Metal based heat dissipation devices have been developed for decades in the fields of electronic packaging and aerospace. With the continuous increase of device power density, higher requirements have been put forward for the thermal conductivity of electronic packaging materials. By combining high thermal conductivity (2200 W/(m ? K)) and low thermal expansion coefficient (8.6 ± 1) × Diamond (10-7/K) composite with metals such as copper and aluminum can be integrated into a "metal+diamond" composite material with high thermal conductivity, adjustable thermal expansion coefficient, and excellent mechanical and processing properties, meeting the stringent requirements of different electronic packaging. It is considered as the fourth generation electronic packaging material.
What are the advantages of diamond/copper composite materials?
Among various metal materials, copper has a higher thermal conductivity (385-400 W/(m ? K)) and a relatively lower coefficient of thermal expansion (17) compared to other metals such as aluminum × 10-6/K), with the addition of a smaller amount of diamond reinforcement, the thermal expansion coefficient can be matched with the semiconductor, and it is easy to obtain higher thermal conductivity. It not only meets the stringent requirements of electronic packaging today, but also has good heat resistance, corrosion resistance, and chemical stability, which can meet the requirements of extreme service conditions such as high temperature and corrosive environments to a greater extent, such as nuclear power engineering, acid-base and dry wet cold hot alternating atmospheric environments.
How to prepare it?
Due to the high interface energy and poor wettability between diamond and copper, the resulting interface thermal resistance is large, which hinders the improvement of the thermal conductivity of the composite material. Therefore, in practical applications, in order to achieve excellent comprehensive performance of the material, in addition to pre metallizing the diamond surface or alloying the copper matrix, the preparation process also needs to comprehensively consider various factors such as the interface bonding between diamond and copper matrix, as well as the dispersion of diamond in the copper matrix.
There are currently many methods for preparing diamond/copper composite materials, such as powder metallurgy, chemical deposition, mechanical alloying, spray deposition, casting, etc. Among them, powder metallurgy has become one of the commonly used preparation methods due to its simple preparation process and excellent performance of composite materials. This method can mix Cu powder and diamond particles evenly through ball milling, and then sintering to prepare composite materials with uniform microstructure. As a crucial step in powder metallurgy, sintering molding is related to the final quality of the finished product. The commonly used sintering processes for the preparation of Cu/diamond composite materials currently include hot pressing sintering, high-temperature and high-pressure sintering, and discharge plasma sintering.
01. Hot press sintering
Hot pressing sintering method is a diffusion welding forming method. As a traditional method for preparing composite materials, its main process is to mix the reinforcement and copper powder evenly, place them in a specific shaped mold, heat them in the atmosphere, vacuum or protective atmosphere, and apply pressure in a uniaxial direction to make the forming and sintering process proceed simultaneously. Due to the fact that the powder is sintered under pressure, the flowability of the powder is good, the density of the material is high, and residual gases in the powder can be discharged, thereby forming a stable and firm interface between diamond and copper, improving the bonding strength and thermal physical properties of the composite material.
Zhang et al. prepared copper/diamond composites with a thermal conductivity of up to 721 W/(m ? K) using hot pressing sintering method after pre metallization of diamond.
Advantages: The ratio of diamond to copper powder can be freely adjusted according to actual needs, and as a traditional preparation method for composite materials, the process is more mature and the preparation conditions are simple,
Disadvantages: This method relies on the control of sintering parameters and the addition of active elements to optimize interface bonding, while being constrained by equipment and molds. It is also subjected to axial unidirectional pressure, resulting in smaller material sizes and a more singular shape.
02. Ultra high temperature and high pressure sintering
The mechanism of ultra-high pressure and high temperature method is similar to that of hot pressing sintering method, except that the applied pressure is relatively large, usually 1-10 GPa. By using higher temperature and pressure, the mixed powder can be quickly sintered and formed in a short period of time. In order to achieve high pressure, the commonly used equipment is a hexagonal top ultra-high pressure press. In a high-pressure cubic cavity, high density composite materials can be obtained by simultaneously applying pressure on six faces, and the powder inside the cavity is subjected to forces from all six faces simultaneously.
Yoshida et al. successfully prepared diamond/copper composites with a thermal conductivity of 742 W/(m ? K) under high temperature and pressure conditions of 1150-1200 ℃ and 4.5 GPa. Among them, the diamond particle size is 90-110 μ m. The volume fraction is 70%.
Advantages: High density, short preparation time, high efficiency. In the case of high volume fraction of diamond, the phenomenon of direct bonding between high-temperature and high-pressure diamonds can bring ultra-high thermal conductivity.
Disadvantages: Special equipment and high conditions are required to achieve this, which is expensive and cannot fully solve the problem of difficult bonding between diamond and copper.
03. Spark plasma sintering
Spark plasma sintering (SPS) is the process of applying high-energy pulse current to a powder and applying a certain pressure to excite plasma between particles. The high-energy particles generated by the discharge collide with the contact surface between particles, which can activate the particle surface and achieve ultra fast densification sintering.
Gan Zuoteng et al. prepared diamond/copper composites with a thermal conductivity of 503.9 W/(m ? K) under the conditions of sintering temperature of 800-1000 ℃, sintering pressure of 30 MPa, heating rate of 100 ℃/min, and holding time of 5 minutes after chrome plating pretreatment of diamond.
Advantages: During the SPS sintering process, there is an active interaction force between the powder particles, which requires a low sintering temperature (usually 800-950 ℃), low pressure (50-80 MPa), extremely short time, and saves energy.
Disadvantages: The sintering process is difficult to accurately control, and there are certain difficulties in controlling the interface composition and thickness. The density of the prepared composite material is slightly low, and complex workpieces cannot be prepared.
Summary
Diamond/copper composite materials not only have high thermal conductivity (often up to 600 W/(m ? K)), but also have a coefficient of expansion that matches electronic semiconductor packaging materials. Powder metallurgy has become one of the commonly used preparation methods due to its simple preparation process and excellent performance of composite materials. However, due to the inability to completely solve the interface thermal resistance problem between diamond and copper, and the difficulty in preparing complex shaped workpieces, its application is limited. In the future, research on high thermal conductivity copper based composites should focus on improving the interface bonding and thermal conductivity performance between the thermal conductivity reinforcement and the matrix (such as pre metallization of diamond and copper based alloying), in order to optimize the comprehensive thermal conductivity performance of the composite materials and achieve economic and efficient application of high thermal conductivity copper based composites.