EN IEC 62812:2019
(Main)Low resistance measurements - Methods and guidance
Low resistance measurements - Methods and guidance
IEC 62812:2019 specifies methods of measurement and associated test conditions that eliminate or reduce the influence of adverse phenomena in order to improve the attainable accuracy of low-resistance measurements. The methods described in this document are applicable for the individual measurements of the resistance of individual resistors, and also for resistance measurements as part of a test sequence. They are applied if prescribed by a relevant component specification, or if agreed between a customer and a manufacturer.
Messung niederohmiger Widerstände - Verfahren und Leitfaden
Mesures de faibles résistances - Méthodes et recommandations
L'IEC 62812:2019 spécifie les méthodes de mesure et les conditions d'essai associées qui éliminent ou réduisent l'influence des phénomènes défavorables afin d'améliorer la précision réalisable des mesures de faibles résistances. Les méthodes décrites dans le présent document s'appliquent aux mesures individuelles de la valeur de résistances individuelles, ainsi qu'aux mesures de résistances dans le cadre d'une séquence d'essai. Elles sont appliquées si elles sont prescrites par une spécification de composant pertinente ou si elles font l'objet d'un accord entre un client et un fabricant.
Meritve nizke upornosti - Metode in navodila (IEC 62812:2019)
Na meritve upornosti običajno vplivajo različni pojavi, na primer serijska upornost na poti merjenja, samosegrevanje ali neohmske lastnosti. Če je vpliv teh pojavov na merjenje upornosti sprejemljiv ali ne, je odvisno od obsega posameznega učinka v primerjavi z upornostjo in zahtevano natančnostjo. Tako se tveganje za napačne meritve upornosti poveča z zmanjšano upornostjo in zaostrovanjem dopustnega odstopanja.
Ta dokument določa merilne metode in povezane preskusne pogoje za odpravo ali zmanjšanje vpliva škodljivih pojavov, da bi izboljšali dosegljivo natančnost meritev nizke upornosti.
Metode, ki so opisane v tem dokumentu, se uporabljajo za posamezne meritve upornosti posameznih uporov ter za meritve upornosti kot del preskusnega zaporedja. Metode se uporabljajo, če jih predpisuje specifikacija ustreznega sestavnega dela ali se o tem dogovorita kupec in proizvajalec.
General Information
Relations
Standards Content (Sample)
SLOVENSKI STANDARD
01-oktober-2019
Meritve nizke upornosti - Metode in navodila (IEC 62812:2019)
Low resistance measurements - Methods and guidance (IEC 62812:2019)
Messung niederohmiger Widerstände - Verfahren und Leitfaden (IEC 62812:2019)
Mesures de faibles résistances - Méthodes et recommandations (IEC 62812:2019)
Ta slovenski standard je istoveten z: EN IEC 62812:2019
ICS:
31.040.01 Upori splošno Resistors in general
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.
EUROPEAN STANDARD EN IEC 62812
NORME EUROPÉENNE
EUROPÄISCHE NORM
July 2019
ICS 31.040.01
English Version
Low resistance measurements - Methods and guidance
(IEC 62812:2019)
Mesures de faibles résistances - Méthodes et Messung niederohmiger Widerstände - Verfahren und
recommandations Leitfaden
(IEC 62812:2019) (IEC 62812:2019)
This European Standard was approved by CENELEC on 2019-06-06. CENELEC members are bound to comply with the CEN/CENELEC
Internal Regulations which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.
Up-to-date lists and bibliographical references concerning such national standards may be obtained on application to the CEN-CENELEC
Management Centre or to any CENELEC member.
This European Standard exists in three official versions (English, French, German). A version in any other language made by translation
under the responsibility of a CENELEC member into its own language and notified to the CEN-CENELEC Management Centre has the
same status as the official versions.
CENELEC members are the national electrotechnical committees of Austria, Belgium, Bulgaria, Croatia, Cyprus, the Czech Republic,
Denmark, Estonia, Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia,
Lithuania, Luxembourg, Malta, the Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden,
Switzerland, Turkey and the United Kingdom.
European Committee for Electrotechnical Standardization
Comité Européen de Normalisation Electrotechnique
Europäisches Komitee für Elektrotechnische Normung
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2019 CENELEC All rights of exploitation in any form and by any means reserved worldwide for CENELEC Members.
