An efficient algorithm is presented for simulating passive microstrip layouts consisting of arbitrary shaped circuitry embedded in a shielded stratified media using an area of influence (AI) approach with a full-wave integral-equation-based numerical solver. The procedure involves introducing an effective area centered at each basis-function within which the interaction of that basis function with others is taken into account in filling Method of Moment matrix. It is shown that by properly choosing the radius of the area of influence in most of cases we can reduce the computation time up to 40% and still maintaining an excellent agreement with the full analysis.