Etching your own copper panels #5: Exposing and Developing

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Blog entry by splintergroup posted 11-01-2016 07:34 PM 1631 reads 0 times favorited 3 comments Add to Favorites Watch
« Part 4: Applying the Resist Part 5 of Etching your own copper panels series Part 6: Etch and Finish »

So now I have an image mask and a copper plated panel with a layer of dried photo resist. Next up is to get the image onto the copper by exposing it to the proper light.

The Light
Different brands/types of photo resist require different wavelengths or colors of light to activate. I don’t know of any that do not require Ultra Violet (UV) light. Withing the UV spectrum are several “bands” of color, most notably are the UV-A and UV-B bands.

UV-A is the spectrum the resist I’m using requires. To get this spectrum, I spent about $20 for a fingernail polish UV lamp (used to dry UV nail polish). I’ve seen these lights at Walmart for this price, but free shipping from Amazon gives me the same deal.

I initially tried the UV-C light from a pond water sterilizer and it had very limited effect. I understand that transparency film at these wavelengths is almost opaque so that may have been part of the cause.

Also within this spectrum are fluorescent black lights and good old free sunshine. The fluorescents are perfect since they allow me to precisely time the exposure. Sunlight works, but the timing depends on how hazy it is, clouds, time of year, etc.

The Setup

This is a view of my simple setup for these small panels.

The lower surface is something flat (a piece of mirror tile). Nothing special here, you just want things steady. The light base is positioned 7” above this lower surface. Exposure time depends on the light strength. There is some wiggle room, but using the coffee cans as support keeps me consistent between setups.

The setup for the board is as follows. This is all done in a room with subdued incandescent light to avoid accidentally exposing the panel before I’m ready.
The board is placed photo resist paint side up. The transparency mask is placed toner side down, aligned with the edges of the panel as required. Remember that I had flipped the image so that it would look correct when I placed it toner side down. The same is true for ink jet printed transparencies too! Doing this places the light blocking surface directly against the resist surface. If it was oriented such that it was on top of the transparency film, a small amount of light could refract behind the toner and expose the resist where it was not intended. The effect is small but real. If you have a lot of fine details, it is best to optimize your setup.

I then cover the mask/panel with a sheet of clean, clear glass to keep everything flat.

Light On!

My light has two, 9-watt lamps. At 7” away, I expose for 10 minutes. This is where you need to be consistent, but in reality, 8 to 15 minutes would also work just as well.
Expose too long and you risk the light that inevitably bleeds through the masks black areas to begin transforming the resist. Too short of time and the resist may not fully transform and get washed away during development.

The developer for this resist is regular washing soda (sodium carbonate). Not to be confused with baking soda (sodium bi-carbonate), you can get this cheap at the grocery store (lifetime supply). I’d avoid anything that has a scent or color added.

I mix the following solution:
1 quart/liter warm (85 deg F/30 deg C) with 1/8 cup/20 grams washing soda (1/8 cup is about 2 TBSPs).
If it is too weak, development will take a lot longer, way too strong and you risk stripping away all of the resist!

This stuff is not bad or harsh (it is a laundry product), but I still advise wearing gloves and eye protection in case of splashing.

I place this into one of my wife’s cooking dishes (Shhhhhh….!). I also have a 5-gallon bucket of rinse water nearby for cleaning.

Place the panel into the developer (you still have the lights dim, right???)

I use a cheap brush to gently sweep over the surface after a few minutes to clean away the debris.
In a minute or two, you should start seeing results. The developer will wash away un-UV exposed resist.

After a total of about 5 minutes, it is all done!
Look closely to see if any fine details still have resist where it is not wanted. The brush is useful for cleaning these areas out. Don’t leave it in for too long, eventually it will eat away all the resist.

The panel is dunked into the bucket of clean water and rinsed off. Do not rub it too hard, just gently brush the surface with a paper towel while the board is submerged. I use a quick blast of compressed air to clear the rinse water off.

This is the results!

Everything that is blue will be copper after etching. Everything that is exposed copper will be gone and all that will be left is the black fiberglass core.

The developing solution was sufficient to develop 4 of these boards. Each board took maybe a minute longer. The solution turns blue from the dissolved resist, so I usually replace it when I can no longer see the board clearly.

As soon as the board appears dry, it can be etched.

The washing soda seems to be fairly standard as a developer, but always best to be sure!
The light source can be a cheap curly CFL blacklight bulb or even a standard CFL if exposure time is longer. I made a test mask consisting of a grid of lines and a 4”x6” PCB. The mask was setup with my light as usual, but I covered everything with a thin piece of cardboard. I started the exposing with only a small portion of the mask/PBC showing from behind the cardboard. Every two minutes I’d slide the cardboard back another 1/2”. This was repeated until the entire board was showing. I effectively now had a board with a set pattern that had each been exposed 2 minutes longer than the previous pattern. I etched this board and examined the results to determine which time-slice gave the best results (10 minutes).

My lights total 18 Watts (9+9). An 18W CFL blacklight or bug-zapper should be similar so if you don’t test exposure times like I did, 10-12 minutes at 7” should be good.

3 comments so far

View Mean_Dean's profile


7057 posts in 4392 days

#1 posted 11-01-2016 09:03 PM

Interesting to actually see how the resist and UV light works. I’m getting a much clearer picture of how this works!

-- Dean -- "Don't give up the ship -- fight her 'till she sinks!" Capt James Lawrence USN

View CaptainSkully's profile


1615 posts in 4803 days

#2 posted 11-02-2016 03:27 PM

Super great, useful, informative! Thanks SO much for doing this! I may be able to get a clock made for Christmas this way.

The negative of a negative thing was a little bit of a mind-bender. I expected the pips and kokopelli figure to be black, with the background copper. Interesting twist…

-- You can't control the wind, but you can trim your sails

View splintergroup's profile


5864 posts in 2467 days

#3 posted 11-02-2016 03:39 PM

Glad to share!

It is a bit of a mind bender. Usually when I think of something etched, I picture that the bulk of the material is untouched and only the lines are actually etched. For this project, the center figure and pips would indeed be “black” with the remaining areas staying copper.

I’m making a number of these boxes and there will be a few done this way. I guess I just didn’t want it to look like I just used a black ink stamp on a sheet of copper 8^)

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