Layout and Routing of PCB Components

In professional automotive electronics, 90% of PCB design lies in component placement, while routing constitutes the remaining 10%. A well-designed PCB can yield significant benefits, such as improved efficiency and enhanced electrical characteristics.

For instance, to enhance work efficiency, attention should be given to the spatial arrangement of traces to prevent the need for rerouting due to space limitations. Similarly, to avoid difficulties during soldering, the positioning of components and consideration of board margins are crucial factors.

Furthermore, achieving an aesthetically pleasing and easily debuggable PCB board requires careful consideration of the overall layout. These aspects should be planned in advance to achieve a symmetric, tidy, and visually appealing PCB design.

Of course, when it comes to the same circuit diagram, there can be countless routing schemes as designing a circuit board is also an artistic process. Different individuals may have varying aesthetic standards. Hence, we do not define rigid standards for PCB layout and routing. However, we can provide a basic approach that designers can use to create their own ideal PCB boards.

Techniques for PCB Layout

Understand the physical limitations of the PCB

Before placing components, determine the positions of mounting holes, edge connectors, and the mechanical dimensions of the PCB.

Understand the PCB fabrication process

Consider the assembly process, testing procedures, whether space needs to be reserved for V-grooves in PCBs, and component soldering techniques.

Allow breathing space for integrated circuits (ICs)

When placing components, leave a minimum distance of 350 mil between them, and allocate even more space for ICs with numerous pins.

Maintain consistent orientations for identical components

For identical components, strive to maintain a consistent alignment. This facilitates assembly, inspection, and testing of the PCB, while ensuring consistent soldering points.

Minimize crossing of traces

Reduce the crossing of traces by adjusting component positions and orientations. This saves a significant amount of effort during subsequent routing.

Place edge components first

Components that cannot be moved freely due to mechanical constraints, such as external connectors, switches, and USB ports, should be placed first.

Avoid conflicts between components

Absolutely avoid overlapping solder pads or component edges when routing on a small PCB. It is best to maintain a distance of 40 mil between all components.

Place components predominantly on one side

Components on a PCB are typically placed using automated component placement machines. If all components are placed on one side, the PCB production process can be completed in a single pass, saving production time and costs.

Ensure consistency between chip pins and component polarity

Disorderly polarity and orientation of components on the PCB hinder successful soldering.

Align component positions with the schematic

During schematic design, the positioning of components (minimizing trace length and crossings) is already optimized. Therefore, placing PCB components according to their positions on the schematic is a more reasonable approach.

Rules for PCB Routing

Routing Direction Control Rule

Ensure that adjacent layers have orthogonal routing directions. Avoid routing different signal lines in the same direction on adjacent layers to reduce unnecessary interlayer crosstalk. If it is difficult to avoid this situation due to board structure limitations (such as certain backplanes), especially for high-speed signals, consider using ground planes to isolate different routing layers and ground signal lines to isolate different signal lines.

Open Circuit Check Rule

Generally, routing with one end left floating (Dangling Line) is not allowed. This is primarily to avoid the creation of an "antenna effect" and minimize unnecessary interference radiation and reception, which may lead to unpredictable results.

Impedance Matching Check Rule

Routing widths of the same network should be consistent. Changes in line width can cause uneven impedance characteristics of the line. When transmitting at high speeds, reflections can occur. This situation should be avoided as much as possible in the design.

Routing Length Control Rule

Also known as the short line rule. In the design, try to keep the routing length as short as possible to minimize interference issues caused by excessively long traces, especially for critical signal lines such as clock lines. It is essential to place their oscillators close to the devices.

Corner Rule

Avoid sharp angles and right angles in PCB design, as they can cause unnecessary radiation and have poor process performance.

Decoupling Rule

Add necessary decoupling capacitors on the printed circuit board to filter out interference signals on the power supply and stabilize the power signal.

Ground Loop Rule

Follow the minimum loop rule, which means that the area of the loop formed by the signal line and its return path should be as small as possible. The smaller the loop area, the less radiation it emits externally, and the less it is affected by external interference.

Power and Ground Plane Integrity Rule

In areas with dense via connections, attention should be paid to avoiding connections between vias in the power and ground planes, which can lead to partitioning of the plane layers, compromising their integrity, and increasing the loop area for signal lines in the ground plane.

Shielding Protection

Similar to the ground loop rule, it aims to minimize the loop area of the signal. It is commonly used for important signals such as clock signals and synchronization signals.

Closed Loop Check Rule

Prevent the formation of self-loops for signal lines across different layers. This is a common issue in multilayer board designs, and self-loops can cause radiation interference.

Isolated Copper Area Control Rule

The presence of isolated copper areas can lead to unforeseen issues. Therefore, connecting isolated copper areas to other signals helps improve signal quality. Typically, isolated copper areas are connected to ground or removed.