Looking for a switch

I would hope that such devices do not exist, because they would be doomed to fail mechanically.

SMD means you have only the solder joints to keep your part in place. For the setup you're looking at that would mean every push of the button exerts exactly the kind of force solder joints to PCB pads really can't stand: shear-pull away from the board. That mechanical setup practically begs for the pad and a piece of track to be torn loose off the base material, leaving your whole button dangling in the wind.

SMD push buttons exist, but there's a reason they always go on the front, where the push force can be passed via the back of the switch housing directly to the PCB, without stressing the solder joints.

yep reverse mount smd wouldn't work unless they were glued down or something

you can get reverse mount:

-Lasse

I have seen transformers mounted with part of the unit protruding through the board, but still soldered on the top side. Maybe a switch like this exists, it recesses into the board from the top, not stressing the solder joints but is very low profile on the top side.

Rick

Personally if the switch is so close to the outside, that you need to save height, I would be looking at touch sensors anyway, just areas of PCB track.

But those come with their own can of worms attached that's bigger and heavier than an entire PC keyboard's worth of conventional push buttons.

SMD push buttons are routinely below 4 mm in height, and can be had down to 1.2 mm. Rubber-mat-with-conductor-pads type buttons are of similar height. If you can fit neither of those, then your PCB is entirely too close to the front panel ;-P

You could use the Forth methodology. If the problem is too hard to solve, solve a different problem.

Rick

my immediate guess is that he wants to use the pcb as the front panel

All components on the back side, button actuators protruding through holes in the PCB, add some reverse mount leds silkscreen what ever text needed

-Lasse

I saw a patent for a tact switch like this, but it was not listed as being assigned to anyone.

You didn't mention if you needed push-push or momentary. For momentary, how about a snap dome, they are pretty low profile. I've used a few from snaptron on prototypes and I think Omron also makes them.

Close. Using the PCB as an end panel for an extruded enclosure. Small LCD mounted on the inside pointing out through a hole. Switches mounted on the backside poking through holes, laser cut Mylar overlay on the front.

Hans concern about the switch lead fragility is something I haven't thought about and needs to be considered.

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I think that's less a concern about the fragility of the lead itself, and more a concern about the connection between the pad and the FR-4. I think you'll wind up with a perfectly sturdy solder joint, still connected to the pad and a dangling bit of trace that's ripped clean off.

What about tactile dome switches? Some of those can be had at only about a half millimeter.

I've seen problems with surface mount connectors coming loose from PCBs when used frequently. I've seen both the solder joint come loose and the pad pull off the PCB. Solder is not really all that strong if subjected to repeated stress. It seems to crack easily. On my designs

Rick

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if you add an overlay anyway maybe just some pads on the pcb and these?

-Lasse

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My experience has been that the solder suffers from stress fractures.

Exactly. A solder joint consists of three regions of material: the copper pad on one side, solder on the other, and an alloy of the two in between. Now here's the problem: that alloy is _brittle_, and weak. It'll crack under bending or shearing loads much more easily than either of its constituent materials. It's also seriously vulnerable to Varying stress (i.e. vibration, or repeatedly applied stress).

In a nutshell the OP's design would be setting up a race between two failure modes. The only question would be which of the two joints will fail first: pin--solder--pad, or pad--glue--board. But it's pretty damn certain that one of them would fail.

If you want to make the joint even more brittle, such that dry joints appear over time or temperature cycling. Add Gold !

Yes soldering onto gold plated contacts lifts gold plating into the solder making the solder even more brittle.

A lot of avionics and military specs for components specify parts to have a solder finish, NO Gold. If you do double desolder on parts you have assay the solder used to remove gold to calculate the full amount of gold has been removed.