A novel method was developed to determine equivalent circuits for plastic packages. The method first divides the package shapes into basic shapes including one or two lead pins, and determines the S parameters of each basic shape using an electromagnetic field simulator. Next, the equivalent circuits and parameters of each shape are determined by a successive fitting technique using a microwave circuit simulator, and these are combined to achieve the total equivalent circuit for the package. The results agree well with measured results. © 1995 John Wiley & Sons, Inc.