PCB Technical - PCB circuit board layout and routing

layout

First, consider the PCB size. When the PCB size is too large, the printed lines will be long, the impedance will increase, the anti-noise ability will decrease, and the cost will also increase; if the PCB size is too small, the heat dissipation will not be good, and adjacent lines will be easily disturbed.

After determining the PCB size, determine the location of the special components. Finally, according to the functional units of the circuit, all the components of the circuit are laid out.

The following principles should be observed when determining the location of special components:

Try to shorten the wiring between high-frequency components as much as possible, try to reduce their distribution parameters and mutual electromagnetic interference. Components that are susceptible to interference should not be too close to each other, and input and output components should be kept as far away as possible.

There may be a high potential difference between some components or wires, and the distance between them should be increased to avoid accidental short circuits caused by discharge. The components with high voltage should be arranged as far as possible in places that are not easily reachable by hands during debugging.

Components weighing more than 15g should be fixed with brackets and then welded. Those components that are large, heavy, and generate a lot of heat should not be installed on the printed circuit board, but should be installed on the chassis bottom plate of the whole machine, and the heat dissipation problem should be considered. Thermal components should be far away from heating components.

For the layout of adjustable components such as potentiometers, adjustable inductors, variable capacitors, and micro switches, the structural requirements of the whole machine should be considered. If it is adjusted inside the machine, it should be placed on the printed circuit board where it is convenient for adjustment; if it is adjusted outside the machine, its position should match the position of the adjustment knob on the chassis panel.

The position occupied by the positioning hole of the printed board and the fixed bracket should be reserved. When laying out all the components of the circuit according to the functional units of the circuit, the following principles must be met:

Arrange the position of each functional circuit unit according to the circuit flow, so that the layout is convenient for signal circulation, and the signal is kept in the same direction as possible.

Take the core component of each functional circuit as the center and lay out around it. The components should be evenly, neatly and compactly arranged on the PCB. Minimize and shorten the leads and connections between the components.

For circuits operating at high frequencies, the distributed parameters between components must be considered. Generally, the circuit should be arranged in parallel as much as possible. In this way, it is not only beautiful, but also easy to install and weld. It is easy to mass produce.

The components located at the edge of the circuit board are generally not less than 2mm away from the edge of the circuit board. The best shape of the circuit board is rectangular. The aspect ratio is 3:2 to 4:3. When the size of the circuit board is larger than 200x150mm, the mechanical strength of the circuit board should be considered.

wiring

The principle of wiring is as follows:

The wires used for the input (control end) and output end should try to avoid parallel adjacent neighbors. It is best to add a shielded ground wire to avoid feedback coupling. For the input (control terminal) and output wires outside the PCBA board, shielding (using shielded wires) should be taken, or the input (control terminal and output wiring harness should be bound separately to prevent signal coupling interference.

The minimum width of the printed circuit board conductor is mainly determined by the adhesion strength between the conductor and the insulating substrate and the current value flowing through them. When the thickness of the copper foil is 0.05mm and the width is 1～15mm, the temperature will not be higher than 3 degree Celsius through the current of 2A, so the wire width of 1.5mm can meet the requirements.

For integrated circuits, especially digital circuits, a wire width of 0.02 to 0.3 mm is usually selected. Of course, as long as possible, use as wide a cable as possible, especially the power cable and the ground cable. The minimum spacing of wires is mainly determined by the worst-case insulation resistance and breakdown voltage between the wires. For integrated circuits, especially digital circuits, as long as the process permits, the pitch can be as small as 5-8mm.

The corners of the printed conductors are generally arc-shaped, and the right angle or the included angle will affect the electrical performance in the high-frequency circuit. In addition, try to avoid using large-area copper foil, otherwise the copper foil will expand and fall off when heated for a long time. When a large-area copper foil is required, it is best to use a grid shape. This will help to eliminate the volatile gas generated by the heating of the adhesive between the copper foil and the substrate.

Pad

The center hole of the pad is slightly larger than the diameter of the device lead. If the pad is too large, it is easy to form a false solder. The outer diameter D of the pad is generally not less than (d+1.2) mm, where d is the lead diameter. For high-density digital circuits, the minimum diameter of the pad can be (d+1.0) mm.

PCB and circuit anti-interference measures

The anti-jamming design of the printed circuit board has a close relationship with the specific circuit. Here, only a few common measures of PCB anti-jamming design are explained.

Source line design

According to the size of the printed circuit board current, try to increase the width of the power line to reduce the loop resistance. At the same time, make the direction of the power line and ground line consistent with the direction of data transmission, which helps to enhance the anti-noise ability.

Ground wire design

The principles of ground wire design are:

Separate digital ground from analog ground. If there are both logic circuits and linear circuits on the circuit board, they should be separated as much as possible. The ground of the low-frequency circuit should be grounded in parallel at a single point as much as possible. When the actual wiring is difficult, it can be partially connected in series and then grounded in parallel. The high-frequency circuit should be grounded at multiple points in series, the ground wire should be short and leased, and a grid-like large-area ground foil should be used around the high-frequency component.

One of the conventional methods of PCB desig n is to configure appropriate decoupling capacitors on each key part of the printed board.

Capacitor leads should not be too long, especially for high-frequency bypass capacitors.