As discussed earlier, a digital circuit represents and manipulates information encoded as electric signals that can assume one of two voltages – logic-high voltage (or Vdd) and logic-low voltage (or GND). A digital circuit requires a power supply that can produce these two voltages, and these same supply voltages are also used to encode information in the form of two-state, or binary signals. Thus,if a given circuit node is at Vdd, then that signal is said to carry a logic ‘1’; if the node is at GND, then the node carries a logic ‘0’. The components in digital circuits are simple on/off switches that can pass logic ‘1’ and logic ‘0’ signals from one circuit node to another. Most typically, these switches are arranged to combine input signals to produce an output signal according to basic logic relationships. For example, one well-known logic circuit is an AND gate that combines two input signals to produce an output that is the logic AND of the inputs (i.e., if both input1 and input2 are a ‘1’, then the output is a ‘1’).
Circuit components are manufactured with exposed metal pins (or leads) that are used to fasten them to the PCB both mechanically (so they will not fall off) and electrically (so current can pass between them). The soldering process, which provides a strong mechanical bond and a very good electrical connection, is used to fasten components to the PCB. During soldering, component leads are inserted through the holes in the PCB, and then the component leads and the through-hole plating metal are heated to above the melting point of the solder (about 500 to 700 degrees F). Solder (a metallic compound) is then melted and allowed to flow in and around the component lead and pad. The solder quickly cools to form a strong bond between the component and the PCB. The process of associating components with reference designators, loading them into their respective holes, and then soldering them in place comprises the PCB assembly process.
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