I came across a PCB, and I found a silver strip behind the crystal. What is that strip? Has anyone used it?
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I wish I could find an image of this, but that is a ground pad.
When mounting larger crystals, like the one shown in Peter's answer, it is normal to bend the leads so the can is laid flat on the board to limit the height of the board envelope. (Note it does little with the crystal shown in your image.)
Unfortunately, when mounted flat in that position the thing is rather easy to move and will catch on things and bend up, eventually breaking the leads in extreme circumstances. The thing will also vibrate in a mechanically noisy environment.
As such it needs to be tied down.
Note the two holes on either sides of the pad. It is common to see a strap, made of an offcut of an axial component lead, bent to fit over the crystal and soldered into those holes with a further dab of solder where the strap touches the case on top to form a good ground for the case. Other times you may see a single hole at the top of the crystal for a single wire post or hook instead.
The reason for the big pad itself is a little less obvious than it first appears. The can on crystals is normally isolated from the pins, and as such it is prudent to solder the case down to a ground pad. (Why they don't have three pins to make it easy to ground the case has always eluded me...)
However, why the pad needs to be as large as the can is not so clear.
Crystals don't get hot so it is not for heat-sink reasons, and since the crystal is already enclosed in its own little Faraday cage, when grounded, E.M. radiation reasons don't make much sense either.
Personally, I think the large pad is more to prevent you from running other signals under the crystal. Doing so with a poorly grounded can could interfere with the crystal oscillations.
The white silver pad is for bending the crystal to side and soldering the same so that it also gets grounded.
Now there are multiple reasons that I have come across, some of which are:
You can get more information on these links: