Some small principles of PCB copy board process
2023-12-07
1. The selection of printing wire width basis
The minimum width of a printed wire is related to the amount of current flowing through the wire:
If 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, and the line width is too wide, the wiring density is not high, and the board area increases, in addition to increasing the cost, it is not conducive to miniaturization.?If the current load is calculated as 20A/square millimeter, when the thickness of the copper clad foil is 0.5mm, (generally so much,) then the current load of 1mm (about 40mil) line width is 1A, therefore, the line width is 1-2.54mm (40-100mil) To 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, and on the low-power digital circuit, in order to improve the wiring density, the minimum line width can be 0.254-1.27mm (10-15mil). In the same board, the power cord. The ground wire is thicker than the signal wire.
2. Line spacing
When it 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 300V, when the line spacing is 1mm (40mil), the maximum withstand voltage between the lines is 200V, therefore, on the circuit board of medium and low voltage (the voltage between the lines is not greater than 200V), the line spacing is 1.0-1.5mm (40-60mil) in the low-voltage circuit, such as the digital circuit system, there is no need to consider the breakdown voltage, as long as the production process allows, it can be very small.
3. Pads
For a 1/8W resistor, a pad lead diameter of 28mil is sufficient.
For 1/2W, the diameter is 32mil, the lead hole is large, and the width of the copper ring of the pad 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 it is difficult to install the component.
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 is difficult to cut the material.
5. The principle of component layout
A: General principle: In PCB design, if the circuit system has both digital circuits and analog circuits. As well as high-current circuits, they must be arranged separately to minimize the integration between the various systems in the same type of circuit, according to the signal flow direction and function, block and partition placement components.
B: The input signal processing unit, the output signal driving element 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 the input and output.
C: Component Placement Direction: Components can only be arranged in both horizontal and vertical directions. Otherwise, it shall not be plugged in.
D: Component spacing. For MDF boards, small components, such as small power resistors, capacitors, diodes, and other discrete components are related to plug-ins, soldering processes, when wave soldering, the component spacing can be 50-100mil (1.27-2.54mm) manual can be larger, such as 100mil, integrated circuit chips, the component spacing is generally 100-150mil
E: When the potential difference between the components is large, the spacing between the components should be large enough to prevent the discharge phenomenon.
F: In the IC that has been inducted, the capacitor should be close to the power supply of the chip. Otherwise, the filtering effect will deteriorate. 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 capacitor is 0.01-0.1uF, and the capacity of the capacitor is generally selected according to the reciprocal of the system operating frequency F.In addition, a 10uF capacitor and a 0.01uF ceramic capacitor are also required between the power line and the ground wire at the inlet of the circuit power supply.
G: The hour hand circuit element is as close as possible to the clock signal pin of the MCU chip to reduce the connection length of the clock circuit. And it's better not to trace the line below.
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