I am just toying with ideas at the moment, and I am pondering the possibility of reverse mounting SMD tactile buttons in a PCB to form a front panel.
(for those of you who don't know, "reverse mounting" a component is the act of mounting it upside down [some components are designed for this] on the underside of a PCB so it protrudes through a hole in the board.)
I have been looking at various SMD tactile buttons, but I can't find anything that I think would really fit the bill.
The requirements are that it have external legs (not legs that wrap underneath the body), that it would mount nice and flat when inserted into a hole and that the legs are large enough to provide a mechanically sound connection that would withstand pushing the button from the other side.
Tab-size wise, the "normal" SMD tactile buttons look best:
However, the curved nature of the legs makes it somewhat awkward to mount cleanly - it would require further cutouts around the hole to accomodate them.
Buttons with flatter legs tend to have very small and flimsy legs:
So, I am wondering:
a) Has anyone here actually reverse mounted a tactile switch before
b) If so, how well did it work, and
c) Any good recommendations for a switch to use.
That doesn't solve the problem of mounting them cleanly and strongly. And those are through hole. I need SMD. Let me just draw a picture of what I mean...
The "reverse" of the PCB forms a nice tidy front panel. All things like buttons, LED displays, etc reverse mounted so they protrude through the PCB, and silk screen for things drawn on the PCB. All the soldering, components, etc are on the back of the front panel.
That looks like pretty much like a standard tactile switch stuck thru a square hole in a board. Perhaps you should make the square loles in your board, then use a small flat punch to flatten out the solder pins on the tactile switches for soldering to the board.
I don't think we are. Having mechanically actuated parts reverse mounted leaves you liable to some ham fisted operater shoving them thru the board. You know what folks go when they get frustrated - punch the buttons harder, and repeatedly.
Parts that aren't touched, or ultmately hidden behind a bezel, are one thing.
Switches held on the board with the minimal size of mounting pads that hold them is not good long term.
Are you planning on having the face of the circuit board exposed as well? Then you're also looking at the high likelyhood of static discharge being directly exposed to your circiuts, and blown components that way.
I don't think we are. Having mechanically actuated parts reverse mounted leaves you liable to some ham fisted operater shoving them thru the board. You know what folks go when they get frustrated - punch the buttons harder, and repeatedly.
Parts that aren't touched, or ultmately hidden behind a bezel, are one thing.
Switches held on the board with the minimal size of mounting pads that hold them is not good long term.
That's exactly why I'm here now asking for button opinions - does anyone know of anything surface mount that's rugged enough to survive.
CrossRoads:
Are you planning on having the face of the circuit board exposed as well? Then you're also looking at the high likelyhood of static discharge being directly exposed to your circiuts, and blown components that way.
Only "open" in so far as it'll be covered with white solder mask, which is pretty insulating - I mean, have you actually seen white solder mask? It's like a layer of plastic.
And the components on this board would be minimal to support the interface devices. The main gubbins would be on a separate board inside the housing.
Only with through hole components I see, which I want to avoid as it'll spoil the look of the panel. Also, I tend not to use Digikey as the website is one of the worst on the internet. Oh, and only one of them is acually available in the UK in quantities of less than 1000. Plus the shipping cost (unless you spend a £50, which I'm not about to do on just buttons) is extortionate.
Also, I tend not to use Digikey as the website is one of the worst on the internet.
That is total bullshit! They are the easiest to use, best filtering for parts website out there - always my first choice when looking for parts.
Male & female headers, no one does those well except for places that specialize in those kinds of parts, such as http://www.phoenixent.com/. Not sure that helps you in the UK.
As for reverse mounted SMD switches, you may be left with reforming leads, SMT or otherwise, to get parts. How many do you think you'll need?
Also, I tend not to use Digikey as the website is one of the worst on the internet.
That is total bullshit! They are the easiest to use, best filtering for parts website out there - always my first choice when looking for parts.
Oh come on now, you HAVE to be kidding me! With a poduct index page that's about 17 miles long, no way of knowing what the options are that you have actually selected in the parametric search, and hence no way of undoing part of your selection, I would call that completely unusable! It's godawful!
Male & female headers, no one does those well except for places that specialize in those kinds of parts, such as http://www.phoenixent.com/. Not sure that helps you in the UK.
I have my suppliers for that kind of thing, thanks, and they're a very reasonable price. And no they don't "specialiSe" in them.
As for reverse mounted SMD switches, you may be left with reforming leads, SMT or otherwise, to get parts. How many do you think you'll need?
I don't know yet, as this is all ideas at the moment. I'd like to have it P&P-able if possible though, so no manual re-forming allowed.
What does Farnell/Newark/RS offer?
Bugger all really, and RS's prices tend to be a little over the top for my taste. I buy one product from them, and one product alone, because the only other place I can get it at the moment is Digikey, and with their shipping costs even RS comes in cheaper.
I have looked at reverse mounted SMD LEDS before... (but never for switches.. so I'll be curious to see what you find/come up with)..
also.. a problem "I" faced (with the reverse LEDS stuff) was:
1.) how to make the HOLE/window/cut 'legally' with the design rules? to mount the LED (part)? usually need to be 'xx' away from any edge.. parts not covering/outside of the PCB outline..etc.
post back on any reverse mount switches you DO find.
I totally agree that being able to do something like this would be a good way to make a cheap front panel, especially if all the logic was on the reverse side, then the project PCB is also it's own panel.
How many are we talking about? A solution that involves some manual "adjusting" of the legs is ok for a few off, but not for production.
EDIT: Have a look at the Omrom B3U series, they have 3 mounting tabs (two are every large) so the mechanical aspects may be ok. Not sure they would look great though.
The B3U's look like they would be strong enough... until you realise the tabs are only 0.5mm x 1.4mm in size. The entire button, tabs included, is 4mm wide, and the main body area is 3mm wide, leaving 1mm for both tabs.
A bit on the small side... If it were only bigger it'd be pretty much perfect.
Here's a little device Dave Jones (EEV Blog) has made that uses reverse-mounting as a front panel. He only really has an LED as a proper reverse mounted component, and slide switches and terminals as through-hole components:
I guess I could settle for through-hole if they were nice enough, and the leads hidden under the body. I would be very low volume initially, and I guess it wouldn't be too bad having them part assembled for larger runs, and then manually solder a couple of buttons.
So plan B: some nice through-hole buttons with concealed legs that would look nice exposed on the surface of a panel...?
I suppose I could just scrap the whole reverse mounting idea for buttons if the buttons are tidy enough. I have just found something which might do admirably. It's surface mount, not through hole, but most of the contact is underneath the body with only a small tab to each side. And they are available in red, green, grey and white:
The Omron B3AL1000P, about 6mm², the price is reasonable, and the reflow profile is within my abilities to conform to. I could compromise for those buttons I guess.
What about things like the NKK JL series (http://www.nikkaiswitches.com/default.asp), they are PTH with no visible legs. I can't find them on their useless web site but if you look around you may stumble across them (no bloody search facility, WTF?)
Hi, looking at the diag with reply #2.
I see that you need a tactile switch that will mount in the hole and solder up under the PCB.
Although you can get ones with tabs that allow this way of mounting them, the problem is the strength of the switch because it has no backing, most tactiles are designed to have back support.
So is there a problem with you backing up the PCB with another blank PCB with some thin, ie 1mm, cushioning material to make up for the irregularities in the back of the switch.
This way you have the switch mounted low in the front PCB with a rear PCB to provide the support the switch was designed for?
I have had to do this with some temperature controllers that had to be refurbished and the slim profile tactiles have not been available.
