IEC 60068-3-13:2016

IEC 60068-3-13 ®
Edition 1.0 2016-05
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Environmental testing –
Part 3-13: Supporting documentation and guidance on Test T – Soldering

Essais d'environnement –
Partie 3-13: Documentation d'accompagnement et guide sur les essais T –
Brasage
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IEC 60068-3-13 ®
Edition 1.0 2016-05
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Environmental testing –
Part 3-13: Supporting documentation and guidance on Test T – Soldering

Essais d'environnement –
Partie 3-13: Documentation d'accompagnement et guide sur les essais T –

Brasage
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 19.040 ISBN 978-2-8322-3359-7

– 2 – IEC 60068-3-13:2016  IEC 2016
CONTENTS
FOREWORD . 4
1 Scope . 6
2 Normative references. 6
3 Terms, definitions and abbreviations . 6
3.1 Terms and definitions . 6
3.2 Abbreviations . 7
4 Overview . 7
4.1 Factors influencing the formation and reliability of solder joints (ability to be
soldered) . 7
4.2 Physics of surface wetting . 8
4.3 Quality and reliability of solder joints . 10
5 Component soldering – Processes . 10
5.1 General considerations . 10
5.1.1 Components' ability to be soldered . 10
5.1.2 Soldering processes . 12
5.1.3 Soldering defects . 12
5.1.4 Geometrical factors which may influence the soldering result . 12
5.1.5 Process factors . 12
5.1.6 Material factors . 12
5.2 Solder . 13
5.3 Grouping of soldering conditions . 13
5.4 Ability to be soldered . 13
5.5 Moisture sensitivity of components . 13
5.6 Relation between storage time/storage conditions and solderability . 14
5.6.1 Natural and accelerated ageing . 14
5.6.2 Oxidation . 14
5.6.3 Growth of intermetallic layers . 14
5.6.4 Effect of ageing to wetting characteristics . 14
5.6.5 Test conditions for accelerated ageing . 15
5.7 Place of soldering tests in testing . 16
6 Soldering tests . 17
6.1 General . 17
6.2 Solder . 18
6.3 Fluxes . 18
6.4 Test equipment . 18
6.5 Evaluation methods . 18
6.5.1 Criteria for visual inspection . 18
6.5.2 Criteria for quantitative evaluation of the wetting characteristic . 19
6.5.3 Special cases . 19
6.6 Acceptance criteria . 19
7 Soldering tests – Methods . 19
7.1 General principles . 19
7.2 Survey of test methods . 19
7.3 Bath test . 22
7.4 Reflow test . 23
7.4.1 With/without solder land . 23

7.4.2 Selection of solder paste (flux system and activity grade) . 23
7.5 Soldering iron test . 23
7.6 Resistance to dissolution of metallization and soldering heat . 23
7.6.1 General . 23
7.6.2 Limitations . 23
7.6.3 Choice of severity . 24
7.7 Wetting balance test . 24
7.7.1 General . 24
7.7.2 Test methods available . 25
7.7.3 Limitations . 25
8 Requirements and statistical character of results . 25
Bibliography . 27

Figure 1 – Sessile drop of solder on oxidised copper . 8
Figure 2 – Sessile drop of solder plus flux on clean copper . 9
Figure 3 – Sessile drop equilibrium forces . 9
Figure 4 – Typical soldering processes . 12
Figure 5 – Soldering tests for devices with leads . 21
Figure 6 – Soldering tests for SMDs . 22

Table 1 – Solder process groups . 13

– 4 – IEC 60068-3-13:2016  IEC 2016
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
ENVIRONMENTAL TESTING –
Part 3-13: Supporting documentation and guidance on Test T – Soldering

