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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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Although overhead transparency acetate works as a transfer film medium, you get better overall results if you use transfer film specifically designed for the job. Check out the Appendix at the back of this book for a list of several online resources that sell transfer film designed for making printed circuit boards.
You need a plain paper copier or printer in top-notch working condition, otherwise the image that the transfer film records turns out gray and grainy. If you donít have such a copier nearby, take your printed circuit board artwork to your local copy center. Many neighborhood copy shops use state-of-the-art high speed plain paper copiers that produce jet-black images on clear acetate film. We think the few extra pennies that you spend are well worth it.
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Chapter 12: Building Your Own Printed Circuit Boards 261
By the way, some copiers require that you use acetate sheets with a leading white strip. The copier uses this strip to sense when a piece of paper feeds through critical parts of the machine. Without the strip, the acetate may jam. Before buying a box of transparency film for your copier, verify what kind of film your copier works with. Most stationery stores donít let you return opened boxes of copier supplies.
Flip-flop, flop-flip
As with the photographic method, which you can read about in the section ďCreating a PCB Photographically,Ē earlier in this chapter, you need to make sure that you have the circuit board image oriented the right way. The artwork must be right-reading, meaning it shouldnít be reversed left-to-right.
You have to again reverse original artwork that you get from a book or magazine that the publishers already reversed once (left-reading, or mirror image).
You can verify that you have the correct orientation by
Using a plain paper copier that can reverse the image. Some of the top
copier models have this feature.
^ Making one copy of the artwork onto the transfer film and then turning the film over and using it to make a second copy. Place a piece of white paper behind the transfer film for a clean background. This method isnít ideal, but it works in a pinch.
You can tell if the image is right- or left-reading by looking at the text that accompanies the PCB layout. The image is right-reading if the text appears normally. If the text appears backwards, the image is left-reading.
Getting a good image
After you copy the artwork to the transfer film, look carefully for washed-out areas or areas where toner hasnít completely adhered. Gently touch it with a soft tissue to make sure the toner is firmly bonded to the sheet. Some copiers donít fuse toner well to overhead transparency film. If you find out that your copier canít cut the transparency-film mustard, try a different copier which may have a higher heat setting.
If the image looks good, protect the transparency by covering it with a blank sheet of paper. If you accidentally scrape toner off the film, you can cause a void in the printed circuit.
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262 Part V: A Plethora of Projects
Transferring the layout to copper clad
After going through the steps in the previous two sections, youíre ready to transfer the toner from the transparency to the copper clad of the printed circuit board.
Follow these steps to prepare the board:
1. First, clean the board thoroughly by using a household cleanser, such as Ajax or Comet, and a sponge. After cleaning be sure to not touch the copper. Handle the board by the edges only.
Be absolutely sure there is no oil or dirt on the board. The copper should be bright and shiny.
2. Place the PCB copper-side up on an ironing board.
3. Cut the transfer film to size so that itís not larger than the board.
4. Carefully place the transfer film over the board so that the toner side faces the copper clad.
5. Secure the film into place with some strips of masking tape, but donít tape over toner areas.
6. Place a small piece of cheese cloth or several layers of paper towel over the PCB and transfer film. (Cheese cloth is a thin cotton material you can get in the housewares aisle of your nearby department or grocery store. You can also use plain white paper towels (no pretty pastel flowers printed on them, please).
Now, itís time to transfer the artwork to the board. If youíre using transfer sheets specifically designed for making PCBs, refer to the instructions that come with the transfer sheet.
The following steps provide general recommendations for the best way to transfer if youíre using ordinary transparency film:
1. Set your clothes iron to cotton-linen or medium-high heat.
2. Let the iron warm up and then apply the iron to the board. Move the iron back and forth in slow, even strokes, as if youíre ironing your very best shirt.
Be careful to keep the cheese cloth or towel flat to avoid wrinkles.
3. Apply steady and firm pressure for 15 to 20 seconds (see Figure 12-5).
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