Principles of printed circuit board PCB process

- Aug 23, 2017-

1, printed wire width selection basis:

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

The line width is too small, the printed wire resistance is big, the voltage drop on the line is also big, affects the circuit performance.

The line width is too wide, the wiring density is not high, the board area increases, besides increasing the cost, it is also not conducive to miniaturization

If the current load is calculated at 20A/ mm2, the current load of the 1MM (about 40MIL) linewidth is 1A when the copper foil thickness is 0.5MM,

Therefore, the linewidth of 1--2.54MM (40--100MIL) can meet the general requirements of the application of large power equipment, plate on the ground and power, based on the size of the power, may be appropriate to increase the line width, while in the digital circuit of the small power, in order to improve the wiring density, the minimum line width 0.254--1.27MM (10--15MIL) will be able to meet.

In the same circuit board, the power line and the earth wire are thicker than the signal line

2, line spacing:

When is 1.5MM (about 60MIL), line insulation resistance is greater than 20M in Europe, the maximum pressure can reach 300V line, when the line spacing is 1MM (40MIL), the line between the maximum voltage is 200V, therefore, in low voltage (line voltage not more than 200V) of the circuit board, line spacing 1.0--1.5MM (40--60MIL) in low voltage circuits, such as digital circuit system, without considering the breakdown voltage, as long as the production permit, can be very small.

3. Pad:

For the resistance of 1/8W, the lead diameter of the pad is 28MIL, that's enough,

As for 1/2W, diameter 32MIL, pin hole is too large, the copper pad ring width is reduced, resulting in the pad adhesion decreased. The lead hole is too small, easy to fall off, with difficulty. Sowing element

4, draw the circuit frame:

The shortest distance between the border line and the component pin pad can not be less than 2MM, which is generally reasonable (5MM), otherwise it is difficult to load material

5, component layout principle:

A, general principles: in the design of PCB, if the circuit system and digital circuit and analog circuit. And the high current circuit, it must separate layout, make the system coupling between the minimum in the same type of circuit, according to signal flow and function block partition, component placement.

B, the input signal processing unit, the output signal drive element should be close to the circuit board edge, make the input and output signal lines as short as possible, in order to reduce the interference of input and output

C, component orientation: the component can only be arranged in two directions, horizontal or vertical. Otherwise, the plug-in is not allowed

D, element spacing. For medium density board, small components, such as small power resistors, capacitors, diodes, spacing and plug-in of discrete components to each other, welding, soldering, element spacing can take 50-100MIL (1.27--2.54MM) by hand can be a little bigger, such as 100MIL, integrated circuit chip, element spacing is generally 100--150MIL

E, when the potential difference between components is large, the component spacing should be large enough to prevent discharge

In the F, just into the IC close to the chip decoupling capacitance of power ground pins. Otherwise autumn filtering effect will become worse. In digital circuits, in order to ensure the reliability of digital circuit system between each digital integrated circuit chip and power supply are placed IC decoupling capacitors. Decoupling capacitors using a ceramic capacitor capacity of 0.01~0.1UF to capacity of coupling capacitors selection according to the general system of the working frequency of F reciprocal choice. In addition, between the power line of the power circuit and at the entrance of the wire is also need to add a capacitor 10UF, and a 0.01UF ceramic capacitor.

G, hour circuit elements as close as possible to the microcontroller chip clock signal pin, in order to reduce the length of the clock circuit, and the following is best not to go line

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