ASTM B667-97(2003)e1
(Practice)Standard Practice for Construction and Use of a Probe for Measuring Electrical Contact Resistance
Standard Practice for Construction and Use of a Probe for Measuring Electrical Contact Resistance
SIGNIFICANCE AND USE
Electrical contact resistance is an important characteristic of the contact in certain components, such as connectors, switches, slip rings, and relays. Ordinarily, contact resistance is required to be low and stable for proper functioning of many devices or apparatus in which the component is used. It is more convenient to determine contact resistance with a probe than to incorporate the contact material into an actual component for the purpose of measurement. However, if the probe contact material is different from that employed in the component, the results obtained may not be applicable to the device.
Information on contact resistance is useful in materials development, in failure analysis studies, in the manufacturing and quality control of contact devices, and in research.
Contact resistance is not a unique single-valued property of a material. It is affected by the mechanical conditions of the contact, the geometry and roughness of contacting surfaces, surface cleanliness, and contact history, as well as by the material properties of hardness and conductivity of both contacting members. An objective of this practice is to define and control many of the known variables in such a way that valid comparisons of the contact properties of materials can be made.
In some techniques for measuring contact resistance it is not possible to eliminate bulk resistance, that is, the resistance of the metal pieces comprising the contact and the resistance of the wires and connections used to introduce the test current into the samples. In these cases, the measurement is actually of an overall resistance, which is often confused with contact resistance.
SCOPE
1.1 This practice describes equipment and techniques for measuring electrical contact resistance with a probe and the presentation of results.
1.2 The values stated in SI units are to be regarded as standard. The values given in parentheses are for information only.
1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to become familiar with all hazards including those identified in the appropriate Material Safety Data Sheet for this product/material as provided by the manufacturer, to establish appropriate safety and health practices, and determine the applicability of regulatory limitations prior to use.
General Information
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Standards Content (Sample)
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Designation:B667–97(Reapproved2003)
Standard Practice for
Construction and Use of a Probe for Measuring Electrical
Contact Resistance
This standard is issued under the fixed designation B 667; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
e NOTE—Keywords were added editorially in December 2003.
1. Scope high spots, which are often called “asperities” or, more
commonly, a-spots. The current flow lines are then forced to
1.1 This practice describes equipment and techniques for
constrict as they funnel through these tiny areas. If oxide films
measuring electrical contact resistance with a probe and the
or other insulating layers interfere with these metal-to-metal
presentation of results.
contacts, the contact resistance will be higher than when such
1.2 The values stated in SI units are to be regarded as
layers are absent (see 4.4 for bulk resistance limitation).
standard. The values given in parentheses are for information
3.2.2 contact resistance probe—an apparatus for determin-
only.
ing electrical contact resistance characteristics of a metal
1.3 This standard does not purport to address all of the
surface. Probe, in this instance, should be distinguished from
safety concerns, if any, associated with its use. It is the
the classical tool whose function it is to touch or move an
responsibility of the user of this standard to become familiar
object.
with all hazards including those identified in the appropriate
Material Safety Data Sheet for this product/material as pro-
4. Significance and Use
vided by the manufacturer, to establish appropriate safety and
4.1 Electrical contact resistance is an important characteris-
health practices, and determine the applicability of regulatory
tic of the contact in certain components, such as connectors,
limitations prior to use.
switches,sliprings,andrelays.Ordinarily,contactresistanceis
2. Referenced Documents required to be low and stable for proper functioning of many
devicesorapparatusinwhichthecomponentisused.Itismore
2.1 ASTM Standards:
convenient to determine contact resistance with a probe than to
B 542 Terminology Relating to Electrical Contacts and
incorporate the contact material into an actual component for
Their Use
the purpose of measurement. However, if the probe contact
3. Terminology material is different from that employed in the component, the
results obtained may not be applicable to the device.
3.1 Definitions—Many terms used in this practice are de-
4.2 Information on contact resistance is useful in materials
fined in Terminology B 542.
development, in failure analysis studies, in the manufacturing
3.2 Definitions of Terms Specific to This Standard:
and quality control of contact devices, and in research.
3.2.1 contact resistance—the resistance to current flow
4.3 Contactresistanceisnotauniquesingle-valuedproperty
between two touching bodies, consisting of constriction resis-
of a material. It is affected by the mechanical conditions of the
tance and film resistance.
contact, the geometry and roughness of contacting surfaces,
3.2.1.1 Discussion—Constriction resistance originates in
surface cleanliness, and contact history, as well as by the
the fact that mating surfaces touch in most cases at only their
material properties of hardness and conductivity of both
contacting members. An objective of this practice is to define
This practice is under the jurisdiction ofASTM Committee B02 on Nonferrous
and control many of the known variables in such a way that
Metals and Alloys and is the direct responsibility of Subcommittee B02.11 on
valid comparisons of the contact properties of materials can be
Electrical Contact Test Methods.
made.
