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.