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Electrostatic discharge generator (ESD) test methods and standards

1.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, with only a few steps, the accumulated electric charge in the human body can reach more than 10-6 coulombs (depending on the resistance between the shoes and the carpet). The average capacitance in the "system" (human/carpet/earth) is about tens to hundreds of pF, and the potential voltage generated should reach 15 kV. There are many different current pulses when studying electrostatic discharges generated by different human bodies. The rise time of the current waveform is between 100 ps and 30 ns. Electronic engineers have found that electrostatic discharge mainly occurs when the human body comes into contact with semiconductor devices, which can lead to breakthroughs in several layers of semiconductor materials, causing irreversible damage. Electrostatic discharges and subsequent changes in electromagnetic fields can damage the normal operation of electronic devices.


2. Electrostatic discharge immunity test method

The domestic electrostatic discharge standard GB/T17626.2 describes the possibility that human bodies charged by friction in low humidity environments may discharge when in contact with equipment. The electrostatic discharge reactance interference test simulates two situations: ⑴ The discharge and impact of direct contact with equipment by equipment operators on equipment operation; ⑵ When equipment operators touch adjacent equipment, they care about the impact of the equipment.

The former is called direct discharge (direct discharge of equipment), and the latter is called indirect discharge (indirect impact on equipment operation through discharge to adjacent objects). The consequences of electrostatic discharge are:

(1) The semiconductor devices in the equipment are damaged by direct discharge, resulting in permanent failure of the equipment.

(2) The near field electromagnetic field changes due to discharge (either direct or indirect), resulting in equipment malfunction.


3. Electrostatic discharge immunity standard

GB17626.2, YY0505, GB/T18268, EN55024, EN61547, etc


4.Electrostatic discharge simulator

The high-voltage vacuum relay is the only device capable of generating repetitive and high-speed discharge waveforms. The 150pF capacitance in the circuit represents the energy storage capacitance of the human body. When 330 Ω hand-held keys and other metal tools are used, the resistance represents the resistance of the human body. According to this standard, Using this human discharge model (including capacitance and resistance values) to describe electrostatic discharge is sufficiently severe. It can be predicted from the discharge current waveform (according to the standard, the current waveform when the discharge electrode contacts a 2 Ω resistor as a current sensor) that it is rich in harmonic components, which increases the severity of the test

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As a verification of electrostatic discharge generators (which is the key to ensuring the comparability and repeatability of test results for different brands of discharge generators), the standard requires an oscilloscope with a bandwidth of at least 1 GHz measured at four different voltages.


5. Discharge method in electrostatic discharge test

There are two types of electrostatic discharge tests: direct discharge and indirect discharge. According to the standard, contact discharge is the first choice for direct discharge, and air gap discharge can only be conducted in places where contact discharge is not possible. For indirect discharge, the standard is to use a metal plate to simulate the discharge object near the equipment under test. Due to the metal plate, contact discharge is the preferred method for indirect discharge.

The reason why this standard uses contact discharge as the preferred discharge method is that it has experienced the shortcomings exposed by the predecessor of the IEC 61000-4-2 standard, IEC, during the implementation of the 801-2 standard. The IEC801-2 standard takes air gap discharge as the only discharge method, and it is found that the test results of this discharge method have poor repeatability and comparability.


The main reason for this situation is that the discharge current waveform of the air gap discharge is affected by the following factors:

① The influence of the surface shape of the tested equipment on the electrode field strength distribution;

② The impact of environment (such as temperature, humidity, and air pressure) on discharge;

③ The speed at which the discharge electrode approaches the equipment under test;

④ (For example, 8) Effect of discharge voltage on the discharge current waveform spectrum The typical value of the kV discharge current rise time is 1 ns to 5 ns; The current rise time above 8 kV can reach 5~30 ns).

In addition, the IEC air gap discharge is the only discharge method in the 801-2:1984 standard, and it was also related to the inability to find suitable measuring instruments at that time (the bandwidth of the oscilloscope was insufficient to observe a waveform with a rise rate of 1 ns).

The drawbacks of air gap discharge to test results published in 1991 were corrected in the draft IEC801-2 standard, which decided to use contact discharge as the preferred discharge method. Air gap discharge remains the test method for non-conductive surface devices such as plastic chassis or surface composite insulated metal enclosures. This situation continued until the issuance of the IEC61000-4-2 standard.


6. Type test in electrostatic discharge laboratory

Electrostatic discharge is divided into type test and field test. According to standards, laboratory type testing is the only test method for equipment qualification and certification. Field testing is mainly used for site conditions due to the impact of the site environment, and cannot be used as a qualification test. Field testing can only be conducted with the consent of the user and manufacturer.


I. Electrostatic discharge test configuration

Due to the steep current waveform of electrostatic discharge, the front line has reached 0.7 to 1 ns, and the harmonic component contained therein should reach at least 500 MHz. Therefore, the standardization of laboratory test configurations is the key to ensuring the repeatability and comparability of test results. Configurations can be customized by users, as specified in the standards, which are summarized as follows:

(1) The materials of the horizontal coupling plate (only for desktop devices) and the vertical coupling plate (the latter with insulating support) are the same as those of the reference floor. Each of the two coupling plates has one, and both ends are connected to a 470 k Ω resistance cable and the reference floor to release electrostatic charges during the discharge test. The required electrical resistance is capable of withstanding the discharge; The entire cable has insulation protection to avoid short circuits to the ground plate.

