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Printed Circuit Board (PCB) |
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PCBs are the way to go if you need a circuit that is very reliable, durable, and visually attractive. Otherwise, the extensive time taken is not worth your while, and a well-soldered circuit on pre-drilled grid board will be just fine. Also, if the circuit hasn't been carefully designed and flawlessly tested on a breadboard, or if there's any chance you would need to make a change, then use a pre-drilled board.
This explanation assumes you will be using a PCB with copper on only one side. (get 2 sided if you've got a very dense network of connections). If you can fit everthing on a single board, large or small, then that saves some work with ribbon cables.
It helps to buy an etching kit. A kit with a resist
ink marker and dissolved etchant is the most compatible with the following
instructions.
Start with your schematic. On a new paper, with a rectangle drawn to scale to represent the board's perimeter, use your problem-solving mind to figure out the best way of arranging the parts on the board. Draw dots to represent pads, through which pins will be inserted and soldered to. If, after drawing these pads, you can't remember what components they were meant for, you can draw each component's above-board shape, but do it lightly so there won't be confusion about which lines are meant to be copper lines. Create the components' layout as from the overhead perspective (so that the numbering of ic pins progresses in the usual direction). Remember that IC pins are 2.5 mm apart (unless surface mount). Then use your problem-solving mind to draw lines or traces to connect pads. If you realize there are a lot of traces that can't get from point A to point B without crossing over each other, you may need to go back and change the component layout. If there are only a couple of crossover situations, they are manageable if you are willing to solder a wire "bridge" over the opposing traces. (ie. a wire above the board, just like another component)
CAD software is out there that will do this automatically. You give it a schematic, and it runs an algorithm to create layouts that become more and more optimal.
You will need to transfer the diagram's mirror image to the copper underside of the board, drawing on the copper with a Sharpie permanent black marker. Ink from markers like these is called resist ink. It will prevent etchant from corroding copper; therefore, where ink is present, there will be a conductive layer of copper, and where there's no ink there will be no copper.
If the design and board are small, you can just invert the image in your head and draw the inversion directly on the PCB. Otherwise, flip the paper over, insert carbon paper between it and the copper layer, secure the drawing so it won't budge more than 0.5mm by taping it tightly around behind the board, and finally trace over the lines firmly with a ballpoint pen. In this case you'll be left with very light lines on the copper which will guide you when you trace over with the marker. Always make sure your marker lines are thoroughly black throughout, and have no inkless scratches. (after etching, there could be a hair-thin interruption in your copper trace). Obviously, a half-dry marker won't help. Note that going over dry ink with a wet marker will peel away the dry ink, unless you dab carefully. For each component pin, draw a nice big circular, or 2mm*3mm oval or rectangular pad.
If you used software to place and route, then it should be able to print the trace diagram. Transferring to PCB involves a special paper, chemicals, and a UV light (?).
Whenever the board is covered in etchant, handle it with a plastic tool (iron/steel will rust), but be careful not to scratch the resist ink. Soak the PCB in etching solution (get pre-dissolved solution), inside a plastic tray. Follow instructions from the web, such as 4mm deep liquid, copper side down, 15~30 minutes. Chemical action will be slower if you don't stir from time to time. I say that the time to stop etching is when you see only yellow in the inter-trace gaps instead of pink copper (upon lifting up the board with a plastic tool and looking underneath). When finished, remove the board and rinse under water very well to stop the chemical action. Bottle used etchant and take to a hazardous disposal place. Or, if you insist on hurting your plumbing and also damaging important bacteria cultures at your local sewage treatment plant, then flush it down the toilet. (Reusing solution next time is an option, but you may need to add some new solution to the mixture if the etching process slows down or stops.)
At this point the board will be completely clear of copper between the traces, but the traces will still be covered with resist ink. Rub with resist-dissolving alcohol (avail. from an electronics store) in a paper towel. Rinse again.
Use a specialized (very thin) drill bit, with electric drill or dremel tool, to drill the holes. Drill into the copper side of the board. My method is, before drilling, to start a small dent for each hole with a nail and hammer, so that the drill doesn't wander. If one of your pads is too small--that is, extends less than 0.5mm beyond the drill bit--then the drill bit will likely tear it right off the board.
Solder. Don't keep traces heated for too long as they will separate from the board.
Spraying the board's copper side with a protective aerosol is optional, but it protects the copper from oxidation which is accelerated by moisture and greasy fingerprints.
If you plan to attach ribbon cable, the entire ribbon should end up on the underside of the board. Design parallel copper traces that are 1.25 mm apart (half the separation of IC pins) and 5~10 mm long. Strip the entire width of ribbon cable in one piece using a knife (gently cut along top and bottom, and poke knife in between each wire), instead of stripping one wire at a time with wire strippers. Pull the stripped section of insulation off in a straight direction. Do not twist the stranded wires; just make sure all strands are grouped together and extending forward. When soldering, the key is to start by tinning the wires, and tinning the copper traces separately (so there's a bead of solder and flux on each trace). Lay the ribbon on the board. The solder will fuse after a quick touch with the soldering iron. If any solder or wire strand is bridging between two traces, heat up both traces and scrape a thin screwdriver in between, in a motion that "chases away" the soldering iron. The last step is to strap the ribbon down so it won't wiggle around and break off. Use a solid wire to strap around the insulated part of the ribbon cable, insert the two ends of that wire into two extra PCB holes, and bend the ends so they stay in place.