IEC TR 60068-3-12:2022

IEC TR 60068-3-12
®


Edition 3.0 2022-10
TECHNICAL
REPORT

colour
inside


Environmental testing –
Part 3-12: Supporting documentation and guidance – Method to evaluate a
possible lead-free solder reflow temperature profile
IEC TR 60068-3-12:2022-10(en)

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IEC TR 60068-3-12

®


Edition 3.0 2022-10




TECHNICAL



REPORT








colour

inside










Environmental testing –

Part 3-12: Supporting documentation and guidance – Method to evaluate a

possible lead-free solder reflow temperature profile

























INTERNATIONAL

ELECTROTECHNICAL


COMMISSION





ICS 19.040; 31.190 ISBN 978-2-8322-5818-7



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


® Registered trademark of the International Electrotechnical Commission

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– 2 – IEC TR 60068-3-12:2022 © IEC 2022
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
4 Determination of an envelope reflow profile . 7
4.1 Temperature-time-envelope curve . 7
4.2 Diagram of a theoretical envelope reflow profile . 8
4.3 Key parameters of the envelope reflow profile . 8
5 Temperature profile measurements . 11
5.1 Determining the measurement locations. 11
5.2 Selection and attachment of thermocouples . 12
5.2.1 Types of thermocouples . 12
5.2.2 Preparation of thermocouples . 13
5.2.3 Attachment of thermocouples . 13
5.2.4 Influence of attachment method and operator on measurement results . 16
5.3 Temperature gradient. 16
5.3.1 Gradient calculation . 16
5.3.2 Sampling rate . 18
5.4 Analysis, comparison and overlay of different reflow profiles and best
practice . 18
5.5 Measuring equipment . 19
6 Tolerance analysis of the measurement chain . 19
7 Optimizing a temperature profile . 21
7.1 General procedure . 21
7.2 Description of a typical test board and the used reflow oven . 21
7.3 Schematic envelope reflow profile for the example board . 22
7.4 Preparation of test board . 24
7.5 Possibility of temperature profiling optimization with simulation tools . 25
7.6 Iteration steps for finding reflow equipment setup . 27
8 Examples of envelope reflow profiles . 30
8.1 Key data for two different solders . 30
8.2 Example of a qualification temperature profile for component used in lead-
free reflow soldering (SAC) . 31
Bibliography . 33

Figure 1 – Schematic envelope reflow profile . 8
Figure 2 – Recommended temperature measurement locations on a test board . 12
Figure 3 – X-ray of a sheath thermocouple. 12
Figure 4 – Example of a) acceptable and b) unacceptable attachment of the
thermocouples . 13
Figure 5 – Examples of good and bad thermocouple attachment . 15
Figure 6 – Thermocouples (TC) fixed to an LED. 16
Figure 7 – Results of the same test board prepared by different methods . 16
Figure 8 – Gradient calculation . 17

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IEC TR 60068-3-12:2022 © IEC 2022 – 3 –
Figure 9 – Example of a gradient calculation on a temperature-time curve . 18
Figure 10 – Overlay of different reflow profiles (origin at oven entry) . 19
Figure 11 – Overlay of different reflow profiles (overlap at start of peak zone) . 19
Figure 12 – Measurement chain . 20
Figure 13 – Description of a test board (electronic assembly) . 22
Figure 14 – Envelope reflow profile for the test board . 24
Figure 15 – Thermal images of the test board after cooling down from 150 °C . 24
Figure 16 – Geometric and thermal description of the test board . 26
Figure 17 – Geometric and thermal description of the reflow soldering equipment. 26
Figure 18 – Predicted reflow profile with help of simulation (blue band) . 27
Figure 19 – Overlay envelope curves of the temperature-time curves of three profiling
steps, 0201-chip solder joint . 29
Figure 20 – Overlay envelope curves of the temperature-time curves of three profiling

steps, ERU25 solder joint . 30
Figure 21 – Exemplary qualification reflow temperature profile for the qualification of
components intended for use in assemblies with a wide variation of thermal masses . 32

