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METHOD OF MEASUREMENT OF LOW RESISTANCE
1
Scope
This international standard specifies a method of measurement for low resistance less than 1 ohm.
2
Nomative reference
The following referenced documents are indispensable for the application 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 60115-1:2008, Fixed resistors for use in electronic equipment – Part 1: Generic specification
IEC 60068-2-58:2004, Environmental testing – Part 2-58: Tests – Test Td: Test methods for
solderability, resistance o dissolution of metallization and to soldering heat of surface mounting
devices (SMD)
3
Tems and definitions
For the purpose of this document, the following terms and definitions apply.
3.1
Low resistance
the resistance less than 1 ohm which easy affected electrode and probe contact in resistance
measurement.
3.2
Kelvin connection
connection method for resistance measurement that uses separate pairs of current-carrying and
voltage-sensing electrodes to eliminate the influence of contact and test lead resistances than twoterminal sensing.
NOTE Kelvin connection is also known as Four-terminal sensing, 4-wire sensing or 4-point probe method.
4
4.1
Method of measurement
Kelvin (4 point) connections
For purpose of measurement on low resistance, Kelvin (4 point) connections shall be used to eliminate
the inappropriate influence of the measuring circuit, e.g. contact resistance and wiring resistance. The
measuring circuit consists of current supply terminals and voltage sensing terminals as shown in
Figure 1.
The current passing through the voltage sensing terminals (I LEAD ) is much lower than the measuring
current (I RX ) thus the voltage drop across the lead wire of voltage sensing terminals is comparatively
very small.
Hence the resistance of the specimen is defined by the following formula. (See Figure 1)
U R ＝U M
(1)
I＝I LEAD ＋I RX ＝I RX
(2)
R X ＝UR ／I RX ＝U M ／I
(3)
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I
I+
R LEAD
U+
R LEAD
I LEAD
+
CS
VM
I RX
+
UM
UR
-
-
U-
R LEAD
I-
R LEAD
RX
Key:
VM
Voltmeter
R LEAD
Resistance of lead wire
RX
Resistance of specimen
CS
Current source
UM
Measured voltage
UR
Voltage across specimen resistor
I
Current of current supply terminal
I LEAD
Current of voltage sensing terminal
I RX
Current of specimen resistor
Figure 1 – Kelvin (4 point) connections
4.2
Thermal electromotive force
Measurement error occurs at the connecting point, if there is a temperature difference between
dissimilar metal arising from Seebeck effect. If the error cannot be ignored, the following methods shall
be used.
4.2.1
Current inversion method
Measurement results of positive and negative current can compensate thermal electromotive force.
As shown in Figure 2, thermal electromotive force V EMF which occurs between dissimilar metal has
constant polarity regardless of current polarity, thermal electromotive force can avoid by 2 times
measurements by added polarity reversal voltage. Measured voltage U M+ comes from added positive
voltage as shown in Figure 2 a),
U M ＋ = V EMF ＋I・R X
(1)
and added to polarity reversed voltage as shown in Figure 2 b), comes to;
U M － = V EMF －I・R X .
(2)
Then the average is exclusive of thermal electromotive force.
U M = (U M ＋ －U M － ) / 2 = I・R X
(3)
Hence the resistance of the specimen shall be calculated from the following formula;
R X = (U M ＋ －U M － ) / (2・I)
(4)
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V EMF
V EMF
－
＋
I
RX
UM
RX
I
＋
－
b) Negative polarity
a) Positive polarity
Key:
RX
UM
V EMF
I
UM
Resistance of specimen
Measured voltage
Thermal electromotive voltage
Measured current
Figure 2 – Current inversion method
4.2.2
Offset correction method
The measuring current shall be switched on and off in a series of cycles as shown in Figures 3.
Measured voltage when applied measuring current include thermal electromotive force as shown in
Figure 3 b).
