Measurement procedures for materials used in photovoltaic modules - Part 6-3: Adhesion testing for PV module laminates using the single cantilevered beam (SCB) method

IEC TS 62788-6-3:2022 describes the single cantilevered beam (SCB) test, useful for characterizing adhesion in photovoltaic (PV) modules. This document offers a generalized method for performing the test, with the expectation that best practices for utilizing this test method will be developed for specific applications.
This document provides a method for measuring the adhesion energy of most interfaces within the photovoltaic (PV) module laminate. This method provides a measure of adhesive energy, via the critical energy release rate, and so is more useful for comparing adhesion of different specimen types; e.g. different materials, module or coupon samples, or materials before and after stress exposure.

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Publication Date
18-Aug-2022
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19-Aug-2022
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IEC TS 62788-6-3:2022 - Measurement procedures for materials used in photovoltaic modules - Part 6-3: Adhesion testing for PV module laminates using the single cantilevered beam (SCB) method
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IEC TS 62788-6-3
Edition 1.0 2022-08
TECHNICAL
SPECIFICATION
colour
inside
Measurement procedures for materials used in photovoltaic modules –
Part 6-3: Adhesion testing for PV module laminates using the single cantilevered
beam (SCB) method
IEC TS 62788-6-3:2022-08(en)
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---------------------- Page: 2 ----------------------
IEC TS 62788-6-3
Edition 1.0 2022-08
TECHNICAL
SPECIFICATION
colour
inside
Measurement procedures for materials used in photovoltaic modules –
Part 6-3: Adhesion testing for PV module laminates using the single
cantilevered beam (SCB) method
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 27.160 ISBN 978-2-8322-5561-2

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

® Registered trademark of the International Electrotechnical Commission
---------------------- Page: 3 ----------------------
– 2 – IEC TS 62788-6-3:2022 © IEC 2022
CONTENTS

FOREWORD ........................................................................................................................... 4

INTRODUCTION ..................................................................................................................... 6

1 Scope .............................................................................................................................. 7

2 Normative references ...................................................................................................... 7

3 Terms and definitions ...................................................................................................... 7

4 Apparatus ........................................................................................................................ 9

4.1 Load frame ............................................................................................................. 9

4.2 Loading tab ........................................................................................................... 10

5 Width-tapered cantilever beam ...................................................................................... 10

5.1 General ................................................................................................................. 10

5.2 Beam design ......................................................................................................... 11

5.3 Beam selection ..................................................................................................... 11

6 Test method .................................................................................................................. 12

6.1 Specimen preparation ........................................................................................... 12

6.2 Measurement procedure ....................................................................................... 12

6.3 Analysis ................................................................................................................ 14

6.3.1 Critical adhesion energy, G .......................................................................... 14

7 Report ........................................................................................................................... 15

Annex A (informative) Summary of background theory, and how this method can be

generalized ........................................................................................................................... 16

A.1 Background theory ................................................................................................ 16

A.2 Beam materials ..................................................................................................... 17

Annex B (informative) Guidance for specific use cases ........................................................ 18

B.1 General ................................................................................................................. 18

B.2 Adhesion test coupons .......................................................................................... 18

B.2.1 Backsheet / encapsulant adhesion ................................................................. 18

B.2.2 Backsheet interlayer adhesion ....................................................................... 18

B.2.3 Glass/encapsulant adhesion .......................................................................... 19

B.2.4 Adhesion between different encapsulants ...................................................... 19

B.2.5 Cell/encapsulant (coupons) ........................................................................... 20

B.3 Modules ................................................................................................................ 20

B.3.1 General ......................................................................................................... 20

B.3.2 Targeting a specific interface in a module ...................................................... 20

Annex C (informative) Reference engineering diagrams for loading tab and beam ............... 23

Annex D (normative) Using a reference compliance curve to calculate G ........................... 26

D.1 General ................................................................................................................. 26

D.2 Procedure ............................................................................................................. 26

D.2.1 Beam compliance measurement .................................................................... 26

D.2.2 Definition of empirical parameters .................................................................. 27

D.2.3 Validation of reference parameters ................................................................ 27

D.2.4 Generating a custom set of α, β and γ parameters .......................................... 28

D.2.5 Calculation of crack length a for adhesion specimens ................................... 29

Annex E (informative) Methods for measurement of final debond length a ........................... 30

E.1 General ................................................................................................................. 30

E.2 Aluminum foil method............................................................................................ 30

