On all but the simplest PCBs, wires must be printed on more than one surface of fiberglass to allow for all the required component interconnections. Each surface containing printed wires is called a layer. In a relatively simple PCB that requires only two layers, only one piece of fiberglass is required since wires can be printed on both sides. In a more complex PCB where several layers are required, individual circuit boards are manufactured separately and then laminated together to form one multi-layer circuit board. To connect wires on two or more layers, small holes called vias are drilled through the wires and fiberglass board at the point where the wires on the different layers cross. The interior surface of these holes is coated with metal so that electric current can flow through the vias. Most Digilent boards are simple four or six layer boards; some more complex computer circuit boards have more than 20 layers.
In a PCB that uses through-hole technology, holes are drilled through the pads so that component leads can be inserted and then fastened (soldered) in place. In a PCB that uses surface-mount technology, component leads are soldered directly to the pads on the surface. Each set of pads (or holes) in the PCB is intended to receive a particular component. To identify which component must be loaded where, reference designators are printed on the circuit board immediately adjacent to the pads using a silk-screen process. A parts list links a designated set of pads to a physical component by describing the component and assigning it a particular reference designator. The reference designators guide assemblers and testers when they are working with the PCB. Many components must be placed into the PCB in a particular orientation. By convention, components that require a particular orientation have one lead designated as pin 1. On the PCB, a square pad or silkscreen indicator typically denotes pin 1.
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