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So your first step is to decide if you want to make a positive photographic copy of the artwork or a negative copy. You make the copy on a thick film, like the film in a 35mm camera (only larger and without the holes!):
A negative inverts the polarity of the image; black areas become clear, and white areas become black.
A positive retains the polarity of the image; black areas remain black, and white areas become clear.
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256 Part V: A Plethora of Projects
The art of layout drawings and schematics
Although both layout drawings and schematics are representations of a circuit, there are differences. A schematic shows the components of the circuit and what connections run among them. A layout drawing essentially shows the traces you use to make the connections that are
called for on the schematic. While all the connections are shown in a schematic as lines, the lines and components don't accurately reflect the physical layout on a final board. In a layout drawing, these connections are shown as they will actually appear on the board.
Have a local printer produce a photo negative or positive of the original artwork that you find in a magazine or book. It costs about $10 or so, but it can really help when you have to make multiple copies of a circuit board.
As an alternative, you can try making a positive mask with a sheet of transparency film. The local copy shop can probably do a better job (they get deeper, darker blacks) than your printer at home. If you photocopy the original onto transparency film, be sure to avoid any sizing errors. Many copiers automatically apply a 1- to 2-percent enlargement, and this size change can slightly alter the dimensions of the solder pad and hole spacings. Be sure to get an exact 1:1 copy.
If you take the artwork to a print shop, they can usually make a negative for less money than a positive because fewer steps are involved. When you make a photocopy onto transparency film, you always get a positive image. Whether you use negative or positive art, be sure to select the proper sensitizer and developer to match. More about this stuff in the section “Positively or Negatively Sensitized,” immediately following.
Creating the film has two most important aspects: The black areas need to be completely black and filled in, and any thin lines in the circuit can’t break up. Closely inspect the film under a bright, even light to look for imperfections. Sometimes, you can just fill in the missing black areas with a Sharpie® or similar marking pen.
Positively or negatively sensitized
We made a bit of a fuss about making a positive or negative in the previous section. Here’s where it matters: You have to be sure to use the right kind of sensitized copper clad or sensitizer (also called photoresist) spray to match the artwork that you’re using. The sensitizer, whether the manufacturer already applied it to the board, or you spray it on, is what makes the copper “pick up” the pattern from the artwork.
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Chapter 12: Building Your Own Printed Circuit Boards 257
Most circuit board layouts in magazines and books are designed as film positives where the black portions represent areas that contain resist, and the etchant washes away the white areas.
If you make a film positive to use as the mask, you must use a positive-acting sensitizer. When you expose the board to light, the dark part of the artwork stops anything underneath it from being exposed. When you dip the board into the developer, the unexposed portions take on a darkish coating of the resist. When etched, the part of the board under this resist remains. The rest of the board is removed, because there is nothing protecting the copper. Conversely, a negative-acting sensitizer turns the clear areas of the artwork into resist. You end up with a board that matches the mask, as long as the artwork and sensitizer/developer match, positive-wise or negative-wise.
You can buy sensitizer solution that you spray onto ordinary copper clad boards. Or, you can buy boards that come already sensitized, which is a lot easier. You can tell they’re sensitized because they come in black plastic baggies. Boards that somebody else sensitizes cost you more, but the time savings are worth it.
You also need a developer that’s compatible with the type of sensitizer that you’re using. When you buy sensitizer spray, the manufacturer usually packages the correct developer with it. Be sure to not mix a positive sensitizer with a negative developer, or your board pattern won’t come out correctly.
Mirror, mirror on the PCB
Okay, one more thing to be careful about: Be sure that you transfer the layout to the board with the proper orientation. Reversing the layout makes a mirror-image of the board, leaving you with a pretty useless circuit board. If you reverse the layout, the connections for any integrated circuits are backwards; at best, the circuit just doesn’t work, and at worst, it can burn out components. Spend some time thinking about how the layout transfers to the copper clad, and be sure that you don’t reverse the layout.