Does PCB positive photoresist and related chimistry deteriorate over time (without usage)?

Question

For creating PCBs, I use the following process:

• Project the layout on a PCB (with a pre-applied positive photoresist layer)
• Reveal the mask with sodium metasilicate pentahydrate saturated solution
• Etching with Na2Si2O8 solution
• Clean the mask with acetone

This process has been successful tens of times until last usage (like 3 years ago)(yes, long time).

Now, trying to reveal the mask with the already prepared sodium metasilicate (3 years old solution) does not seem to have any effect on the photoresist (same box, 3 years old).

My hypotheses are:

• The photoresist has degradated over time, making it less sensitive to light.
• The sodium metasilicate has degradated over time (maybe absorbing some substance from air, as the bottle was not well closed and water evaporated).

Is this deterioration of PCB photoresist or sodium metasilicate known? Which? (or should I look into some other direction)

Answer 1

Different photoresists have different compositions, but all the ones I know have expiration dates that range between 1 and 3 years, which doesn't mean that they won't work anymore, just means that the manufacturer can not guarantee the original specs will still stand.

In my experience, older photoresists would need longer UV exposition time and longer revealing times.

Regarding the sodium metasilicate solution, I was under the impression that if not properly closed, it will actually absorb water from the air and dilute the concentration, but you described that water evaporated from it. In that case maybe it re-crystalized, check the bottom of the container for crystals, not sure how easy would be to re-dissolve them back into the solution.

Hope this helps.