Advanced Micro Devices Inc (AMD)

[kpf]

elmer

The most simple I could think of for is that increased number of connections cannot not be done without an additional layer due to architectural layout-restrictions. (That was the humble thought behind "needs another layer")

But additional layer(s) could be necessary for all sort of reasons, most of them are foreseen and considered designwise. But then unexpected issues occur as well, e.g. what AMD encountered in 130nm node, induction phenomenon requiring not only one, but two additional layers from Thoroughbred A to B stepping to design it out.
These sort of things can be very nasty as you can hardly guess how long it will take to understand what is going on. K.