Microstrip change ground plane

Hi to all

I'm currently working on a standard 4-layer PCB (Signal, Gnd Plane, 1mm FR4 ,VCC, plane, Signal) with microstrip.
Useful frequencies up to around 4 GHz.

Most MWave IC's are based on a QFN package so when using the top signal plane and the GND plane at 0.2 mm distance this gives me a comfortable 50 ohm strip around 10 mils width.
Now, near the end of the chain I would like to place a lumped component filter in 0603 and the pads are quite large compared to the 10 mil microstrip, same goes for the center conductor of the end launch sma connector.

Therefor I would like to switch from ground plane and use the VCC plane as ground reference under those areas which gives me a wider 50 ohm trace more comfortable to solder components on.
Is this feasible and what is the best way to achieve this? Remove parts of the layer 2 gnd plane with a keep out area?

I would stay with the low substrate height and calculate the pad capacitance into the lumped circuit, and use smaller (0402 or 0201) parts.

Otherwise place a ground area on the VCC layer below the cutout and dense via fences to connect it with the primary ground layer.

FvM said:

I would stay with the low substrate height and calculate the pad capacitance into the lumped circuit, and use smaller (0402 or 0201) parts.

Otherwise place a ground area on the VCC layer below the cutout and dense via fences to connect it with the primary ground layer.

Click to expand...

0603 is mostly for easy prototyping of the filter and the higher Q of the SMD coils. I'd rather not change the footprint just yet.
The cutout on layer 2 and vcc ground plane does seem like the way the go. Perhaps this is worth simulating in an 3D EM simulator.

If anyone has some more useful comments on practical caveats I'd like to hear them.
I'm also wondering about whether to taper the micro strip over the ground plane transition.

To place both, the signal and ground currents on the same layer you can use a CPW transmission line (without ground plane under). In most of the applications this have better performances than Microstrip transmission line.