The ultimate solution for ESD electrostatic problems

Static electricity is a kind of natural phenomenon that people are very familiar with. Many of the functions of electrostatic have been applied to military or civilian products, such as electrostatic precipitator, electrostatic spraying, electrostatic separation, electrostatic copying and so on. However, electrostatic discharge ESD (electro-static discharge) has become a hazard to electronic products and equipment, resulting in the malfunction of electronic products and equipment and even the damage of parts. The scale of modern semiconductor devices is increasing and the working voltage is getting lower and higher, which leads to the sensitivity of semiconductor devices to external electromagnetic disturbance. ESD for the interference caused by the circuit, components, CMOS circuit and interface circuit caused by the destruction of more and more people pay attention to. ESD for electronic devices has also begun to be used as an important part of electromagnetic compatibility testing in national and international standards.

[Color= #0000FF] 1. Electrostatic causes and hazards [/color]
Static electricity is two kinds of dielectric coefficients of different material friction, positive and negative polarity of the charge is accumulated in two curvier on the formation. When two objects are in contact, one tends to attract electrons from the other, thus forming different charge potentials. In human body, the friction between clothes and skin is one of the main reasons for human being charged.

When the static power is in contact with other objects, there is a charge flow according to the charge neutralization mechanism, which transmits enough power to counteract the voltage. During the transmission of high-speed electricity, the potential damage voltage, current and electromagnetic field will be generated, and the object will be destroyed in serious time, which is electrostatic discharge. National Standard Definition: Electrostatic discharge is a different electrostatic potential of the curvier with each other near or direct contact caused by the charge transfer (gb/t4365-1995), generally with ESD. ESD can cause serious damage to electronic equipment or malfunction of the operation.

Electrostatic damage to the device is both overt and covert. Hidden damage was not visible at the time, but the device became more vulnerable, under the conditions of overvoltage, high temperature, and very easy to damage.

ESD two main destruction mechanism is: the thermal failure of the equipment caused by the ESD current, induced by the ESD high voltage caused by insulation breakdown. Two kinds of damage can occur simultaneously in one device, for example, an insulation breakdown can trigger a large current, which further results in thermal failure. In addition to easy to cause circuit damage, electrostatic discharge is also very easy to cause interference to electronic circuits. Electrostatic discharge has two ways to interfere with electronic circuits. One is conduction interference, the other is radiation interference.

[Color= #0000FF] 2. Construction of digital products and ESD problems [/color]
Now all kinds of digital products are becoming more and more powerful, but the circuit board is getting smaller and higher integration. and are more or less equipped with some interface for human-computer interaction, so there is the ESD problem of human electrostatic discharge. General digital products need to carry out ESD protection parts are: USB interface, HDMI interface, IEEE1394 interface, antenna interface, VGA interface, DVI interface, key circuit, SIM card, headphones and other types of data transmission interface.

ESD can cause the product to work abnormally, crash, or even damage and cause other safety problems. Therefore, before the launch of the product, domestic or foreign testing departments are required to conduct ESD and other surge impact testing. In which the contact discharge needs to reach ±8kv, the air discharge needs to reach ±15kv, which puts forward a higher requirement for ESD design.

[Color= #0000FF] 3. ESD problem solving and protection in digital products [/color]

[Color= #0000FF structure design of]3.1 products [/color]
If the release of static electricity as a flood, then the main solution is similar to water, that is, "plugging" and "sparse." If we design the product has an ideal shell is airtight, static electricity will not be in, of course, there is no electrostatic problem. But the actual shell in the lid of the common gap, and many also have metal decorative pieces, so be sure to pay attention to.

One is to use the "blocking" method. Increase the thickness of the shell as much as possible, that is, increase the distance between the shell and the circuit board, or increase the distance of the shell air gap by some equivalent method, which can avoid or greatly reduce the energy intensity of ESD.

Through the improvement of the structure, the air gap between the shell and the internal circuit can be increased so that the energy of ESD is greatly weakened. According to experience, 8kV of ESD is generally attenuated to zero after a distance of 4mm.

Second, the "sparse" method, can be sprayed with EMI paint on the inside of the shell. EMI paint is conductive, can be seen as a metal shielding layer, so that the static conductivity on the shell, and then the shell and the PCB (printed circuit Board) to connect the static electricity from the ground guide away. This method can not only prevent static electricity, but also can effectively suppress EMI interference. If there is enough space, you can also use a metal shielding cover to protect the circuit, metal shielding cover to connect the PCB GND.

