Choosing a four layer stackup was a really instructive point for the purposes of his blog post, but kind of a bad rule of thumb to carry forward on a professional level.
PCBs are built in a sandwich stack. The middle dielectric layer, or "prepreg", is way thicker than the dielectric layers in the outer layers. This doesn't seem like a big deal, but for the speeds that a DDR interface runs at, it creates a higher impedance return loop for whatever you end up routing on layer 3. (Typically DQs, in my experience.)
You can avoid this entirely by building with a six layer stackup, and routing your DRAM signals on layers 1/2/3, keeping layer 2 as an unbroken ground plane. The result: nice, low impedance return tracks.
Yes, Jay is right - most of the time, this doesn't create problems. But when it does, it's pretty challenging to fix without a board spin, and when you're doing this professionally, that loses you time, creates risk, and is generally hella stressful.
Well said. And we're not even mentioning what happens when you start adding other criss-crossing traces for parallel LCDs and cameras into the mix. Those usually creep into the 30-40 MHz range, 18-24 lines per device with no differential lines. And I haven't even thought about USB-C yet.
And you still need to pass radiated emissions testing. Even those right-angle DIMM connectors for the SOMs spray RF noise everywhere when you put video on them.
PCBs are built in a sandwich stack. The middle dielectric layer, or "prepreg", is way thicker than the dielectric layers in the outer layers. This doesn't seem like a big deal, but for the speeds that a DDR interface runs at, it creates a higher impedance return loop for whatever you end up routing on layer 3. (Typically DQs, in my experience.)
You can avoid this entirely by building with a six layer stackup, and routing your DRAM signals on layers 1/2/3, keeping layer 2 as an unbroken ground plane. The result: nice, low impedance return tracks.
Yes, Jay is right - most of the time, this doesn't create problems. But when it does, it's pretty challenging to fix without a board spin, and when you're doing this professionally, that loses you time, creates risk, and is generally hella stressful.