Ref. No. EN IEC 62812:2019 E
European foreword
The text of document 40/2665/FDIS, future edition 1 of IEC 62812, prepared by IEC/TC 40
"Capacitors and resistors for electronic equipment" was submitted to the IEC-CENELEC parallel vote
and approved by CENELEC as EN IEC 62812:2019.
The following dates are fixed:
• latest date by which the document has to be implemented at national (dop) 2020-03-06
level by publication of an identical national standard or by endorsement
• latest date by which the national standards conflicting with the (dow) 2022-06-06
document have to be withdrawn
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CENELEC shall not be held responsible for identifying any or all such patent rights.
Endorsement notice
The text of the International Standard IEC 62812:2019 was approved by CENELEC as a European
Standard without any modification.
In the official version, for Bibliography, the following notes have to be added for the standards
indicated:
IEC 60115-2 NOTE Harmonized as EN 60115-2
IEC 60115-8 NOTE Harmonized as EN 60115-8
IEC 60301 NOTE Harmonized as EN 60301
IEC 61249-5-1 NOTE Harmonized as EN 61249-5-1
Annex ZA
(normative)
Normative references to international publications
with their corresponding European publications
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.
NOTE 1 Where an International Publication has been modified by common modifications, indicated by (mod), the relevant
EN/HD applies.
NOTE 2 Up-to-date information on the latest versions of the European Standards listed in this annex is available here:
www.cenelec.eu.
Publication Year Title EN/HD Year
IEC 60068-1 - Environmental testing - Part 1: General and guidance EN 60068-1 -
IEC 60115-1 (mod) 2008 Fixed resistors for use in electronic equipment - Part EN 60115-1 2011
1: Generic specification
- - + A11 2015
IEC 60294 - Measurement of the dimensions of a cylindrical EN 60294 -
component with axial terminations
IEC 62812 ®
Edition 1.0 2019-05
INTERNATIONAL
STANDARD
NORME
INTERNATIONALE
Low resistance measurements – Methods and guidance
Mesures de faibles résistances – Méthodes et recommandations
INTERNATIONAL
ELECTROTECHNICAL
COMMISSION
COMMISSION
ELECTROTECHNIQUE
INTERNATIONALE
ICS 31.040.01 ISBN 978-2-8322-6870-4
– 2 – IEC 62812:2019 © IEC 2019
CONTENTS
FOREWORD . 4
1 Scope . 6
2 Normative references . 6
3 Terms and definitions . 6
4 Resistance measurement phenomena . 7
4.1 General . 7
4.2 Lead and contact resistance . 7
4.3 Self-heating . 9
4.4 Variation of resistance with temperature . 10
4.5 Thermoelectric e.m.f. . 12
4.6 Peltier effect . 15
5 Methods of measurement . 16
5.1 General . 16
5.2 Four-wire resistance measurement . 16
5.3 Offset compensation method . 19
5.4 Current inversion method . 22
5.5 Differential current inversion method . 25
5.6 Short-term trigger method . 28
6 Connecting the specimen . 32
6.1 Resistors with lead wires for soldered assembly . 32
6.1.1 Connecting leaded resistors in a test fixture . 32
6.2 Resistors with solder terminations for surface mount assembly . 33
6.2.1 Connecting SMD resistors on a test substrate. 33
6.2.2 Connecting SMD resistors in a test fixture . 35
7 Information to be given in the relevant component specification . 36
Annex A (normative) Letter symbols and abbreviated terms . 37
A.1 Letter symbols . 37
A.2 Abbreviated terms . 38
Annex B (informative) Test results of soldering pad with Kelvin connection for surface
mount resistors . 39
B.1 General . 39
B.2 Test procedures . 39
B.2.1 Test substrates . 39
B.2.2 Test method . 41
B.3 Measurement result and studies . 42
Bibliography . 45
Figure 1 – Resistance measurement using two-wire sensing . 8
Figure 2 – Variation of resistance with temperature (random example) . 10
Figure 3 – Resistances on a resistor with lead wires . 11
Figure 4 – SMD chip resistor on a PCB . 12
Figure 5 – Thermoelectric e.m.f. . 13
Figure 6 – Thermocouples on a resistor with lead wires . 14
