Author Topic: How photosensitive is photosensitive board?! And related photo etching qns...  (Read 2438 times)

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Offline seb1982

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Hi everybody!

I'm going to try etching with with photosensitive board and UV light for the first time.

My question is: just how sensitive to light is the board likely to be?  As in, will it ruin it if natural daylight hits the board at all, or would a couple of seconds here or there be OK?

I don't have any specialist lighting other than the UV tubes, so should I get everything lined up in dim daylight, or should I throw a red LED array onto a spare breadboard to light it by?  I've no idea what exact wavelength the random LEDs in my drawer are, though...!

Also, I've got a bit of scrap photoboard to test exposure times.  I'm doubling up the transparency to try and get the black bits more solid (my 600dpi laser isn't that good quality), so do you think testing exposure time at 30 second increments from 2 mins to 3.30 would be a good starting point?

Ooh, and finally (and this is probably a really stupid question, I know, but it will put my mind at rest!) - after developing the exposed board, it'll be OK to leave etching it for a few hours, won't it?

Thanks ever so much, as ever, for your wise counsel and advice!  :-+
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Offline SeanB

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It is not terribly sensitive, and will survive a few minutes in non direct sunlight. To expose if you do not have a UV source generally means around 10-15 minutes in direct sunlight, with a thin sheet of glass on top to keep the toner in contact with the surfave ( otherwise it bleeds in to the side and makes a thin trace) and after exposure you can leave it covered with dark card for a few hours before developing, when you will be able to see the exposed areas and the non exposed ones from the colour.
 

Online mikeselectricstuff

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There is no need for anything like darkroom conditions - just be sensible, like avoid direct sunlight, only peel the covering off immediately before use, and develop straight after exposure. If you need to leave it, just cover it up, or face-down for single-sided.
Exposure times can be in the range of 2-8 mins depending on manufacturer and UV tube strength, and leeway for a usable board about +/- 30 secs, depending on level of detail.
Be sure to use a silicate based developer, not sodium hydroxide, as this gives a lot more tolerance between "develop" and "strip"
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Offline seb1982

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Excellent!

Thanks very much for the replies.  Time to bite the bullet then, I guess!
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Offline lewis

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Exactly what Mike said. The sensitivity of the photoresist depends on the laminate manufacturer, I use the FR4 from Rapid electronics which requires 150 seconds exposure in the UV box. The stuff from Farnell made by Kelan requires over 200 seconds. Development time is significantly longer for the Kelan laminate too, but is much more resistant to stripping in the developer solution.

The 150 seconds is almost critical. If it's <140 seconds it won't develop properly, if it's >160-170 seconds UV bleeds through the dark bits on the transparency and you get patchy over-etching.

As for developing, I use Sodium Metasilicate heavily diluted - about 100ml Na2O3Si to 5L of tap water - heated gently to 25 Celsius. Newly made developer will develop the board in about 10 seconds, if the solution has been used several times before it will take a couple of minutes.

Neither board will like exposure to direct sunlight for more than a few seconds, or fluorescent light for more than 30 seconds. Depending on the sensitivity of the photosensitive coating, you'll find you get patchy development and poor etch with too much exposure to ambient light. (I keep new laminate in a light-proof drawer even through it has the black backing on it.) Basically, ensure you only peel off the black backing just before use. You'll need to experiment with exposure and development time, and concentration of the developer. It all depends on the manufacturer of your laminate, and how strong your UV light is.
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Offline seb1982

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That's very useful extra info, Lewis - thank you - my boards are the FR4 from Rapid too.  And I seem to have the same Sodium Metasilicate developer solution as Mike has in his photos on his website, so that adds to the confidence levels!
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Offline seb1982

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Just thought I'd come back and show the fruits of my labour!  Apologies for the poor quality photo, but it was very dark by the time I'd finished!

Very, very happy with the results of my little test - absolutely no over or under etching, and perfectly good traces down to 4/5 mil.  Exposure time was bang on 2'30".

Thanks again for your help!

« Last Edit: May 13, 2013, 08:02:16 am by seb1982 »
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Offline hlavac

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I have found that working in dark with just orange LED strip lighting makes the best results for me.
Things like extreme sensitivity to exposition time and apparent under/overexposition and bad coverage/holes in big areas happen exactly because of the resist being partially exposed already by daylight.
With partially exposed photoresist there is no room for error because the window between covered and uncovered is so small and inconsistent between areas.
I thought it was bad coverage on big areas from laser printer, but coverage problems disappeared magically since I have been more careful about lighting, and I'm using the same laser printer and tracing paper...
I am using 20W UV exposition light made from 160 UV LEDs and exposition time of just 40 seconds and results are consistently excellent (0.2mm traces possible!).
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