I too have some questions:
- Where do you think your process is deficient?
- What process are you using, Positive or Negative, contact or not?
- What chemicals are you using?
I built my system some time ago from odd bits, fish tank pumps and the like. I can only tell you of the results of my experiments.
It is clear that heating and bubbling the etch improves the consistency of the results. Heating speeds up the reaction. Bubbling ensures that
the concentration of the active ingredients in the etch is uniform. Without bubbling, large areas of exposed copper result in local dilution
of the acid in contact with the PCB, this leads to uneven etch times, with some sections being over-etched.
If you are using pre-coated resist boards, the age of the coating is important. Old boards take longer to develop and may need stronger
developer concentrations. Again it is more important that the pre-coated boards are stored in suitable environment. The colder, darker and dryer
the better. I store in the garage, in a black plastic sack.
Changes in storage temperature may result in micro cracks in the resist. Heat acts the same as light in hardening the resist. I use positive
resist because the process is simple. The professionals use a negative resist process which means that tracks tend to get thicker rather than
be undercut during the etch process.
I have had good results using pre coated overhead film in an Epson Stylus photo printer. This printer has software that allows very dark
over printing. I had intended to see if any of the other colours were opaque to UV
light. The transmission of visible light is not a good indicator of performance at UV wavelengths, where the photo reaction takes place. The
reason for looking at other colours is that some ink colours flow better than others and fill in the gaps between the ink dots. I now have a Cannon S800 printer but have not tried it in this
In this application contrast ratio is key to good results, hence the uses of overhead film. Note, bending the film after printing can lead to
micro cracks in the ink tracks, resulting in breaks in the final PCB track. It is also very important to keep the process clean, avoiding
grease, dust and hair on the master film and unexposed PCB. I use an air extraction system in the exposure box, and wear a lint free hood and
gloves when performing the photographic bits of the process.
To obtain good contact between the master film and the PCB during exposure, I print the film so that it is the ink side of the film in
contact with the resist of the PCB at the point of exposure. The film is held in place by a thin sheet of flexible polythene plastic. The plastic
is held down by a vacuum generated by a fan mounted under the exposure table in a sealed box. The table being perforated for this purpose. This
avoids the need to use a glass sheet to hold the film in place. Glass, other than quartz, being a poor transmitter of UV light.
To obtain double sided boards, I buy double sided resist coated copper board. I cut the board a bit bigger than is required. I expose one side,
develop and drill reference holes. I then remove the plastic cover from the unexposed side, and use the reference holes to align the second side
film. A red light in my exposure box allows me to align the holes with the film. This work well, but I am sure this is not the best method. I
would like to expose both sides at the same time, and leave the drilling till later. I think this would require the film for both sides to be
mounted in some form of jig. The jig might need to be constructed of quartz glass sheets to act as support for the film, and allow per
I use Potassium Hydroxide solution as the developer for the resist. It is important that this solution is kept out of contact with the air as
much as possible as it reacts with CO2. So I use a sealed tank and
agitate the developer by placing the tank on a mechanical rocker table, driven by an electric motor. Again this ensures no local dilution of the
I use a home brew of acid etch, made up of a mix of Ferric Chloride, Hydrochloric acid, and a few drops liquid detergent in pure water. The
detergent lowers the surface tension of the acid solution allowing it to wet the PCB. (Warning, do not add too much detergent or you will have a
foaming mass of liquid, bubbling out of the tank, eating all in its path. A Calcium Hydroxide solution is handy for such events)
I make up my own Ferric Chloride solution. Which I filter before adding to the etch mixture. The Hydrochloric acid stops the solution from going turbid with solid
copper salts. The solution starts clear brown and turns a clear green in colour. The reason for all this is to stop solids from being deposited
on the PCB blocking the etch action. I use distilled water to make up the etch. This avoids putting lime scale and other salts in the tank.
Lime changes the PH, and over time leads to floating solids in the solution.
My acid tank is heated by a 100W light bulb, mounted in a black tin box on which the plastic acid tank stands. A diode contained in a length
of sealed fish tank air tube measures the acid temperature. Low and High voltages circuits being isolated by opto and transformer
I never liked the idea that the 240 volt heater should be immersed in the acid. A fish tank pump provides the bubbles.
My PCBs float horizontal in the tank, but vertical is better.
I hope this helps.
I tin plate using a home brew electrolysis tin solution. The only problems with this are that once mixed the chemicals have a very limited
life, are expensive and very poisonous.
PVC gloves, and face mask are required at all times.