Vias and Solder Stop Mask – How Does it Work?
EAGLE generates by default a solder stop mask for each Via (also for Pads and SMDs, of course). This means the Via is free of coating material. The solder stop mask is drawn automatically in the layers 29, tStop, for the top side and 30, bStop, for the bottom side. The size of the solder stop mask can be determined in the Design Rule’s Mask settings. See the values for Stop. By default the value is fixed to 4 mils. Minimum and Maximum are set to the same values therefore. If you want to have a diameter-dependent mask you could also define a certain percentage. The resulting value can be limited by a minimum and a maximum. In order to have vias coated, EAGLE allows you to set the solder stop Limit in the Design Rules’ Mask tab. Here you can define a value which is dependent on the drill diameter of the via. Let’s assume you would like to set the Limit to 0.012 inch. Now all vias in the layout up to a drill diameter of 0.012 inch will be covered with coating material. All those vias that