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About 1080 Prepreg for getting 40 Ohms Impedance in Top and Bottom layers

Bermell , 06-29-2022, 04:09 AM

For the routing of DDR3 memory chips and FPGA, I need 40 Ohms impedance for single tracks, and 80 Ohms for differential pairs.

I think i can get these values in external layers (top and bottom), using a 1080 prepreg (70 micrometers and Er=4,4).
I mean, for top layer, i have the following configuration

1.- 35um copper foil
2.- 65um(real) 1080 prepreg
3.- GND plane as reference

The problem, is about the manufacturer of the PCB. They do not recommend me to use that prepreg because it gives them a lot of problems.
This PCB manufacturer doesn't recommend to use a 75 micrometers (dielectric) core either.

My question is what do you think about it? Is 1080 prepreg a very rare dielectric ? Must i choose other PCB manufacturer ?

Thanks a lot in advance

qdrives , 06-29-2022, 01:44 PM
1080 prepreg is about the most standard there is. However, you probably have a different problem in that the fabricator wants at least 2 layers of prepreg. They also want it to be 3 times the thickness of the copper. With 3x35um is 105um and 1080 being ~75um, it would be to thin.
For thicker dielectric you could simply use wider traces. The opposite could be more problematic.
Thin cores are expansive.
Bermell , 06-29-2022, 02:34 PM
Thank you for yor answer, qdrives.

If you check the Stackup of the OpenRex board (picture attached), I think there is a 1080 prepreg too. So I think it is perfectly possible to use the 1080 dielectric individually.

For 40 Ohms, the width of tracks is very big.

Thanks a lot

qdrives , 07-01-2022, 02:03 PM
Yes, but that stack-up has 18um copper. You mention 35um.
Thicker copper also means wide tracks and spacing.
robertferanec , 07-08-2022, 12:33 AM
I would like to add, don't forget about plating. That will add around 20um on the top of the foil. The total copper thickness on openrex is 43um (see the finished thickness column).

PCB manufacturer should not have problems with the stackup as used on OpenRex (even our local PCB manufacturer did it). Maybe I would ask what exactly is the problem (maybe they simply dont want to use 75um prepreg as it is very thin or they don't normally stock that material).
binayak , 07-28-2022, 12:01 AM

Ask the fabricator what exact problem they say they will face?

What do you mean by a " real" 65um prepreg? Does real mean "pressed"?
Normally the prepreg thickness decreases during the fabrication process. You need to calculate trace impedances based on pressed thicknesses.

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