1. Whether it is a high-frequency electrical connector or a low-frequency electrical connector, contact resistance, insulation resistance, and dielectric withstand voltage (also known as electrical strength) are the most basic electrical parameters to ensure that electrical connectors can work normally and reliably. Usually, electrical The quality consistency inspection of the technical conditions of connector products has clear technical index requirements and test methods. These three inspection items are also an important basis for users to judge the quality and reliability of electrical connectors.
However, according to the author's years of experience in testing electrical connectors, there are many inconsistencies and differences in the specific implementation of relevant technical conditions between manufacturers and between manufacturers and users. Differences in factors such as operating methods, sample handling, and environmental conditions directly affect the accuracy and consistency of test results. To this end, the author believes that it is very beneficial to improve the test reliability of electrical connectors to conduct some special discussions on the problems existing in the actual operation of these three conventional electrical performance test items.
In addition, with the rapid development of electronic information technology, a new generation of multi-function automatic testers is gradually replacing the original single-parameter tester. The application of these new test instruments will greatly improve the detection speed, efficiency, accuracy, and reliability of electrical properties.
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2 Contact resistance test
2.1 Principle of action
Observing the surface of the connector contacts under a microscope, although the gold plating is very smooth, bumps of 5-10 microns can still be observed. It can be seen that the contact of the paired contact pair 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 smoothness of the surface and the magnitude of the contact pressure, the difference between the two can reach several thousand times. The actual contact surface can be divided into two parts; one is the real metal-to-metal direct contact part. That is, the contact micro-points without transition resistance between metals, also known as contact points, are formed after the interface film is damaged by contact pressure or heat. This part accounts for about 0% of the actual contact area of 5-1. The second is the parts that are in contact with each other after contaminating the film through the contact interface. Because any metal has a tendency to revert to its original oxide state. In fact, there are no really clean metal surfaces in the atmosphere. Even very clean metal surfaces exposed to the atmosphere can quickly form an initial oxide film of a few microns. For example, it only takes 2-3 minutes for copper, 30 minutes for nickel, and 2-3 seconds for aluminum to form an oxide film with a thickness of about 2 microns on the surface. Even the particularly stable precious metal gold will form an organic gas adsorption film on its surface due to its high surface energy. In addition, dust and the like in the atmosphere also form a deposited film on the contact surface. Therefore, from a microscopic analysis point of view, any contact surface is a contaminated surface.
In summary, the real contact resistance should be composed of the following parts;
1) Concentrate on resistance!
The resistance exhibited by the contraction (or concentration) of the current line when the current passes through the actual contact surface. Call it concentrated resistance or contraction resistance.
2) Membrane resistance
Sheet resistance due to contact surface films and other contaminants. From the analysis of the contact surface state; the surface fouling film can be divided into a firmer film layer and a looser impurity contamination layer. Therefore, to be precise, the membrane resistance can also be called interface resistance.
3) Conductor resistance!
When actually measuring the contact resistance of the contacts of the electrical connector, it is all carried out at the contact terminals, so the actual measured contact resistance also includes the conductor resistance of the contacts outside the contact surface and the resistance of the lead 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 a temperature coefficient.
For the convenience of distinction, the concentrated resistance plus the thin film resistance is called the real contact resistance. The actual measured resistance including the conductor resistance is called the total contact resistance.
In the actual measurement of contact resistance, a contact resistance tester (milliohm meter) designed according to the principle of the Kelvin bridge four-terminal method is often used. The resistance R consists 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 that occurs when current flows through the electrical contacts of the contact surfaces. When large currents flow through high resistance contacts, excessive energy consumption and dangerous overheating of the contacts can occur. Low and stable contact resistance is required in many applications so that the voltage drop across the contacts does not affect the accuracy of circuit conditions.
In addition to milliohm meters, voltammetry and amperometric potentiometers can also be used to measure contact resistance.
In the connection of weak signal circuits, the set test parameter conditions have a certain influence on the contact resistance test results. Because oxide layers, oil, or other contaminants will adhere to the contact surface, film resistance will develop between the surfaces of the two contact sites. Since films are poor conductors, contact resistance increases rapidly with increasing film thickness. Membranes undergo mechanical breakdown under high contact pressure or electrical breakdown under high 0 voltage and high current. However, for some small connectors, the contact pressure is very small, the working current and voltage are only MA and MV levels, the film resistance is not easily broken down, and the increase in contact resistance may affect the transmission of electricity. Signal.
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 breakdown on the contact piece, the test circuit AC or DC open circuit peak voltage It is not more than 20MV, and the current is not more than 100MA during AC or DC testing.
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 CO contact under voltage and current application conditions that do not alter the physical contact surface or alter the non-conductive oxide film that may be present. The applied open circuit test voltage shall not exceed 20MV, and the test current shall be limited to 100MA. This level of performance is sufficient to represent the performance of the contact interface at low levels of electrical excitation. The purpose of the contact resistance test method is to measure the resistance between the ends of a pair of mating contacts or between the contacts and the measuring gauge by means of a specified current. Typically this test method applies a much higher specified current than previous test methods. Complying with the national military standard GJB101 "General Specification for Small Circular Quick Separation Environmental Resistant Electrical Connectors"; the current during measurement is 1A. After connecting the contact pairs in series, measure the voltage drop across each contact pair and convert the average value to contact resistance. value.
2.2 Influencing factors
Mainly affected by factors such as contact material, positive pressure, surface state, working voltage, and current.
1) Contact material
The technical conditions of electrical connectors stipulate that the contact heads of the same specification made of different materials have different contact resistance evaluation indicators. For example, according to the general specification GJB101-86 of the small round quick separation environment-resistant electrical connector, the contact resistance of the mating contact with a diameter of 1MM, copper alloy ≤ 5MΩ, iron alloy ≤ 15MΩ.
2) Positive pressure
The positive pressure of a contract is the force generated by the surfaces in contact with each other, perpendicular to the contact surface. With the increase of the positive pressure, the number and area of the contact micro-points also gradually increased, and the contact micro-points transitioned from elastic deformation to plastic deformation. Since the concentrated resistance gradually decreases, the contact resistance decreases. The contact positive pressure mainly depends on the contact geometry and material properties.
3) Surface condition
The first contact surface is a looser film formed by mechanical adhesion and deposition of dust, rosin, oil, etc. on the contact surface. Due to the particulate matter, the film is easily embedded in the microscopic pits of the contact surface. The area decreases, the contact resistance increases, and it is extremely unstable. Second, the fouling film formed by physical adsorption and chemical adsorption is mainly chemical adsorption on the metal surface, which is generated with the migration of electrons after physical adsorption. Therefore, some products with high-reliability requirements, such as aviation electrical connectors, must have clean assembly and production environmental conditions, perfect cleaning processes, and necessary structural sealing measures, and users must have good storage and use environmental conditions.
4) Use a voltage
When the operating voltage reaches a certain threshold, the film layer of the contact sheet will be broken down, and the contact resistance will drop rapidly. However, because the thermal effect accelerates the chemical reaction near the film, it has a certain repairing effect on the film. Therefore, the resistance value is non-linear. Around the threshold voltage, small fluctuations in the voltage drop can cause the current to vary by a factor of perhaps twenty or tens of times. Contact resistance varies widely, and without understanding this non-linear error, errors can occur when testing and using contacts.
5) Current
When the current exceeds a certain value, the Joule heat () generated by electrification at the tiny point of the contact interface will soften or melt the metal, affecting the concentrated resistance, and thereby reducing the contact resistance.