A two-dimensional integral equation formulation is used to model electromagnetic scattering from metallic structures embedded in substrates. The new method involves a fast evaluation scheme for the Green's function of the substrate by using perfectly matched layers to obtain closed-form expressions as a series of leaky and Berenger modes. Efficient summation of these series is performed by means of the Shanks transform. The modal series can be applied to calculate all interactions, except for the self-patch contribution. The latter can be approximated with good accuracy by using the closed-form quasi-static part of the Green's function. The new technique is illustrated by studying scattering from wires embedded in a microstrip substrate.