IEC TR 60068-3-15:2024

Environmental testing - Part 3-15: Supporting documentation and guidance - Vacuum-assisted reflow soldering

IEC TR 60068-3-15:2024 describes vacuum-assisted soldering considering the thermal profiling, soldering methods, suitability of the components and vacuum features of soldering systems. It is based on practical experiences from manufacturers, component, material, and soldering systems suppliers. It supports manufacturers by providing information about the functionality of vacuum and effect of vacuum on components performance.

General Information

• Status: Published
• Publication Date: 05-Feb-2024
• ICS: 19.040 - Environmental testing
• ICS: 31.190 - Electronic component assemblies
• Technical Committee: TC 91 - Electronics assembly technology
• Drafting Committee: WG 3 - TC 91/WG 3
• Current Stage: PPUB - Publication issued
• Start Date: 06-Feb-2024
• Completion Date: 23-Feb-2024

Overview

IEC TR 60068-3-15:2024 is a Technical Report from the IEC that provides supporting documentation and guidance for vacuum-assisted reflow soldering. It synthesizes practical experience from manufacturers, component and material suppliers, and soldering-system vendors to explain how vacuum integrates with reflow processes (convection, vapour phase, contact) and how it affects solder joint quality and component performance. The report is intended to help electronics assemblers and designers understand the functionality of vacuum, thermal profiling, and vacuum equipment features when targeting reduced voiding, improved thermal management, or hermetic sealing.

Key topics and technical requirements

• Voids in solder joints: classification of macro-voids, causes, and the impact of voiding on solder joint reliability. (References to industry classifications such as IPC-7095 and IEC TR 61191-8 are noted.)
• Vacuum-assisted soldering processes: combinations of reflow techniques with vacuum (convection with vacuum, vapour phase with vacuum, contact soldering with vacuum) and typical temperature–pressure–time curves.
• Thermal profiling under vacuum: guidance on how vacuum interacts with thermal profiles and process timing to achieve consistent reflow and mitigate voids.
• Vacuum parameters and physical basics: influence of pressure, time, and vapour pressures (outgassing) on soldering outcomes and component integrity.
• Component suitability and failure modes: effects of pressure differentials on sensitive components (blow holes, gas bubbles, housing bursts, bulging electrolytics), and how vacuum ramping strategies can reduce risks.
• Vacuum equipment constraints: practical considerations such as chamber size, time-to-vacuum, and cycle-time impacts on throughput and process control.
• Typical defects: descriptions and images of defects associated with vacuum-assisted reflow - bridging, splattering, component drop/shift, and explosive outgassing - and mitigation approaches (e.g., stepwise vacuum application).

Practical applications and target users

• Electronics assemblers and contract manufacturers seeking to reduce voiding in BGAs, QFNs, LEDs, and power components.
• OEM design and process engineers specifying thermal profiles and component compatibility for vacuum reflow.
• Quality, reliability, and failure-analysis teams evaluating solder-joint performance and hermeticity.
• Solder paste, flux, and equipment suppliers optimizing materials and vacuum-capable reflow ovens.
• Industries requiring high thermal or RF performance, hermetic sealing, or tight void specifications (e.g., aerospace, medical, power electronics).

Related standards and guidance

• IEC TR 61191-8 (solder joint inspection/voids) and IPC-7095 (design/implementation guidance for BGA assemblies) are relevant references for void definitions and acceptance criteria.
• ISO 857‑2 provides the baseline definition for reflow soldering (liquidus ≤ 450 °C).

IEC TR 60068-3-15 is a practical resource for implementing vacuum reflow soldering reliably while balancing process capability, component suitability, and equipment limitations.

