Uputstvo za izradu stampanih plocica

Thanks to Robin Moorshead for the following procedure to make silicate developer solution:
Take 200cc of "water glass", add 800cc of distilled water and stir. To this add 400g of sodium hydroxide (caustic soda).
Important safety precautions: The sodium hydroxide solid must NEVER be handled, use disposable gloves. When the sodium hydroxide dissolves in water it produces a great deal of heat so it must be added a little at a time and each portion allowed to dissolve before more is added. If the solution becomes very hot leave it to cool before adding more sodium hydroxide.
The solution is VERY caustic and it is particularly damaging to the eyes, use eye protection when making and handling it.
It also ruins clothing. Water glass is also known as "sodium silicate solution" and "egg preserver", It used in fire proofing fabrics, for waterproofing walls and making "chemical gardens". Caustic soda is used for clearing drains and available from any chemist. The solution cannot be made by dissolving solid sodium silicate.
This solution will be the same strength as the concentrate, and so will need diluting - about 1 part concentrate to 4 to 8 parts water, depending on the photoresist used and temperature.



Comment:

The pH of the solution just described should be about 13.5. In my experience various brands of positive-sensitized PCB require different strengths of developer. Some developers are simply sodium carbonate, which can be considered like sodium silicate in which the silicon atom has been replaced by carbon. The pH of carbonate developers is probably lower (i.e., less aggressive) than the silicate based ones. I was unable to develop Injectorall boards using sodium carbonate solution alone. However, I have used a 0.5 M solution of KOH for many years without problems. To a very rough approximation, a 0.5 M KOH solution should have a pH of about 13.5 as well, or a bit less. Note: The Moorshead recipe has approximately 10 moles of NaOH per liter which is then diluted 1:5 to 1:9. Therefore the final concentration of NaOH is about 1 to 2 molar. Because of the buffering capacity of the silicate, however, the solution should not be as aggressive as a neat solution of sodium hydroxide in water at the same concentration.

If I were to experiment with the Moorshead recipe, I would probably stick to a 1:9 dilution of the stock concentrate, or maybe even a 1:10 dilution. I would also start with about half as much NaOH (i.e., 200 g/L) to see how it works.