Introduction To The ESD of PCB In Detail

Recently, I have been doing ESD test of electronic products, and it is found from the test results of different products that this ESD is a very important test: if the circuit board is not well designed, the introduction of static electricity will cause the crash of the product or even the damage of components. Previously, we only noticed that ESD would damage components, but we did not think that we should pay enough attention to electronic products.

ESD is also known as Electro-Static Discharge. From the learned knowledge can know, static electricity is a natural phenomenon, usually through contact, friction, electrical induction and other ways to produce, its characteristics are 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 time of action. For electronic products, if the ESD design is not well designed, the operation of electronic and electrical products is often unstable, or even damaged.

There are two methods for ESD discharge testing: contact discharge and air discharge. Contact discharge is to discharge the test equipment directly. Air discharge, also known as indirect discharge, is caused by the coupling of strong magnetic field to adjacent current loops. The test voltage of these two kinds of tests is generally 2KV-8KV, and the requirements are different in different regions. Therefore, before design, it is necessary to make clear the market for the product.

In the electrostatic protection design is usually divided into three steps: to prevent the external charge into the circuit board and produce damage; Prevent external magnetic field from damaging the circuit board; Prevent hazards from electrostatic field.

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 often used in design, typically by connecting a key signal line with an avalanche diode to the ground. This method utilizes the ability of avalanche diodes to respond quickly and clamp stably, which can consume the accumulated high voltage in a short time to protect the circuit board.

2. The use of high voltage capacitor circuit protection.

This is usually done by placing a ceramic capacitor of at least 1.5 KV in the I/O connector or critical signal position and keeping the cable as short as possible to reduce the inductance of the cable. If the low voltage capacitor is used, it will cause damage of the capacitor and the loss of protection.

3. The use of iron oxide magnetic beads for circuit protection.

Ferroxide beads can attenuate ESD current very well, and can also suppress radiation. When faced with a two-pronged problem, a ferrooxide bead is a good choice.

4. Spark gap method.

This method is seen in a material, the specific practice is in the copper sheet composed of microstrip line layer using the tip of mutual alignment of the triangle copper sheet, triangle copper sheet is connected to one end of the signal line, the other triangle copper sheet connected to the ground. When there is static electricity, it generates a tip discharge and consumes electricity.

5. Using the LC filter method to protect the circuit.

The filter composed of LC can effectively reduce the high-frequency static electricity into the circuit. The inductive property of the inductor can well suppress the high-frequency ESD entering the circuit, while the capacitance can divert the high-frequency ESD energy to the ground. At the same time, this type of filter can also smooth the edge of the signal and reduce the RF effect, and the performance of the signal integrity has been further improved.

6. Multi-layer board for ESD protection.

When funds permit, the choice of multilayer boards is also an effective means of preventing ESD.

With a complete ground plane close to the routing in multilayer panels, the ESD can be coupled to the low impedance plane more quickly, thus protecting the critical signal function.

7. The circuit board peripheral protection belt protection method.

This is usually done by drawing wires around the circuit board without the added layer of solder.

Connect the cable to the housing when conditions permit, and take care that the cable does not form a closed loop, so as not to form a ring antenna and introduce more trouble.

8. CMOS devise or TTL device with clamp diode is used to protect the circuit.

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

9. Decoupling capacitors are mostly used.

These decoupling capacitors should have low ESL and ESR values. For low-frequency ESD, the decoupling capacitors reduce the area of the loop. Due to the effect of their ESL, the electrolyte action is weakened and the high-frequency energy can be better filtered.

To sum up, although ESD is terrible and may even bring serious consequences, it is effective to prevent ESD current from flowing into PCB only by protecting the power supply and signal line on the circuit. Among them, my eldest brother often says a “good grounding of a board is the king”, hope this sentence can also bring the effect that breaks skylight to everyone.