Do I need to ground a 4 pad crystal?

Thread Starter

Tim Fernandez-Hart

Joined Nov 30, 2018
12
I used a crystal in my design from the SparkFun library without properly checking it. The schematic has two wires, but the footprint has 4 pads.

Unfortunately I have already had my PCB manufactured and only two of these pads are connected to my IC, so I'm hoping that this crystal will do the trick:
https://abracon.com/Resonators/abm3b.pdf

However, the datasheet says that the two pins that are not connected to the crystal should be grounded. This is going to be tricky unless I start hacking at my PCB's with a knife to fudge some sort of connection to a ground plane.

Old crystals had only two pins by design. What will happen if I don't connect the two ground pins for this crystal?
 

cmartinez

Joined Jan 17, 2007
8,218
The grounding pads, other than also strengthening the hold as wolframore's just said, connect the crystal's casing to ground so as to turn it into a shield against EMI, increasing its reliability.
 

Wolframore

Joined Jan 21, 2019
2,609
The grounding pads, other than also strengthening the hold as wolframore's just said, connect the crystal's casing to ground so as to turn it into a shield against EMI, increasing its reliability.
absolutely... that's what case grounding is for... also blocks unintentional RF emissions
 

MisterBill2

Joined Jan 23, 2018
18,167
Of course, if the item is indeed only a crystal AND if you have the correct pads connected, no problem. BUT if the device is a crystal oscillator module then you also need a power supply connection. So the concern is which do you have?
 

Thread Starter

Tim Fernandez-Hart

Joined Nov 30, 2018
12
Well I am in the midst of a steep learning curve (the most productive kind) but thankfully the schematic is for just a crystal.

I didn't thoroughly check the design before buying, and I just jumped in and bought a crystal oscillator which required a VCC and GND. Reading that datasheet made me realise my mistake. I now know the difference between an oscillator, and a plain crystal, and the different form factors that these may come in.

In answer to Wolframore, no I haven't tested my plain crystal yet, its in the post and should arrive next week. I'm hopeful that it just works.
Will I need to shield against EMI? Could I do that with the whole case? i.e. could I solder a wire to it and attach that to a GND via? This is a DSP board for audio based on the ADAU1701 from analogue devices.

Thank you to everyone who has replied.

Tim
 

Wolframore

Joined Jan 21, 2019
2,609
Many folks fix their first version PCB using wires. Grounding it with a wire should work. Try without it first. It might be fine depending on what the end use is and what other components are around it.

Example if your crystal is a fundamental (or harmonic) freq for a RF circuit it may have an effect... it really depends... reminds me of the discussions about superheterodyne mixing in RF class... it can get complex. The grounded case avoids much of that. In your situation it might not matter.
 

Thread Starter

Tim Fernandez-Hart

Joined Nov 30, 2018
12
Thanks Wolframore. Just so its clear in my mind, when you say "grounding it with a wire should work" your suggesting I solder a wire to the crystal case and attach that to a GND somewhere?

I will try without it first, with my fingers crossed.
 

Wolframore

Joined Jan 21, 2019
2,609
Thanks Wolframore. Just so its clear in my mind, when you say "grounding it with a wire should work" your suggesting I solder a wire to the crystal case and attach that to a GND somewhere?
that is correct - repairs happen like this:
upload_2019-4-18_15-44-7.png

This guy forgot to run traces for SPI - my OCD is bothered by the components not being very straight.

upload_2019-4-18_15-50-39.png

and yet more boo boos - eek there's way too much solder for my liking...
upload_2019-4-18_15-52-1.png
 
Last edited:

Thread Starter

Tim Fernandez-Hart

Joined Nov 30, 2018
12
Fantastic!

So I'll hope for the best, but be ready to fudge as required.

Many thanks to everyone who has taken the time to reply. Especially Wolframore. Those pictures are great!

Have a good easter everyone.

Tim
 

MisterBill2

Joined Jan 23, 2018
18,167
I have seen circuit boards with a wire soldered across the crystal to hold it to the board. But those were through-hole mount crystals with wire leads through holes. And years ago I did buy some oscillators, only to discover that they were VERY small and so I had to add wires to all 4 holes to mount them But it worked perfectly and the customer was totally pleased and they are still working now ten years later.
 

djsfantasi

Joined Apr 11, 2010
9,156
And if needed and you’ve soldered a ground wire, a couple drops of epoxy under the unsoldered pads can help stabilize the crystal case.
 
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