Curved or 45 degrees for Cap Sense traces?

[timb]

Hey guys, anyone with experience routing Cap Sense traces?

I'm working on a board that has a large number of buttons connected in a matrix configuration (4 rows * 8 columns). I've followed Cypress's Cap Sense design guide for the PCB, however there's something I'm uncertain about...

In the aforementioned design guide (in addition to app notes from TI and others) they recommend to always use round-rect, circular or oval pads for the buttons themselves; they say sharp corners (like on squares and triangles) can cause issue with the capacitive fields.

I've got the traces routed using standard 45 degree bends, now I know that's not the same as a 90 degree corner, but it's still a lot sharper than a circle!

So, should I reroute using curves? Or will 45 degree bends be alright?

I've never routed a Cap Sense board before, so maybe I'm overthinking this. I figured I might as well check, since it's bugging me.

Thanks!

[NiHaoMike]

You want to minimize stray capacitance to ground and other touch traces. I don't see the shape of the turns being significant. (Most touch controllers operate from a few tens of kHz to a few MHz.)

[Dielectric]

45-degree bends should be fine, I've never bothered with actual curves on the sense traces.  CapSense is only a few hundred kHz.

Everything else will hose you up though.  Double-triple check your ground return path and make sure it's solid all the way back to the PSoC.  If it goes through more than one via, you've messed up.  Same if you're using shield.  Use shield around the keys and along the sense traces if you can, and enable the shield on sensors too.  I like shield

[timb]

45-degree bends should be fine, I've never bothered with actual curves on the sense traces.  CapSense is only a few hundred kHz.

Everything else will hose you up though.  Double-triple check your ground return path and make sure it's solid all the way back to the PSoC.  If it goes through more than one via, you've messed up.  Same if you're using shield.  Use shield around the keys and along the sense traces if you can, and enable the shield on sensors too.  I like shield

Thanks! I ended up looking at some of their demo boards and they all use 45 degree traces. In retrospect it makes sense.

And yes, I've got shield coming directly from a the PSOC, through 4 pins of a 20-pin low profile mezzanine connector and off that connector directly into a group of vias that takes it to the top of the board.

The top of that board is completely open with no traces, allowing the shield to completely surround all buttons.

Here's a PDF of the layout. (Note: Layout is magnified by 400% in that PDF; actual board dimensions are 95x65mm.)

I'm using the "Pie Slice" style circular buttons (that Cypress used on their Matrix Buttons demo board) for the main keys. I wanted smaller round-rect buttons for the less often used stuff, so I rigged up them up based on something I saw in a non-Cypress app note. (You have *no* idea how hard it was to create round-rect pads in DipTrace. Seriously.) Finally, the four function buttons above the LCD cutout are non-matrix connected (which ended up being a lot easier than trying to route four columns up there and then back; I have a version where I did that, but it's not ideal).

Anyway, comments welcome!