1. Selection basis for the width of printed wires: the minimum width of printed wires is related to the magnitude of the current flowing through the wires: if the wire width is too small, the resistance of the just-printed wires is large, and the voltage drop on the line is also large, which affects the performance of the circuit, and the line width is too large. Wide, the wiring density is not high, the board 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 clad foil is 0.5MM, (usually so much), the current load of the 1MM (about 40MIL) line width is 1A, therefore, the line width is taken as 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 minimum The line width can be satisfied by taking 0.254-1.27MM (10-15MIL). On the same board, the power lines. The ground wire is thicker than the signal wire.
2. Line spacing: when it is 1.5MM (about 60MIL), the insulation resistance between lines is greater than 20M ohms, 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 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, it is not necessary to consider the breakdown voltage, as long as The production process allows, it 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 decreased pad adhesion. It is easy to fall off, the lead hole is too small, and the component placement is difficult.
4. Draw the circuit border: the shortest distance between the border line and the component pin pad cannot be less than 2MM, (generally 5MM is more reasonable), otherwise it will be difficult to cut the material.
5. Principles of component layout: A: General principle: In PCB 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 to minimize the coupling between each system. In the same type of circuit, components are placed in blocks and partitions according to the signal flow direction and function.
6. The input signal processing unit and the output signal driving element should be close to the edge of the circuit board, so that the input and output signal lines should be as short as possible to reduce the interference of input and output.
7. Component placement direction: Components can only be arranged in two directions, horizontal and vertical. Otherwise it cannot be used by plugins.
8. 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 welding process. During wave soldering, the component spacing can be 50-100MIL (1.27-2.54MM) Manually Larger, such as 100MIL, integrated circuit chip, the component spacing is generally 100-150MIL.
9. When the potential difference between components is large, the distance between components should be large enough to prevent discharge.
10. The decoupling capacitors that have entered the IC should be close to the power and ground pins 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, an IC decoupling capacitor is placed between the power supply and the ground of each digital integrated circuit chip. Decoupling capacitors generally use ceramic capacitors, with a capacity of 0.01~0.1UF. The choice of decoupling capacitor capacity 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.
11. 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 circuit. And it is best not to run the line below.
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