In short, the ESD design shell needs to pay attention to a lot of places, first of all, try not to let the ESD into the shell, to minimize its access to the energy of the shell. For ESD to enter the inside of the shell as far as possible from the GND guide, do not let it endanger the other parts of the circuit. Metal decorations on the housing must be used with caution, as it is likely to bring unexpected results that require special attention.

[color= #0000FF]3.2 product PCB design [/color]
Now the product's PCB (printed circuit Board) is a high-density board, usually 4-layer board. With the increase in density, the trend is to use a 6-layer board, its design has always been to consider the balance between performance and area. On the one hand, the larger the space can have more space to display components, at the same time, the line width and the width of the line distance, EMI, audio, ESD and other aspects of performance are beneficial. On the other hand, the small digital product design is the trend and needs. Therefore, the design needs to find a balance point. As far as ESD is concerned, there are many places to pay attention to in design, especially about the design of GND cabling and the line spacing. In some products, there is a large number of ESD problems, has been unable to find the cause, through repeated research and experiments, found that the PCB design problems arise. To this end, here summarizes the PCB design should pay attention to the points:

(1) The distance between PCB board edge (including through-hole via boundary) and other wiring should be greater than 0.3mm;
(2) The board edge of the PCB is preferably surrounded by the GND line;
(3) The distance between GND and other wiring is maintained at 0.2mm~0.3mm;
(4) The distance between Vbat and other wiring is maintained in 0.2mm~0.3mm;
(5) Important line such as reset, clock and other wiring between the distance should be greater than 0.3mm;
(6) The distance between the high-power wire and other wiring is kept in 0.2mm~0.3mm;
(7) The different layers of gnd should be connected with as many vias as possible (VIa);
(8) At the end of the paving should try to avoid sharp angle, sharp angle should try to make it smooth.

[color= #0000FF]3.3 product circuit design [/color]
In the design of the shell and PCB, the ESD problem is noted, ESD will inevitably enter the product's internal circuit, especially the following ports: USB interface, HDMI interface, IEEE1394 interface, antenna interface, VGA interface, DVI interface, key circuit, SIM card, Headphones and other types of data transmission interface, these ports are likely to bring the human body's static electricity into the internal circuit. Therefore, ESD protection devices need to be used in these ports.

In the past the main use of static protection devices are varistors and TVs devices, but the common disadvantage of these devices is that the response speed is too slow, the discharge voltage is not accurate, very large capacitance, short life, electrical performance due to multiple use and poor. Therefore, the current industry in general use of professional "electrostatic suppressors" to replace the previous electrostatic protection devices. "Electrostatic suppressor" is a professional solution to the problem of static electricity products, its internal structure and working principle than other products more scientific and professional. It is made of polymer polymer material, the internal diamond molecules in a regular discrete arrangement, when the electrostatic voltage exceeds the device's trigger voltage, the internal molecules quickly produce cutting-edge to the tip of the discharge, electrostatic in an instant vent to the ground. It is characterized by a fast reaction speed (0.5ns~1ns), very low pole capacitance (0.05PF~3PF), and a small leakage current ("the"), which is ideal for protection of various interfaces.

Because the electrostatic suppressors have small volume (0603, 0402), no polarity, fast reaction speed and many other advantages, now the use of electrostatic suppressors as a protective device in the proportion of more and more, in the use of the following points should be noted:
1, place the device as far as possible near the port to be protected;
2, the connection to GND as short as possible;
3, the area of the GND is as large as possible.

The problem with

      ESD  is one of many important issues. In different electronic devices there are different ways to avoid the damage to the circuit. Because the digital products are small in size and large in density, they have unique characteristics in the protection of  ESD . Through a large number of static test experiments proved that the design method of this article to deal with a original ± 2kv  discharge will crash products to protect and improve, in the ± 8kv  of electrostatic discharge can still work stably, played a very good electrostatic protection effect. With the increasingly extensive use of electronic devices, esd  design is a problem that every structural design engineer and electronic design engineer needs to focus on, by constantly summarizing and learning, esd  problems will no longer be a problem.