Standards Content (Sample)

IEC TR 60068-3-15 ®
Edition 1.0 2024-02
TECHNICAL
REPORT
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Environmental testing –
Part 3-15: Supporting documentation and guidance – Vacuum-assisted reflow
soldering
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IEC TR 60068-3-15 ®
Edition 1.0 2024-02
TECHNICAL
REPORT
colour
inside
Environmental testing –
Part 3-15: Supporting documentation and guidance – Vacuum-assisted reflow

soldering
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
ICS 19.040; 31.190 ISBN 978-2-8322-8169-7

– 2 – IEC TR 60068-3-15:2024 © IEC 2024
CONTENTS
FOREWORD . 4
INTRODUCTION . 6
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
4 Voids in solder joints . 7
4.1 Type of voids . 7
4.2 Reasons for voids . 8
4.3 Influence of voiding on solder joint performance . 9
5 Vacuum-assisted soldering processes . 9
5.1 Purpose . 9
5.2 Combination of soldering process with vacuum . 9
5.3 Typical temperature-pressure-time curves . 10
5.3.1 Convection soldering with vacuum . 10
5.3.2 Vapour phase soldering with vacuum . 11
5.3.3 Contact soldering with vacuum . 12
6 Effect of vacuum when reflow soldering . 13
6.1 General . 13
6.1.1 General description . 13
6.1.2 Physical basics . 13
6.1.3 Vacuum parameters . 14
6.1.4 Vapour phase vacuum reflow soldering . 14
6.2 Components in the vacuum reflow soldering process . 15
6.2.1 Influence of pressure differences . 15
6.2.2 Influence of temperature, time, and vacuum . 18
7 Vacuum equipment restrictions . 18
7.1 General . 18
7.2 Chamber size . 18
7.3 Time to reach vacuum level . 19
7.4 Cycle time . 19
7.5 Summary . 19
8 Typical defects after vacuum-assisted reflow soldering . 20
8.1 Typical defect modes occurring at components . 20
8.2 Component defect modes – summary . 24
8.3 Soldering defect modes . 24
8.3.1 Dropping of components . 24
8.3.2 Bridging . 25
8.3.3 Splattering . 25
Bibliography . 27

Figure 1 – X-Ray examples of voids in solder joints in different SMD-Components . 8
Figure 2 – Reduction of voids with low flux soldering & preforms . 8
Figure 3 – Example of a product for vacuum-assisted soldering processes . 10
Figure 4 – Typical profile – vacuum-assisted convection soldering . 11
Figure 5 – Typical profile – vacuum-assisted vapour phase soldering . 12

Figure 6 – Typical profile – vacuum-assisted contact soldering . 13
Figure 7 – Vapour pressure curve of Galden® . 15
Figure 8 – Pressures to be considered . 16
Figure 9 – Vapour pressure curve of water . 17
Figure 10 – Blow Hole Void in/out of metallization . 20
Figure 11 – Gas bubbles at metallization interface . 20
Figure 12 – Gas bubble caused by residues in metallization defect . 21
Figure 13 – Blow out void in solder meniscus . 21
Figure 14 – Aluminium electrolytic capacitors with non-solid electrolyte, bulged . 22
Figure 15 – Composite housing bursts in case of overpressure . 22
Figure 16 – Housing mainly made of plastic bursts in case of overpressure . 23
Figure 17 – Relay lock (polymer dot) blown off. 23
Figure 18 – Housing with adhesive joint bursts in case of overpressure . 24
Figure 19 – An example of bridging on BGA during vacuum assisted soldering . 25
Figure 20 – Optimization with stepwise applying of vacuum to reduce bridging . 25
Figure 21 – Splattering due to explosive outgassing from the solder joint . 26

Table 1 – Combination of soldering processes with vacuum . 10
Table 2 – Molar mass . 17
Table 3 – Combination of soldering processes with vacuum . 19

– 4 – IEC TR 60068-3-15:2024 © IEC 2024
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ENVIRONMENTAL TESTING –
Part 3-15: Supporting documentation and guidance –
Vacuum-assisted reflow soldering

FOREWORD
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IEC TR 60068-3-15 has been prepared by 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
91/1916/DTR 91/1930/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.

