trouble making PCB - evil ink?

I'm currently struggeling (again) to make good PCBs. My method is to print the layout onto transparent plastic foil (that does not block UV) and then use a halogen lamp to get it onto the photo-board.
However, I constantly get results that look like 'solarised' photographies: where there was black on the foil the developer removes the photo layer!!!

I tried several different exposure times and today, it seems, I found something strange. One PCB came out perfectly all right, the other one was 'solarised' again. The only difference was that I printed the foils for the bad one today and the good one several days ago.

Is it possible that the ink somehow reacts with the PCB's photo layer? Did anybody of you experience something similar? (for clarification: I print the foils mirrowed so that the ink side is in contact with the photo layer)

thanks for any hints,
Olaf

hmm never heard of that problem. try printing the stencil non-mirrored so the ink is NOT touching the photoresist. are you using something that is holding the transparency down on the board, like a peice of plexiglass?

[quote author="Svart"]... are you using something that is holding the transparency down on the board, like a peice of plexiglass?[/quote]
... yes my box has a lid with soft foam to hold things flat and tight.

Perhaps the exposure is too long.

hmm i was just thinking that maybe Olafmatt was allowing his stencils to pull up slightly allowing some of the reflected/refracted light to get under it causing a shadow on the photoresist?

Hhm, seems I typed too slow, my message is gone...

Okay, here again: Just uploaded an image of the bad PCB: http://www.akustische-kunst.org/audio/images/badpcb.jpg

The edges are okay, but the inner parts of the ground plane are gone. So I suppose it's not light leaking under the tracks....

Olaf

it's almost like your photoresist isn't fully developed, or the developer mixture is too strong causing it to start to dissolve before you put it in the etchant.. are you using persulfate or ferric chloride? sodium hydroxide as the developer?

I had the exact same problems on a couple PCB's. Unfortunately my solution was to get PCB's from others on the board, or send them off and have them professionally done. For some reason I just couldn't get it to work for me, and quickly it became cheaper just to buy them from a manufacturer or other folks here. That's probably not acceptable to most people on here, but I'm a lot happier this way! :green: :green:

ye, these probs are the reason I stick with the toner transfer method now - so quik and no messing about with trays, chemicals and lights...apart from the etching stage - which is gravy anways with hot FC and a hairdryer :thumb:

seems the exposing/developing method is very good if u get the science right tho...

Olaf - I once had problems very similar to the ones I see in your image. Very frustrating - it looks like there are areas both under and over exposed. Are you coating the boards yourself or are you using pre-coated boards?

[quote author="Svart"]it's almost like your photoresist isn't fully developed,[/quote]

yep
that's what it looks like
... and sometimes the pre-sensitised board just has areas that will never work ... you get that.

sometimes I can see even straight after the exposure and before developing that a board won't work.

I recycle the develop back into the bottles and really wear it out. The colour of the develop gets darker as it gets more used. I save my new develop for the larger more important PCB's and continually use the old develop for the smaller throw away boards.

Practice , practice
once you get the hang of it
... it all just works.

I got a board that looked like that once, but it was due to poor agitation and my FeCl bath not being hot enough (I couldn't find my air pump and I was trying to heat two gallons of the stuff with a hairdryer :roll.

Olaf, why don't you post some more info? What's your setup? What exposure times are you using, what kind of bulb, how far from the board? Are you heating and shaking your FeCl bath?

Peace,
Al.

Oh, also the presensitized board I get has an expiration date printed on the wrapper... I'd check that.

What brand board are you using?

Peace,
Al.

[quote author="olafmatt"]Hhm, seems I typed too slow, my message is gone...

Okay, here again: Just uploaded an image of the bad PCB: http://www.akustische-kunst.org/audio/images/badpcb.jpg

The edges are okay, but the inner parts of the ground plane are gone. So I suppose it's not light leaking under the tracks....

Olaf[/quote]

I have absolutely no experience with home ethcing, but.....
Id still try looking into getting something in to press the film evenly down on the board. Ive never seen it this bad, but that work, this would mean vacuum problems in a machine that develops the film onto the board in 99 out of 100 cases. (It suck the air out of a tray to press the film down onto the board).....

I dont know anything about the chemistry, so I cant comment on that,.

Gustav.

Here some more data about what I did:

The boards are Bungard boards that already come with the photo layer (but it happened before with other brands too). Developer is NaOH, etching bath is (NH4)2S2O8 (aka "Ammoniumpersulfat").

Yesterday I made two PCBs under the excact same conditions, the first one was okay, the seconds one not. The only difference was the age of the ink on the foil. This made me think it could be related to the ink.
Since both were 10x16cm boards I don't think the developer was exhausted. I could actually see in the developer that the second board was going wrong.

Olaf

Just another theory: could it be that the photo layer changes it's sensitivity when it get's warm? The lamp I use for exposure is not very strong (150 watt) but produces a lot of heat, exposure times are 30 minutes. Under the large black areas the temperature will rise more since black absorbs the heat.... But can this change the sensitivity of the board????

Olaf

[quote author="olafmatt"]exposure times are 30 minutes.[/quote]

SAY WHAT?!?! :shock:

I know this doesn't exactly explain what's happening to your boards, but I usually only need 5 or 6 minutes!

Also, I use 4 "black light" neon bulbs, not a regular incandescent type - maybe you're right and too much heat is the culprit...

Or - only speculating - maybe your NaOH bath is too concentrated? or not enough???

Good luck!

Peace,
Al.

Olafmatt

Check with the supplier of the PCB as to the exposure time.

With my pre coated boards, I expose for 30sec, if I use PCB's that I coat with Positiv 20 spray, exposure is 4 min.

My light box has 2 x 18w UV fluorescent tubes about 20cm from the PCB's.

Peter

i frequently let mine sit for 30 minutes or longer since I am usually doing other things at the same time. never had a problem like this. I also use a UV bulb (from my eeprom eraser) no heat involved. try something with no heat.

30 minutes seems to be too much...

I've stopped making pcbs 5 years ago... but never exposed them much than 2-3 min...

Under the large black areas the temperature will rise more since black absorbs the heat.... But can this change the sensitivity of the board????

Click to expand...

Interesting theory, but copper is such a good conductor of heat that there wouldn't be a huge difference in temperature between the black and transparent areas.

From the Bungard website:

Typical exposure times are less than 90 secondes on a set with fluorescent vacuum tubes.

Click to expand...

So to prove or disprove your theory try fluorescent tubes with short exposure times, as others have also suggested.

It still looks to me like a contrast problem. The lower left part of your image shows places where the copper is left behind but where your artwork is blank. Yet your large ground areas are almost completely etched away on the inside.

Are you using an inkjet or laser printer to make your artwork?

For what it's worth - when I switched from transparent film to sheets of tracing paper to print my artwork a lot of contrast issues went away. The toner from my laser printer seems to bond much better to tracing paper than plastic film, making a higher contrast image. Tracing paper seems like an unlikely medium to use - you wouldn't make a photographic negative on it as the 'grain' of the paper would show - but in high contrast applications it seems to work perfectly.

Also if you don't put the ink side down, that is touching the board surface, you risk losing sharpness on your traces. That would particularly be true if you use tracing paper, due to the diffraction of light, or if you use multiple sources of light such as two or more fluorescent tubes.