Table 1 – Temperature-time curve – Units . 8
Table 2 – Envelope points of a reflow temperature-time-profile . 9
Table 3 – Thermocouple attachment methods . 14
Table 4 – Tolerances of the temperature measurement chain . 20
Table 5 – Envelope points at the envelope reflow profile for the test board . 23
Table 6 – Measurement locations on the sample assembly . 25
Table 7 – Settings according to experience . 28
Table 8 – Measurement results for the settings from Table 7 . 28
Table 9 – Settings for second run . 28
Table 10 – Measurement results for the settings from Table 9 . 28
Table 11 – Settings after adjustment of the heating zone temperatures . 29
Table 12 – Measurement results from Table 11, adaptation of heating zone
temperatures . 29
Table 13 – Examples for envelope reflow profile key data for two different solders . 31
Table 14 – Key parameters for a lead-free SAC reflow temperature profile for
component qualification . 32

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– 4 – IEC TR 60068-3-12:2022 © IEC 2022
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________

ENVIRONMENTAL TESTING –

Part 3-12: Supporting documentation and guidance –
Method to evaluate a possible lead-free solder reflow temperature profile

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,
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6) All users should ensure that they have the latest edition of this publication.
7) No liability shall attach to IEC or its directors, employees, servants or agents including individual experts and
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8) Attention is drawn to the Normative references cited in this publication. Use of the referenced publications is
indispensable for the correct application of this publication.
9) Attention is drawn to the possibility that some of the elements of this IEC Publication may be the subject of patent
rights. IEC shall not be held responsible for identifying any or all such patent rights.
IEC TR 60068-3-12 has been prepared by IEC technical committee 91: Electronics assembly
technology. It is a Technical Report.
This third edition cancels and replaces the second edition published in 2014. This edition
constitutes a technical revision.
This edition includes the following significant technical changes with respect to the previous
edition:
a) Extended purpose
Guidance is added on how to create a reflow profile considering the tolerances resulting
from the accuracy of the measuring equipment, preparation method and specifications of
the component manufacturers (components, PCB, solder paste, etc.).

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IEC TR 60068-3-12:2022 © IEC 2022 – 5 –
b) Distinction from existing standards
The envelope profile given in this document does not represent a temperature-time profile
for the qualification of materials but defines the reflow process limits for the soldering of
electronic assemblies.
The schematic temperature-time-limit curves of the envelope profile are derived from
generally valid findings (literature data). Additionally, tolerance considerations are given for
all envelope points of the envelope profile.
In contrast to IEC TR 60068-3-12:2014, the creation of the envelope profile is not primarily
linked to a concrete example.
c) Subclause 8.2 presents an approach for establishing a possible temperature profile for a
lead-free reflow soldering process using SnAgCu solder paste that is taken from
IEC TR 60068-3-12:2014.
d) Synergies with existing standards
Limit values and tolerances from standards and guidelines for the qualification of materials
are included in this document and are listed as examples in the references.
The text of this Technical Report is based on the following documents:
Draft Report on voting
91/1776/DTR 91/1804/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 60068 series, published under the general title Environmental testing,
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 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 TR 60068-3-12:2022 © IEC 2022
INTRODUCTION
The enormous variety of materials and components processed in SMT requires to consider their
thermal properties, especially in reflow soldering.
Since the second edition essentially limited its focus to lead-free soldering, there is a need to
extend the contents in order to cover state of the art reflow soldering processes in general.
Reflow soldering is a joining process using an additional metal (solder) with a liquidus
temperature of 450 °C or less, in which solder paste or preforms are reflowed
(see ISO 857-2:2005).
Reflow soldering can be carried out with the technical processes of convection (air or nitrogen),
condensation (vapour phase), radiation (e.g. infrared) or contact heat as well as with the help
of low pressure (vacuum).
The goal of a qualified reflow soldering process is to create high quality and reliable solder
joints at product level. It is important to avoid soldering defects and damage to components and
printed circuit board.
In addition to the requirements for the formation of reliable solder joints, the specifications of
the connection partners and the production requirements (temperatures, final layers, alloys,
etc.), an adequate process control is an important factor. Primarily the resistance of the
components and circuit boards to solder heat, as well as the specifications of the solder paste
and/or flux, need to be considered. The sum of these physical limits is a theoretical temperature-
time curve for a specific product (see DVS 2613).
This document is intended for engineers (e.g. development, manufacturing technology, work
preparation) and operators (production) responsible for the creation and release of
temperature-time (T-t) profiles for reflow soldering in surface mount technology.
This document initially was prepared by the German DKE GUK 682.2 "Thermal joining
technology in electronics".