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 with the IEC also participate in this preparation. IEC collaborates closely
with the International Organization for Standardization (ISO) in accordance with conditions determined by
agreement between the two organizations.
2) The formal decisions or agreements of IEC on technical matters express, as nearly as possible, an international
consensus of opinion on the relevant subjects since each technical committee has representation from all
interested IEC National Committees.
3) IEC Publications have the form of recommendations for international use and are accepted by IEC National
Committees in that sense. While all reasonable efforts are made to ensure that the technical content of IEC
Publications is accurate, IEC cannot be held responsible for the way in which they are used or for any
misinterpretation by any end user.
4) In order to promote international uniformity, IEC National Committees undertake to apply IEC Publications
transparently to the maximum extent possible in their national and regional publications. Any divergence
between any IEC Publication and the corresponding national or regional publication shall be clearly indicated in
the latter.
5) IEC itself does not provide any attestation of conformity. Independent certification bodies provide conformity
assessment services and, in some areas, access to IEC marks of conformity. IEC is not responsible for any
services carried out by independent certification bodies.
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
members of its technical committees and IEC National Committees for any personal injury, property damage or
other damage of any nature whatsoever, whether direct or indirect, or for costs (including legal fees) and
expenses arising out of the publication, use of, or reliance upon, this IEC Publication or any other IEC
Publications.
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.
International Standard IEC 60068-3-13 has been prepared by IEC technical committee 91:
Electronics assembly technology.
This first edition cancels and replaces IEC 60068-2-44:1995 and constitutes a technical
revision.
This edition includes the following significant technical changes with respect to the previous
edition:
– information for lead-free solders are added;
– technical update and restructuring.

The text of this standard is based on the following documents:
FDIS Report on voting
91/1345/FDIS 91/1356/RVD
Full information on the voting for the approval of this standard can be found in the report on
voting indicated in the above table.
This publication has been drafted in accordance with the ISO/IEC Directives, Part 2.
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 publication will remain unchanged until
the stability date indicated on the IEC web site under "http://webstore.iec.ch" in the data
related to the specific publication. At this date, the publication will be
• reconfirmed,
• withdrawn,
• replaced by a revised edition, or
• amended.
– 6 – IEC 60068-3-13:2016  IEC 2016
ENVIRONMENTAL TESTING –
Part 3-13: Supporting documentation and guidance on Test T – Soldering

1 Scope
This part of IEC 60068 provides background information and guidance for writers and users of
specifications for electric and electronic components, containing references to the test
standards IEC 60068-2-20, IEC 60068-2-58, IEC 60068-2-69, IEC 60068-2-83, and to
IEC 61760-1, which defines requirements to the specification of surface mounting
components.
2 Normative references
The following documents, in whole or in part, are normatively referenced in this document and
are indispensable for its application. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any
amendments) applies.
IEC 60068-2-20:2008, Environmental testing – Part 2: Tests – Test T: Test methods for
solderability and resistance to soldering heat of devices with leads
IEC 60068-2-58, Environmental testing – Part 2-58: Tests – Test Td: Test methods for
solderability, resistance to dissolution of metallization and to soldering heat of surface
mounting devices (SMD)
IEC 60068-2-69, Environmental testing – Part 2-69: Tests – Test Te: Solderability testing of
electronic components for surface mounting devices (SMD) by the wetting balance method
IEC 60068-2-83, Environmental testing – Part 2-83: Tests – Test Tf: Solderability testing of
electronic components for surface mounting devices (SMD) by the wetting balance method
using solder paste
IEC 61760-1, Surface mounting technology – Part 1: Standard method for the specification of
surface mounting components (SMDs)
IEC 62137-3, Electronics assembly technology – Part 3: Selection guidance of environmental
and endurance test methods for solder joints
3 Terms, definitions and abbreviations
3.1 Terms and definitions
For the purposes of this document the following terms and definitions apply.
3.1.1
solderability
ability of the lead, termination or electrode of a component to be wetted by solder at the
temperature of the termination or electrode, which is assumed to be the lowest temperature in
the soldering process within the applicable temperature range of the solder alloy
________________
A new edition (third edition) is currently under consideration.

