General Principles of PCB Design

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To ensure optimal performance of an electronic circuit, the layout of the components and wiring of the wires must be given careful consideration. In order to design a good-quality, low cost PCB, the following general principles should be followed:

1. Layout

Firstly, the size of the PCB should be considered. If the PCB size is too large, the printed lines are long, which increases the impedance, reduces noise resistance, and also increases the cost. If the size is too small, heat dissipation is not good, and adjacent lines are susceptible to interference. Once the PCB size is determined, the location of the special components should then be decided. Finally, all the components of the circuit should be laid out according to the functional unit of the circuit.

When determining the location of a particular component, the following principles should be observed:

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

(2) There may be a high potential difference between some components or wires. The distance between them should be increased to avoid accidental short circuits caused by discharge. Components with high voltage should be placed as far as possible from where they are handled during debugging.

(3) Components weighing more than 15g should be fixed by brackets and then welded. Those components that are large, heavy, and generate a lot of heat should not be mounted on the printed board, but should be installed on the chassis of the whole machine, and heat dissipation should be considered. The thermal element should be kept away from the heating element.

(4) For the layout of adjustable components such as potentiometers, adjustable inductors, variable capacitors, microswitches, etc., the structural requirements of the whole machine should be considered. If it is adjusted inside the machine, it should be placed on the printed board to facilitate adjustment. If it is adjusted outside the machine, its position should be compatible with the position of the adjustment knob on the chassis panel.

(5) The position occupied by the printing plate positioning hole and the fixing bracket should be left.

When laying out all the components of a circuit according to the functional unit of the circuit, the following principles must be met:

(1) The position of each functional circuit unit should be arranged according to the flow of the circuit, so that the layout facilitates signal circulation and keeps the signal as consistent as possible.

(2) The layout should center around the core components of each functional circuit. Components should be arranged evenly, neatly, and compactly on the PCB. Leads and connections between components should be minimized and shortened.

(3) For circuits operating at high frequencies, the distribution parameters between components should be considered. In general, the circuit should be arranged in parallel as much as possible. This way, it is not only beautiful but also easy to load and weld, and easy to mass produce.

(4) Components located at the edge of the board should be at least 2mm away from the edge of the board. The optimal shape of the board is rectangular, with an aspect ratio of 3:2 to 4:3. When the board surface size is greater than 200x150mm, the mechanical strength of the board should be considered.

General Principles of PCB Design

2. Routing

The principles of wiring are as follows:

(1) Try to avoid using wires at the input and output terminals. If required, add a ground wire between the wires to avoid feedback.

(2) The minimum width of the printed photoconductive wire depends on the adhesion strength between the wire and the insulating substrate, and the current value flowing through them. When the copper foil thickness is 0.05mm and the width is 1~15mm, a wire width of 1.5 mm is sufficient for a current of 2A, and the temperature will not be higher than 3 ° C. For integrated circuits, especially digital circuits, a wire width of 0.02 to 0.3 mm is usually chosen. However, use wide lines as much as possible, particularly for power and ground lines. The minimum spacing of the wires primarily depends on the worst-case interline insulation resistance and breakdown voltage. For integrated circuits, especially digital circuits, the pitch can be as small as 5 to 8 mm, as long as the process allows.

(3) The curved corner of the printed conductor should ideally take a circular arc shape to avoid any impact on the electrical performance in high-frequency circuits. Avoid using right angles or angles. Also, try to avoid using large areas of copper foil because it tends to expand and shed when heated for a long time. If large areas of copper foil must be used, it is best to use a grid. This helps eliminate volatile gases generated by the heat of the adhesive between the copper foil and the substrate.

3. Pad

The pad's central hole is marginally bigger than the device lead's diameter. A solder joint cannot form because of the pad's size. When d is the lead aperture, the pad outer diameter D is typically not less than (d + 1.2) mm. The minimal pad diameter for high-density digital circuits can be (d + 1.0) mm.

PCB with anti-interference circuitry

Printed circuit boards' anti-jamming design is directly tied to the particular circuit. Only a few standard PCB anti-interference design measures are covered here.

1. Power cord design

Try to widen the power line and lower the loop resistance in accordance with the printed circuit board's current. Additionally, the ground and power lines' directions coincide with the direction of data transmission, further improving the anti-noise capacity.

2. Ground design

The principle of ground design is:

(1) The simulated ground and the digital ground are divided. As much as feasible, the logic and linear circuits on the board should be kept apart. The low-frequency circuit's ground ought to be connected in parallel to a single point. Partially connecting and grounding in parallel is an option if the actual wiring proves to be challenging. The ground cable should be short and leased, the high-frequency circuit should employ multi-point series grounding, and large-area foil that resembles a grid should be used as much as possible to surround the high-frequency components.

(2) The grounding wire should be as thick as possible. If the grounding wire uses a very thin line, the ground potential changes with the change of the current, which reduces the noise immunity. Therefore, the ground wire should be thickened so that it can pass three times the allowable current on the printed board. If possible, the grounding wire should be 2~3mm or more.

(3) The grounding wire constitutes a closed loop. In a printed circuit board composed only of digital circuits, the grounding circuit is mostly formed into a ring circuit to improve the anti-noise capability.

3. Untwisting capacitor configuration

Appropriately placing decoupling capacitors at various crucial printed board locations is a standard practice in PCB design.

The following is the general untwisting capacitor setup principle:

(1) A 10–100 uF electrolytic capacitor is attached to the power input terminal.

(2) A 0.01pF ceramic capacitor ought to be included with every integrated circuit chip. If the printed board is too small, a 1~10pF capacitor can be positioned per 4–8 chips.

(3) The decoupling capacitor needs to be placed directly between the chip's ground and the power cord for devices like RAM and ROM storage units that have poor anti-noise capabilities and significant power supply variations during shutdown.

(4) The capacitor leads should not be too long, especially the high-frequency bypass capacitors must not have leads.

In addition, you should also pay attention to the following two points:

(1) When the printed board has contacts, relays, buttons, and other parts. When operating them, a big spark discharge occurs, and the discharge current needs to be absorbed by the RC circuit depicted in the drawing. R often takes 1-2K, while C typically takes 2.2–47UF.

(2) CMOS has a very high input impedance that is prone to induction. As a result, while using the power source, it must be grounded or connected to the unoccupied connector.

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