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IEC TR 60068-3-12:2022 © IEC 2022 – 7 –
ENVIRONMENTAL TESTING –

Part 3-12: Supporting documentation and guidance –
Method to evaluate a possible lead-free solder reflow temperature profile



1 Scope
This part of IEC 60068, which is a Technical Report, describes the creation of temperature-time
profiles (in specific envelope profiles) for reflow soldering of electronic assemblies, considering
tolerances resulting from the accuracy of the measuring equipment, preparation method and
specifications of the manufacturers of components, circuit boards, solder paste, etc.).
The envelope profile given in this document does not represent a temperature-time profile for
qualification but defines the reflow process window for the soldering of electronic assemblies.
Qualification profiles can be found, for example, in IEC 60068-2-58 for resistance to soldering
heat, or in IEC 60749-20, IEC 61760-4 and IPC/JEDEC J-STD-020E for moisture sensitivity
classification of components.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
No terms and definitions are listed in this document.
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
4 Determination of an envelope reflow profile
4.1 Temperature-time-envelope curve
According to IEC 61191-1:2018, 8.2.2, manufacturers of electronic assemblies need to
determine the parameters of a soldering process as follows:
"The process shall include, as a minimum, a reproducible time/temperature envelope
including the drying/degassing operation (when required), preheating operation (when
required), solder reflow operation, and a cooling operation."
The necessary envelope points for the creation of a temperature-time-envelope curve result
from the respective minima and maxima of the data for the solder heat resistance of the
components and PCBs, their minimum solderability temperatures, as well as from the
specifications of the solder paste and/or flux. The cycle time (time per electronic assembly) and
the process time (T to T end) influence each other.
0 3
The temperature-time curve is described by the units in Table 1.

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Table 1 – Temperature-time curve – Units
Physical quantity Symbol Units Remark
Temperature T °C or K ∆T, Temperature differences in kelvin (K)
1 K = 1 °C
0 °C = 273,15 K
Time t s or min
Gradient G K/s ∆T/∆t

4.2 Diagram of a theoretical envelope reflow profile
Figure 1 shows the maximum (upper line in red) and minimum (bottom line in green) theoretical
temperature-time curves for a reflow soldering process with all envelope points defining these
two curves. The reflow soldering profile measured in each case is expected to be located within
the boundaries of the envelope profile, as shown in Figure 1.

NOTE The calculation of gradients is described in 5.3.
Figure 1 – Schematic envelope reflow profile
4.3 Key parameters of the envelope reflow profile
Table 1 describes the main requirements for setting the envelope points for defining the upper
and lower limits of an envelope reflow profile.
The envelope points take into account material-specific data, in particular the resistance to
soldering heat, as well as findings for the reliable formation of the solder joints (alloy properties,
microstructure formation, etc.). Therefore, there are, for example, temperature-time curves
which are measured on the components (soldering heat resistance) and temperature-time
curves which are measured on or in the solder joint (soldering). In addition, the manufacturing
expectations regarding the cycle time of the individual product need to be taken into account.

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IEC TR 60068-3-12:2022 © IEC 2022 – 9 –
The "Tolerance" column in Table 2 needs to be completed by the manufacturer of the electronic
assemblies. The measurement errors are treated as "inset" limits, which means that the
estimated measurement error (e.g. 5 K for the temperature measurement chain) is subtracted
from the upper limit values, and added to the lower limit values. This is to ensure that the limit
values cannot be exceeded.
In the "Comment" column in Table 2, corresponding notes are given for each parameter.
Table 2 – Envelope points of a reflow temperature-time-profile
Envelope point Explanation Typical value Tolerance Comment
T, temperature (y-axis)

T T > room
Start Reference temperature to determine the
0 0
temperature reflow profile.
temperature
Temperature is significantly above room
Typical: 50 °C
temperature – consider the production needs.
Temperature at which the recording begins.
T Lower +5 K Consider solder paste recommendations and
1
preheating needs of PCB-Assembly.
temperature
IEC 61760-1:2020, 6.2.2
IPC/JEDEC J-STD-020E: T (s = soak)
smin
T Upper −5 K Consider solder paste recommendations and
2
preheating needs of PCB-Assembly.
temperature
IEC 61760-1:2020, 6.2.2
IPC/JEDEC J-STD-020E: T (s = soak)
smax

