ISM PCB and RF effects of a shield and top PCB fill.

[trevorth]

I have designed an small RF card whereby a shield is intended to be placed over the top with only a small gap for the antenna. Theres an Class 1 Bluetooth amplifier in there with some RF switches.

My layer setup is:

Layer 1 (Signal)
Ground
VCC
Layer 2 (Signal)

and with the shield:

Shield
Layer 1 (Signal)
Ground
VCC
Layer 2 (Signal)

...whereby layer 2 doesnt have much on it at all and is 90% filled with ground. One thing that concerns me slightly is resonance once the shield is placed ontop. To this end, I havent filled the top layer with ground around the RF microstrip.

I have read extracts from various papers and a good chunk of Complete Wireless Design (Cotter W.) but Im still unsure as to weather it will be an issue. Maybe not filling around the microstrip will be more of an issue... I notice many boards I look at without a shield include a ground fill which makes sense, but with, a chamber is created. Although this is no more between ground, past the top layer and upto the shield. :-?

I wondered if anyone had any experience in this area?

 

[FvM]

5 layer ("with the shield") is no reasonable stackup. You'll go for either 4 or 6 layer.

I understand that "shield" refers to a stripline design with signal embedded between two ground planes. It should use via fences to avoid ground plane modes.

 

[trevorth]

I meant to explain that the PCB is 4 layer with an additional shield:

**broken link removed**

Therefore, although you can stitch and fence the layers around the microstrip, you cant do much with the free space above the top layer, and below the RF shield where the circuit is enclosed.

This is what Im wondering....
If there is literature on this issue.
If its a problem at all.

 

[FvM]

Impedance-wise, the shield is just a small bar across the microstrip, SMD shielding cases have usually voids for the signal traces. A shield must have multiple ground plane connections anyway.

All in all, I don't understand which problem you are exactly addressing in your post.

If you have copper pours /ground fills beneath the RF signals traces, they should use via fences. Presuming a continuous ground plane in your 4-layer board, the difference betweeen ground copper pours or void upper layer isn't so important.