‘Negative’ PCBs – how?

[An OtherParticipant @another21905]

Perhaps one of the chemists/physicists here can explain a problem I had making a Printed Circuit Board.

I have made my own PCBs for more years than I care to remember by printing a 'photo-negative' transparency, then laying photo-sensitive board on this, and exposing it to UV light using a UV lightbox. The result is the copper I wish to keep is protected by the photo-negative, and the copper I wish to remove is exposed to UV light. The board is then 'developed' in sodium hydroxide, before etching. I have successfully made hundreds of PCBs by this method.

Today, I wanted to make another board (set of limit switch mounts for a CNC machine). I produced the 'photo-negative' in the same manner I always do – and this is where I was hit by encroaching senility – first I placed the PCB board on the light-box, copper side up, then placed the photo-negative on top of it (In the wrong order, and I know, but it comes to all of us eventually!).

I then exposed it for the usual time, and developed as usual. Then I had the first inklings something was not right – the developed board seemed to be a very slightly fuzzy negative of what was required. At this point I had not realised I had placed the negative/board in the lightbox in reverse order. Eventually I realised what I had done (so the board was scrap), but then began to wonder how this can happen – how did light get through (or around) the board to create a pattern on the top of the board – the side away from the UV light? And why was the resulting pattern 'reversed' – that is, the track areas were exposed (so would later etch away), and the areas between tracks left solid?

I could just imagine light leaking around the board to affect the top side, but this had the photo-negative on it, and when the UV light-box lid is closed, the whole 'sandwich' is pressed down by a layer of black plastic foam, so it is hard to see how this leakage happens. The board is 1.5mm thick fibre-glass, coated on one side with copper, which would also block light, I assume. So why did the pattern appear on the side of the board away from the UV light, and why was it negative?

Once I realised what had happened (but not why), I used the same photo-negative and a board from the same batch to make the PCB correctly, and it came out perfectly, so I don't believe there was a problem with the board.

I know the mistake I made, but am at a loss to explain why it worked as it did, so am interested to see any suggestions.

[An OtherParticipant @another21905]

[BazyleParticipant @bazyle]

The pcb is translucent especially if you are using UV light.

[Martin KyteParticipant @martinkyte99762]

Posted by Bazyle on 11/08/2021 01:01:01:

The pcb is translucent especially if you are using UV light.

The substrate is but not the copper layer(s).

If I can assume that the print was on the side of the transparancy in contact with the PCB as is usual to produce maximum sharpness of track edges. I would go for a heat effect. The board warms and the toner reacts with the photo resist either chemically or mechanically and makes it slightly porous to the etch. On a normal board there is always some dimpling where the etch resist is not quite impervious.

Comments welcome.

regards Martin

[Alan Wood 4Participant @alanwood4]

This has been nagging at me since posted. First as is tradition on the forum, some self indulgent waffle ..

In the early days in business we used to make our own PCBs. We used crepe tapes to create the positive track side artwork and for the ground plane side (these were RF boards) we created a negative artwork with the tape lands that represented the clearances we needed in the ground plane for the component through connections. Artwork was created at 2:1 scale and a local printer produced positive and negative photographic films for us to make the boards. We had to specify that the film had the image emulsion on the lower side so that it was directly in contact with the photo resist. If this wasn't done we found the UV 'crept' around the edge of the artwork and we had a poor resolution etch.

(Interestingly with such a double sided board we would tape the track side and ground plane side artwork together on the longest dimension plus a 1/2" or so at each end and then slide the resist coated board in between the sandwich and tape it to one artwork only. The second side artwork would naturally align itself when the board was turned over to expose that side).

The need to have the emulsion on the film directly in contact with the resist was the critical aspect in getting a good resolution etch. The 'creep' with the emulsion on the wrong side was not consistent. If we had a wrong side emulsion print we had to move the UV source quite a way from the board surface to stand a chance of getting anything useable from the etch. (The source was a modified sun lamp).

On the basis of these experiences I am making a wild guess that your film is acting as a waveguide to the UV source. The UV is coming 'around' the board and entering the cut edge of the film and being transmitted across the board surface 'inside' the film. The film is trapped against the black foam pressure pad and the combination of the actual artwork black areas and the foam related black areas are affecting the internal co-efficient of reflection of the film surfaces to allow the UV to selectively escape and expose your board photo resist. It is a trick worthy of a magic show.