T Liquidus Note the difference between solidus and
3
temperature liquidus temperature. The solder alloy is
completely fluid (liquidus temperature).
Consider solder paste recommendations.
IEC 61760-1:2020, 6.2.2
IPC/JEDEC J-STD-020E): T (T = liquidus
L L
temperature

T Maximum peak T is below the The maximum peak temperature is either the
Pmax P
temperature maximum allowed temperature at the
classification
termination (e.g. dissolution of metallization)
temperature T (i.e.
C
or the maximum temperature measured on
the max. soldering
the package top side (e.g. moisture/reflow
temperature T of
4
sensitivity of non-hermetic components).
the component)
IEC 61760-1:2020, 6.2.2: T
4
T = T – 5 K
pmax C
IEC 60068-2-58:2015, 7.6.4.4: T
4
IPC/JEDEC J-STD-020E: T
P
Consider: T = T – 5K,
P C
T : classification temperature
C
IPC TM 650 2.6.27A

T Minimum peak T + 15 K The minimum peak temperature is normally
pmin 3
temperature the temperature at the termination of the
component (solder joint). Reaching the
minimum peak temperature enables the
solderability.
IEC 60068-2-58:2015, 6.6.5
IPC-7093, IPC-7095,
IPC/JEDEC J-STD-020E
T = (T + 15 K)
pmin 3

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– 10 – IEC TR 60068-3-12:2022 © IEC 2022
Envelope point Explanation Typical value Tolerance Comment

T End Reference temperature to determine the end
E
temperature of the reflow profile.
Temperature is significantly below the solidus
temperature. Consider the production needs.
Temperature at which the calculation ends.
t, time (x-axis)

t
Min. preheating Minimum time between T and T
1min
1 2
time
Consider that t is part of the time to peak.
1
Consider solder paste and component
recommendations.
IEC 61760-1:2020, 6.2.2

t Max. preheating Maximum time between T and T
1max 1 2
time
Consider that t is part of the time to peak.
1
Consider solder paste and component
recommendations.
IEC 61760-1:2020, 6.2.2
t Process time
2
from T to T
0 3
(end of peak)

t Min. time above
Typical: ≥ 30 s Time above T
3min
3
liquidus
Consider the solder alloy recommendations.
temperature
t in IEC 61760-1 and IEC 60068-2-58
2
t in IPC/JEDEC J-STD-020E
L
Additional reference: DVS 2613

t Max. time above Typical: ≤ 90 s Time above T
3max 3
liquidus
temperature Consider the component recommendation.
Additional reference: DVS 2613
t Time to peak  Time between T and T
4 0 P
Process time
Consider the solder paste recommendation
from T to T
and component specification.
0 P
t in IEC 61760-1 and IEC 60068-2-58
4

t Time on peak Corresponding to Consider the component specification.
5
the component
The heat resistance of components limits t .
specification (T ) 5
C
IPC/JEDEC J-STD-020E, Time (t )* within
p
5 K of the specified classification
temperature (T )
C
t in IEC 61760-1 and IEC 60068-2-58
3
Ramp rate
G, gradient ∆T∕∆t
See the instructions of the component and
material suppliers.
G Max. heating −0,5 K/s Consider the solder paste and component
1
gradient to T , recommendation
2
preheating
IEC 61760-1:2020, 6.2.2
G Max. heating −0,5 K/s Consider the component recommendation
2
gradient from T
2
IEC 61760-1:2020, 6.2.2,
to T
4 IPC/JEDEC J-STD-020E
Max ramp-up rate (3 K/s)

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IEC TR 60068-3-12:2022 © IEC 2022 – 11 –
Envelope point Explanation Typical value Tolerance Comment
G Max. cooling +0,5 K/s The cooling gradient is negative therefore the
3
gradient from T
tolerance is positive.
4
to T
3
Consider the component recommendation.
IEC 61760-1:2020, 6.2.2,
IPC/JEDEC J-STD-020E
Max. ramp-down rate (−6 K/s)
G Max. cooling +1 K/s The cooling gradient is negative therefore the
4
gradient from T tolerance is positive
...