Idea for a UV exposure box

[and101]

I'm currently looking at designing a new UV exposure box for developing PCBs and had an idea for making it but before I go ahead and start work I wanted to post the idea on here and see if anyone else had tried it or can see why it wouldn't work.

Most of the led based exposure boxes that people make seem to use a grid of leds sat a few centimetres away from the developing surface.  The biggest problem with that method is you end up with a fairly bulky box, especially if it is double sided, and the leds will not give an even spread of light across the surface.

The idea I had was to use the LED backlight from a modern LCD display and replace the white LEDs with ultraviolet versions.  This would give you a large fairly evenly lit surface that is only a few millimetres thick.  The light output would be lower but that would just mean longer exposure times.

The second part of my idea was that with modern LCD panels like you find on ipads and other tablets the resolution of the display is getting close to that of a printer so could you keep the modified back light attached to the lcd panel and display the circuit to be etched directly onto the lcd doing away with the need for transparency.  The display on the newer iPads is bonded directly to the glass so hopefully the distance between the LCD and the PCB would be close enough that you wouldn't get blurring on the edges.  You can get replacement panels on ebay for around £40 so I would need to develop some sort of display driver to drive two lcd panels and in theory I could have a UV exposure unit that can make double sided PCBs up to 19cm by 14cm and is less than an 10mm deep.

There are probably lots of reasons why that will not work which is why I thought I would post it here before I go on ebay and start buying LCD panels.

[kurt]

It is possible to make a fairly slim double sided box with leds alone. You just need a lot of them  I built mine using high density LED strips and the total height is about 9cm. http://kavionic.com/blog/DIY_1188_LED_Dual_Sided_UV_Radiation_Unit

[and101]

If the LCD idea doesn't work I will go with a normal LED design.  I found some surface mount UV leds on Aliexpress for about £5 per hundred which have a 120° spread so I was thinking of making some 10cm square panels with LEDs at 5mm spacing with the leds on one side and the driver circuit on the back.  If I designed them so the pcbs could be placed side by without any spacing between the leds then I could make a uv box as big as I want and just turn on the panels needed for the size of the PCB i am developing which would save quite a bit of power.

At the moment I have a UV box which uses fluorescent tubes that I built into an old flatbed scanner case.  It works fine for single sided boards but I would like to make something a bit smaller that does double sided pcbs.  I don't have much space in my workshop so the smaller I can make it the better.

The idea behind using LCDs was that it would make alignment for double sided boards very easy as a simple hinge design would mean the two lcd panels would always align in exactly the same place and and would remove the step of having to print out the designs onto transparency.

[Kalvin]

I built an exposure box using twelve 3W 395-405nm UV LEDs, driven by three small constant current LED driver modules. I was first thinking of implementing the box using those small UV LEDs, but as a lazy person I ended up using twelve high-power UV LEDs as I hated the idea of wiring all those small LEDs and resistors. The exposure time is 1.5 minutes, which is quite nice. Maybe the LEDs wavelength is not quite optimized for the PCB or the glass I am using to keep the film and the PCB together, but it gets the job done. At the moment the box is used as single sided, but it can be upgraded to double sided very easily, if needed. The distance between the LEDs and the PCB was determined experimentally so that the light was even across the PCB. With my LEDs the distance is about 7cm.

[kurt]

Using an LCD screen as the mask is in deed a very interesting idea. Would be awesome if you could make that work. I did some research about that too some time ago, and one of the pages I landed on was this: http://www.openservo.com/forums/viewtopic.php?t=688&sid=4faa5eb0a584f573712232e894e769e2.

Unfortunately the tests proved that the particular screen tested with there could not do it. But that was an old PC monitor and not an ipad. The ipad screen does indeed sound like a much better candidate for the job. I'm not sure if the crystals are opaque enough to work as a mask even if the resolution is high enough and the distance small enough to make a sharp image. And there is a worry about UV actually degrading the crystals over time. Whether this factors are indeed a show stopper or not, I don't know.

But if you start doing some experiments with this I will be very curious about the results!

