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Pictures shown below is a typical 32.768kHz crystal, commonly use in Real Time Clock circuit (e.g: DS1307 & DS1337)

Pic #1Pic #2

Referring to an earlier question posted here, it is a good practice that ground planes should be beneath the crystal. But is it compulsory that we should ground the crystal's body/case too (like what they did in these pictures)? And if yes, what happens if we didn't grounded the case?

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  • \$\begingroup\$ what happens if by mistake the body acquires some static charge from somewhere and touches somewhere on spots on this or other pcb????.......... \$\endgroup\$ – perilbrain Oct 4 '12 at 18:22
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One reason may be more mechanical than electrical. Crystals are mechanically vulnerable, and may easily be damaged by shock and (ironically) vibration. Fixing the housing to a larger mass may reduce the effects of those.

Note that, while most likely there will be electrical contact between case and ground in the second picture, I would not rely on it without adding a solder blob at the point of contact. Check it out: hold your DMM's probes on the case to measure resistance. You may have to slightly rub over the surface to see the expected 0 Ω.

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I suspect that this may be to short out a parasitic capacitance between the crystal and other parts of the circuit that could affect the frequency of the crystal.

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The can is not all that much heavier than the crystal inside. If mechanical energy gets wasted shaking a comparatively responsive can, the resonance quality will be quite worse and the leads and solder points will get more of a strain than desirable. Tieing the can down makes sure that the electrical energy is just spent on making the crystal vibrate: the whole board has enough mass that it can be considered fixed at frequencies of 32768Hz.

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    \$\begingroup\$ You didn't actually answer OPs question on whether or not grounding the case is necessary and why/why not. \$\endgroup\$ – I. Wolfe Mar 24 '15 at 16:16

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