Evaluation of metal-matrix composites baseplates with anisotropic thermal conductivity inserts

In order to fulfill the increasing demands on thermal management of high performance power modules we investigated the usability of heat spreading via materials with anisotropic thermal conductivity lambda. As a vivid example we took a look at macroscopic anisotropic inserts integrated into metal baseplates. We used annealed pyrolytic graphite (APG) as insert material with lambda = 1600 Wm(exp -1)K(exp -1) in two and lambda = 10 Wm(exp -1)K(exp -1) in one spatial direction. First we evaluated the influence of geometry and orientation of those inserts on the heat dissipation of typical power modules and compared them to isotropic baseplates using steady-state thermal FEM simulations. Common heat spreading was shown to be detrimental and we suggest the use of increased thermal conductivity in the vertical instead of the horizontal direction. Next we investigated the long term stability of cast aluminum-APG composite baseplates via thermal cycling and transient plane source as well as scanning acoustic microscope measurements. The interfaces between matrix and inserts show deterioration over time while the effective lateral thermal conductivity remains constant.