Abstract: One important issue in characterizing the electromagnetic immunity of the microcontroller is the location for disturbance injection. This paper investigates the dependence of the immunity on the type of the pin under test. The paper constructs models of the propagation paths of the electromagnetic interference(EMI) for supply pins, digital input/output pins and oscillator pins. Through analysis on the property of the EMI propagation paths, the paper defines a set of fingerprint parameters to characterize the immunity of microcontrollers in the middle frequency range, and derives immunity equations to calculate these parameters. The measurement results performed on various microcontrollers present the validation of the immunity equations. The conclusion helps to understand the immunity mechanism of complicated integrated circuits and establish the criterion for the pin selection and the immunity comparison of different microcontrollers.