UV etching, consistency problems with etch

Other Hardware General Electronics
1

Well, I've finally started to explore UV etching. I'm using photoresist sheets I got from Ebay, an iron on low temp (60C) to apply the photoresist, and followed the suggestions I got here.

Thing is, I'm getting consistency problems, specifically with the etching. Some turn out perfect, others not.

Even though the board is clean of photoresist, I'm getting zones that won't etch, see picture. And it's always this kind of defect. I'm using Peroxide + HCL in 2:1 ratio, at 50-60C temperature. Initially I thought the problem was that my solution was old, so I bubbled air through it with a fish pump and tested it on an old piece of copper clad, worked perfect. But then on the circuit with the resist, no result. No etching on a huge area in the center.

This one I tried with brand new HCL and peroxide. And got the result in the picture. After 10 minutes I even tried taking it out and running one of those scotch brite pads over the side areas, didn't help. For some reason, the copper isn't etching there.

This particular copper clad had the photoresist applied about two weeks ago, but stayed dark in a drawer for that time. It was exposed under a CFL (26W generic supermarket brand) for 20 minutes. Raw photoresist sheets are also stored in the darkness, under at least 4 layers of protection, so, no chance for anything strange there.

I'm using Oxyclean for developer, about 2 tsp to a cup of water. Photoresist comes off easy within 1 - 2 minutes with the help of a brush and scotch brite pad.

Anyone have similar problems?

(pay no attention to the purple stain, that was actually me showing someone why you couldn't take the resist off with acetone :grin: )

20150215_203216_resize.jpg

2

Suspect you have contamination (oxide or finger grease?) on that part of the board
before applying the photoresist. Wouldn't store your own boards, use immediately
the resist is dry - pre-prepared boards probably have antioxidants or other chemistry
to preserve the copper surface in a pristine state. Normally copper grows an oxide
layer in presence of air.

3

What are the resist sheels. ?
post a link

What developer are you using.

if these are negative resist I see an immediate problem

Do a test.
Cover one half of a board with kitchen foil.
Expose and etch.

Oxyclean is not a name I know , what is the chemical name.
Ok I found it.
Sofium carbonate and sodium metasilicate are both known developers the other constituents are not.
Do a test as I described to eliminate exposure as a possibility.

4

My experience is as follows, I am using negative resist though.

Boards have to be etched within a day or two of application, wait a week and they stick all over.

Development 3 percent solution of sodium carbonate (washing soda crystals).

Sodium metasilicate is normally used for positive resist.

1 percent in oxy clean , that's not enough , it needs to be 5 percent.

Your problem is exposure or development . The test I suggested should clear that up.

mmelendeze:
Photoresist comes off easy within 1 - 2 minutes with the help of a brush and scotch brite pad.

I guarantee you are not getting all the resist off.

If its negative resist your exposure is wrong.

If its positive resist development is wrong

5

Looks like over-exposure to me. No experience of using the CFL lamps but I just expose to the sunshine on a cloud free day and 5 seconds is about right for me. If it is over exposed it might look as if the resist is removed on development but if you look with a 10x lens and scrape the exposed part with a sharp point you can see that a very thin film remains.

Make sure that your master is in good contact all round. You might have had some light leaking in at the edges.

Russell.

6

russellz:
Looks like over-exposure to me. No experience of using the CFL lamps but I just expose to the sunshine on a cloud free day and 5 seconds is about right for me. If it is over exposed it might look as if the resist is removed on development but if you look with a 10x lens and scrape the exposed part with a sharp point you can see that a very thin film remains.

Make sure that your master is in good contact all round. You might have had some light leaking in at the edges.

Russell.

I think its both under and overexposure.

This is typically what you get when blacks on the master are not dense enough.

Negative working has less latitude than positive.

The only iron on resist available is negative working AFAIK..

I would be delighted to be proved wrong though as iv been looking for positive for a decade.

7

Boardburner2:
I think its both under and overexposure.

This is typically what you get when blacks on the master are not dense enough.

Yes, you need good quality film (I use Xerox brand) and ink jet printer set in photo mode. Laserjets are no good.

Russell.