[kurt]

I built an exposure box using twelve 3W 395-405nm UV LEDs, ... Maybe the LEDs wavelength is not quite optimized for the PCB or the glass I am using to keep the film and the PCB together, but it gets the job done.

My UV box use the same wavelength, and it worked very well for the dry-film laminate I use as etch resist. But when I tested it with the liquid paint I use for solder masks I got disappointed at first. It seemed to harden, but got a tacky surface.

That turned out to be caused by oxygen inhibiting the polymerization. By covering the paint with cellophane plastic it hardened just as well as when using my old fluorescent tube based UV source (I normally do this anyway to spread it and prevent dust from collecting, but skipped it for the initial test).

The dry-film laminate are already covered with a protective layer of plastic, so by leaving that on until the exposure is done no oxygen have access. I guess the higher energy in the shorter wavelengths emitted from the tubes managed to polymerize the paint even in the presence of oxygen.

[Kalvin]

Here is discussion about using the iPad Retina LCD panels with the UV light (for the 3D SLA printer, but the principle is the same):

http://www.buildyourownsla.com/forum/viewtopic.php?f=12&t=2113&start=50

It seems that some LCD panels may contain polarizers which will block the UV wavelengths.

Edit: Fixed the link.

[Kalvin]

I built an exposure box using twelve 3W 395-405nm UV LEDs, ... Maybe the LEDs wavelength is not quite optimized for the PCB or the glass I am using to keep the film and the PCB together, but it gets the job done.

My UV box use the same wavelength, and it worked very well for the dry-film laminate I use as etch resist. But when I tested it with the liquid paint I use for solder masks I got disappointed at first. It seemed to harden, but got a tacky surface.

That turned out to be caused by oxygen inhibiting the polymerization. By covering the paint with cellophane plastic it hardened just as well as when using my old fluorescent tube based UV source (I normally do this anyway to spread it and prevent dust from collecting, but skipped it for the initial test).

The dry-film laminate are already covered with a protective layer of plastic, so by leaving that on until the exposure is done no oxygen have access. I guess the higher energy in the shorter wavelengths emitted from the tubes managed to polymerize the paint even in the presence of oxygen.

kurt, thank you very much for this information. I have purchased some UV-curable soldering mask paint but haven't tried it yet. With your excellent information I may have saved quite a lot of trial & error effort. Thank you!

[kurt]

Glad to be of help. I'm planning to make a write-up about all the PCB making techniques I have found by trial and error on my web site, but have not gotten around to it yet. I will probably post about it in another thread soon. Right now I'm a bit worried about derailing this thread to much. I wan't to know if it is possible to use an LCD screen as a photomask too :-)

[Kalvin]

Here is another thread of UV light and LCD:

http://www.buildyourownsla.com/forum/viewtopic.php?f=12&t=84

Lots of discussion of polarizers used in the LCD panels and the proper wavelengths. It seems that the 400nm is the minimum usable wavelength which will pass through the polarizers of the LCD panel. So, the usable UV LED wavelength seems to be 405-410nm.

[and101]

Here is discussion about using the iPad Retina LCD panels with the UV light (for the 3D SLA printer, but the principle is the same):

http://www.buildyourownsla.com/forum/viewtopic.php?f=12&t=2113&start=50

It seems that some LCD panels may contain polarizers which will block the UV wavelengths.

Edit: Fixed the link.

Thanks for the link.  It looks like that idea isn't going to work then, unless I can find a polarizer that doesn't block UV.  I might still try using the backlight from and LED TV as that will give me the low profile that I want even if I can't use the LCD part.

[Kalvin]

Here is discussion about using the iPad Retina LCD panels with the UV light (for the 3D SLA printer, but the principle is the same):

http://www.buildyourownsla.com/forum/viewtopic.php?f=12&t=2113&start=50

It seems that some LCD panels may contain polarizers which will block the UV wavelengths.

Edit: Fixed the link.

Thanks for the link.  It looks like that idea isn't going to work then, unless I can find a polarizer that doesn't block UV.  I might still try using the backlight from and LED TV as that will give me the low profile that I want even if I can't use the LCD part.

