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@gnarf I mean they're fanning out 676 signals from a die with tight pitch solder bumps to a 27x27mm BGA array. There's not much space to work with so you need your tracks to be thin.

And with DDR4 2667 and 32 Gbps SERDES support in the highest speed grade impedance control isn't an option, it's absolutely mandatory.

Without knowing the Dk of the substrate hard to be sure but I expect 50 ohm traces are 50um or less in width.