IEC 62925:2016

IEC 62925 ®
Edition 1.0 2016-12
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Concentrator photovoltaic (CPV) modules – Thermal cycling test to differentiate
increased thermal fatigue durability

Modules photovoltaïques à concentration (CPV) – Essai de cycles thermiques
pour la détermination de la durabilité renforcée à la fatigue thermique

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IEC 62925 ®
Edition 1.0 2016-12
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Concentrator photovoltaic (CPV) modules – Thermal cycling test to differentiate

increased thermal fatigue durability

Modules photovoltaïques à concentration (CPV) – Essai de cycles thermiques

pour la détermination de la durabilité renforcée à la fatigue thermique

INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 27.160 ISBN 978-2-8322-3725-0

– 2 – IEC 62925:2016 © IEC 2016
CONTENTS
FOREWORD . 3
INTRODUCTION . 5
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 6
4 Sampling . 7
5 Marking . 7
6 Testing . 8
7 Rating. 8
8 Report . 8
9 Modifications . 9
10 Test procedure . 9
10.1 General . 9
10.2 Cell evaluation . 9
10.2.1 Purpose . 9
10.2.2 Procedure . 10
10.2.3 Requirements . 10
10.3 Thermal cycling. 10
10.3.1 Purpose . 10
10.3.2 Test sample . 10
10.3.3 Procedure . 10
10.4 Outdoor exposure test . 12
10.4.1 Purpose . 12
10.4.2 Procedure . 12
Bibliography . 13

Figure 1 – Number of required cycles as a function of average temperature ramp rate
in order to complete an equivalent test . 11
Figure 2 – Overall test time as a function of temperature ramp rate . 12

INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
CONCENTRATOR PHOTOVOLTAIC (CPV) MODULES –
THERMAL CYCLING TEST TO DIFFERENTIATE
INCREASED THERMAL FATIGUE DURABILITY

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
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patent rights. IEC shall not be held responsible for identifying any or all such patent rights.
International Standard IEC 62925 has been prepared by IEC technical committee 82: Solar
photovoltaic energy systems.
The text of this standard is based on the following documents:
FDIS Report on voting
82/1185/FDIS 82/1210/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 62925:2016 © IEC 2016
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.
INTRODUCTION
IEC 62108 defines IEC requirements for the design qualification of concentrator modules for
long-term operation in general open-air climates. This standard, IEC 62925, is not compulsory
with but will supplement IEC 62108 by providing tests that differentiate thermal fatigue
durability of concentrator modules for deployment in a larger range of applications and
climates.
– 6 – IEC 62925:2016 © IEC 2016
CONCENTRATOR PHOTOVOLTAIC (CPV) MODULES –
THERMAL CYCLING TEST TO DIFFERENTIATE
INCREASED THERMAL FATIGUE DURABILITY

1 Scope
This document defines a test sequence that will quickly uncover CPV module failures that
have been associated with field exposure to thermal cycling for many years. This document
was specifically developed to relate to thermal fatigue failure of the HCPV die-attach, however,
it also applies, to some extent, to all thermal fatigue related failure mechanisms for the
assemblies submitted to test.
IEC 62108, the CPV module qualification test already includes an accelerated thermal cycle
sequence in one leg of the testing, however, the parameters of that test only represent a
qualification level of exposure. This test procedure applies more stress and will provide a
route for comparative testing to differentiate CPV modules with improved durability to thermal
cycling and the associated mechanical stresses.
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 62108:2016, Concentrator photovoltaic (CPV) modules and assemblies – Design
qualification and type approval
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
durability
system’s capacity to resist degradation specific for a particular stress or set of stresses
Note 1 to entry: The durability with respect to all relevant stresses shall be assessed in order to gain information
about the anticipated performance of the system.
3.2
reliability
probability for a system to perform and maintain its designed function in specific conditions for
a specified period of time
Note 1 to entry: In this context the term reliability shall be accompanied by three numbers: a) a specified period of
time, b) a criterion that quantifies its performance at that time and c) the probability that the criterion will be met.

