The fundamental goal of PCB design is to optimize the performance of the electronic circuit. Both the layout of components and the routing of traces are critical factors. To achieve a high-quality and cost-effective PCB design, the following general principles should be followed:
1. **Layout**
The first step is to consider the size of the PCB. If the PCB is too large, the trace lengths will increase, leading to higher impedance, reduced noise immunity, and increased costs. On the other hand, if the PCB is too small, heat dissipation will be poor, and adjacent traces may interfere with each other. After determining the PCB size, the placement of special components should be addressed. Finally, components should be laid out according to the functional units of the circuit.
(1) Minimize the distance between high-frequency components to reduce their parasitic inductance and capacitance, as well as to mitigate mutual electromagnetic interference. Components that are sensitive to interference should be spaced farther apart, and input and output components should be placed as far from each other as possible.
(2) Certain components or traces may experience significant potential differences. In such cases, increase the spacing between them to prevent accidental short circuits caused by discharge. High-voltage components should be positioned in areas that are less likely to be accidentally touched during debugging.
(3) Components weighing over 15g should be secured with brackets before soldering. Large, heavy, and heat-generating components should not be mounted directly on the PCB. Instead, they should be installed on the chassis bottom plate, where proper heat dissipation can be addressed. Additionally, thermal components should be placed far from other heat-generating components to avoid overheating issues.
(4) When laying out adjustable components such as potentiometers, adjustable inductors, variable capacitors, and micro switches, the overall system’s structural requirements must be considered. If the adjustment is done inside the device, the components should be placed on the PCB in a location convenient for adjustment. If the adjustment is external, the component’s position should align with the corresponding adjustment knob on the chassis panel.
(5) The placement for positioning holes on the PCB and the fixed mounting brackets should be reserved, based on the circuit’s functional units. When arranging all the components, the following guidelines must be adhered to:
1) Position each functional circuit unit according to the signal flow, ensuring that the layout facilitates smooth signal routing and keeps the signal direction consistent wherever possible.
2) Arrange the core components of each functional circuit at the center, and lay out surrounding components in an even, neat, and compact manner on the PCB. Minimize the length and number of connections between components to reduce parasitic effects.
3) For circuits operating at high frequencies, distributed parameters between components must be considered. The layout should, as much as possible, keep components arranged in parallel. This approach not only improves aesthetics but also simplifies installation, soldering, and mass production.
4) Components located near the edge of the PCB should typically be no less than 2mm away from the board’s edge. The optimal shape for a PCB is rectangular, with an aspect ratio between 3:2 and 4:3. For boards larger than 200x150mm, the mechanical strength of the PCB should be taken into account.
2. **Wiring Principles**
The principles for routing wires are as follows:
(1) Input and output wires should avoid running parallel and adjacent to each other. It is advisable to insert ground wires between them to minimize feedback coupling.
(2) The minimum width of the printed traces is mainly determined by the bond strength between the trace and the insulating substrate, as well as the current carried by the trace. For a copper foil thickness of 0.05mm and a trace width of 1–15mm, with a current of 2A, the temperature rise will be no more than 3°C. Therefore, a trace width of 1.5mm should be sufficient. For integrated circuits, especially digital circuits, trace widths of 0.02–0.3mm are typically used. In general, wider traces should be used wherever possible, particularly for power and ground traces. The minimum trace spacing is determined by the required insulation resistance and breakdown voltage between traces. For digital circuits, the trace spacing can be as small as 5–8mm, depending on the manufacturing process.
(3) The corners of printed traces should generally be rounded, as right angles or sharp turns can negatively impact electrical performance, especially in high-frequency circuits. Additionally, large-area copper pours should be avoided, as they can expand and delaminate when exposed to prolonged heat. If a large area of copper must be used, a grid pattern is recommended to allow for the release of volatile gases generated by the heating of the adhesive between the copper and the substrate.
3. **Pad Design**
The center hole of a pad should be slightly larger than the diameter of the component lead. If the pad is too large, it can cause poor soldering. The outer diameter (D) of the pad should generally be at least (d + 1.2) mm, where (d) is the lead diameter. For high-density digital circuits, the minimum pad diameter can be (d + 1.0) mm.
4. **PCB Noise Immunity Measures**
The anti-jamming design of a PCB is closely linked to the specific circuit. Below are some common strategies for improving PCB noise immunity:
(1) **Power Trace Design:** The width of the power trace should be sized according to the current requirements to reduce loop resistance. Additionally, the power and ground traces should be oriented in the same direction as the data transmission lines to enhance noise immunity.
(2) **Ground Design Guidelines:**
1) Separate the digital ground from the analog ground. If the PCB includes both logic and analog circuits, it is best to keep these grounds separated. For low-frequency circuits, try to maintain a single-point ground connection. In cases where this is difficult, a series connection followed by a parallel ground can be used. For high-frequency circuits, ground connections should be made at multiple points in series, and the ground traces should be kept as short as possible. Large-area ground planes should be placed around high-frequency components whenever feasible.
2) The ground traces should be as wide as possible. Narrow ground traces can cause voltage drops that affect ground potential, reducing noise immunity. It is recommended that ground traces be thick enough to carry three times the maximum current allowed on the PCB. In practice, ground traces should ideally be 2–3mm or more in width.
3) The ground circuit should form a closed loop. For PCBs consisting primarily of digital circuits, grounding in a loop configuration generally improves noise immunity.
(3) **Decoupling Capacitor Configuration:** One standard method for decoupling is to place capacitors at key points in the PCB. General principles for decoupling capacitor placement include:
1) A 10–100µF electrolytic capacitor should be placed across the power input, with 100µF being the ideal choice if possible.
2) Each integrated circuit (IC) should have a 0.01µF ceramic capacitor. If board space is limited, a 1–10pF capacitor can be used for every 4–8 ICs.
3) For components with poor noise immunity and significant supply voltage fluctuations (e.g., RAM, ROM), a decoupling capacitor should be placed directly between the power and ground pins of the chip.
4) Keep capacitor leads as short as possible, especially for high-frequency bypass capacitors. Additionally, consider the following:
When handling components like contactors, relays, or switches, which can generate large spark discharges, an RC circuit should be used to absorb the discharge current. Typically, R values range from 1–2kΩ, and C values are between 2.2–47µF.
CMOS inputs are highly susceptible to induction due to their high impedance, so unused terminals should either be grounded or connected to a positive power supply to prevent interference.
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