Recently, ESD testing of electronic products, from different product test results found that the ESD is a very important test: If the circuit board design is not good, when the introduction of static electricity, will cause the product to crash or even components The damage. Previously only noted that ESD will damage the components, did not expect, for electronic products have to pay enough attention.

ESD, that is, we often say that the electrostatic discharge (Electro-Static discharge). From the learned knowledge can know that static electricity is a natural phenomenon, usually through contact, friction, electrical induction between the way, which is characterized by a long time accumulation, high voltage (can produce thousands of volts or even tens of thousands of volts of static electricity ), Low power, small current and short duration of action. For electronic products, if the ESD design is not designed, often resulting in unstable operation of electronic products, or even damage.

ESD testing is usually done in two ways: contact discharge and air discharge. Contact discharge is directly on the device under test discharge; air discharge, also known as indirect discharge, is a strong magnetic field on the adjacent current loop coupling. These two test test voltage is generally 2KV-8KV, different regions are not the same, so before the design, we must first clear the product for the market.

The above two cases is for the human body in contact with electronic products, due to human body charge or other causes of electronic products can not work and the basic test. The following figure shows the air humidity statistics for some areas in different months of the year. It can be seen from the figure that Lasvegas has the least moisture throughout the year and that special attention should be paid to the protection of ESD in the region.
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On the PCB board ESD design of those practical experience 1

Humidity is not the same everywhere, but at the same time in a region, if the air humidity is not the same, resulting in static electricity is not the same. The following table shows the collected data, from which we can see that static electricity increases as the air humidity decreases. This also indirectly illustrates the northeastern winter, when the hair dryer is caused by a great cause of static sparks.
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On the PCB board ESD design of those practical experience 2

Since electrostatic protection is so dangerous, how do we protect it? We are in the electrostatic protection design is usually divided into three steps: to prevent external charge into the circuit board and damage; to prevent the external magnetic field on the circuit board damage; to prevent electrostatic field generation The harm.

In the actual circuit design, we will use one or more of the following methods to carry out electrostatic protection:

1, avalanche diode for electrostatic protection.

This is also a design often used in a method, the typical practice is in the key signal line in parallel to an avalanche diode to the ground.
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On the PCB board ESD design of those practical experience 3

The method is the use of avalanche diode rapid response and has the ability to stabilize the clamp, can be consumed in a short period of time to accumulate the high voltage and then protect the circuit board.

2, the use of high-voltage capacitors for circuit protection.

This practice usually places a ceramic capacitor with a voltage of at least 1.5KV at the position of the I / O connector or key signal, while the cable is as short as possible to reduce the inductance of the cable. If the use of low voltage capacitors, will cause damage to the capacitor and lose the role of protection.

3, the use of ferrite beads for circuit protection.

Ferrite beads can be very good attenuation of ESD current, and also inhibit radiation. When faced with two problems, a ferrite bead will be a very good choice.

4, spark gap method.

This method is seen in a material, the specific approach is made of copper in the microstrip line with the use of cutting-edge alignment of the triangle of copper, triangular copper one end connected to the signal line, another triangular copper Connect to ground. When there is static electricity will produce cutting-edge discharge and then consume power.

5, the use of LC filter method to protect the circuit.

LC filter can effectively reduce the high-frequency electrostatic into the circuit. Inductance of the anti-characteristics can be a good suppression of high-frequency ESD into the circuit, and the capacitor has diverted the high frequency of ESD energy to the ground. At the same time, the type of filter can also smooth the edge of the signal and smaller RF effects, performance aspects of the signal integrity has been further improved.

6, multi-layer board for ESD protection.

When the funds allow the case, the choice of multi-layer board is also a means to effectively prevent ESD. In the multi-layer board, because there is a complete ground plane near the alignment, this can make ESD more quickly coupled to the low impedance plane, and thus protect the role of key signals.

7, the circuit board to protect the protection of the method of protection.

This method is usually around the circuit board to draw the wiring without the wiring layer. In the case of conditions permitting the connection to the shell, and pay attention to the line can not constitute a closed ring, so as to avoid the formation of a loop antenna and the introduction of greater trouble.

8, using a clamped diode CMOS devices or TTL devices for circuit protection.

This method is the use of the principle of isolation for the protection of the circuit board, because these devices have a clamp diode protection, in the actual circuit design to reduce the complexity of the design.

9, the use of decoupling capacitors.

These decoupling capacitors have low ESL and ESR values. For low-frequency ESD, the decoupling capacitor reduces the area of ​​the loop, and the effect of the ESL is reduced by the effect of the ESL. The

In short, although the terrible ESD, and even bring serious consequences, but only to protect the circuit power and signal lines, it can effectively prevent the ESD current into the PCB. Among them, my boss often said, "a good grounding of the board is king," I hope this sentence can also bring you the effect of breaking the skylight.