Some small principles of PCB copying process

Time:2019-10-19| Author:admin


1: Printed wire width selection: 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 wire is large, which affects the performance of the circuit. If the width is small, the wiring density is not high, and the board area is increased. In addition to increasing the cost, it is also disadvantageous for miniaturization. If the current load is calculated at 20 A/mm 2 , when the thickness of the copper clad is 0.5 MM (generally so much), the current load of 1 MM (about 40 mil) line width is 1 A, so the line width is 1 - 2.54. MM (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 according to the power. On the low-power digital circuit, in order to improve the wiring density, the minimum The line width is 0.254 - 1.27MM (10 - 15MIL). In the same board, the power cord. The ground line is thicker than the signal line.


2: Line spacing: When it is 1.5MM (about 60MIL), 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 1MM (40MIL), the maximum withstand voltage between lines is 200V. Therefore, on a circuit board with medium and low voltage (line voltage not greater than 200V), the line spacing 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. The production process allows, can be small.


3: Pad: For a 1/8W resistor, a pad lead diameter of 28 MIL is sufficient, while for a 1/2 W, the diameter is 32 MIL, the lead 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 lead hole is too small, and component mounting is difficult.


4: Draw circuit frame: The shortest distance between the frame 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 principle: A: General principle: In PCB design, if there are digital circuits and analog circuits in the circuit system. As well as high-current circuits, they must be laid out separately, so that the matching between the systems can be minimized in the same type of circuit, and the components are placed in the direction of signal flow and function, partitioning, and partitioning.


6: Input signal processing unit, the output signal driving 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 of input and output.


7: Component placement direction: components can only be arranged in both horizontal and vertical directions. Otherwise it is not allowed in the plugin.


8: Component spacing. For medium-density boards, 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 spacing can be 50-100 MIL (1.27 - 2.54MM). Larger, such as 100 MIL, integrated circuit chip, component spacing is generally 100 - 150 MIL.


9: When the potential difference between components is large, the component spacing should be large enough to prevent discharge.


10: In the IC, the tantalum capacitor is close to the power supply line of the chip. Otherwise the filtering effect will be worse. In the digital circuit, in order to ensure reliable operation of the digital circuit system, an IC decoupling capacitor is placed between the power supply of each digital integrated circuit chip and the ground. The tantalum capacitor is generally made of a ceramic capacitor, and the capacity of the capacitor is 0.01~0.1UF. The selection of the capacity of the tantalum capacitor is generally selected according to the reciprocal of the operating frequency F of the system. In addition, a 10UF capacitor and a 0.01UF ceramic capacitor are required between the power and ground wires at the entrance of the circuit power supply.


11: The hour hand circuit component is as close as possible to the clock signal pin of the microcontroller chip to reduce the length of the connection of the clock circuit. And it is best not to go offline.

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