3.3
concentrator receiver
group of one or more concentrator cells and secondary optics (if present) that accepts
concentrated sunlight and incorporates the means for thermal and electric energy transfer. A
receiver could be made of several sub-receivers. The sub-receiver is a physically standalone,
smaller portion of the full-size receiver
[SOURCE: IEC 62108:2016, 3.4]
3.4
concentrator module
group of receivers, optics, and other related components, such as interconnection and
mounting, that accepts unconcentrated sunlight. All above components are usually
prefabricated as one unit, and the focus point is not field adjustable
Note 1 to entry: A module could be made of several sub-modules. The sub-module is a physically stand-alone,
smaller portion of the full-size module.
[SOURCE: IEC 62108:2016, 3.5]
3.5
concentrator assembly
group of receivers, optics, and other related components, such as interconnection and
mounting that accepts unconcentrated sunlight. All above components would usually be
shipped separately and need some field installation, and the focus point is field adjustable
Note 1 to entry: An assembly could be made of several sub-assemblies. The sub-assembly is a physically
standalone, smaller portion of the full-size assembly.
[SOURCE: IEC 62108:2016, 3,6]
4 Sampling
For non-field-adjustable focus-point CPV systems or modules, a number of modules that
contain at least 20 separate PV cells are required. Test failure is based on the failure of 5 %
of the submitted cells; therefore submission of more than 20 cells is encouraged.
If coordinated between the submitting manufactured and testing laboratory, disassembly of
the submitted module(s) into a separate receiver or receivers for the thermal cycling
sequence is acceptable and encouraged. This approach may be desirable in order to reduce
thermal mass thereby enabling the possibility to achieve a faster temperature ramp rate.
Following the thermal cycling sequence, the receiver shall be reassembled into its module
configuration for the outdoor exposure. The original submission of receivers for test is also
acceptable, provided they may subsequently assembled into a module for the required
outdoor exposure.
5 Marking
Each receiver or module section should carry the following clear and indelible markings:
• name, monogram, or symbol of manufacturer;
• type or model number;
• serial number;
• polarity of terminals or leads (color coding is permissible);
• maximum system voltage for which the module or assembly is suitable;
• nominal maximum output power and its tolerance at specified condition;

– 8 – IEC 62925:2016 © IEC 2016
• the date, place of manufacture, and cell materials should be marked, or be traceable from
the serial number.
If representative samples are used, the same markings as on full-size products should be
included for all tests, and the marking should be capable of surviving all tests sequences.
6 Testing
If some test procedures in this document are not applicable to a specific design configuration,
the manufacturer should discuss this with the certifying body and testing agency to develop a
comparable test program, based on the principles described in this standard. Any changes
and deviations shall be recorded and reported in details, as required in Clause 8, item j).
7 Rating
The rating system is based on the number of cycles completed with total cell failure less than
5 %. Cell failure is defined in 10.2.3 as a shorted cell or an inactive cell due to an electrical
open. The number of required cycles according to Formula (2), 10.3.3, is equivalent to five
times the qualification level of thermal cycling testing (IEC 62108) and therefore earns a
rating of 5Q upon successful completion (< 5 % cell failure). There also exists the possibility
to achieve a rating higher or lower than 5Q by completing more or less cycles than defined by
Formula (2). Rating shall be based on integer intervals according to Formula (1):
N
C
rating= 5
(1)
N
R
Where
N is the number of completed cycles with a total cell failure of less than 5 % and,
C
N is the number of required cycles according to Formula (2).
R
The rating achieved shall be an integer product of Formula (1) and will default to the next
lower rating if N is in between an integer interval. For instance, if Formula (1) results in 4,9,
C
the rating received is 4Q.
Every interval of cycling represents an amount of service life. Therefore the 5Q rating is
intended to correspond with a minimum service life with total cell failures less than 5 % due to
thermal fatigue related failure mechanisms.
It is up to the discretion of the submitting party and testing laboratory as to what interval of
testing is attempted and at what interval(s) the outdoor evaluation and performance testing
are made.
8 Report
Following testing, a certified report of the qualification tests, with details of any failures and
re-tests, should be prepared by the test agency. Each test report should include at least the
following information:
a) a title;
b) the name and address of the laboratory, and the location where the tests were carried out,
if different from the address of the laboratory (such as on-site location);
c) unique identification of the test report (such as the serial number), and on each page an
identification to ensure that the page is recognized as a part of the test report, and a clear
identification of the end of the test report;

