Impacts of electromagnetic (EM) interference (immunity) on operation of LSI circuits in a QFP-packaged and PCB-mounted environment are studied. EM power injection to a power-supply system leads to malfunction, where the power is translated into voltage bounces through combined on- and off- chip impedances, affecting power supply and ground, as well as signal nodes in a die, seen from on-chip waveform measurements. A lumped power-supply impedance model and the minimum amplitude of voltage bounce induced by EM power for malfunction, both of which can be derived from external measurements to a given packaged LSI, formulate an EM interference model that is helpful in the PCB design toward high immunity. The technique can be generally applied to systems-on-chip applications.