Small Principles of PCB Design and Technology

1. Selection of printed wire width:

The minimum width of a printed wire is related to the amount of current flowing through the wire:

If the width of the printed wire is too small, the resistance of the printed wire is large, and the voltage drop on the line is large, which will affect the performance of the circuit.

If the wire width is too wide, the routing density is not high, and the PCB area is increased.

In addition to increasing costs, it is not conducive to miniaturization. If the current load is calculated at 20 A/mm2, when the thickness of the copper-clad foil is 0.5mm, the current load of 1mm line width is 1 A. Therefore, the wire width of 1--2.54mm (40--100mil) can meet the general application requirements. You can appropriately increase the ground and power supply on the high-power equipment board according to the power, and you can also increase the line width appropriately. On a low-power digital circuits, in order to improve the wiring density, the minimum line width should be 0.254-1.27mm, which is 10-15mil. In the same PCB, the power and ground wires are thicker than the signal lines.

2. Line Spacing: When it is 1.5MM, about 60mil, the insulation resistance between circuits is greater than 20M ohms, and the maximum withstand voltage between the lines can reach 300V. When the line spacing is 1mm and about 40mil, the maximum withstand voltage between the lines is 200V. Therefore, on a PCB with a voltages between low and medium voltage lines not greater than 200V, the lin spacing is 1.0-1.5mm, which is 40-60mil. For example, in a digital circuit system, it is not necessary to consider the breakdown voltage, as long as the production process allows it, the breakdown voltage can be very small.

3. Pad: For 1/8W resistors, a pad lead diameter of 28mil is sufficient. But for 1/2W, the pad lead diameter needs to be 32mil. If the lead hole is too large, the width of the copper ring of the pad is relatively reduced, resulting in a decrease in the adhesion of the pad and easy fall off. However, if the lead hole is too small, it is difficult to mount the component.

4. Draw circuit frame: The shortest distance between the frame line and the component pin pads cannot be less than 2MM, generally 5MM is more reasonable, otherwise laying-off will be difficult.

5. Component layout principle:

A: General principle: In PCB design, if there are digital circuits, analog circuits, and high-current circuits in the circuit system, they must be laid out separately to minimize the integration between the systems. In the same type of circuit, components are divided according to signal flow and function.

6. The Input signal processing unit and output signal driving components should be close to the edge of the PCB, so make the input and output signal lines as short as possible to reduce input and output interference.

7. Component placement direction: Components can only be arranged in horizontal and vertical directions, otherwise they must not be used in plug-ins.

8. Components pitch: For medium density boards, the distance between small components (such as low power resistors, capacitors, diodes, and other discrete components) is related to the plug-in and soldering process. When wave soldering, the component pitch can be 50-100mil (1.27-2.54mm), if manually operated, the component pitch can be larger, such as 100mil. For integrated circuit chips, the component pitch is generally 100-150mil.

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

10. The decoupling capacitor that has entered the IC should be close to the chip’s power and ground pins, otherwise the filtering effect will be worse. In digital circuits, to ensure the reliabe operation of digital circuit systems, IC decoupling capacitors are usually placed between the power and ground of each digital integrated circuit chip. Decoupling capacitors usually use ceramic capacitors with a capacity of 0.01~0.1UF. The selection of the decoupling capacitor capacitance is generally based on the inverse of the system operating frequency F. In addition, a 10UF capacitor and a 0.01UF ceramic capacitor should be connected between the power and ground wire at the circuit power inlet.

11. The clock circuit components should be as close as possible to the clock signal pins of the SCM system chip to reduce the wiring length of the clock circuit. It is better not to route under the pins.