Murata Crystals

[gcary]

The RT1021 and RT1176 EVKs use a Murata XRCGB crystal.  The RT1062 EVK uses a different crystal, but I suspect the same restriction applies.  Here is a link to the specification for one of the 24 MHz XRCGB crystals:

https://www.murata.com/products/productdata/8801057439774/SPEC-XRCGB24M000F1H01R0.pdf 

Section 12-6 (12. 使用上の注意 Caution for Use) reads as follows:

12-6
基板最上面のグランドや信号パターンは水晶振動子の下部に配置しないよう注意して下さい。
Ground or signal path on the top of substrate should not be located underneath crystal unit.

In all three designs one of the crystal signals is routed underneath the crystal, which seems like a bad thing to do because fields from that signal run close to the piezoelectric element and could interfere.  I'm sure it is a minor issue, but I thought I would point it out since it would be a best practice not to do that.

Also, in all 3 EVK designs the No Connect pins are connected to ground.  The datasheets for the Murata and TXC Corp crystals do not recommend connecting them to ground.  I'm doubtful that connecting them to ground is a problem. If the pads are connected to a ground pour without thermal spokes, that would provide more mechanical strength, but perhaps could be an issue with reflow because of the thermal mass difference between the pads.

As for ground being underneath the crystal, there is no way I'm going to cut a hole in the ground plane under the crystal!

If anyone believes I have made a mistake in this analysis, please let me know.  Otherwise it is a heads up for people doing layouts.

Greg

[gusarambula]

Hello Greg,

You are correct in your analysis based on the recommendation from the crystal’s manufacturer, and while these may seem not to affect the design too greatly at first sight, I agree that it’s important to consider to have the most robust design both structurally and in regards to electromagnetic interference.

I checked the Hardware Design Guides and fortunately the recommendations listed there do not contradict the manufacturer’s datasheet. I have seen that the EVKs do not always have the best or most optimized design. It may even happen that the EVK do not follow the Hardware Design Guide recommendations and in these situations, we always recommend following the recommendations on the design guide.

Thank you for sharing these findings, I’m sure they will help other Community users.

Regards,
Gustavo

[gcary]

Hi Gustavo,

Thank you for validating my thoughts.  I'm glad it will be useful to others.  One other note about these crystals is that you can't use them with water-soluble flux because they are not hermetically sealed and cannot be washed.  Fortunately that is not a problem for most people because no-clean flux is pretty much the norm these days.

Regards,

Greg