Electrostatic discharge test (ESD)

4. Electrostatic discharge test (ESD)
Overview of Electrostatic Discharge Immunity Test
The national standard for electrostatic discharge immunity test (ESD) is GB/T17626.2 (equivalent to the international standard IEC61000-4-2).
Electrostatic discharge (ESD) is a natural phenomenon. Experience has shown that when people walk on a synthetic fiber carpet, through the friction between the shoes and the carpet, as long as walking a few steps, the accumulated charge on the human body can reach 10-6 coulombs or more ( It depends on the resistance between the shoe and the carpet. In such a "system" (human/carpet/earth) the average capacitance is about tens to hundreds of pF, and the possible voltage should be 15kV. Study different human body production Electrostatic discharge, there will be many different current pulses, the rise time of the current waveform is between 100ps and 30ns. Electronic engineers have found that electrostatic discharge occurs when the human body contacts the semiconductor device, which may cause several layers of semiconductor material to strike. Wear, causing irreparable damage. Electrostatic discharge and subsequent changes in the electromagnetic field may jeopardize the normal operation of the electronic device.
Electrostatic discharge immunity test
GB/T17626.2 domestic electrostatic discharge standard describes the human body charged by friction in low humidity environment. The human body with electricity can discharge the equipment during contact with the equipment. The electrostatic discharge immunity test simulates Two situations: (1) the discharge of the equipment when the equipment operator directly touches the equipment, and the impact of the discharge on the operation of the equipment; (2) the influence of the equipment operator on the equipment concerned when touching the adjacent equipment.
The former case is called direct discharge (direct discharge to the device); the latter case is called indirect discharge (by the discharge of adjacent objects, indirectly constitutes the impact on the operation of the device). The possible consequences of electrostatic discharge are:
(1) By direct discharge, causing damage to the semiconductor device in the device, resulting in permanent failure of the device.
(2) The near-field electromagnetic field change caused by discharge (may be direct discharge or indirect discharge), causing malfunction of the equipment.
Electrostatic discharge immunity standard
GB/T 18268
GB 4824
Simulation of electrostatic discharge immunity test
The high-voltage vacuum relay is currently the only device capable of generating repeating and high-speed discharge waveforms. The 150pF capacitor in the line represents the storage capacitor of the human body, and the 330Ω resistor represents the body resistance of the human body when holding the key and other metal tools. The human body discharge model (including capacitance and resistance value) to describe the electrostatic discharge is sufficiently harsh. From the discharge current waveform (the standard specifies the current waveform when the discharge electrode is used as a current sensor for the 2 Ω resistance contact discharge), it is foreseen that it is extremely rich. Harmonic composition, so it increases the severity of the test. As a check for the electrostatic discharge generator (this is the key to ensure the comparability and repeatability of the test results of different brands of discharge generators), the standard requirements are 4 Measurements are made at different voltages, using an oscilloscope with a bandwidth of at least 1 GHz.
Laboratory equipment electrostatic gun (Figure 3-1)
Discharge method in electrostatic discharge test
There are two kinds of electrostatic discharge tests: direct and indirect. The standard stipulates that direct discharge is the preferred method for contact discharge. Only when the contact discharge cannot be used, the air gap discharge is used.
For indirect discharge, the standard is to use metal plates to simulate the discharge objects in the vicinity of the equipment under test. Because it is a metal plate, the contact discharge is the preferred discharge method for indirect discharge.
The reason why the standard discharge discharge is the preferred method is because it has experienced the deficiencies exposed during the implementation of the IEC801-2 standard of the IEC61000-4-2 standard. The IEC801-2 standard uses air gap discharge as its only discharge method. In the standard implementation, the test results of this discharge mode are relatively poor in repeatability and comparability. The main reason for this situation is that the discharge current waveform of the air gap discharge is subject to the following factors:
1 the discharge electrode is close to the speed of the device under test;
2 the influence of the shape of the surface of the device under test on the field strength distribution of the electrode;
3 the impact of the environment (such as temperature, humidity and pressure) on the discharge;
4 The effect of discharge voltage on the spectrum of the discharge current waveform (for example, the 8kV discharge current rise time is typically 1ns - 5ns; the current rise time above 8kV may reach 5 - 30ns).
In addition, the air gap discharge is the only discharge method in the IEC801-2:1984 standard, and it is also related to the failure to find a suitable measuring instrument at that time (the bandwidth of the oscilloscope is not enough to observe the waveform with a rising rate of 1 ns).
The various drawbacks caused by air gap discharge on the test results were corrected in the draft IEC801-2 standard published in 1991. The draft decided to use contact discharge as the preferred discharge method. Air gap discharge is still reserved as a device for non-conductive surfaces ( Test method such as plastic case, or metal case with surface insulation. This case continues to the IEC61000-4-2 standard.
Type testing in electrostatic discharge laboratories
Electrostatic discharge is divided into two types: type test and field test. The standard test stipulates that the type test of the laboratory is the only test method used for equipment identification and certification. The field test is subject to the on-site environment and is mainly used for the scene situation, but not As an identification test. The field test can only be carried out after the unanimous consent of both the user and the manufacturer.
Electrostatic discharge test configuration
Since the current waveform of the electrostatic discharge is very steep, the front edge has reached 0.7-1 ns, and the harmonic components contained in it must be at least 500 MHz. Therefore, the standardization of the test configuration in the laboratory is one to ensure the repeatability and comparability of the test results. The key. The configuration can be made by the user. The standard stipulates this, which comes down to the following points:
(1) The reference grounding plate adopts 0.25mm copper plate or aluminum plate (the aluminum plate is easy to be oxidized and used with caution). If other metals are used, the thickness is at least 0.65mm. The actual size of the reference grounding plate is not limited, and all the requirements are exceeded. Refers to the ground equipment) or the test table countertop horizontal coupling plate (for desktop equipment) more than 0.5m on each side. The reference grounding plate should be connected to the protective earthing wire of the test room.