d) name and address of client, where appropriate;
e) description and identification of the item tested;
f) characterization and condition of the test item;
g) date of receipt of test item and date(s) of test, where appropriate;
h) identification of ramp rate and number of cycles used (Formula (1));
i) reference to sampling procedure, where relevant;
j) any deviations from, additions to, or exclusions from the test method, and any other
information relevant to a specific tests, such as environmental conditions;
k) measurements and examinations supported by tables, graphs, sketches, and photographs
as appropriate after completion of each cycling interval and any failures observed;
l) a statement of the estimated uncertainty of the test results, where relevant;
m) a signature and title, or equivalent identification, of the person(s) accepting responsibility
for the content of the report, and the date of issue;
n) where relevant, a statement to the effect that the results relate only to the items tested;
o) a statement that to maintain the rating, the manufacturer shall report to and discuss with
the certifying body and testing agency every change they made;
p) a statement that the report shall not be reproduced except in full, without the written
approval of the laboratory.
A copy of this report should be kept by the manufacturer for reference purposes.
9 Modifications
Any changes in design, materials, components, or processing of the modules and assemblies
may require a repetition of this test sequence. Manufacturers shall report to and discuss with
the certifying body and testing agency every change they have made.
10 Test procedure
10.1 General
The test sequence is as follows:
a) Cell evaluation.
b) Thermal cycling.
c) Outdoor exposure.
d) Cell evaluation.
If it is desired to periodically evaluate the test articles at the designated cycling intervals,
steps a) and d) are combined and the sequence repeated.
10.2 Cell evaluation
10.2.1 Purpose
The purpose of this evaluation is to determine if any of the included PV cells have failed
through testing. This document is intended to evaluate the thermal fatigue durability of the
CPV die-attach. When the die-attach cracks due to thermal fatigue, the thermal resistance of
this layer increases and ultimately causes thermal runaway of the attached PV cell when
placed on-sun. Thermal runaway of the PV cell typically results in an electrical short across
the cell that is detected with the following evaluation. Provisions to detect an electrically open
cell are also provided.
– 10 – IEC 62925:2016 © IEC 2016
10.2.2 Procedure
a) All PV cells are forward biased to a level that causes electroluminescence emission of a
pristine cell.
b) If a cell or cells fail to emit, they may be individually probed to determine if an electrical
short exists. This step may consist of an evaluation of the cell’s dark current and voltage
response. This electrical method of determining if each individual cell is an electrical short
may be substituted for step a) if desired.
For some cell technologies, the electroluminescence emission is in the visible spectrum;
therefore the emission from each cell may be visually evaluated. In the case the emission is
not within the visible spectrum, the emission may be evaluated by use of a camera with a
quantum efficiency that overlaps the normal electroluminescence emission of those cells.
If the module or string of cells biased presents as electrically open, the location of the open
shall be determined and electrically jumped in order to continue the thermal cycling sequence.
To locate the open, continuity checks and/ or forward biasing smaller portions of the string
until the open is found should be conducted.
10.2.3 Requirements
Any cell that fails to emit and/or is evaluated as an electrical short is considered a failure for
the purpose of this standard. It is at the discretion of the submitting party and test laboratory
to ascertain the root cause of the failure. Only if, through this root cause analysis, it can be
demonstrated that thermal fatigue is not the cause of cell failure, the failed cell may be
removed from the failed population.
Any cell that is removed from the active string of the module by an electrical open is also
considered a failure for the purpose of this document.
If a cell is initially shorted prior to the first thermal cycling interval, it should not be considered
in the failed population and be replaced by a pristine cell.
10.3 Thermal cycling
10.3.1 Purpose
The purpose of the thermal cycling test is to determine the ability of the receivers to withstand
thermal mismatch, fatigue, and other stresses caused by rapid, non-uniform or repeated
changes of temperature.
10.3.2 Test sample
Either module(s) or their contained receiver(s) are applicable for the thermal cycling test.
10.3.3 Procedure
Thermal cycling is to be conducted between –40 ˚C and 110 ˚C with dwell times of 5 min at
±5 ˚C of the dwell temperature. The average temperature ramp rate (𝑇̇) is variable, with a
maximum of 30 ˚C/min, and will set the required number of cycles (N ) according to Formula
R
(1).
−0,166
𝑁 = 105,56 + 2787,8 𝑇̇ (2)
R
The number of required cycles for any ramp rate is considered as an equivalent test. Figures
1 and 2 are provided to illustrate this relationship and its consequence on overall test time.
NOTE 1 Thermal cycling parameters are based on:
a) IEC 62108: A 2,5 ˚C/min ramp rate results in 5x the number of TCA2 cycles;