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IEC TS 62788-6-3:2022 © IEC 2022 – 3 –

E.3 Light method ......................................................................................................... 30

E.4 Pull-apart method ................................................................................................. 30

Bibliography .......................................................................................................................... 32

Figure 1 – Diagram of the loading connection using a clevis grip ............................................ 9

Figure 2 – Schematic of load frame with a) a platen for securing test coupon, and b)

modified to sit on top of a PV module .................................................................................... 10

Figure 3 – Photos of the loading tab alone, and attached to the beam .................................. 10

Figure 4 – Width-tapered beam ............................................................................................. 11

Figure 5 – Typical width-tapered cantilever beam load/displacement curve ........................... 13

Figure 6 – Example of an a measurement on glass/encapsulant/cell specimens ................... 14

Figure B.1 – Top view of backsheet and encapsulant beam coupons .................................... 21

Figure B.2 – Cross-sectional view of backsheet and encapsulant beam coupons .................. 22

Figure C.1 – Schematics of loading tab ................................................................................. 24

Figure C.2 – Schematics of beam ......................................................................................... 25

Figure D.1 – Photo of a beam prior to start of the calibration measurement .......................... 26

Figure D.2 – Photo of a beam at the end of the calibration measurement .............................. 27

Figure D.3 – Plot used for generating beam calibration curves with the empirical fits

according to Formula (D.2) using values from Table D.1 ....................................................... 29

Figure E.1 – Illustration of debond length measurement with a cohesive zone ...................... 31

Table 1 – Typical adhesion strengths .................................................................................... 12

Table D.1 – Reference empirical fit parameters .................................................................... 27

Table D.2 – Example read points for fit evaluation ................................................................ 28

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– 4 – IEC TS 62788-6-3:2022 © IEC 2022
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
MEASUREMENT PROCEDURES FOR MATERIALS
USED IN PHOTOVOLTAIC MODULES –
Part 6-3: Adhesion testing for PV module laminates
using the single cantilevered beam (SCB) method
FOREWORD

1) The International Electrotechnical Commission (IEC) is a worldwide organization for standardization comprising

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

IEC TS 62788-6-3 has been prepared by IEC technical committee 82: Solar photovoltaic energy

systems. It is a Technical Specification.
The text of this Technical Specification is based on the following documents:
Draft Report on voting
82/2012/DTS 82/2057A/RVDTS

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 Technical Specification is English.

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IEC TS 62788-6-3:2022 © IEC 2022 – 5 –

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

described in greater detail at http://www.iec.ch/standardsdev/publications.

A list of all parts in the IEC 62788 series, published under the general title Measurement

procedures for materials used in photovoltaic modules, 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 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 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.
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– 6 – IEC TS 62788-6-3:2022 © IEC 2022
INTRODUCTION

This document describes the single cantilevered beam (SCB) test, useful for characterizing

adhesion in photovoltaic (PV) modules. This method is grounded in fundamental concepts of

beam and fracture mechanics [1] , [4], and allows for a quantitative measurement of adhesion

strength. A method for calculating the debond length, a has been developed as an option to

physical measurement.

PV modules are multi-layer structures that depend on adequate adhesion between each layer

to ensure their reliable operation. Adhesion testing is described in current IEC PV standards for

module safety qualification (IEC 61730-2) and component characterization (IEC 62788 series).

The most commonly used tests are peel tests at either 180° for components (IEC TS 62788-2

test and IEC 62788-1-1), or at 90° for modules (IEC 61730-2 MST 35).

Peel tests are in practice simple to carry out, and provide a peel strength value, different from

adhesion strength. Viscoelastic properties of the polymeric material and the mechanics of the

pull tab have a strong influence on the result, making these tests of limited value in comparing

either different materials, or the same material after stress exposures.

In the SCB method, an elastic width-tapered cantilever beam is adhered to the sample. When

the beam is loaded at its apex, delamination will initiate at the weakest interface and advance

upon continued loading. This measurement allows for calculation of the critical value of the

, which is the adhesion property for a given material interface. The value
energy release rate, G

defined by this method is less dependent of the viscoelastic properties of the polymeric material,

and so more useful for measuring differences or changes in adhesive strength.

The SCB method can be conducted at either the coupon or module level. Because it does not

require using the backsheet as a pull tab, it is more likely to able to test the adhesion of a thin

outer layer of the backsheet. These considerations give this test method good flexibility to use

in applications related to PV modules. Examples for several specific use cases are provided.

