Photoresist exposure time

[m-Kresol]

Hey guys,
I stayed up all night to finish my first PCB in eagle and I want to etch the PCB today after work in the lab. I got some positive photoresist FR4 plates yesterday. Unfortunately, there was no instructions and I can't find a solid answer to the following question:
How long do I have to expose the photoresist to usual desk-light?
Alternatively, we've got several UV lamps and chambers, but I'm afraid the might be too intense or that my mask isn't black enough, so the traces won't come out as I want them too.
Any help is appreciated! Thanks

[mah62]

Hi Felix

Experimentation is the only way because different UV sources are of different strengths
and from varying distances from the board.

Get a piece of board, some artwork and a piece of card. while exposing, edge the card along every 30 seconds or a minute so that each section of the board is exposed for different times. keep a log and let your eye be the judge.

I had spent hours trying the iron on method with very inconsistent results. Tried the UV way and, Bingo! Flawless every time. Much cleaner sharper traces and this is coming from a guy who usually fucks everything up!

Mark

[mah62]

......oh, and if your mask isn't black enough then everything will be over exposed. If so, then double up the artwork film you are using. Use glue to make sure the two pieces are aligned. Hold them up to a light source and wiggle them about. I'm using ordinary tracing paper and get fine results. Smudging was a problem on this medium at first but found this was solved by printing the image at least halfway down the page.

Hope this helps!

Mark

[m-Kresol]

thanks mark,
i feared that the only way of knowing is testing. I was hoping for a "universal" method. guess I'll have to sacrifice one of the boards ;(

[mah62]

.....you don't have to sacrifice the whole board, just a slim piece will do. Any artwork will suffice, too. Besides it's definitely worth it.

Mark

[davent]

Yeah just a narrow strip 1cm wide or so. I've found with various brands of boards it usually needs ~9-10 minutes exposed about 3cm from a regular fluorescent tube. I would do the exposure test so the range of times were a minute apart and ranged from 8-15 minutes. The times are long so an acceptable exposure will be had over a fairly wide time range, if you have a UV light, times will be much shorter as will be the acceptable time window.

[m-Kresol]

Just to update you:
I did not do the testing first and just went for it and got lucky!
I pulled the protective layer of in out yellow light room (UV free) and then taped the mask on. After 5 min in a UV chamber, 2x9W lamps approx. 8 cm distance from the PCB, I developed the mask in 10g/l NaOH solution. Took about 2-3 minutes to develop, so I'll go with 13 g/l the next time.
I then etched with a almost saturated FeCl3*6H2O solution (160g in 200ml water). That took about 10 min. The results turned out great except for some minor flaws here and there. I'm really happy how it turned out.