If you have to work on a particularly dense board, you can’t really get away from using small vias: <=0.3mm/12mil drill bits and <=0.125mm/5mil annular rings. While unavoidable, using small vias and particularly small annular rings poses a problem during manufacturing, since there is inevitably some variability with the drill placement and the smaller the annular rings (the metal ring left around the drilled hole) the more likely you are to end up with a broken via. One easy solution to improve your yield of error-free boards is using something called ‘teardrops’. They fan the trace out before it joins the via, making a ‘teardrop’ shape, effectively increasing the size of the annular ring and reducing the risk of faulty vias.
Most packages include some means to create teardrops, and Eagle is no exception. The default installation includes a ULP called ‘teardrops.ulp’, which you can run by simply typing ‘ULP teardrop’ when your board file is open. One word of caution, though … you should only run the teardrop ULP on a seperate, renamed copy of your board file since the changes are irreversable!. Make a copy of your board names xxx_teardrops.brd, make sure the original schematic is closed, and then run “ULP teardrop”. Give it a try and you should get an idea of why they can be useful on tight-pitch boards like the image above, which was from a 6mil/6mil 4-layer board with 0.3mm drill holes and 0.1mm annulars.