An impact of layer stack-up on EMI

Investigation of a server shows the heatsink of the CPU module as a primary component of the EMI coupling path. In order to identify the specific noise source and coupling path to the heatsink, a series of experiments were defined to provide support for one source and eliminate others. Based on experiments with two different versions of the CPU module, a stack-up related design guideline is proposed: a ground layer should be the first entire plane (as opposed to V_cc) on the active component side of the board. If there are known IC sources that switch significant currents with the outputs unloaded at nanosecond rise and fall times on both sides of the board, then ground should be the first entire plane on both sides of the board when feasible