Note 1 to entry: The term “solderability” is often used in combination with the term “test”, indicating a specific
method to evaluate the wettability or ability to be soldered of a surface under worst case conditions (soldering
temperature and contact time with solder). It is not to be confused with the concepts “ability to be soldered” (see
4.1, 5.1.1) or “soldering ability” (see 3.1.4).
3.1.2
resistance to soldering heat
ability of the component to withstand the highest temperature stress in terms of temperature
gradient, peak temperature and duration of the soldering process, where the temperature of
the component body is within the applicable temperature range of solder alloy
3.1.3
wettability
intrinsic property of the termination material to form an alloy with the solder
Note 1 to entry: Wettability depends on the base metal used to produce the termination or, in the case of a plated
termination, the condition and material used to plate the base metal.
3.1.4
soldering ability
ability of a specific combination of components to facilitate the formation of a proper solder
joint
Note 1 to entry: See 3.1.3, wettability.
3.2 Abbreviations
SMD Surface mounted device
SMT Surface mounting technology
THD Through-hole mounting device
THT Through-hole mounting technology
THR Through-hole reflow soldering
4 Overview
4.1 Factors influencing the formation and reliability of solder joints (ability to be
soldered)
The conditions of ease of production and the reliability of a soldered joint can be classified in
three groups, as follows.
a) The joint design, determined by the choice of the two metallic elements to be joined (their
shape, size, composition, etc.) and of the assembly method (relative position, initial
fastening, etc.).
b) The wettability of the surfaces to be joined.
c) The conditions adopted for the soldering operation (temperature, time, flux, solder alloy,
equipment, etc.).
The choice of conditions of groups a) and c) concerns the manufacturer of equipment or
subassemblies, who shall know the importance of each of the conditions and the limits of their
variation. Condition b) depends to a large extent on the component manufacturer, except in
cases of unusual handling or storage conditions by the equipment manufacturer. The
wettability of surfaces needs to be defined with whatever degree of precision is necessary to
allow the equipment manufacturer to choose conditions of classes a) and c) appropriate to
that wettability. On the other hand, components of satisfactory surface quality will not
necessarily prevent rejectable joints arising from faults in joint design or joining conditions.

– 8 – IEC 60068-3-13:2016  IEC 2016
This often complex overlapping of responsibilities between component manufacturers and
equipment manufacturers creates a need to be able to define with considerable precision the
wettability of component terminations or, more generally, the solderability of components.
4.2 Physics of surface wetting
In order to obtain wetting between a substrate and molten solder, the tin in the solder shall
react with the substrate to form an alloy. In order to form an alloy the tin and the substrate
has to come into molecular contact. In order to do this the surface of both the molten solder
and the substrate shall be free from contamination.
In order to better understand how molten solder spreads over a substrate, and what
determines solderability, the surface tension property of the solder needs to be examined.
A free droplet of molten solder held in free space will form into a globule shape, just as a free
drop of water will form into a spherical shape. The droplet is held in this shape by the surface
tension force of the molten solder. Inside the droplet the atoms are uniformly surrounded by
other atoms, and the net force on them is zero, ignoring thermal motion. At the surface there
is an imbalance in the inter-atomic attraction forces, as the surface atoms experience a net
force into the body of the droplet.
The complete system tries to adopt a shape that has the minimum free energy, which means
the minimum surface-to-volume ratio. This situation is achieved when the molten solder forms
into a sphere. The strength of the surface tension force is determined by the bond energies
between the atoms within the molten solder.
If the molten sphere of solder is placed onto a heated, oxidised copper plate, the shape of the
sphere is depressed by gravity, to form a sessile drop, as shown in Figure 1 below.
Solder
Oxide layer
IEC
Figure 1 – Sessile drop of solder on oxidised copper
If a suitable flux is added to the sessile drop on the oxidised copper, the oxide layer will be
removed from the copper and the solder, and the tin in the solder will react with the copper to
form an intermetallic layer, allowing the solder to spread, as shown in Figure 2 below.

Solder
Diffusion layer
IEC
Figure 2 – Sessile drop of solder plus flux on clean copper
The final shape of the spreading solder will depend on the surface tension forces acting at the
interfaces. Solid and solid-liquid interfaces also exert a surface tension force, and all try to
reduce their surface areas to a minimum to attain a minimum free energy. As a result
equilibrium is reached whereby the net force at the advancing solder front is zero.
Figure 3 below shows the forces acting at the advancing solder front. The surface tension of
the solid copper in air is balanced by the surface tension between the liquid solder and the air,
and the liquid solder and the solid copper.
γ
LA
θ
γ
γ LS
SA
IEC
Figure 3 – Sessile drop equilibrium forces
The resulting forces at the advancing solder front can be written as follows:
γ = γ + γ cos θ
SA LS LA
where
γ is surface tension between solid copper and air;

SA
γ is surface tension between liquid solder and solid copper;
LS
γ is surface tension between liquid solder and air.

LA
This equation is known as Young’s equation. The contact angle θ can be used as a measure
of the degree of spreading obtained. The smaller the contact angle, the greater the spreading,
and the better the wetting obtained.
If the cohesive forces within the solder are greater than the adhesive forces between the
solder and the copper, then the solder will remain as a non-spreading sessile drop, and the
contact angle will be greater than 90°. If the adhesive forces exceed the cohesive forces, then
it is energetically favourable for the solder to react with the copper and spread outward,
reducing the contact angle below 90°.

