I do it all the time..
It takes some serious effort to stack it all up so the LCD bezel is fairly flush and the buttons are precisely positioned.
Ultimately you need the button tops about exactly flush with the outer face of the panel. The switches need to be short throw tactile type with the typical 200~450 force constants.
For an overlay the only functional company I've ever found is [link markingsystems.com]markingsystems.com[/url]
Overlays are VERY nice but they aren't cheap in small quantities. You're usually looking at about $500 for a 100 pieces.
If you're using the product in normal low use mode the standard tiny button tops are fine thru a real overlay. If you're talking heavy use, then I suggest you chose a button with optional button caps and use a bigger snap-on button cap to spread the forces translated thru the overlay to the switch to reduce the chance of imprinting the overlay. You need a generous hole in the panel face to allow the overlay to deform enough to reach the button stroke length. This means for a "no cap" button about a 0.75" hole or larger. It's ok for the neighboring holes to overlap. If you use button caps you'll want a bigger hole yet.
You measure the from the top of the button to the top face of the board. Subtract the panel face thickness and that leaves the standoff distance needed to fix the board behind the face panel. Often you will need a stack-up of a standoff and a nylon flat washer to get it right with OTS components. Places like Mcmaster-Carr have lots of different washer thicknesses to get the correct one.
Once the buttons/panel/standoff aspect is fixed and complete you then turn to the display. Use whatever standoffs/washers you need to place the face bezel into the LCD cutout in the face panel. You don't want it to touch the back of the overlay. Give it some space 30mil or so. It's not critical so that's why its setup follows the critical button placement setup.
The hardest part when using an LCD is getting the buttons to have actuators that are tall enough to put them beyond the LCD.
Remember when you apply a 'real' overlay the instant its adhesive touches something that's it. There is no do-over in any sense of the word.
Oh, the standoffs to hold the board behind the face panel are normally Pemstuds of the appropriate length and screw size to do the job. This provides a bumpless face on the product for the overlay. You want just studs so all stand-offs can be inexpensive unthreaded nylon types you just drop on the Pemstuds and follow with the board then Kepnuts.
Keith Cress
kcress -
