Key Switch Footprint

A project log for Aerodox

[WIP] Keyboard based on Ergodox. But wireless?

simon-merrettSimon Merrett 05/11/2020 at 01:222 Comments

I think I may have managed to make a key switch footprint for the Kailh hot swap PCB inserts/adapters. 

The reason I want to be able to hot swap my key switches is because I may not want to keep the first ones I try (probably box browns). Here's a footprint which I think is reversible and will save time during layout.

The only downside I can see at this stage is that the diode symbol could be confusing. Bear in mind that if we're looking down on the PCB, the hot swap insert is connected to the bottom layer (green) and the diode direction we want is in the bottom silkscreen (magenta). I did this because the surface mount diodes are likely to get reflowed on the same side as the hot swap inserts. Obviously, if you are placing a through hole diode, you are less constrained by the reflow, so you may choose to hand solder it on the other side of the PCB, in which case you still need to heed the diode direction on the bottom layer, not the top layer. The rule is "correct diode direction is on the same side as your hot swap insert pads".

Here's the 3D view of the front 

And here's the back

There are only two "user" pins in this footprint - top left rectangle pad and bottom left "D" pad. Note that none of the pads down the bottom are PTH. They are copper annuli and non-plated holes, which adds another layer of complexity for the through-hole-diode constructor but achieves the symmetry required to allow the PCB to work on both sides. It was painful and I needed to use custom pad shapes in the footprint editor, as well as putting the soldermask and paste layer apertures separate to the main pads.

I thought I was done and then I realised that the mirrored pads that weren't being used would be expecting joins in the ratsnest, so I added more traces and makeshift vias (PTH with same pin number and without soldermask). This one is trying to not get in the way of the LEDs that are in some footprints. 

Finished footprint (for now)

I'll see how it goes when I try and lay the board out. Schematic next.


peanutman wrote 08/21/2023 at 11:09 point

Cool idea! Has this worked out for you in practice? 

A lot of the other reversible footprints force you to place the switch upside down on the other side, so I might want to try something like you did.

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Simon Merrett wrote 08/21/2023 at 12:54 point

Yes, although I recall I may have had an issue with the orientation of the diodes - definitely check the silkscreen orientation with respect to the way you set your matrix up.

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