PractICal Experience of ESD design of PCB -- How to Produce High Quality PCB

Recently, we are doing ESD test on electronic products. From the test results of different products, we find that this ESD is a very important test: if the circuit board is poorly designed, the introduction of static electricity will cause the product to crash or even damage the components. Previously, I only noticed that ESD would damage components, but I didn't expect that we should pay enough attention to electronic products.

ESD, also known as Electrostatic discharge. From the knowLEDge learned, we can know that static electricity is a natural phenomenon, which is usually generated through contact, friction, induction between electrical appliances and other ways. It is characterized by long-term accumulation, high voltage (which can generate thousands of volts or even tens of thousands of volts of static electricity), low electricity, SMAll current and short action time. For electronic products, if the ESD design is not well designed, electronic and electrical products are often unstable, or even damaged.

There are usually two methods for ESD discharge test: contact discharge and air discharge. Contact discharge is to discharge the equipment to be tested directly; Air discharge, also known as indirect discharge, is caused by coupling of strong magnetic field to adjacent current loop. The test voltage of these two tests is generally 2KV-8KV, and the requirements are different in different regions. Therefore, before design, it is necessary to find out the MARKet for the product.

The above two conditions are basic tests for electronic products that cannot work due to electrification of human body or other reasons when human body contacts electronic products. The following figure shows the air humidity statistics of some regions in different months of the year. It can be seen from the figure that the humidity in Lasvegas is the lowest all year round, and the electronic products in this area should pay special attention to ESD protection.

The humidity situation is different all over the world, but at the same time, if the air humidity is different in a region, the static electricity generated is also different. The following table is the collected data, from which it can be seen that static electricity increases with the decrease of air humidity. This also indirectly explains the reason why the electrostatic sparks generated during hair removal in winter in the north are very large.

Since static electricity is so harmful, how can we protect it? When we design electrostatic protection, we usually take three steps: prevent external charges from flowing into the circuit board and causing damage; Prevent external magnetic field from damaging the circuit board; Prevent hazards caused by electrostatic field.

In the actual circuit design, we will use one or more of the following methods for static protection:

1. Avalanche diode for static protection.

This is also a method often used in the design. The typical method is to connect an avalanche diode in parallel on the key signal line to the ground.

This method utilizes avalanche diode's fast response and stable clamping ability, which can consume accumulated high voltage in a short time to protect the circuit board.

2. High voltage capacitors are used for circuit protection.

In this practice, the ceramic capacitor with a withstand voltage of at least 1.5KV is usually placed at the I/O connector or key signal position, and the connecting line is as short as possible to reduce the inductance of the connecting line. If a capacitor with low withstand voltage is used, the capacitor will be damaged and the protection will be lost.

3. Iron oxide magnetic beads are used for circuit protection.

Iron oxide magnetic beads can attenuate ESD current and inhibit radiation. When faced with two problems, a ferrite bead is a good choice.

4. Spark gap method.

This method is seen in a piece of material. The specific method is that the microstrip line layer composed of copper sheets is composed of triangular copper sheets whose tips are aligned with each other. One end of the triangular copper sheet is connected to the signal line, and the other is connected to the ground. When there is static electricity, it will produce tip discharge and then consume electric energy.

5. Adopt LC filter to protect the circuit.

LC filter can effectively reduce high-frequency static electricity entering the circuit. The inductive reactance of the inductor can well prevent high-frequency ESD from entering the circuit, while the capacitor shunts the high-frequency energy of ESD to the ground. At the same time, this type of filter can smooth the signal edge with less RF effect, and its performance has been further improved in signal integrity.

6. ESD protection is provided for multilayer boards.

With the permission of funds, selecting multilayer boards is also an effective means to prevent ESD. In the Multilayer board, since there is a complete ground plane close to the wiring, ESD can be more quickly coupled to the low impedance plane, thus protecting the role of key signals.

7. The protection method of leaving protective tape around the circuit board.

This method is usually used to draw wiring without assembly and welding layer around the circuit board. If conditions permit, connect the wiring to the housing. At the same time, note that the wiring cannot form a closed loop, so as to avoid causing more trouble by forming a loop antenna.

8. CMOS devices with clamping diodes or TTL devices are used for circuit protection.

This method uses the isolation principle to protect the circuit board. Because these devices are protected by clamping diodes, the complexity of the design is reduced in the actual circuit design.

9. Decoupling capacitors are often used.

These decoupling capacitors should have low ESL and ESR values. For low-frequency ESD, the decoupling capacitors reduce the area of the loop. Because the role of its ESL weakens the role of the electrolyte, it can better filter out high-frequency energy.

In a word, although ESD is terrible, it may even bring serious consequences. However, only by protecting the power supply and signal lines on the circuit, the ESD current can be effectively prevented from flowing into the PCB. Among them, my boss often said that "a good grounding of a board is the king's way", and I hope that this sentence can also bring about the effect of breaking the skylight.

PCB manufacturers, PCB designers and PCBA manufacturers will explain the practical experience of ESD design of PCB - how PCB can produce high-quality PCBs.