Figure 7 – Resistance measurement affected by thermoelectric e.m.f. . 15
IEC 62812:2019 © IEC 2019 – 3 –
Figure 8 – Four-wire resistance measurement . 17
Figure 9 – Offset compensation method for resistance measurement . 19
Figure 10 – Current and voltage in the offset compensation method . 20
Figure 11 – Current inversion method for resistance measurement . 22
Figure 12 – Current and voltage in the current inversion method . 23
Figure 13 – Current and voltage in the differential current inversion method . 26
Figure 14 – Example of resistor specimen. 31
Figure 15 – Connecting leaded resistors in a test fixture . 32
Figure 16 – Resistance of cylindrical copper lead wires . 33
Figure 17 – Soldering pad of test substrate for Kelvin (four-point) connections . 34
Figure 18 – Resistance of PCB conductor tracks with 35 µm copper thickness. 35
Figure 19 – Example for connecting SMD resistors on a test fixture . 36
Figure B.1 – Lengths of soldering pad . 40
Figure B.2 – Position of voltage sense conductor . 40
Figure B.3 – Thickness of the solder printing screen and position of sense line . 43
Figure B.4 – Position of voltage-sensing line. 43
Figure B.5 – Soldering pad length . 44
Figure B.6 – Recommended soldering pad . 44
Table 1 – Relative Seebeck coefficients of selected metals. 13
Table A.1 – Letter symbols . 37
Table B.1 – Thickness of solder printing screen . 41
Table B.2 – Table of test conditions . 42
– 4 – IEC 62812:2019 © IEC 2019
INTERNATIONAL ELECTROTECHNICAL COMMISSION
____________
LOW RESISTANCE MEASUREMENTS –
METHODS AND GUIDANCE
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 62812 has been prepared by IEC technical committee 40:
Capacitors and resistors for electronic equipment.
The text of this International Standard is based on the following documents:
FDIS Report on voting
40/2665/FDIS 40/2671/RVD
Full information on the voting for the approval of this International Standard can be found in
the report on voting indicated in the above table.
This document has been drafted in accordance with the ISO/IEC Directives, Part 2.
IEC 62812:2019 © IEC 2019 – 5 –
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.
– 6 – IEC 62812:2019 © IEC 2019
LOW RESISTANCE MEASUREMENTS –
METHODS AND GUIDANCE
1 Scope
Resistance measurements are typically compromised by a variety of phenomena, for example
serial resistance in the measurement path, self-heating or non-ohmic properties. Whether the
effect of such phenomena on a resistance measurement is acceptable or not depends on the
magnitude of each effect in comparison to the resistance and to the required accuracy. Hence,
the risk of erroneous resistance measurements increases with decreasing resistance and with
a tightening of the permissible tolerance.
This document specifies methods of measurement and associated test conditions that
eliminate or reduce the influence of adverse phenomena in order to improve the attainable
accuracy of low-resistance measurements.
The methods described in this document are applicable for the individual measurements of
the resistance of individual resistors, and also for resistance measurements as part of a test
sequence. They are applied if prescribed by a relevant component specification, or if agreed
between a customer and a manufacturer.
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 60068-1, Environmental testing – Part 1: General and guidance
IEC 60115-1:2008, Fixed resistors for use in electronic equipment – Part 1: Generic
specification
IEC 60294, Measurement of the dimensions of a cylindrical component with axial terminations
3 Terms and definitions
For the purposes of this document, the terms and definitions given in IEC 60115-1 and the
following apply.
A list of used letter symbols and abbreviated terms is provided in Annex A.
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
electromotive force
e.m.f.
difference in potential that gives rise to an electric current
IEC 62812:2019 © IEC 2019 – 7 –
3.2
thermoelectric e.m.f.
