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Consider:

PCB

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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    \$\begingroup\$ It's not silver, it's tin or more specifically lead-tin. \$\endgroup\$ – winny Aug 5 '17 at 9:28
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    \$\begingroup\$ Or 98% tin, 2% silver, if the PCB is less than five years old. \$\endgroup\$ – Brian Drummond Aug 5 '17 at 10:34
  • \$\begingroup\$ @winny I guess the asker meant silver the colour rather than silver the metal. (Though your comment is still useful.) \$\endgroup\$ – David Richerby Aug 6 '17 at 15:36
  • \$\begingroup\$ @DavidRicherby That's the difference between silver and silver colored. For gold it's easier - golden. \$\endgroup\$ – winny Aug 6 '17 at 16:19
  • \$\begingroup\$ Yeah , I meant the colour. \$\endgroup\$ – Karthik ziffer Aug 7 '17 at 5:00
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Here's a picture to help you understand better. The crystal's case can be soldered onto boards to secure them.

Enter image description here

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    \$\begingroup\$ Finally someone actually explains how it should look like. \$\endgroup\$ – pipe Aug 5 '17 at 10:20
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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.

Enter image description here

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.

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    \$\begingroup\$ +1 If the crystal can is not soldered to the hold-down wire, but merely in casual contact, the pad would provide better shielding of the high-Z nets if contact is lost due to corrosion etc. \$\endgroup\$ – Spehro Pefhany Aug 5 '17 at 16:35
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It's a solder pad for a full-size (HC-49/U) crystal, for mechanical purposes. Clearly it has no use when a small (HC-49/US) crystal such as the one in the photograph is used.

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    \$\begingroup\$ Just a casual comment, so it has nothing to do with shielding noise to the other side of the PCB, or work as a heat sink? Or perhaps that's just two positive side effects. \$\endgroup\$ – Harry Svensson Aug 5 '17 at 8:05
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    \$\begingroup\$ If you're putting anything on the other side of a PCB from a crystal then you slept through a very important part of PCB Layout 101. \$\endgroup\$ – Ignacio Vazquez-Abrams Aug 5 '17 at 8:08
  • \$\begingroup\$ Nice response \$\$ \$\endgroup\$ – Harry Svensson Aug 5 '17 at 8:10
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It'd be used to horizontally mount a larger crystal such as the following HC-49. The two through-hole pads either side are often used along with some wire to secure them in place, plus you may also find those connected to ground to provide some additional shielding.

enter image description here

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  • \$\begingroup\$ The strip is connected to the output pin of TSOP 1738 photodiode. I think the strip reduces the noise by reaching the microcontroller . Am I right ? \$\endgroup\$ – Karthik ziffer Aug 5 '17 at 8:08
  • \$\begingroup\$ YES .. It connects to ground \$\endgroup\$ – Karthik ziffer Aug 5 '17 at 8:13
  • \$\begingroup\$ Also note the two holes. These can be used to solder a jumper wire hold down across the can further securing and grounding the crustal case. \$\endgroup\$ – Trevor_G Aug 5 '17 at 15:18
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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:

  • The metal can enclosing the crystal acts as a Leyden jar and an effective RF shield. There is little or no reason it needs to be grounded. The main reason the case is sometimes soldered is for mechanical stability.
  • Precaution to prevent oscillations at the wrong frequency.
  • If you're going to be touching the crystal case a lot, then you may want to ground it, so that the clock is solid even though you touch it. We're using 24 MHz crystals in one application and if you get your finger within 1/4 inch (6 mm) of the can the crystal speed will change. So we ground them to keep them stable. Once in production, no one will be touching it, so no grounding after we get done with the design/programming cycle.

You can get more information on these links:

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  • \$\begingroup\$ "Precaution to prevent oscillations at the wrong frequency." How does it help? \$\endgroup\$ – Oskar Skog Aug 5 '17 at 16:19

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