Given the thickness of the film, I am sure someone far brighter than me will do the maths.

Otherwise this is total and utter speculation.

[An OtherParticipant @another21905]

Bazyle – I concede the substrate may be transparent to UV light, but how does the pattern get around to the other side of the copper? – if the substrate is transparent, then the light would impinge on the side of the copper 'stuck' to the substrate. The inverted print pattern appears on the outside (top) of the copper away from the UV lamp.

Martin – this was the way I was going, but I still have doubts – the UV tube is contained in an aluminium case, and the case has a 4mm thick glass (not plastic) above the lamp – the gap between lamp and glass is about 3 cms. The board/transparency sits on top of the glass. The exposure time is 70 seconds (in my case), and the lamp is off before starting (no preheating) and goes off at the end of the exposure, so do you think there would be any appreciable temperature rise? – bear in mind that the copper board is sitting on top of the glass, copper side up, and the pattern appears on the top side. The transparency is placed on top of the copper with the ink pattern against the copper (for maximum resolution if done properly). I tried placing my hand on the glass to see if it got warm (I know, but I'm too old to care!), and could not feel any perceptible rise in temperature after 2 minutes – a bit crude, but I have no easy way to measure the surface temperature of the glass.

Alan – a good idea, so I thought I would test it. The exposure area of the UV box is slightly smaller than A4, and the transparency material I use (Avery Inkjet Transparency) is A4, so I made sure the transparency was squarely placed over the exposure area – i.e. the edges are screened by the aluminium edges of the box – and the effect still happens. I agree this may not be perfect for 'screening' the edge of the transparency – but light has then got a very contorted path to the edge of the transparency. The resulting 'positive' pattern is also quite clear – it is slightly 'fuzzier' than one produced correctly, but if it was the correct 'way round', it would be perfectly useable – I would have thought an attenuated light path through the transparency would produce a poor reproduction.

The effect is reproduceable, so I'll try and post some photos if I get time, I'll also try a different transparency material.

[pgk pgkParticipant @pgkpgk17461]

I used to use a lot of UV-set dental pastes (in my past life). On one occasion the UV source died as I picked it up to set a filing I'd just placed. I found I was able to set the material effectively using the light source and fibreoptic probe from my endoscope system – a xenon bulb?.
My point being that unless all sources of leakage from your UV box and processed in a dark room then there will be other sources of light with a proportion in the sensitive spectrum.

pgk

[Martin KyteParticipant @martinkyte99762]

OK, so I just did the experiment with our light box. Thermocouple on top of the board lamps underneath. 4.8 degrees rise in temperature in 5 min exposure time. It's the radiant energy of the photons that heats the board not the thermal effect of the lamps. It's not a huge rise in temperature. Starting point was 19 degrees in our aircon workshop.

You seem to have brighter lamps than we do for a normal 70 sec exposure time. However we would normally expose for 3 to 4 minutes to give correct exposure and the heating is dependent on the total incident radiation so my 5 minutes would be more exposed but not too much.

Try doing your inverted print without turning the lamps on just to eliminate (or not) the UV from the effect just in case it's a purely chemical effect between the toner and the photo-resist. The other idea is turn the film so that the toner is not in contact with the board but with the UV on.

regards Martin

[An OtherParticipant @another21905]

PGK – The light box is produced specifically for PCB exposure, and when shut, the aluminium box does not seem to provide much chance of extraneous light ingress – it closes tightly, and is clamped shut by the catches. Internally there is about a 1cm thick black foam cushion which presses down on the board, transparency – I usually do process in a 'fairly' dark room, because I need to handle the board for maybe a minute with no protection, to set it up in the box – then I just leave it in the room. I'll take a look and see if I can improve that, but again I have to note that the quality of the 'reverse developed' board is fairly good – much better than I would have expected under low light exposure conditions.

