How to make printed circuit boards

This guide will explain to the electronics hobbyist how to produce a professional quality PCB easily and efficiently, without professional etching equipment.

Equipment

You will need to build two pieces of equipment: an ultraviolet exposer and an etching tank.

An ultraviolet exposer is simple to make, build a small wooden box with glass on the top and place a 2 or 4 tube (4 tube is better) fluorescent light fixture in the bottom. Instead of putting regular white fluorescent tubes in it, place blacklight bulbs in it.

There are many different variations of etching tanks. The basic idea is to agitate the etching solution with your circuit board inside, and optionally, for faster etching, heat the solution. The two most common ways of agitating the solution is to blow bubbles through it with an air pump (as in a bubble-etch tank) or to hold the circuit board out of the solution and use a pump to spray the solution onto the board. When constructing an etching tank, NEVER use metal parts and fittings. Circuit boards are usually etched with ferric chloride, which attacks metal (which is what we want). Your best bet is to use PVC tubing and fittings.

Safety glasses, heavy rubber gloves, and an apron are also recommended for most steps where chemicals are involved, and safety glasses should be worn whenever drilling circuit boards as fibreglass particles irritate the eyes.

A drilling stand is recommended for straight holes. Tungsten-carbide bits should be used. You will need 3 or 4 0.8mm bits, 1 or 2 1.0mm bits, and 1 each of 1.2 to 1.5 for larger components.

You should also have a guillotine for cutting boards to size. A hacksaw will work, but is tedious, time consuming, and tends to dull the blade quickly.

Supplies

You should always use good-quality positive pre-coated photoresist fibreglass board. Check for scratches on the coating and on the protective covering before using. Boards can be purchased at many electronics supply houses. Boards should be handled in subdued light. Its not necessary to use a safe light, but they should not be exposed in direct sunlight and minimize unnecessary light exposure.

It is recommended that you use a silicate-based developer such as sodium metasilicate pentahydrate. Sodium hydroxide works, but is highly sensitive to temperature, concentration, and is very caustic. If you absolutely have to use sodium hydroxide, to mix it correctly: add 200cc of "water glass" (sodium silicate solution) to 800cc of distilled water. Then add 400g of sodium hydroxide. When adding the sodium hydroxide to the solution, take care _never_ to touch it with your hands, and add it a small amount at a time, as it produces a lot of heat when it dissolves.

Ferric chloride hexahydrate is recommended as an etchant. Its cheaper and easier to obtain than most of the alternatives. It attacks most metals, so it should be stored and handled with only ceramic and plastic apparatus. The FeCl comes as a powder or granules, it should be dissolved in warm water until saturated.

Step 1: The Artwork

The first step in the circuit board production process is to design the layout of the circuit board. Producing good-quality PCB artwork is beyond the scope of this article (however, I may write an article on this subject later). Lay tracks and pads out with careful condsideration to scale, the size of devices, current, and soldering.

A mirror-image positive of the artwork (i.e. black=copper) should be printed exactly to scale on acetate, preferably on a laser printer or photocopied onto the acetate. The idea is to get the black as dark as possible, as it will be blocking the ultraviolet light. The artwork is mirrored because when it is exposed, it should be face-down on the board, leaving no space between the image and the board to prevent blurriness.

Step 2: Exposing the Circuit Board

If producing a double-sided circuit board, tape both halves of the artwork together to form a "pocket" for the board, making sure they are exactly lined up.

If producing a single-sided circuit board, the artwork should be taped to the board taking care to leave no gap between the artwork and the board itself.

The board and artwork should then be placed face down on the exposer. The time to leave them on depends on the brand of board, and your exposer. You should put test pieces on first and expose them for 2-8 minutes in 30 second intervals to determine the best exposure time. As a general rule, overexposure is better than underexposure when exposing PCBs. After exposure, you should be able to see a faint image of the artwork on the circuit board.

Step 3: Etching the Circuit Board

note: you should ALWAYS wear gloves, safety goggles, and an apron for this step, etchant is caustic and can damage eyes and permanently stain clothing

Time for etching a board depends on how fresh the etchant is, agitation, and temperature. Fresh, hot etchant can etch the board in under 5 minutes, and as long as an hour without heat or agitation. Etching boards faster produces a sharper image, slow etching may lead to blurriness.

The etched board should be thoroughly rinsed and dried. Methanol (Methylated Sprits, available at most paint suppliers and hardware stores) or Isopranol (available at any pharmacy) can be used to remove the remaining photoresist from the board.

Step 4: (Optional) Tinning

Tin-plating a circuit board is optional, but makes soldering a lot easier by preventing the copper from oxidizing upon contact with air. If you decide not to tin the board, leave the photoresist on to protect it from oxidization.

