Prepreg Dk vs. frequency

[Mercury]

Hi guys,
I'm designing an 8-layer impedance controlled PCB. The fastest signals on it will be 303 MHz DDR3. My PCB manufacturer recommended me to use the Panasonic R-1650M prepreg material. The 1080 prepreg has the dielectric constant, Dk, specified as:

1MHz: 4.6
1GHz: 4.1

The 303 MHz digital signals obviously includes 303MHz, 606MHz, 909 MHz...etc components. So my question is, which number should I use for impedance calculation?

Thanks and best regards,
George

 

[marce]

This is where "signal integrity verification" comes into its own. The best way to do a DDR3 interface is to lay it out using the required design constraints (set up skews between: data lanes, clock, address bus, control lines etc) getting the lengths as near as possible, leaving room for length matching. One of the first mistakes people make when first laying out DDR memory interfaces is not leaving enough room between traces for the automatic length matching routines, spread the traces out, especialy the shorter ones as these will require the most serpentines.
To determine the frequencies of interest you need to know the rise time of the signals between the memory controller and the memory chips, from this you can determine the knee frequency which will give you the spectral content of the signals. From this you can calculate the impedance at all frequencies, or...
Use simulation with a 3D field solver, it will work it all out for you, check timing etc etc it is the best way to do these types of interface and takes all the guess work out of the equation.
For the final impedance of the PCB, you want it as near as possible to 50 Ohms, here you PCB manufacturer is your friend, they usually have tools from Polar, that will work out dialectric thicknesses, trace widths etc depending on your stack up, prepreg etc and can build your PCB to match your required impedance.~Finaly there are numerous application notes, data sheets etc regarding DDR3 interfaces, some will have reccomended stack up, layout guides etc. Depending on the controller, the floor area for the memory you will probably have to use traces between 0.10mm - 0.15mm, so you can approximate impedances and work out your stack up using tools such as Saturn PCB Toolkit.