Martin: the 70 seconds exposure time was arrived at experimentally – I exposed a number of sample pieces at differing times, until I arrived at what seemed to provide the best definition – 70 to 90 seconds in my case. Longer times (~ 200 secs) ruin the board – it appears grossly over-exposed, and when etched, small details disappear, so I think you are correct, and I would appear to have a brighter lamp than you. I have also noticed that old sensitized board has similar effects. From what you say, the board itself would be heated – I have to admit I did not check that (something else to check!). I am not sure what the wattage of the UV lamps are (will check), but the box is sold as a professional UV developing box for PCBs, so I just cracked on and used it. It does do the job perfectly if I load it up correctly – this effect we are discussing was due to my error in loading the board and transparency incorrectly.

I have tried the transparency 'turned over' so the ink is not in contact with the board – no difference, but I will try as you suggest and leave the lamp off, to see if it is some other cause (can't imagine what would do it though!)

Incidentally, I use an inkjet printer to produce the transparencies. It can be set to a very dense print, and I much prefer it to the Laser printer. I cannot seem to achieve anything like the same density of print with the laser (big disappointment – I bought the laser for this work – now its just a door-stop) – if I examine the transparency from a laser under a lens, it seems to show very thin lines which transferred to the PCB, whereas the inkjet printer produces a solid dense printout. (its an Epson 3150) – it has access to almost every printing parameter I could wish for, and it is possible to set the density of each colour (i use black) to a very wide range, and also brilliance and contrast.

I do appreciate that the laser printer could also be set up incorrectly, but does not seem to be able to produce as dense printout as the inkjet, and the inkjet is so much quicker.

Thanks for the thoughts, everyone – this has got me puzzled.

[Martin KyteParticipant @martinkyte99762]

We used to use a laser printer (with a high temperature fuser) and as you say for wide tracks you get a 'hollowing' of density down the middle. This is due to the charge distribution on the immage roll, you always get best density at the edges of lines. We use a PCB 'mill' now so have not done wet etch for around 6 years so I can't do a test this end.

Have you actually etched the board just to ensure that it's not just discolouring ?

regards Martin

PS None of the other engineers here have any suggestions either.

(LMB Electronics Workshop)

[An OtherParticipant @another21905]

Hi, Martin.

It seems you are making PCBs on a professional basis – mine are just for hobby use, although I have been doing them for a long time. I use the traditional etch method with a 'bubble' etch bath, but I now have a small CNC router which I intend to try and use for milled boards – reason being I have a box full of 'rejects' over the years using the photosensitive etch method, and they are scrap unless I use a different method. I find 'spray-on' photolacquer is pretty useless. I was making limit switch boards for this router when I made my mistake and noticed this effect.

I have etched one of the boards – its definitely 'reversed'. I will try to get some photos, but my camera is giving me trouble, so I can't produce good close-ups immediately.

For info, my lightbox has 4 x 14 Watt tubes covering an area about 5mm less than A4 dimensions. It is made by Gie-tech GmbH in Germany. I usually use genuine Bungard photo-sensitive glass-fibre board, usually 1.5 mm thick, both single and double sided.

The effect is reproducible – I tried again this afternoon, and it did the same thing. I also tried the transparency both ways up, to see if anything was transferred to the board, or a chemical effect, but no difference. I also tried, as you suggested, putting a sample in the box without the UV light on, and nothing happened.

I did think that it was possible that if the ink on the transparency was not fully dry, it could transfer to the board, but on second thoughts, the copper which is now 'exposed' is under the ink, so when it is etched, it disappears – if ink was transferred to this area, it would at least hinder etching, if not stop it altogether – so its very puzzling.

I appreciate your interest and efforts, and of your colleagues, to find an answer – I must admit I am thoroughly stumped – its going to keep me thinking for a while yet. If I come up with anything, I'll add it to this thread and 'bump it up'

Just an idea, since it is reproducible, if the 'inverted' effect, however it happens, could be fine-tuned to produce fine detail, it would be a simple way to produce a double-sided board from a single transparency! – I might get rich yet!

Best Regards,

[Martin KyteParticipant @martinkyte99762]

Just because we are getting paid to make boards doesn't mean that the methodology is any different at least for the wet etch process. You are not doing anything too different that we did for many years. I tend to agree with you regarding spray on photo-resist. You really need a spin coater to get a good even cover.

regards Martin

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