ESD (electrostatic discharge) is an abbreviation for electrostatic discharge.
Non-conductive objects generate static charges by friction, heating, or contact with other electrostatically charged objects.
When an electrostatic charge accumulates a gradient (electric field gradient) in a particular electric field, it will lead to an arc, or attractive force (mechanical attraction).
The phenomenon in which energy is released by an electric arc due to the accumulation of static electricity in a non-conductor is called ESD.
Electrostatic protection (ESD) of sheet metal case
1-1 Factors that affect the charging of objects
Conductor — Charges are easily neutralized and do not accumulate static charges.
Non-conductor — Large resistance and charge are not suitable for resonance (recombination). As a result, electric charge accumulates.
The larger the relative permittivity (permittivity) sheet metal between two contact materials (non-conductors), the easier it is to be charged with static electricity.
If the surface resistance of the material is greater than 109Ω / □, it is likely to be charged with static electricity.
2. The lower the relative humidity in the air, the more likely it is to be charged with static electricity.
ESD parameter characteristics
Basic relationship of ESD: V = Q / C
Q is the amount of static electricity carried by the object. When Q is fixed, the smaller the capacitance of an electrostatically charged object, the higher the ESD voltage emitted.
Generally, the capacitance of women is higher than that of men, and the capacitance of the general human body is 80pfd to 500pfd.
One of the main causes of IC component failure is the voltage released by ESD. The human body usually has an electrostatic discharge voltage of 10 to 15 kV due to friction, and the ESD voltage that can be generated does not exceed the upper limit of 35 to 40 kV. The lower limit of the ESD voltage that the human body can detect is 3 to 4 kV.
W = 1/2 * CV2
Typical ESD energy is about 17 millijoules, i.e. when C = 150 pfd, V = 15 kV
W = 1/2 * 150 * 1012 * (15 * 103) 2 = 17 * 103 Joules
The static electricity carried by an object has positive and negative points. When a component moves toward a reverse bias due to a particular polarity, the component is prone to destruction.
5. Rise time (tr)
Rise time — ESD start pulse (PULSE) The time required for the peak value of the ESD current of 10% to 90%.
Duration — Time elapsed between 50% of ESD start pulse and 50% of drop pulse
Discharging with a sharp tool minimizes ESD rise time and maximum current.
The occurrence of ESD can be divided into five stages.
- 1. Generates corona discharge and RF radiation.
- 2. Advanced electric field discharge (pre-destruction E field)
- 3. Electric field discharge collapse (collapse)
- 4. Magnetic discharge (Discharge H-Field)
- 5. Current flows and a transient voltage is generated.
1-2 ESD problems of electronic devices
Direct discharge to electronic components
- (1) Based on MOS (Metal Oxide Semiconductor) devices
- (2) If the ESD voltage exceeds the breakdown voltage of the oxide layer (SiO2, etc.), the parts will be destroyed.
- (3) Due to the electric field
Damage caused by electric current
- (1) BIPOLAR (Schottky, TTL) DEVICE main
- (2) When the ESD current reaches 2 to 5 A, the high heat (I 2 t) due to the Joule effect burns the IC.
- (3) Due to magnetic field
Direct discharge to electronics housing
When an electrostatically charged human body comes into contact with the metal case of an electronic device, if the device is grounded, ESD current will flow directly to the ground wire. Otherwise it may flow through the electronic component and then to the ground, damaging the component.
Since the ESD current is transmitted to the ground via the path with the lowest impedance, if the dynamic impedance of the ground line is lower than the impedance of the cabinet to the ground / desktop, there may be a case that is transmitted to the ground , which may cause radiation interference to the electronic circuit.
3. Indirect discharge
This is because the ESD current is transmitted to the ground via the path having the lowest impedance. If the dynamic impedance of the ground line is lower than the impedance of the cabinet from ground to desktop, it can be transmitted to ground, which can cause radiation interference to the electronics.
Indirect discharge —- This means that electrostatically charged objects will not be discharged directly to the equipment department they are in contact with. Due to ESD PILSE, electromagnetic field radiation affects electronic components. 1-3 ESD protection design
- Component level
- Board level (PCB level)
- Cable level
- Residential level (residential level)
One or two of them are not related to organizational design
Keep the following in mind about the flat and power cables in the cabinet:
- 1. Do not use cables that are too long.
- 2. The cables should not be too close to the housing seams to prevent the induction of ESD noise.
- 3. Prevent the cable from contacting the inner surface of the metal housing. If you prevent the housing from undergoing electrostatic discharge, the cables will interfere.
2. Cabinet level
The most important thing to note is the enclosure shield and grounding. When it comes to shields, the requirements for ESD and EMI are exactly the same. ESD should pay attention to the following:
- All externally accessible metal parts (such as switches) must be connected to the housing and cannot be floated.(1) Apply ESD current to the PCB.(2) Secondary discharge or radiation interference due to charge saturation.
- Do not use screws that are too long so that ESD does not cause internal radiation interference.
- In the plastic casing gap design, the gap length should be as long as possible to avoid ESD discharge or ESD radiation.