This document offers a generalized method for performing the test, with the expectation that

best practices for utilizing this test method will be developed for specific applications.

Examples of this method being employed to quantify and define the threshold values of

encapsulant and backsheet adhesion for PV module reliability may be found in the literature [1]

through [5].
___________
Numbers in square brackets refer to the Bibliography.
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IEC TS 62788-6-3:2022 © IEC 2022 – 7 –
MEASUREMENT PROCEDURES FOR MATERIALS
USED IN PHOTOVOLTAIC MODULES –
Part 6-3: Adhesion testing for PV module laminates
using the single cantilevered beam (SCB) method
1 Scope

This part of IEC TS 62788 provides a method for measuring the adhesion energy of most

interfaces within the photovoltaic (PV) module laminate.

In contrast to other adhesion tests in general use, this method provides a measure of adhesive

energy, via the critical energy release rate, and so is more useful for comparing adhesion of

different specimen types; e.g. different materials, module or coupon samples, or materials

before and after stress exposure.

This is a “weakest link” test, meaning that the weakest interface is the one most likely to fail in

a given test. Adhesion of a specific layer may be difficult to intentionally measure if there is a

weaker interface in the system.
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 TS 61836, Solar photovoltaic energy systems – Terms, definitions and symbols

ISO 7500-1, Metallic materials – Calibration and verification of static uniaxial testing machines

– Part 1: Tension/compression testing machines – Calibration and verification of the force-

measuring system
3 Terms and definitions

For the purposes of this document, the terms and definitions given in IEC TS 61836 apply, as

well as the following.

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
cantilevered beam

beam supported at only one end such that the slope and deflection of that end is ideally zero

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– 8 – IEC TS 62788-6-3:2022 © IEC 2022
3.2
mechanical compliance

measure of the extent of deformation due to the action of external forces (reciprocal of stiffness)

Note 1 to entry: Unit (preferred): m/N.
3.3
adhesive failure

de-bonding occurring between the adhesive and the adherent, to be differentiated from

cohesive failure within the adhesive material
3.4
cohesive failure
crack propagating within the adhesive during adhesion test, e.g. peel test
3.5
adhesive energy
specific energy (in J/m ) released during separation of two material layers
3.6
critical adhesive energy
critical strain energy release rate necessary to promote crack growth
3.7
debond length

measured length of specimen from the apex of the tapered beam to the end of the debonded

area
3.8
load-line displacement
displacement measured along the loading axis of a load frame
3.9
unfixed beam length

length of the beam between the clamp and the tip, used to determine the compliance calibration

of the beam
3.10
compliance calibration method

method used to calculate the debond length based on the measured compliance at various

crack lengths for a specific beam
3.11
plastic deformation
permanent, non-recoverable deformation
3.12
cohesive zone

trailing area adjacent to the debond edge that may consist of cavitation, voids and ligaments

within the adhesive
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IEC TS 62788-6-3:2022 © IEC 2022 – 9 –
4 Apparatus
4.1 Load frame

A properly calibrated load frame shall be used that can be operated in a displacement control

mode with a constant rate of 10,0 µm/s. A load cell with a capacity of 200 N is recommended.

The load frame shall conform to the requirements of ISO 7500-1.
The load frame shall be equipped with the following:

• a clevis grip link that couples the load train to the loading tab attached to the specimen,

Figure 1. The link should be ≥ 30 mm between the centres of the connection points, and

each end of the link shall be able to rotate freely about the clevis pin orthogonal to the

specimen plane. The clevis pin should be 1,0 mm steel or material of equal or greater elastic

modulus.

• a platen opposite the loading grip to which the test specimen is secured, Figure 2a); or, the

load frame may be modified to sit on the specimen (e.g., PV module) Figure 2b).

• a displacement indicator capable of monitoring and recording load-line displacement. The

displacement indicator shall indicate the load-line displacement within an accuracy of

10 μm.

