Direct Laser Imaging with 3018 CNC

[ranthalion]

I've started using dry film photoresist for quick prototyping, but lining up transparency films and getting the exposure right is taking a bit more time than I was hoping for. 

Does anyone have any experience using a laser cutter/cnc to do direct laser imaging with photosensitive dry film? I was thinking of trying it with one of the inexpensive 3018 cnc kits and a 500 mw 405nm laser. 

I know it's incredibly cheap to get pcbs now, but my intention for this would be to create one-off prototypes.  I'd like to be able to handle tssop and 1208 (maybe even 0806) components.

[jmelson]

I've started using dry film photoresist for quick prototyping, but lining up transparency films and getting the exposure right is taking a bit more time than I was hoping for. 

Here's my system.  I get a sheet of clear plastic the same thickness as the board.  I put matching fiducial marks on the phototools, front and back.  I lay the bottom phototool on a light table, and glue a scrap of PC board material to one side.  I then place the clear plastic on it, and lay the other phototool on top and align the fiducials.  Then, I glue the top film to the PC board scrap, and align until I'm happy with the alignment, and put a weight on the glued area, and wait for the glue to dry.  I usually use rubber cement.  Not real strong, but it allows you to adjust the alignment.  If you use contact cement, you'd better have the alignment perfect because you can't shift it after it sticks.

Then, you pick up the film sandwich and slide the board in it, put in a double-sided vacuum frame and expose both sides without interrupting the vacuum.  This maintains the alignment between the two films.

Now, you said something about transparencies.  That implies laser printers.  You may be able to make decent one-sided films on a laser printer, and for 0805 and SOIC parts, that should work fine.  However, unless you use really large via pads, or are doing REALLY small boards, the two films won't line up well enough over the whole board, due to mechanical-to-paper errors in the printer.

I use a home-built laser photoplotter to make my phototools, and they are accurate to a few thousandths of an inch over a foot or so.
I mostly use it to make solder stencils for commercially-made boards, so it has to match their accuracy pretty closely.

TSSOP is another whole ballpark than 0805, and I think you will have to tune everything in your process up very carefully for that to work.

Jon

[ranthalion]

Thanks for the input.  That sounds like a pretty sophisticated set up. I'll have to try to rubber cement idea when I run into a double sided board.

I see a lot of isolation routing, but I haven't seen anyone else using these lasers for photoresist, so that makes me think it probably isn't a good idea.  For the quick one-offs, I've always managed to get by with single sided boards.  I'm lining up the transparencies to get the mask dark enough. 

[langwadt]

https://youtu.be/1hFNj86L7sk

[ranthalion]

Thanks.  I found that video before, but passed over it since it was using spraypaint for the mask.  There's a subsequent video showing photoresist. 

[jmelson]

Thanks for the input.  That sounds like a pretty sophisticated set up. I'll have to try to rubber cement idea when I run into a double sided board.

I see a lot of isolation routing, but I haven't seen anyone else using these lasers for photoresist, so that makes me think it probably isn't a good idea.  For the quick one-offs, I've always managed to get by with single sided boards.  I'm lining up the transparencies to get the mask dark enough.

I think there are some guys who got the laser to photoresist to work.  One of the problems is the speed.  My photoplotter is a raster-scan machine, it produces 0.6" of plot of any complexity per minute.  The drum rotates at 600 RPM, and it lays down 1000 lines/inch.  I did hack up a UV beam attached to a Calcomp pen plotter to expose film.  It took a long time, like 45 minutes to draw even a simple PC board, with the beam writing up and down the traces over and over to get the desired width.

My photoplotter exposes Kodak typesetter film, and the exposed areas are QUITE black.  I then use that phototool to expose the resist with 4 blacklight fluorescent tubes for only ONE minute!  If you expose too long, light leaks under the edges of the dark spots and causes the resist to not completely wash away, and then you have trouble etching the board.

Jon
Jon