AlSi7Mg/SiC/70p (AlSiC) is used for heat sinks because of its good thermal conductivity combined with a low coefficient of thermal expansion (CTE). These properties are important for power electronic devices where heat sinks have to provide efficient heat transfer to a cooling device. A low CTE is essential for a good surface bonding of the heat sink material to the insulating ceramics. Otherwise mismatch in thermal expansion would lead to damage of the bonding degrading the thermal contact within the electronic package. Therefore AlSiC replaces increasingly copper heat sinks. The CTE mismatch between insulation and a conventional metallic heat sink is transferred into the MMC heat sink. The stability of the interface bonding within a MMC is critical for its thermal properties. In situ thermal cycling measurements of an AlSi7Mg/SiC/70p MMC are reported yielding the void volume fraction and internal stresses between the matrix and the reinforcements in function of temperature. The changes in void volume fractions are determined simultaneously by synchrotron tomography and residual stresses by synchrotron diffraction at ESRF-ID-15. The measurements show a relationship between thermal expansion, residual stresses and void formation in the MMC. The results obtained from the in situ measurements reveal a thermoelastic range up to 200 °C followed by plastic matrix deformation reducing the volume of voids during heating. A reverse process takes place during cooling. Thus the CTE becomes smaller than according to thermoelastic calculations. Damage could be observed after multiple heating cycles, which increase the volume fraction and the size of the voids. The consequence is local debonding of the matrix from the reinforcement particles, which leads to an irreversible reduction of the thermal conductivity after multiple heating cycles.