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A list of all parts in the IEC 60068 series, published under the general title Environmental
testing, can be found on the IEC website.
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– 6 – IEC TR 60068-3-15:2024 © IEC 2024
INTRODUCTION
As defined in ISO 857-2, 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.
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.
Sometimes it is not possible to achieve the required void level for an assembly only with
methods listed above despite the use of all technical possibilities.
Regarding void-induced asymmetrical stress constellations, a reduction of voiding can lead to
a mitigated stress condition within the solder joints.
Various technical requirements only tolerate very small void dimensions. To achieve these
requirements, vacuum-assisted soldering can be applied with the above mentioned reflow
soldering processes.
In some product applications, a hermetic seal is required. The reliable fulfilment of this
requirement is very demanding to the process technology especially complex assemblies.
Vacuum-assisted soldering creates significantly more consistency in the results here.
Further benefits of vacuum-assisted soldering are improved thermal management or high
frequency performance (contour adaptation, mitigation of blow holes).
Vacuum-assisted soldering, however, requires a different equipment with more complex
structure and process control. Since the vacuum process has an impact on the process time,
the suitability of the components and solder paste that are used need to be checked.

ENVIRONMENTAL TESTING –
Part 3-15: Supporting documentation and guidance –
Vacuum-assisted reflow soldering

1 Scope
This part of IEC 60068 describes vacuum-assisted soldering considering the thermal profiling,
soldering methods, suitability of the components and vacuum features of soldering systems. It
is based on practical experiences from manufacturers, component, material, and soldering
systems suppliers. It supports manufacturers by providing information about the functionality of
vacuum and effect of vacuum on components performance.
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 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
4 Voids in solder joints
4.1 Type of voids
After soldering, many different macro and micro disturbances of the solder structure can be
detected in the solder joints with e.g. X-Ray inspection, ultrasonic inspection or cross
sectioning. Some of them represent the so-called voids which are divided according to their
causes and type.
The definitions and classification of different void types can be found for example in IPC-7095,
IPC-7093 or IEC TR 61191-8. This document describes the use of vacuum to prevent so-called
macro voids. Figure 1 shows examples of macro voids in solder joints of different component
types.
– 8 – IEC TR 60068-3-15:2024 © IEC 2024

QFN Power LED Power Resistor
Reproduced with permission by Rehm Thermal Systems GmbH
Figure 1 – X-Ray examples of voids in solder joints in different SMD-Components
4.2 Reasons for voids
The research results to date lead to the essential finding that the mechanisms of void formation
depend on many factors. When soldering with conventional solder pastes, the main factor is
the use of flux. After remelting of the solder alloy, flux residues are trapped on the surfaces of
the circuit board (PCB) and the soldered components. At high temperatures, these evaporate,
and the products of these outgassing are trapped in the solder joint volume in the form of gas
bubbles, i.e., voids. With a reduction of the flux amount, the proportion of voids can be reduced,
as shown in the example in Figure 2.

Reproduced with permission by Indium Corporation of Europe
Figure 2 – Reduction of voids with low flux soldering & preforms
In addition, the void formation is influenced by the interaction of solder paste, quality and type
of surface finish (PCB and component) and geometry of the solder joint. As a rule, void ratios
in the range of 20 % to 50 % can occur by soldering under normal pressure / condition.
Depending on the application and the required minimum void ratio, with vacuum-assisted
soldering void ratios below 10 % can be realized.

