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One thing that all schematics have in common is the utter inability to drive a motor, or blink an LED, or filter out noise, or do any of the other useful and interesting things that we expect electrical systems to do. A schematic is, after all, just a drawing. To actually accomplish something with a circuit, we need to translate its schematic into physical components and physical interconnections. Simple schematics can often be realized on a breadboard, but the vast majority of circuit designs enter the physical realm in the form of a printed circuit board, or PCB for short.
The Structure of a PCB
A very basic printed circuit board is a flat, rigid, insulating material that has thin conductive structures adhering to one side. These conductive structures create geometric patterns consisting of, for example, rectangles, circles, and squares. Long, thin rectangles function as interconnections (i.e., the equivalent of wires), and various shapes function as connection points for components.
A printed circuit board such as the example in the image has only one conductive layer. A single-layer PCB is very restrictive; the circuit realization will not make efficient use of available area, and the designer may have difficulty creating the necessary interconnections.
A conductive interconnection is called a trace, and connection points for components are called pads (for pins that rest on the surface of the board) and through-holes (for pins that are inserted into holes drilled in the board). Basic PCB design consists of arranging pads and through-holes so that components can be properly installed, and then connecting these pads and through-holes using traces.
Not all drilled holes are for through-hole components. We often need to transfer a signal or supply voltage from one PCB layer to another, and this is accomplished using small, conductive holes called vias.
Many PCBs also include mounting holes, which have a mechanical rather than an electrical function and therefore don’t need to be plated. The term “plating” in this context refers to conductive material that has been deposited onto the interior of a drilled hole.
A PCB layer that consists entirely of one large copper pour is called a plane layer. We frequently use an internal layer as a ground plane and create ground connections by placing vias next to component pins.
A through-hole or via begins as a circle of copper and then becomes a hole when a drill bit passes through the circle (ideally through the center of the circle). The term annular ring refers to the width of copper that remains after the hole has been drilled.
Printed circuit boards include a variety of “supplemental” information that have no role in the electrical functionality of the device. For example, reference designators uniquely identify components, dots indicate proper component orientation, and project titles or serial numbers help us to keep track of the many circuit boards that accumulate in a lab. We refer to this information as the silkscreen.
PCB believes that many people are familiar with PCB circuit boards, which may be heard frequently in daily life, but may not be familiar with PCBA, and may even be confused with PCB.
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