Connector contact resistance inspection technology

The surface of the connector contact was observed under a microscope, and although the gold plating layer was very smooth, a convex portion of 5-10 μm was observed. It will be seen that the contact of the pair of contacts that are inserted is not the contact of the entire contact surface, but the contact of some points scattered on the contact surface. Principle of action The surface of the connector contacts is observed under a microscope. Even though the gold plating is very smooth, a 5-10 micron raised portion can still be observed. It will be seen that the contact of the pair of contacts that are inserted 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 contact pressure, the difference between the two can be several thousand times. The actual contact surface can be divided into two parts; the first is the direct contact between the metal and the metal. That is, contact micro-dots without transition resistance between metals, also known as contact spots, which are formed by contact pressure or thermal damage to the interface film. This part accounts for about 5-10% of the actual contact area. The second is the part that contacts each other after contaminating the film 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, and even a very clean metal surface, once exposed to the atmosphere, will quickly form a few microns of initial oxide film. For example, copper is only required for 2-3 minutes, nickel is about 30 minutes, aluminum is only required for 2-3 seconds, and an oxide film layer having a thickness of about 2 μm can be formed on the surface. Even a particularly stable precious metal gold, due to its high surface energy, forms an organic gas adsorption film on its surface. In addition, dust or the like in the atmosphere also forms a deposited film on the surface of the contact. Thus, any contact surface from a microscopic analysis is a contaminated surface. In summary, the true contact resistance of the connector should be composed of the following parts: 1) Centralized resistance. When the current passes through the actual contact surface, the current line shrinks (or concentrates) to show the resistance. Call it a concentrated resistor or a shrink resistor. 2) Film resistance The film resistance caused by contact with the surface film and other contaminants. From the state of the contact surface analysis; the surface contamination film can be divided into a relatively solid film layer and a loose impurity contamination layer. Therefore, the film resistance can also be referred to as interface resistance. 3) Conductor resistance When actually measuring the contact resistance of the contact of the electrical connector, it is performed at the contact terminal, so the actual measured contact resistance also includes the conductor resistance of the contact member other than the contact surface and the lead wire itself. The conductor resistance mainly depends on the electrical conductivity of the metal material itself, and its relationship with the ambient temperature can be characterized by a temperature coefficient. For the sake of distinction, the concentrated resistance plus the layer resistance is called the true 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 (millimeter) designed according to the four-terminal method of the Kelvin bridge is often used. The special fixture is clamped at the ends of the contact part of the contact to be tested, so the total contact measured is actually measured. The resistor R is composed of the following three parts and can be represented 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 current flows through the electrical contacts of the contact surface of the contact. If a large current is passed through the high-impedance contact, excessive energy consumption may occur and the contact may be exposed to dangerous overheating. In many applications, the contact resistance is required to be low and stable so that the voltage drop across the contacts does not affect the accuracy of the circuit conditions. In addition to the milliohm meter, the contact resistance can also be measured by the volt-ampere method, the amperometric-potentiometer method. In the connection weak signal circuit, the set test parameter conditions have a certain influence on the contact resistance detection result. Because the contact surface will be coated with an oxide layer, oil or other contaminants, the surface resistance of the two contact surfaces will be generated. Since the film layer is a poor conductor, as the thickness of the film layer increases, the contact resistance increases rapidly. The film will mechanically break down under high contact pressure, or electrical breakdown will occur at high voltage and high current. However, the contact pressure of some small connectors is very small, the working current voltage is only mA and mV level, the film resistance is not easy to be broken down, and the contact resistance increase may affect the transmission of electrical signals. One of the contact resistance test methods in GB5095 "Electrical and Mechanical Equipment for Electronic Equipment" target=_blank>Electrical Components Basic Test Procedures and Measurement Methods", "Contact Resistance - Millivolt Method" stipulates that in order to prevent the film on the contact layer from being broken down, The open circuit peak voltage of the test circuit AC or DC should be no more than 20mV, and the current in the AC or DC test should be no more than 100mA. In the GJB1217 "Electrical Connector Test Method", there are "low level contact resistance" and "contact resistance". Two test methods. The basic content of the low-level contact resistance test method is the same as the contact resistance-millivolt method in GB5095 above. The purpose is to evaluate that the contact does not change the physical contact surface or change the non-conductive oxidation that may exist. The contact resistance characteristics of the film under voltage and current conditions. The applied open circuit test voltage does not exceed 20 mV, and the test current should be limited to 100 mA. The performance at this level is sufficient to exhibit the contact interface under low-level electrical excitation. Performance. The contact resistance test method is designed to measure the ends of a pair of mating contacts or contacts and measurements through a specified current. The resistance between the gauges. Generally, the specified current applied by this test method is much larger than the previous test method. For example, the national military standard GJB101 "General specification for small circular fast separation and environmentally friendly electrical connectors"; When the current is 1A, after the contact pair is connected in series, the voltage drop of each pair of contacts is measured, and the average value is converted into the contact resistance value. http://?itemid=360 Edit: (Hardware Business Network Information Center)

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