Primary heat dissipation method of IC Chip is still air-cooling at present. This paper presents a new machining method of air-cooled plate fin heat sinks, namely orthogonal planing. The surface of plate fins machined by this method are rough enough to augment its specific surface area; the rough surface can also disturb laminar sub-layer of turbulent thermal boundary layer, thus the thermal resistances of thermal boundary layer are reduced. Therefore, heat dissipation efficiency of heat sink can be enhanced further. The focus of plate fin heat sinks forming by orthogonal planing is how to machine flat chips. A new pattern of chip curl that is called down-curl can be observed when rake angle of tool is rather large and cutting thickness is very thin. As cutting thickness increases, down-curl chips become flat and then up-curling. There is a transition area that chips are not curled between down- and up-curl. The conditions that chips are not curled are obtained through investigating influences of rake angle and cutting thickness on chip curl, and flat plate fins with rough surface are produced.