ESD is the abbreviation of Electro Static Discharge in English, and the Chinese meaning is electrostatic discharge: the transfer (transfer) of charge caused by direct contact or the induction of an electrostatic field between two objects of different potential. It can be seen that static electricity and electrostatic discharge (ESD) are completely different physical concepts or physical processes. One is "silent" and the other is "moving". Electrostatic discharge is often accompanied by the transmission of electricity, the generation of current, and the emission of electromagnetic fields.1. What is static electricity
Static electricity is an excess or deficiency of static charge on the surface of an object.
1. Static electricity is a kind of electrical energy that remains on the surface of objects.
Static electricity is the result of a local imbalance between positive and negative charges;
Static electricity is generated by the transfer of electrons or ions.
2. An electrostatic phenomenon is a general term for electrical phenomena that occur when charges are generated and destroyed.2. Three elements of electrostatic discharge
● B: A certain accumulated static charge.
● M: Discharge path such as metal contact, ground, or low resistance discharge path.
● D: Device sensitive to static electricity.3. The principle of generating static electricity
Electrons move around the nucleus, and in the presence of an external force, they will de-orbit, leave the original atom and invade other atoms.
External force includes various energies such as kinetic energy, potential energy, thermal energy, chemical energy, electromagnetic energy, etc.
Atom A is positively charged due to the absence of electrons and is called a cation.
Atom B is negatively charged due to the increase in the number of electrons, which is called an anion.4. the harm of static electricity in the electronics industry
1. Electrostatic adsorption of dust, reduction of the insulation resistance of components (shortened service life).
2. Electrostatic discharge damages, damages components, and cannot work normally: When the discharge current is too large (instantaneously high current can reach tens of A) during electrostatic discharge, excessive heat energy will be generated, which will break the component parts.
Three phenomena of failure:
● Thermal breakdown: P-N damage;
● Breakdown of the dielectric: destruction of the oxide layer;
● Metal Evaporation: The metal wire is vaporized and opened.
3. The amplitude of the electromagnetic field emitted by electrostatic discharge is very large (up to several hundred V/m), and the frequency spectrum is extremely wide (from tens of megabytes to several gigabytes), causing interference or even damage to electronic products.5. electrostatic discharge test standard
IEC 61000-4-2 is currently considered the de facto standard for ESD testing. my country's national standard (GB/T 17626.2-1998) is equivalent to IEC 61000-4-2.
Regular consumer tests use 4 kV contact voltage and 8 kV air voltage, while some high standard industries such as automotive use 8 kV and 15 kV air voltage.6. Circuit level ESD protection standard
1. Parallel unloader
Commonly used discharge devices include TVS, zener diodes, varistors, gas discharge tubes, etc.
Stiner diodes. Using the reverse breakdown characteristics of zener diodes can protect devices sensitive to electrostatic discharge. But zener diodes usually have a capacitance in the tens of pF, which will cause signal distortion for high speed signals (eg 500 MHz). Zener diodes are also good at absorbing power surges.
TVS (Transient Voltage Suppressor): TVS is a solid state diode designed to prevent ESD transient voltage from damaging sensitive semiconductor devices. Compared with traditional Zener diodes, TVS diodes have a larger P/N junction area. This structural improvement makes TVS more resistant to high voltages and also reduces the voltage cutoff rate, so they are useful for protecting low voltage portable devices. devices. The safety of the voltage circuit has a better effect.
The transient power and transient current characteristics of a TVS diode are directly proportional to the transient area. The junction of this diode has a large cross-sectional area to withstand high transient currents caused by lightning and ESD. TVS will also have junction capacitance, typically 0.3 pF to tens of pF. There are unipolar and bipolar TVS, so be careful when using them.
Varistors: Varistors can also effectively suppress instantaneous high voltage shocks. This type of device has a non-linear voltage-current dependence (impedance), and the cut-off voltage can be 2–3 times the initial cut-off voltage. . This function is suitable for electrostatic protection or surge protection of lines and devices that are not sensitive to voltage, such as power circuits, key input terminals, etc. The price of varistors is much lower than TVS, but the protective effect is not as good as that of TVS, and varistors have an aging life.
2. Series impedance
Generally, the ESD discharge current can be limited by series resistors or magnetic beads to achieve the antistatic purpose. As it shown on the picture. For example, a mobile phone port with high input impedance can be protected with a set of 1 kΩ resistors, such as ADC, GPIO input, buttons, etc.
3. Add Filter Network
Using a filter to filter out the main energy can also provide protection against static electricity.
For low frequency signals such as GPIO input, ADC and audio input, 1k+1000PF capacitors can be used for ESD protection, cost can be ignored, and performance is as good as varistors. +50PF varistors (described below Comprehensive protection measures), the effect is better, and experience has shown that the protective effect sometimes exceeds TVS.
For RF antenna microwave signal, if TVS tube, pressure sensitive and other capacitive devices are used for ESD protection, the RF signal will be attenuated, so the TVS capacitance must be very low. , which increases the cost of electrostatic discharge measures. For microwave signals, a tens of nH inductance can be connected in parallel to ground to provide a discharge path for static electricity that has little effect on microwave signals. For 900 MHz and 1800 MHz mobile phones, 22 nH inductance is often used. It can absorb a lot of energy on the main energy spectrum of static electricity.
4. Multilayer ESD Board
If funds allow, choosing a multilayer board is also an effective ESD protection. In a multilayer board, since there is a full ground plane near the track, it can make the ESD communicate faster with the low impedance plane, thus protecting the role of key signals.
5. Wrap the board with protective tape
This method usually draws traces around the PCB without solder layers. If conditions allow, connect the trace to the shell, and at the same time, note that the trace cannot form a closed loop, so as not to form a loop antenna and cause more trouble.
In short, although ESD is terrible and can even cause serious consequences, if the power supply and signal lines in the circuit are well protected, ESD current can effectively be prevented from flowing into the PCB. The above content is for reference, I hope it can help you.