Primer — Printed Circuit Board

If you’re building a circuit from scratch, rather than using a known design, you should prototype it first. One way is to string it up on a piece of perf board, as shown at right. Our “Hello World” blinker circuit has a 555 timer chip, a battery, a capacitor, an LED, and three resistors.

The underside of our proto-board is a “spaghetti patch” of wiring that will inevitably lead to problems. Even with this simple example, it’s easy for wires to touch and short out. With all wires shielded, it’s still ugly — inelegant and difficult to decipher and debug. This is a powerful reason for making a PCB.

Start with your circuit’s schematic diagram, then lay out the PCB design based on the actual sizes of your
components. You can create your design with any drawing, paint, or design program. I use Eagle CadSoft
(www.cadsoft.de), which is shareware.

First, produce your layout with components drawn in life-size. Print this out double-size for later reference. Then generate your final layout by removing the parts outlines.

You’re designing the underside of the board, so your layout should be a mirror image of the arrangement you’d see looking from above. Also, you’ll turn the printing to face the copper you’re etching, so lettering should be backward.

Indicate parts placement and the
location of Pin 1 on any chip. For future reference, you should also list the design’s date and a revision number.

And with a small design, make multiple copies side-by-side; this increases the odds that at least one will work.

Place the Techniks Press-n-Peel PCB transfer film in the tray of a laser printer or photocopier. One side of each sheet is shiny, while the other side is dull. Place the film so that the printing will come out on the dull side.

When using a laser printer, you want to minimize the possibility of damaging or curling the film. Use the “manual feed” feature to avoid making the film roll around the drum.

Once the film is printed, check it against a strong light for any defects. The film should be dark and no light should come through the traces.

The PCB copper board should be as clean as possible so that the design adheres to it. Scrub it with dish soap and a scouring pad. Do not use steel wool, because it leaves tiny pieces of steel behind on the copper, which interfere with the etching.

Water should flow easily off the copper board. Dry the board to a polished finish with a lint-free cloth.

You should use the board as soon as possible after cleaning, because otherwise the copper will begin to slowly oxidize in the air, diminishing its ability to take the design transfer and to etch well. Make sure not to touch the copper with bare hands.

Note: do not touch the
copper with your bare hands, since sweat and oil likewise interfere with the transfer and etching processes.

This is the most challenging step of making PCBs at home. First, cut the transparency to the same size as the PC board.

Find a fairly hard surface to iron on, such as a thick, wooden cutting board or a scrap piece of wood.

Set the iron to the polyester/rayon setting (medium heat on most irons). Once the iron has come up to temperature, lay it down directly on the wood to heat it. The warm wood will help with the adhesion and transfer of the film. Proper temperature control is critical; you will find it only through experimenting with your iron.

After about five minutes of warming the wood, lift the iron and lay the PCB down copper-side up. Align the film dull/printed-side down on the copper, and begin ironing firmly over the sheet, never lingering longer than four or five seconds in any one place. You want to avoid smearing the design.

Continue ironing for about 45 seconds, or longer if the copper board is thick. Then remove the board and let it air-cool, or cool it faster by putting it under cold water.

Note:Warm the ironing surface for five minutes before ironing the PCB.

Settings:Set iron on Polyester/Rayon.

Alignment: Cut the transparency to the same size as the PC Board.

Examine the board for obvious errors. You can fix broken traces by filling in the gaps with a permanent marker such as a fine-tipped Sharpie. Use a hobby knife to separate any traces that have smeared together. Take the time to do a good, thorough job with this step, which can make or break your final product. For this reason, it’s smart to print several designs, rather than pinning all of your odds on one.

The chemicals used in etching are
corrosive, and their fumes are irritating. Follow proper safety procedures, wear a mask (or work in a well-ventilated area), and protect your eyes.

There are different types of etching chemicals: dry and liquid. Dry chemicals are less expensive, but require dilution, as directed on the packaging. For this project, we recommend ammonium persulfate, available at lab and electronics supply stores. You will need about 1 quart of etching solution (etchant) to etch a 6″ board.

Etching time is related to temperature. A small increase in temperature drastically reduces the time needed to etch a board, but too high a temperature makes the etchant too aggressive and gives off fumes. Aim for a temperature of around 125 degrees Fahrenheit (52°C).

Design houses have temperature-controlled etching tanks. For etching at home, you just need a stove, a large pot, and a plastic tray for the etchant.

Fill the pot halfway with water and heat to a simmer, but not a boil.

Put the PCB in the tray and place the tray on the simmering pot. Then slowly pour the etching solution into the tray.

The steam from the pot will slowly heat the etchant and increase the rate of etching. It should take no more than 15 to 30 minutes to do a proper etch. Gently agitate the tray during this time. This will remove bubbles, speed the etching, and make it more uniform. As shown in the middle picture on the facing page, you’ll see copper dissolving away from the PC board.

When it cools, remove the board from the tray and thoroughly wash with water to remove all traces of the etchant. Wash again with a little dish soap to help neutralize any remaining acid.

Now the copper of the board has to be exposed for soldering. Use an abrasive pad or fine-grain (400 grit) sandpaper to scrub off the black film layer and reveal the copper underneath.

Drill the holes that the component leads fit into. If you’re making many boards, it helps to use a drill press, or at least a power drill or Dremel, because hand-drilling can get tedious. The drill bits should have the same diameter as the etched holes.

Finally, if you repeated your design, cut the boards apart. Choose the best board to solder the components onto.

Use the double-sized parts placement diagram you printed earlier, and solder the parts into the PCB. If the board is an early revision, I always use sockets for any chips, as shown in the bottom photo, so I can remove them without having to desolder. If all is done right, then you’ve got a complete, functioning circuit on a beautiful little board.