How to create boards with vias and straps with Eagle ?

Eagle is a great tool to design PCB board. I’ve used it to design my new mainboard. In its non-profit version, you can build board with two layers (bottom, top). But I only have single sided copper boards. So I used to deactivate, declare as forbidden the top layer. Often, Eagle can’t create 100% routes, so I manually creates vias and straps (straps are wires, not tracks, maybe not the correct english word). Quite annoying, time consuming…
Following is a simple tutorial to let Eagle build its own vias and straps to reduce manual actions. This is a two steps approach
    1. Since I want to have the minimum straps, I declare the top layer as forbidden. This forces Eagle to create as tracks as possible on the bottom layer.
    2. Then (and if not 100% routing done), I activate the top layer, but declare it as costly. Eagle will finish its work, still creating bottom tracks, and adding small top tracks through vias, which I can then convert (consider as) to straps.
Just a reminder here. Since I use a PCB pen, which can’t produce accurate results (because of the pen and I), I configure Eagle to create quite large tracks. 32mil works for me. This constraint will help while actually drawing tracks.
Same things while drilling holes. Drilling is quite critical, it can easily destroy your board.
So, first, top layer is deactivated. This is defined in the Design rules menu entry.
One eagle has done its job, 79.8% routing is done. Not one top tracks is created. Still several routes need to be created. Here’s the second step…
This time, the top layer is activated…
… but it has a high cost compared to the bottom. Without this setting, Eagle will long top tracks, which can’t considered as short straps. [Edit: cost value on top layer must also be set to 99 -- or the like -- for all Optimize tabs, otherwise Eagle will create long route during optimisation]
Now 100% routing is done. Several short top tracks were created (red). Most can be considered as straps.
  And some definitively can’t… This is where I still need manual work. For instance, few top tracks are going under components here. Since wires (straps) are quite thick, there’re problems here. This occurs when Eagle directly connect tracks with pads. I didn’t find yet where to forbid this.
I need to move the bottom tracks outside of components, and recreate the straps. There’s still manual intervention, but let Eagle optimizing routing produces better results than humans can do (as least me…).


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