Measurement Methods and Influencing Factors of Contact Resistance
[Introduction] When observing the surface of connector contacts under a microscope, even though the gold-plated layer appears exceptionally smooth, microscopic protrusions measuring 5 to 10 micrometers can still be detected. It becomes evident that when a pair of mating contacts is engaged, the contact does not occur across the entire contact surface; rather, it is concentrated at discrete points scattered across the surface. Consequently, the actual contact area is invariably smaller than the theoretical contact area. Depending on the degree of surface smoothness and the magnitude of contact pressure, this discrepancy can sometimes reach several thousand times.
Principle of Contact Resistance
When observing the surface of connector contacts under a microscope, even though the gold-plated layer appears extremely smooth, protrusions measuring 5 to 10 micrometers can still be detected. It becomes evident that when a pair of mating contacts is engaged, the contact does not occur over the entire contact surface; rather, it is concentrated at discrete points scattered across the contact area. Consequently, the actual contact area is invariably smaller than the theoretical contact area. Depending on the degree of surface smoothness and the magnitude of contact pressure, this discrepancy can sometimes reach several thousand times. The actual contact area can be divided into two distinct parts: First, the portion where true metal-to-metal contact occurs—namely, microscopic contact points between metals with no transitional resistance. These are also referred to as contact spots, which are formed when the contact pressure or thermal effects break down the interfacial oxide film. This first part accounts for roughly 5% to 10% of the total actual contact area. Second, the portion where contact occurs through contamination films formed at the interface. This is because any metal has a tendency to revert to its original oxidized state.
In fact, no metal surface in the atmosphere is truly clean. Even surfaces that appear remarkably clean will quickly develop a thin initial oxide layer—just a few micrometers thick—once exposed to the atmosphere. For example, copper forms such an oxide layer in as little as 2 to 3 minutes; nickel takes about 30 minutes; and aluminum requires only 2 to 3 seconds to produce an oxide layer roughly 2 micrometers thick on its surface. Even the exceptionally stable precious metal gold, despite its high surface energy, will form a thin film of adsorbed organic gases on its surface. Moreover, particles of dust and other airborne contaminants in the atmosphere can also deposit themselves on contact surfaces, forming additional layers. Thus, from a microscopic perspective, any contact surface is inherently contaminated.
How to measure contact resistance
Contact resistance is the resistance encountered when current flows through a pair of closed contact points. Such measurements are typically performed on components such as connectors, relays, and switches. Contact resistance is generally very low, ranging from microohms to several ohms. Depending on the type of device and its application, the measurement methods may vary. ASTM Method B539, “Measurement of Contact Resistance in Electrical Connections,” and MIL-STD-1344 Method 3002, “Low-Signal-Level Contact Resistance,” are two commonly used methods for measuring contact resistance. As a general rule, the Kelvin four-wire method is often employed for measuring contact resistance.
Measurement method:
Figure 4-42 illustrates the basic configuration used to measure the contact resistance of a single electrical contact. An ohmmeter with four-terminal measurement capability is employed to avoid including lead resistance in the measurement results. The terminals of the current source are connected to the two ends of the contact pair. The sense terminals, on the other hand, should be connected as close as possible to the points where the voltage drop across the contact occurs. The purpose of this arrangement is to prevent the inclusion of voltage drops caused by the test leads and bulk resistance in the measurement results. Bulk resistance refers to the total resistance of the contact when it is assumed to be a solid metal body of uniform geometric dimensions, with the actual contact area having zero resistance. For devices designed with only two leads, it can sometimes be challenging to implement a four-wire connection. The specific form of the device determines how it should be connected. In general, tests should be conducted as closely as possible to the device’s normal operating conditions. When placing voltage probes on the sample, care must be taken not to disturb the mechanical integrity of the sample. For example, soldering the probes could induce unwanted changes in the contact. However, in certain situations, soldering may be unavoidable. Each connection point on the contact being tested may generate thermoelectric EMFs. Nevertheless, these thermoelectric EMFs can be compensated for by reversing or biasing the current.
Dry circuit testing
Typically, the purpose of testing contact resistance is to determine whether oxidation or the accumulation of other surface films at the contact points has increased the resistance of the device under test. Even a very brief period of excessively high voltage across the device’s terminals can damage this oxide layer or surface film, thereby compromising the validity of the test. The voltage level required to break down such a film typically ranges from 30 mV to 100 mV.
During testing, an excessively high current flowing through the contacts can also cause subtle physical changes in the contact area. The heat generated by the current can soften or even melt the contact points and the surrounding regions. As a result, the contact area increases, leading to a reduction in contact resistance.
To avoid such issues, the dry-circuit method is typically used for contact resistance testing. A dry circuit is one in which the voltage and current are limited to levels that cannot cause any physical or electrical changes in the state of the contact junction. This means that the open-circuit voltage is 20 mV or lower, and the short-circuit current is 100 mA or lower.
Since the test current used is very low, a highly sensitive voltmeter is required to measure the voltage drop, which typically falls within the microvolt range. Because other testing methods might induce physical or electrical changes at the contacts, dry-circuit measurements of the device should be performed before conducting any other electrical tests.
