When I prepare a double layer DIY PCB using Press-n-Peel I have a problem aligning the bottom and top layer so that the holes and vias match on the copper between layers. Can you suggest any easy method that would minimize the wrong alignment?
Not easy, but I use an involved method that uses "alignment pins" to get perfect alignment every time. This works very well with the toner transfer and photo paper method and might work for press and peel as well.
Dot the edges of your layout with evenly spaced pads used for alignment holes. 3 - 4 per side is sufficient. You may need to distribute more into the center as well if the board is largish. Use really small drill holes in the pads, so that you can accurately find the center.
Print the two sides to your transfer paper and make an additional copy with just the alignment holes.
Transfer the alignment holes to your board and use this as a guide to drill holes. As an alternative, you could tape a normal paper copy of the holes to the board and use that as the guide. But, I think it's more accurate to actually transfer it. The size of holes should just small enough to pass pins or component leads through. I like to use 18mil bits and use clipped off resistor leads as the alignment pins that I collect from previous projects.
After drilling, remove the toner by sanding or acetone. You should be left with the bare copper again and drilled holes.
Drill out the two transfer paper layout sides with the same bit. Note, I hand drill both the board and transfer paper holes, so that they are perfectly centered. This can be tedious especially if you use thick boards.
Trim down the two transfer paper layouts sides, so that they are slightly smaller than the board. This is important because you need to tape them down after the alignment procedure.
Align the two transfers over each side of the board, so that you can pass the alignment pins straight through the set of holes. Then adjust each pin, so that they are all more or less perpendicular. This should make everything aligned. Carefully, tape down both transfer sides to the board securely and finally remove the pins.
I have done something similar, it can be done two ways.
Use two points on the pcb as reference points, drill, and align each separately.
Hold both pieces up to the light, get both aligned, and staple or tape end of sheets together so it folds like a notebook. This requires an extra two inches or so on the end to allow flexing around the end of the board. Sometimes I also tape it to the board to keep it aligned.
I use the "envelope method" in conjunction with the toner transfer (ironing version). It is similar to the 2nd method that Erik described in his answer. I like it because it does not require you to drill any holes in the board to make the alignment. It consists of the following steps:
- Add registration marks to both sides of your board (top and bottom). I usually place four 150 mil crosses (with 16mil traces) on the corners of the board.
- Print both layers of your board on the transfer paper. I print the layers in glossy ink jet paper on a laser printer.
- Place the pieces of paper with both layers together, against each other and align the registration marks against a bright light. You'll know the papers are aligned when the bar from the crosses merge with each other. Then you should see that the drill holes in both layers align as well.
- Now the tricky part. WITHOUT MOVING the pieces of paper, tape 2 or 3 ends of the paper sheets together, making an envelope. You'll use the end(s) that are not taped to insert the blank board in the next step. If the sheets move, remove the tapes and go back to step 3.
- Insert the clean blank board onto the envelope and iron both sides of the board as usual. Take care when started ironing the first side of the board and are about to turn the it around the first time to iron the other side, so that the tapes don't fall apart. The tapes may be molten because of the heat. That happened to me once. The 2nd layer fell off the board while the first had already been transfered. If that happens, scrub the board and start over again.
To help with the process, I also do the following:
- I only solder the top layer on vias, never on pads. If I have to move the signal from one layer to the other, I never do it on pads. Instead, I bring the signal from the pad to a via, a few mm away from the pad and move the signal to the other layer there. This way, only the vias have to be carefully aligned between layers.
- I design my boards so that its VIAS are as large as possible (usually 70 mil wide, with 20 mil drill holes);
Here's a link to a description of the alignment process that I find helpful. Scroll down to the section Aligning Masks for Double Sided Boards.
Include a couple registration marks, or use some vias or through-holes for alignment. Attach one side, drill the holes, then use those holes to align the other side.
In my experience, a typical printer introduces enough distortion that even with the most care, the alignment between the layers can't be really great. For layouts I'll be making in the basement, I make all the pads, traces, and vias as fat as I possibly can to accommodate distortions from the printer, my inaccurate manual drilling, etc.
My first method was to stick each side down onto an L shaped piece of scrap PCB. That way you remove parallax errors between the layers and can get much better registration.
I now do it as follows:
Heat melt the first side onto the board and remove the backing.
Drill the holes based on registration marks on the PCB.
Register the second side using pins.
This works better since registration of the holes from the first side is absolute. Drill registration errors can be removed when I perforate the second side.
Downside is you have to be careful not to overheat the second side, or you lift / destroy the first side. It won't work through a laminator unless you pre-etch the first side and protect it. I'm still working on making this a reliable and repeatable process.
My company used to make our own PC boards in-house. This was back in the early '80s and there weren't any quick-turn PCB fab houses around. We used DuPont Riston film with a proper laminator and had one of the local graphics houses make our negatives from the 2:1 mylar master artwork. The sensitised PCB material was held in a glass vacuum frame with the negative sandwiched between the PCB and the glass.
Making double-sided boards was always a trial until we discovered a technique that worked extremely well. What we did was to make a "L" shape out of un-etched PCB material, tape one negative to both arms of the "L" shape, then flip the whole thing over onto our light table and carefully line up the other negative. This 2nd negative was then taped only to the long arm. Because we were using our light table for alignment, we were able to achieve alignment accuracy down to a few (very few) mils.
In using this, we just flip the 2nd negative open, tuck the exposed PCB into the corner of the "L", flip the 2nd negative back down on top of the PCB, then put the whole thing into the glass vacuum frame and expose. When the exposure was finished, flip the vacuum frame over and expose the other side.
This technique should be directly applicable to toner transfer.
I know I'm kinda late for this, but check this video, it shows how to make it easy and very accurate. Basically almost the same as Ricardo explained, but it's better to see it in action.
Video is in Russian, but you don't need to know it to understand what happens.
P.S. Method marked as answer here is pain in the ass, tried it many times and it is much more harder than that on the video. I tried to make it this way many times and had many troubles, that would probably lead to wasted time or even boards(if you etch not-aligned layout).