U M1 = V EMF + I・R X
(1)
U M2 equal to thermal electromotive voltage without measuring current as shown in Figure 3 c).
U M2 = V EMF
(2)
Compensated voltage which subtracted thermal electromotive force can be calculated from the
following formula.
U M = U M1 – UM2
(3)
U M = (V EMF + I・R X ) – V EMF
(4)
U M = I・R X
(5)
The resistance of the specimen can be calculated by dividing voltage U M by measuring current I.
Rx=U M / I
(6)
Measuring cycle
ON
Measuring current
OFF
Measurement of
Thermal electromotive force
a) Measuring cycle for offset correction method
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V EMF
V EMF
＋
＋
RX
I
RX
U M1
U M2
－
－
b) Measuring current on cycle
c) Measuring current off cycle
RX
Resistance of specimen
V EMF Thermal electromotive voltage
I
Measuring current
Figure 3 – Offset correction method
4.3
Self-heating
Temperature rise occurs from self-heating by measuring current. If the temperature coefficient of the
resistor is such that this effect cannot be ignored the following methods shall be used.
4.3.1
Short-term trigger method
Short-term measuring current pulse shall be supplied by one time trigger to avoid temperature rise by
self-heating of specimen.
4.3.2
Comparison with measurement reference resistor method
Measuring current shall apply to series-connected measured standard resistor and specimen as shown
in Figure 4. Each voltage drop of standard resistor and specimen shall be measured by switching
between them. The resistance of the specimen shall be calculated from the ratio of the measured
voltages and the measurement standard resistance from the following formula;
U M1 = I・R X
(1)
U M2 = I・RREF
(2)
R X = V M1 ・R REF / V M2
(3)
RX
SW
I
SW
VM
R REF
Key:
RX
Resistance of specimen
R REF Measured standard resistance
I
Measuring current
Figure 4 – Comparison with measurement reference resistor
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5
5.1
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Measurement procedure
Leaded resistors and without mounting on a test substrate
When the resistance is to be measured without mounting on a test substrate, Kelvin (4 point) contact
probe shall be used as shown in Figure 5.
For leaded resistors distance from the resistor body shall be within 5 mm to avoid resistance of lead
wire, unless otherwise specified in the detail specification.
Contact condition ( position, pressing force and probe head durability) should be stable. The self
heating and thermal electromotive force of resistors should be considered to mitigate the effect on
measurement of resistance.
Distance from resistor body
Current
probe
I+
Voltage
sensing probe
Voltage
sensing probe
U-
U+
Current
probe
I-
Voltmeter
Current source
NOTE1 Kelvin (4 point) contact probe consists of alignment mechanism, current source and voltmeter,
2 current probes and 2 voltage-sensing probes.
Figure 5 – Kelvin (4 point) contact probe
5.2
Surface mount resistors
When the resistance value is to be measured after mounting on a test substrate in accordance with
IEC 60115-1 4.31, Kelvin (4 point) connection to the soldering pads shall be as shown in Figure 6.
Voltage-sensing lines shall be attached inside the soldering pad to eliminate the resistance component
of soldering pad and solder joint.
The resistor should be mounted on the test substrate same attitude with normal use. Detail soldering
condition should be prescribed in relevant specification.
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Surface mount resistor
Solder
Test substrate
Current-carrying lines
Voltage sensing lines
Key:
□ solderable area
■ non solderable area, conductor covered with solder resist
Figure 6 – Soldering pad of Test substrate for Kelvin (4 point) connections
When the resistance value is to be measured without mounting on a test substrate, Kelvin (4 point)
contact probe shall be used as shown in Figure 7.
The contact position to the resistor’s electrode of current probe should be edge side and voltage
probes should be bottom side unless otherwise defined in the detail specification.
Resistor
Electrode
Edge side
Bottom side
Current probes
I+
Voltage
probes
U-
U+
Voltmeter
Current source
Figure 7 – Kelvin (4 point) contact probe
I-