PCB Technical - Five principles of PCB board craftsmanship

Five principles of PCB board craftsmanship

1. The basis for selecting the width of the printed wire: the minimum width of the printed wire is related to the size of the current flowing through the wire: PCB board line width is too small, the resistance of the printed wire is too large, the voltage drop on the line is also large, which affects the circuit performance of the circuit board, If the line width is too wide, the wiring density is not high, and the PCB area increases. In addition to increasing the cost, it is also not conducive to miniaturization. If the current load is calculated at 20A/mm2, when the thickness of the copper foil is 0.5MM, (generally For so many, the current load of 1MM (about 40MIL) line width is 1A, therefore, the line width of 1-2.54MM (40-100MIL) can meet the general application requirements, the ground wire and power supply on the high-power equipment board, According to the power, the line width can be increased appropriately. In the low-power digital circuit, in order to improve the wiring density, the minimum line width can be satisfied if the minimum line width is 0.254–1.27MM (10–15MIL). In the same circuit board, the power and ground wires are thicker than the signal wires.

2. The line spacing of the PCB board: when it is 1.5MM (about 60MIL), the insulation resistance between the lines is greater than 20M ohms, and the maximum withstand voltage between the lines can reach 300V. When the line spacing is 1MM (40MIL), the maximum resistance between the lines The voltage is 200V. Therefore, on the circuit board of medium and low voltage (the voltage between the lines is not greater than 200V), the line spacing is 1.0-1.5MM (40-60MIL). In low-voltage circuits, such as digital circuit systems, it is not necessary to consider the breakdown voltage. As long as the production process permits, it can be small.

3. Pad: For 1/8W resistance, the diameter of the pad lead is 28MIL, and for 1/2W, the diameter is 32MIL, the lead hole is too large, and the width of the copper ring of the pad is relatively reduced., Resulting in a decrease in the adhesion of the pad. It is easy to fall off, the lead hole is too small, and the component placement is difficult.

4. Draw the circuit border of the circuit board: The shortest distance between the border line and the component pin pad should not be less than 2MM, (generally 5MM is more reasonable) otherwise it will be difficult to blank.

5, the principle of component layout:

A. General principles: In the PCB board design, if the circuit system has both digital circuits and analog circuits. And high-current circuits, they must be laid out separately to minimize the coupling between the systems. In the same type of circuit, according to the signal flow direction And function, divide into blocks, place components in districts.

B. Component placement direction: Components can only be arranged in two directions, horizontal and vertical. Otherwise, they cannot be used in plug-ins.

C. When the potential difference between the components is large, the component spacing should be large enough to prevent discharge.

D. Input signal processing unit, output signal drive components should be close to the side of the circuit board, make the input and output signal lines as short as possible to reduce the interference of input and output.

E. Component spacing. For medium-density boards, small components, such as low-power resistors, capacitors, diodes, and other discrete components, the spacing between each other is related to the plug-in and the welding process. During wave soldering, the component spacing can be 50-100MIL (1.27-2.54). MM) The manual can be larger, such as taking 100MIL, integrated circuit chip, the component spacing is generally 100-150MIL.

F. Before entering the IC, the capacitor should be close to the power supply and ground pin of the chip. Otherwise, the filtering effect will be worse. In the digital circuit, in order to ensure the reliable operation of the digital circuit system, the power supply of each digital integrated circuit chip IC decoupling capacitors are placed between the ground. Decoupling capacitors generally use ceramic capacitors with a capacity of 0.01 ~ 0.1UF. A 10UF capacitor and a 0.01UF ceramic capacitor should also be added between the power line and the ground line.

G. The hour hand circuit components are as close as possible to the clock signal pins of the microcontroller chip to reduce the wiring length of the clock circuit. And it is best not to route the wires below.