E
T
potential difference occurring at the junctions of dissimilar conductors when a temperature
difference exists between the junctions
3.3
low resistance
resistance for which the predictable error when measured with a conventional two-wire
sensing method is significant in comparison to the required precision or to the stated
tolerance
3.4
four-wire sensing
Kelvin sensing
four-terminal sensing
four-point sensing
electrical impedance measuring technique using separate pairs of wires for carrying the
measuring current and for sensing the potential difference in order to eliminate the impedance
contribution of wiring and contact resistances
3.5
two-wire sensing
conventional electrical impedance measuring technique using one pair of wires for carrying
the measuring current and for sensing the potential difference on the same wires
4 Resistance measurement phenomena
4.1 General
The measurement of a low resistance usually relies on the measurement of a low voltage,
which requires a number of precautions against typical detrimental phenomena such as offset
voltages, radio frequency interference, electromagnetic interference, electrical noise, or non-
ohmic contacts. However, these phenomena are not discussed here as they are not
specifically related to the measurement of resistance.
The voltage to be measured increases with an increase of the measuring current, which may
also result in effects which are adverse to the measurement. Such phenomena are discussed
in Clause 4.
4.2 Lead and contact resistance
A conventional method for measuring a resistance is to use a constant current source with a
known (or measured) output current and a voltmeter for measuring the voltage across the
unknown resistor, while the connection is built with a single pair of test leads, as shown in
Figure 1.
– 8 – IEC 62812:2019 © IEC 2019
R
I L
M
I
V
I
U U R
V R x
CS
VM
R
L
IEC
Key
CS current source
VM voltmeter, measuring voltage U
V
R lead resistance, including contact resistance to the specimen
L
R resistance to be measured
x
I supply current from current source
O
I current passing through the voltmeter
V
I current passing through the unknown resistor
M
Figure 1 – Resistance measurement using two-wire sensing
In this circuit, the source current I splits up into the current I passing through the path with
0 M
the unknown resistor and the current I passing through the voltmeter, where I depends on
V V
the measured voltage U and the voltmeter's impedance R .
V V
II + I (1)
0 MV
U
V
I = (2)
V
R
V
The voltmeter measures the following voltage drop of current I along both lead and contact
M
resistances R , plus along the unknown resistor R :
L x
U =I ⋅ 2RR+ (3)
( )
VM L x
This leads to the apparent result of the resistance measurement, R′, based on the measured
voltage U and the known sourced current I :
V 0
U I 2RR+
V M Lx
′
R= = ⋅(2RR+=) (4)
Lx
2RR+
I II+
Lx
0 MV
1+
R
V
With I → 0, which is the case if R >> (2R + R ), the apparent result tends towards:
V V L x
UI
VM
R′= = ⋅( 22RR+=) RR+ (5)
Lx Lx
I I + 0
0 M
This final apparent result still bears the error ∆R of
=
IEC 62812:2019 © IEC 2019 – 9 –
′
ΔR=R−=R 2R (6)
xL
This error will only be negligible if (2R ) << R , where the negligibility depends on the required
L x
accuracy for the measurement of R .
x
EXAMPLE 1 A 1 m copper wire with a cross section of 0,5 mm has a resistance of 35 mΩ. Using a pair of these
wires for two-wire sensing for measuring a 100 mΩ resistor results in an unacceptable error of 70 %. The current
passing through the voltmeter due to its limited impedance is not likely to gain any significance on the error figure.
EXAMPLE 2 Using the same circuit for measuring a 10 Ω resistor results in 0,7 % error, while first assuming the
current through the voltmeter to be zero. This 0,7 % error may be acceptable if the relative tolerance of the
resistance is given as ±10 %, but not if it is only ±1 %.
Using a voltmeter in this circuit with an impedance of 1 MΩ results in only a −0,001 % additional error, which is not
significant compared to the error caused by the lead wires. If the voltmeter, however, has an impedance of only
10 kΩ, the additional error is −0,1 % and thus may no longer be negligible.
EXAMPLE 3 For a resistor of 1 kΩ, measured as above, even the seemingly small error of only 0,007 % renders
the described circuit useless, if it is a high precision type with, for example, a relative tolerance of ±0,01 %.
Using a voltmeter in this circuit with an impedance of 1 MΩ results in the additional error of −0,1 %. Comparing the
absolute error contributions, this influence is even larger than the error caused by the lead wires.
