Home made two sided PCBs alignment techniques - Page 1
Forum section: electronics / general

author: akis
author_rank: Frequent Contributor
timestamp: August 15, 2014, 05:29:24 pm
content: On an older threads we discussed tracing and other papers, and a few members mentioned their techniques for two sided PCBs and alignment.I would like to discuss just the process of alignment and exposure of a two sided board.The way I do it :1. Print additional thick-ish guide lines as an L-shape around the printed layout.2. Align PCB with the L shape guide lines, expose one side, turn over, align on the other tracing paper and expose other side.The problems there are that the PCB itself is not perfectly square so when trying to align it it will be off on one side of the "L" guide lines. When you turn it over to do the other side, you will have to remember how exactly it did not align well before, to try and match the misalignment. For large boards this results in holes being out of true, and therefore the results are more or less random-ish. The guide lines certainly help but are not panacea.Please tell me how you do it, including the L shape PCB idea.1) stapling the two tracing papers together -- how is that even accurate? I tried to tape two small pieces of tracing paper together and it took multiple attempts

author: Refrigerator
author_rank: Super Contributor
timestamp: August 15, 2014, 05:56:46 pm
content: Here, watch this.

author: JuKu
author_rank: Frequent Contributor
timestamp: August 15, 2014, 08:59:25 pm
content: I don't do boards myself anymore, but when I did, I drilled two holes at opposite corners and aligned the masks to them.

author: sotos
author_rank: Frequent Contributor
timestamp: August 16, 2014, 08:58:08 am
content: Quote from: JuKu on August 15, 2014, 08:59:25 pmHave you found I cheap way for 2-3 boards?

author: DJohn
author_rank: Regular Contributor
timestamp: August 16, 2014, 08:07:50 pm
content: I use something very similar to the method in Refrigerator's video. Put some alignment markers on both sides. Leave a very wide margin around the board. Align and stick the two sheets together (I use tape rather than superglue, but the details don't matter). Slide the board inside, taping it to the sheets as well so it doesn't move when it's turned over.

author: mazurov
author_rank: Frequent Contributor
timestamp: August 16, 2014, 09:51:56 pm
content: When I mill boards I only need to make sure the board is square to the mill when I flip it. Therefore the sequence is mill one side, drill it, route the outline, flip the board, make sure it is square to the mill, then align the spindle using any of the holes drilled in step 2 as a reference; finally, mill the other side.

author: nctnico
author_rank: Super Contributor
timestamp: August 17, 2014, 12:06:18 am
content: Quote from: akis on August 15, 2014, 05:29:24 pmIMHO the best way to do it is using an L shaped piece of PCB material (or anything with the same thickness as the PCB you want to make) and stick the layouts to it. Slip the PCB in between and expose. I tried several other methods but this is the easiest way I have ever used. Alignment within 0.2mm is not a problem.

author: wraper
author_rank: Unknown
timestamp: August 17, 2014, 01:18:15 am
content: IMO making aliment holes are not precise enough for a larger board with a small pads/holes. I print transparencies with a thin "+" aliment marks on all corners. Align them precisely and fix them together with aluminium adhesive tape (keeps shape better than polymer ones) on two opposite sides. Now I just put a board inside, fix it a bit with adhesive tape so it don't move, and then expose one side at a time. Don't need to do alignment each time if need more then one board. Sticking transparencies on both sides is essential to get good alignment especially on larger boards. When exposing I just put a piece of glass on top so transparency is pushed down to the PCB.

author: sleemanj
author_rank: Super Contributor
timestamp: August 17, 2014, 02:11:41 am
content: Ireallyneed to get around to making a video series about how to make PCBs, one of these days...Anyway, the way I do alignment, for UV exposure on a single side exposure rig (so expose, flip, expose) is thisI print the artwork using myGerber To Etchable PDFthis produces on a single sheet the top and bottom sides (appropriately mirrored) separated by a specific distance, with the same physical corner of the PCB marked, and with an extra border with a small extra space.Then on my exposure rig, which is powered by a load of UV leds through a diffuser, built into the remnants of a multifunction printer's scanner bed, I have a "T square" arrangement made out of a couple of rulers. Not coincidentally, the vertical bar of the square is the same width as the space between the top and bottom art.Then I turn on the white light mode of my box and slide the artwork under the tsquare (the square is attached to the glass only at the top edge with tape like a hinge so I can slide the artwork right under it securely. You can see that those marked "physical corners" now are on the inside corner of each side of the square. When I am satisfied that both sides are evenly aligned with regard to the square I put a couple bits of tape down to stick the art to the glass just to stop it moving.(this was just chucked in,not quite aligned but you see the idea)I've already cut the PCB to size, laminated the dry film, and located the "squarest corner" of the pcb, which I will use as the marked physical corner, I colour in the edges of the PCB at that corner with a marker pen to remind me.I place the PCB on the appropriate side of the artwork and push it firmly into the corner of the T. Then I use the inside corner of a plastic set-square I cut out as a stop on the opposite corner, and tape it in place just to stop it moving.(pcb not shown)I then put a piece paper on the top side of the PCB to protect it from exposing, and a sponge or similar on top of that to make sure it gets an even pressure, close the lid, and clamp it down (I have a wooden bar across the lid and a couple of brackets either side, then use a couple of quick-clamps to pull ti down), expose.Then it's simply a matter of opening it up, flipping the PCB over onto the other side of the artwork and away you go.

