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PCB Layout Knowledge


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PCB common wiring rules

1. The digital, analog, and DAA signal wiring areas are pre-divided on the PCB.
2. Digital and analog components and corresponding wiring should be separated as far as possible and placed in their respective wiring areas.
3. The high-speed digital signal traces should be as short as possible.

4. Keep sensitive analog signal traces as short as possible.

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5. Reasonably distribute power and ground wires.
6. DGND and AGND are separated on the ground.
7. Use wide wires for power supply and critical signal routing.
8. The power line and ground line should be as radial as possible, and the signal line should not be looped.
9. The digital circuit is placed near the parallel bus/serial DTE interface, and the DAA circuit is placed near the telephone line interface.

10. The wiring of small discrete devices must be symmetrical, and the leads of SMT pads with relatively dense spacing should be connected from the outside of the pads, and direct connection in the middle of the pads is not allowed.

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11. Key signal lines are given priority: key signals such as power supply, analog small signal, high-speed signal, clock signal and synchronization signal are given priority.
12. The principle of wiring density priority: start wiring from the device with the most complicated connection relationship on the single board. Start wiring from the most densely connected area on the board.

13. Acute angles and right angles should be avoided in PCB design, resulting in unnecessary radiation, and the performance of PCB production process is not good.

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14. There should be no through holes on the patch pad, so as to avoid the loss of solder paste and cause the component to be soldered. Important signal lines are not allowed to pass through the pins.

PCB high frequency circuit wiring

1.Reasonably select the number of PCB layers. Using the middle power layer (VCC layer) and ground layer (GND layer) can play a shielding role, effectively reducing parasitic inductance and parasitic capacitance, and can also greatly shorten the length of wiring and reduce cross-interference between signals.

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2. Routing method: must follow the corner method of 45°, do not use the corner of 90°. As shown below:

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3. Interlayer wiring direction: should be perpendicular to each other, the top layer is horizontal, and the bottom layer is vertical, which can reduce the interference between signals.

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4. Ground wrapping: Ground wrapping processing for important signals can significantly improve the anti-interference ability of the signal, and it can also wrap multiple interference signals so that it cannot interfere with other signals.

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5. Add a decoupling capacitor (decoupling capacitor): add a decoupling capacitor at the power supply end of the IC.

6. High-frequency choke: When there are public grounds such as digital ground and analog ground, a high-frequency choke device should be added between them. Generally, a high-frequency ferrite bead with a wire in the center hole can be used.

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7. Copper laying: Increasing the grounding area can also reduce signal interference (Dead copper needs to be removed during the copper laying process).

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8. Wire length: The shorter the wire length, the better, so that the interference received will be reduced. Of course, not all traces are only short. For example, DDR traces pay attention to the equal length between clock, address, and data traces, so you will see many serpentine traces specially designed to increase the length.

Wiring of special components

1. High-frequency components: The shorter the connection between high-frequency components, the better. Try to reduce the distribution parameters of the connection and the electrical interference between them. The components that are easy to interfere should not be too close.

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2. Components with high potential difference: The distance between components with high potential difference and wiring should be increased to avoid accidental short circuit damage to components. In order to avoid the occurrence of creepage, it is generally required that the distance between the copper foil wires with a potential difference of 2000V should be greater than 2mm.

3. Overweight components: Overweight components should be fixed with brackets.

4. Heating and heat-sensitive components: Note that heating components should be kept away from heat-sensitive components. High heat components should be evenly distributed.

Important parameters of PCB layout design

1. Width of copper track (Track): 0.3mm for single panel, 0.2mm for double panel.

2. Minimum gap between copper foil wires: 0.3mm for single panel, 0.2mm for double panel.

3. The minimum distance between the copper foil line and the edge of the PCB is 1mm, the minimum distance between the component and the edge of the PCB is 5mm, and the minimum distance between the pad and the edge of the PCB is 4mm.

4. The pad diameter of general through-hole mounting components is twice the inner diameter of the pad.

5. Electrolytic capacitors should not be close to heating elements, such as high-power resistors, transformers, high-power triodes, three-terminal regulated power supplies, and heat sinks. The distance between the electrolytic capacitor and these components should not be less than 10mm.

6. There should be no copper foil wires (except grounding) and components within 5mm outside the radius of the screw hole.

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7. In large-area PCB design (more than 500 square meters), in order to prevent the PCB from bending when passing the tin furnace, a gap of 5mm to 10mm wide should be left in the middle of the PCB without placing components, so as to prevent the PCB from bending layering.

8. Each PCB should mark the direction of the tin furnace with a hollow arrow.

9. When wiring, the direction of the IC in the DIP package should be perpendicular to the direction of the tin furnace, and try not to be parallel to avoid short circuit with tin.

10. When the wiring direction changes from vertical to horizontal, it should enter from the direction of 45°.

11. The power line width should not be less than 18mil; the signal line width should not be less than 12mil; the CPU inlet and outlet lines should not be less than 10mil (or 8mil); the line spacing should not be less than 10mil.

12. The wiring on the board surface should be properly dense. When the density difference is too large, it should be filled with mesh copper foil, and the mesh should be larger than 8mil (or 0.2mm).

13. No wiring is allowed within the area within ≤1mm from the edge of the PCB and within 1mm around the mounting hole.

14. Warning signs should be printed on the silk screen layer near components such as fuses, fuse resistors, AC 220V filter capacitors, and transformers on the PCB.

15. The distance between the live wire and the neutral wire of the AC 220V power supply part should not be less than 3mm. The distance between any wire in the 220V circuit and the low-voltage components, pad, and track should be no less than 6mm, and the high-voltage mark should be printed on the silk screen. The weak current and strong current should be separated by thick silk screen wires to warn maintenance personnel to operate carefully.

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