The demand for power electronic packages is constantly growing. With an increasing power loss of electrical components, the thermal performance of is gaining more attention on the PCB-level. The application of relatively thick Cu cores in PCB modules is beneficial with respect to high levels of ampacity and heat dissipation. Applicable PCB designs require a more or less complex structure and/or arrangement of several cores. For instance, gaps between several Cu cores are demanded for high-voltage applications, while cavities may be of a certain advantage when additional components need to be integrated on a high level. In our work, we investigated the reliability using repeated thermal shocks of PCB test vehicles featuring cavities within Cu cores of 2 mm thickness. After 1000 cycles the specimens do not show any significant damage which may result in an operational failure. Additionally, we investigated deliberately introduced defects at the Cu-resin interface by means of thermography.