PCB Blog - Some small principles of PCB board technology

The width of the PCB board wire is related to the amount of current flowing through the wire:

1. If the line width is too small, the resistance of the wire just printed is large, and the voltage drop on the line is also large, which affects the performance of the circuit. If the line width is too wide, the wiring density is not high, and the board area increases. In addition to increasing the cost, it is not conducive to miniaturization. If the current load is calculated at 20A/mm2, when the thickness of the copper-clad foil is 0.5MM, (usually so much), the current load of the 1MM (about 40MIL) line width is 1A, so the line width is 1--2.54 MM (40--100MIL) can meet the general application requirements. The ground wire and power supply on the high-power device board can be appropriately increased according to the power size. In the low-power digital circuit, in order to improve the wiring density, the line width can be increased. A wide range of 0.254--1.27MM (10--15MIL) can be satisfied.

In the same circuit board, the power line and the ground line are thicker than the signal line:

2. Line spacing: When it is 1.5MM (about 60MIL), the insulation resistance between lines is greater than 20M ohms, and the withstand voltage between lines can reach 300V. When the line spacing is 1MM (40MIL), the withstand voltage between lines is 200V. Therefore, on the circuit board of medium and low voltage (line-to-line voltage is not more than 200V), the line spacing is 1.0--1.5MM (40--60MIL). In low-voltage circuits, such as digital circuit systems, there is no need to consider the breakdown voltage, as long as the production Process allows, can be very small.

3. Pad: For a 1/8W resistor, 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 pad copper ring 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 frame: the short distance between the frame line and the component pin pad should not be less than 2MM, (generally 5MM is more reasonable), otherwise it will be difficult to cut the material.

5. Component layout principles:

1) General principle: In PCB board design, if there are digital circuits and analog circuits in the circuit system at the same time. As well as high-current circuits, they must be laid out separately, so that the coupling between each system can be achieved in the same type of circuit, and components are placed in blocks and partitions according to signal flow and function.

2) The input signal processing unit and the output signal driving components should be close to the edge of the PCB board, so that the input and output signal lines should be as short as possible to reduce the interference of input and output.

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

4) Component spacing: For medium-density boards, and 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 welding process. When wave soldering, the component spacing can be 50-100MIL (1.27--2.54 MM) Manual can be larger, such as taking 100MIL, integrated circuit chip, the component spacing is generally 100--150MIL.

5) When the potential difference between components is large, the distance between components should be large enough to prevent discharge.

6) The decoupling capacitor in the IC should be close to the power and ground pin of the chip. Otherwise, the filtering effect will be poor. In the digital circuit, in order to ensure the reliable operation of the digital circuit system, IC decoupling capacitors are placed between the power supply and the ground of each digital integrated circuit chip. The decoupling capacitors generally use ceramic capacitors. The capacity of the decoupling capacitor is generally selected according to the reciprocal of the system operating frequency F. In addition, a 10UF capacitor and a 0.01UF ceramic capacitor should also be added between the power line and the ground line at the entrance of the circuit power supply.

7) The clock hand circuit components should be as close as possible to the clock signal pins of the single-chip microcomputer chip to reduce the wiring length of the clock PCB board, and do not route the wires below.