Hitachi Cable is pleased to announce broad improvements made to the thermal properties of L-COP (Low expansion COPper), which is used as a material in heat sinks for semiconductor devices.

L-COP is a composite material characterized by low thermal expansion coefficient particles of Cu₂O (cuprous oxide) dispersed inside the high thermal conductivity Copper matrix. Through changing the content and distribution of Cu₂O, it becomes possible to keep the thermal conductivity in the range of 90-390W/m K (see note 1 below), and the thermal expansion in the range of 8-17x10-⁶/K (see note 2 below). L-COP was developed at the Hitachi Research Laboratory of Hitachi, Ltd., while Hitachi Cable will produce it commercially as a material for copper heat sinks.
Until recently, L-COP was manufactured through the powder metallurgy method. However, the application of hot extrusion technology as a method of manufacturing in an effort to improve high thermal conductivity and low thermal expansion was considered by the Company. As a result, Cu₂O particles were arranged in the direction of extrusion through hot extrusion, and as a result it became possible to produce differing thermal properties (thermal anisotropy). In comparison to the conventional method of producing L-COP, this new method improve thermal conductivity by at least 30% and thermal expansion by at least 10%. In particular, the large improvements in thermal conductivity are thanks to enhancements in the crystal continuity of the copper that is the conduit for the heat follows due to hot extrusion.

The power module heat sink made by hot extrusion technology is an example of the use of thermal anisotropy of L-COP manufactured through hot extrusion. In this case, the direction of extrusion of the L-COP is made perpendicular to the semiconductor chips, that is to say perpendicular to the heat sink, and the direction and angle of the extrusion are arranged parallel to the semiconductor chips. Through this arrangement, it is possible to efficiently diffuse the heat produced by the semiconductor chips perpendicular to the chip. Furthermore, with the parallel direction of the semiconductor chips, the heat expansion of the L-COP is small, so the heat stress that is caused by the difference between the insulation board and the expansion of the L-COP is relaxed and the reliability of the module increases.

The Cu-W (cupric tungsten) alloy and the Al-SiC (Aluminum Silicon Carbide) composite material are already being used as heat sink materials that work in the same way as L-COP, but costs for both materials and production have been prohibitive, processing has been difficult, and it was not possible to form it into shapes other than sheets. The L-COP, which was developed to solve these problems, will take the lead in properties thanks to the aforementioned improvements in thermal properties.

1	When there is a 1degree temperature difference in each unit length, the quantity of heat that is transferred in the unit area in the unit time.
2	The ratio of the length direction that expands with every 1degree increase.