Electronic components are often assembled and interconnected on a flat surface known as a circuit board. The several types of existing circuit boards may be divided into two broad categories: those intended for prototype or experimental circuits, and those intended for production and/or commercial sale. Circuit boards used for experimental work are often referred to as breadboards, or protoboards. Breadboards allow engineers to construct circuits quickly, so that they can be studied and modified until an optimal design is discovered. In typical breadboard use, components and wires are added to a circuit in an ad hoc manner as the design proceeds, with new data and new understanding dictating the course of the design. Since breadboard circuits exist only in the laboratory, no special consideration need be given to creating reliable or simple-to-manufacture circuits—the designer can focus exclusively on the circuit's behavior.
In contrast, circuit boards intended for production or commercial sale must have highly reliable wires and interconnects permanent bonds to all components, and topographies amenable to mass production and thorough testing. And further, they must be made of a material that is reliable, low-cost, and easy to manufacture. A fiberglass substrate with copper wires (etched from laminated copper sheets) has been the printed circuit board (PCB) material of choice for the past several decades. The Digilent® board is a simple example of such a board. Note that most often, production circuit board designs are finalized only after extensive breadboard phases. Components are permanently affixed to production boards using the soldering process. Figure 2 displays the layers of PCB.
Production circuit boards typically start out as thin sheets of fiberglass (about 1mm thick) that are completely covered on both sides with very thin sheets of metal (typically copper). A “standard” circuit board might use a 1 ounce copper process, which means that one ounce of copper is evenly spread across 1 square foot of circuit board. During the manufacturing process, wire patterns are “printed” onto the copper surfaces using a compound that resists etching (hence the name Printed Circuit Board, or PCB). The boards are subjected to a chemical etching process that removes all exposed copper. The remaining un-etched copper forms wires that will interconnect the circuit board components and small pads that define the regions where component leads will be attached.
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 help to 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.
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.
The unloaded PCB appears green because thin sheets of green plastic have been applied to both sides (otherwise the PCB would appear pale yellow). Called solder masks, these sheets cover all exposed metal other than the component pads and holes so that errant solder can't inadvertently short (or electrically connect) the printed wires. All metal surfaces other than the exposed pads and holes (i.e., the wires) are underneath the solder mask. Not infrequently, blue or even red solder masks are used.
Circuit components are manufactured with exposed metal pins (or leads) that are used to fasten them to the PCB both mechanically (so they won't 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.
Examine the Digilent board, and note the printed wires on either side. Wires on one side go largely “north and south” while wires on the other side go largely “east and west”. The perpendicular or Manhattan arrangement of wires on alternate layers is very common on multi-layer PCBs. Locate some vias, and note that they connect wires on opposite sides. Locate various components, their hole patterns, and associated reference designators. Identify pad 1 for the various components. Note that the through-holes are somewhat larger than the vias, and that component leads can easily be inserted into their through-holes, but not into vias.