SMD crystal package modification question

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Hi all,

I like to use crystals vs resonators for AVR boards (both Arduino and home made standalone). To replace the resonator on an Arduino board, a 2 smd no-lead package of size 3.2mm x 2.5mm in needed. I unsolder the resonator, remove the center PCB trace (gnd) with a razor blade and hot air solder on the crystal. Of course I would like to use the same type of crystal for standalone boards as well (the package neatly fits between adjacent pads of a protoboard).

My problem / question is this: the only value crystal I can get in the 2 smd 3.2x2.5 package is 16.0000 MHz. I prefer to use 22.1184 MHz. crystals to get "integer perfect" serial baud rates.

Unfortunately, the only package that this frequency is available in (in the 3.2x2.5 size) is 4 smd. The 4 smd parts have 4 pads on the bottom, a diagonal pair are the crystal and the other diagonal pair are case ground.

Obviously, soldering these to an Arduino board will produce a direct short across the MCU crystal pins.

My idea is to either solder a small piece of wire to each ground pad and use it to rip the pads off the package, or else use a razor blade to get under the pads and peel them off the package (or if the ground pads ONLY contact the case from the outside, cut the connection between the pads and the case with a razor blade).

I know this seems barbaric, but I don't see any other way to do it.

So, the questions:

* Will the crystal work OK without the case grounded?
* Should I solder a tiny wire from the case to ground to re-establish the ground?
* Will removing the pads the way I described destroy the crystal?
* Anyone have a better idea?

Any input will be appreciatted.

Gentlemen may prefer Blondes, but Real Men prefer Redheads!

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Krupski wrote:
Obviously, soldering these to an Arduino board will produce a direct short across the MCU crystal pins.
Can place the patch on top of Kapton tape (polyimide)

Likewise with a snippet of one-sided PCB stock, cut traces, flux, solder, bind/glue/epoxy, wire the patch.

0.8mm FR4 can be cut with scissors otherwise metal snips.

 

"Dare to be naïve." - Buckminster Fuller

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Why not use a small leaded crystal?  Just solder the leads to wherever they need to go

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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Krupski wrote:
* Will the crystal work OK without the case grounded?
Yes though the AVR's crystal oscillator will be more susceptible to EMI (oscillator's input is very high impedance, its output impedance is "low" though not low); no issue with a ground plane under all of the oscillator's current loops or with that one PCBA in a complete shielded enclosure.

Krupski wrote:
* Should I solder a tiny wire from the case to ground to re-establish the ground?
Optional; a mega324PB's MHz low power crystal case is grounded.

A concern is de-lamination of internal parts of the crystal; might consider conductive epoxy carefully cured by a heat gun (crystal within its reflow limits for temperature and duration)

Solder paste and a heat gun may be more in-hand (in your refrigerator and tool box)

ATmega324PB_Xplained_Pro

[page 3, C1]

via ATmega324PB Xplained Pro

 

"Dare to be naïve." - Buckminster Fuller

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Krupski wrote:
My problem / question is this: the only value crystal I can get in the 2 smd 3.2x2.5 package is 16.0000 MHz. I prefer to use 22.1184 MHz. crystals to get "integer perfect" serial baud rates.

Isn't 22+MHz outside spec for AVR.  (even tho it seems an obvious spec point to be using )

Even 20MHz has issues with Xtal oscillator not starting, on some AVRs.

If you do want to overclock to 22+MHz, maybe an external powered oscillator is better ?

 

Krupski wrote:
.. My idea is to either solder a small piece of wire to each ground pad and use it to rip the pads off the package, or else use a razor blade to get under the pads and peel them off the package (or if the ground pads ONLY contact the case from the outside, cut the connection between the pads and the case with a razor blade). I know this seems barbaric, but I don't see any other way to do it. So, the questions: * Will the crystal work OK without the case grounded? * Should I solder a tiny wire from the case to ground to re-establish the ground? *

Other choices could be some paper tape over the unused pads, to prevent them soldering.

I've also used a 'dead bug' approach when testing xtals on boards not designed for them & a slice of double sided tape.

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Just get something very small like this....you can solder it to any pads in the vicinity   https://uk.rs-online.com/web/p/crystal-units/1849417/

No need to go ripping up pads & traces or tearing apart components!!

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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avrcandies wrote:

 

Just get something very small like this....you can solder it to any pads in the vicinity   https://uk.rs-online.com/web/p/crystal-units/1849417/

No need to go ripping up pads & traces or tearing apart components!!

 

 

That's not the point. Here is a pic of one of my Arduino MEGA 2560 boards with a size 3225 crystal installed in place of the resonator:

 

(click for full size)

jpg image

 

 

I also use this package on protoboards:

 

 

(click for full size)

jog image

 

 

You see, I don't want to build an ugly kludge with wires onto random pads......

 

Attachment(s): 

Gentlemen may prefer Blondes, but Real Men prefer Redheads!

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You see, I don't want to build an ugly kludge with wires onto random pads......I

You can just cut the xtal leads short, "J" bend them & solder to the pads...no wires & it should look perfectly fine.  Not sure about "random pads"--solder to the two pads the xtal is supposed to connect with.

 

Certainly, to your point, an smt part that fits would be better, but this may be better than hacking up parts.

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Thu. Oct 24, 2019 - 12:20 AM
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Krupski wrote:

You see, I don't want to build an ugly kludge with wires onto random pads......

 

Did you check the 4 pad ones, to confirm the unused pads do connect ?   You may be able to simply solder them, & you may find a slightly smaller package rotated somewhat, can solder better.