4.3 Self-heating
The measuring current I passing through the unknown resistor with its resistance R causes
M x
dissipation of the power P
R
PI ⋅ R (7)
RM x
The dissipation P produces a temperature rise on the unknown resistor, which depends on
R
the ability of the test assembly or fixture to dissipate heat to the environment, expressed as
the thermal resistance R . The steady-state temperature rise ∆ϑ on the unknown resistor
th R∝
is
Δϑ R⋅ P (8)
R ∞ th R
which adds to the ambient temperature next to the specimen, ϑ , and thereby leads to the
amb
steady-state temperature ϑ on the unknown resistor of
R∝
ϑ ϑ +Δϑ ϑ +⋅R P (9)
R am∞∞b R amb th R
NOTE The heat conduction out of the unknown resistor is considered to be a linear system for the purpose of this
specification. This is based on the general observation that radiation and convection from the body of most low-
power resistors only have a minor share in the total heat dissipation. A more complex consideration can be suitable
for large resistors where radiation and convection from the body's surface prevail over conduction through the
terminals or lead wires.
The temporal rise of the temperature ϑ (t) on the unknown resistor before reaching the
R
steady state is determined by the thermal time constant τ of the unknown resistor in its test
th
assembly or fixture:
t
−
τ
th
ϑϑ(te) +Δϑ ⋅−(1 ) (10)
x amb R ∞
Knowledge of the thermal time constant τ is necessary for measurements aiming at the
th
steady state and for determination of the timing of switched measurements alike.
=
= =
=
=
– 10 – IEC 62812:2019 © IEC 2019
The raised temperature on the unknown resistor due to self-heating not only affects the
specimen, but also spreads the heat to the test assembly or mounting and affects those parts
of the measurement circuit as well. Therefore, the raised temperature will be root cause of the
variation of resistance with temperature, as discussed in 4.4, and of the thermoelectric e.m.f.,
as discussed in 4.5.
Self-heating is decreased by reducing the measuring current I as much as possible while
M
still providing the required voltage for a measurement with the desired accuracy. However,
setting the measuring current is not a common feature with resistance meters. Other options
to reduce the self-heating are to activate the measuring current for a short period only, as
discussed in Clause 5, and of course to enhance the heat flow from the specimen and the test
fixture.
4.4 Variation of resistance with temperature
One of the reference conditions prescribed in IEC 60115-1 for measuring the resistance is the
reference temperature of 20 °C. For practical reasons, however, most tests and
measurements are permitted to be executed under standard atmospheric conditions for
testing as defined in IEC 60068-1, which includes a permissible range for the ambient
temperature from 15 °C to 35 °C.
If measured with sufficient accuracy, a resistor measured at 15 °C or at 35 °C will not show
the same resistance as when measured at 20 °C. In fact, there is a variation of resistance
with temperature for almost every type of resistor, which typically does not follow a linear
relationship. The slope and the amount of variation depend substantially on the technology
and manufacturing of the resistor and in some cases also on the actual resistance.
+α
max
ϑ
ϑ ϑ
min max
20 °C
−α
max
IEC
Key
α temperature coefficient of resistance
ΔR resistance change
Figure 2 – Variation of resistance with temperature
(random example)
As a specification figure for resistors, the limitation of the permissible range for such
resistance variation in a given temperature range is usually given by a pair of symmetrical
linear slopes through the reference point at 20 °C, +α and −α as shown in Figure 2.
max max
The value α is the absolute value of the specified temperature coefficient of resistance, or
max
TCR.
−6
EXAMPLE 1 Thick film chip resistors of 100 mΩ or lower are typically offered with a TCR of 500 ⋅ 10 /K or above.
Measuring the resistance at 35 °C results in a possible deviation of ±0,75 % from the resistance at the reference
temperature 20 °C. Such a deviation may be acceptable if the relative tolerance of the resistance is given as ±10 %,
but not if it is only ±1 %.
∆R
IEC 62812:2019 © IEC 2019 – 11 –
−6
High-precision resistors made in thin film or metal foil technology may be offered with a TCR of only 5 ⋅ 10 /K.
Measuring such a resistor at 15 °C or at 25 °C results in a possible deviation of only ±0,002 5 % from the true
resistance at the reference temperature. This very small deviation may again be unacceptable in light of a specified
relative tolerance of those resistors of only ±0,01 %, or even better.
