Detail the ESD of PCB board

- Sep 15, 2018-

Recently, I have been doing ESD testing of electronic products. According to the test results of different products, this ESD is a very important test: if the circuit board is badly designed, the introduction of static electricity will cause the product to crash or even damage the components.I have only noticed that ESD can damage components, but I did not expect that enough attention should be paid to electronic products.

ESD, also known as Electro-Static discharge.It can be known from the learned knowledge that static electricity is a natural phenomenon, which is usually generated through contact, friction, induction among electric appliances, etc., which is characterized by long time accumulation, high voltage (which can produce thousands of volts or even tens of thousands of volts of static electricity), low electricity, small current and short acting time.For electronic products, if the ESD design is not well designed, the operation of electronic products is often unstable or even damaged. 

When doing ESD discharge test, two methods are usually adopted: contact discharge and air discharge.The contact discharge is to discharge the test equipment directly.Air discharge, also known as indirect discharge, is a strong magnetic field coupled to the adjacent current loop.The test voltage of these two kinds of tests is usually 2kv-8kv, and the requirements are different in different regions. Therefore, before designing, it is necessary to figure out the market for the product. 

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

Detail the ESD of PCB board

Humidity varies around the world, but in an area, if air humidity is different, so is static electricity.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 northern winter, when taking off the sweater produces the static electricity spark very big reason. 

Detail the ESD of PCB board

Since static electricity is so harmful, how can we protect?In electrostatic protection design, we usually take three steps: to prevent the external charge from flowing into the circuit board and causing damage;Prevent external magnetic field from damage to circuit board;Prevent the harm from electrostatic field. 

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

1. Avalanche diode for electrostatic protection.This is often used in design, typically by connecting an avalanche diode to the ground in parallel with a key signal line. 

Detail the ESD of PCB board

The method USES the avalanche diode to respond quickly and has the ability of stable clamping, which can consume the high voltage of accumulation in a short time and then protect the circuit board. 

2. Use high voltage capacitance for circuit protection. 

This method usually places the ceramic capacitance with a voltage of at least 1.5kv at the position of the I/O connector or key signal, and the connection line is as short as possible to reduce the inductance of the connection line.If low voltage capacitance is used, the capacitance will be damaged and lose protection. 

3. Use ferric oxygen magnetic beads for circuit protection. 

Ferroxomagnetic beads can well attenuate ESD currents and also suppress radiation.When faced with two problems, an iron oxide bead is a good choice. 

4. Spark gap method. 

The method is seen in one material, in which the microstrip layer of copper is formed using a triangular copper sheet with points aligned, one end of which is connected to the signal line and the other to the ground.When there is static electricity, the tip will discharge and consume electricity. 

5. Protect the circuit by means of LC filter. 

LC filter can effectively reduce the high frequency electrostatic entry circuit.The inductive resistance of the inductor can well inhibit the high-frequency ESD into the circuit, and the capacitance shunt the high-frequency energy of ESD to the ground.At the same time, this type of filter can also round the signal edge and small RF effect, the performance of the signal integrity has further improved. 

6. Multi-layer board for ESD protection. 

Selecting multiple layers is also an effective means of preventing ESD when funds are available.In the multi-layer board, a complete ground plane is close to the routing line, which enables the ESD to be coupled to the low impedance plane more quickly, thus protecting the role of key signals. 

7. Protective tape on the circuit board. 

This method is usually to draw the line around the circuit board without the composite welding layer.Connect the walkthrough to the enclosure if conditions permit, and note that the walkthrough does not form a closed loop to avoid further trouble in forming a loop antenna. 

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

This method makes use of the isolation principle to protect the circuit board. As these devices are protected by clamping diode, the complexity of the design is reduced in actual circuit design. 

9. Decoupling capacitance is mostly adopted. 

These decoupling capacitors should have low ESL and ESR values. For low-frequency ESD, the decoupling capacitors reduce the area of the loop, and the electrolyte effect is weakened due to the effect of ESL, which can better filter out high-frequency energy. 

In conclusion, although the ESD is terrible and even has serious consequences, it can effectively prevent ESD current from flowing into the PCB only if the power supply and signal line are well protected.Among them, my eldest brother often said that "a good grounding of a board is the king", I hope that this sentence can also give everyone the effect of breaking the skylight.