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: the line width is too small, the resistance of the printed wire is large, and the voltage drop on the line is large, which affects the performance of the circuit. The line width is too Wide, the wiring density is not high, the board area increases, in addition to increasing costs, it is not conducive to miniaturization. If the current load is calculated as 20A / mm2, when the thickness of the copper clad foil is 0.5 MM, (usually so many), the current load of 1MM (about 40 MIL) line width is 1 A, so the line width is taken as 1-2.54 MM (40-100 MIL) 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 size. On the low-power digital circuits, in order to improve the wiring density, the minimum Line width can be satisfied by taking 0.254-1.27MM (10-15MIL). In the same circuit board, the power cord. The ground wire is thicker than the signal wire.
2: Line spacing: When it is 1.5MM (about 60 MIL), the insulation resistance between the lines is greater than 20 M ohms, and the maximum voltage between the lines can reach 300 V. When the line spacing is 1MM (40 MIL), the maximum voltage between the lines is 200V Therefore, on the circuit board of medium and low voltage (the voltage between the lines is not more than 200V), the line spacing is taken as 1.0-1.5 MM (40-60 MIL). In low voltage circuits, such as digital circuit systems, it is not necessary to consider the breakdown voltage, as long Production process allows, can be very small.
3: Pad: For the 1 / 8W resistor, the pad lead diameter is 28MIL is sufficient, and for 1 / 2 W, the diameter is 32 MIL, the lead hole is too large, and the pad copper ring width 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: The shortest distance between the border line and the component pin pad cannot be less than 2MM, (generally 5MM is more reasonable) otherwise, it is difficult to cut the material.
5: Principle of component layout: A: General principle: In PCB design, if there are both digital circuits and analog circuits in the circuit system. As well as high-current circuits, they must be laid out separately to minimize coupling between systems. In the same type of circuit, components are placed in blocks and partitions according to signal flow direction and function.
6: Input signal processing unit, output signal drive element should be close to the circuit board side, make the input and output signal line as short as possible, in order to reduce the interference of input and output.
7: Component placement direction: Components can only be arranged in two directions, horizontal and vertical. Otherwise, plug-ins are not allowed.
8: Element spacing. For medium density boards, the spacing between small components such as low 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). Larger, such as taking 100MIL, integrated circuit chip, component spacing is generally 100-150MIL.
9: When the potential difference between the components is large, the spacing between the components should be large enough to prevent discharges.
10: In the IC, the decoupling capacitor should be close to the power supply ground pin of the chip. Otherwise, the filtering effect will be worse. In digital circuits, in order to ensure the reliable operation of digital circuit systems, IC decoupling capacitors are placed between the power supply and ground of each digital integrated circuit chip. Decoupling capacitors generally use ceramic chip capacitors with a capacity of 0.01 ~ 0.1 UF. The selection of decoupling capacitor capacity is generally based on the reciprocal of the system operating frequency F. In addition, a 10UF capacitor and a 0.01 UF ceramic capacitor are also required between the power line and the ground at the entrance of the circuit power supply.
11: The hour hand circuit component should be as close as possible to the clock signal pin of the single chip microcomputer chip to reduce the connection length of the clock circuit. And it is best not to run the wire below.