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Analysis of impact on ESD test repeatability


 According to the definition of the standard IEC 61000-4-2/GBT17626.2 , electrostatic discharge is the charge transfer caused by objects with different electrostatic potentials approaching or directly contacting each other.

       When the human body is dressed in a fabric of insulating material and is insulated from the ground, a certain amount of charge may accumulate when moving on the ground. Electrostatic discharge occurs when the human body comes into contact with equipment that is connected to ground.

       The electrostatic discharge test mainly checks the discharge (direct discharge) caused by people or objects in contact with the equipment, and the impact on the operation of the equipment caused by the discharge (indirect discharge) of people or objects on nearby objects of the equipment. During electrostatic discharge, a discharge current of 1 to 50A can be generated within 0.5 to 20ns . Although the current is large, the energy is very small because of the short duration. Therefore, general electrostatic discharge will not cause harm to people, but it may cause destructive harm to electronic products such as integrated circuit chips.

1 ) Test environment conditions

i. Climate environment ambient temperature 15°C ~30°C

           Relative humidity 30%~60%

           Atmospheric pressure86kPa ~ 106kPa

ii. Electromagnetic environment the environment that does not affect the test results

2 ) Test arrangement

Although ESD testing is very easy , it is one of the least repeatable of all EMC tests. When doing electrostatic discharge tests, we often encounter this situation: the same tested product passed in laboratory A , but failed in laboratory B. The reasons for this situation can be summarized and analyzed as follows possible:

①Due   to the difference of test equipment, the test results are different

②Due   to the different test environments, the test results are different

③Due   to the different testers, the test results are different

For the above reasons, the specific analysis is as follows:

1. Due to the different test equipment, the test results are different

According to the standard IEC 61000-4-2 : 2008 , the requirements of the electrostatic discharge simulator:

 


The regulation of electrostatic discharge current waveform, the main four key indicators are: first peak current I p ; rise time Tr ; current value I 30 at 30ns ; current value I 60 at 60ns . It can be seen from the above table that the allowable deviations of these four indicators are relatively large, the first peak current I p is ±15% , the rise time Tr is ±25% , the current value I 30 at 30ns is ±30% , and at 60ns The current value of I 60 is ±30%  .

The following is a comparison of the output waveforms of 4 standard-compliant ESD simulators, which belong to different companies.

      It can be seen from the waveforms that there is still a big difference. Some waveforms have serious ringing, but they all meet the standard requirements.

The most direct reason for the difference in electrostatic discharge test results is that the ringing of the waveform directly affects the test results due to the difference in the output waveform. In the case of a relatively large waveform change, the frequency change in each discharge cycle is different, so it cannot be repeated. The possibility of testing is relatively large, so the pursuit of waveform quality is the key to solving the problem, and it is very important to choose a good electrostatic test equipment. At present, there are no less than 10 manufacturers of electrostatic testing equipment in the market , but in terms of industry recognition and market share, DITO ( 16.5kV ) and ESD 30N ( 30kV ) produced by Switzerland EMTEST are the most convincing.

HERE 16.5kV                          

 

ESD 30N30kV

2. Due to the different test environments, the test results are different

The environmental factor that has the greatest impact on the repeatability of electrostatic tests is humidity. The impact of changes in air humidity on discharge parameters is shown in the figure below:

Studies have shown that the gas pressure under laboratory conditions is significantly affected by the ambient humidity, and the gas pressure will affect the electron drift speed. Therefore, the greater the ambient humidity, the greater the pressure, and the discharge current will also increase.

      Therefore, the air humidity has a great influence on the electrostatic discharge test.

3. Different testers lead to different test results

Different testers will definitely have some differences when conducting electrostatic discharge tests, but the test standards are the same (IEC 61000-4-2) , and the test methods are all described in detail in the standard, so The differences produced by different testers are very limited, and the main reason analysis is not done here.

      Based on the above analysis, the main factors affecting the ESD test are the test equipment and the test environment (mainly humidity), so in order to obtain more reliable test results, it is recommended to choose brand equipment with high market recognition (such as EMTEST), and to strictly control the test Environment, preferably a relatively sealed environment, so as to effectively control the air humidity.

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