Good news , you don't have to take it apart or touch any screw, you can fix it even with phone working..
But watch out for the adhesive between layers not to get on actual contacts.
I have got adhesive on contacts and i had to dissasemble the phone, isolate the keyboard and clean all adhesive with isopropyl alcohol. Lucky me.
This guide will cover following problems when pressing the keys:
The key needs to be pressed hard to register
The key doesn't register at all
The key registers two times when pressed hard
You will need a sharp knife.
The problem does not come from dust.
Gently remove the black cover starting ftom right , near that charging port
This is what you get
Gently remove this Layer
Now watch out for this flex that goes under the contacts
Edit: this flex does not seem to affect kb functionaity but leave it there just in case
And remove this layer----this has the actual problem
Now you will see 4 heads in the middle of the metal part that is actually touching the contacts
With a sharp knife press on the center of the metal part to make a small pit/cavity, something like those 4 pits/cavity, but NOT an actual hole.
WARNING dont do this on the phone, this layer must be removed from the phone.
Now you will see that the metal part tends to stay pressed, now you come from behind an do the same until it comes FIRMLY back
Also don't abuse that methal part because in the end the key feedback may differ from other keys.
Now put everything back watching for this flex
Edit: the flexible tab seems to have all the keys connected and i think it has ground (negative polarity) on it to help on key press,
like the situation when that metal touchs only the centre flex contact.
And you should have a perfectly functional key
It might be harder to press, so press once harder and it will level out with other keys.
But.. if u are some of those lucky enough to really mess up that metal part you can always
1.Switch metal parts with keys that you never use.. like menu or search or sym.. there must be one that you didn't touch in the last month. )
2.Borrow that part from retired phones like i did in the following photos
In the last image you can see clearly that i cut that thing from htc vox's keyboard and i glued it there with paper glue
Paper glue will do the job, don't try with poxipol or other hard adhesives for metal wood or anything like that.
Now the key must have the good old feedback.
if the black rubber does not stick or even falls off you can put paper glue on the left and right of all the keys (those two big empty spaces on sides) between al layers,
After i used all those methods i really recommend the last two ones (switch or replace) because it's quite hard to make it have perfect feedback and last for a good amount of time with that feedback.
Dont forget to thank if i helped you .
I attached the images in case they are removed and also posted a link to dropbox below.
you can take those metal clickers from pcb buttons too