How to best align sheets for double sided pcb. - Page 1
Forum section: electronics / general

author: Dan Moos
author_rank: Frequent Contributor
timestamp: June 09, 2016, 12:11:19 am
content: I'm learning the photo resist method, and early signs tell me it's the way to go. Just way fewer ways to screw up. Different ways, but still fewer.Anyway, I'm not certain what the best way to align my transparencies for a double sided board.I've seen video of folks taping one end together, but that seems like it would still allow movement. In fact, to me, if you taped two sheets together, and tried to slip a board between them, they'd have to offset to accommodate the board.I have tried 3/16" fiducials with cross hairs in them, and using a third print as a tool to drill these holes into the board, so I can use the holes to align my sheets. That seems to work, but now I'm concerned that I'm leaving the boards exposed to light too long while I'm fooling around lining things up.I'd love to use a similar system to what the pros use, i.e., holes in every sheet and board, and rods to thread everything on to , but I can't see how to do that and have the necessary pane of glass on top of the stack.What works well easily? Btw, I'm using my scrap component pins for vias, and will only ever connect 50 mil traces with them. In other words, I don't need amazing yet. But someday I might want more.

author: D3f1ant
author_rank: Frequent Contributor
timestamp: June 09, 2016, 05:43:29 am
content: If you setup a nice yellow light you can work for a very long time without it affecting the photoresist.

author: Brutte
author_rank: Frequent Contributor
timestamp: June 09, 2016, 06:51:43 am
content: The alignment is easy with 4 vernier scales on four sides of artwork. I did not know how to add a vernier scale directly in a KiCAD (pcb software that I used) so I exported both Cu layers to Gimp (bitmap) as two separate layers and then four verniers were added at about 1" offset on each edge.IIRC one scale had a period of 24 stripes/inch and the other had a period of 25 stripes/inchSo 24*25=600 and they matched only once per inch (my printer is 600dpi).If the bottom layer was shifted w.r.t top layer by more than 1 pixel (2-3) then it was clearly visible on the scale.

author: KL27x
author_rank: Super Contributor
timestamp: June 11, 2016, 06:50:42 pm
content: delete

author: jpanhalt
author_rank: Super Contributor
timestamp: June 11, 2016, 07:35:03 pm
content: I just tape the two transparencies to make a sleeve about two inches wider than the board on both sides and put the board inside. If the bow is symmetrical, there is no error. If one side is flat and the other bowed, a 1/16" error at 3" is approximately 0.0006" (0.016 mm). Is your printer that accurate? What about natural stretch of the transparency as it goes through the heated rollers of the toner?John

author: nctnico
author_rank: Super Contributor
timestamp: June 12, 2016, 12:56:32 am
content: The best way is to use an L shaped piece with the same thickness as the PCB material. If you put a square outline on the PCB layout you can line the transparent sheets up with the inside of the L, stick them to the L and slip the PCB in between the sheets. Then again it makes more sense to order the boards nowadays so you have a solder mask, plated vias and silk screen.