PCB Tech - Practical experience of ESD design of circuit boards-how PCBs produce high-quality PCBs

Recently I am doing ESD testing of electronic products. From the test results of different products, it is found that this ESD is a very important test: if the circuit board is not well designed, when static electricity is introduced, it will cause the product to crash or even the components. Damage. In the past, I only noticed that ESD would damage the components, but I didn't expect to pay enough attention to electronic products.

ESD is what we often call Electro-Static discharge. From the learned knowledge, it can be known that static electricity is a natural phenomenon, which is usually generated through contact, friction, induction between electrical appliances, etc. It is characterized by long-term accumulation and high voltage (which can generate thousands of volts or even tens of thousands of volts of static electricity. ), low power, low current and short action time. For electronic products, if the ESD design is not well designed, the operation of the electronic and electrical products is often unstable or even damaged.

Two methods are usually used when doing ESD discharge tests: contact discharge and air discharge. Contact discharge is to directly discharge the equipment under test; air discharge is also called indirect discharge, which is generated by the coupling of a strong magnetic field to adjacent current loops. The test voltage for these two tests is generally 2KV-8KV, and the requirements are different in different regions. Therefore, before designing, we must first figure out the market for the product.

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

Humidity conditions are different in different parts of the world, but at the same time in a region, if the air humidity is not the same, the static electricity generated is also different. The following table is the collected data, from which it can be seen that the static electricity increases as the air humidity decreases. This also indirectly explains the reason why the static sparks generated when taking off the sweater in the northern winter are very big.

Since static electricity is such a great hazard, how do we protect it? When designing static electricity protection, we usually take three steps: prevent external charges from flowing into the circuit board and cause damage; prevent external magnetic fields from damaging the circuit board; prevent electrostatic field generation The harm.

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

1. Avalanche diodes for electrostatic protection.

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

This method is to use the avalanche diode to respond quickly and have the ability to stabilize the clamping, which can consume the concentrated high voltage in a short time to protect the circuit board.

2. Use high-voltage capacitors for circuit protection.

In this approach, ceramic capacitors with a withstand voltage of at least 1.5KV are usually placed in the I/O connector or the position of the key signal, and the connection line is as short as possible in order to reduce the inductance of the connection line. If a capacitor with low withstand voltage is used, it will cause damage to the capacitor and lose its protection.

3. Use ferrite beads for circuit protection.

Ferrite beads can attenuate ESD current very well, and can also suppress radiation. When faced with two problems, a ferrite bead is a very good choice.

4. Spark gap method.

This method is seen in a piece of material. The specific method is to use triangular copper with the tips aligned with each other on the microstrip line layer composed of copper. One end of the triangular copper is connected to the signal line, and the other is the triangular copper. Connect to the ground. When there is static electricity, it will produce sharp discharge and consume electrical energy.

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

The filter composed of LC can effectively reduce the high frequency static electricity from entering the circuit. The inductive reactance characteristic of the inductor is very good to inhibit high-frequency ESD from entering the circuit, while the capacitor shunts the high-frequency energy of the ESD 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 has been further improved in terms of signal integrity.

6. Multi-layer board for ESD protection.

When funds permit, choosing a multilayer board is also an effective means to prevent ESD. In the multi-layer board, because there is a complete ground plane close to the trace, this can make the ESD couple to the low impedance plane more quickly, and then protect the role of key signals.

7. The protection method of leaving a protective tape on the periphery of the circuit board.

This method is usually to draw traces around the circuit board without welding layer. When conditions permit, connect the trace to the housing. At the same time, it should be noted that the trace cannot form a closed loop, so as not to form a loop antenna and cause greater trouble.

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

This method uses the principle of isolation 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. Use decoupling capacitors.

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

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