PCB Design and Layout Guidelines

PCB design and layout is an important and complex skill that requires knowledge, experience and patience. Efficient design can not only save money but can also improve the functionality of the product. To help the process, there are many PCB design tools such as Advanced Design Suite, OrCAD, ARES and so on. This article discusses certain guidelines that have been drawn up to inform PCB designers about most common mistakes and to help them through the design and layout phase.

The first task in PCB layout is to make a raw layout of the PCB by allotting the available space on the PCB to different blocks of circuit. This helps in two critical initial decisions

a. Separate areas for different components, communication ports or antennae are identified.
b. Most critical tracks of the circuit are identified and the rest of the layout becomes centralized which leads to a more convenient design.

Generally, we want to fit all the components in the smallest possible space which becomes critical in the case where miniaturized PCB is required. The PCB is mostly mounted in a device after manufacturing using some space of the board; therefore, the critical components are placed so that they don’t get damaged at that stage. PCB layers are decided on the basis of the design complexity. Adding more layers adds more cost but at the same time, complicated circuits can be fitted in lesser space.

The tracks or traces, which are essentially lines of copper deposits, are designed following strict guidelines. The strictness of the design guidelines for copper thickness, trace width and holes differ for different types of PCB.

a. The trace width can change the radiation pattern in case of an RF design where changing a track width can dramatically change the impedance, however, for all types of PCB, the trace width decides the resistance therefore it has to be kept so that electrical properties of the circuit are not affected.
b. The copper thickness is kept greater for the outer layers and lesser in case of inner layers in a multi-layered design. Trace clearance is another important factor, traces or tracks have to be kept at a safe distance where they don’t electrically interfere.
c. Almost every PCB has holes drilled in it. Since drilling holes requires a greater effort and there is greater probability of the PCB manufacturing fault during the drilling process, it is advisable to reduce the number of holes.

Multi-layer PCB design helps in saving space in case of a complicated circuit where we have to save space. Different layers are connected using via-holes. In multi-layer PCB, it is recommended to have two separate layers for ground and power. This not only helps in dissipation of heat but also reduces chances of miniature tracks to form antennas. The cost-to-benefit ratio of a multi-layered design is often carried out before manufacturing a multi-layered PCB. Multilayer PCB Stack-up design info is available in the following link: http://blog.7pcb.com/wp-content/uploads/2012/09/Multi-layer-stack-up7_24_2012.pdf

Every track in a PCB has a certain resistance and therefore we have to estimate the heating issues that might develop in certain designs. We can incorporate heat sinks in such cases. For RF designs, PCB design and layout is the key. Often by incorporating circular bends instead of rectangular, signal strength is preserved. For RF mostly one ground plane is necessary and inputs are kept as far away from outputs as possible.