Symmetric Stripline vs Asymmetric Stripline

[tennythomas]

Hi,

Do u know how to choose symmetric stripline or asymmetric stripline for pcb layer stack up?

If yes, then pls give me an example.

Thanks in advance.

 

[edaedaeda]

There is more than one case for each.

A case where I would use a smmetrical stack for impedance control would be when I want signals to evenly propagate to reference planes.

Lets say I have a transmission line that needs to route on an internal layer. This TRL is coming onto board through a coaxial cable. If I were to maintain the balance between reference planes it would be benificial to signal integrity. Making this TRL co-planar would even be better. But doing this does open another can of worms when thinking of inductance and capacitance across planes and signals.

A case where I would use an asmmetrical stack would be when I want a signal or signals to propagate to nearest reference plane. This is usually a good idea when designing high frequency signals where you need to minimize effects of inductance/capacitance.

Lets say I have a stackup where:
layer2 is gnd
layer3 is sig
layer4 is sig
layer5 is gnd

I have a differential pair on layer3 that needs to be close coupled. Also I need to minimize inductance/capacitance along this signal. I would want to calculate impedance to assure + and - are propagating to eachother more than they are to reference plane. I also want to make sure I minimize the return path of current on this line by keeping reference plane layer2 gnd only.

There are entire discussions and many approaches to controlled impedance and there is usually more than one way to think about.

Eda