An insulated metal substrate (IMS) is a circuit board comprising an insulating layer on a metal base plate. The insulating layer is made from epoxy resin incorporating dense inorganic fillers with high thermal conductivity. Because the substrates have high thermal conductivity, they are used in applications where electric parts generate intense heat, such as inverters, amplifiers, motor drivers and so on. It is expected that the insulating layer has higher thermal conductivity as the use of an IMS is expanded. Therefore, the influence of percolation on the equivalent thermal conductivity of an insulating layer is considered. The effect of the volume fraction of inorganic filler on the equivalent thermal conductivity of insulating layer in IMS is experimentally investigated. The equivalent thermal conductivity of insulating layer as a function of volume fraction of filler is estimated by FEM and Monte Carlo technique together. The acquired value of percolation threshold volume fraction is the same grade as the previous reported value. Based on these experimental and numerical results, an effective thermal conductivity of a filler which contains surrounding interfacial region is evaluated. The effective thermal conductivity of an irregular filler is presumed smaller than that of a spherical filler. It is noted that the control of filler size and shape is important for the formation of high thermal conductivity of an insulating layer. In addition, an improved equation for the equivalent thermal conductivity of insulating layer in IMS is proposed. The predictive values from the equation for insulating layer in an improved IMS agree with experimental results.

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