Each layer in EAGLE is dedicated to a given function. Placing each item in the appropriated layer is highly recommended. This article presents some important layers and describes it functionality.
Layer 1 – Top
The first layer contains the top side tracks and the top side copper pour (if used).
Layer 16 – Bottom
This layer contains the bottom side tracks and the bottom side copper pour (if used).
Layer 17 – Pads
This layer contains the through-hole pads.
Layer 18 – Vias
This layer contains the through-hole vias.
Layer 19 – Unrouted
This layer contains the unrouted tracks, i.e. the airwires (rubberbands). Here almost all the board is routed, there is just a few unrouted tracks:
Layer 20 – Dimension
This layer contains the board outlines and circle for holes.
Layer 21 – tPlace
This layer contains the top side silk screen. It usualy contains the component outlines. Care must be taken not to cover any ares that have to be soldered. It is also possible to create additional and rather betterlooking silk screen for documentation purposes in layer 51, tDocu. This may indeed cover soldered areas, since it is not output along with the manufacturing data.
Layer 22 – bPlace
This layer contains the bottom side silk screen (see layer 21 for more details).
Layer 23 – tOrigins
This layer contains the top side component origins. It contains the origin cross for each component. Top side components can be moved or modifyed only if this layer is visible.
Layer 24 – bOrigins
This layer contains the bottom side component origins (see layer 22 for more details). Bottom side components can be moves only if this layer is visible.
Layer 25 – tNames
This layer contains the top side service print. It usualy contains the component names and may appear on the PCB as the silk screen.
Layer 26 – bNames
This layer contains the bottom side service print (see layer 25 for more details).
Layer 27 – tValues
This layer contains the top side component value. It usualy contains the component value and appears on the PCB as the silk screen and service print.
Layer 28 – bValues
This layer contains the bottom side component value (see layer 27 for more details).
Layer 44 – Drills
This layer contains the conducting through holes. It is usually used for pads (of through hole components) and vias.
Layer 45 – Holes
This layer contains the non-conducting holes. It is usually used for s used for mounting holes.
Layer 46 – Milling
This layer is dedicated to milling. If the board manufacturer has to mill oblong holes, you have to draw the milling contour of oblong holes in this layer. Any other inner cutouts in the board are drawn in the same way. Draw the milling contours in this layer. Note that the board outline is not concerned and must be designed in the layer 20 (Dimension).
Layer 47 – Measures
This layer contains the measurement. It is not used during the manufacturing process, it is just display for information.