Electronic – RF PCB Design Basics

high speedpcbRF

I want to get some knowledge in RF PCB design. Therefore I have some questions about this, in order to make some clarification.

What is the purpose of the golden ring around the PCB, often seen on RF designs, like this?

bare PCB
Image source

I suppose it is used as some kind of shield. But, what is it protecting – the board from the external EM sources or the environment from the board's EM radiation? Why is it golden and not masked (painted)? Are there some rules for designing such rings (e.g. ring width, shape, via mechanics, …)? And, finally, in some cases, there are rings that are not completely golden, like around the CC3200 in the following image below. What is the difference?

TI CC3200 Launchpad PCB
Image source

Best Answer

I believe the first image you posted is meant to accommodate a machined EMI shield that is screwed down. You can see that it would provide EMI shielding as well as isolating the circuits from each other. The metal is plated so it makes contact with the shield (maybe just ENIG).

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The second one you posted is for a cheaper TI part. It looks like this design could accommodate a soldered down sheet metal shield for EMI. These are much cheaper and if this small can be placed during automated assembly.

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It's easier to take off the more expensive screwdown shield, but you can get some sheet metal ones with pull off tops. They also make board clips now that you can put on your board and snap the shield into.