IEC TR 61760-5-1:2024

IEC TR 61760-5-1 ®
Edition 1.0 2024-01
TECHNICAL
REPORT
colour
inside
Surface mounting technology –
Part 5-1: Surface strain on circuit boards – Strain gauge measurement applied to
chip components
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IEC TR 61760-5-1 ®
Edition 1.0 2024-01
TECHNICAL
REPORT
colour
inside
Surface mounting technology –
Part 5-1: Surface strain on circuit boards – Strain gauge measurement applied to

chip components
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 31.190  ISBN 978-2-8322-8160-4

– 2 – IEC TR 61760-5-1:2024 © IEC 2024
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
4 Damaging mechanisms of chip type ceramic components . 7
4.1 Surface strain by board bending. 7
4.2 Typical cracking modes . 8
4.3 Measurement of local strain . 9
5 Crack prevention . 9
5.1 Strain control . 9
5.2 Critical board design factors . 9
5.2.1 Distance from circuit board cutting position . 9
5.2.2 Distance from screw clamping position . 9
5.2.3 Mounting direction . 10
5.2.4 Warpage . 10
5.2.5 Orientation of the component related with the loading point . 10
5.3 Critical assembly process factors . 10
5.3.1 General . 10
5.3.2 Circuit board singulation . 10
5.3.3 Circuit board fitting into the housing. 10
5.3.4 Screw clamp . 10
5.3.5 Manual handling . 11
5.3.6 ICT (In-Circuit Test) / FT (Functional Test) / Programming on assembly
level . 11
6 Example of an instruction on board preparation for strain measurement . 11
6.1 General . 11
6.1.1 Introductory statement . 11
6.1.2 Theory of strain gauge measurement . 11
6.1.3 3-axes strain measurement . 12
6.2 Position of strain gauges (example) . 13
6.2.1 General . 13
6.2.2 Determination of critical positions on a circuit board . 13
6.2.3 Rules for determination of sample size for performing the investigations . 13
6.3 Attachment of strain gauges (example) . 13
6.3.1 Step 1 – Remove components . 13
6.3.2 Step 2 – Polish the attachment area . 14
6.3.3 Step 3 – Indication of attachment point . 14
6.3.4 Step 4 – Application of instant glue . 15
6.3.5 Step 5 – Attachment of strain gauge . 16
7 Typical mistakes and faults occurring in practice . 16
7.1 Strain gauge attachment . 16
7.2 Wrong type of strain gauge used . 16
7.3 Circuit board without components is used . 16
7.4 Irregular stress is not considered . 16
7.5 Insufficient measurement settings . 17

8 Evaluation of data and report . 17
9 Assembly process control . 18
9.1 Machine/Process capability . 18
9.2 Machine maintenance . 18
Annex A (informative) Examples and relevant processes . 19
A.1 Typical measurement results – Press-fit operation, Example 1 . 19
A.2 Typical measurement results – Press-fit operation, Example 2 . 20
A.3 Typical measurement results – Housing assembly by screwing . 22
A.4 Typical critical processes . 23
Bibliography . 24

Figure 1 – Mechanical stress by board bending . 8
Figure 2 – Strain simulation . 8
Figure 3 – Typical bending crack at a ceramic capacitor . 8
Figure 4 – Longitudinal stress . 8
Figure 5 – Diagonal stress . 9
Figure 6 – Strain control and bending strength . 9
Figure 7 – Strain during screwing . 11
Figure 8 – Resistor bridge for strain measurement . 12
Figure 9 – 3-axes strain gauge and maximum principle strain . 12
Figure 10 – De-soldering of components . 14
Figure 11 – Polishing the attachment area . 14
Figure 12 – Marking of the attachment point . 14
Figure 13 – Application of glue by transfer from polyethylene sheet . 15
Figure 14 – Direct application of glue . 15
Figure A.1 – Dosing control unit for exhaust treatment system equipped with a
connector using compliant press-fit technology . 19
Figure A.2 – Top and bottom side of circuit assembly of dosing control unit with strain
gauges replacing passive multilayer chip capacitors . 19
Figure A.3 – Strain measurement evolution for the different strain gauges during the
press-in process . 20
Figure A.4 – Press fit – Strain measurements using different strain gauges and
positions . 21
Figure A.5 – Screwing – Strain measurements using different strain gauges and
positions . 22

Table A.1 – Strain measurement results with different gauges and placements . 21
Table A.2 – Strain measurement results with different gauges and placements . 22

– 4 – IEC TR 61760-5-1:2024 © IEC 2024
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
SURFACE MOUNTING TECHNOLOGY –
Part 5-1: Surface strain on circuit boards –
Strain gauge measurement applied to chip components

