Push button design for small gadget

Question

I'm looking at designing a small gadget (think something like a Fitbit in size) where the case has a couple of buttons on it. For various reasons the buttons have to be around the narrow side (edge) of the case. I am fairly lost as to what the standard button mechanical design for this type of application would be. I mean, I know about (much bigger) buttons with a cantilever or a linear spring or spring and pivot, or rubber dome switches, but here... the whole thing it tiny! The largest buttons that can fit are roughly 12x5mm, and the "mechanism" has to fit in a few mm of thickness.

The prototype has oval plastic buttons which are ~12x5mm and 4mm deep, and C&K PTS840 tactile switches on a PCB behind them. These switches have a tiny little plunger 1.7x1mm with 0.2mm of travel that the buttons can hit. In general the buttons can push the plunger just fine. However there isn't really anything keeping the buttons in place or restricting them to move in a straight line, they kinda rattle around in holes in the case and can both tilt / jam at an angle and fall out.

I think this is probably a solved problem for cell phones / smartwatches / whatever, but I don't know what the standard solutions are. Could you please let me know what the major different of approaches are and what their pros and cons are?

P.S. This would be designed for injection molding in the future; PC/ABS material. Wall thickness is around 1.2-1.4mm.

UPDATE I took apart some sports watches; they all have buttons with a metal spring loaded plunger that goes through a O-ring seal and hits a tactile switch. The total height of the mechanism is about 5mm (from the outside surface of the button to the switch on the PCB). That's a good mechanism (and waterproof) but I don't have room to fit something like that; and it probably can't be easily made out of just plastic parts. I'm still looking for a good solution.

Answer 1

You can get switches designed to be soldered onto the edge of a PCB and actuated from the side. It's your responsibility to make the buttons.

Get some cheap electronic watches and tear them down, and maybe some broken cell phones, ditto.

I learn about this sort of stuff by getting onto a distributor website (DigiKey and Mouser in the US) and looking at what they have available. If I'm seriously looking, I'll go from there to manufacturer's websites.

Answer 2

As you mentioned in your update you're going to want to start with a PCB edge mounted tactile switch. These can be quite small. So now what you've got to do is add a button.

• A common solution would be to use a small plastic button with a retaining flange. You've said you're worried about this rattling or jamming, but 1. the travel is very short, so it won't jam. And 2. The button will be so small that the rattling will be inaudible. The buttons in your prototype sound like they were designed with too much linear travel.

• Another option is to make the button part of the case, and retain it using a living hinge. For a cheap product this is probably the way to go, because it eliminates a part.

• A 3rd, weirder option is to make the case flexible enough that the button can be pressed through it. Unfortunately this is not well suited to an edge button.

A flexible sticker or silicone coating can be added over the preceding assemblies to add some water/dust resistance.