Contact resistance principle and composition

The resistance presented by the "contact pair" conductor is called contact resistance.
Generally, the contact resistance is required to be less than 10-20 mohm. Some switches are required to be less than 100-500uohm. Some circuits are very sensitive to changes in contact resistance. It should be pointed out that the contact resistance of a switch is the maximum value of the contact resistance allowed in a number of contacts.
Contact Area Contact resistance
On a circuit board, it refers specifically to the contact point between the gold finger and the connector, and the resistance presented when current passes through. In order to reduce the formation of oxides on the metal surface, the positive gold finger part and the negative card clip of the connector are usually plated with metal to suppress the occurrence of its "carrying resistance". There is also contact resistance when the plug of other electrical appliances is squeezed into the socket, or between the guide pin and its socket.

Working principle
Observe the surface of the connector contact under a microscope. Although the gold-plated layer is very smooth, you can still observe a 5-10 micron raised part. You will see that the contact of the plugged-in pair of contacts is not the contact of the entire contact surface, but the contact of some points scattered on the contact surface. The actual contact surface must be smaller than the theoretical contact surface. Depending on the surface smoothness and the size of the contact pressure, the difference between the two can be up to several thousand times. The actual contact surface can be divided into two parts; one is the part where the real metal directly contacts the metal. That is, the contact micro-points between the metals without transition resistance, also known as contact spots, which are formed after the interface film is destroyed by contact pressure or heat. The part accounts for about 5-10% of the actual contact area. The second is the part that contacts each other after the film is contaminated through the contact interface. Because any metal has a tendency to return to the original oxide state. In fact, there is no truly clean metal surface in the atmosphere. Even if the metal surface is very clean, once exposed to the atmosphere, it will quickly form an initial oxide film layer of several microns. For example, copper only needs 2-3 minutes, nickel about 30 minutes, and aluminum only needs 2-3 seconds, and a thickness of about 2 microns can be formed on its surface. Even the precious metal gold, which is particularly stable, will form an organic gas adsorption film on its surface due to its high surface energy. In addition, dust in the atmosphere will also form a deposition film on the surface of the contact. Therefore, from a microscopic analysis, any contact surface is a contaminated surface.

In summary, the real contact resistance should be composed of the following parts;
1) Concentrated
resistance When the current passes through the actual contact surface, the resistance displayed due to the contraction (or concentration) of the current line. It is called concentrated resistance or contraction resistance.
2) Film resistance
The film resistance formed by the contact surface film and other contaminants. From the analysis of the contact surface state; the surface contamination film can be divided into a more solid film layer and a looser impurity contamination layer. So to be precise, the film resistance can also be called interface resistance.
3) Conductor resistance
When the contact resistance of the electrical connector contact is actually measured, it is carried out at the contact lead end, so the actual measured contact resistance also includes the conductor resistance of the contact outside the contact surface and the lead wire itself. The conductor resistance mainly depends on the conductivity of the metal material itself, and its relationship with the ambient temperature can be characterized by the temperature coefficient.

For the sake of distinction, the concentrated resistance plus the film resistance is called the real contact resistance. The actual measured resistance including the conductor resistance is called the total contact resistance.
When measuring the contact resistance, a contact resistance tester (milliohmmeter) designed according to the principle of the Kelvin bridge four-terminal method is often used. Its special fixture is clamped at both ends of the terminal part of the contact piece to be measured. Therefore, the total contact resistance R actually measured is composed of the following three parts, which can be expressed by the following formula: R= RC + Rf + Rp, where: RC—concentrated resistance; Rf—film resistance; Rp—conductor resistance.
The purpose of the contact resistance test is to determine the resistance generated when the current flows through the electrical contact on the contact surface of the contact piece. If a large current passes through a high-resistance contact, it may cause excessive energy consumption and cause the contact to overheat dangerously. In many applications, the contact resistance is required to be low and stable so that the voltage drop on the contact does not affect the accuracy of the circuit condition. In
addition to the milliohmmeter, the volt-amperemeter method and the ampere-potentiometer method can also be used to measure the contact resistance.
In the circuit connecting weak signals, the set test conditions have a certain influence on the contact resistance test results. Because the contact surface will be attached with oxide layer, oil or other pollutants, the surface of the two contact parts will produce film resistance. Since the film layer is a poor conductor, the contact resistance will increase rapidly with the increase of film thickness. The film layer will mechanically break down under high contact pressure, or will break down electrically under high voltage and high current. However, the contact pressure designed for some small connectors is very small, and the working current and voltage are only mA and mV level. The film resistance is not easy to be broken down, and the increase of contact resistance may affect the transmission of electrical signals.
One of the contact resistance test methods in GB5095 "Basic Test Procedures and Measurement Methods for Electromechanical Components for Electronic Equipment", "Contact Resistance-Millivolt Method", stipulates that in order to prevent the film layer on the contact part from being broken down, the open circuit peak voltage of the test loop AC or DC should not be greater than 20mV, and the current in the AC or DC test should not be greater than 100mA.
In GJB1217 "Test Methods for Electrical Connectors", there are two test methods: "Low Level Contact Resistance" and "Contact Resistance". The basic content of the low-level contact resistance test method is the same as the contact resistance-millivolt method in the above-mentioned GB5095. The purpose is to evaluate the contact resistance characteristics of the contact under the conditions of voltage and current that do not change the physical contact surface or the possible non-conductive oxide film. The applied open circuit test voltage does not exceed 20mV, and the test current should be limited to 100mA. The performance at this level is sufficient to show the performance of the contact interface under low-level electrical excitation. The purpose of the contact resistance test method is to measure the resistance between the two ends of a pair of mating contacts or between the contact and the measuring gauge through a specified current. The specified current usually applied by this test method is much larger than that of the previous test method. As stipulated in the military standard GJB101 "General Specification for Small Circular Quick Separation Environmental Resistant Electrical Connectors"; the current is 1A during measurement, and after the contact pairs are connected in series, the voltage drop of each pair of contact pairs is measured, and the average value is taken to convert it into the contact resistance value.

