Since ESD failures can cause mobile phones to work abnormally, crash, or even damage and cause other safety issues, the Ministry of Industry and Information Technology has forcibly required ESD and other surge current tests for mobile phone products during network access tests before they go on sale .
ESD standard documents
The current standard for electrostatic discharge immunity test GB/T17626.2 in my country is equivalent to the 1995 IEC 61000-4-2 standard. Since its implementation on December 1, 1999, it has been revised and improved many times. In the IEC 61000-4-2 standard, specific test models, test environments, test equipment, test platforms and judgment standards are specified.
In addition, the more common ones are JESD22-A114-B (American Electronic Industry Association Standard, Electrostatic Discharge Sensitivity Test of Electronic Components under Human Body Model Conditions ), and ESDSTM5.1: American Electrostatic Discharge Association Standard, Electronic Components under Human Body Model Conditions ESD sensitivity test).
Test model and environment
For mobile phone ESD testing, the human body model (HBM) is the mainstay. The electrostatic voltage on the human body usually reaches 8-10kV, and even as high as 12-15kV in a dry environment and chemical fiber clothes.
Human Body Model There are two types: contact discharge (Contact Discharge, ESD simulator electrode held in contact with the mobile phone, the internal simulator discharge switch excitation discharge) and the air discharge ( Ai R & lt Discharge, discharge electrodes of the ESD simulator near the device, by the The simulator breaks down the air statically and generates a discharge current). Table 1 is a table of human body discharge levels specified in the standard.
The IEC 61000-4-2 standard, in order to make the environmental parameters affect the test results as much as possible, stipulates:
Ambient temperature: 15~35℃
Relative humidity: 30%~60%
Atmospheric pressure: 86~106kPa
test instrument
ESD test requires the following equipment
1. Electrostatic discharge generator
The main specifications of the electrostatic discharge generator are as follows:
Energy storage capacitor : 150pF±10%
Discharge resistance : 330Ω±10%
Charging resistance: 50MΩ~100MΩ
Output voltage: contact discharge 8kV (nominal value)
Air discharge 15kV (nominal value)
Allowable deviation of output voltage indication: ±5%
The electrostatic discharge generator is mainly composed of a DC voltage generator, a charging capacitor, a charging resistor, a discharging resistor, a discharging switch and a discharging head. The DC high voltage source charges the 150pF capacitor, and the capacitor instantly charges the end of the discharge loop, thereby simulating the electrostatic discharge process. The equivalent circuit shown in Figure 1
Figure 1. Equivalent circuit diagram of electrostatic discharge generator
2. Standard test bench
The IEC 61000-4-2 standard has certain requirements for the configuration of the test bench. The architecture diagram is shown in Figure 2.
Figure 2. Test bench architecture diagram
The specific configuration requirements of the test bench in the standard are as follows:
1. A 0.8m high wooden table placed on the ground reference plane
2. Horizontal coupling plate (HCP) placed flat on the table , with an area of 1.6m*0.8m
3. The vertical coupling board (VCP) placed on the desktop, with an area of 0.5m*0.5m
4. A 0.5 mm thick insulating pad to isolate the mobile phone cable from the HCP; the ground reference plane should be a copper or aluminum alloy sheet with a thickness of not less than 0.25mm, and the area of other reference planes should not be less than 1 square meters, and each side should extend at least 0.5m away from the ECU or coupling board, and connect it to the ground of the power system
5. Coupling plate: the same metal and thickness as the ground reference plane should be used, and 470kΩ resistors should be set to connect to the ground reference plane through each section.
ESD test procedure
The ESD test steps of mobile phones are as follows:
1. Adjust the working mode of the phone: standby mode or call mode
2. Select the discharge mode of the electrostatic gun: contact discharge and air discharge
3. Discharge frequency: at least 10 times per point, each discharge interval is 1s, after each discharge, the ground will lead to the residual charge
Result judgment
1. Air discharge
±5kV, ±8 kV: The mobile phone to be tested has no abnormalities, and the function is normal, it will judge Pass, otherwise it will judge Fail
±15kV: non-permanent fault is judged as Pass, otherwise judged as Fail
2. Contact discharge
±3kV, ±4 kV: The phone to be tested has no abnormalities, and if the function is normal, it will judge Pass, otherwise it will judge Fail
±8kV: non-permanent fault is judged as Pass, otherwise judged as Fail
What is the function of the electrostatic test coupling board connected to two 470K resistors
In the electrostatic discharge experiment, in the electrostatic discharge test environment, there are two coupling boards each with a strip
Two ground wires with 470K resistance. Often have customers
Asked, why should I connect two 470Kr resistors? Do not use one resistor, but two, is there any special meaning?
According to the standard, an indirect discharge test in contact discharge must be carried out during electrostatic discharge. Specifically, a horizontal coupling board is placed on the test table and a horizontal coupling board is placed on the side at the same time.
Vertical coupling plate, and then use an electrostatic gun to discharge the horizontal and vertical coupling plates to see if the equipment is affected. This test is mainly to simulate the human body
Discharge other objects on the side of the test device (simply holding a key in hand) to indirectly test whether the test device is affected.
According to the standard, both the horizontal coupling plate and the vertical coupling plate are grounded through two 470K resistors, and the two resistors are distributed at both ends of the grounding wire.
coming:
1. What do these two resistors do?
2. Why use two 470K resistors?
3. Why are these two resistors at both ends of the ground wire?
The first question may be known to everyone, because the discharge of the horizontal coupling plate and the vertical coupling plate affects the equipment through transient electromagnetic and electrostatic fields, so this
The two resistors are to prevent the electrostatic charge from quickly venting along the ground line, and the electrostatic field cannot be generated; in addition, the high impedance is easy to accumulate charge, and the final charge is also
Accumulated until these resistances are released.
The second problem is that the resistance of the human body to ground is approximately
1M, two 470K resistors are exactly the same;
The third problem is to put two resistors on both ends. Because the impedance is not matched, the current generated by the static charge will reflect back and forth between the two resistors until it disappears.
Reduce the current flowing to the ground.
