Slow etching

[Adam]

I'm trying to etch a pcb, using uv sensitive pcb, a process I've done many times before and am now having problems with,

the pcb exposes fine and fully develps fine leaving etch resist tracks on a bare copper board, I then warm the ferric chloride and try etching, it goes very slow, and after a few hours the majority of the board is etched, but there are still some areas not etched, even though it was fully developed, also the tracks are going thin, i've tried, new developer, new ferric chloride, warming the ferric chloride more (would too much heat damage it, becuase it it does, I may as well put it in the bin now), and just at my wits end, i've wasted too much board as it is

 

[panic mode]

I was using ferric chloride for years (same batch...) and it was
getting slower over time but not nearly as bad as what you describe.
I was also warming it up to speed up the process but didn't try
to boil it ;-)
Since this was same old over-used batch I noticed that etching
was faster if the board was closer to the surface.
I would simply leave PCB floating with copper side in the etchant.
For double sided didn't work too well because flipping PCB to do
the other side would take longer than submerging the board at the
begining of the process.
There are only two things you might have problem with:
etchant (which you have changed) and development process.
If it is ferric chloride, just put in peace of PCB that was never coated.
See if it works. To check development process make a scratch on
surface that should be etched. If it was developed correctly, you
will not see any difference. If you see etching of the scratched area
to go much faster then the rest, you found the problem...

 

[pandaemonium]

There's no problem with over heating the ferric chloride except for fumes, so I wouldn't over heat it. You can actually bake your board at a low temperature to harden the resist if you don't plan on removing the resist latter. I assume you used a pre-mixed ferric chloride? there are powder forms you need to mix with water (always add the powder to the water, not the water to the powder!), make sure the powder to water ratio is correct. If you are hand agitating your boards, its more difficult to get good agitation if you us too much ferric chloride, the board should be laying in about 1 inch of fluid. What your problem sounds like to me is under exposure, your board may look like it has fully developed, but very thin film may still partially exist that is not readily visible, try exposing the board for another 1-5 minutes and see what happens.
Also, does this occur mostly at the edges? if there are rough edges on the board, the transparency film may not be making full contact with the board. Use a fine metal file such as a nail file to file off any burs.

 

[Dean Huster]

What I don't see in your "mix" or the answers posted so far, is agitation. If the board just sits there, it's like sitting still in a tub of hot bathwater. Pretty soon, you start to feel cool but if you move a bit, fresh hot water swirls around you and warms you right up. Same with the ferric chloride. The reaction will slow to a stop if the liquid is allowed to just sit there, the "used up" chemistry sitting next to the copper so that fresh etchant can't get in. If you agitate the board often, or better yet, install a bubbler to provide constant agitation, you can shorten your etching time to anywhere from 1/10 to 1/100 of what it used to be.

And as per the above suggestions, fresh, warm etchant is always a plus. Also be sure that you're etching in a glass or plastic tray, not metal. A metal tray will have a reaction of its own and eat up your etchant -- or vice versa.

And I'll always prescribe a change to ammonium persulfate as it's my etchant of choice.

Dean