(2) The reference ground plate shall be made of the above copper or aluminum plate (aluminum plate is easy to oxidize, and should be used with caution). If other metals are used, the thickness shall be at least 0.65 mm or more. The actual size of the reference ground plate is infinite, and it is required that each side of the reference ground plate exceed the tested equipment (ground equipment) or the water level coupling plate (for desktop equipment) by more than 0.5 m. The ground plate shall be connected to the protective ground wire of the laboratory.

(3) All connecting wires (including the grounding cable on the reference ground plate, the connecting cable with resistance on the coupling plate, the grounding wire from the discharge gun to the reference ground plate, etc.) must maintain a low impedance connection.

(4) For desktop equipment, covering a 0.5mm insulating plate on the horizontal coupling plate requires no significant accumulation of electric charge during the test. In desktop equipment testing, the horizontal coupling plate is at least 0.1m larger than each side of the test product. If the test product is too large, either choose a larger test bench; Alternatively, select two identical test benches to place the test product. The horizontal coupling plates on the table top do not need to be welded together, but can be covered with a piece of metal of the same material at the combination of the two tables, as long as the pressure on each table top is above 0.3 m. However, the horizontal coupling plates of the two tables need to be connected to the reference floor using a resistance wire.

(5) For ground equipment, there should be a 0.1m high insulation support on the reference ground plate, with the test piece and the test piece cable placed on the insulation support.


(6) Other precautions

A. There should be no walls or other metal objects (including instruments) within 1m from the test object.

B. During discharge, the ground return line of the discharge gun and the surface of the test piece should be kept at least 0.2m apart to avoid additional induction between them, which may affect the test results.

C. Layout according to actual conditions (including power lines, signal lines, including power lines, signal lines, mounting pins, etc.) The grounding wire should be grounded according to the manufacturer's regulations (if there is no grounding wire, it will not be grounded), and no additional grounding wire is allowed.


II. Electrostatic discharge test method

The standard stipulates that when the equipment under test is working properly, the parts that people can touch are those that require electrostatic discharge testing (except for the housing, such as control keyboard, display screen, indicator light, knob, keyhole, power cord, etc.).

During the test, the equipment under test was in normal working condition.

Before the formal start of the test, the tester quickly scans the surface of the test product at a discharge rate of 20 times per second to find sensitive parts of the test product (parts that cause the number of jumps and abnormal movements of the test product during the scanning process should be recorded at the key parts of the formal test, and several test points should be added around them during the formal test).

During the formal test, the discharge is conducted at a speed of 1/second (also specified as 1/5 second for products), so that the test product can respond in a timely manner. Typically, each selected point is discharged 20 times (10 times are positive and 10 times are negative).

In principle, contact discharge should be used in all places where contact discharge can be used. For painted housings, if the manufacturer does not explain insulation, during the test, use the tip of the discharge gun to pierce the paint film discharge test piece. If the manufacturer explains insulation, change to air gap discharge. Air gap discharge should use a semicircular head electrode. Before each discharge, the discharge gun should be removed from the surface of the test piece, and then the discharge gun should slowly approach the test piece until the discharge occurs. In order to improve the repeatability and comparability of the test results, the discharge electrode should be perpendicular to the test surface.

Indirect discharge:

① For a horizontally coupled plate, the discharge gun is discharged through contact discharge with a vertical distance of 0.1 m from the test piece.

② For vertical coupling plates, the coupling plate should be placed in a distance of 0. 1 m from the test object. The discharge gun should discharge perpendicular to the center of the vertical edge of the coupling plate. Indirect discharge tests should be conducted using vertical coupling plates in the four vertical directions of the test product.


7. Electrostatic discharge field test

Field testing is the testing of equipment under installation conditions. Generally, after on-site equipment is frustrated, testing is conducted through negotiation between the manufacturer and the user.

For the layout of the field test, in order to facilitate the connection of the electrostatic discharge gun ground return cable, a reference ground plate should be laid on the ground and kept at a distance of 0.1m from the test product and its system. The material and thickness of the reference ground plate are the same as the requirements of the laboratory configuration. If conditions permit, the size of the ground plate is 0.3m × The reference floor is securely connected to the site, or to the ground terminal of the test product and its system. The ground return line of the generator is connected to the reference ground plane near the discharge point of the test product and its system.


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When the test product and its system are installed on a metal table that is not securely connected to the site, the metal table should be connected to the reference ground through a 470k Ω resistance wire to prevent static electricity accumulation.


8. Electrostatic discharge level

The standard divides the test level into four levels: contact discharge is set at 2kV, 4kV, 6kV, and 8kV respectively; Air gap discharges are set at 2kV, 4kV, 8kV, and 15kV, respectively.


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The selection of the test level is related to environmental factors (the drier 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 (along with the qualification evaluation criteria for the tested product).


9. Review of electrostatic discharge standards

① Due to the low uncertainty of contact discharge, contact discharge is the preferred standard discharge.

② Contact discharge has a very steep rise time, so the discharge current waveform contains very rich harmonic components. Even if a relatively low test voltage is selected, more rigorous test results can be obtained than the same level of high-voltage air gap discharge.



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