Signals propagate at the speed-of-light in free space. However, this speed varies dramatically depending on the surrounding dielectric materials. Each layer, of a multilayer PCB, can have a very different propagation speed. This is particularly important for the latest high-speed DDR3/4 memory devices.
The new 'Relative Signal Propagation' feature, of the ICD Stackup Planner, allows you to not only match the length of busses, but takes this one step further by automatically simulating the appropriate length required to match the delay exactly. The integrated field solver simulates the inductance, capacitance and flight time, of each signal layer, to quickly give you the results you need to effectively route memory busses.
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