4.3 Influence of voiding on solder joint performance
A solder joint has several functions on an assembly: electrical contact, mechanical fixation and
thermal connection. These functions are classified here into two areas: mechanical integrity
and thermal performance. Macrovoids in solder joints can have a negative impact on both areas.
More information can be found in IPC-A-610, IPC-7095, IPC-7093, IEC TR 61191-8.
5 Vacuum-assisted soldering processes
5.1 Purpose
Vacuum has been used for many years in reflow soldering technology as an additional process
step. Depending on the combination of soldering method with vacuum, the void content in solder
joints of different products can be reduced.
In vacuum-assisted soldering, the pressure-time profile is recorded in addition to the
temperature profile.
The parameters of vacuum profiling are:
a) The vacuum steps and the vacuum level (the reached minimum pressure), pressure in
Pascal (Pa) referred to zero;
b) the frequency of vacuum use and the holding time of the specified vacuum level in seconds
(s)
While in convection soldering with vacuum, the vacuum is applied above the liquidus
temperature of the solder, in vapour phase and conduction soldering with vacuum the vacuum
can be applied at any time of the process. Figure 4, Figure 5 and Figure 6 demonstrates
examples of temperature-pressure time profiles for vacuum-assisted convection, condensation
and conduction soldering. The so-called pre-vacuum can be used to change atmospheric gases,
dry assemblies or even pastes.
To reduce voids, the vacuum is applied in all reflow soldering processes above the liquidus
temperature of the solder. Since the application of vacuum requires additional time, the time
above liquidus is extended. For the time above liquidus, however, the specifications given by
common norms and standards must be complied with.
In general, the requirements and limitations of the temperature-time envelope curve apply to
the profiles of vacuum-assisted soldering processes as well. More information can be found in
IEC TR 60068-3-12. This applies in particular to the time above liquidus.
The cycle time of the complete soldering process depends on the vacuum level and the holding
time of vacuum. Usually, the cycle time will be increased by addition of a vacuum step due to
speed limitations of vacuum chamber for the opening and closing as well as for the evacuation
and venting procedures.
5.2 Combination of soldering process with vacuum
All reflow soldering technologies, such as convection, condensation (vapour phase) and
conductive (contact heat) processes are currently available on the market with additional
vacuum technology.
Depending on the technology used, the vacuum process can be used before and during the
preheating process as well as during the soldering process, in the molten phase of the solder
joint. Table 1 contains different vacuum-assisted soldering processes and typical parameters.
In Figure 3 an example of a product for the different vacuum-assisted soldering processes is
shown.
– 10 – IEC TR 60068-3-15:2024 © IEC 2024
Table 1 – Combination of soldering processes with vacuum
Minimum
Soldering Typical additional time
Frequency of Vacuum Holding Examples of
process application of above liquidus
use level time products
vacuum for applying of
vacuum
[hPa] [s]* [s]**
Power
Convection Above liquidus Continuously 10 to 100 0 to 60 > 14 electronics,
lightning
High thermal
masses,
At all
Condensation Multiple times 10 to 100 0 to 60 > 9 maximum
temperatures
temperature
limitation
Module in
At all Continuously, power
Contact heat 0,2 to 150 0 to 360 > 9
temperatures multiple times electronics, Die-
Attach
* At minimum vacuum level
** To reduce the void formation, a vacuum is applied at a temperature above the liquidus of the solder alloy. In
order to generate a reliable solder joint, the time above liquidus needs to be considered. The application of the
vacuum needs additional time above the liquidus temperature of solder alloy. This time consist of times for
transportation of the assembly into the vacuum chamber, closing the chamber, evacuation down to the vacuum
level, vacuum holding time and venting of the chamber.

Reproduced with permission by Semikron-Danfoss.
Figure 3 – Example of a product for vacuum-assisted soldering processes
5.3 Typical temperature-pressure-time curves
5.3.1 Convection soldering with vacuum
In a convection soldering system, the vacuum chamber is usually situated after or is a part of
the peak zones.
Figure 4 shows the example of a temperature-pressure-time curve of a vacuum-assisted
convection soldering process.
Figure 4 – Typical profile – vacuum-assisted convection soldering
5.3.2 Vapour phase soldering with vacuum
In this process, the vacuum can be applied selectively before (solder is solid), during (solder is
liquid) and after the peak zone of the reflow soldering process. For a vapour soldering process,
a vapor phase (VP ) medium is used as a heat transfer medium.
Purpose of vacuum:
a) before
...