IEC 63251:2023 © IEC 2023
B.4 Characteristics of the optic fibres
Generally, the properties of the POF are a large core size (over 250 µm) and low thermal
endurance below 85 °C. The properties of the GOF are a small core size (under 50 µm) and
high thermal endurance above 1 000 °C.

Annex C
(informative)
Example of reflow assembly simulation test results
C.1 General
The reflow assembly simulation process is a preconditioning test to be performed before
carrying out the high thermal shock process. The FOECBs are subjected to high thermal shock
during the soldering process (see Figure C.1).

Figure C.1 – LED chip mounted FOECB
C.2 Results of reflow assembly simulation test for a LED chip mounted FOECB
with GOF
An example of an FOECB having a LED chip mounted with GOF is shown in Figure C.1 and is
tested for the reflow assembly simulation process. Its results are shown in Figure C.2:
– reflow test condition: 260 °C, 20 s, 3 cycles;
– test result: no optical loss variance, but slight deformation.

Figure C.2 – Appearance of a LED chip mounted FOECB after
the reflow assembly simulation test

– 16 – IEC 63251:2023 © IEC 2023
C.3 Results of reflow assembly simulation test for a transparent FOECB with
GOF for display applications
A transparent FOECB with GOFs is tested for the reflow assembly simulation process, and its
results are shown in Figure C.3:
– reflow test condition: 260 °C, 20 s, 3-cycle reflow;
– test result: no variance in optical and electrical losses, but a small colour change.

Figure C.3 – Appearance of a transparent FOECB with GOFs after
the reflow assembly simulation test
C.4 Results of reflow assembly simulation test for a polyimide (PI) based
FOECB with GOF
A polyimide based FOECB with GOF is tested for the reflow assembly simulation process, and
its results are shown in Figure C.4:
– reflow test condition: 260 °C, 20 s, 3 cycles;
– test result: no variance appears in optical and electrical losses after the reflow test except
the pad portions of the Cu circuit are corroded.

Figure C.4 – Appearance of a PI based FOECB with GOF after
the reflow assembly simulation test

C.5 Results of reflow assembly simulation test for a polymer-based FOECB
A polymer-based FOECB is tested for the reflow assembly simulation process, and its results
are shown in Figure C.5:
– reflow test condition: 260 °C, 20 s, 3 cycles;
– test result: the test specimen is severely deformed.

Figure C.5 – Appearance of a polymer-based FOECB after
the reflow assembly simulation test

– 18 – IEC 63251:2023 © IEC 2023
Annex D
(informative)
Example of thermal shock endurance test results
D.1 General
In many cases, the assembled FOECB is subject to operation under optically and electrically
supplied powers and to thermal shocks during the operation cycles. The rapid temperature
change associated with the thermal shock can cause a detrimental effect to the contact portions
of the FOECB due to the fact that the optical and electric circuits absorb and give off heat at
different rates.
Since the optical and electrical circuits in the FOECB are made of different materials,
probabilities for delamination, deformation, and optical and electrical property changes are high
for a sudden thermal shock due to the mismatch of their coefficients of thermal expansion.
D.2 Results of thermal shock endurance test for an FOECB with GOF
An FOECB with GOF is tested for the thermal shock processes, and its result is shown in
Figure D.1:
– thermal shock test condition: -55 °C ~ 140 °C, 30 min/cycle, 1 000 cycles;
– test result: no variation in the optical loss through the optic circuits is observed except minor
physical deformation and delamination.

Figure D.1 – Appearance of an FOECB with GOF after the thermal shock test

Annex E
(informative)
Example of humidity storage test results
E.1 General
FOECBs with GOFs are likely to be exposed to high temperature and high humidity
environments for a long period of time during transportation and storage.
E.2 Results of humidity storage test for an FOECB with GOF
An FOECB with GOF is tested for the humidity storage test, and its result is shown in Figure E.1:
– humidity storage test condition: begin with the original normal test conditions of 85 °C / 85 %
RH, 1 000 h, and then follow the main test under a highly accelerated temperature and
humidity stress test (HAST) condition (130 °C / 85 % RH, 24 h);
– test result: no variance in optical loss of the optic circuit of the FOECB is observed except
slight deformation. Exposure to high temperature and high humidity conditions for a long
period of time causes corrosion on the electric circuit pad area.

