Barry...Mr (ex) TV man:
I will throw in one more technique, of my own, that I used back in the 60's when I made circuitry using "pull" xstrs from IBM com-
puter boards. Nothing like we have available today for fab was available in those days !
I tend to still use it today,for low tech fabrication of a simple, single unit,if no hair thin traces are required nor dense- multi pin IC sockets....eg >~16 are involved.
After you get your component template/ layout planned for the foil side of the board, drill all of the holes that will be required.
Take a sheet of ~#600 grit wet/n/dry paper and use just enuff pressure to knock down any drill burrs / ridges / ferrules, to get a flat surface again. Clean its copper surface with a
Scotchbrite pad with a touch of copper cleaning powder. When you get a clean-rosy copper, rinse off, using a toothbrush's bristles to help in the cleaning out of the caked powder in the holes. Make a final water cleansing rinse and let the board dry. Lastly, do a Degreasing/
cleaning of the copper surface with an aggressive
...non waterbearing...solvent/degreaser like acetone/methyl ethyl ketone/xylene toluene or lacquer thinner. Cleanse and make NO finger contamination after that.
Cut a piece of CONTACT paper (plastic)...matte Clear or White works best...just larger than your PCB blank and cover the copper side. Do a pressure rolling to assure optimum adhesion.
Take a sewing needle that is large enough to not pass thru the drilled holes in the PCB, when inserted from the foil side....pierce thru to mark all holes.
Place your template /or/ draw your foil paths on the CONTACT paper...a soft lead pencil will let u correct errors. Once all foil paths are completed, the points where lands/pads for component leads are required, can be drawn as appropriately larger squares. On transistors, triangular pads seem to work best. I've always
had zero/error aligning the artwork to the holes, compared to having a walking drill bit later on. The next step is to take a clear plastic ruler and an EXACTO with a new #11 blade and just cut thru the CONTACTS marked foil paths, not over-extending your cut, and then taking the tip of the knife to lift the CONTACT out of the path just made, exposing the clean
copper. When the final copper circuitry path has been completed, and all checked out, the resist stage is ready to be done.This requires the board to be placed at work level ~1 ft in front of you and a hand held lightbulb/floodlight be held behind the bd ~1-2 ft. It will be manipulated as you spray.The resist will be FLAT BLACK KRYLON spray paint. Start at the top of the board and at a CONSTANT 9-12 in distance from the board make linear sweeps across the board from top to bottom ,til coated.
The purpose of the frontal lighting is to produce a reflection of your coating going onto the surface. This will give you a PRECISE evaluation of the amount of coating going on, and its UNIFORMITY. Test this technique on a piece of cardboard/wood, etc if you're uncertain of, or want to hone your skill level. This should reveal just how sloppy/non-uniform your previous spray paint jobs probably were.
Anytime I spray paint project cases,furniture,
cars....this is the ONLY way to do it.(Unless you have access to one of Detroit's programmed spray robots!)
Your PCB resist coating should have been just enough to have adequately coated the copper,but not enough to have filled up the CONTACT
"troughs". When just dry, carefull peel off the
surplus Contact covering, revealing the clean bare copper that is to be etched away.Any add on resist repairs can be made with a bit of paint shot into the cans plastic cap and applied with a toothpick tip. It does have a quick drying time though!
Etch away the copper in an agitated ,warm Ferric Cloride solution. When thru etching, cleanse the resist from the board with any of the afore-
This technique was the result of not being satisfied with any the resist Pens (SHARPIES) or the other family of them, that required
multiple overstrokes of the fine lines that they produced. The success of this resist medium is the decreased "filming / coagulation" characteristics of the formulation of the matte/ flat surface mixes. Try using a gloss version, and if you pull off the CONTACT masking too soon, the paint will have crept under
the masking medium. Let it dry too long and the paint will have filmed and will either tear or pull away from the foil.Touchy-touchy-touchy!
If a layout for an IC socket holes was required I still prefer the dry transfer lettering patterns, put on just after the degreasing
stage and then they were just barely exposed by a window cut into the clear CONTACT at masking stage, so that interconnecting foil paths could be made to them.<p>73's de Edd<p>[email protected]
<p>[ August 27, 2002: Message edited by: Edd Whatley ]</p>