Pick a layer and stick with it
One of the nice things about newer, faster busses like DDR3 and DDR4 is on-die termination. They are nice because you don’t have a bunch of components clogging up your routing layers, and I would say more importantly, it limits your required layer transitions which can help make your boards quieter. So take advantage of the fact that you don’t have to route out to a terminator and try to keep your layer transitions limited to underneath the IC. The area under the IC is a particularly exciting location on the board, and also happens to be the best place for layer transitions.
In a recent aricle in Printed Circuit Design and Fabrication Magazine, I discuss the EMI problems associated with layer transitions. The article can be found here: http://pcdandf.com/cms/component/content/article/171-current-issue/9656-designers-notebook
Bascially, a stitching via is needed anywhere you transition a signal between layers, to provide a continuous return current path for the signal. I have discussed return current paths in several previous blogs, but to summarize, broken return path = radiating signal. This is why a stitching via is needed when a signal transitions between layers. Well, more precisely, a stitching “thingy” – in the case that the different return paths are planes with the same voltage (usually ground), a via will work. But if they are at different potentials, a capacitor should be used. Where on the board are there the most vias and capacitors? Just around the IC. This makes it the best place to do any layer transitioning.
Posted January 7th, 2013, by Patrick Carrier
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