parts-man73:
That did it… I had to lower the grid to 10 mil before it did anything though. What is the minimum grid spacing? Would it be the same as the board manufactures min spacing (i.e. 8 mil) ?
Parts-man,
No. The PCB manufacturer’s min spacing is there to improve yields. You may place the parts or traces on any position, subject to the uncertainty (tolerance) of your tools and the board house’s tools. In EAGLE that’s a 0.00001 inch grid. Using a very fine grid would create a huge data base and slow down the computations! Better to use the default of 50mil and switch to, say, a 10mil grid when SMT parts are missed by the auto router or manual routing.
What happens in your case is the ‘hot spot’ used by the auto router to start or end a trace is on the SMT pad centre, which in turn is off grid for relatively course grids of 50mils or 100mils. I’ve run into the same issues as I move from a PTH (Plated Through Hole) PCB world to one with SMT parts (on both side of a two layer board).
When the PCB design standard was based on a 100mils (0.1 inch) grid, and parts manufacturers tooled their products to fit on a 0.1 inch grid, PCB design was simple and had few rules.
With SMT parts, particularly in EAGLE although I’m sure other design environments are similar, the pads can be of varying sizes and not tied to a standard grid.
For example, the common SMT passives built on “1206” size (found in the EAGLE RCL.lbr) have 0.024 x 0.035 pads (SMT1 pads), spaced 0.075 either side of the component’s origin (centre).
The pads in turn have their “hot spots” for the auto router in the middle of the pad, so the co-ords are (-0.075, 0) and (+0.075, 0) respectively.
If the part is placed on a 100mil grid the “hot spots” are now off grid and will be ignored by the auto router (with the default 100mil Routing Grid), even though the part’s centre is on grid.
Reducing the Routing Grid to 75mil (or lower) and the auto router will now see these parts and route them.
Take a few minutes to try this for yourself by starting a new schematic, placing and connecting a few SMT parts and letting the auto router hook them up. If the auto router misses them, change it’s grid from 50mil default (found in the Tools > Auto… > General tab) to, say 10mil.
Another lesson I learned the hard way: When you place the SMT parts in a new board layout be careful to put them on the grid. If you don’t or if they get ‘bumped’ off grid there will be issues with routing and DRC.
An easy way to do this is to type MOVE and the reference designator, say, C1, R1, etc. The part will jump to the grid lines, and then jump from grid line to grid line to be placed with the mouse cursor.
Good Luck and Comments Welcome!
Peter