– 10 – IEC 60068-3-13:2016  IEC 2016
The surface tension between solid and air, γ , will be high when the solid is free from oxides,
SA
sulphides, chlorides, hydrocarbons and other surface contaminants, which will all reduce the
surface tension.
For the surface tension between liquid and solid, γ , to be low, a metallurgical bond has to
LS
be formed between the tin and the substrate.
The surface tension between liquid solder and air, or flux film, will depend on the solder alloy,
the soldering temperature and the flux used to solder the parts. The surface tension of the
alloy can be markedly affected by the impurities in the solder. Very small levels of impurity
can have a large effect on the surface tension. This is because the surface tension of a liquid
is determined by the surface composition of the solder and not the composition of the bulk of
the solder. Impurities with low surface energies will rapidly segregate to the surface of the
liquid, reducing the surface tension, γ .
LA
Impurities in the solder alloy, and changes to the alloy composition may also affect the
surface tension between the liquid and the solid, altering the intermetallic formation, and can
also affect the surface tension between the solid and the air, affecting the diffusion process
across the solid, ahead of the liquid front.
Alloy additions or impurities may also affect the spreading and wetting properties of an alloy,
by altering the viscosity of the liquid solder.
4.3 Quality and reliability of solder joints
The quality of solder joints is characterised by wetted area, wetting angle, microstructure and
specific visual criteria.
One factor affecting the reliability of electronic assemblies is solder joint microstructure, which
in turn depends on the thermal conditions under which the solder joint solidifies. Both the bulk
microstructure of the solder and the intermetallic layer structure at the interfaces between
solder and component termination should be taken into consideration.
IEC 62137-3 gives guidance to test methods for the evaluation of solder joint reliability under
consideration of the above described four elements.
5 Component soldering – Processes
5.1 General considerations
5.1.1 Components' ability to be soldered
Because of the large variety of processing conditions a component can no longer simply be
classified as suitable e.g. for “flow soldering” or “reflow soldering”, or “lead-free soldering”.
Specific attention should be given to the fact, that the suitability of a component for “lead-free
soldering” cannot be stated because of the variety of lead-free solder alloys and processing
conditions. Typical soldering processes and related process conditions are described in
IEC 61760-1.
To be suitable for a certain soldering process a component shall fulfil the following
requirements:
a) material and surface of the component termination shall be suitable to be soldered with
the solder alloy and soldering method;
b) it shall possess thermal characteristics (thermal demand) small enough for a temperature
sufficiently higher than the liquidus of the solder alloy used, to be reached and maintained
for the length of time for wetting to occur;

c) it shall withstand without short-term or long-term change the thermal stresses associated
with the soldering cycle (including rework and possible repair by soldering iron);
d) it shall withstand without short-term or long-term damage the mechanical and chemical
stresses accompanying cleaning operations for the removal of flux residues. Cleaning
considerations are not emphasized in this Guide.
Thus, certain components containing lubricated mechanical parts (e.g. switches), or being
unsealed are sensitive to contamination (e.g. relays, potentiometers), or containing plastic
material with poor heat resistance (e.g. certain capacitors with thermoplastic dielectric), shall
be carefully selected for mass-soldering operations because of their inability to withstand one
or more of the stresses associated with the process.
For these reasons careful distinction shall be made between the processability (ability to be
soldered) of the component, which refers to the total suitability for industrial soldering, and
the wettability of the termination, which refers only to the ease of coating the termination with
solder. Unfortunately, these concepts are often confused in ordinary language, and such
confusion can prevent smooth running of production.
Furthermore, unsuitability of a component for soldering under the general conditions specified
(see below) does not mean that its terminations cannot be soldered to a printed circuit board
or other support. It entails only that it is necessary to take special precautions depending on
the condition it does not satisfy, such as having thermally sensitive insulation, or
incompatibility with some or all solvents. Only defective wettability of the terminations
prevents the use of soldering for mounting the component. This quality is of prime importance,
but does not exclude consideration of the others.
The standardised tests referred to here are all directed to simulating some part of the effects
of this set of conditions.
The appropriate choice of a group of these tests, in conjunction with electrical and mechanical
measurements, allows to answer the question: "ls this component solderable by the methods
normally used in electronics?" This is one of the questions which the equipment manufacturer
shall consider before putting a component on a soldering line.
The principle of each standardised test and the degree of information it supplies are defined
in Clause 7.
In this way the component specifier can, in full knowledge of the reasons, select the number
and type of tests needed to establish the behaviour of the component during soldering, as
well as the requirements that shall be determined in every case to reflect the general
requirements of the method of manufacture.
Similarly, the person conducting the tests will appreciate the degree of information given.

