Particle reinforced metal matrix composites are developed for heat sink applications. For power electronic devices like IGBT modules (Insulated Gate Bipolar Transistor) a baseplate material with high thermal conductivity combined with a low coefficient of thermal expansion is needed. Commonly AlSiC MMC are used with a high volume content of SiC particles (~ 70 vol.%). To improve the performance of these electronic modules particle reinforced materials with a higher thermal conductivity are developed for an advanced thermal management. For this purpose highly conducting diamond particles (TC ~ 1000 W/mK) are embedded in an Al matrix. These new diamond reinforced MMC were investigated concerning their thermal fatigue mechanisms compared to the common AlSiC MMC. Differences in reinforcement architecture and their effects on thermal fatigue damage were studied by in situ synchrotron tomography during thermal cycling.