• a load-sensing device capable of monitoring and recording the total load carried by the

specimen. This device shall indicate the load with an accuracy over the load range(s) of

interest within 0,1 N.
Dimensions in millimetres
Figure 1 – Diagram of the loading connection using a clevis grip
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– 10 – IEC TS 62788-6-3:2022 © IEC 2022
a) b)

Figure 2 – Schematic of load frame with a) a platen for securing test coupon, and b)

modified to sit on top of a PV module
4.2 Loading tab

The loading tab connects to the beam and to the loading pin of the Clevis joint. The preferred

material for the loading tab is stainless steel, although aluminium may also be used. To provide

a low friction surface, a sapphire jewel bearing is recommended for the contact with the loading

pin. This should be inspected for damage prior to each test, and cracked bearings shall be

replaced. Photos of a loading tab are shown in Figure 3. A reference engineering design is

provided in Annex C.
Figure 3 – Photos of the loading tab alone, and attached to the beam
5 Width-tapered cantilever beam
5.1 General

The beam is usually considered disposable and used only once. After cleaning, it may be reused

if evaluated to ensure no permanent deformation has occurred. Recovering a deformed beam

is not recommended.

Design parameters for the beam include both physical dimensions and material properties.

Annex A describes a range of beams which may be used in the context of this document. Two

specific designs are included in this specification, with the selection to be made based on the

maximum expected adhesion energy, G , of the system to be measured.
max
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IEC TS 62788-6-3:2022 © IEC 2022 – 11 –
5.2 Beam design

A variety of materials and beam designs can be used according to theory as described in

Annex A. For simplicity, this document specifies a single material, Grade 5 Ti-6Al04V, and

specific design parameters as given in Figure 4. Thickness shall be either 0,8 mm or 1,6 mm,

as appropriate for different adhesion strengths (5.3).
A reference engineering diagram is provided in Figure C.1.

NOTE Annex A describes considerations for other materials, in particular for situations where very low adhesion

energy measurements are targeted, or when adhesion to both beam and specimen is problematic with a titanium

beam.
Dimensions in millimetres
Figure 4 – Width-tapered beam
5.3 Beam selection

The optimum beam thickness is based on the expected adhesion strength value, with typical

adhesion strengths for some PV interfaces shown in Table 1. The 0,8 mm titanium beam has

2 2

been found useful for a range from ~20 J/m to 1 200 J/m , and the 1,6 mm beam for a range

2 2

from 100 J/m to 2 500 J/m . For measurement of very low adhesion energies, a different beam

material should be used; see Annex A.

Adhesion energy of an interface after environmental exposures can vary significantly, to near

zero. For comparison of adhesion energy before and after environmental exposures, the same

beam is recommended to be used for both, even for very low adhesion energies.

Highest sensitivity will be obtained with the most compliant beam which does not permanently

deform during the measurement, so the titanium beam with thickness of 0,8 mm is a useful

starting point. If the thin beam is used and visibly deformed after the test, the thicker beam

should be used. If the thick beam is pulled off the substrate without elastically bending, this

indicates a lower adhesion energy, and the thinner beam should be used.
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– 12 – IEC TS 62788-6-3:2022 © IEC 2022
Table 1 – Typical adhesion strengths
Interface G
max
J/m
Backsheet – encapsulant (initial) 200 to 800
Backsheet interlayers (initial) 200 to 2 300
Encapsulant glass (initial) 1 200 to 2 500
Encapsulant – cell (initial) 1 200 to 2 000
6 Test method
6.1 Specimen preparation

A general procedure is provided below, with examples for specific use cases provided in

Annex B.
a) Prepare the test material

1) If the interface of interest is in the form of two different materials, laminate them in a

manner which replicates the bonding in the application.

2) If a flexible substrate is used (e.g. for backsheet interlayer, or backsheet to-

encapsulant), or a weak rigid substrate (e.g. a silicon cell) fix it to a strong rigid substrate

(e.g. thick glass).

• The size shall be large enough to completely hold the beam, with width greater than

h, and length greater than b.

• It may improve beam-specimen adhesion if the surface to be attached to the beam

is abraded. Then, clean the surface with isopropyl alcohol and allow to dry.

b) Abrade the surface of the selected beam to be bonded with a 150 grit sandpaper or similar,

and clean with isopropyl alcohol.

c) Adhere the tapered-width portion of beam to the top layer of the test specimen using a thin

layer of adhesive, leaving the rectangular portion unattached. A two-part structural adhesive

is recommended, using manufacturer’s instructions for mixing and shelf life. A weight (1 kg

to 5 kg) should be placed on top of each beam specimen to ensure even adhesive coverage

and a thin bond line. Care should be taken to ensure uniform weight distribution along the

length of the beam. Use of silica spheres in the epoxy can help to assure a minimum

thickness across the interface. Remove any excess adhesive from around the beam while

tacky but before it is fully cured.

NOTE 1 One example is 3M DP4M epoxy adhesive. This information is provided for the convenience o

...

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