Current edition approved Dec. 9, 2003. Published December 2003. Originally
approved in 1980. Last previous edition approved in 1997 as B 667 – 97.
4.4 Insometechniquesformeasuringcontactresistanceitis
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
not possible to eliminate bulk resistance, that is, the resistance
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
ofthemetalpiecescomprisingthecontactandtheresistanceof
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. thewiresandconnectionsusedtointroducethetestcurrentinto
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
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B667–97 (2003)
the samples. In these cases, the measurement is actually of an 6. Design Aspects
overall resistance, which is often confused with contact resis-
6.1 The probe is mounted on one end of a pivoted beam, a
tance.
cantilever, or a coil spring. Force is applied by dead weight,
compression of the spring, bending of the cantilever, or
5. General Description of a Probe
electromagnetically.
5.1 A probe generally includes the following:
6.2 Probe holders have been designed so that force may be
5.1.1 Fixtures for holding specimens of varied size and
applied to the contact and to an electronic load cell which is
shape and for attaching electrical leads to them.
mounted between the probe contact and a micrometer spindle
5.1.2 A mechanism that applies a measurable load to the
that can be advanced. An alternative design is to mount the
specimen that can be increased, decreased, or held constant.
specimen on the load cell and to advance the probe directly
5.1.3 A shock mounted table to prevent any indigenous
with the micrometer spindle. Load and contact resistance are
vibrations from inadvertently altering the conditions at the
the usual parameters measured and recorded simultaneously.
contact interface.
6.3 A probe can be made by mounting a U-shaped free-
5.1.4 Areference surface (the probe) that is pressed against
standing gold wire to the micrometer spindle (see Fig. 1(b)).
the specimen and which is normally made of a noble metal.
The load is measured after the probe is observed (preferably
Noble metals such as pure gold are used because they are
electrically) to first touch the specimen from a preliminary
substantially free of oxide films and have the best likelihood of
calibration (with a load cell) of micrometer advance versus
obtaining reproducible results.
load. In some cases, where very small (to tens of milligrams)
5.1.5 A current source with current and voltage measuring
forces are used, it may not be necessary to know the load
instrumentation for determining contact resistance. Ordinarily,
precisely. In such cases, fine (for example, 50-µm diameter),
contact resistance is determined at dry circuit conditions to
straight, or U-shaped gold or platinum wires can be used as the
avoid changes that may occur due to voltage breakdown or
probe.
heating at the contact interface.
6.4 The apparatus must be isolated from vibration to avoid
5.2 Additionalelectricalcircuitrymaybeincludedtopermit
damaging the surface film that may exist at the interface to be
related measurements to be obtained, such as the voltage
evaluated.The slightest movement can translate into extremely
breakdown or the current versus voltage characteristics of
large stresses at the tops of small asperities. Likewise, bounce
film-covered surfaces.
should be avoided when touching the probe to the specimen.
5.3 Probes are also convenient for determining the depen-
Vibration may reveal itself as a noisy signal when contact
dence of contact resistance on sliding or wipe when a slide is
resistance is continuously monitored electronically.
incorporated in the specimen holder. This permits the probe to
6.4.1 For sensitive surfaces, a preliminary run should be
be moved small measurable distances after loading.
made on as-received (uncleaned) test specimens of the same
surface material as the samples to be measured. If the
vibration-inducedfluctuationsaregreaterthan10 %,additional
See ASTM Standard B 539, Measuring Contact Resistance of Electrical
Connections (Static Contacts), in the Annual Book of ASTM Standards, Vol 03.04. antivibrational measures should be taken.
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B667–97 (2003)
FIG. 1 Arrangement of Current and Voltage Leads to Probe and to Specimen (Typical)
6.4.2 Wipe should not be introduced when contact resis- 7. Requirements of the Probe Contact
tance versus load characteristics are being measured, since as
7.1 The probe is normally made with a pure gold surface,
little as a few micrometers of lateral movement can drastically
although other noble metals can be used. The probe should be
change the contact resistance of samples having films.
smooth and have a large radius of curvature to minimize the
possibility that it may damage the specimen surface. An
NOTE 1—Some variation of contact resistance with time under load has
beenfoundtooccurformanymaterials. Itisthereforerecommendedthat, exception to this latter recommendation are the wire probes
for continuously monitored runs, the resistance at the final applied load
that are generally designed for low normal loads (6.3).
should also be recorded after a fixed dwell time, usually 10 to 30 s.
7.1.1 One early probe design that has seen much use is a
3.2-mm diameter solid gold rod having a hemispherical end.
6.5 The power supply shall be capable of delivering a
Such probes have been used exten
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