Influencing factors
Mainly affected by factors such as contact material, positive pressure, surface state, operating voltage and current.
1) Contact material
The technical conditions of electrical connectors stipulate different contact resistance assessment indicators for the same specification of mating contacts made of different materials. For example, the general specification GJB101-86 for small circular quick-detach environmentally resistant electrical connectors stipulates that the contact resistance of mating contacts with a diameter of 1mm is ≤5mΩ for copper alloy and ≤15mΩ for iron alloy.
2) Positive pressure
The positive pressure of the contact refers to the force generated by the surfaces in contact with each other and perpendicular to the contact surface. As the positive pressure increases, the number and area of ​​contact micro-points also gradually increase, and at the same time, the contact micro-points transition from elastic deformation to plastic deformation. As the concentrated resistance gradually decreases, the contact resistance decreases. The positive contact pressure mainly depends on the geometric shape and material properties of the contact.
3) Surface state
The surface of the contact is a loose film formed by mechanical deposition of dust, rosin, oil, etc. on the contact surface. This film is very easy to embed in the microscopic pits on the contact surface due to the particulate matter, which reduces the contact area, increases the contact resistance, and is extremely unstable. The second is the pollution film formed by physical adsorption and chemical adsorption. The main adsorption on the metal surface is chemical adsorption, which is produced after physical adsorption and electron migration. Therefore, for some products with high reliability requirements, such as aerospace electrical connectors, there must be clean assembly production environment conditions, perfect cleaning processes and necessary structural sealing measures. The user unit must have good storage and operation environment conditions.
4) Use voltage When
the use voltage reaches a certain threshold, the contact film layer will be broken down, and the contact resistance will drop rapidly. However, due to the thermal effect, the chemical reaction in the area near the film layer is accelerated, which has a certain repair effect on the film layer. Therefore, the resistance value is nonlinear. Near the threshold voltage, a small fluctuation in the voltage drop may cause the current to change within a range of twenty times or dozens of times. The contact resistance changes greatly. If you do not understand this nonlinearity, you will make mistakes when testing and using the contact.
5) Current
When the current exceeds a certain value, the Joule heat () generated by the tiny points on the contact interface will soften or melt the metal, which will affect the concentrated resistance and reduce the contact resistance.

Problem Discussion
1) Low-level contact resistance test
Considering that the contact film layer may break down mechanically under high contact pressure or break down electrically under high voltage and high current. The contact pressure designed for some small-volume connectors is quite small, and the use occasion is only at the mV or mA level. The film resistance is not easy to be broken down, which may affect the transmission of electrical signals. Therefore, the national military standard GJB1217-91 electrical connector test method specifies two test methods. That is, the low-level contact resistance test method and the contact resistance test method. The purpose of the low-level contact resistance test is to evaluate the contact resistance characteristics of the contact under the conditions of voltage and current that cannot change the physical contact surface or the non-conductive oxide film that may exist. The applied open circuit test voltage does not exceed 20mV, and the test current should be limited to 100mA. The performance at this level is sufficient to meet the performance of the contact interface under low-level electrical excitation. The purpose of the contact resistance test is to measure the resistance between the two ends of a pair of mating contacts or between the contact and the measuring gauge through a specified current, and this specified current is much larger than the former, usually specified as 1A.
2) Single hole separation force test
To ensure reliable contact of the contact, it is crucial to maintain a stable positive pressure. Positive pressure is a direct indicator of contact pressure and significantly affects contact resistance. However, since it is difficult to measure the positive pressure of the contact in the plugged state, the single hole separation force that measures the contact from static to moving in the plugged state is generally used to characterize that the pin and the socket are in contact. Usually, the separation force requirements specified in the technical conditions of electrical connectors are determined by experimental methods, and its theoretical value can be expressed by the following formula.
F=FN·μ
, where FN is the positive pressure and μ is the friction coefficient.
Since the separation force is restricted by both the positive pressure and the friction coefficient. Therefore, it cannot be assumed that a large separation force means a large positive pressure and reliable contact. Now, with the improvement of contact manufacturing accuracy and surface plating quality, reliable contact can be guaranteed by controlling the separation force at an appropriate level. The author found in practice that if the single hole separation force is too small, it may cause a transient signal interruption when subjected to vibration impact load. It is more effective to evaluate contact reliability by measuring the single hole separation force than by measuring the contact resistance. Because in actual inspection, contact resistance parts rarely fail, and the contact resistance of the jack with low and out-of-tolerance separation force of the single hole is often still qualified.
3) Passing the contact resistance test does not mean reliable contact.
In many actual use occasions, automobiles, motorcycles, trains, power machinery, automation instruments, and military connectors such as aviation, aerospace, and ships are often used in dynamic vibration environments. Experiments have shown that only testing whether the static contact resistance is qualified cannot guarantee reliable contact in dynamic environments. Connectors with qualified contact resistance often still experience momentary power failure during simulated environmental tests such as vibration, impact, and centrifugation. Therefore, for some connectors with high reliability requirements, many designers have proposed that it is best to conduct 100% dynamic vibration tests on them to assess contact reliability. Recently, Japan's Naike Company launched a small desktop electric vibration table used in conjunction with a continuity tester, which has been successfully used in contact reliability tests for many civilian wiring harnesses.