Proto services for keyboard membranes? - Page 1
Forum section: electronics / projects

author: Kremmen
author_rank: Super Contributor
timestamp: May 20, 2012, 07:40:36 am
content: The header realy says it. I have a CNC refurbishing project in need of a couple of different control panels. While physical pushbuttons and switches are nice, with the number of switches needed i would hate to invest like $40 per good quality Omron guard button. So what i am looking for is same kind of service as for proto PCBs, only for keyboard membranes. Any sources would be appreciated.

author: yanir
author_rank: Regular Contributor
timestamp: May 20, 2012, 12:30:44 pm
content: I'd like the same! I've been searching for some time and haven't found anything comparable to the real thing in terms of quality or a complete assembly. I need one for a project too. I was shocked at the cost of a complete assembly tooling costs.For prototyping I've found two options:1) 3d print the silicone keypad using a polyjet 3d printer (in a similar material)Pros: cheapest option if you have a custom design (will cost you hundreds not thousands)Cons: not in the final material, no conductive pads on the bottom to close the circuit (if you need them for your design), polyjet lettering won't look as sharp as final screen printing or laser marking process.2) rtv mold. Will produce a production quality silicone keypadPros: real high quality keypadCons: no conductive pads, non marked, need to pay for rtv mold.IMHO if you just need functionality with a decent appearance the first option is the way to go. The second option is best for real production quality prototypes and the mold can produce low volume runs.Third option is to buy a silicone keypad already produced and design your project around it.I'd really like to know if other people have different solutions to this.Yanir

author: Kremmen
author_rank: Super Contributor
timestamp: May 20, 2012, 12:37:26 pm
content: Thanks for your answer. That is about what i have been able to find out as well.Since i am free to implement what i want, i would be happy with an alternative to put discrete tactile pusbutton switches on a PCB and just have the overlay membrane done.

author: yanir
author_rank: Regular Contributor
timestamp: May 20, 2012, 01:00:20 pm
content: Quote from: Kremmen on May 20, 2012, 12:37:26 pmThat's what I'm doing for my prototype, I've got low profile poly dome switches on the board and I've got a polyjet keypad being made up to overlay on the poly domes. I'll let you know how it goes when I get the parts.For final production I may change the design as volume dictates.

author: free_electron
author_rank: Super Contributor
timestamp: May 20, 2012, 02:07:01 pm
content: Buy sealed switches from MEC , they are in your area .. ( denmark ).Pcb mount , cheap , lots of shapes. And singe you have a cnc you can mill the frontpanel. Look at controlmec and multimec families

author: Kremmen
author_rank: Super Contributor
timestamp: May 20, 2012, 04:35:09 pm
content: Yes the MEC switches would be OK as switches. My question however is more about the overlay membrane and that question was not answered at MEC website. I don't particularly need hig IP class switches since the assumption is that the membrane overlay will isolate the switch board from the ambient.There will of course be an engraved panel, but the key membrane is intended to have the keys and legends printed on. That is the service i am looking for.

author: ciccio
author_rank: Frequent Contributor
timestamp: May 20, 2012, 05:04:47 pm
content: I think that the simplest and least expensive solution is to use standard low profile switches soldered on a PCB.When designing the PCB, panelize it by adding another piece of FR4 to use as an overlay, with holes corresponding to the switches pushbuttons and with holes for spacers of the correct height.The correct height is when the outer keyboard membrane (see later) exerts a slight pressure on the switches' button.The correct hole diameter is about 1 cm, depending on the flexibility of the membrane's material (the more elastic it is, the smaller can be the hole diameter).Then what you need is an overlay membrane printed on a foil of polycarbonate or polyester, and glued to dual adhesive tape to the overlay panel.With a modern ink-jet printer, you can get very good results when printing on the back of the polycarbonate or polyester foil. You do not need to cut holes in the adhesive: the button's top surface will always touch the membrane's back side, so the adhesive will give no problems.If you have some height issue (because of other components (displays or whatever) installed on the switches' PCB), you can even "play" with some extra spacers between switches and membrane.I don't have samples here at home, but if required I'll be happy to take photos of some items I have in the lab.

author: Kremmen
author_rank: Super Contributor
timestamp: May 20, 2012, 07:31:26 pm
content: @ciccio: Thanks, this is exactly the end result i have in mind. Now the only thing is i don't want to try doing the actual membrane myself because a) i don't have a suitable printer and b) i don't have the proper materials and i have no idea where to get them.Basically the end result should be at least as professional quality as a Chinese proto PCB is if talking about PCBs. So it doesn't have to be the best in the world but it should be representative quality and done using the methods normally employed for such things. Maybe the attached picture illustrates what i am looking for.

author: SeanB
author_rank: Super Contributor
timestamp: May 20, 2012, 07:43:17 pm
content: So basically you want a self adhesive cold laminate. You print the key legend on a transparency ( if you want to have a LCD display without making cuts in a paper sheet) or on a photo paper ( good ink quality and good non yellowing properties) and have it cold laminated with a film that has a self adhesive backing.You can leave cutouts in the paper ( or use an ohp transparency as this gives a better result but the print is poorer) if you have a display to show though it. It will give enough to operate most pcb keys.Try a art supplies or graphical printer for the laminating, it is easily available and works for a prototype before you have it done in bulk.

author: Kremmen
author_rank: Super Contributor
timestamp: May 20, 2012, 09:04:29 pm
content: Yes basically that is what i want. I was kind of hoping someone knew a place to have all that done to order. But looks like it will be a DIY task after all.I mean it just seems curious that getting a PCB done is dead easy. Need a front panel? No problem there are numerous shops, some even providing the CAD software to draw them in case you don't have your own. All kinds of materials and color fills and whatnot. But membrane keyboards that are all over the place otherwise - zip.

author: mc
author_rank: Regular Contributor
timestamp: May 21, 2012, 12:08:10 am
content: I spent some time looking at just this a few weeks ago.Using a matrix of tactile buttons as per ciccio's description, then printing your own seems to be the most cost effective method for small runs.For creating the membrane, my plan revolves around printing the design onto a transparet sheet, then sealing that sheet into a laminate pouch.However I've been wondering lately if splitting a laminate pouch into two bits, printing directly onto the adhesive side of the laminate, and then rejoining/laminating them would create a thinner/more flexible panel.

author: ciccio
author_rank: Frequent Contributor
timestamp: May 21, 2012, 10:25:32 am
content: I'm sorry but, due to the great chaos in the lab, I could not find any suitable example.The photos are of a production unit, using a PCB with RAFI illuminated tactile switches (they are really expensive).The panel is milled with a CNC router, but could be replaced by a piece of PCB material.The holes are square to accommodate the switches, that are rather large, but could be round if the switches are smaller.The membrane is actually silk screened polyester, but we built the prototypes using a good transparency film, for use with overhead projectors printed by a standard office Canon ink jet printer.Dual side adhesive film is available from hobby shops or arts supply shops. Buy the best available, press it very well in place using some tools to apply force (a small spoon is suitable, if you do not have other available) and leave it sitting for some hours before applying pressure on it (these are "trade secrets" that a supplier taught me years ago: the adhesive surface is formed by small spheres of adhesive, that mus be broken by applying high punctual pressure, and the adhesive must "cure"for some time)The dual adhesive film is cut with a CNC router, but for the prototype we glued it to the panel, cut the holes with a standard blade cutter, then take away the outer protective paper and glued the printed film.The printed film has the habit of being cut by the hedges of the panel's holes, so it is a good idea to chamfer the hedges using a big de-burring bit, or with a small file and a lot of patience).Best regards