I guess there is still some hope According to the following post, with 405-407nm wavelength and the iPad 3 LCD, the transmission is quite good and the resulting contrast is something like 1:10:

http://www.buildyourownsla.com/forum/viewtopic.php?f=12&t=84&start=130#p5836

I measured this morning the light output with the help of the same LED used as source ( 405-407nm) a multimeter keeping my probe alway in the same position(lens in contact with the LCD) :

- Light switched off , no lcd in a semi dark room = 10mV
- Light switched on , no lcd in a semi dark room= 2540mV
- Light switched on , lcd in place, black slide (RGB = 0,0,0)= about 170mV
- Light switched on , lcd in place, White slide (RGB = 255,255,255)= about 1500mV
- Light switched on , lcd in place, Blue (RGB = 0,0,255) slide = about 900-1000mV
values for Red and Green no measured ...

This doesn't give precise value, but looks like the Ipad3 LCD is not so bad for transmitting 405-407nm light, and can cut "efficiently" when turned to black.

[MFX]

Why not just use tubes, tried, tested, fast, cheap. Not sure what the facination is these days with using LED's, so a box using tubes is a bit more bulky, big deal. This is my double sided unit with vacuum pull down, built it in a day :-



And the sort of boards I can easily make with it:-



Test piece, including 3 tracks running between two 0.1" spaced DIL pads :-

[and101]

I might get an iPad panel from ebay and do some experimenting to see if it will work.  According to this site the iPad panels use a DisplayPort interface instead of the usual LVDS that you find on most panels so I will need to figure out the best way of driving the display.  The iPad retina displays are 2048×1536 so it will need a display buffer of at least 3Mb.

Does anyone have any experience with driving displays with this sort of resolution.  Could you do it with a microcontroller or will I have to use an FPGA?  I could use an iPad to drive it but I don't want to butcher my iPad and it isn't worth spending £400 on a new one just to rip it apart on the off chance that this idea will work, especially as I will need two of them to make a double sided exposure box.

[and101]

Why not just use tubes, tried, tested, fast, cheap. Not sure what the facination is these days with using LED's, so a box using tubes is a bit more bulky, big deal.

I have a tube based exposure unit at the moment but it is only single sided and fairly bulky.  My workshop is a bit limited for space so the idea of a double sided exposure unit that is the same size as two ipads sandwiched together is very appealing.

[NiHaoMike]

I might get an iPad panel from ebay and do some experimenting to see if it will work.  According to this site the iPad panels use a DisplayPort interface instead of the usual LVDS that you find on most panels so I will need to figure out the best way of driving the display.  The iPad retina displays are 2048×1536 so it will need a display buffer of at least 3Mb.

Does anyone have any experience with driving displays with this sort of resolution.  Could you do it with a microcontroller or will I have to use an FPGA?  I could use an iPad to drive it but I don't want to butcher my iPad and it isn't worth spending £400 on a new one just to rip it apart on the off chance that this idea will work, especially as I will need two of them to make a double sided exposure box.

Use an old PC plus a cheap graphics card.

[MFX]

My workshop is a bit limited for space so the idea of a double sided exposure unit that is the same size as two ipads sandwiched together is very appealing.

Lateral thinking, build it into the work surface so when closed it becomes part of it, or a slide out unit, add in the option of switch able white tubes and a piece of frosted perspex and you have a light box for artwork checking/alignment as well, wish I'd done that with mine but I built a separate white light box.

[lapm]

My workshop is a bit limited for space so the idea of a double sided exposure unit that is the same size as two ipads sandwiched together is very appealing.

Lateral thinking, build it into the work surface so when closed it becomes part of it, or a slide out unit, add in the option of switch able white tubes and a piece of frosted perspex and you have a light box for artwork checking/alignment as well, wish I'd done that with mine but I built a separate white light box.

Now thats a nice idea for us space constrained people... 

[SeanB]

Get some solder proof antistatic mat and place over it then it will serve as a place to do soldering as well. Central and you do not have to dedicate a spot for it at all.