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Good button over tact switch design

Page history last edited by Ramon Alarcon 13 years, 5 months ago Saved with comment

Initial question

When designing handheld products that use tact switches, I usually specify a two part button construction. The typical construction comprises a hard plastic button shell and a 2nd shot TPE that actually interfaces with the face of the tact switch. The TPE tends to give a good feel and minimizes rattle.

I'm working on a new project now where the molder wants to make the buttons in the client's product using a single shot of ABS. Before putting my foot down and saying no, I want to check the collective wisdom to see if I've just grown too accustomed to doing it my way. Can anyone think of successful examples where they've seen single shot, hard plastic buttons that press against tact switches. By successful, I mean good feeling and no rattling.

 

What we actually wound up doing and how it worked

TBD - Will update this section when a design has been chosen and results are available.

 

Responses

Response #1

From what I know, we've done the majority of our button designs with a single shot plastic. Usually a spring cantilever is designed in to keep the button up against the coverset to minimize rattling. Tolerance analysis and clever thought to minimize the tolerance gap is needed. It seems to work pretty well. We've also done it the opposite way--preload on a switch that is below the force of actuation. Tougher to do, but prevents the 'double click' feel you sometimes get.

I don't know that we've ever done an additional compliant material in between (via second shot or otherwise), though I can imagine it would give a different feel. It seems like the compliant material would need to be preloaded, thus some force on the switch, so maybe it is similar to the second example I gave. It seems like you may feel spongy as you compress the compliant material enough to then transfer the actuation force.

I know what we usually get anal about is making sure the user force doesn't bottom out on the PCB, but bottoms on the plastic/frame/etc.

This sometimes is tricky given some of the minimal travel of microswitches...often we end up supporting the PCB directly behind the switch to take the load.

 

Response #2

Your approach works well. But it also builds cost & tooling complexity into each part, which adds up, especially if the qtys are high. There are nice tact switches with rubber actuators, which solves the rattling & fit problem. These can be used effectively with simple molded plastic buttons. The correct way to develop the right geometry for the button is to design the button intentionally short (.25-.5mm or so), then fit develop it once T1 tooling is complete. This adds time & cost to your manufacturing ramp, but it is only a 1 time cost. By going through this process, you eliminate the cost of the double shot part in production. 

You should be able to see this in combination of plastic button & tact switch with rubber actuator on many consumer products that use tact switches. Disassemble a few & you'll find it quickly.

 

Response #3

I think your concerns about clicking are well founded. You need compliancy somewhere in the system to take care of this. One possibility would be to add a piece of very low force poron foam in the system... Depends.  But generally speaking, I think you need something to keep the button pressed against the tact switch without activating it. Tricky without something soft.

 

Response #4

You eliminate rattle by preloading the switch or dome - given the typical throw is below 0.2mm it calls for a tight tolerance fit and more care than most suppliers know how to muster. Can make for a highly responsive feel though.

 

Response #5

I accidentally lost the text of this response, but the suggestion was to mold a single part using a polypro resin or a polypro/TPE blend.

 

Response #6

I've designed many such buttons and never used a TPE "cushion" except once when I used it to reach the pcb switch ~ 1" away. I do recall it reduced the audible click and had a nicer feel. But normally I heatstake the button to the bezel etc. and locate the switch as close as tolerance will allow below it and have never experienced rattle. If you are set on TPE your could mold a separate TPE part, add locating posts that fit into pcb holes and fasten it by sandwiching it between the pcb and bezel etc. Or you could pick a tact switch with a long plunger and slip a molded TPE "sock" over it.

 

Original author: Ramon Alarcon (ralarcon@stanfordalumni.org)

Date: 1/4/2011

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