After a circuit has been designed and tested in simulations or on a breadboard, the focus shifts to designing a Printed Circuit Board (PCB) for the circuit. Designing a PCB can be as just as complicated as designing the original circuit and good design is essential for a circuit to work properly and reliably.
Printed circuit boards are designed to provide a physical platform and electrical connections to a set of electrical components that make up a circuit design. To accommodate all of the signals and components possible in a circuit design, PCBs have several physical features to connect components and ensure good operation of the circuit.
Track/Trace - A track or trace on a PCB is an electrical connection that is made between components. They are formed from thin layers of copper and can be made in nearly any width. The width used will depend on the current the trace will carry, the routing space and clearance available between components and other traces.
A wider trace will have lower resistance and greater current carrying capability. The smallest trace width will depend on what your board manufacturer can support, which is generally stated as trace/space resolution, or the minimum width of a trace and the spacing between traces.
Pads - Pads are the area of a PCB where components are soldered to the board. Pad layouts come in a wide variety of shapes and sizes as well as surface mount and through hole variations. Pad sizes, shapes, and dimensions will depend on the component they will be used with as well as the manufacturing process. Pads with through holes should be at least 0.5mm or 1.8 times larger than the hole size to ensure the through hole is aligned correctly.
Vias - Vias are essentially through hole pads that are specifically to connect a trace from one layer of a board to another. Vias are also called plated through holes and do not have an opening to accept through hole components. In dense designs, vias are essential to make all of the required electrical connections. In simpler designs, through hole pads can often serve a dual purpose and reduce or eliminate the need for vias.
Layers - Printed circuit boards are available with one copper layer to more than 64 layers, each with an insulating layer placed between them. Multiple layers are often used to simplify trace routing and are often required in very dense or complicated designs. Multiple layers also allows a full ground plane or power plane to be used which can significantly reduce noise issues and cross talk between traces.
Basic Factors in PCB Design
When designing a PCB, there are several important factors in good design. One of the most important rules, especially in mixed signal design, is to keep analog and digital grounds separate to keep noise and anomalous behavior to a minimum. Often this requires routing grounds by hand as autorouters see a single ground connection between components and route accordingly. Additionally:
*Place capacitors next to every digital IC between its power input and ground.
*Keep high frequency traces as short as possible
*Avoid running high frequency signals over a gap in a ground plane
*Separate analog and digital circuitry
*Use separate analog and digital grounds that are tied back to a common point
*Traces should never have a 90 degree or greater bend. 45 degree bends should always be used to prevent signal reflections and noise.
*Place your power and ground traces first and make them as big as possible
*Avoid using an autorouter as much as possible.
*Do not leave unconnected copper fills on the PCB. They must be connected to ground or removed.
*Avoid placing vias under components whenever possible
*Use through hole components as vias when possible
*Minimize the number of drill sizes used
This article only touches the surface on PCB design factors and guidelines. Other factors that impact PCB design include:
High Speed Signals
Radio Frequency Applications
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