b) Experiment and FEM simulation that gives the relationship between temperature ramp rate and equivalent
number of cycles [1] .
Apply a current equal to 1,25x the CSTC short circuit current when temperature increases
above 25 ˚C and remove at the end of the high temperature dwell. The application of forward
bias during cycling may provide the opportunity to detect cell failure. If, through cycling, the
die-attach cracks through thermal fatigue, the increased thermal resistance may precipitate a
thermal runaway condition in the PV cell causing catastrophic failure. This failure will manifest
as an abrupt drop in the voltage required in applying the constant short circuit current. Since
the intention of this test is to only capture thermal fatigue related failures, any such in situ cell
failure should be examined for its root cause. If evidence implies that a thermal fatigue related
issue was not the cause of the in situ cell failure, the failure should not be considered failed
population.
Module or receiver temperature is monitored and controlled via an RTD or thermocouple
placed as close to the PV cell as possible. Multiple cells should be probed in both the interior
and on perimeter of the module or receiver or on multiple, individual, receivers and the
average temperature used as the test control.
NOTE 2 Modules with better thermal solutions, which result in a lower difference between cell and ambient
temperature when on-sun, are expected to be more durable against thermal fatigue because of the reduced driving
force. Thermocouple placement for cycling temperature control is intended to reward these solutions with a less
severe test. Designs with poor thermal solutions may have a larger temperature difference between thermocouple
location and cell, therefore cycling temperature control from the thermocouple will cause the cell temperature to be
elevated when compared with designs with better thermal solutions.
For the purpose of rating, cycling may be interrupted at predetermined intervals. See Clause 7:
The required number of cycles divided by five creates the cycling intervals. Evaluations may
be made at these intervals; the number of intervals and evaluations is at the discretion of the
submitting party and test laboratory.
2 500
2 400
2 300
2 200
2 100
2 000
1 900
1 800
1 700
+110 °C to –40 °C
1 600
5 10 15 20 25 30
Ramp rate (°C/min)
IEC
Figure 1 – Number of required cycles as a function of average
temperature ramp rate in order to complete an equivalent test
___________
Figures in square brackets refer to the Bibliography.
Required number of cycles
– 12 – IEC 62925:2016 © IEC 2016
+110 °C to –40 °C
5 10 15 20 25 30
Ramp rate (°C/min)
IEC
Figure 2 – Overall test time as a function of temperature ramp rate
10.4 Outdoor exposure test
10.4.1 Purpose
The purpose of the outdoor exposure test is to reveal any failures, if present, from the thermal
cycling sequence. The electrical and thermal connections that are susceptible to thermal
fatigue require the unique combination of solar, thermal and electrical flux from an on-sun
exposure to precipitate catastrophic cell
...