hf materials for digital PCB

[buenos]

hi.

who uses high frequency dielectric materials for high speed PCB designs, who dont, and why?

in an AMD appnote they use standard FR4 for a board, where the highest speed bus is 400MHz DDR memory bus.

for a same speed board, intel uses some HF material, with Er about 3.2. In the appnote, they tell the width and thickness, 4/5 mils. they want 55 ohm striplines. this is only possible with Er=3.2 nad with a HF material.



so is it needed or not?

 

[CDRCDR]

HF materials such as Rogers and Gentek are used in 10Gbps applications. For applications such as several hundered MHz,FR4 is more than enough.

 

[buenos]

with one impedance calculator it needs Er=3.2 for 55 ohm (at 5mil/5mil) with another calculator it needs 4.5 which is the normal FR4.

so i am confused.

Added after 1 hours 25 minutes:

i checked again, ad yes:

high enough impedances and high enough density on inner layers is only possible with low Er materials.

with normal FR4 a 55 ohm trace on inner layers is only possible with thickness of 0.2mm, but then the trace separation on a high speed bus has to be 0.4mm which can not produce dense enough PCB for a very complex circuit with 1000 pin BGAs.