author: DanielS
author_rank: Frequent Contributor
timestamp: August 17, 2014, 02:40:22 am
content: Quote from: wraper on August 17, 2014, 01:18:15 amIt seems to be good enough for PCB manufacturers: many use mechanical alignments using registration pins on their transparencies or transfer films for individual two-sided core boards and then use X-ray alignment to align cores in a multi-layered board. The trick is in the alignment rig.For surface-mount components, the component's alignment itself is not important; what matters is the vias if you want to use (really) small ones. Through-hole holes are usually much larger than mechanical alignment's tolerances when done right.

author: lewis
author_rank: Frequent Contributor
timestamp: August 17, 2014, 10:21:06 am
content: I make double sided PCBs prototype in the lab as follows:1. Print off the top side and bottom side artwork on their own sheet of transparency.2. Add three strips of double sided tape around three edges of the artwork on one of the transparencies, leaving about 7-10mm between the tape and the edge of the artwork. If the board is rectangular, leave one of the long edges of the rectangle (so put tape around the two short edges, and the other long edge).3. Very carefully align the top and bottom transparencies. When I'm happy, peel off the double sided tape protective layer and stick the transparencies together.4. We now have a 'pocket'. Transparencies stuck together on 3 sides, the fourth side is where we insert the blank pcb into the pocket between the transparencies.5. Cut the PCB roughly to shape so it fits snugly in the pocket.6. Expose one side, then the other. The PCB will not move in the transparency pocket when turning over if it was inserted snugly enough.No need to cut the board accurately before exposure, no need for alignment holes, and both layers get perfectly aligned. I've had it within ~3 mil. Be careful when inserting the pcb, there is a risk the transparency can be scratched, although I've never had a problem.For best results, the 'ink' side of the transparency should be in contact with the copper - this gives sharper and more accurate edges. This means mirroring one of the layers before printing.

author: PlainName
author_rank: Super Contributor
timestamp: August 17, 2014, 04:25:36 pm
content: This is one of the advantages of milling PCBs. When I do double sided (not often now - at that point it is less effort to get itead to do them) I put two 3mm holes along the bottom side (off-circuit or not, doesn't matter) about 25-50mm apart. These match similar holes the mill has previously made in its bed (which is a block of perspex, skimmed and ultimately disposable - don't make holes in the mills metal bed).The idea is that the holes in the PCB can be exactly aligned with the holes in the bed via a couple of 3mm pins, and it is now only necessary to specificy one pin as the origin of the PCB. In principle, the mill is zeroed to the pin which is the origin for the side being milled, but in practice I zero the mill to one pin and then ensure that's the pin that's used as origin (so it will be the different one for each side).Apart from zeroing the mill, something I don't have to do with single-sided but is a piece of cake when using Mach3 anyway, there is nothing else special to do. Even making the holes in the right place is trivial since it's automated. The process goes:1. Drill holes as normal2. locate PCB on pins3. mill top side as normal4. flip PCB and locate on pins5. mill bottom side as normal6. mill board outline (if not done at 1. because pin holes are off-PCB)The only fly in the ointment is play on the pins - finding a hard pin that's exactly the right fit for a hole drilled by a jobbing 3mm bit can be a pain, but once done it's done. Having 3 pins/4 holes might make it less of a drag.

author: kc9qvl
author_rank: Regular Contributor
timestamp: August 18, 2014, 07:35:01 am
content: Use a couple of the vias or through holes for alignment pins. Expose one side of the board. Drill a couple of via or through hole. Use the holes to align the other side of the board.

author: PepeK
author_rank: Regular Contributor
timestamp: August 18, 2014, 10:33:07 am
content: Print both sides of a PCB mask on separate transparent foils. Do not forget to include some align marks like border lines or corner crosses. Create an envelope from the foils using an adhesive tape. Insert PCB inside the envelope an apply exposure light.

author: WarSim
author_rank: Frequent Contributor
timestamp: August 18, 2014, 12:13:11 pm
content: Quote from: dunkemhigh on August 17, 2014, 04:25:36 pmMany mills like mine have alignment cameras and pattern skewing.Two holes are drilled just outside the pattern area.Select a side to work on.Find each hole with the camera.Computer skews the pattern to the holes.Largest alignment error using this method, so far has been 0.02mm.No I can not measure this at home. I could see a small variance in via hole to pad alignment with the mills camera. So I took it the mechanical Calibration lab at work. They measured the error as 0.02mm.Sent from my iPad using Tapatalk

author: PlainName
author_rank: Super Contributor
timestamp: August 18, 2014, 12:48:25 pm
content: QuoteI might have to acquire a cameraLast time I tried it was a dead loss, but I must surely have used up my bad karma by now.

author: Fred27
author_rank: Unknown
timestamp: August 18, 2014, 01:54:30 pm
content: Another vote for the "envelope" technique (as I mentioned to akis in a PM). It's really easy. If you tape 3 edges and avoid the corners then it should stay perfectly aligned even as the envelope opens for the PCB.I'd also recommend Ziploc vacuum bags as a cheap and easy way to press the artwork down firmly and evenly across the board. Like a ghetto version of those expensive vacuum exposure units. Make sure you don't have any sharp corners or edges on your PCB that would create a hole.http://www.ziploc.com/Products/Pages/VacuumFreezerSystem.aspxIf you're using a CNC mill for drilling then I've heard that you can drill before etching and align the transparency with the holes/ vias. Sounds like you might get problems with undercutting to me, but I've not tried it. Hmmm... maybe it would work if you drilled a couple of alignment holes outside your board area just for this purpose. A hole in the PCB and a crosshair on the transparencies.

author: EEVblog
author_rank: Unknown
timestamp: August 18, 2014, 01:57:39 pm
content: I just tape both films either side of scrap bit of PCB, one side first, then the other with a thin piece of transparent polycarb between them so you can eyeball through it.Often didn't even need the polycarb, just eyeballed the whole thing.Was always good enough for any double sided job I had.