Five Unknown Principles of PCB Board Technology

PCB board process principle:
1: Selection basis of width of printed wire: The minimum width of printed wire is related to the current flowing through the wire: too small line width, high resistance of the newly printed wire, large voltage drop on the line, which affects the performance of the circuit. Width too wide, the wiring density is not high, the area of the board is increased, besides increasing cost, it is not conducive to miniaturization. If the current load is calculated by 20A/square millimeter, when the thickness of copper foil is covered. At 0.5MM, the current load of 1MM (about 40MIL) line width is 1A. Therefore, the line width of 1-2.54MM (40-100MIL) can meet the general application requirements. The ground wire and power supply on the high-power equipment board can increase the line width appropriately according to the power size. In the low-power digital circuit, in order to improve the wiring density, the minimum line width of 0.254-1.27MM (10-15MIL) can meet the requirements. In the same circuit board, the ground wire is thicker than the signal line.
2: Line spacing: When 1.5MM (about 60MIL) is used, the insulation resistance between wires is greater than 20MO, and the maximum withstand voltage between wires can reach 300V. When the line spacing is 1MM (40MIL), the maximum withstand voltage between wires is 200V. Therefore, in low-voltage circuit board with Medium-low Voltage (line voltage is not more than 200V), the line spacing should be 1.0-1.5MM (40-60MIL). In low-voltage circuit, such as digital circuit system, breakdown voltage need not be considered, as long as the production process permits, it can be used. Very small.
3: Pad: For 1/8W resistance, the diameter of pad lead is 28 MIL, while for 1/2W, the diameter is 32 MIL, the lead hole is too large, the width of pad copper ring is relatively reduced, which leads to the decrease of pad adhesion. Easy to fall off, lead hole is too small, component installation is difficult.
4: Draw the circuit frame: The shortest distance between the frame line and the component pin pad should not be less than 2MM (usually 5MM is more reasonable), otherwise it is difficult to cut.
5: Component Layout Principle:
A General Principle: In the design of PCB board, if there are both digital circuit and analog circuit in the circuit system, and high current circuit, it must be separately arranged so that the coupling among the systems can be minimized in the same type of circuit, and the components can be placed in blocks and zones according to the direction and function of the signal.
B: In the input signal processing unit, the output signal driver should be close to the circuit board edge so that the input and output signal lines are as short as possible to reduce the interference of input and output.
C: Component placement direction: Components can only be arranged horizontally and vertically. Otherwise, plug-ins are not allowed.
D: Component spacing. For medium density plates, small components, such as small power resistors, capacitors, diodes, and other discrete components, the spacing between each other is related to plug-ins and welding process. When wave soldering, the component spacing can be taken 50-100MIL (1.27-2.54MM) manually, such as 100MIL, integrated circuit chips, and the component spacing is generally 100-150MIL.
E: When the potential difference between components is large, the spacing between components should be large enough to prevent discharge.
F: In the digital circuit, in order to ensure the reliability 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 decoupling capacitor generally adopts the ceramic chip capacitor with the capacity of 0.01-0.1UF. The selection of the decoupling capacitor capacity generally depends on the system operating frequency F. In addition, a capacitor of 10 UF and a ceramic chip capacitor of 0.01 UF are added between the power supply line and the ground line at the entrance of the circuit power supply.
G: Clock circuit elements should be as close as possible to the clock signal pins of MCU chips in order to reduce the connection length of clock circuit. And it is better not to go down below.