This study examines the threat of information leakage when digital ICs, which process sensitive information such as cryptographic operations and handling of personal and confidential information, are mounted on printed circuit boards with split ground (GND) planes. We modeled the mechanism of generating such information leakage and proposed a methodology to control it. It is known that the GND plane of a printed circuit board on which digital integrated circuits are mounted should be solid and undivided to ensure signal integrity, power integrity, and electromagnetic compatibility. However, in actual designs, printed circuit boards may have split GND planes to isolate analog and digital circuits, isolate high-voltage and low-voltage circuits, or integrate multi-function electronic control units. Such split GND planes can increase the risk of electromagnetic information leakage. We, therefore, investigated a side-channel attack standard evaluation board, SASEBO-G, which has been reported to leak cryptographic information superimposed on common-mode currents, known as one of the major causes of electromagnetic emanation. Our experimental results showed that the split GND planes were the dominant cause of common-mode (CM) information leakage. Next, we constructed an equivalent circuit model of the dominant leakage mechanism and confirmed that the behavior of side-channel information leakage superimposed in the simulated CM current was consistent with the measured results. We also confirmed that to mitigate side-channel information leakage in CM caused by the potential difference between the split GND planes, the impedance should be reduced in the information leakage band by connecting the GND planes with capacitors, and the like. In addition, the RF band coupling between cables should be weakened if the cables are connected to the split GND planes.