Electrostatic discharge is a natural phenomenon. Experience has shown that when people walk on synthetic fiber carpets, after only a few steps the amount of charge accumulated on the human body can reach 10-6 Coulomb (depending on the resistance between the shoe and the carpet) through the friction between shoes and carpets, and the average capacitance in such a "system" (man / carpet / earth) is about several tens to hundreds of pF, and the potential voltage to be up to 15kV. There will be many different current pulses in the study of electrostatic discharge generated by different human bodies. The rising time of current waveform is between 100ps and 30ns. Electronic engineers have found that electrostatic discharge occurs when the human body contacts with semiconductor devices, which may lead to the breakdown of several layers of semiconductor materials, resulting in irreparable damage. Electrostatic discharge and the subsequent electromagnetic field changes may endanger the normal operation of electronic equipment.
1. Standard for static discharge immunity test
The national standard of electrostatic discharge immunity test (ESD) is GB/T17626.2 (equivalent to international standard IEC 61000-4-2). GB/T17626.2, the domestic standard of electrostatic discharge describes that the human body is charged by friction in low humidity environment.
The charged human body may discharge on the equipment during contact with the equipment. Electrostatic discharge immunity test simulated two cases: a. the discharge of human body to equipment when operator touches the equipment directly, and the influence of the discharge on the work of the equipment; b. the influence of the operator on the equipment when he touches the adjacent equipment. The former is called direct discharge (direct discharge to the equipment); and the latter is called indirect discharge (through discharge to the adjacent equipment, indirectly affect the work of the equipment). The possible consequences of electrostatic discharge are: a. damage the semiconductors in the equipment by direct discharge, resulting in permanent failure of the equipment. b. changes in near-field electromagnetic fields caused by discharge (either direct or indirect discharge), resulting in malfunction in the equipment..
2. ESD immunity test grade
|1a Contact discharge||1b Air discharge|
|Grade||Test Voltage/kv||Grade||Test Voltage /kv|
3. Test Method
Electrostatic discharge test is divided into two types of laboratory type test and field test. The standard stipulates that the type test of laboratory is the only test method used for equipment identification and certification. The field test is restricted by the field environment and is mainly used in the field situation mapping, but cannot be used as the appraisal test. Site tests must be conducted with the consent of both the user and the manufacturer.
1) Laboratory type test of electrostatic discharge
a. Test Configuration
Because the current waveform of electrostatic discharge is very steep, the front line has reached 0.7 ~ 1ns, and its harmonic component must be at least above 500MHz, so the standard of test configuration in the laboratory is a key to ensure the repeatability and comparability of test results. The configuration can be made by the user himself, which is specified in the standard, and comes down to the following:
(1) The reference floor is made of copper or aluminum plate above 0.25mm thickness (aluminum plate is easily to be oxidized, should be used with caution). If other metals are used, the thickness is at least 0.65mm. There is no restrictions regarding the actual size of the plate, but it should at least 0.5m exceed each side of the tested equipment (refers to the ground equipment) or the horizontal coupling plate of the test table (used for desktop equipment). The reference floor shall be connected to the protective ground wire of the laboratory.
(2) Materials of horizontal coupling plate (desk equipment only) and vertical coupling board ( having insulating support) are the same as those of reference floor. One cable with 470kΩ resistor at both ends of the two coupling plates is connected to the reference connection floor in order to discharge the electrostatic charge in the test. The resistor used is required to withstand discharge. The whole cable is insulated to avoid short circuit with the floor.
(3) For desktop equipment, an insulating sheet of 0.5mm is coated on the horizontal coupling plate, which requires that the electric charge is not accumulated obviously. In the desktop test, the horizontal coupling plate is at least 0.1 m larger than each side of the sample. If the test equipment is too large, either use a larger test bed, or use two test beds of the same test stands. The horizontal coupling plate on the two test beds do not have to be welded together, but can be covered with a piece of metal of the same material at the junction, as long as the metal sheet is 0.3m more exceeding the edge of each desktop. But the horizontal coupling board of the two tables is required to be connected to the reference floor through resistance line.
(4) For ground equipment, there should be a 0.1m high insulation support on the reference floor, the test equipment and test cable should be placed on the insulation support.
(5) All connectors (including ground cables of reference floor; cables with resistance on the coupling board; and grounding loop of discharge guns connected to the reference floor, etc.) must be connected with low impedance.
(6) Other points of concern
A. There should be no walls or other metal items (including instruments) within 1m from the tested equipment
B. The test should be as practical as possible (including power lines, signal wires, mounting pins, etc.). Ground wires should be grounded according to the factory regulations (if no ground wires, this is not necessary), and no additional ground wires are allowed.
C. During discharge, the distance between the grounding loop of the discharge gun and the surface of the sample is at least 0.2m to avoid additional induction between each other and affecting the test results.
b. Test method for electrostatic discharge
According to the standard, the parts that can be touched by the hands when the equipment is working normally are the areas where electrostatic discharge tests are required (such parts as controlling keyboards, display screens, indicator lights, knobs, keyhole, power line other than housing and so on are in the scope of examination.)
During the test, the equipment under test is in normal working condition.
Before the formal start of the test, the experimenter quickly scan the surface of the sample at a rate of 20 times/s in order to find the sensitive part of the sample (areas in the scan that cause the specimen to jump in number, or show signs of abnormal movement should be recorded as a key site in the formal test, and several additional sites around the area should be added to the formal test.
During formal testing, the discharge is performed at a rate of 1 time/sec (also some products specified as 1time/5 seconds) so that the sample can respond in time. There are usually 20 discharges at each selected point (of which 10 are positive and 10 are negative).
In principle, contact discharge is used wherever contact discharge can be used. For the coated case, if the manufacturer does not specify that it is insulated, the sample will be discharged by puncturing the paint film with the tip of the discharge gun during the test. If the manufacturer defined that it is used for insulation, use air gap discharge. A semi-circular electrode should be used for air gap discharge. Before each discharge, the discharge gun should be removed from the surface of the sample, and then the discharge gun should be slowly close to the sample until the discharge occurs. In order to improve the repeatability and comparability of the test results, the discharge electrode should be perpendicular to the surface of the specimen.
① For the horizontal coupling plate, the discharge gun discharges perpendicularly at 0.1m away from the sample by contact discharge.
②For the vertical coupling plate, the coupling plate should be placed at a distance of 0.1m from the sample, and the discharge gun should be perpendicular to the center of a vertical edge of the coupling plate. Indirect discharge tests are performed on all four vertical surfaces of the sample using a vertical coupling plate.
2) Electrostatic discharge field test
Field testing is a test of equipment under installation conditions. The equipment is usually tested as a mapping by the manufacturer and the user after site frustration.
Site test configuration: In order to facilitate the connection of the grounding cable of the electrostatic discharge gun, the reference connection floor should be laid on the ground, and kept a distance of 0.1m from the tested sample and its system. Material and thickness of the reference floor are the same as the laboratory configuration requirements. The size of the floor is 0.3m × 2m if conditions permit. The reference floor is connected to the site safety ground, or to the grounding terminal of the tested equipment and its system. The grounding loop of the generator is connected to the reference grounding plane near the discharge point of the tested sample and its system.
When the tested sample and its system are installed on the metal table, and the metal table is not connected to the field safety ground, the metal table should be connected to the reference grounding plane through a resistance line with 470kΩ at both ends to prevent electrostatic accumulation.