Use a microohmmeter or a digital multimeter:
Figure 4-42 shows the basic configuration for four-terminal contact resistance measurement using either a Keithley Model 580 microohmmeter, a Model 2010 digital multimeter, or a Model 2750 digital multimeter data acquisition system. These instruments are capable of automatically compensating for thermoelectric EMF offsets in the sampling circuit by employing a bias compensation mode, and they also feature built-in dry-circuit measurement capability. For most applications, a microohmmeter or a digital multimeter is sufficient for measuring contact resistance. However, if the short-circuit current or the resistance value being measured is significantly lower than the specifications of the microohmmeter or digital multimeter, it becomes necessary to use a combination of a nanovoltmeter and a precision current source instead.
Using a nanovoltmeter and a current source:
Figure 4-43 shows the test configuration for measuring contact resistance using a Keithley 2182A nanovoltmeter and a 2400 Series digital source meter.
The 2400-series instruments force a current through the contact, while the nanovoltmeter measures the voltage drop across the contact. To perform dry-circuit testing, set the digital source meter’s clamp voltage to 20 mV, thereby clamping the open-circuit voltage of the circuit at 20 mV. To ensure that the clamp voltage appears only across the contact terminals and not across the test leads, this digital source meter employs a four-wire measurement mode. This is especially important when using larger currents, since the voltage drop across the test leads can become significantly larger compared to the voltage drop across the contact terminals.
To avoid transient phenomena, be sure to turn off the current source first before connecting or disconnecting the contacts to the test fixture. Connecting a 100-ohm resistor directly across the output terminals of the current source can further reduce transient effects.
The current reversal method can be used to minimize thermoelectric bias. The Delta mode of the 2182A, when paired with a digital source meter instrument, can automatically implement this technique. In this mode, the 2182A automatically triggers the current source to reverse polarity and then measures a reading for each polarity. Subsequently, the 2182A displays the “compensated” voltage value:
Where: I = the absolute value of the test current.
Factors Affecting Contact Resistance
Contact resistance is primarily influenced by factors such as the material of the contact components, normal force, surface condition, operating voltage, and current.
1) Contact material
The technical specifications for electrical connectors stipulate different contact resistance evaluation criteria for mating contacts of the same specification but made from different materials. For example, the General Specification GJB101-86 for Small Circular Quick-Disconnect Environmental-Resistant Electrical Connectors specifies that, for mating contacts with a diameter of 1 mm, the contact resistance shall be no more than 5 mΩ for copper alloys and no more than 15 mΩ for iron alloys.
2) Positive pressure
The normal contact force refers to the force generated at the surfaces in contact with each other and acting perpendicularly to these contact surfaces. As the normal contact force increases, both the number and area of microscopic contact points gradually increase; simultaneously, the microscopic contact points transition from elastic deformation to plastic deformation. Because the concentrated resistance gradually decreases, the overall contact resistance is reduced. The magnitude of the normal contact force primarily depends on the geometric shape of the contacting components and their material properties.
3) Surface condition
The surface of contact components is often covered by a relatively loose surface film formed by the mechanical adhesion and deposition of dust, rosin, oil stains, and other contaminants. This surface film, containing fine particulate matter, readily becomes embedded in the microscopic pits on the contact surfaces, thereby reducing the effective contact area, increasing contact resistance, and rendering the contact highly unstable. The second type of contamination film arises from physical and chemical adsorption; on metal surfaces, chemical adsorption predominates and occurs following physical adsorption accompanied by electron migration. Therefore, for products with high reliability requirements—such as aerospace electrical connectors—it is essential to maintain clean assembly and production environments, implement comprehensive cleaning processes, and adopt necessary structural sealing measures. Moreover, users must ensure proper storage conditions and operating environments.
4) Operating voltage
When the applied voltage reaches a certain threshold, the film layer on the contact surfaces can be broken down, causing the contact resistance to drop rapidly. However, due to the thermal effect, chemical reactions in the vicinity of the film layer are accelerated, which in turn provides a certain degree of self-repairing action on the film layer itself. As a result, the resistance exhibits a nonlinear behavior. Near the threshold voltage, even slight fluctuations in voltage drop can lead to current variations ranging from twenty to several dozen times. This causes significant changes in contact resistance. Failure to understand this nonlinear behavior can result in errors during both testing and actual use of the contacts.
5) Current
When the current exceeds a certain value, the Joule heating generated at the tiny contact points on the interface of the contacting parts causes the metal to soften or melt, thereby affecting the concentrated resistance and subsequently reducing the contact resistance.
Disclaimer: This article is a repost. The purpose of reposting this article is to convey more information; the copyright belongs to the original author. If the videos, images, or text used in this article involve issues related to copyright, please contact us by phone or email to request their removal.
Hot News
How to Use a Multiphase Boost Converter
When the required system voltage exceeds the available supply voltage, a boost converter is typically an ideal solution. However, conventional standard boost topologies are not the only option. In certain application scenarios, phase-shifted multiphase boost converters can deliver superior performance: these converters achieve higher efficiency under heavy-load conditions while significantly reducing the required capacitance values for both the input and output stages.