ESD is an abbreviation of Electro Static Discharge English, meaning of electrostatic discharge: between two objects of different potentials, due to direct contact or electrostatic field induction (transfer). It can be seen that static electricity and electrostatic discharge (ESD) are completely different physical concepts or physical processes. One is "quiet", one is "moving". Along with electrostatic discharge, there is often a transfer of electricity, the generation of current and electromagnetic field radiation.
1 What is static electricity
Static electricity is an excessive or insufficient static charge of the object.
1. Electrostatic is a power, it retains the surface of the object.
Static electricity is the result of positive charge and negative charge to lose balance within a local range;
Static electricity is formed by electronics or ion transfer.
2. The electrostatic phenomenon is the general name of electricity generated during the generation of charge and disappears.
2 electrostatic discharge three elements
Q + m + D = ESD
Q: There must be an electrostatic charge.
M: Discharge pathway, such as metal contact, place to earth or low-resistance.
D: Electrostatic sensitive device.
3 static generating principle
Electronics around the nucleus, an external force is separated from the track, and the original atom is separated from other atoms.
External force includes various energy, such as kinetic energy, bit energy, thermal energy, chemical energy, electromagnetic energy, etc.
A atoms have positive phenomena because of the lack of electron number, called cations.
B atoms with negative electricity due to increasing the number of electrons, called anion.
4 Distortion of static electricity in the electronics industry
1. Electrostatic adsorption dust, reduce component insulation resistance (shorten life).
2. Electrostatic discharge is damaged, making component damage not normal: When the electrostatic discharge is too large (the instantaneous large current can reach several tens of a), producing too high thermal energy will break down the component.
Three kinds of breakdown phenomena:
Hotteath: P-N destruction;
Dielectric breakdown: damage to the oxide layer;
Metal vaporization: The metal wire is turned into vaporization.
3. Electrostatic discharge radiation electromagnetic field is large (a few hundred V / m) spectrum extreme width (from tens of megabytes to several thousand megabytes), interfere with electronic products or even damage.
5ESD test standard
At present, IEC 61000-4-2 is thought to be the factual standard for ESD testing. my country's national standards (GB / T 17626.2-1998) is equivalent to IEC 61000-4-2.
Tests of common consumption class use contact 4kV and air 8kV, some high standard industries such as car classes, will use 8kV and air 15kV.
6 circuit-level ESD protection standard
Parallel discharge device
Commonly used discharge devices include TVS, Zener diode, pressure sensitive resistance, gas discharge tube, and the like.
Zener Diodes: Use the reverse breakdown characteristics of the Zener diode to protect the ESD sensitive device. However, the Zener diode typically has a capacitor of tens of PF, which causes signal distortion to high speed signals (e.g., 500 MHz). Zener diode also has a good absorption of surges on the power supply.
Transient Voltage Suppressor: TVS is a solid diode that is dedicated to preventing sensitive semiconductor devices in ESD transient voltage. Compared to conventional Zener diodes, the TVS diode P / N junction area is larger, and this structure improves the TVS to have stronger high pressure receiving capabilities, and also reduces the voltage cutoff, and thus low work for protecting handheld devices. The safety of the voltage circuit has a better effect.
The transient power and transient current properties of the TVS diode are proportional to the area of the knot. The junction of the diode has a large cross-sectional area, which can handle the high transient current caused by the lightning and ESD. TVs also have a junction capacitor, usually 0.3 PF to dozens of PF. TVs have unipolar and bipolar, pay attention to when using.
Sensitive resistance: Demolic resistance can also be effective instantaneous high voltage impact inhibition, such devices have a nonlinear voltage-current (impedance performance) relationship, and the cutoff voltage can reach 2 to 3 times the initial stop voltage. This feature is suitable for electrostatic or surge protection of lines and devices that are less sensitive to voltage, such as power circuit, buttons input, etc. The pressure-sensitive resistance price is much lower than TVs, but the protective effect is not TVS, and the pressure sensitive resistance has a life aging.
2. Serial impedance
The ESD discharge current can generally be limited by a series resistance or magnetic bead to achieve anti-static purposes. Figure. If the high input impedance of the mobile phone can be string for 1k European resistance, such as ADC, input GPIO, buttons, etc.
3. Add filtering network
Filtering the main energy with the filter can also achieve the purpose of electrostatic protection.
For low frequency signals, such as GPIO input, ADC, audio input can be used with capacitance of 1K + 1000pF to do electrostatic protection, the cost can be ignored, the performance is not comparative pressure resistance, if the pressure sensitive resistance of 1 k + 50 pF (composite protection below Measures), the effect is better, and the experience shows that such a protective effect sometimes exceeds TVS.
For the microwave signal of the RF antenna, if electrostatic protection is made with a TVS tube, pressure sensitive and other capacitive devices, the RF signal is attenuated, so that the capacitance of the TVS is low, which increases the cost of ESD measures. For the microwave signal, it can provide a discharge passage for electrostatic electricity to the electrostatic inductance, which has little effect on the microwave signal. For the 300MHz and 1800 MHz mobile phones often use 22nH inductance. This can absorb a lot of energy on the main energy spectrum of electrostatic electricity.
4. Multi-layer panel for ESD protection
When funds are allowed, selecting multi-layer boards is also a means that effectively prevents ESD. In the multi-laminate, since there is a complete level close to the trace, the ESD can be more quickly coupled to the low impedance plane, thereby protecting the key signal.
5. Rollover protection belt outside the board
This method is usually a trace that does not group welded waves around the board. The trace is connected to the housing in the case of conditions allowed, and it is important to note that the trace cannot form a closed ring to avoid a greater trouble to avoid a ring antenna.
In short, although the ESD is terrible, it will even bring serious consequences, but as long as the power supply and signal line on the circuit can be protected, the ESD will be effectively influent into the PCB. The above is for your reference, I hope to help you.
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