Tungsten Copper Heat Sink Material Hermeticity

Based on high density, high strength, excellent chemical stability and arc ablation resistance, tungsten copper material also has perfect thermal and electrical conductivity, which can be widely used in EDM electrodes, electrical contacts, military industry and heat sink materials. However, tungsten copper electrodes and contacts used for electrical machining has different requirements of properties with tungsten copper used in heat sink. As heat sink materials, tungsten copper not only be supposed to have excellent thermal conductivity, but also should have excellent hermeticity to ensure the equipment working stability. Since any porosity and leakage will lead to the operation failure of the microwave component.

In the early 1980s, tungsten copper has been chosen as heat sink materials, which due to it has the similar coefficient of thermal expansion to some ceramic, and both of them after brazing can not only guarantee the thermal matching, but also form a good thermal channel. Theoretically, W and Cu has big difference in physical and chemical properties, which the difference of melting point of them are as much as 2000℃. And they can not form a solid solution or a compound so that it is only suitable for using powder metallurgy process. And the best process is infiltration, tungsten powder is compacted to a certain density of blank, after sintering and shrinking at high temperature forms tungsten skeleton with a certain density and then at a temperature above the melting point of copper was infiltrated tungsten skeleton.

Since the high hardness and poor plasticity of tungsten powder, using a general manufacturing method infiltrated tungsten copper alloy will be some of the pores, the relative density can only reach about 60%. Densification at high temperatures also have some closing pores in copper infiltration process is not sufficiently filled with tungsten skeleton, reducing the product's tightness. In order to reduce the porosity of the relevant parameters need to be improved so that it can meet the requirement of hermeticity.
Merely rely on raising the temperature of the tungsten blanks shrink to increase the density, which is difficult to control precisely. Such as the deviation of heat matching, it is the major influencing factor of conventional infiltration hermeticity. Some researchers try to ultrafine tungsten powder added to certain activators, such as nickel (Ni), cobalt (Co), iron (Fe), etc., and then mixed with copper powder. After pressing and activated sintering, the relative density of tungsten copper can reach 98 % or more, which can be a good solution to tungsten copper heat sink material hermeticity issue. But it added activator of Ni, Co, Fe and other elements will be Cu and W-phase and phase mutually dissolved, and magnetic Fe, Cu content changes will directly affect the conductivity and the thermal conductivity of the material, so not suitable in the heat sink industry applications.

Therefore, on this basis, the researchers conducted a process optimization, the tungsten powder mixed with a small amount of copper powder, copper powder can effectively preserve this part of the green bodies connected pores, when carried out at high temperature and pressure infiltration of liquid copper can be fully effective filled tungsten skeleton. This section is also known as copper induction of copper, its main role lies in two main aspects, one is to enhance the strength of tungsten blanks, and the other is infiltration process hermeticity guarantee.

Tungsten powder has high hardness, poor plasticity, generally under pressure prone to "arch effect" between the powder. Once the pressure is too large, blank-arch bridge will make a release force is rupture and delaminated, and thus the relative density of tungsten powder can only reach about 60%. But copper powder has good plasticity, which can effectively destroy the "arch effect" between the tungsten powder and plastically deformed. Both of them can fill each other, engage with each other, thereby increasing the density and strength of tungsten copper green. On the other hand, since the initial tungsten skeleton have a uniform distribution of copper, the part of copper changed into liquid and complement with the copper liquid out side the skeleton at 1350℃.

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