Eagle PCB routing: airwire issue

[earckens]

This is an issue I found with the eagle autorouter (yes, I know, it is a piece of crap and I should not use it): sometimes I get an airwire that is connected between a component and the bottom ground plane. I check the part of the ground plane the airwire is connected too but I am unable to find it to be an isolated island. The part of the plane in question is connected to other parts where no airwire issue exists.
Is it possible that I overlook something and the airwire is justified?
Or is it possible that an airwire does not take into account that the island in question is in fact not isolated from the remainder of the plane on the pcb?

 

[unclejed613]

by "airwire" do you mean it populates a jumper, or do you mean it shows that connection as an unresolved "rat's nest" wire?

 

[earckens]

by "airwire" do you mean it populates a jumper, or do you mean it shows that connection as an unresolved "rat's nest" wire?

An unresolved rat's nest wire

 

[unclejed613]

try moving components around near the device that won't connect. sometimes reshuffling the placement of thins is the difference between routing and not routing.

 

[earckens]

try moving components around near the device that won't connect. sometimes reshuffling the placement of thins is the difference between routing and not routing.

So you in effect say that when an airwire remains after autorouting, even if I cannot find it to be connected to an isolated island, that in fact it is justified? And that I may have overlooked an island that in effect ís isolated?

 

[unclejed613]

is there a pad on the board for the component lead? if you got the schematic and the footprints right, there should be a pad for it. if it's not connecting that particular wire, then you have an unroutable pad, that's because the autorouter can't find a way to get there. it's kind of like the old "3 houses and 3 utilities" puzzle where you end up not able to connect one of the utilities to the last house, because you've blocked the path with another line somewhere else. sometimes moving parts around can open up connections between zones that were previously blocked. sometimes, depending on the DRC rules, you can open up blocked zones by changing wire widths, pad diameters and pad clearances.

 

[JonSea]

I think you too don't have a meeting of the minds as far as the question goes.

I believe I have seen meaningless airwires between parts connected but a ground plane but I'm not certain of that.

One thing I have found useful is to shut off all layers but the ground plane layer to inspect for isolated islands. With the silkscreen and other layers out of the way, breaks are easy to see.

 

[earckens]

I think you too don't have a meeting of the minds as far as the question goes.

I believe I have seen meaningless airwires between parts connected but a ground plane but I'm not certain of that.

One thing I have found useful is to shut off all layers but the ground plane layer to inspect for isolated islands. With the silkscreen and other layers out of the way, breaks are easy to see.

Hi JonSea!
I did turn off all layers except ground plane and "unrouted", "vias", "pads" and dimension layer; while I did think that in this particular case there was an unjustified airwire your comment made me look again: there ís a break to the particular island.
So in conclusion would it be safe to say that if Eagle leaves an unresolved airwire there always is a justified reason?

 

[JonSea]

So in conclusion would it be safe to say that if Eagle leaves an unresolved airwire there always is a justified reason?

I think it's safe to say "Look at it and figure it out "

It seems like I'be seen a few that were nothing, but I don't recall for certain. For the board I was working on yesterday, the leftover airwire was the chip's connection to the ground plane. Glad I paid attention to that one!

 

[DGM]

I don't know if this is related, but I have certainly been able to create zero-length traces in Eagle (perhaps an older version than yours).
Often if I find an airwire running off to some nonsensical place, it's because there's an invisible remnant of a ripped-up trace that it's trying to connect back to its net.

 

[earckens]

I don't know if this is related, but I have certainly been able to create zero-length traces in Eagle (perhaps an older version than yours).
Often if I find an airwire running off to some nonsensical place, it's because there's an invisible remnant of a ripped-up trace that it's trying to connect back to its net.

I use v7.6 full; I have not had this issue, but I learned to save often (maybe not related to your issue?).

 

[JonSea]

I think clicking on ratnest will clear those out and remaining airwires will be the shortest distance between connected points.

 

[unclejed613]

I don't know if this is related, but I have certainly been able to create zero-length traces in Eagle (perhaps an older version than yours).
Often if I find an airwire running off to some nonsensical place, it's because there's an invisible remnant of a ripped-up trace that it's trying to connect back to its net.

i had some trouble with another cad package a few years ago, where it would continually fail DRC. it turned out there was a "mystery wire" in the netlist that went from nowhere to nowhere (just two sets of coordinates on a schematic page, and didn't show up when viewing the schematic). schematic and pcb layout software can do strange things once in a while.

 

[DGM]

I think clicking on ratnest will clear those out and remaining airwires will be the shortest distance between connected points.

At least in 5.0.0, running ratsnest doesn't actually clear them, but if there are nearby traces, you can use ripup, target an ambiguous region between traces, and then right-click to cycle through the local segments until you select the zero-length segment. It's similar to trying to rip up a via and not the traces connected to it. I guess you could use the group-select tool too.

 

[JonSea]

Sometimes, the leftover pieces of airwire are handy, like when I want to movd a track slightly. The airwire will show where the ripped up track was, so you can make adjustments.