Ground plane under crystals

Should the can on a crystal be tied to ground, or left floating?

I usually see ground planes under crystals, sometimes with solder mask, sometimes intentionally bare/tinned. If the can should be grounded, it seems sloppy to rely on contact instead of a joint, so why the bare pad instead of solder mask? At

I've seen crystal oscillators stopped (temporarily) or staggered by noisy switch closure or other randomly pulsed EMI, when the case was not grounded. I ground them, even if just with a solder blob - this is easier if the ground plane has no mask.

RL

I don't know yet. I strive to ask "why", rather than ignoring a common practice, or copying it blindly.

It would seem to have benefits either way, but casual contact seems very haphazard and soldering the can doesn't seem right. I suppose I'll get the flavor with the ground pin and remove all doubt about whether it is / should be grounded.

Thanks to all for the comments!

The reason that I put it there, is to form a shield for the crystal leads which run back to the processor. On my boards, you'll see that this ground under the crystal is an isolated finger, and touches nothing except the crystal caps, and the nearest uP ground pin. From there, it joins system ground. I normally don't connect the cans to the plane, in fact I use insulators under the cans to make sure nothing makes any unapproved connections.

Indeed - I'd forgotten about that, and a couple boards I've just grabbed from the junkpile also tie this strap to ground.

The particular crystals I'm using are low-profile cans, so they install vertically instead of bending the leads. But the strap-around-the-can seems to endorse the practice of grounding the can.

Thanks!

Interesting. I presume you've got some pics on your site, so I'll go check this out. Thanks!

OK, I'll bite - why do you want the can floating instead of grounded? (What insulators do you use? And in addition to the solder mask, or only on bare boards?)

I'd think grounding might help EMI emissions, but it seems you're concerned about causing circuit problems? Perhaps when the "noise" being grounded is 0 or 180 degrees to the crystal signal? (Speculating here... my knowledge of crystals and harmonics is limited.)

Thanks!

"Richard H." wrote

Grounding the can is a generally good idea -- as it is a bad idea to have a large metallic object that is not tied to a known voltage.

The extra pin also serves to hold the crystal in place. An unsupported through-hole crystal will otherwise put a mechanical cantilevered load on it's leads when the board is subject to vibration/getting dropped.

A bare-wire strap around the crystal is often used in lieu of the third pin.

I don't know of any reason for a ground plane under the crystal. TTBOMK it is a flourish added by the layout guy. It also saves on etchant.

I have never done it and recently, one of my boards was tested for IEC60945 (very stringent EMC test). It came out right. Millions of receivers/transmitters have been produced in the past were crystals were plugged in sockets. No ground connection. Many of the boards I see have no ground connection. The only ones that have, are the ones where the crystals are mounted horizontally. Here it is just done to prevent them from moving under high G loads. Oh, and SMD crystals do not even have the possibility for a ground connection.

And based on the nature of a crystal (low frequency compared to it's simensions and high Q) I'd like to thing they don't radiate at all.

Meindert

All excellent points. Evidence suggests that bare pads under crystals and/or grounding of restraining straps may be overkill, precautionary, or maybe just superstition. And some folks go out of their way to avoid it. Very curious.

In the end, this is probably the key - whether these emissions are even significant, to the circuit itself or for EMI. And with SMD crystals, there's really no question about how it should be done.

Thanks!

Soldering it makes for good mechanical stability anyway.

PW

That's actually what led me to start wondering here, after a recent batch of boards that didn't have solder mask under the can.

With solder mask, are the extra insulators really needed, or do you work much with bare boards? (i.e., concerned that vibration would damage the mask underneath?)

Again, maybe a moot issue, or a bad example I've run across... on a recent board, I happened to probe the can with my scope and noticed a lot of signal on it. Apparently it's not strong enough to be an EMI issue, but it got me to thinking about grounding the can.

BTW, I didn't find any images on your site that showed your approach to crystal ground pads. Can you provide a link?

Thanks!

I don't know where we get them, but they are little sheet insulators with holes for the leads. I don't want the osc pads shorting to the can. That would make for a lot of antenna, I'd think. I'm not worried about the can itself. The signals on the two leads are nearly the same amplitude, and 180 degrees apart, so in such a small area, they should cancel nicely.

Noise?

Hmmm. Good point, above & beyond all the discussion about grounding.

I've wanted this for other reasons, and it seems to be the one thing I'd like that Eagle doesn't do yet... for some reason, it makes THT pads the same size on top and bottom.

Thanks!

For typical low-profile through-hole crystals, I use a keepout larger than the crystal can in the footprint so that no copper ends up under the crystal itself (on the top layer) other than the pads. I like to see a ground plane under the crystal on some other layer. The mask would "probably" be okay if it visually looks okay with no copper edges showing through on the sides of traces, and if vibration is that bad, the crystal leads will probably fail first.

If there are no stand-offs under the crystal, it's possible for solder to come up through the holes and form a short to the crystal can. It probably would be a good idea to keep the pad size minimum on the top layer consistent with the design rules, and larger on the bottom if desired.

Best regards, Spehro Pefhany

Depends on the pad geometry, and the way the bottom of the crystal is shaped. The insulators make sure it will never be a problem.

I've used that point to check oscillator function. In essence, you're capacitively coupled to the oscillator, on both sides, so whichever side is stronger (output pin) dominates. It's probably best to ground the can, but then you need a manual solder blob, or a three-pin crystal with can ground pin. I haven't seen it be a problem.

I don't have a convenient way to get there from my cad software.

If the crystal is mounted horizontally then I place copper on the top layer to stop me (or the autorouter) using the space for anything else.

Geo