A real resistor not only consists of the resistive element, made of a specific material, but also
incorporates conductors on both sides in order to establish the electrical connection. In some
cases, these conductors are fixed in shape and effective length and therefore should be
included in an overall specification of the resistor. In other cases, such as the axial leaded
resistors shown, for example, in Figure 3, the conductors are supplied with generous lengths
of lead-wire, of which typically only a part is used in the circuit, requiring a suitable
specification of mandatory points of resistance measurement.
A B
l l
1 2
R R R
Lead 1 x Lead 2
IEC
Key
A, B measurement probes
l ,l distance of the point of measurement from resistor body, as measured in accordance with IEC 60294
1 2
Figure 3 – Resistances on a resistor with lead wires
The lead-wires are usually made of a high conductivity metal, which naturally comes with a
TCR much higher than the TCR of the resistive element. A typical choice would be copper
2 −6
with an electrical conductivity of γ = 58 m/Ωmm and a TCR of α = 3 800 ⋅ 10 /K.
Cu Cu
EXAMPLE 2 A resistor of 100 mΩ, may be supplied with copper lead wires of 0,5 mm in diameter. With the
measurement probes attached at a distance of 5 mm on each side, the pair of lead wires contributes 0,88 mΩ to
the total resistance at 20 °C, and 0,93 mΩ at 35 °C, which is almost a potential error of 1 %.
With the measurement probes attached at a distance of 21 mm on each side, the lead wires contribute 3,7 mΩ to
the total resistance at 20 °C, and 3,9 mΩ at 35 °C, which is almost a potential error of 4 %.
For SMD chip resistors, the separate contributions may be not as striking as in the above
example of a leaded resistor. However, Figure 4 illustrates, for the chip resistor, the presence
of the resistive element on the top side and the conductors around each edge to the point of
contact with the electrical circuit on the PCB. Naturally, each of these elements is featured
with its own electrical conductivity and TCR, which all contribute to the total measurable
resistance of a specimen.
– 12 – IEC 62812:2019 © IEC 2019
R T
S
P
IEC
Key
R resistive element
T termination
S solder joint
P printed circuit board
Figure 4 – SMD chip resistor on a PCB
EXAMPLE 3 A chip resistor, size RR3216M, may be manufactured with terminations consisting of a 3 µm nickel
layer and a 5 µm tin layer on top, wrapped around the edges. With a termination width of 1,6 mm and an effective
conductor length assumed to be 1 mm, the conductor resistance amounts to 7,8 mΩ on each side, or 15,6 mΩ in
total. If the resistor was, for example, specified to be 100 mΩ, then this termination resistance would already
represent about 15,6 % of it, and every 64 µm difference in the effective conductor length would change that
contribution by 1 %.
If the resistor was measured at 35 °C, the combined conductor resistance amounts to 16,8 mΩ, which is due to an
−6
effective TCR of approximately 5 400 ⋅ 10 /K for both termination layers combined.
4.5 Thermoelectric e.m.f.
Any solid conductor subjected to a temperature gradient features a displacement of charge
carriers through thermal diffusion, leading to a movement of electrons towards the cold end.
Hence, an electrical field establishes itself from the hot to the cold end.
Two different materials combined to form a loop with both junctions subjected to different
temperatures result in the flow of a continuous thermoelectric loop current. Opening the loop
within any one of the materials results in a measurable thermoelectric e.m.f., E , as shown in
T
Figure 5.
The thermoelectric e.m.f., known as Seebeck effect, depends on the two involved materials
and on the temperature difference between the two junctions. The Seebeck effect is not linear
and depends on the actual temperatures. For a limited temperature range, however, it is
possible to assume sufficient linearity and therefore to calculate the thermoelectric e.m.f., E ,
T
from the difference of the Seebeck coefficients α of the two joined metals and the
S
temperature difference of their junctions:
E αα− ⋅ ϑϑ− (11)
( ) ( )
T SB SA 2 1
=
IEC 62812:2019 © IEC 2019 – 13 –
VM
A A
Ε
T
B
ϑ ϑ
2 1
IEC
Key
VM voltmeter, measuring the thermoelectric e.m.f., E
T
A, B wire materials
ϑ , ϑ temperature of the wire junctions
1 2
Figure 5 – Thermoelectric e.m.f.
