- Published: Friday, 20 June 2014
- Written by Jon Chandler
- Hits: 4499
It's desirable that connections to pads on a circuit board are correctly aligned to the center of the pad. A few tips follow to aid in doing this.
Difficulties arise when pads are not on the selected grid in Eagle. This can happen when mixing parts with imperial and metric units or when it's required to position a component to align with a panel or another board.
The illustration to the right shows the ideal case for a DIP package. The pad pitch is 0.1" and the grid spacing is 0.05" as shown by the faint gray lines. The traces intersect with the center of the pads.
The figure below shows what happens when pads aren't on the grid. In this case, there's a DIP package with 0.1" pitch pads and a header with 2mm pitch. The grid is set to 1mm and the header is aligned to the grid. The connections to the header align with the center of the pads.
The connections to the DIP package don't align well. Every other connection is so far off, the trace connects to the edge of the pad. This can result in clearance problems, particularly when traces are run between pads, and a weak point where the trace and pad intersect.
Fortunately, this problem is easy to remedy. If we start laying out our trace from the pad not on the grid, it will start from the center of the pad, and the trace will stay on the same line in either the horizontal or vertical direction until the direction is changed. Start from the non-aligned pad, and head towards the aligned pad and things will work out. The bottom traces in the illustration below show this.
This becomes an even bigger problem with the close pads on SMT components, as shown below; some of the traces end up shorting adjacent pads. Starting from the non-aligned pad works here too.
When using square packages like the TQFP shown here, it's frequently useful to rotate the package 45° to gain more room for routing. The above trick doesn't work for 45° traces, so we quickly end up with a mess!
We need a different method to deal with rotated packages and 45° degree traces. In this case, the pitch of the TQFP leads is 0.8mm. If we resort to that nearly forgotten high school trigamometry class, we can multiply the pitch of the leads by the sin of the angle to define a new grid spacing such that our rotated pads are on the grid:
0.8 × sin(45) = 0.566
If we reset the grid to 0.566mm long enough to lay out our connections to the TQFP, then reset the grid to our normal selection, we can use the trick above to make smooth connections.
Look at the grid pattern above and you'll see the 45° traces align perfectly with the center of the pads. One detail not shown in the illustration above is that the chip must be aligned to the new grid for this to work – the crosshairs at the center of the chip must be aligned to the the grid. The easiest way to accomplish this is to use the I tool to position the chip at (0, 0) and then drag the chip where you want it with the move tool.