With the printed circuit board (PCB), the electronics industry moved from a labor intensive manufacturing plan to a more heavily automated format. PCBs were designed to factor in component size, circuit needs and automated "component stuffing" machine capabilities to make boards that could be manufactured in large quantities to become cost-effective. Knowing how to do this is the heart of PCB design.
The Board in the PCB
Most PCB designs are done on a fiberglass board, with G-10 being one of the most common types. This material is cheap, sturdy, light, easy to cut and drill cleanly and is a good insulator. Being a good insulator is important for electronic circuits, since the current should flow in specific areas and nowhere else. The other attributes are important for keeping down costs.
The Printed Circuit in the PCB
Instead of connecting electronic components by soldering wires to the leads, the PCB uses metal lines printed onto the fiberglass board to make the connections. Whether the line is made by depositing metal onto a printed line or by printing a line onto a metalized board and etching away everything but the line, the result is an easily repeatable metal etch-line on each board. Regardless of the metalizing process, designating the etch-lines is most often done by a photographic process in which a photo-sensitive resist is exposed to a mask pattern and strong light. The unwanted parts of the resist are washed away, and the remaining pattern determines where the metal lines will run on the PCB.
Circuit Design Concerns
When laying out a PCB, the type of circuit being made must be considered. The location of high power-dissipating components such as power transistors or transformers must be factored in so hot spots are not created. Components that give off strong electromagnetic (EM) fields must be placed far from EM-sensitive components or must be shielded. Also, power inputs, grounds, inputs and outputs must be near the edges so the PCB can connect to the rest of the system. These PCB design factors often force the disconnect between the PCB component layout and the clean and logical circuit associated with the board.
A PCB Must be "Buildable"
To be useful for a manufacturing customer, the PCB design must take into account any limitations on the component insertion equipment. Components too tightly packed for the robots to install defeat the purpose of using automation. The designer must have an appreciation of the insertion equipment before starting the PCB layout. This includes any automated soldering considerations as well as limiting any steps that could require hand work by a manufacturing technician.
Final Coating
To give a PCB a better chance of survival in the field, many boards receive a final coating of a clear insulating spray. This coating seals the PCB from moisture and corrosion but also reduces the dissipation of heat into the air. If the PCB is to have a protective coating, the design must factor in space for all of the components to receive a complete surrounding of spray or dip as well as giving more space for hot components.
So, What Is PCB Design?
PCB design is a balancing act requiring the designer to factor in a variety of sometimes conflicting needs. The design must accurately create the circuit while being "buildable," isolating sensitive components from "noisy" ones and doing this in the most cost-effective manner.
