Misconception of thermal spreading angle and misapplication to IGBT power modules

This paper analyzes the widely used 45 degrees thermal spreading model in IGBT package design and quantifies error in application to both single and multilayered package structures. The results are compared with finite element analysis (FEA). For single-layer heat transfer problem, the spreading angle model with a 45 degrees assumption provides a less than 20% conservative error of thermal resistance for a certain substrate layer thickness range, but is not applicable to multi-layered structures. For two or more layered structures, as commonly found in direct bonded copper (DBC) substrates and used in multiple-chip power modules (MCPMs), the 45 degrees fixed-angle method cannot capture the behavior of the heat transfer problem nor accurately predict the temperature of critical points for design. The method introduces substantial underestimation of junction temperature dependent on layer thickness ratios. An in-depth literature review was conducted and little, if any, concrete basis for the 45 degree assumption was found. Guidelines for using more accurate spreading-angle calculations are provided for the practice engineer.