minimum UV exposure time

[mik3ca]

I recently wasted 2/3rds of a circuit board and the last third barely passed in quality.

The first attempt failed with some traces missing but I could guess because the T5 insect killing lights haven't warmed up yet. I exposed it for 5 minutes and then developed for about one minute using a plastic container of 1 part of local sodium hydroxide (aka MGchemicals "developer") mixed with 10 parts water.

The second attempt also failed the same way but with traces missing in other places. The only thing I did different this time was increase exposure time to 11 minutes.

Now for the third attempt, I exposed for 5 minutes then this time, I developed for about one minute using a plastic container of 1 part of sodium silicate (DP-50 developer from Kinsten) and 4 parts of water. Yes I followed their directions. The board looked decent in the solution. So I took it out and after etching, it seems the traces were legible yet instead of pure dark tracks, I get them grainy, meaning there are thousands of little dots (like spots the same color as that of a circuit board without the copper layer) inside the traces.

In all three tests, I did the following:

1. For the artwork, I went to a local print shop to print the art dark with a professional lazer printer on a sheet of transparency and the image comes out very dark.

2. For exposing, I used this UV exposure unit. I had the knob turned all the way to the right in all tests.

Details of that unit are found here: **broken link removed**

Things were working better before the lights that came with the unit eventually burned out. Sadly there were no labels to the original lights so I ended up researching and ended up buying UV lights from the UK.

The listing is located here: http://www.ebay.ca/itm/6x-8W-T5-Ult...nsect-Fly-Killer-Mosquito-Zapper/322199166926

I replaced every light with the ones from ebay before running all three tests.

I think I read somewhere the 368 in the model number 368BL means 368nm which is between 350 and 400nm (the range for PCB presensitized layer). So the only thing that comes to my mind is that I should probably warm up the unit first with no board for 5 minutes, then after expose my board for 2 1/2 minutes only since I'm using light in the PCB layer range and probably the lights that were originally included in the unit were of higher wavelength?

or is 2 1/2 minutes still too long?

I'm a bit confused, but I don't want to have the developer strip away the good tracks.

 

[gophert]

It is normal to tinker a bit to find the right conditions for your boards and light source. First some basics...

1) make sure your printed surface on the film is touching the copper. You may have to print "mirrored" image to the transparency. If you don't do this, the photomask is one transparency film thickness away from the board and you will get light leakage.
2) use binder clips to hold a glass plate onto your board to pinch the transparency tightly to the board and prevent any ripples in the plastic from allowing light leakage under your traces.

3) testing correct timing....
A) make a test pattern of some traces About a 25mm x 100 mm with 0.010", 0.015", 0.020"... traces that run 100 mm length.
Set up your board to etch but slight a very thin piece of 25mm x 150 mm foil between the board transparency (and position so that only 10 mm of board is exposed to light). Most of the board should be uncovered.

B) set the board into the light box and expose for 30 seconds. The pull the foil to expose 10mm MORE board.
C) repeat "B" (above) 9 more times until foil is completely pulled from the board.
(You now have a board exposed at 30 second intervals form 30 seconds to 5 minutes.

4) Develop per instructions EXCEPT, wear good rubber gloves and use a soft fine-grained make-up remover sponge to gently "wash" the exposed photo-resist from the board. Don't go by time, just pull it out as soon as you see a nice crisp version of your circuit. Usually 30 seconds to a minute with the nice surface agitation.

5) Dry the board and look at the quality of the traces. You may need to repeat above but leave the board in the box an extra 5 minutes after the last step to get 5:30 to 10minute exposure.

6) Pick the conditions that work best.

Note that one thickness of the foil is between the photomask (transparency) and board so you may not have perfectly crisp lines but you should be able to determine which is the "best" range quite easily. Also, the range may be broad. I use a simple "cold" color fluorescent bulb at 50 mm from the tube and get the same great results from 12 to 16 minutes. I use two stacked layers of Transparency from an HP laser printer. On HP printers, the big areas don't darken as well as narrow lines for some reason so I have to stack.

Finally, if you have supper dark prints and ritually no UV light coming through, then exposure time is a no issue - go as long as you want. Since you are experiencing over-exposure issues, you may not be as dark as you think ( or you may need to review the basics in steps 1 and 2). I usually just copy-paste my circuit to have two on one sheet and, after printing, just cut them and stack them so I have no wasted materials and no extra cost.

Good luck.

 

[mik3ca]

1) make sure your printed surface on the film is touching the copper.

I always do this.

2) use binder clips to hold a glass plate onto your board to pinch the transparency tightly to the board and prevent any ripples in the plastic from allowing light leakage under your traces.

That might be an option of the past but I'm using a UV exposure unit (see picture in my question). The unit itself has metal clips that allows me to press the board next to a glass plate

 

[DGM]

One test you can do is make a test pattern such as a dot grid or a mesh of fine lines.
Place the film against the board, and mask off the majority of the entire thing with aluminum foil.
Decide on a plausible minimum and maximum time, and then incrementally slide the foil out from under the PCB/film over that interval.
That way, you can get a gradient of exposure across a uniform pattern to help narrow down the timing.
Having such a pattern on hand is handy in case any part of the process changes and you need to troubleshoot it again.

Also, IDK about your lamps, but I certainly have to let mine warm up before use ... but I'm using a HID arc lamp.
Low pressure lamps do change output as they warm up, but just not as noticeably.

 

[Wirth's Law]

Not sure how much help this will be, but I had gone through the same process quite a few years ago; and had maybe gone through 3 boards before I finally got the right combination of exposure time and etch time.

Basically, I had started with the same materials (laser transparency from the local office/print shop). My work area was lit with a 60 W red incandescent "party" bulb, which seemed to cast just enough light to keep me from tripping over things. To keep the transparency flat throughout the whole exposure, I placed borosilicate beaker glass on top to keep it pressed down. The UV light sources were two black light florescent lamps from the hardware store that operated very close to the visible spectrum near 400 nm and shorter. If I can find the part number, I'll post them too. The minimum board to UV lamp spacing was about 6 cm. I recall the exposure time I used with those lamps was about 2 to 4 minutes. I'm guessing your light source puts out more power, so maybe consider my times to be on the excessive side.

Anyway, I also used the same developer solution (sodium hydroxide from MG Chemicals) diluted with deionized water at the same ratio you used. The foam mixing brush was strictly for stirring the liquid, and it never made direct contact with the board. Like gophert mentioned, I really just kept an eye on the board during the mixing, and stopped as soon as I could see the well-defined circuit pattern.

The etchant I used muriatic acid from a swimming pool supply shop. Just like in the development process, I mostly worked off visual appearances instead of a set amount of time. Of course, safety is #1, so I also had splash-proof goggles, a face shield, acid smock, gloves, and closed shoes.