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 PCB board line width is too small, and the voltage drop on the printed wire has a large resistance, which will greatly affect the circuit performance of the circuit board; If the width is too wide, the wiring density will not be high, and the increase in PCB area will not only increase 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 this much) Then the current load of 1MM (about 4OMIL) line width is 1A. Therefore, the line width of 1-2.54MM (40-100MIL) can meet the general application requirements. The line width can be increased appropriately, and in order to increase the wiring density on low-power digital circuits, the minimum line width is 0.254-1.27MM (10-15MIL). In the same circuit board, the power line and ground line are thicker than the signal line.
2. The line spacing of the PCB board: When the line spacing is 1.5MM (about 60MIL), the insulation resistance between the lines is greater than 20M ohms, and the maximum withstand voltage between the lines can reach 30OV. When the line spacing is 1MM (4OMIL), the maximum resistance between the lines The voltage is 200V. Therefore, the line spacing on the circuit board of medium and low voltage (the voltage between lines is not greater than 200V) 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 , can be very small.
3. Pad: For 1/8W resistors, the diameter of the pad lead wire is 28MIL is enough, but for 1/2W, the diameter of the 32MIL lead hole is too large, and the width of the pad copper ring is relatively reduced. Lead to a decrease in the adhesion of the pad. It is easy to fall off, the lead hole is too small, and the components are difficult to install.
4. Draw the circuit border of the circuit board: 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 will be difficult to cut the material.
5. Component layout principles:
A. General principles: In PCB board design, if there are digital circuits and analog circuits and high-current circuits in the circuit system at the same time, the layout must be separated to minimize the coupling between the systems. In the same type of circuit, according to the signal flow direction And function, divide into blocks, and place components in partitions.
B. Component placement direction: Components can only be arranged in horizontal and vertical directions. Otherwise, they cannot be inserted.
C. When the potential difference between components is large, the spacing between components should be large enough to prevent discharge.
D. 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.
E. Component spacing. For small components of medium density boards, such as small power resistors, capacitors, diodes, and other discrete components, the spacing between them is related to the plug-in; the welding process is related. During wave soldering, the component spacing can be 50-100MIL (1.27-2.54 MM) Manual can be larger, such as 100MIL, integrated circuit chip, component spacing is generally 100-150MIL.
F. The IC decoupling capacitor that has been added should be close to the power supply and ground wire 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 power supply and the power supply of each digital integrated circuit chip IC decoupling capacitors are placed between the grounds. 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, the A 10UF capacitor and a 0.01UF ceramic capacitor also need to be connected between the power line and the ground wire.
G. The clock circuit components 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 wire below.
