UV Exposure times

[akis]

I am about to create my first home made PCB using photo resist board. I have a UV box, and a transparency with the artwork. All I need to do is place the artwork on the glass, then the board on top of it, close the lid, and set the timer for a few seconds.

I am not sure how long I need to set the UV timer for, does anyone know?

[ElektroQuark]

It is "system-dependant" (PCD-light source distance, light power, light type, ...) so you will have to test.
Try at 30s, 60s, 120s, 240s, select the best result and them fine tune exposure time.

[eliocor]

The best way to identify the correct exposure is using a STOUFFER scale.
Here some more details on its usage:
http://www.stouffer.net/using21step.htm
http://www.inkjetnegative.com/images/RNP/how_to_use_a_step_wedge.htm
http://www.ulano.com/FAQ/FAQexposure.htm

The price of this scale is rather low and it simplify your life a lot!

[wraper]

It depends, you should make test board where you cover parts of it after some time of exposure. So you can test different exposure times on the one board. Start from 10-15s. It also depends on transparency contrast. If it is bad you will have a hard time as unwanted places will be also exposed. You would need very precise exposure time. If contrast is really good then exposure time is very tolerant and can be changed by couple of times without huge impact on board quality. Which photoresist do you use? From my experience, positive 20 and other positive ones are not worth to mess with. Especially if you print photo-mask on regular inkjet printer (laser is usually even worse). Negative dry films are the way to go as they are more process tolerant, have better resistance to etchant and overall result in better quality.

[akis]

I used 3 minutes as a starting point because that is what was mentioned on the photo-resist PCB instructions.

I then developed it with mediocre results. As it can be seen the sides of the board have not got exposed adequately. The problem is the UV exposure - it is not uniform and it is stronger in the middle than on the sides. I am not sure yet how to fix this problem on my UV box.

I attach pictures of the problems:

1) The UV machine with the artwork on top in a dark room - now I can clearly see the uneveness of the light... Considering the UV box contains 6 tubes inside I am very surprised.

2) The board as it was being developed - you can see the photoresist is not coming cleanly off the edges of the board because it has not been exposed to UV light correctly

3) The board as it is being etched - again the copper is not coming cleanly off the edges - this is puzzling because much of the surviving copper is not at all covered by photoresist yet it is not coming off - I have no idea why this is happening.

[wraper]

Do you use Bungard pre-coated positive pcb? It needs very long exposing compared to negative films. I gave up on it. Wasted a lot of time, pain to get decent results with home printed photomask. It either is not developing or traces are etched through (developing OK but don't withstand ferric chloride). Bought cheap laminator and Riston film photoresist. No problems since then.

[madires]

Do you use Bungard pre-coated positive pcb? It needs very long exposing compared to negative films. I gave up on it. Wasted a lot of time, pain to get decent results with home printed photomask. It either is not developing or traces are etched through (developing OK but don't withstand ferric chloride). Bought cheap laminator and Riston film photoresist. No problems since then.

Got no problems with Bungard PCBs. I'm using an exposure box with 4 small tubes, semi-transparent drawing paper (laser printed) and the exposure time is about 4,5 minutes. As all the others wrote, you have to find out the time required for the materials you use by trial and error. If you change any material later on you have to do that again.

[wraper]

Got no problems with Bungard PCBs. I'm using an exposure box with 4 small tubes, semi-transparent drawing paper (laser printed) and the exposure time is about 4,5 minutes. As all the others wrote, you have to find out the time required for the materials you use by trial and error. If you change any material later on you have to do that again.

Should be ok if you can print very good black. It was not possible with my previous printer however. With  current printer maybe it would be ok as it gives better black. However dry film still would remain better choice as boards with the photoresist are not cheap, especially bigger ones. I got 200x30 cm roll of Riston mm540 for ~15USD total, that is enough for a LOT of boards. http://www.tech-place.com/en/photosensitive-materials/23-photosensitive-film.html

[ElektroQuark]

I use Bungard PCBs with 4 min exposure. No problems at all.
You can see my setup HERE (description in Spanish, ask here if you need any info).

Then I used an unnamed PCBs which only need 90 s exposures. But they where very bad, using Bungard boards has been a great improvement.

[wraper]

I use Bungard PCBs with 4 min exposure. No problems at all.
You can see my setup HERE (description in Spanish, ask here if you need any info).

Then I used an unnamed PCBs which only need 90 s exposures. But they where very bad, using Bungard boards has been a great improvement.

What I can see on Photos if not perfect at all. I got something like this on good tries. I inspect my PCBs with microscope so my "good" may be different from yours. By etching through I mean tiny dots on traces which usually are not visible with naked eyes (but affects soldering). I think that could be avoided if I used different etching solution (or better printer).

[akis]

I noticed my traces are narrower than they should. is this because of:

1) over UV exposure
2) under/over-development
3) over - etching ?

[wraper]

I noticed my traces are narrower than they should. is this because of:

1) over UV exposure
2) under/over-development
3) over - etching ?

Do you put transparency with printed side on the board? Otherwise sides will be also exposed. It is also possible if transparency have bad blacks. Overall with positive photoresist tracks are slightly narrower and slightly wider with negative ones.

