Membranes: "sealed" vs. "open" contacts?

[keyb_gr]

Confused n00b alert!

I am somehow under the impression that membrane-based keyboards may be built with different kinds of contacts.

First there are conventional rubber dome 'boards, which have graphite coating on the rubber dome to make contact ("direct contact") - that kinda screams "wear!!!1" to me (provided the manufacturer does not have the expertise to keep the graphite there and/or decided to skimp on gold plating, and the number of rather short-lived rubber dome 'boards indicates that this is not uncommon).

However, there also seem to be other variants which basically work like membrane keyboards with stuff on top to apply pressure in the right spot - buckling spring and Cherry FTSC ("full travel sealed contact") apparently belong to this camp. Here the contacts are not exposed. It still took some research to make this reliable (as indicated by old IBM papers) but the better-known implementations seem to hold up very well with use.

Can anyone shed some more light on this? I'm all ears...

[huha]

I've hardly seen any rubber dome keyboards with exposed graphite contacts. I don't know for sure, but it might just be cheaper to make two plastic foils with traces printed on them, one with holes and plain rubber domes instead of one plastic foil with traces and rubber domes with graphite coating.
In terms of wear!!!1, I'd say that's somewhat of a no-brainer with rubber dome boards; the domes themselves begin to feel bad soon enough.

-huha

[Ulysses31]

My very old rubber dome board literally has a dome under each key, like an upside-down cup.  Bonded to the inside of each cup is a small black conductive rubber disc, which shorts two traces on the PCB when the key is pressed.  For whatever reason this conductive rubber disc wears out, though you can't tell by looking.  You can buy conductive rubber liquid in vials to repair these things, if need be.  From what i've read, most so-called rubber-dome boards are not like this though.

[lowpoly]

First there are conventional rubber dome 'boards, which have graphite coating on the rubber dome to make contact ("direct contact") - that kinda screams "wear!!!1" to me (provided the manufacturer does not have the expertise to keep the graphite there and/or decided to skimp on gold plating, and the number of rather short-lived rubber dome 'boards indicates that this is not uncommon).

That's not a membrane board. Current rubber domes use membrane technology (no pcb required). Today conductive rubber is used for chiclet keyboards, think TV remote controls. Of course there are exceptions.