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DIY problems with an enormous 508mm board

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ampdoctor:
I've got a question similar to the one asked several days ago regarding large copper boards and maybe any discussion here will help out that poster as well. 

I've got a design I need to prototype that comes out to 508x108 mm.  Clearly this is enormous by DIY standards. Due to mechanical constraints I can't bust it up into smaller boards so I'm stuck with this size.  I looked around for places that would do one off's and the price is sky high until you get into larger quantities. The price point becomes problematic for a design that I'm still not sure won't need tweaking or adjustments in either the circuit itself or the board layout. So unless I want to hand over a lung or my first born I'm somewhat forced into figuring out a way to prototype this thing myself. 

So here's the question. Has anybody ever tried to home brew a double sided board this large and if so how did you do it?

Obviously it's larger than a single sheet of A4 or any other size of commonly available paper. So I'm thinking I'd have to come up with some sort of method that won't blow the registration between layers out the window. I've got a few thoughts on how I might approach this using either toner transfer or uv exposure methods.

Using toner transfers I might be able to carefully cut the sheets and butt them up against each other. This seems a bit impractical given that should I get the indexing pins wrong along with the typical paper shrinkage it's a disaster waiting to happen.  There would most likely also be a lot of rework with a sharpie after the fact as well. 

Another option I've thought about is printing on clear vinyl with several overlays that fractionally overlap so I wind up with a single sheet that's large enough, then expose using sunlight instead of a light box. I'm also batting around the idea of using a UV emulsion coating that gets used with silkscreens where the unexposed areas can be washed off with tap water.

I'm in some uncharted waters here, so any comments, ideas, or suggestions would be greatly appreciated.

free_electron:
Iteadstudio will do it.
I've done really largo boards with them. They have a category for it

sleemanj:
Find some A2 size tracing paper of a good grade (110gsm is what I use), art shops do sell it. A2 is 590 something mm long.
Go to a printery with a large format printer that can handle A2, shouldn't be too hard (although try and find a laser rather than an inkjet).
Get artwork printed onto the paper.
Dry film laminated on to your PCB, couple panes of glass, couple clamps, sunlight exposure.  You might have to do a couple of test runs first to get the timing right.
From then on, standard developing and etching.

ampdoctor:

--- Quote from: free_electron on February 26, 2014, 05:49:23 am ---Iteadstudio will do it.
I've done really largo boards with them. They have a category for it
--- End quote ---
I checked out iteastudio, and unless I'm missing something somewhere the only thing I was able to find was a category for boards larger than 30cm. For a board this size their schedule gives me a cost of about $150/board with a minimum order of 5 units...unless I'm misreading something. On a board that may need a respin 750 bucks is pretty pricey. If you're a corporation yea sure no big deal. Just write it off as R&D costs, but for the average Joe....OUCH!




--- Quote from: sleemanj on February 26, 2014, 07:46:33 am ---Find some A2 size tracing paper of a good grade (110gsm is what I use), art shops do sell it. A2 is 590 something mm long.
Go to a printery with a large format printer that can handle A2, shouldn't be too hard (although try and find a laser rather than an inkjet).
Get artwork printed onto the paper.
Dry film laminated on to your PCB, couple panes of glass, couple clamps, sunlight exposure.  You might have to do a couple of test runs first to get the timing right.
From then on, standard developing and etching.
--- End quote ---
I've never really thought about tracing paper as a viable solution.  At first blush I would think that you would have a tough time getting a sharp image due to the nature of the medium and the way it diffuses light somewhat.  Since you offered it as a suggestion is it safe to assume you've gone this route before. And if so how well does it work in general?

sleemanj:

--- Quote from: ampdoctor on February 27, 2014, 02:44:35 am ---I've never really thought about tracing paper as a viable solution.  At first blush I would think that you would have a tough time getting a sharp image due to the nature of the medium and the way it diffuses light somewhat.  Since you offered it as a suggestion is it safe to assume you've gone this route before. And if so how well does it work in general?

--- End quote ---

Tracing paper works just fine as a medium for photo artwork, 12/12 is my standard parameters but I etch down to 10/10 in a pinch, and could do 8/8 if I really really really really wanted to.

Of course it's not as transparent as, well, transparent stuff, but it's pretty good.  My standard exposure (dry film) is currently 53 seconds under UV leds, and if under sun, well about the same really, you just have to experiment a bit.

I use a single print, some people double it up and do a longer exposure, but I find it's better to just have one print and a shorter exposure myself. 

After developing the dry film, I re-expose for about 5 minutes to fully harden off the traces.

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