1: Selection basis of printed wire width: the minimum width of printed wire is related to the current flowing through the wire: if the line width is too small, the resistance of just printed wire is large, the voltage drop on the line will be large, which will affect the performance of the circuit. If the line width is too wide, the wiring density will not be high, and the board area will increase. In addition to increasing the cost, it is not conducive to miniaturization. If the current load is calculated in 20A / mm2, when the thickness of copper foil is 0.5mm, (generally so much), the current load of 1 mm (about 40mil) line width is 1a. Therefore, the line width of 1-2.54mm (40-100mil) can meet the general application requirements. According to the power, the ground wire and power supply on the high-power equipment board can be appropriately increased according to the power, In order to improve the wiring density, the minimum line width is 0.254-1.27mm (10-15mil). In the same circuit board, the power line. The ground wire is thicker than the signal wire.
2: Line spacing: when the line spacing is 1.5mm (about 60mil), the insulation resistance between lines is greater than 20m Ω, and the maximum withstand voltage between lines can reach 300V. When the line spacing is 1mm (40mil), the maximum withstand voltage between lines is 200V. Therefore, on the circuit board with medium and low voltage (the voltage between lines is not greater than 200V), the line spacing is 1.0-1.5mm (40-60mil) in low-voltage circuit, such as digital circuit system, it is not necessary to consider breakdown voltage, as long as the production process allows, it can be very small.
3: Pad: for 1 / 8W resistance, the diameter of pad lead wire is 28mil, while for 1 / 2W, the diameter is 32mil, the lead hole is too large, and the width of pad copper ring is relatively reduced, resulting in the decrease of pad adhesion. It is easy to fall off, the lead hole is too small, and it is difficult to install the components.
4: Draw circuit border: 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 is difficult to cut.
5: Component layout principle: A: general principle: in PCB design, if there are both digital and analog circuits in the circuit system. In the same type of circuit, according to the signal flow direction and function, the components are placed in different areas.
6: The input signal processing unit and the output signal driving element should be close to the circuit board to make the input and output signal line as short as possible to reduce the input and output interference.
7: Component placement direction: components can only be arranged horizontally and vertically. Otherwise, it is not allowed to plug-in.
8: Element spacing. For medium density board, the distance between small components, such as low-power resistor, capacitor, diode and so on, is related to the plug-in and welding process. For wave soldering, the distance between components can be 50-100mil (1.27-2.54mm) and larger manually. For example, if 100mil is used, the spacing between components is generally 100-150mil for integrated circuit chip.
9: When the potential difference between components is large, the distance between components should be large enough to prevent discharge.
10: In the IC, the decoupling capacitance should be close to the power supply 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 IC decoupling capacitor is placed between the power supply and the ground of each digital integrated circuit chip. The decoupling capacitor is usually made of porcelain chip, and the capacity is 0.01 ~ 0.1uF. The selection of decoupling capacitance capacity is generally based on the reciprocal of system operating frequency f. In addition, a 10uF capacitor and a 0.01uF ceramic capacitor should be added between the power line and the ground wire at the entrance of the circuit power supply.
11: The clock circuit components should be close to the clock signal pin of single chip microcomputer chip as far as possible to reduce the connection length of clock circuit. And it's better not to run the line below.