To reduce parasitic mismatches in analog design, we usually care about the property of symmetric placement for symmetry groups,which would form several symmetry islands in a chip. However, routing is greatly affected by placement results. If modules with nput or output ports are placed arbitrarily in a symmetry island, the routing wires, which connect these modules with other modules outside the island, may induce unwanted parasitics coupling to signals, and thus circuit performance is deteriorated. This phenomenon can not be identified by a cost function, which only considers placement area and total wire length. Therefore, we would like to introduce the necessity of considering boundary constraint for the modules with input or output ports in symmetry islands. Based on ASF-B* tree , we explore the feasible conditions for 1D and 2D symmetry islands to meet this constraint. Further, a procedure is presented to maintain the easibility for each ASF-B* tree after perturbation. Experimental results show that our approach guarantees the boundary property for the modules with input or output ports in symmetry islands.