Room temperature tin-plating crystals are the best to tin boards with. They produce a good finish in a short amount of time. Made-up tinning solution deteriorates over time, so you should only make up exactly as much as you need. Take care to avoid contamination, clean and dry the pan before tinning, and keep a pair of tongs to be used only for tinning.

Ensure the copper is absolutely clean before tinning. Remove all residual photoresist and scrub the surface with steel wool until it is shiny.

Put the board in the tinning solution. After about 30 seconds, the copper tracks should turn silver. Leave the board in the solution for another 5 minutes, agitating it occasionally. Rinse the board and rub it dry with a paper towel to remove any remaining tinning crystal deposits.

Step 5: Drilling

note: safety glasses should be worn when drilling.

Pads should be drilled with consideration to the sizes of terminals on components to be installed. Find datasheets on components before drilling to find out the correct size.


If you followed all these steps correctly, you should have a good professional quality printed circuit board.

If you want to produce a circuit board cheaply and don't particularly care about quality, you can manually draw your pattern on a copper clad board without the photoresist coating. My recommedation for this method would be:

1. Tape a printout of your board design on the copper clad board.
2. Punch a small indentation in the center of each pad using a nail and hammer or something similar. This will help keep the drill centerred as you begin each hole.
3. Drill all of the holes. (Yes, before etching.)
4. Remove the taped-on board layout and copy it using a Sharpee. (Other types of permanent markers may work as well.) This will stop the copper underneath from being etched away, but you will probably need to apply several coats, and possibly take the board out midway through etching, rinse it, add more ink, and continue to help prevent slight cracks in the traces.
5. If you're doing a double-sided board, draw the other side. You can use the pre-drilled holes to align it.
6. Etch as above
7. Rinse and sand off the ink. I strongly recommend wearing a dust mask of some sort, as you do not want to breathe the dust from this.
8. Inspect the board for any broken traces that may have accidentally etched through. You should do this even if you're using the photoresist method above.

The main problem with drilling first is that a small amount of copper will etch away from the inside of the pads if you're not careful.

The resulting board won't look anywhere as nice as professional quality boards, and you can't have traces as thin, but it does work and with fewer, cheaper supplies.

The toner transfer method

The toner transfer method works well for printed circuit boards that are too complex, or have physical tolerances too exacting, for hand-drawn boards, and it avoids some of the bother of photoresist methods (or the expense of sending your board to a fab house). It is also a very convenient way to make boards if you've done the board layout on a computer.

The method works by using special paper to transfer the diagram printed by a laser printer (or xerox machine) onto the copper surface. Xerox toner happens to make a reasonably good etch resist, and so you can just dunk the board into etchant at that point. Once it's etched, the toner scrubs off fairly easily.

Procedure

For this, you'll need plain copper-clad board (not coated), a xerographic printer or copier (e.g. a laser printer; inkjets won't do), some toner-transfer paper, an iron, an etching tank, and the usual tools such as a drill, saw, shear, etc.

The first step is, as always, to lay out the board. There are many programs, free and commercial, for doing this on a computer; in a pinch you could use a decent drawing or diagramming program. The end result is an image you can print that is black where you want the copper to be, and white elsewhere. The artwork has to be mirror-reversed, because of the transfer step.

You print the layout onto toner-transfer paper, which is a somewhat glossy coated paper whose gloss dissolves in water. You have to use a laser printer for this. If you have another sort of printer, or if you decided to draw the layout by hand, you can use a xerox machine to copy the image onto the transfer paper. The point is that the paper works with toner, but not with other kinds of ink.

Transferring the toner to the board is the tricky part. First clean and dry the board as described elsewhere in this node: it should be a bright pink color. Place the board, then the paper (aligned so that the artwork matches up with the board), then a thin towel or a napkin. Take a hot clothes iron — no steam! — and press it against this stack for a while. It will take a small amount of practice to learn how long to do this, how hard to press, how to do it without smudging the transfer, etc. 45 seconds is about right.

Once this is done, dunk the board and the paper (which will now be stuck to the board) into some water and wait. Eventually the paper will curl away from the board, leaving the diagram on the board. (If you've ever made kit models with decals, the process is similar.) Take the board out and dry it gently. Touch up any gaps or pinholes using a resist pen.

From this point, the toner transfer method is the same as the others: etch with ferric chloride or ammonium persulfate, wash thoroughly and scrub the toner off with steel wool, drill and cut if needed, and populate it.

Details and Asides

Cleaning the board can be done with fine steel wool without any solvents more noxious than isopropyl alcohol. Clean copper is very light in color, almost salmon pink. It oxidizes in air, so clean it shortly before you use it.

The boards made using this method are good enough to use surface-mount packages such as SOIC or 0604 discretes. (You could probably go even smaller, if you're careful.) For some circuits — especially if you can use surface-mount parts to avoid having to drill holes — it's actually faster to make a board than it is to wire it up using normal prototyping techniques. And the final product is much nicer, smaller, and more rugged.