Getting a pattern on the board
From least to most desirable:
- Masking tape
- Drawing pattern directly on board using a resist pen
- Drafting tape/patterns on board as resist
- Photgraphic methods
- Toner transfer systems
These things form the resist on the board to save what you don't want etched away by the chemical solution.
NOTE: BEFORE PUTTING DOWN ANY RESIST, CLEAN THE BOARD WITH AN ABRASIVE CLEANER LIKE AJAX OR PUMICE, SOAP AND WATER! Finger oils are acidic, and will discolor the board and keep resist/tinplate from sticking. Try not to touch the copper after the board's cleaned and dried.
Masking tape, drafting patterns, and resist pens are all manual methods, prone to misalignment and fraught with error. Also, board made like this are usually one-of-a-kind and not reproducable. One advantage to using premade rub-on patterns such as those available from Datak is that the spacing of the IC pins will be correct.
Photographic methods used to be more popular before toner transfer came along. Used to be, you exposed a sensitized board to a negative of your pattern in the darkroom, then processed the board like a photograph. Nasty chemicals, horrible odors, and errors in alignment characterized this process.
Recently, positive-image pre-sensitized circuit boards are now available that can be exposed in direct contact to your pattern printed as a transparency (traces black, unwanted copper as clear). Only one step is required to develop them, and only a simple #2 photoflood bulb is needed for exposure. I have used these, with good results.
Toner transfer sheets (like from Techniks ) are available that print useable resist right from your laser printer. (And, if you don't have a laser printer at home, usually a school or office supply store can print your layouts for you. I have used Kinko's a number of times to do this.) The only catch here is that you have to print the pattern as a mirror image. Most printers and layout software can do this.
One you have printed your mirrored pattern onto the toner transfer sheet, you carefully iron the pattern onto the board, ink side down. Use a household iron on the lowest setting, with a sheet of printer paper in between the iron and toner transfer paper. Let the toner transfer cool (I've dunked the whole thing in water to "anneal" the plastic) before you carefully, SLOWLY peel it off, leaving your resist behind.
Getting rid of excess copper
- Milling / cutting (Xacto,dremel, mill)
- Etching: Ammonium Chloride / Ferric Chloride
If you're cutting, be careful. We all tend to forget that those Xacto knives are very much like the surgical scalpels they were designed after! They can cut into the hand deeply and quickly, leaving a clean cut that bleeds profusely. BE CAREFUL!
In etching, the most important thing is to have HOT, CLEAN etchant. I dump etchant that's been used more than twice (down the sink is okay, so long as you rinse it with plenty of water). You want the etchant hot to the touch, like scorching hot bath water. Around 110-120 degrees is great. The hotter it is, the faster it etches, and the quicker you get that acid away from your precious traces! The slower it etches, the more undercutting you have, running the risk of obliterating narrow traces. I've seen it happen.
Use a Pyrex or plastic tray to etch in. DON'T USE ANYTHING METAL, EVEN IF IT'S COATED! A microscopic crack in a ceramic-coated metal vessel could allow the etchant to eat right through the wall. Cover the board in about 1/2" deep of etchant.
While it's etching, gently slosh it around, bringing fresh etchant into contact with the bare copper. Or, use a little motor with a magnet mounted on it, and a magnetic stirrer from a lab store. I used to do that when I was etching a lot.
Also, use work clothes, tongs, gloves and be careful not to get it on your skin. Ferric Chloride STAINS! I call it "Green Vulcan Blood" :-)
A 3"x4" board, with about half the copper taken off, should take around 30 minutes to etch. If it takes much longer, it's too cold or you're not sloshing it around! |
