Five principles of PCB technology

1: Print line width selection: The minimum width of the printed line is related to the current flowing through the line: the line width is too small, the resistance of the printed line is large, and the voltage drop on the line is large, which affects the performance of the circuit. The width, the wiring density is not high, and the board area is increased. In addition to increasing the cost, it is not conducive to miniaturization. If the current load is calculated as 20A / mm 2 , when the thickness of the copper foil is 0.5MM (generally), the 1MM (about 40MIL) line width current load is 1A, so the line width is 1-2.54MM (40-100MIL). ) can meet the general application requirements, the ground wire and power supply on the high-power device board can be appropriately increased according to the power level, and in the low-power digital circuit, in order to increase the wiring density, it can pass 0.254-1.27MM (10-15 dense) Ear) to meet the minimum line width. In the same board, the power and ground lines are thicker than the signal lines.


2: Line spacing: When it is 1.5MM (about 60 MIL), 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 1 MM (40 mil), the maximum withstand voltage between the lines is 200V. Therefore, on a circuit board having a medium-low voltage (line voltage not greater than 200V), the line pitch is 1.0-1.5 MM (40-60 MIL). In a low voltage circuit such as a digital circuit system, it is not necessary to consider the breakdown voltage. The production process allows, can be small.


3: Pad: For a 1 / 8W resistor, a pad diameter of 28 MIL is sufficient, and for a 1 / 2W, a diameter of 32 MIL, the pin hole is large, and the pad copper ring width is relatively reduced. , causing the adhesion of the pad to decrease. It is easy to fall off, the guide hole is too small, and the component is difficult to play.


4: Draw the circuit border:The shortest distance between the boundary line and the component pin pad can't be less than 2MM (usually 5MM is reasonable), otherwise it is difficult to cut.


5: Component layout principles´╝Ü

General Principle: In PCB board design, if the circuit system has both digital and analog circuits and high current circuits, it must be laid out separately to minimize matching between systems in the same type of circuit, depending on signal flow and function, block , partition placement component.

B: Input signal processing unit, the output signal drive component should be close to the edge of the board, so that the input and output signal lines are as short as possible to reduce the interference between the input and output.

C: Component placement direction: components can only be arranged in horizontal and vertical directions. Otherwise, you should not use them in the plugin.

D: component spacing. For MDF, small components, such as small power resistors, capacitors, diodes, etc., the spacing between discrete components is related to the plug-in, soldering process. When soldering, the component pitch can be 50-100 MIL (1.27--2.54MM), which can be hand-made, such as 100 MIL, integrated circuit chip, and the component pitch is generally 100-150 MIL.

E: When the potential difference between the elements is large, the element spacing should be large enough to prevent discharge.

F: In the IC, the tantalum capacitor is close to the power supply of the chip. Otherwise, the filtering effect will be worse. In digital circuits, in order to ensure reliable operation of the digital circuit system, a power source is placed in each digital integrated circuit chip and the IC is placed between ground to remove the tantalum capacitor. Tantalum capacitors are usually made of ceramic capacitors. The capacity is 0.01~0.1UF. The choice of tantalum capacitor capacity is generally chosen based on the reciprocal of the system operating frequency F. In addition, at the entrance to the circuit power supply, a 10UF capacitor and a 0.01UF ceramic capacitor are required between the power supply and ground. .

G: The hour hand circuit component is as close as possible to the clock signal pin of the MCU chip to reduce the connection length of the clock circuit. It is best not to route underneath.