PCB Board Reliability Design -3

D. Decoupling Capacitor Configuration

In the DC power supply loop, the change of the load will cause the power supply noise. For example, in digital circuits, when the circuit changes from one state to another, a large spike current will be generated on the power line, forming a transient noise voltage. The configuration of decoupling capacitors can suppress the noise generated by load changes, which is a common practice in the reliability design of printed circuit boards. The configuration principles are as follows:

●A 10～100uF electrolytic capacitor is connected across the power input. If the position of the printed circuit board allows, the anti-interference effect of using an electrolytic capacitor above 100uF will be better.

● Configure a 0.01uF ceramic capacitor for each integrated circuit chip. If the printed circuit board space is small and cannot be installed, a 1-10uF tantalum electrolytic capacitor can be configured for every 4-10 chips. The high-frequency impedance of this device is particularly small, and the impedance is less than 1Ω in the range of 500kHz-20MHz. And the leakage current is very small (less than 0.5uA).

●For devices with weak noise capability and large current changes during turn-off, and storage devices such as ROM, RAM, etc., a decoupling capacitor should be directly connected between the power line (Vcc) and ground (GND) of the chip.

●Decoupling capacitor’s lead wire cannot be too long, especially the high-frequency bypass capacitor cannot take lead wire.

E. The Size of Printed Circuit Board And The Layout of The Device

The size of the printed circuit board should be moderate. If it is too large, the printed line will be long and the impedance will increase, which will not only reduce the noise resistance, but also increase the cost. If it is too small, the heat dissipation will not be good, and it will be easily interfered by adjacent lines.

In terms of device layout, like other logic circuits, the devices related to each other should be placed as close as possible so that a better anti-noise effect can be obtained. The time generator, crystal oscillator, and the clock input of the CPU are all prone to noise, so they should be closer to each other. Devices that are prone to noise, low-current circuits, and high-current circuits should be kept away from logic circuits as much as possible. If possible, separate circuit boards should be made. This is very important.

A lot of practical experience has shown that the use of a reasonable device arrangement can effectively reduce the temperature rise of the printed circuit, so that the failure rate of devices and equipment is significantly reduced.

The above is an introduction to the general principles of reliability design. We can understand that the design of PCB boards is very important. These reliability designs have a direct relationship with the quality of the finished circuit board, and also affect the application performance of the circuit board on the product.

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