Placement and routing are the processes of designing a printed circuit board (PCB) that involve component placement and the creation of connections between them. It is an important and critical step in PCB design. Component placement has a significant impact on the overall design schedule, while routing itself forms a smaller part of this process.
Designing a PCB is a combination of art and science. It requires technical knowledge and an understanding of various factors such as trace width, layers, schematics, and ground plane design. The artistic aspect lies in component placement, and since there is no single correct way to position them, PCB design is a highly creative process. If the same schematic were given to 100 experts, it is likely that we would get 100 different layouts. Therefore, PCB design is considered a truly artistic process.
These rules are not listed in any specific order and can generally be applied to any PCB design. They serve as a useful guide for beginners.
The first step is to determine the maximum allowed size of the printed circuit board (PCB). The number of layers depends on the performance and complexity requirements. There are several general design rules that need to be known and taken into account when designing manufacturable, functional, and reliable PCBs.
The following list provides some of these rules, although it is not exhaustive:
These guidelines help ensure that the proposed PCB is efficient, functional, and reliable.
For components with controlled power consumption, it is advisable to use ground planes or planes for heat dissipation. When dealing with high-current traces, it is important to consider an acceptable voltage drop for the connection. Transitions between layers for high-current traces should be designed with reliability in mind. It is recommended to use two to four vias in each layer transition and multiple vias in layer transitions to increase reliability, reduce resistive and inductive losses, and improve thermal conductivity.
Creating a ground plane is also an important aspect, which can be achieved by either using a large copper area on a single-layer board or dedicating an entire layer as a ground plane on multi-layer boards. After adding the ground plane, all components that need to be connected to the ground should be interconnected using vias.
Real copper traces have a certain resistance, which means that there will be voltage drop, power loss, and heating when current flows through them. The trace width should be dimensioned based on the estimated current passing through it. Power traces should be wider as they carry the entire current. The typical thickness of copper traces on a PCB is around 35 microns.
These points are important in PCB design and serve to ensure proper functionality, reliability, and efficiency of electronic devices.
When placing components on a PCB, it is advisable to minimize trace lengths and avoid 90-degree angles. Instead, it is better to use two 45-degree bends. The reason is that during the PCB manufacturing process, an outer corner with a 90-degree angle may only be slightly etched, which can affect the trace impedance. Using a 45-degree bend shortens the electrical path between components and reduces the risk of reflections of high-speed signals from a sharp corner, which could cause interference.
It is also desirable to keep digital and analog grounds separate. Voltage and current spikes from digital circuits can cause interference in analog circuits, which can affect their proper functioning.
Proper component placement and routing require knowledge of technical rules and recommendations. There are many specific guidelines for designing trace widths, layers, schematics, and other aspects. Although there is no single correct way to place components, creativity and the designer's experience play an important role in achieving an optimal result.
The result of the placement and routing process is the final arrangement of the PCB, which is depicted in a drawing. The drawing shows electronic components with their packages and their positions on the final PCB, interconnected by traces between their terminals.
