PCB Tech - What types of components are generally used for PCB?

On the premise of meeting the functional requirements of the circuit, components with high sensitive voltage thresholds should be used as much as possible. Because the electrostatic discharge sensitivity of a PCB board is determined by the component with the lowest sensitive voltage threshold in the circuit board. In addition to implementing the correct selection, it should also be noted that the sensitive voltage thresholds of components with the same function and model but with different PCB manufacturers may vary greatly. Components from the same PCB manufacturer but with different production batches, It may also be different.

1: Limiting the output current can avoid the locking effect of the CMOS circuit

The lock-in effect is a unique failure mode of CMOS circuits. This is because there are parasitic PNP transistors and NPN transistors in the internal structure of CMOS circuits, and a parasitic PNPN thyristor structure is formed between them, so The lock-in effect of CMOS circuits is sometimes

Also called the thyristor effect. This kind of interlocking positive feedback structure may be triggered by external factors (such as electrostatic discharge), and current will flow on the PNP tube (or NPN tube), and then through another parasitic NPN tube (or PNP tube) to make the current flow Amplify, and due to positive feedback, the current becomes larger and larger and eventually burned. It can be seen that limiting the current so that it cannot reach the level of maintaining the locked state is one of the issues to be considered for CMOS devices when designing printed circuit boards. The common solution is to use a resistor to isolate each output terminal from its cable, and use two high-speed switching diodes to clamp to VDD (drain power) and VSS (source power) with the cable. .

2: Use filter network

Sometimes a long input cable is required between the CMOS circuit system and the mechanical contacts, which increases the possibility of electromagnetic interference, and a filter network should be considered. At the same time, long input lines must be accompanied by larger distributed capacitance and distributed inductance, which can easily form LC self-excited oscillation, especially when there is a negative oscillation voltage at the input terminal, which may lead to protection

The diode of the network is burnt out. The way to solve this problem is to connect a resistor in series at the input end. Its resistance can be selected according to the formula R=VDD/1mA. For example, when VDD=10V, then R=10 kΩ.

3: RC network

Where feasible, for the sensitive input terminals of bipolar devices, an RC network composed of a resistor with a larger resistance value and a capacitor of at least 100pF can be used to reduce the impact of electrostatic discharge. However, if the circuit characteristics have requirements, two parallel diodes that can be clamped to 0.5V in either polarity can be used to short the input to ground. In this way, the interference to the input characteristics is reduced.

4: Avoid floating input pins of CMOS devices

Avoid floating the input terminals of CMOS devices that have been soldered to the circuit board. At the same time, note that all unused redundant input leads on the CMOS device are not allowed to float. This is because once the input terminal is left floating, the input potential will be in an unstable state, which will not only destroy the normal logical relationship of the circuit, but also cause electrostatic breakdown and external noise due to its high input resistance.

Acoustic interference and other phenomena. The redundant input terminal should be dealt with separately according to the function of the circuit. For example, the redundant input terminals of the AND gate and NAND gate circuit should be connected to VDD or high level; the redundant input terminals of the OR gate and NOR gate should be connected to VSS or low level.