Seebeck coefficients can be given as absolute figures or relative to a second material. For
metallic conductors, the coefficients typically are in a range of one to a few tens of microvolts
per Kelvin, while for doped semiconductor materials they are rather in the order of a millivolt
per Kelvin. Table 1 gives the relative Seebeck coefficients α of a number of potentially
S
relevant metals joined with platinum, or joined with copper.
Table 1 – Relative Seebeck coefficients of selected metals
α to Platinum α to Copper
S S
Metal
µV/K µV/K
Chrome nickel +22 +14,5
Iron +18,3 +10,8
Brass +11 +3,5
Copper +7,5 ±0
Silver +7,3 −0,2
Gold +7 −0,5
Lead +4,4 −3,1
Tin +4,2 −3,3
Aluminium +3,9 −3,6
Platinum ±0 −7,5
Nickel −15 −22,5 ®
1 −33 −40,5
CuNi44 (e.g. Constantan )
The cited figures are stated for a reference temperature of 0 °C.
Seebeck coefficients α are traditionally also given in mV/100 K.
S
Even junctions of laboratory connectors are reported to generate a thermoelectric e.m.f. since
the specific materials used for their production are chosen by the manufacturer. Connectors
specified for low thermal activity may show coefficients less than a tenth of a microvolt per
____________
Constantan® is an example of a suitable product available commercially. This information is given for the
convenience of users of this document and does not constitute an endorsement by IEC of this product.
– 14 – IEC 62812:2019 © IEC 2019
Kelvin, while junctions of regular products may show coefficients in excess of one microvolt
per Kelvin.
Semiconductor materials are typically not relevant for the construction of resistors, except for
copper oxide, for which a Seebeck coefficient of 1 mV/K with respect to copper is reported.
Hence, special attention should be applied to any attempt at contacting a copper surface.
Most resistors are built by connecting a number of different metals in series.
– A wirewound resistor may just have its Constantan® resistive wire welded to the copper or
copper-clad steel lead-wires on both sides, or may have the resistive wire welded to the
rim of steel termination caps to which the lead-wires are welded in the centre.
– An axial leaded film resistor may have steel termination caps press fitted onto the nickel
chrome resistive film, before the copper lead-wires are welded to their centres.
– An SMD chip resistor may have its nickel chrome resistive film in planar contact with the
inner layer of the termination system, which typically ends in a tin layer on a nickel layer.
These devices are either contacted in a test fixture by tips or clips, which most likely are of a
different metal than the device's lead wire or termination. Or they are soldered to a circuit
board, where typically a tin alloy builds the joints to the printed copper conductors. Figure 6
shows the sequence of different metals and the serial connection of resulting thermoelectric
e.m.f. for the example of a film resistor with lead wires, which are clamped in a test fixture.
A B
R
x
Au Au
ϑ ϑ ϑ
R1 Rx R2
ϑ ϑ
L1 L2
Cu Fe CrNi Fe Cu
ϑ ϑ ϑ ϑ ϑ ϑ
L1 R1 Rx Rx R2 L2
A
B
R R R
Lead 1 x Lead 2
E E E E
E T Cu-Fe T Fe-CrNi T CrNi-Fe T Fe-Cu E
T Au-Cu T Cu-Au
IEC
Key
A, B measurement probes
Figure 6 – Thermocouples on a resistor with lead wires
All these connections of different metals are likely to generate each a thermoelectric e.m.f. if
the individual joints connected in series are at different temperatures (Seebeck effect). The
self-heating and respective heat flow as described in 4.3 is a common cause of different
temperatures along the series connection of metallic joints.
The combined thermoelectric e.m.f. of an assembled resistor may be zero if the series of
metallic joints as seen from the middle of the resistor to both sides is identical, and if the
temperatures on these pairs of identical joints are identical, too. This, however, typically
requires the heat flow from the resistor to split evenly into two equivalent heat flow paths.
IEC 62812:2019 © IEC 2019 – 15 –
Using a resistance material with high thermoelectric e.m.f. versus copper (e.g. CuNi44, known
as Constantan®) is critical, especially for low value resistors, for example below 10 mΩ.
