To reduce the separation of coolant flow from the high temperature surface of a substrate, by tilting a passage hole at a specific angle to the substrate, by making the depth of an exit point smaller than the length of the passage hole, and by contacting the coolant flow with the area which is larger than that of the high temperature surface.

The surface 12 of a substrate 10 is a hot surface, while a surface 14 is a cold surface. A passage hole 16 passes through a hole length part 18, and comes out on the cold surface by a hole bottom 20. The distance between both surfaces is equal to the thickness of a normal substrate, and this distance is within the range of about 20 to 2000 mil. The hole is located at a certain angle to the horizontal surface of the normal substrate, and it is in the range of about 20 to 45° in many cases. The angle of the hole is determined by the shape of parts, cooling conditions, the air flow on the substrate, and so on. In order to maximize the cooling efficiency, the coolant flow coming off of the hole preferably contacts with the hot surface 12 as much as possible. Moreover, it is important that the exit point, for example the depth of a crater, is smaller than the length of the passage hole. This depth is preferably reduced about 50% of the length of the passage hole.