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How Harmful is Static Electricity to LED Chips?

How Harmful is Static Electricity to LED Chips?

1. Generation Mechanism of Static Electricity

In general, static electricity is generated by friction or induction.

Friction static electricity is caused by the movement of electric charge in the process of contact friction or separation of two objects. The static electricity left by the friction between the conductors is usually relatively weak, which is due to the strong conductivity of the conductor. The ions generated by the friction will move together and neutralize quickly in the friction process and at the end. After the insulator is rubbed, it may produce high electrostatic voltage, but the charge is very small. This is due to the physical structure of the insulator itself. In the molecular structure of insulator, it is very difficult for electrons to move freely away from the binding of atomic nucleus, so only a small amount of molecules or atoms can be ionized as a result of friction. 

Induced static electricity is an electric field formed by the movement of electrons in an object under the action of electromagnetic field. Generally, induced static electricity can only be generated on the conductor. The effect of space electromagnetic field on insulator can be ignored. 

2. Electrostatic Discharge Mechanism

The 220V city electricity can kill people, but people can't kill people with thousands of volts on the body. What's the reason? The voltage at both ends of the capacitor meets the following formula: U=Q/C. According to this formula, when the capacitance is very small, a small amount of charge will produce a high voltage. Generally, the capacitance of our body and objects around us is very small. When we generate electric charge, a small amount of electric charge will also generate a high voltage. Due to the small amount of charge, when discharging, the current formed is very small, the time is very short, the voltage can not be maintained, and the very short time will be reduced. As the human body is not an insulator, the static charges accumulated in all parts of the body will be collected when there is a discharge path, so it feels like that the current is larger and there is a sense of electric shock. After static electricity is generated in conductors such as human body and metal objects, the discharge current will be relatively large.

For materials with good insulation performance, one is that the amount of charge generated is very small. On the other hand, the generated charge is difficult to flow. Although the voltage is high, only the charge at the contact point and a small area nearby can flow and discharge when there is a discharge path somewhere, while the charge at the noncontact point cannot discharge (because it is insulator). Therefore, even if there are tens of thousands of volts, the discharge energy is very small. It is shown in Figure 8. 

Therefore, although the static voltage of plastic turnover box, packaging foam and chemical fiber carpet is very high, the discharge energy is very small. 

3. Electrostatic Hazards to Electronic Components

Static electricity will do harm to the LED pixel screen. It's not the only "patent" of LED, but also the common diode and triode made of silicon materials. Even buildings, trees and animals can be damaged by static electricity.

So, how does static electricity damage electronic components? I don't want to say something irrelevant, and just talk about semiconductor devices, and it's limited to diode, triode, IC, LED. 

The damage of electricity to semiconductor components is ultimately the participation of current. Under the action of current, the device is damaged by heat. To have current, there must be voltage. However, the semiconductor diode has a PN junction. Whether it is forward or reverse, the PN junction will have a voltage range of blocking current. The forward barrier is low, while the reverse barrier is much higher. In a circuit, where the resistance is high, the voltage is concentrated. However, when the voltage is applied forward to the LED and the external voltage is less than the threshold voltage of the diode (the size corresponds to the material band gap width), there is no forward current, and the voltage is all applied to the PN junction. When the voltage is applied to the LED in reverse direction and the external voltage is less than the reverse breakdown voltage of the LED, the voltage is all applied to the PN junction. At this time, there is no voltage drop at the false soldering point of the LED, the bracket, the p-area and the n-area, because there is no current. When the PN junction breaks down, the external voltage will be shared by all the resistors on the circuit. Where the resistance is large, the voltage borne by which part is high. As far as LEDs are concerned, it is natural that the PN junction bears most of the voltage. The thermal power generated on the PN junction is the voltage drop on it multiplied by the current value. If the current value is not limited, too much heat will burn the PN junction, and the PN junction will lose its function and pass through.

Why is the IC afraid of static electricity? Because the area of each component in the IC is very small, and the parasitic capacitance of each component is very small (often the circuit function requires the parasitic capacitance to be very small). Therefore, a small amount of electrostatic charge will produce a high electrostatic voltage, and the power tolerance of each component is usually very small, so the electrostatic discharge will easily damage the IC. But ordinary discrete components, such as ordinary small power diodes and small power triodes, are not very afraid of static electricity, because their chip area is relatively large, and their parasitic capacitance is also relatively large, so it is not easy for general statics to accumulate high voltage on them. Due to the thin gate oxide and small parasitic capacitance, low-power MOS transistors are easy to be damaged by static electricity. Usually, they leave the factory after three electrodes are short circuited after packaging. In use, it is often required to remove the short route after welding. However, due to the large chip area of high-power MOS tubes, general static electricity will not damage them. So you will see that the three electrodes of the power MOS tube now have no short circuit protection (the early manufacturers still short them before leaving the factory).

Led actually has a diode in fact. Its area is very large compared with each component in IC. So the parasitic capacitance of LED is relatively large. Therefore, in general, static electricity can not damage the LED.

In general, the static electricity, especially the static electricity generated on the insulator, will have a high voltage, but the discharge charge is very small, and the duration of the discharge current is very short. However, the voltage of the static electricity induced on the conductor may not be very high, but the discharge current may be very large, and it is often a continuous current. This is very harmful to electronic components. 

4. Why does Static Electricity Damage LED infrequently?

Let's first look at an experimental phenomenon. There is a piece of the metal iron plate having 500V static electricity. Put the LED screen panel on the metal plate (pay attention to the method of putting to avoid the following problems). Do you think the LED will be damaged? Here, if the LED is to be damaged, it should usually be added with a voltage greater than its breakdown voltage, that is, the two electrodes of the LED should contact the metal plate at the same time and have a voltage greater than the breakdown voltage. As the iron plate is a good conductor, the induced voltage is equal everywhere on it. The so-called 500V voltage is relative to the ground, so there is no voltage between the two electrodes of LED, so naturally, there will be no damage. Unless you touch one electrode of the led to the iron plate, the other electrode connects to the ground or other conductors with conductors (hands or conductors without insulating gloves).

The above test results indicate that when LED is in electrostatic field, one electrode must contact with the electrostatic body, and the other electrode must contact with the ground or other conductors are damaged. In actual production and application, with the small volume of LED, there is little chance of such things, especially in batch. An accident is possible. For example, the LED is on the electrostatic body, and one electrode contacts the electrostatic body, and the other electrode is just suspended. At this time, someone touches the suspended electrode, which may damage the LED.

The above phenomenon tells us that the electrostatic problem can not be ignored. Electrostatic discharge is to have a conductive circuit, and there is no electrostatic damage. When only a small amount of leakage occurs, the problem of accidental electrostatic damage can be considered. If it happens in large quantities, it is more likely to be a problem of chip contamination or stress.

  1. WILLIAMS, M. W. (n.d.). What Creates Static Electricity? American Scientist.

  2. Electrostatic Discharge: What is Electrostatic Discharge & How to Prevent It. (2019, February 27).

  3. How ESD Affects Electronics: ESD effects. (n.d.). Electronic Notes.

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