Same trace on top and bottom of PCB - Page 1
Forum section: electronics / general

author: kkbb
author_rank: Contributor
timestamp: July 08, 2018, 08:00:15 pm
content: Hello All,I have two identical parts on the top and bottom whose pads are connected.Is there a way to connect the pads for the parts by running a trace simultaneously on the top and the bottom (and joining the traces via a via)?As an example, the image shows the same pins being connected on the IC1 and IC2. The top and bottom traces are on top of each other, and connect the identical pads of the components.Thanks.Victor.

author: wraper
author_rank: Unknown
timestamp: July 08, 2018, 08:15:34 pm
content: Placing ICs on both sides like on the picture is does not work. You can't just mirror the pads on the opposite side because pinout will be opposite to what an actual IC has.

author: jpanhalt
author_rank: Super Contributor
timestamp: July 08, 2018, 08:29:00 pm
content: If you are brave, you can bend the pins on the bottom IC up and connect that way. You can use an interface board, but the best way as wraper implied, is to offset the IC's and connect the pins in the usual way.

author: kkbb
author_rank: Contributor
timestamp: July 08, 2018, 08:30:08 pm
content: Yes, I am aware of that. Please forget about the IC. The question was about placing two traces exactly on top of each other. I just drew that to show what I was attempting to do in the real PCB (which has 100s of these traces)That image was just to illustrate the point that I wanted the traces to go on top of each other, and if that can be done as one operation (instead of drawing the top trace, placing the via and then drawing the bottom trace exactly below the top).

author: jpanhalt
author_rank: Super Contributor
timestamp: July 08, 2018, 08:48:05 pm
content: I use Eagle 7.70 and have never had a problem with traces on top of one another. There may be other reasons not to do that, but from the standpoint of designing the board , it has never been a problem.What problem are you getting, and what CAD program are you using?

author: kkbb
author_rank: Contributor
timestamp: July 08, 2018, 08:58:19 pm
content: I think I did not explain my self. So my apologies.The question was not if it was ok to place traces on top of each other, but to bind the routing tool so that when you start routing, it simultaneously places both top and bottom traces.For a small project, I understand that you can first place the top route, then a via and then place the bottom route exactly below the top. But is there a way of avoiding placing the bottom route (and have eagle automatically place the bottom route when the top route is drawn).

author: wraper
author_rank: Unknown
timestamp: July 08, 2018, 09:07:24 pm
content: It's not something that you should do normally. If you need more current capability, just make a wider trace.

author: kkbb
author_rank: Contributor
timestamp: July 08, 2018, 09:15:34 pm
content: It is not for current capability.This is for having a unique requirement where two identical high density connectors are placed back to back and I want to connect their pads. As an example, a high density plug connector on the top and the receptacle (with the same footprint as plug) on the bottom.

author: jpanhalt
author_rank: Super Contributor
timestamp: July 08, 2018, 09:27:44 pm
content: Quote from: kkbb on July 08, 2018, 08:58:19 pmWell, that was definitely not clear until now. So far as I know, you cannot do that automatically with Eagle or KiCad, nor can I think of any common reason why you may want to do that. Of course, there may be the rare exception, but the mirror effect on devices connected to the traces will eventually do you in.Why don't you show a real example of what you want to do? I suspect this is more an imaginary issue than one a software provider might want to offer.

author: montemcguire
author_rank: Regular Contributor
timestamp: July 14, 2018, 05:09:49 am
content: Unless you have hundreds of traces to route like this, it's not that tedious to route one side the way you want it, route the other side sloppily, and then using the 'info' tool, copy the trace coordinates for each segment from the good layer to the 'rough' layer until they're the same. Good use of a grid can make the initial 'sloppy copy' pretty close, or maybe even the same as the 'good' layer, reducing the need to hand tweak one layer's traces to match the other layer's traces.

author: ebastler
author_rank: Super Contributor
timestamp: July 14, 2018, 05:37:16 am
content: Quote from: kkbb on July 08, 2018, 09:15:34 pmI trust that these a meant to be SMD parts?! Nobody has mentioned it, I think, but your example at the beginning of this thread would have a bit of a problem, simply because the pins of those two ICs collide...

author: MarkL
author_rank: Unknown
timestamp: July 16, 2018, 02:41:43 am
content: I'm also not clear why you'd want to do this, but here's a manual way that doesn't involve copying coordintes or re-routing every trace:Completely route only one layer of the traces you want duplicated. For this discussion, let's say it's the top and you want a duplicate on the bottom.Show only the top layer and group the traces you want duplicated.Copy the group and place the copy in a blank area on the board.Now group the copied traces and change the layer of the group to the bottom.One at a time, change the name of each copied trace to the original name. Each copied trace will be called the original name with a "1" appended. Get rid of the "1". Each time you will be asked if you want to connect "<originalname>1" to "<originalname>". Say yes. This is the most annoying step if you have a lot of traces, but I don't see a way to automate it.Move the group into position so that it's exactly under the original top traces.You now have the same copper pattern on the top and bottom, and they are identified as the same signal name. If you have thru-pads or thru-vias on each original trace, you should have no airwires between the top and bottom traces.Works fine on at least v7.2

author: borghese
author_rank: Regular Contributor
timestamp: July 22, 2018, 02:19:25 pm
content: But the components are not mirrored?

author: MarkL
author_rank: Unknown
timestamp: July 22, 2018, 07:57:45 pm
content: Quote from: borghese on July 22, 2018, 02:19:25 pmWhen I say "Show only the top layer", it does mean only the top layer and nothing else, e.g. "display none top;". That way, components, pads, and other items are not copied as part of the group, and don't get mirrored. The group only contains the copper traces.After renaming and when placed exactly under the top layer, any vias and pads will connect the top traces to the bottom copy automatically. Any missing connectivity will show up as airwires to be routed manually as usual.Again, I'm not sure this is what the OP wanted to achieve, but I've used this method for increased current capacity between thru-hole components.Another method, which works most of the time, is to paste the traces to be copied onto an empty board in an area that wouldn't overlap with anything in the real board. Then use "export-board.ulp" to create a script of the copied traces in "temp.scr". Then go back to the real board and execute "temp.scr". This should regenerate all the traces with the same names. Then group them, change the layer, and reposition the group as before.This saves having to rename everything, but it doesn't always re-draw the traces correctly for me (like miters are sometimes forgotten). It will also display errors about non-existing layers and trying to create new signal names, but it will work anyway. "export-board.ulp" is in the user contributed ULP area on the web site. It could probably be fixed up to do this duplication task better. YMMV.

author: kkbb
author_rank: Contributor
timestamp: July 26, 2018, 08:38:41 pm
content: Thanks Mark.That worked.