Investigation of mutual inductive coupling in RF stacked-die assemblies

3D packaging is extensively used in digital applications, and is under consideration for analog RF applications as well. A key problem, however, is the coupling of strong output signals from front-end passive devices into the local oscillator of a transceiver chip. The main mechanism for this is mutual inductive coupling. In this study, a test structure has been designed, built and measured to examine the problem of mutual inductive coupling in stacked-die assemblies. Simulated results show good agreement with measurement, and can be used to predict the coupling. Additional simulation results are used to devise rough placement guidelines for the assembly to avoid excessive coupling. Surprisingly, even for modest amounts of lateral offset between coils on a stacked passive die and the tank coil of the underlying local oscillator, acceptably low levels of coupling can usually be obtained.