Detailed connector detection

　　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 dielectric strength) are the most basic electrical parameters to ensure that the electrical connector can work normally and reliably. Usually, clear technical index requirements and test methods are listed in the routine delivery inspection items of group A and group B of the quality consistency inspection of the electrical connector product technical conditions. 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 many years of practice in electrical connector inspection; at present, there are many inconsistencies and differences in the specific implementation of the relevant technical conditions between the production plants and between the production plants and the use plants, often due to the instruments and test tooling used. The difference in factors such as operating methods, sample processing and environmental conditions directly affect the accuracy and consistency of the test results. For this reason, the author believes that it is very beneficial to improve the reliability of electrical connector inspection by conducting some special discussions on the problems existing in the actual operation of the current three conventional electrical performance inspection items.

　　In addition, with the rapid development of electronic information technology, a new generation of multifunctional automatic testers is gradually replacing the original single-parameter testers. The application of these new test instruments will greatly improve the testing speed, efficiency and accuracy and reliability of electrical performance.

　　specific:

　　2 Contact resistance test

　　2.1 Principle of action

　　Observing the surface of the connector contact under a microscope, although the gold-plated layer is very smooth, you can still observe the convex part of 5-10 microns. You will see the contact of the mating pair of contacts, 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 degree of surface smoothness and contact pressure, the difference between the two can be several thousand times. The actual contact surface can be divided into two parts; one is the direct contact part between real metal and metal. That is, the contact micro-points with no transition resistance between metals, also known as contact spots, are formed after the interface film is destroyed by contact pressure or heat. This part accounts for about 5-10% of the actual contact area. The second is the part that touches each other after contaminating the film through the contact interface. Because any metal has a tendency to return to its original oxide state. In fact, there is no truly clean metal surface in the atmosphere. Once exposed to the atmosphere, even a very clean metal surface will quickly form an initial oxide film of several microns. For example, copper takes only 2-3 minutes, nickel takes about 30 minutes, and aluminum takes only 2-3 seconds, and an oxide film with a thickness of about 2 microns can be formed on the surface. Even the particularly stable precious metal gold, due to its higher surface energy, will form a layer of organic gas adsorption film on its surface. In addition, dust in the atmosphere will also form a deposited film on the surface of the contact. Therefore, any contact surface is a contaminated surface from a microscopic analysis.

　　In summary, the true contact resistance should consist 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. This is called concentrated resistance or shrinking resistance.

　　2) Film layer resistance

　　The resistance of the film due to contact with the surface film and other contaminants. Analyze from the contact surface state; the surface pollution film can be divided into a firmer film layer and a looser impurity pollution layer. Therefore, to be precise, the film layer resistance can also be called the interface resistance.

　　3) Conductor resistance!

　　The actual measurement of the contact resistance of the contacts of the electrical connector is carried out at the lead-out end of the contact, so the actual measured contact resistance also includes the conductor resistance of the contact outside the contact surface and the lead wire itself. Conductor resistance mainly depends on the conductivity of the metal material itself, and its relationship with the ambient temperature can be characterized by temperature coefficient.

　　In order to facilitate the distinction, the concentrated resistance plus the film layer resistance is called the true contact resistance. The actual measurement 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, and its special fixture is clamped at both ends of the terminal part of the tested contact, so the actual measured total contact The resistance R 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 layer resistance; Rp—conductor resistance.

　　The purpose of the contact resistance test is to determine the resistance generated when current flows through the electrical contacts of the contact surface of the contact. If a large current flows through the high-resistance contacts, excessive energy consumption may occur and the contacts may cause dangerous overheating. In many applications, the contact resistance is required to be low and stable, so that the voltage drop on the contacts does not affect the accuracy of the circuit condition.

　　In addition to the milliohmmeter, the volt-ammeter method and the ampere-potentiometer method can also be used to measure the 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 the contact surface will be attached with an oxide layer, oil stains or other contaminants, film resistance will occur on the surfaces of the two contact parts. Since the film is a poor conductor, as the thickness of the film increases, the contact resistance will increase rapidly. The film layer will be mechanically broken down under high contact pressure, or electrical breakdown will occur under high zero voltage and large current. However, the contact pressure designed for some small connectors is very small, the working current and voltage are only mA and mV, the film resistance is not easy to be broken down, and the increase in contact resistance may affect the transmission of electrical signals.

　　One of the contact resistance test methods in GB5095 "Basic Test Procedures and Measurement Methods of Electromechanical Components for Electronic Equipment", the "Contact Resistance-Millivolt Method" stipulates that in order to prevent the film on the contact from being broken down, the test loop is AC or DC. The open-circuit peak voltage 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 GB5095. The purpose is to evaluate the contact resistance characteristics of the contact under the condition of adding voltage and current that does not change the physical contact surface or does not change the non-conductive oxide film that may exist. The applied open circuit test voltage should 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 plug-in contacts that pass a specified current or between the contact and the measuring gauge. Generally, the prescribed current applied by this test method is much larger than that of the previous test method. As stipulated in the National Military Standard GJB101 "General Specification for Small Circular Quick-Separation Environmental Resistant Electrical Connectors"; the current is 1A during measurement, after the contact pairs are connected in series, the voltage drop of each contact pair is measured, and the average value is converted into contact resistance. value.

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