Guard Ring

[JohnsonMiller]

Hi Guys,

I have seen a bare metal (solder mask removed) ring around some sensitive analog or RF circuits. Those rings are usually stitched with ground connected vias. I was wondering what help this structure does? My doubt is due to that fact that those area are open to air and I believe without closed metallic case we cannot claim about noise immunity, any opinion?

BR,

[mairomaster]

Ring guards shield against interference coming from surrounding signals on the same layer of the board. Also the via stitching (if dense enough) forms a cage, that shields to a certain amount the inner layers as well. Metal can can be soldered over the ring guard if shielding against interference coming from the outside is required.

[JohnsonMiller]

Thank you mariomaster, I appreciate your helping hand!
I got it, so can we say that in those board outer layers shall not be used, just only internal layers?
What is help of solder mask removing? if we are not going to attach metal cage.
What help does the gold immersion, can we expect same physical property with hot-air finishing?

[mairomaster]

I don't really know what is the benefit of the solder mask removal over the guard, if you don't want to cover it with a metal can. Also I don't think ENIG has a particular advantage in this case compared to HASL. If you have really important sensitive signals, you can use a ground fill on top/bottom layers in the ring area and only use internal layers for the tracking. The RF field is a science on itself though, I don't have much experience in that.

[nikhil2402]

Hi All,

How does a ground polygon poured over many layers (usually this is the case in many PCBs) with many vias for good grounding differ from guard rings ? Both of them are trying to achieve the same thing right ? or I am missing something here.

Nikhil

[robertferanec]

We discussed some things about the ring also here: http://www.fedevel.com/designhelp/fo...dual-shielding