[akis]

Yes printed side of artwork touches the PCB and there is a very firm lid to keep it all squashed down. It is an easy fix: I will simply make my tracks a bit thicker from now on.
However I still would like to know if it is one of the above reasons, so I can eliminate the problem.

[madires]

Should be ok if you can print very good black. It was not possible with my previous printer however. With  current printer maybe it would be ok as it gives better black.

My laser printer prints dark traces but larger areas are not solid black. For a really nice solid black I use Solvent 50 (for removing stickers/labels) and that works great. And it's cheaper than special toner sprays.

[wraper]

Maybe you use silicate developer, I couldn't get that locally. It became apparent that it is cheaper to get negative photoresist than order better developer so I choose that way. For negative ones developer is sodium carbonate, cheapest junk you can get, and works with half transparent photomask.

[madires]

Maybe you use silicate developer, I couldn't get that locally. It became apparent that it is cheaper to get negative photoresist than order better developer so I choose that way. For negative ones developer is sodium carbonate, cheapest junk you can get, and works with half transparent photomask.

I'm using NaOH, €6 for 250g and 10g are needed for 1l. That makes 25l and nearly lasts forever :-)

[wraper]

I got 250g Na2CO3 for ~2EUR (could get cheaper if searched better), same concentration 

[madires]

I got 250g Na2CO3 for ~2EUR (could get cheaper if searched better), same concentration 

I can't beat that :-) But I prefer buying PCBs already coated with positive photoresist for convenience. Haven't seen any negative pre-coated ones.

[ElektroQuark]

I use Bungard PCBs with 4 min exposure. No problems at all.
You can see my setup HERE (description in Spanish, ask here if you need any info).

Then I used an unnamed PCBs which only need 90 s exposures. But they where very bad, using Bungard boards has been a great improvement.

What I can see on Photos if not perfect at all. I got something like this on good tries. I inspect my PCBs with microscope so my "good" may be different from yours. By etching through I mean tiny dots on traces which usually are not visible with naked eyes (but affects soldering). I think that could be avoided if I used different etching solution (or better printer).

Sorry, my bad. I only wanted to show my setup. That PCB was made with a no name (and bad) PCB. Now I have changed to Bungard (exposure times changed from 90s with no name PCB to 240s with Bungard). And the quality now is orders of magnitude better.

[wraper]

I can't beat that :-) But I prefer buying PCBs already coated with positive photoresist for convenience. Haven't seen any negative pre-coated ones.

It takes couple of minutes to apply film with laminator (it is the same film PCB fabs use BTW). First cold run and then hot. So no hassle at all. Some do it without laminator.

[sleemanj]

. So no hassle at all. Some do it without laminator.

I apply negative film using a clothes iron mostly (obviously for boards smaller than the irons plate).  On  a very cool setting just a couple clicks above minimum.  Stick the film down, cover with a piece of paper, sit the iron on top for about 30 seconds, then gently rub it with the iron, especially along the edges.

Any air bubbles that got in, I prick and then rub-down with the iron point to smooth them out.

Temperature is the the key, must not be too hot, too hot and the polymer blisters. 

Have board and film cold when attaching the film, makes it easier (less inclined to bond on contact so you can reposition a bit).

[wraper]

I stick board on a piece of paper and stick film in one end. Then feed it to the laminator while holding the film at the same time. So film meets the pcb on laminator rolls, therefore no air bubbles. Then hot run to make better adhesion.

[akis]

Please remind me what is the advantage of applying the film on the PCB at home rather than buying a ready made photo resist coated PCB?

[sleemanj]

Please remind me what is the advantage of applying the film on the PCB at home rather than buying a ready made photo resist coated PCB?

1. Cost, film is cheaper
2. Flexibility, apply film to any size board, which you can cut, drill, sand, clean, etc to your heart's content before applying film
3. Screw up an exposure with a precoated pcb then you have something not useful (unless you use it with a different method), with film, strip off the film and start again.
4. No dark space required, work under normal household lighting conditions
5. Film is negative acting which may be advantageous if your print is a bit light
6. Develop and strip safely with Sodium Carbonate (Washing Soda, from any supermarket), no need for somewhat more dangerous Sodium Hydroxide (lye, the stuff used in soap manufacture and for example in Fight Club).

[madires]

1. Cost, film is cheaper
2. Flexibility, apply film to any size board, which you can cut, drill, sand, clean, etc to your heart's content before applying film
3. Screw up an exposure with a precoated pcb then you have something not useful (unless you use it with a different method), with film, strip off the film and start again.

If you just focus on cost (excluding time :-), those are valid points.

4. No dark space required, work under normal household lighting conditions
5. Film is negative acting which may be advantageous if your print is a bit light

Never needed a dark room for photoresist coated boards.

6. Develop and strip safely with Sodium Carbonate (Washing Soda, from any supermarket), no need for somewhat more dangerous Sodium Hydroxide (lye, the stuff used in soap manufacture and for example in Fight Club).

Another good point, washing soda is cheap and simple to dispose. I'm lucky because we got a regularly free disposal of dangerous waste for households organized by the county.