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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
Download (direct link): electronicsfordummies2005.pdf
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Preparing the PCB for etching
After you make a mask and sensitize the copper clad (or purchase presensitized copper), you’re ready to actually make the printed circuit board.
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258 Part V: A Plethora of Projects
Here’s what you do to make the board:
1. In a darkened room (remember, this is a photographic process so you need to work in the dark!), spray a coating of sensitizer onto a clean copper-clad board.
2. Place the film over the sensitized board and insert both in a suitable exposure holder, like the picture frame in Figure 12-4.
To be really fancy, you can get photographic holders at a photography shop, but picture frames work just as well.
Figure 12-4:
Use a picture frame to hold your circuit board and artwork during exposure.
3. Note the orientation of the film and correct it, if necessary.
Make sure that you haven’t reversed the film, or you may etch the board with a mirror-image of the circuit layout. Checking the film’s position requires some thought on your part, so don’t rush through it.
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Chapter 12: Building Your Own Printed Circuit Boards 259
4. Place the film negative so that the emulsion (the dull side) faces the copper clad. This step helps produce a sharper image on the board.
Note, however, that you may need to reverse the film (emulsion side facing out) if the original artwork that you’re copying from a printed magazine or book is reversed. Some magazines print circuit artwork in reverse (called left-reading), to make it easier for them to transfer the image using the dry toner transfer method, described in the section, “Creating a PCB By Using the Transfer Film Method,” later in the chapter.
Let there be tight: Exposing and developing the board
You can expose the sensitized board in a number of ways. If the sun is out and about, you can expose the board to its ultraviolet rays. Exposure time varies between just a few minutes to over 15 minutes. The many makes and brands of photoresist require different exposure times, so check the instruction sheet that came with the photoresist that you’re using.
You can reduce exposure time by using an ultraviolet tanning lamp. Place the lamp a foot or two away from the board so that the light falls evenly on the board. Don’t put the lamp so close that the edges of the board are in shadow.
Remember that UV tanning lamps give off ultraviolet rays, so don’t expose yourself to these rays for long periods, unless you actually enjoy the stinging sensation of sunburn! Also, wear tanning bed goggles that keep the UV rays from entering your eyes to avoid permanent damage to your retinas. Better yet, don’t use an ultraviolet lamp. Wait for a sunny day and expose the board in the great outdoors.
Which end is up? (Or down or left or right?)
If you want to make sure that you orient the film properly, try this approach. Look at the component layout diagram that you can usually find printed with the circuit layout. This diagram shows where you should place each component on the board. The shaded portions of the component layout diagram show where the pads and traces are located on the underside of the board.
Now look at the film and orient it to match the shading of the component layout diagram. Most boards aren't symmetrical, so you can easily tell the left side from the right side. Place the sensitized board into its exposure holder. Finally, flip the film over (left to right) and place it over the board.
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260 Part V: A Plethora of Projects
After you expose the board, you have to develop it in a suitable developing solution. Mix the developer according to the instructions the manufacturer included with the package. After you mix it, pour the developer into a shallow plastic tray — the kind you find at a photography store is perfect. Place the exposed board into the developer liquid. Developing times vary from one manufacturer to another. Follow the solution manufacturer’s recommended times for developing.
After developing, you’re ready to etch the board, which we detail in the section titled “Showing You My Etchings: Etching the Circuit Board,” later in this chapter.
Creating a PCB by Using the Transfer Film Method
The photographic method that we describe in the previous section requires that you use sensitized copper clad, a negative (or a positive), and exposure to ultraviolet light. All in all, this process means a lot of fuss if you’re making a slew of boards.
If you plan on making just one or two circuit boards from pre-printed artwork, you may want to consider the transfer film method. The transfer film method involves nothing more than a sheet of clear acetate — the type used for overhead transparencies — and a plain paper copier or laser printer.
When you copy the artwork onto the transparency, it fuses black toner onto the acetate sheet. You transfer the toner from the transparency to the copper clad using the heat from a clothes iron. The toner provides an effective resist to the etchant solution that you use to dissolve unwanted copper from the circuit board. Although the method may seem anything but high-tech, it works quite well.
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