Electronics > Manufacturing & Assembly

PCB development - Sodium Carbonate

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So, my PCB sits in the warm Sodium Carbonate solution too long.  As I read the reporting of others,
the SC bath ought to take 4 - 5 minutes.  After 30 minutes, the copper areas that need to be etched
still have areas that are covered.

PCB is exposed in sunlight - about 3 minutes or so.

Photo Resits Film is applied on top of a wet board that has been sanded with fine sandpaper.  I don't
have a laminator but use a heat gun and vigorous rubbing with a dry paper tower to affix the film to
the PCB.

Currently using two doubled-up transparencies.

Any suggestions?  I've got another board ready for exposure,.

Jerry W
Warrenton, VA

My first guess would be exposure time. If you have another board ready, use it for testing different exposure times. For example you can expose only a part of the board for one minute, covering most of it with some cardboard. Then you move the cardboard slightly, exposing more of the board to the sun, wait one minute more, move again, expose more, etc... and at the end expose all of the board for 3 minutes.
Then at the end you'll have a part of the board that has been exposed for 3 minutes, a part that has been 4 minutes, etc...
Then put the board in the bath and see how the different parts reacts to find the best exposure time. I agree it shouldn't be more than 5 minutes to develop it in the bath.

Sunlight is not a reliable UV source.  Barely perceptible Cirrostratus cloud, haze and pollution can adsorb a significant proportion of the UV solar irradiance, together possibly as much as 50% near sea level, and it is also reduced by the cosine of the angle of incidence.   Exposure times will therefore vary significantly seasonally, and day to day even if you carefully position the board normal to the incident sunlight, and your only hope of reproducible results would be to either incrementally expose a test strip and develop it to determine the optimum exposure time, immediately before exposing boards, or to correct the exposure times with the aid of a UV lightmeter.

Probably too long exposure, but also too-strong sodium carbonate solution seems to slow the developing (it may be additives in the laundry powder I use of course that do that). 30g of Sodium Percarbonate (aka Peroxyhydrate, aka Laundry Whitening Powder) per Litre of water is what I use.

I also use a vibrating toothbrush to gently assist.

In terms of exposure you want the minimum exposure you can get away with in order to develop it cleanly.  After you have developed it you can (and should) put the developed board under UV for a longer period of time to harden it (at least twice as long as developing).  Then inspect to see if there are any bits you missed with debeloper (if so you can put back into the developer and clean those areas again, or scrape it away with a pointy thing or whatever).

The sun is a rather variable UV source unless you are in a really sunny spot at high altitude. Even with UV lamps one should do a series of exposure tests (e.g. a small area and uncover in steps like 1,2, 4, 8,16 min) to get the right exposure.  With the sun it may very well take way longer than 3 minutes. The time also depends on the base material and age - with the material I used a few years ago the needed exposure got longer the older the boards.


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