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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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have an error in your circuit, you probably have to chuck the whole thing and start again.
Thatís why you really, really need to inspect the board for errors in layout, missing traces and pads, skips in the resist pattern, and other gremlins that produce a poor result before you actually etch the board:
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If you created the resist pattern from artwork appearing in a book or magazine, compare your board with the printed layout. Follow the traces from pad to pad and note any discrepancies. If you find any, youíll have to redo the artwork or fix any problems before you begin etching.
If you created the resist pattern from your own design, or by using the direct-etch method, carefully review your work and compare it against a schematic or paper drawing. Be sure that pads and traces arenít too close together. At a minimum, all pads and traces should be X2nd of an inch apart, but more is always better here.
Repairing a board after a shoddy etching job ó if you can do it at all ó is time-consuming and frustrating.
Cleaning the board ó carefully, please!
After you inspect the board, wet a cotton ball with isopropyl alcohol and gently clean the exposed parts. Donít apply much alcohol because some types of resist may melt or distort when exposed to alcohol. Also, let the alcohol dry completely before immersing the board in the etchant fluid.
Use isopropyl alcohol with a minimal water content. General purpose isopropyl alcohol that you buy at the drug store can have 30- to 40-percent water content. The more water mixed in with the alcohol, the more chance you have that the water will damage the resist. Look for so-called technical grade isopropyl alcohol, available at chemical supply outlets and school lab suppliers.
Kvetching about etching
Etching can be dangerous ó not only to your health, but also to your wardrobe. Most circuit board etchants, whether in liquid or powder form, are toxic and highly caustic. Never allow the etchant chemical to come into contact with your skin or your clothes. If you do get some etchant on your fingers or hands, wash it off immediately.
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Chapter 12: Building Your Own Printed Circuit Boards 267
Types of etchant chemicals
Perhaps the most common etchant chemical for making printed circuit boards is ferric chloride. You can buy it in both liquid and concentrated dry (or paste) form. The liquid often comes already diluted and ready for use. But you will have to dilute dry powder or paste in water. We recommend the concentrated form because you can mix it with hot water. The hotter the water, the faster the etching process. (But never exceed 135 degrees Fahrenheit or else the
chemical may bubble and get all riled up, and splash over you and everything near you.)
Ammonium persulfate is gaining in popularity as a PCB etchant. It is available in both liquid and crystal form. Folks in the know about chemicals and safety generally consider ammonium persulfate safer than ferric chloride, though both chemicals are toxic. Handle both with extreme care.
Because etchant stains skin and clothing, avoid wearing your best party clothes when etching. Instead, wear a smock, your least favorite pair of pants, and old shoes. Also, wear eye protection to prevent the etchant from injuring your peepers if it splashes onto your face.
Wear gloves to protect your hands against burns and stains. Choose gloves that let you work almost as well as if you didnít have gloves on at all. (So donít use those old gardening gloves for this kind of work.) Disposable plastic or latex gloves do a good job.
Prolonged exposure to etching solution fumes can seriously injure you, so be sure to etch your circuit boards only in a well-ventilated area. All etchant solutions give off fumes, which can do serious harm to the mucous membranes in your nose and throat. You donít necessarily notice the effect right away. You may etch one or two boards and not be aware of the fumes. But an hour or two later, you feel an intense burning in your nose or throat that can last up to several days.
Store unused etchant solution in a dark-colored plastic bottle designed for photographic chemicals and keep the bottle in a dry, dark, cool place. Clearly label the bottle with its contents and keep it away from children.
Mixing the etchant
If the previous section didnít scare you away from ever touching etchant, even with a ten foot pole, check out this section for the mad scientist portion of the process ó mixing the etchant.
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268 Part V: A Plethora of Projects
Talking of trays and tongs
No matter what type of etching solution you use, always pour it into plastic containers. Avoid all metals because the etchant reacts with them. Be sure that the stopper or cap for your containers aren't metal, either, and that they don't have any metal parts inside.
You mix certain types of etchant with warm water. While stirring, chemical reaction heats up the etchant even more, so be sure to use a plastic tray that can hold very hot water. Processing trays for photography generally make ideal
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