In addition to taking some measures on the circuit, we can also take some measures on the PCB when designing the PCB:
The ground line adopts a single point common ground;
If possible, try to have a ground wire near each signal line, so that it is convenient to discharge static electricity through parasitic capacitance;
Static electricity is generally a tip discharge, so you can deliberately set an acute angle trace in the circuit, and then design electrostatic protection at this acute angle; other traces are as wide and short as possible;
Deliberately reserve some installation resistors and capacitor positions on the line. When it is impossible to eliminate static electricity in production, increase resistance, capacitance, etc. to eliminate static electricity, use ion wind curtains or other types of anti-static equipment.
The effect of electrostatic discharge on the human body does not seem obvious, but in the production process of electronic components, or in the process of installation, debugging and inspection of electronic products, if the static electricity is not eliminated, it will affect the production or reduce the product quality. Especially in the semiconductor device and microcircuit manufacturing industries, device failure is caused by electrostatic discharge.
With the rapid development of science and technology, the rapid rise of high-tech industries such as electronics, post and telecommunications, and aerospace, especially the demand for electronic instruments, instruments and equipment, are becoming more and more compact, multi-functional and intelligent. High-density integrated circuits (such as MCUs) have become an indispensable part of the electronics industry's requirements. Such devices have the characteristics of short line spacing, thin lines, high integration, fast operation speed, low power, low withstand voltage and high input impedance, which makes these devices more and more sensitive to static electricity. The energy of electrostatic discharge (ESD) has little effect on traditional electronic components, which is difficult to detect. However, in these high-density integrated circuit components, both MOS devices and bipolar devices may be affected by electrostatic electric fields and electrostatic discharge currents. The phenomenon of "soft breakdown" caused by failure or caused by people to be discovered, leaving potential hidden dangers to the whole machine, directly affects the quality, life, reliability and economy of electronic products.
Component breakdown damage caused by electrostatic discharge is the most common and serious electrostatic hazard in the electronics industry. It is divided into immediate failure and delayed failure.
Immediate failure is a one-time breakdown of the component dielectric, burnout or permanent failure. Delay failure is caused by the performance degradation of the device or the degradation of the parameter index, which means that even if the product has passed all the inspections and tests, it may still be invalid after being sent to the customer.
Here are a few situations that cause failure:
Soft breakdown of the PN junction and reduced product reliability;
The chipping of the single crystal silicon metal in the chip breaks down, and the product rejection rate increases;
The breakdown of the leads inside the chip increases the rejection rate of the product.
The relationship between electronic components and ESD voltage damage
Different devices are sensitive to electrostatic discharge (ESD). This difference is due to the design of the device and the different impurity composition incorporated into the device. The relationship between common components and ESD voltage damage is shown in Table 1.
As can be seen from Table 1, VMOS devices are one of the more sensitive devices for static electricity, and such devices are often referred to as electrostatic sensitive devices (SSD). The 30V is called the electrostatic safety voltage.
People are the main body in the production of electronic products. Due to the constant movement of the human body, and the friction, contact and separation between people and other objects such as the floor and clothes, it is the main static power source, which is caused by various activities of the human body. The electrostatic voltage is about 0.5~2kV, and the electrostatic voltage will increase by more than 10 times in a low humidity environment.
The impact of static electricity on home appliance manufacturing and design prevention
It can be said that static electricity is ubiquitous, which puts high demands on home appliance manufacturing.
The static electricity generated by the machine can be grounded by the machine, which can effectively solve the damage caused by the static electricity generated by the machine to the home appliance.
For artificially generated static electricity, you can find the source of its production and solve it from the source. Our idea is simple: to provide a venting channel to the source static that allows it to dissipate static electricity before it reaches the sensitive component. Through analysis and actual production experience, the most vulnerable to electrostatic damage is the MCU, and the damage is also the biggest. Once damaged, the entire appliance will lose its effectiveness. Therefore, we generally take the following simple methods to solve static electricity for the MCU.
RC circuit or single resistance, single capacitor circuit
We know that resistors are energy-consuming components, so placing an appropriately sized resistor on an electrostatic path would be an ideal choice and not costly.
The design idea of the circuit (Fig. 1) is to place the components with strong antistatic capability in a place with high static electricity, and keep the components with weak antistatic ability away from the static power source, and the MCU is placed on the end of the static sensitive device.
Single diode circuit
For some circuits, adding a resistor to the path may affect its inherent function. You can pull up or pull down a diode on the power supply (Figure 2). We know that static electricity is very fast, so the diode is required to react at a fast speed. Otherwise, it will not discharge static electricity. We usually use the IN4148 with high cost performance (of course, the TVS tube will work better, but the cost will need to be higher).
RCD circuit
For larger static electricity, or to achieve higher antistatic capability, an RCD (resistor, capacitor, diode combination) circuit (Figure 3) can be used, but the circuit cost is higher than the above two.