1. The basis for selecting the width of the printed wire:
the minimum width of the printed wire is related to the current flowing through the wire: if the line width is too small, the resistance of the printed wire will be large, and the voltage drop on the line will be large, which will affect the performance of the circuit. Wide, the wiring density is not high, and 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/square millimeter, when the thickness of the copper clad foil is 0.5MM, (generally so much), the current load of the 1MM (about 4OMIL) line width is 1A, therefore, 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 equipment board can be appropriately increased according to the power level. On low-power digital circuits, in order to increase the wiring density, the minimum The line width can be satisfied by taking 0.254-1.27MM (10-15MIL). In the same circuit board, the power line. Ground wires are thicker than signal wires.
2. Line spacing:
When the line spacing 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 circuit boards 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 circuits, such as digital circuit systems, there is no need to consider the breakdown voltage, as long as the production Technology permits, it can be very small.
3. Pad:
For 1/8W resistors, the diameter of the pad lead wire is 28MIL is enough, while for 1/2W, the diameter is 32MIL, the lead hole is too large, and the width of the pad copper ring is relatively reduced , leading to a decrease in pad adhesion. It is easy to fall off, the lead hole is too small, and it is difficult to install and install components.
4. Draw the circuit frame:
the shortest 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:
A: General principles: In PCB design, if there are both digital circuits and analog circuits in the circuit system. As well as high-current circuits, the layout must be separated to minimize the coupling between the systems. In the same type of circuit, according to the signal flow and function, place components in blocks and partitions.
6. The input signal processing unit and the output signal drive components should be close to the edge of the circuit board, so that the input and output signal lines are 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 is not allowed in the plug-in.
8. Component spacing.
For medium-density boards, the distance between small components, such as small power resistors, capacitors, diodes, and other discrete components, is related to the plug-in and welding process. During wave soldering, the component spacing can be 50-100MIL (1.27—2.54MM) and can be done manually. Larger, such as 100MIL, integrated circuit chip, component spacing is generally 100——150MIL
9. When the potential difference between components is large, the spacing between components should be large enough to prevent discharge.
10. The decoupling capacitor of the IC that has been added should be close to the power supply and ground wire pins 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, 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 need to 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 as close as possible to the clock signal pin of the microcontroller chip to reduce the connection length of the clock circuit. And it's best not to wire below.
