To design a PCIe*8 carrier board for a XC7K160T module, what is the requirement for the PCB?
The 16 pairs are all adjacent to each other on the 0.6mm pitch B2B connector of the module with ~20mm span , the signals on the card edge span ~40mm. The module is planed to be placed as close to the PCIe connector as possible. But *8 still means some horizontal routing and vertical spaces for routing.
For example the vertical space is 20mm, then all signals are in a (20-40mm)*20mm area, then trace length on the carrier board won't be longer than 40mm, suppose the signal rise time is 100ps, then the trace length is several times the rise length, then impedance should matter even on this small area, and I'm not sure whether will this be a "work by luck" design with such many connections (chip to module to carrier to motherboard to chipsets).
It is said PCIe3.0 works on FR4 but is at the upper limit of FR4 https://www.intel.com/content/dam/doc/guide/pci-express3-phy-implementation-considerations-idf2009-presentation.pdf, even 10GT/s's eye won't open well.
FR4 also may have many grades, what is the requirement and what to tell the PCB manufacture?
The carrier board is very simple, can very low cost 4 layer board from small vendors be used (can they even replace FR4 with some lower grade material)?
Best Answer
Bog standard FR4, the type used as standard by PCB manufacturers is not really up to the task of Gen3 PCIe.
The weave is much too coarse which means that as the pair runs along the board, the impedance changes continuously due to the dielectric constant varying. The image below tells a thousand words. Standard FR4 is on the left.
In order to avoid degrading the signal so much, for high speeds like this we tend to use a higher quality substrate. This can range from higher quality FR4 all the way to very expensive exotic ceramic materials like Rogers.
We've had reasonable success over short distances (~10cm or so) using a comparatively low cost "High-Tg FR-4" material - this is basically still fibreglass, but the weave is much more tightly packed, as shown in the right hand diagram of the picture, which means the dielectric constant doesn't vary as much. In particular we use VENTEC VT-47, though there are many high-Tg materials out there, it will depend on what your fab house has available.
For longer distance traces or going through multiple connections such as backplanes, you will have to experiment or model to see if your drivers can cope. In this case you could go for a more expensive Rogers substrate. Alternatively an option we've used in the past is to add a PCIe Redriver IC (such as DS80PCI402), which will allow you to tune the drive strength of your differential pairs to compensate for some of the effects of impedance mismatch.