Simulation based selection of the molding compound for power packaging based on the case study of D2PAK

Abstract

This paper presents the utilization of simulation techniques even before the first prototype is manufactured or first product failure is observed during the testing phase, the true potential of numerical simulation in realizing validated designs is achieved. The obtained advantages in terms of reduced time, cost & effective design are the essential prerequisites for any organization to remain competitive. The present paper illustrates the material selection based on simulation driven design approach for a D2PAK used in automotive electronics. Selection of epoxy based encapsulation for the components is under consideration. Virtual Design of Experiments (VDoE) has been employed to explore the design space consisting of material properties of the encapsulation. VDoE have been carried out to finalize the mechanical & thermal properties of the encapsulation for the given construction of component. The optimum material properties obtained from the aforesaid process has been verified by warpage measurements of D2PAK using Digital Image Correlation (DIC) technique. Five material properties which characterize the mechanical response of plastic encapsulation are glass transition temperature (Tg), coefficient of thermal expansion below and above glass transition temperature (Tg), modulus of elasticity and curing shrinkage. The value of permitted curing shrinkage is decided by evaluating the out of plane deformation of bi-material specimen consisting of molding compound & copper. Here a VDoE is carried out to determine the optimum shrinkage value from a range of material properties of molding compound. The mechanical response at the discrete points on the design space is obtained by executing Finite Element (FE) based simulation in ANSYS. Response surface is extracted by regression analysis using Cornerstone tool. Further optimum values of remaining mechanical properties & thermal conductivity of the encapsulation is zeroed in by evaluating the mechanical & thermal response of the D2PAK on a Printed Circuit Board (PCB) substrate. A second run of VDoE is carried out to evaluate parameters such as plastic strain energy density in wire bonds & creep strain energy density in solder pad of D2PAK for the given transient power profile. The D2PAK constructed with the optimum molding compound is verified by measuring it's out of plane deformation using DIC technique. The measurement results are compared with the response surface values obtained by regression analysis. It has been shown that the molding compound selected for the D2PAK from such a method results in the best possible thermal & mechanical performance for the constraints under consideration. Additionally it underlines the simplicity & effectiveness of simulation driven approach using VDoE technique.