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Electronics for dummies - McComb G.

McComb G., Boes E. Electronics for dummies - Wiley publishing, 2005. - 433 p.
ISBN: 0-7645-7660-7
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250 Part V: A Plethora of Projects_
Anatomy of a Circuit Board
Before we get into how to make a printed circuit board, we take a closer look at what makes up the typical PCB:
You make a printed circuit board by gluing a very thin sheet of copper over a plastic, epoxy, or phenolic base. This copper sheet is called cladding and represents the foil side of the board. We show an example of this bit of copper in Figure 12-1. It looks pretty boring because here it’s just a blank canvas. It can become almost anything at this point.
To make the final board, you etch away specific portions of the copper, leaving just the printed circuit design. We talk about the exact methods of laying out the circuit and etching the board in more detail in the section “Showing You My Etchings: Etching the Circuit Board,” later in this chapter.
You produce the circuit with pads and traces:
• Pads: These contact points for components are generally round or rectangular in shape (these are called “donuts” in electronics parlance). After you etch the circuit board, you drill a hole in the center of each pad. You mount the components on the top of the board with their leads poking through the holes. You then solder each component lead to the board at the board’s pads.
• Traces: These wires of the circuit board run between the pads to electrically connect the components together. See Figure 12-2 for an example of traces.
Printed circuit boards can be either single- or double-sided:
• Single-sided boards are copper clad on only one of their sides. You mount the components on the other side.
• Double-sided boards are copper clad on both sides; you often use these boards when you’re working with a very complex circuit. It’s hard to make your own double-sided PCBs, but you can design them and have them made for you. We tell you more about PCB manufacturers in the last part of this chapter.
More advanced circuit boards have multiple layers. An insulating covering keeps the layers from shorting out. Multi-layer circuit boards are way beyond what most people can make on their own, so we just mention them here and go on our merry way.
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Chapter 12: Building Your Own Printed Circuit Boards 251
Figure 12-2:
Finished PCBs use pads for soldering on components and traces in place of wiring.
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252 Part V: A Plethora of Projects
How the Copper Gets onto the Circuit
There’s more than one way to skin a cat, and there’s more than one approach to building a circuit board. To begin with, you can use a number of methods to form the pads and traces that ultimately turn a sheet of copper into a bona fide PCB.
Here’s how most commercial manufacturers make circuit boards. While you may not create your own circuit boards in the same way, it’s helpful to know how the process is typically done
1. First, they coat the copper with a light-sensitive chemical layer called the sensitizer, also known as resist or photoresist.
2. Next they place an exact-size film negative of the circuit board layout drawing over the copper clad and expose it.
Just as in processing a photograph, they expose the negative by bathing the board in light, in this case, strong ultraviolet light. The light passes through the negative and strikes the sensitized copper underneath.
3. After exposing the board, they dip it in a resist developer (this is messy stuff but a necessary part of the process).
What comes out from the developer is a copper-clad board where the portions of the copper that weren’t exposed to light have turned black or a dark gray. As a point of reference, they often refer to this process as the positive method; the negative method produce blacks or grays on those areas that are exposed to light.
4. As a final step, they dip the board in etchant solution.
The etchant is a strong acid-like liquid that eats away at the copper. The black/gray areas resist the action of the etchant, forming the circuit pattern on the board. (You can find more information about etchant and etching in the section “Showing You My Etchings: Etching the Circuit Board,” a little later in this chapter.)
You can duplicate this photographic method in your own shop, although the process takes a lot of time and can cost you a boatload. You can purchase all the chemicals that you need from specialty electronics supply outfits; check out the Appendix at the end of this book for a list of sources. However, the rest of this chapter concentrates on simpler methods that are probably more up your alley.
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Chapter 12: Building Your Own Printed Circuit Boards 253