(2) The material of the horizontal coupling plate (only for the desktop device) and the vertical coupling plate (the latter has the insulating bracket) is the same as the reference grounding plate. The two coupling plates each have a cable with a 470kΩ resistor at both ends and a reference connection. The floor is connected to discharge the electrostatic charge in the test. The required resistance is required to withstand the discharge; the entire cable is insulated to avoid short circuit with the ground plate.
(3) For desktop equipment, a 0.5mm insulating sheet is placed on the horizontal coupling plate, and the board is required to not accumulate charge in the test. In the bench test, the horizontal coupling board is at least 0.1m larger than each side of the sample. If the sample is too large, either use a larger test bench; or use two identical test stands to place the test piece. The horizontal coupling plates on the table do not need to be welded together, but can be combined at the junction of two tables. A piece of metal of the same material can be pressed more than 0.3m from each tabletop. However, the horizontal coupling plates of the two tables are required to be connected to the reference grounding plate by resistor wires.
(4) For ground equipment, a 0.1m high insulation support shall be provided on the reference grounding plate, and the test sample and test cable shall be placed on the insulating support.
(5) All connecting wires (including the grounding cable of the reference grounding plate; the connecting cable with resistor on the coupling plate; and the grounding return of the discharge gun to the reference grounding plate) must maintain a low impedance connection.
(6) Other places to pay attention to
A. There should be no walls or other metal objects (including instruments) within 1m of the test sample.
B. The test samples in the test should be laid out as much as possible according to the actual situation (including power lines, signal lines and mounting feet, etc.). The grounding wire should be grounded according to the manufacturer's specifications (there is no connection without the grounding wire). Additional grounding wire.
C. When discharging, the grounding return line of the discharge gun should be at least 0.2m away from the surface of the sample to avoid additional induction between each other, which will affect the test results.
II. Electrostatic discharge test method
The standard stipulates that when the equipment under test is working normally, the parts that can be touched by the human hand are the parts that need to be subjected to the electrostatic discharge test (such parts, except the case, such as the control keyboard, display screen, indicator light, knob, Keyholes, power cords, etc. are all within the scope of assessment).
During the test, the equipment under test was in normal working condition.
Before the test is officially started, the tester quickly scans the surface of the test sample at a discharge rate of 20 times/second to find the sensitive part of the test sample (where there is a part of the scan that causes the sample to be beaten and the movement is abnormal). The key examination sites during the formal test shall be recorded and additional test points shall be added around it during the formal test).
In the formal test, the discharge is performed at a rate of 1 time/second (the product is also specified as 1 time/5 seconds), so that the sample can be responded to. It is usually discharged 20 times for each selected point (10 of which are positive) And 10 times are negative).
In principle, all places where contact discharge can be used are contact discharge. For galvanized casings, if the manufacturer does not specify insulation, the tip of the discharge gun is used to puncture the paint film to discharge the sample. If the manufacturer's instructions are for insulation use, then use air gap discharge. For the air gap discharge, a semi-circular head electrode should be used. Before each discharge, the discharge gun should be removed from the surface of the test sample, and then The discharge gun slowly approaches the sample until the discharge occurs. To improve the repeatability and comparability of the test results, the discharge electrode should be perpendicular to the surface of the sample.
Indirect discharge:
1 pair of horizontal coupling plates, the discharge gun discharges vertically in the contact discharge mode 0.1m away from the sample.
2 pairs of vertical coupling plates, the coupling plate should be placed 0.1m away from the test piece, and the discharge gun should be discharged perpendicular to the center of a vertical side of the coupling plate. Vertically coupled plates should be used for the four faces in the vertical direction of the test piece. Do an indirect discharge test.
Electrostatic discharge field test
Field test is a test carried out under the installation conditions of the equipment. Usually, after the equipment is frustrated on the spot, it is tested by the manufacturer and the user.
For the on-site test arrangement, in order to facilitate the connection of the ground return cable of the ESD gun, the reference grounding plate should be laid on the ground and kept at a distance of 0.1 m from the sample and its system. The material and thickness of the reference grounding plate are The requirements for laboratory configuration are the same. When the conditions permit, the size of the grounding plate is 0.3m × 2m. The reference grounding plate is safely connected to the site, or connected to the grounding terminal of the sample and its system. The ground return line is connected to the reference ground plane near the sample and its system discharge point.
When the test sample and its system are installed on a metal table, and the metal table is not securely connected to the site, the metal table should be connected to the reference ground plane through a 470kΩ resistor wire at both ends. Prevent static electricity accumulation.
Electrostatic discharge test level
The standard divides the test level into four levels: the contact discharge is set to 2kV, 4kV, 6kV and 8kV, respectively; the air gap discharge is set to 2kV, 4kV, 8kV and 15kV, respectively.
The choice of test level is related to environmental factors (the dryer the environment, the higher the test voltage level). However, for a specific product, it is often given in the corresponding product family standard or product standard test (with the test sample). The conformity assessment criteria are also given).
Electrostatic discharge test standard review
The reason why the standard uses contact discharge as the preferred method of discharge is because there are fewer uncertainties in contact discharge.
2 Contact discharge has extremely steep rise time, so the discharge current waveform contains extremely rich harmonic components. Even if a lower test voltage is selected, it can be more strict than the higher voltage air gap discharge in the same level. Test Results.
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