Speed up etching times

Like many people who post on the internet, I've used online guides for etching my own PCBs. I use press-n-peel blue and a laminator (a big chunky metal thing built by Alphatec which is just perfect) to create toner-transfer etch masks. Like most other people, I put my ferric chloride into a shallow, flat container and etch by gently rocking it from side to side, so that the etchant "washes over" the PCB:



The other day, after making up three failed Invisible Instrument PCBs and putting all my messy PCB making equipment away I decided to quickly throw a board together to try out a new idea. Instead of pouring the ferric chloride into a shallow tub, I suspended the board on a piece of wire and just dunked it in and out of the etchant (I always replace my ferric chloride into a large coffee jar - the batch I have at the minute has been reused regularly for about 12 months and is still going strong!)



The peculiar thing was the board etched in no time!
Normally I heat up the etchant by placing the shallow tub into a larger container filled with hot water. This time, I etched "cold" but inbetween dunking, lifted the board completely clear of the etchant. It's well known that agitating a copper board in ferric chloride speeds up etching (and that heating the etchant gently also reduces etching times). What I hadn't realised until now was that exposing the board to the air during etching also seems to improve etching times too.



In fact, I noticed that the board etched from the top, down. i.e. the parts of the board that were out of the ferric chloride for longer actually etched quicker. While this seems counter-intuitive, maybe some chemistry genius will explain how oxidisation (or something) improves etching time.......

Anyway, it works for me. And from now on, my ferric chloride will stay in it's large coffee jar - my brown-stained plastic tubs have been junked and if anyone asks, I'll tell them I'm a dunker, not a rocker.