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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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4. Wait 10 to 15 seconds, then gently lift the cloth.
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Chapter 12: Building Your Own Printed Circuit Boards 263
Figure 12-5:
Use a clothes iron to transfer the toner from the transparency to the copper clad.
5. Carefully peel back a small corner of the film to see if the toner has transferred to the copper clad.
6. If the toner hasn’t completely transferred to the copper, replace the film and apply the iron for another 15 to 20 seconds.
You have to experiment with the exact time to achieve proper toner transfer. It may take as little as 15 seconds or as long as a minute. Don’t try to speed things up by increasing the temperature setting of the iron. This increase makes the transparency film wilt, causing a distorted transfer of the circuit layout.
Be sure to QC (Quality Control) your Work!
After you’ve transferred the toner, wait for the film to cool. If it’s hot to the touch, leave it for a minute or two. Then gently remove the spent film and discard it; you can’t use it again.
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264 Part V: A Plethora of Projects
Carefully inspect the pattern on the copper clad. The toner should be well adhered with few, if any, voids. Use a magnifying glass (3x or 4x) to get a close-up view. If you see voids or skips, fill them in with a fine-tipped resist pen, which you can find at most electronics stores, or with a Sharpie®. The pen uses a jet-black ink that resists the action of the etching solution.
You may find that you need to press the toner into the copper clad using a small wooden or rubber roller. The pressure of the roller helps fuse the toner to the copper. We have had good success with a wooden wallpaper seam roller, such as the one in Figure 12-6. Be careful to burnish the toner onto the copper with a rolling action, not a scraping action, or you may scratch off the circuit layout.
Figure 12-6:
Use a wooden seam roller to burnish the toner into the copper clad.
You now have the board ready for etching, which you can find out about in the section “Showing You My Etchings: Etching the Circuit Board,” later in this chapter.
Choosing a Method for Making Your Own Circuit Layouts
Can’t find any artwork on which you want to base your circuit board? You’re probably glad to hear that you can also make a printed circuit board completely from scratch.
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Chapter 12: Building Your Own Printed Circuit Boards 265
You can try two popular methods for making a board from scratch:
^ The direct-etch method: With this method, you apply the resist directly to the copper clad. Dry transfer direct-etch kits, such as those made by Datak, provide pads for ICs and other components, as well as spools of thin black tape for the traces. Refer to the Appendix for Web sites that sell dry transfer kits. You apply the resist just like you make signs, using dry transfer lettering. The direct-etch method is practical for designs where you want to make just one board.
^ Draw the layout: You can use a computer and plotter, ink pen, or other method to draw your layout and then use that drawing as a master to make one or more PCBs. The master is essentially a film negative or positive that you use to expose the surface of the sensitized copper clad.
You can buy pre-sensitized boards or apply the sensitizer to a standard board with a brush or spray can. Exposure typically requires a strong short-wave ultraviolet light source, such as a tanning lamp.
Showing You My Etchings: Etching the Circuit Board
Creating the resist pattern on a new sheet of printed circuit board material, as we describe in the section “Making Your Own Circuit Layouts,” earlier in this chapter, really only gets your board one third of the way done. For the next step, you need to etch that board to remove the unwanted copper. The copper that remains forms the printed circuit that makes your project work.
You use something called etchant to etch your board. Etchant is a caustic (meaning it can burn you) chemical that dissolves copper. It’s not like some acid that a monster in a B-movie oozes out dissolving everything in its path; etchant doesn’t fizzle away the copper on contact. The etching process actually takes several minutes. The copper that the resist pattern doesn’t protect dissolves away first. The etchant finishes its job when it gets rid of all the copper in the exposed, resist-free areas.
(The final third of the PCB-making process involves drilling the holes for the components, which we cover in the section “Final Prep and Drilling,” later in this chapter.)
First step: Inspecting the board
Think of etching as an unforgiving process. In the steps leading up to this process, you can modify or redo your work, to a certain extent. But when you reach the etching stage, you’re making a commitment: After you etch, if you
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266 Part V: A Plethora of Projects
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