Static electricity is a natural phenomenon that people are very familiar with. Many functions of static electricity have been applied to military or civilian products, such as electrostatic dust removal, electrostatic spraying, electrostatic separation, electrostatic copying, etc. However, ESD (Electro-Static Discharge) has become a hazard to electronic products and equipment, causing functional disorders and even component damage to electronic products and equipment. The scale of modern semiconductor devices is getting larger and larger, and the operating voltage is getting lower and lower, resulting in the sensitivity of semiconductor devices to external electromagnetic disturbances. Problems such as interference caused by ESD to circuits and damage to components, CMOS circuits and interface circuits have attracted more and more attention.

1. Causes and harm of static electricity

Static electricity is formed when two materials with different dielectric coefficients rub against each other, and charges of positive and negative polarity accumulate on the two objects respectively. When two objects come into contact, one tends to attract electrons to the other, thus creating different charging potentials. As far as the human body is concerned, static electricity generated by friction between clothes and skin is one of the main causes of charging of the human body.

The two main damage mechanisms of ESD are: the heat generated by the ESD current leads to thermal failure of the equipment; and the excessive voltage induced by ESD leads to insulation breakdown. Both types of damage may occur simultaneously in a device; for example, insulation breakdown may trigger large currents, which in turn can lead to thermal failure.

In addition to easily causing circuit damage, electrostatic discharge can also easily cause interference to electronic circuits. There are two ways that electrostatic discharge interferes with electronic circuits. One is conducted interference and the other is radiated interference.

2. The structure of digital products and their ESD issues

Nowadays, various digital products are becoming more and more powerful, but circuit boards are getting smaller and smaller, and their integration level is getting higher and higher. And all of them are more or less equipped with some interfaces for human-computer interaction, so there is an ESD problem caused by electrostatic discharge from the human body. The parts that require ESD protection in general digital products include: USB interface, HDMI interface, IEEE1394 interface, antenna interface, VGA interface, DVI interface, button circuit, SIM card, headset and other various data transmission interfaces

ESD may cause products to work abnormally, crash, or even be damaged and cause other safety issues. Therefore, before the product is put on the market, domestic or foreign testing departments require ESD and other surge impact tests. Among them, contact discharge needs to reach ±8kV, and air discharge needs to reach ±15kV, which puts forward higher requirements for ESD design.

3. ESD problem solving and protection in digital products

3.1 Product structural design

If the released static electricity is regarded as a flood, then the main solutions are similar to water control, which is "blocking" and "draining". If the product we design has an ideal shell that is airtight, static electricity will not be able to enter, and of course there will be no static electricity problem. However, the actual shell often has gaps when the cover is closed, and many have metal decorative pieces, so you must pay attention to it.

3.2 Product PCB design

The PCB (Printed Circuit Board) of current products are all high-density boards, usually 4-layer boards. As density increases, the trend is to use 6-layer boards, whose design has always required a balance between performance and area. On the one hand, the larger the space, the more space there is for placing components. At the same time, the wider the line width and line spacing of the traces, which is good for EMI, audio, ESD and other aspects of performance. On the other hand, compact design of digital products is a trend and need. Therefore, you need to find a balance point when designing. As far as ESD issues are concerned, there are many things that need to be paid attention to in design, especially the design and line spacing of GND wiring, which are very particular. There are big ESD problems in some products, and the cause has never been found. Through repeated research and experiments, it was found that the problem was in the PCB design. To this end, here is a summary of the key points that should be paid attention to in PCB design:

(1) The distance between the PCB board edge (including the through-hole Via boundary) and other wiring should be greater than 0.3mm;

(2) It is best to surround the edges of the PCB with GND traces;

(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 at 0.2mm~0.3mm;

(5) The distance between important lines such as Reset, clock, etc. and other wiring should be greater than 0.3mm;

(6) The distance between high-power lines and other wiring is maintained at 0.2mm ~ 0.3mm;

(7) There should be as many through holes (VIa) connected between GNDs on different layers as possible;

(8) When paving the final floor, sharp corners should be avoided as much as possible, and sharp corners should be smoothed as much as possible.

3.3 Product circuit design

In the design of the casing and PCB, after paying attention to ESD issues, ESD will inevitably enter the internal circuit of the product, especially the following ports: USB interface, HDMI interface, IEEE1394 interface, antenna interface, VGA interface , DVI interface, button circuit, SIM card, headset and other various data transmission interfaces. These ports are likely to introduce static electricity from the human body into the internal circuit. Therefore, ESD protection devices need to be used in these ports.

The issue of ESD is one of many important issues. There are different ways to avoid harm to circuits in different electronic devices. Since today's digital products are small in size and high in density, they have unique features in ESD protection. Through a large number of electrostatic test experiments, it has been proved that using the design method in this article, a product that would have crashed at ± 2kV discharge has been protected and improved, and it can still work stably under ± 8kV electrostatic discharge, which has played a very good role in reducing electrostatic discharge. protective effect. With the increasingly widespread use of electronic equipment, ESD design is a key concern of every structural design engineer and electronic design engineer. Through continuous summary and learning, ESD issues will no longer be a problem!