How can I remove artifacts from a flat surface after using a boolean modifier? I tried to model a simple keypad, I got this model after using a boolean modifier on an array of cylinders and a cube:



The face of the cube had way too much edges and as suggested here Cutting hole into a face with a line loop? I used limited dissolve and got this:



What would be a good way not to get the rendering artifacts on the plane?

EDIT:

The face normals look like that:
The problem was solved by setting Shading to flat in the Object Tools Panel  T