This paper presents a time-domain visualization method for tracing electromagnetic (EM) radiation associated with information leakage from cryptographic ICs on the printed circuit board (PCB) surface. In recent years, security threats based on EM analysis attacks on cryptographic devices are attracting considerable attention due to their relative simplicity in practice. Some of the most cost-effective countermeasures against such attacks can be implemented at the PCB level. In order to implement such countermeasures effectively, critical parts (i.e., information sources and information propagation paths) on the board should be identified in advance. The key idea behind this identification is to calculate a correlation between measured EM traces and EM intensity values estimated from correct information (secret key) in the time domain. Transient analysis can reveal information propagation paths even if the EM signal carrying information is weak in comparison with noise generated from other components. Through an experiment, we confirm that EM radiation associated with information leakage can be traced even in situations where the information signal is obscured by background noise.