layoutpcbrouting
I have been using 10 mil trace with 10 mil spacing. The PCB vendors quote that they like traces down to 7 mil. But then I ran across a PDF showing how to fan out a QFP to get all the signals accessible. They use millimeters because the QFPs are packaged with 0.4mm or 0.6mm pitches.
They also make an argument that using a 0.05mm grid approximates mils, but mm allows you to route buses in between the vias and pads.
Should I be using mil or mm when I am routing a PCB?
Look for application notes for your PHY and magnetics. The manufacturer would know best in regards to what works with their parts.
Generally there is no ground/power or routing under the magnetics and try to avoid ground/power under the TX/RX pairs. If you can't route the whole trace without a ground/power plane under it, leave the plane under it. It is worse if you go over a break in the plane.
For termination, check with manufacturers of the PHY and magnetics. Like you said, there are a few different schemes, the manufacturer should know best about their device.
We follow what I described above at work and don't have any problems with ethernet.
The Eagle autorouter is a decent tool, and I use it a lot. However, like any tool, you have to know how to use it well and understand its limitations. If you are just expecting to throw everything at the autorouter, you will be dissappointed. No current auto router, and probably for a number of years to come, can do that for anything beyond contrived or toy problems.
You say there are settings in the Eagle autorouter you don't understand and never mess with. This is a bad attitude, and probably a good part of your problem. There is no set of control parameters that works on all boards. Even within 2 layer boards there are various tradeoffs. You absolutely have to read the manual and adjust the parameters for your particular situation.
For two layers boards, I often try to keep most of the bottom layer a ground plane. I therefore use the top layer for interconnects as much as possible, and the bottom layer for short "jumpers" to make the routing topology work out. In this case, I set a high cost for routing in the bottom layer.
Before autorouting, you have to look at the board and think about the critical areas that you can't explain to a autorouter. For example, you want to keep the loop currents of a switching power supply local and off the main ground plane. The same holds true for high frequency currents local to a digital chip, like bypass caps and crystal with its caps. If you are using the pseudo ground plane layer as I described above, then you want to manually connect every ground connection immediately to the ground plane with its own via. That leaves maximum room on the top layer for routing everything else.
The process of routing a board even when letting the auto router do most of the grunt work looks like this:
Here is a Eagle autorouter control file I used on a two layer project with the bottom layer a ground plane to the extent possible:
; EAGLE Autorouter Control File [Default] RoutingGrid = 4mil ; Trace Parameters: tpViaShape = Round ; Preferred Directions: PrefDir.1 = * PrefDir.2 = 0 PrefDir.3 = 0 PrefDir.4 = 0 PrefDir.5 = 0 PrefDir.6 = 0 PrefDir.7 = 0 PrefDir.8 = 0 PrefDir.9 = 0 PrefDir.10 = 0 PrefDir.11 = 0 PrefDir.12 = 0 PrefDir.13 = 0 PrefDir.14 = 0 PrefDir.15 = 0 PrefDir.16 = * Active = 1 ; Cost Factors: cfVia = 50 cfNonPref = 5 cfChangeDir = 2 cfOrthStep = 2 cfDiagStep = 3 cfExtdStep = 0 cfBonusStep = 1 cfMalusStep = 1 cfPadImpact = 4 cfSmdImpact = 4 cfBusImpact = 0 cfHugging = 3 cfAvoid = 4 cfPolygon = 10 cfBase.1 = 0 cfBase.2 = 1 cfBase.3 = 1 cfBase.4 = 1 cfBase.5 = 1 cfBase.6 = 1 cfBase.7 = 1 cfBase.8 = 1 cfBase.9 = 1 cfBase.10 = 1 cfBase.11 = 1 cfBase.12 = 1 cfBase.13 = 1 cfBase.14 = 1 cfBase.15 = 1 cfBase.16 = 5 ; Maximum Number of...: mnVias = 20 mnSegments = 9999 mnExtdSteps = 9999 mnRipupLevel = 50 mnRipupSteps = 300 mnRipupTotal = 500 [Busses] @Route Active = 1 cfVia = 10 cfChangeDir = 5 cfBusImpact = 4 cfPolygon = 25 cfBase.16 = 10 mnVias = 0 mnRipupLevel = 10 mnRipupSteps = 100 mnRipupTotal = 100 [Route] @Default Active = 1 [Optimize1] @Route Active = 1 cfVia = 99 cfNonPref = 4 cfChangeDir = 4 cfExtdStep = 1 cfHugging = 1 cfPolygon = 30 cfBase.16 = 10 mnExtdSteps = 20 mnRipupLevel = 0 mnRipupSteps = 100 mnRipupTotal = 100 [Optimize2] @Optimize1 Active = 1 cfNonPref = 3 cfChangeDir = 3 cfBonusStep = 2 cfMalusStep = 2 cfPadImpact = 2 cfSmdImpact = 2 cfHugging = 0 cfPolygon = 40 mnExtdSteps = 15 [Optimize3] @Optimize2 Active = 1 cfVia = 80 cfNonPref = 2 cfChangeDir = 2 cfPadImpact = 0 cfSmdImpact = 0 cfPolygon = 50 mnExtdSteps = 10 [Optimize4] @Optimize3 Active = 1 cfVia = 60 cfNonPref = 1 cfPolygon = 60 cfBase.16 = 12 [Optimize5] @Optimize4 Active = 1 cfVia = 40 cfNonPref = 0 cfPolygon = 70 cfBase.16 = 14 mnExtdSteps = 5 [Optimize6] @Optimize5 Active = 1 cfVia = 20 cfBase.16 = 16 [Optimize7] @Optimize6 Active = 1 cfBase.16 = 18 [Optimize8] @Optimize7 Active = 1 cfBase.16 = 20
Best Answer
While the metric system is arguably saner, those that have been doing PCBs for much longer than myself have are pretty adamant about using mils. I think it has something to do with the machines used by manufacturers. That being said, if you have a board house in mind, and they spec their capabilities out mm, then yea you'll probably get a neater routing by picking a grid that is "natural" to your components. Typically though, you're going to have a mix of parts that are mm and mil based, so you'll likely have a "grid mismatch" regardless. I pretty much always use a 25mil grid and things usually turn out "good enough."