OK a few (!) words about the process.
I process the PCB resist on a one-shot basis, as was common with film processing in the latter days of film photography.
I use Sodium Hydroxide (caustic soda, caustic hydroxide) for preference rather than proprietary resist developer, it's cheap. I do the processing in flatbottomed plastic tubs which came free from the Chinese takeaway. I have some bigger dishes too from the old photography days.
I mix a quarter boiling water with threequarters cold tap water to a pint (two mugfuls) and add a shallow level teaspoon of granulated hydroxide and stir immediately. This is not strong enough to burn my fingers as long as it is washed off quickly so I can handle the wet board in comparative safety once the exposed resist has dissolved. You may have sensitive skin. WEAR RUBBER GLOVES. A little physical manipulation is sometimes necessary to remove some stubborn resist where perhaps the illumination has been poor. SODIUM HYDROXIDE is both CAUSTIC and POISONOUS so TAKE CARE when handling it and dispose of it carefully immediately after use. It CAN go into the sewage, it is frequently used for clearing drains.
I have been handling strong acids and alkalis since I was a child, I learned not to take them lightly. I remember a demonstration I saw done with strong caustic hydroxide solution. A small amount was boiled in a beaker and a small piece of meat thrown in. It quickly dissolved into a brown colour. Sodium hydroxide can do this to you. If you come into contact with concentrated hydroxide or solid hydroxide wash the affected area with vinegar to neutralise it, then douse freely with water. Use water alone if no vinegar is available. Exposed caustic absorbs water from the air to form a strong solution on the surface. Cap the container tightly immediately after removing a small quantity.
Caustic-free developers are available. If you are uncertain about using caustic, use one of these instead.
Experience has taught me how few crystalline balls of ferric chloride will adequately process the size of board I am working on. I dissolve them in a small quantity of recently boiled water and slide in the PCB. The solution does not stay hot enough to finish the board in one go, so I zap the solution with 20 seconds in the microwave once I can feel that it has cooled down. Used ferric chloride contains dissolved copper. It kills marine life and should not be disposed of into sewage systems.
More recently I started to use Hydrochloric Acid and Hydrogen Peroxide for etching. In the past I have also used Ammonium Persulphate.
Ammonium persulphate has the advantage that it doesn't mark things with a yellow stain like ferric chloride. It has the disadvantage that it has a shorter life when made up into solution. It shares the disadvantage with ferric that it's only available from specialist suppliers. Being an impatient kind of guy I prefer to be able to go out to a local shop and get the things I want. Hydrochloric acid and hydrogen peroxide fall into that category. Chemists here sell 10, 20 and 30 volume peroxide. 30 is the strongest and best value for money. Hardware stores sell hydrochloric acid (spirits of salt).
CONCENTRATED HYDROCHLORIC ACID (MURIATIC ACID) IS A DANGEROUS SUBSTANCE COMPARABLE WITH CAUSTIC HYDROXIDE. Observe the same handling precautions with it. If acid comes in contact with your skin treat the affected area with sodium bicarbonate (baking soda) and water or a solution of sodium carbonate (washing soda). Rinse with an excess of water.
WATER in large quantities is the best treatment for strong acid or alkali in the eyes. Seek medical advice if splashed in the eye.
MIx the acid and peroxide about 2:1 initially. Add the acid to the peroxide, not vice-versa, a drop of water (or peroxide) added to acid can get extremely hot and turn to steam, splattering acid around.
This mixture produces a blue compound of copper which goes into solution. Apparently this compound itself acts as an etchant when well oxygenated and can be revived by blowing water through it. I just add more peroxide when the colour gets dark. I may post further information on this when I have more experience, I've only been using this chemistry for a couple of months. Like all etchants it works better when warm. Again, DO NOT DISPOSE OF COPPER CONTAINING COMPOUNDS INTO THE SEWAGE SYSTEM.
DISSOLVED COPPER IS HIGHLY INIMICAL TO MARINE LIFE. TAKE ADVICE ON ITS DISPOSAL FROM YOUR LOCAL AUTHORITY AND MAKE SURE IT DOES NOT END UP IN THE SEA.
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