Square shaped face has a strong forehead and jaw line.
There are a few suitable eyebrow shapes for a square face. A thick strong arch will create a more balanced look because the angles of the arch take away from the strong facial features.
Another option is to go with a soft gentle arch, sticking with a thick brow to take away from the square shape and draw attention to rounded features of the eyes. Or, a thick brow with high peaked arch. Either option uses a thick brow and a peak to draw away from the boxy look of a square face.