Kicad: Improved Magnetism

The magnetism patches recently posted to the kicad-devel list add "magnetic" vias and traces. Here are a few examples how magnetic traces help to make tidier layouts when off-grid traces are involved (which is pretty common with SMT components). All examples are with 10 mil traces and a 25 mil grid.

Join an off-grid trace

We first connect the 0603 component on the left. Then we try to add the one on the right to the trace. Without magnetic traces (left), we can't hit the center of the trace if coming from the 0603 chip (unless we adjust the grid), so the best we can do is run a new trace from the pad of the TQFP chip, and then branch towards the 0603.

With magnetic traces (right), we can just draw the trace from the 0603, and get a magnetic lock as soon as we touch the trace.

Come from an off-grid pad

In this example, we want to join an on-grid trace (between the 0603 component and pad 1 of the TQFP part), but our origin (pad 8) isn't on the grid. Without magnetic traces, the trace veers off to meet the grid and we get a little acid trap.

Again, with magnetic traces, the new trace snaps right to the center of the old trace at the y-coordinate of pad 8.

The symmetric Y

32 pin parts of the ATmega48/88/168 family have GND on pads 3, 5, and 21. It's convenient to connect them with a "Y" structure under the chip. Unfortunately, it's usually not possible to make this symmetric. The best we can do is to run traces from pad 21 to pad 3 and then from pad 21 to pad 5, and try to make them branch at roughly the same location.

With magnetic traces, we just make the first connection out of pad 21, and then draw a trace from pad 3 or 5 and snap to the corner, getting a perfectly symmetrical "Y".

Start from off-grid traces

Starting from off-grid traces is also difficult. This case is not very common, but it could occur in a scenario like the one below: we start with a trace between the two 0805 components, from which we'd like to draw another trace (in the middle). Note that a direct connection between the two 0805 components doesn't touch a single grid point.

We can bring the first trace to the grid, but this looks a little messy and also requires us to draw two traces for the connection between the 0805 components. With magnetic traces, we can just start right from the middle of the trace, obtaining a perfect "T".