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
co-operation on all questions concerning standardization in the electrical and electronic fields. To this end and
in addition to other activities, IEC publishes International Standards, Technical Specifications, Technical Reports,
Publicly Available Specifications (PAS) and Guides (hereafter referred to as “IEC Publication(s)”). Their
preparation is entrusted to technical committees; any IEC National Committee interested in the subject dealt with
may participate in this preparatory work. International, governmental and non-governmental organizations liaising
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
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9) IEC draws attention to the possibility that the implementation of this document may involve the use of (a)
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shall not be held responsible for identifying any or all such patent rights.
IEC TR 61760-5-1 has been prepared by IEC technical committee 91: Electronics assembly
technology. It is a Technical Report.
The text of this Technical Report is based on the following documents:
Draft Report on voting
91/1915/DTR 91/1927/RVDTR
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 Report 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
described in greater detail at www.iec.ch/standardsdev/publications.
A list of all parts in the IEC 61760 series, published under the general title Surface mounting
technology, 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, 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 TR 61760-5-1:2024 © IEC 2024
INTRODUCTION
This Technical Report applies to electronic and electromechanical circuit board assemblies and
describes current best-practices for dealing with mechanical stress induced cracks in the body
of surface-mount ceramic components soldered onto circuit boards.
Circuit boards are becoming smaller and thinner, design margins are decreasing, and
components are becoming more sensitive to mechanical stresses. In consequence in-depth
strain control on a circuit board is getting more and more important to prevent mechanical
damage to components.
This Technical Report is an informative document which serves to illustrate the technically
feasible options and provides a basis for customer and supplier discussions and agreements.
It is based on many years of experience of component manufacturers and users in measuring
surface strain on circuit board surfaces during various assembly processes. It is not intended
to be regarded as a specification or standard. Formulations and data expressed in the form of
provision such as requirements or recommendations do not claim to be provisions and are just
suggested as the results of the discussion.
Related standards are gathered in the bibliography.

SURFACE MOUNTING TECHNOLOGY –
Part 5-1: Surface strain on circuit boards –
Strain gauge measurement applied to chip components

1 Scope
This document describes examples of methods using electrical strain gauges for determination
of critical mechanical stresses in assembly processes. These stresses can damage chip type
ceramic components, causing so called “bending cracks”. Area-array components are excluded
from the scope of this document.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60194-2 and the
following apply.
ISO and IEC maintain terminological 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
principal strain
maximum and minimum normal strains in a plane, always perpendicular to each other and
oriented in directions for which the shear strains are zero
3.2
maximum principal strain
maximum value of principal strain developed in the component body
Note 1 to entry: Failure of a material or component will occur when the maximum principal strain developed in the
body exceeds the limiting value of strain for a certain component.
4 Damaging mechanisms of chip type ceramic components
4.1 Surface strain by board bending
When a board is bent, lands are pulled outwards and generate mechanical stress on the solder-
joints, electrodes or components (Figure 1, Figure 2). This mechanical stress causes defects,
for example a bending crack in a ceramic capacitor (Figure 3). The root cause of this defect is
the local strain at the surface on which the component is mounted.

– 8 – IEC TR 61760-5-1:2024 © IEC 2024

Figure 1 – Mechanical stress by board bending

Reproduced with the permission of Murata Manufacturing
Figure 2 – Strain simulation
Reproduced with the permission of Murata Manufacturing
Figure 3 – Typical bending crack at a ceramic capacitor
4.2 Typical cracking modes
Bending stress can occur in any direction, see Figure 4 and Figure 5. The cracks were made
visible by grinding from the board side. The position and shape of cracks can be used to
estimate the direction of stress. The local strain at the position at which the component is
mounted causes the cracks. Therefore, even if a board does not look bent, this defect could
occur by local surface stress or short time impact.

Reproduced with the permission of Murata Manufacturing
Figure 4 – Longitudinal stress

Reproduced with the permission of Murata Manufacturing
Figure 5 – Diagonal stress
4.3 Measurement of local strain
Local strains can occur in any direction, see Figure 4 and Figure 5. 3-axes strain gauges
provide a full assessment of the strain state. The use of 1-axis or 2-axes measurements
provides limited information on the strain state.
5 Crack prevention
5.1 Strain control
To prevent bending cracks described in 4.1 and 4.2, strain control on a PCB is needed. The
bending strength of SMD for example can be determined by the so-called substrate bending
test, as provided in IEC 60068-2-21. However, the real mechanical strength of a surface mount
component depends on various design and process factors (e.g. land size, solder quantity/type,
PCB material/type, Cu foil thickness, strain rate, etc.). Furthermore, considering the strain
measurement accuracy, measurement repeatability, and irregular stresses in assembly
processes, a certain safety margin is needed, see Figure 6.

Figure 6 – Strain control and bending strength
5.2 Critical board design factors
5.2.1 Distance from circuit board cutting position
Components mounted close to a cutting or singulation position are at risk. A slit can reduce the
cutting stress if it is placed between the component and the cutting line. Also, a deep v-groove
can reduce the cutting stress.
5.2.2 Distance from screw clamping position
Components mounted close to a screw clamping position are at risk.

– 10 – IEC TR 61760-5-1:2024 © IEC 2024
5.2.3 Mounting direction
Risk can be reduced, if a components’ longitudinal axis is oriented perpendicular to the
maximum principle strain.
5.2.4 Warpage
Warpage can be the cause of surface stress when a circuit board is flattened when fitted into
the housing.
5.2.5 Orientation of the component related with the loading point
The mechanical load to components is influenced by their orientation relative to the forces
applied to the board.
5.3 Critical assembly process factors
5.3.1 General
In general, an assessment of all processes which involve mechanical loads applied to populated
circuit boards is needed. This implies that such an assessment needs to start after the first
reflow cycle. All process steps, including solder paste printing for the second reflow side (top
side), conveyor movement, pick-and-place etc. occurring after the first reflow cycle need to be
considered. Further information on suc
...