Using a trim router for routing shallow hinge mortises proves a no-brainer. A full-size router can tip or wobble when you balance it on a workpiece edge (a door, for example). But a trim router, with its narrow base, light weight, and low center of gravity, makes the job easy. Use a template with a top-bearing dado cleanout bit (with a small cutterhead), as shown. Square up the corners, if needed, with a chisel.