– 12 – IEC 60068-3-13:2016  IEC 2016
5.1.2 Soldering processes
Figure 4 shows typical soldering processes grouped into types.
Reflow Special soldering
Flow
soldering processes
soldering
Wave
Soldering iron
Convection
(double, single)
(with or without IR support)
Hot air
Selective
Hot plate
(mini-wave, solder pot,
Vapour phase
dip soldering)
Induction / microwave
soldering
Laser
Hot bar
IEC
Figure 4 – Typical soldering processes
5.1.3 Soldering defects
The series IEC 61191 and IEC 61192 provide information about requirements for soldered
electrical and electronic assemblies and related workmanship standards.
• Non wetting, dewetting
• Tombstoning
• Shifting
• Wicking
• Bridging
5.1.4 Geometrical factors which may influence the soldering result
• Land pattern design
• Component geometry
• Component terminal geometry
• Insertion hole diameter
• Annular ring
5.1.5 Process factors
• Time – Temperature profile
• Temperature spread (different temperatures at solder joints)
• Atmosphere (air, nitrogen)
5.1.6 Material factors
• Solder paste, solder alloy
• Flux activity
5.2 Solder
The composition of the solder alloy affects the surface tension of the liquid solder. Relatively
small concentrations of impurities in the solder can have a marked effect on the wetting
properties of the solder. Thus, the solder alloy used for soldering and for tests shall be
described in the relevant specification.
5.3 Grouping of soldering conditions
The melting temperatures of lead free solder alloys selected for industrial processes are
significantly different from those of tin lead solder alloy. Moreover, the melting temperatures
of present solder alloys are different from each other but can be clustered in groups. The
ability of the SMD to withstand the typical temperature and dwell time conditions shall match
the exposure to the process temperature groups using the selected alloys.
The following groups of soldering processes in Table 1 are given as a guideline for selecting
the severities for the wetting and resistance to soldering heat tests against the specified
soldering heat profile.
Table 1 – Solder process groups
Process temperature 1 2 3 4
group Low Medium Medium-high High
Typical solder alloy
Sn-Bi Sn-Pb Sn-Ag-Cu Sn-Cu
family
Flow – (235 to 250)°C (250 to 260)°C (250 to 260)°C
Reflow (170 to 210)°C (210 to 240)°C (235 to 250)°C –