EXAMPLE A resistor of 1 mΩ, made of CuNi44 and connected with copper wires, is measured with a current of
2 A in order to achieve a measurable voltage drop of 2 mV. A temperature difference of 5 K between the two
resistor terminals leads to a thermoelectric e.m.f. of −202,5 µV, which adds to the intended voltage drop and
thereby causes the resulting voltage and thus the resistance result to deviate by 10,1 %.
E
T1
R
L
U U R
V R x
CS VM
R
L
E
T2
IEC
Key
CS current source, supplying current I
VM voltmeter, measuring voltage U
V
R lead resistance, including contact resistance to the specimen
L
R resistance to be measured
x
Figure 7 – Resistance measurement affected by thermoelectric e.m.f.
Thermoelectric e.m.f. is superimposed upon the voltage generated on the unknown resistor by
the sourced current, see Figure 7, and, therefore, can seriously affect the accuracy of a
resistance measurement. Less obvious is that the distribution of currents in a node will also
be affected if thermoelectric e.m.f. affects only one of the branches.
This issue of thermoelectric e.m.f. may be of static or of dynamic nature, depending on
whether the temperature on the specimen and on the assembly or fixture has reached the
steady state or not. Clause 5 discusses methods with a potential of eliminating the influence
of a resulting thermal e.m.f. from the resistance measurement result.
4.6 Peltier effect
Another cause for a temperature difference, the Peltier effect, will remain unavoidable, even if
the symmetrical heat flow proposed in 4.5 would be able to balance the internal sources of
thermoelectric e.m.f. The Peltier effect is creating a temperature difference when a d.c.
current is passed through the joint of two different materials. Hence, the variety of materials
sequentially joined together in a resistor is likely to result in a sequence of temperature
differences when such a resistor is subjected to a d.c. current, for example for measuring its
resistance.
The heat generated by the Peltier effect is proportional to the d.c. current and the
thermoelectric e.m.f. of the joined pair of materials. The temperature difference rises
exponentially with a time constant to a terminal value, which are both determined by the
design of the resistor and by the ability of the test fixture to dissipate heat to the environment.
The resulting temperature difference is reversed when the d.c. current is reversed.
EXAMPLE 1 A 100 µΩ resistor, manufactured by welding copper terminations to each side of a strip of CuNi44
resistive alloy, is measured in a suitable fixture with a current of 300 A in order to achieve a measurable voltage
drop of 30 mV. The measuring current by means of the Peltier effect generates heat on the material joints, which
have been observed to result in a temperature difference of 30 K after being loaded for 3 min.
– 16 – IEC 62812:2019 © IEC 2019
This temperature difference generated by the Peltier effect in turn generates a thermoelectric
e.m.f. through the Seebeck effect, which again is likely to cause a significant error to the
resistance measurement.
EXAMPLE 2 As a consequence of the conditions shown in the above Example 1, the temperature difference of
30 K between the two resistor terminals leads to a thermoelectric e.m.f. of −1,2 mV, which adds to the intended
voltage drop and thereby causes the resulting voltage and thus the resistance result to deviate by 4 %.
5 Methods of measurement
5.1 General
The discussion in Clause 4 has illustrated that – aside from more general low-signal
measurement uncertainties – the measurement of low resistance is likely to be affected by a
number of very basic phenomena. These phenomena, each by itself, are likely to have
detrimental effects on the achievable accuracy of the resistance measurement. These effects
may result in substantial measuring errors, which could finally render a low-resistance
measurement useless.
An obvious first measure to avert the detrimental influence of lead and contact resistance as
demonstrated in 4.2 is to apply the offset correction provided by most current meters. This
method requires the contact points to be connected directly, without any resistance between
them, which may not be applicable in a real test fixture without imposing further influence.
Also, since the resistor under test is not involved in this method, it is not possible to
compensate for any influence by its materials and of self-heating under the actual
measurement.
Clause 5 describes a variety of methods with a potential to eliminate the effect of one or more
of the described phenomena and discusses the particular background of each method. It also
identifies limitations which still can persist for and possible conditions under which such
limitations can be waived.
5.2 Four-wire resistance measurement
A very common method for the elimination of lead and contact resistances from the act
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