Director of Signal Integrity Engineering at Rambus, Shawn Nikoukary, recently published a piece in eeweb.com/EE Times about crosstalk (xtalk) as being the most challenging signal integrity (SI) issue associated with GDDR6.
Nikoukary says, “the main reason is that the circuits used to remove crosstalk from signals are complicated and require a large power/area overhead to be effective. There are several types of active and passive crosstalk cancellation methods, such as passive mutual capacitive couplings and crossed continuous time linear equalization (CTLE) and crossed decision feedback equalization (DFE) circuits.”
But he reminds SoC and system designers to keep in mind that it is much easier to design a channel that is low in crosstalk, than trying to correct it in the receiver.
It’s important to have a good handle on and fully understand crosstalk in order to design a robust GDDR6 interface, he emphasizes. “Crosstalk is due to capacitive and inductive coupling between two signals and it is categorized as Near End and Far End XTalk (NEXT/FEXT), Nikoukary explains.
Further, he adds it is highly recommended that all data lines (DQ signals) running at higher data rates be routed as a strip line. The largest contributor of the crosstalk in GDDR6 channel is from the vertical transitions such as vias, plated through holes (PTHs) and BGAs which are placed near each other. Great care needs to be taken to shield vias and design a BGA that will have a low crosstalk pin assignment.
“The majority of the crosstalk in GDDR6 systems results from the ball arrangement and mis-assignment in a BGA package,” according to Nikoukary. “This has a major impact on crosstalk and is especially true for certain applications where the PTHs are placed under the BGA. For example, networking systems tend to have large PCB stack-up, very thick PCBs that utilize through vias. These long through vias placed under the BGA cause large amounts of coupling and crosstalk if designed properly.”
Summarizing his key points, Nikoukary explains that “ideally, to eliminate this crosstalk, a very low crosstalk BGA assignment for the package is essential. In a good BGA ball map, ground and power shielding is placed between signals or blind and buried vias are used in the dense area under the BGA to minimize crosstalk.”