In floorplanning of VLSI design, it is useful if users are allowed to specify some placement constraints in the packing. One particular kind of placement constraints is to pack some modules on one of the four sides: on the left, on the right, at the bottom or at the top of the final floorplan. These are called boundary constraints. In this paper, we enhanced a well-known slicing floorplanner to handle these boundary constraints. Our main contribution is a necessary and sufficient characterization of the Polish expression, a representation of the intermediate solution in a simulated annealing process, so that we can check these constraints efficiently and can fix the expression in case the constraints are violated. We tested our algorithm on some benchmark data and the performance is good.