Figure E.1 – Appearance of an FOECB with GOF after the humidity storage test
E.3 Results of humidity storage test for an FOECB with POF
An FOECB with POF is tested for the humidity storage test, and its result is shown in Figure E.2:
– humidity storage test condition: begin with the original normal test conditions of 85 °C / 85 %
RH, 1 000 h, and then follow the main test under a HAST condition (130 °C / 85 % RH 24 h);
– test result: the POF circuit portions are melted after the HAST process.

– 20 – IEC 63251:2023 © IEC 2023

Figure E.2 – Appearance of an FOECB with POF after the humidity storage test

Bibliography
IEC 60793-2 (all parts), Optical fibres – Part 2-X: Product specifications
IEC 62496-3-1, Optical circuit boards – Part 3-1: Performance standards – Flexible optical
circuit boards using unconnectorized optical glass fibres
ISO 5626, Paper – Determination of folding endurance
IPC-6012DA 2016-April, Automotive Applications Addendum to IPC-6012D Qualification and
Performance Specification for Rigid Printed Boards
IPC-TM-650 2.6.27, Thermal Stress, Convection Reflow Assembly Simulation Optic fibre
IPC-TM-650 2.4.31A, Folding, Flexible Flat Cable

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– 22 – IEC 63251:2023 © IEC 2023
SOMMAIRE
AVANT-PROPOS . 24
1 Domaine d’application . 26
2 Références normatives . 26
3 Termes et définitions . 26
4 Méthode d’essai . 27
4.1 Généralités . 27
4.2 Échantillon d’essai . 27
4.3 Processus d’essai . 28
4.3.1 Description générale de l’essai . 28
4.3.2 Préconditionnement . 29
4.3.3 Essai . 29
4.3.4 Récupération . 30
4.3.5 Mesures finales . 30
5 Rapport . 31
Annexe A (informative) Exemple des résultats d’essai de perte optique de courbure
sur fibres optiques de verre courantes . 32
Annexe B (informative) Exemple de méthode de préparation d’échantillons d’essai du
circuit optoélectrique (type de fibre optique) . 33
B.1 Généralités . 33
B.2 Procédés de fabrication des FOECB avec fibres optiques (POF, GOF) . 33
B.3 Procédés de fabrication des FOECB avec guides d’ondes optiques
polymères . 34
B.4 Caractéristiques des fibres optiques . 35
Annexe C (informative) Exemple de résultats d’essai de simulation d’assemblage par
refusion . 36
C.1 Généralités . 36
C.2 Résultats de l’essai de simulation d’assemblage par refusion pour un
FOECB à puce LED avec des GOF . 36
C.3 Résultats de l’essai de simulation d’assemblage par refusion pour un
FOECB transparent avec des GOF pour applications d’affichage . 37
C.4 Résultats de l’essai de simulation d’assemblage par refusion pour un
FOECB à base de polyimide (PI) avec des GOF . 37
C.5 Résultats de l’essai de simulation d’assemblage par refusion pour un
FOECB à base de polymère . 38
Annexe D (informative) Exemple de résultats d’essai d’endurance au choc thermique . 39
D.1 Généralités . 39
D.2 Résultats de l’essai d’endurance au choc thermique pour un FOECB avec
des GOF . 39
Annexe E (informative) Exemple de résultats d’essai de stockage humide . 40
E.1 Généralités . 40
E.2 Résultats de l’essai de stockage humide pour un FOECB avec des GOF . 40
E.3 Résultats de l’essai de stockage humide pour un FOECB avec des POF . 40
Bibliographie . 42

Figure 1 – Représentation schématique du FOECB (vue du dessus) . 27
Figure 2 – Schémas des échantillons d’essai de FOECB du type de fibre . 28
Figure 3 – Schéma des échantillons d’essai FOECB du type de fibre . 29

Figure A.1 – Montage d’essai pour perte de courbure . 32
Figure A.2 – Perte optique en fonction du diamètre de courbure . 32
Figure B.1 – Structure groupée des échantillons d’essai de FOECB formée sur une
seule feuille . 33
Figure B.2 – Fabricati
...