5.4 Ability to be soldered
The ability to be soldered is determined mainly by the following three properties of a
component.
• Solderability of components
The determination of solderability can be made at the time of manufacture, at receipt of the
components by the user, or just before assembly and soldering.
• Thermal demand
It is necessary to bring the joint area to the soldering temperature. It is possible that the
component design will allow the heat being applied to the joint area to be drained away into
the component body, causing the temperature at the joint site to fall too low to produce an
adequate solder joint. Preheat may be used to overcome thermal demand issues.
• Resistance to soldering heat
The component shall be able to withstand the thermal stress of the soldering process without
any loss of functionality. This is particularly important with current assembly methods where
components may experience rapidly changing thermal gradients.
The result of this definition is that a matrix of soldering tests standards have evolved, which
measure some or all of these three properties individually or in some cases a combination of
the first two properties (see 7.2).
5.5 Moisture sensitivity of components
The relevant specification may prescribe a moisture soak procedure to determine the
sensitivity of a component against the influence of humidity during storage to the component
body.
– 14 – IEC 60068-3-13:2016  IEC 2016
NOTE 1 As distinguished from moisture soak the sensitivity of the component terminal surface against humidity is
described by accelerated ageing (see 5.6.4).
Typical soak conditions are: 85 °C/85 % r.h., 85 °C/60 % r.h., 60 °C/60 % r.h., 30 °C/60 % r.h.
Duration of moisture soak (168 h to 696 h) depends on the diffusion speed of water and the
absorption characteristics of the material. Thus, it needs specific investigation.
NOTE 2 Examples for suitable soak procedures for semiconductor components may be found in IEC 60749-20 or
J-STD-020. Applicable pre-drying and soak conditions for other types of components are under consideration. See
also IEC 61760-4.
5.6 Relation between storage time/storage conditions and solderability
5.6.1 Natural and accelerated ageing
Ageing is the natural process by which the solderability of a component decreases with time.
The correlation between natural ageing and accelerated conditions (“accelerated ageing”) is
difficult to be determined and cannot be generalized.
The majority of component terminations are formed from a base material over which a
solderable coating is applied to retain the solderability of the termination. It is common
practice to plate a barrier layer over the base metal, before applying the solder coating,
particularly if the base metal has a high solubility in solder.
5.6.2 Oxidation
Tin forms an oxide SnO with the atmosphere, which forms a protective layer on the substrate.
The rate of oxidation is accelerated by temperature and moisture in the atmosphere.
Lead also forms an oxide PbO with the atmosphere, but generally SnO is formed
preferentially as the tin has a greater affinity for oxygen.
Sulphur levels in the atmosphere are generally low and tin and lead have very little reaction
with sulphur at low concentration levels. Silver, however, reacts with sulphur at very low
levels to form a black sulphide layer, which reduces the solderability of the substrate.
Nitrous oxide (NO ) and chlorine both react with tin and lead to form lead nitrate (PbNO ) and
2 3
tin and lead chlorides (SnCl ) (PbCl ). Lead nitrate forms a non-protective coating, which is
2 2
difficult to solder. Both the chlorides are also non-protective and again reduce solderability
(although the lead chloride is usually reduced to lead nitrate which cannot be soldered with
normal electronic grade fluxes).
5.6.3 Growth of intermetallic layers
The vast majority of electronic component terminations are coated with tin or tin-lead, and so
most of the intermetallic layers contain tin. We have just seen that while the solder is molten
the intermetallic layer is continually forming and being dissolved. In the solid state migration
of the tin in the solder towards the intermetallic layer still continues, combined with diffusion
of the substrate through the intermetallic, resulting in an increase in the thickness of the
intermetallic layer into the solder coating.
This process is proportional to the square root of the temperature and is significant even at
room temperature.
5.6.4 Effect of ageing to wetting characteristics
Typical ageing effects are:
• degradation of metallic layers (oxidation);

• degradation of organic constituents of galvanic platings;
• growth of intermetallic phases through solderable surface;
• cracking of the solderable surface.
Degradation of wetting normally occurs in three distinct phases.
a) Firstly the wetting time starts to increase as the solderability is reduced by the formation
of oxides or corrosion products on the solder surface.
b) Then there is a phase where no further deterioration occurs as the solder oxide layer
protects the solder from further oxidation, by reducing the diffusion through the oxide layer
and a chemical passivation of the surface.
c) In the third phase the intermetallic layer has grown through to the surface of the solder
coating, and the wetting time again begins to increase.
Intermetallic growth is a major factor in the deterioration of the solderability of a solderable
substrate, but the initial degradation is due to the reaction of the solderable coating with the
atmosphere.
5.6.5 Test conditions for accelerated ageing
The natural ageing process of a component is a very complicated process that would be
impossible to accurately reproduce for each component, as we would need to be able to
predict the storage environment and temperature over a long period. We therefore need to
compress the ageing process into a much shorter time so that we can predict how
components will age as they enter the factory.
Clearly, it is impossible to produce an ageing method that will provide the same ageing
mechanism as natural ageing. The international solderability specifications include a number
of methods to accelerate the ageing process and provide data parallel to natural ageing,
although the exact mechanism could never be the same.
Typical ageing conditions are (see IEC 60068-2-20):
• dry heat: at 155 °C for 2 h, 4 h, 16 h;
• damp heat: at 40 °C, 93 % r.h. for 4 days, 10 days;
• steam: in steam for 1 h, 4 h;
• unsaturated pressurized vapour: at 125 °C, 85 % r.h. for 4 h.
These methods either affect the surface of the solder by use of moisture or a corrosive
atmosphere, or accelerate the rate of intermetallic growth by the use of high temperatures.
Care has to be taken when using these methods as we have already seen that different
pollutants in the atmosphere will produce different modifications to the solder surface.
The aim of accelerated ageing methods is to compress one or two years of natural ageing into
a few hours. We have seen that this is not possible as the mechanism of the ageing process
changes as we increase the temperat
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