What Are the Differences Between Diamond-Copper, Diamond-Aluminum and Diamond-SiC Composites?

A thermal engineer is designing the heat spreader for a 1,000 watt processor. The heat flux at the die surface exceeds 300 watts per square centimeter. Copper cannot spread the heat fast enough, and aluminum is worse. The engineer turns to Diamond Composite Materials, which combine diamond particles with a metal or ceramic matrix to reach thermal conductivities several times higher than copper. But there are three main systems to choose from: diamond-copper, diamond-aluminum, and diamond-silicon carbide. Each has a different thermal conductivity ceiling, a different coefficient of thermal expansion, a different density, and a different interface engineering challenge. This guide compares the three systems and explains how to select the right one.

1. Why Does the Choice Between Three Diamond Systems Come Down to Thermal Conductivity Versus CTE Matching?

The purpose of a diamond composite is to move heat away from the device while matching the thermal expansion of the device and the substrate. If the thermal conductivity is too low, the device overheats. If the coefficient of thermal expansion is too high, the solder joint or the die cracks during thermal cycling. The three diamond systems sit at different points on this trade-off. Diamond-copper has the highest thermal conductivity ceiling, but its coefficient of thermal expansion is higher than that of silicon. Diamond-aluminum is lighter, but its conductivity is lower than diamond-copper. Diamond-silicon carbide has a coefficient of thermal expansion that is closer to silicon, but its conductivity depends on the residual silicon content and the interface quality. In our factory, we produce Diamond Composite Materials for thermal management, and we help customers compare the three systems. WuYi TianYao New Material Tech.Co.,Ltd. has been developing diamond composites for over 8 years.

The selection is therefore not a matter of picking the highest number on a datasheet. It is a matter of matching the material to the device, the substrate, and the cooling method. A material with a thermal conductivity of 800 watts per meter kelvin is useless if its coefficient of thermal expansion is so high that the die cracks after 100 thermal cycles.

2. What Performance Boundaries Separate Diamond-Copper, Diamond-Aluminum, and Diamond-SiC?

The three systems have different performance envelopes. Diamond-copper typically reaches a thermal conductivity of 600 to 800 watts per meter kelvin, with reported values above 900 in laboratory samples. The coefficient of thermal expansion is 8 to 12 parts per million per kelvin, and the density is 6.0 to 6.7 grams per cubic centimeter. Diamond-aluminum reaches 450 to 700 watts per meter kelvin, with a coefficient of thermal expansion of 7 to 10 parts per million per kelvin and a density of 3.0 to 3.2 grams per cubic centimeter. Diamond-silicon carbide reaches 300 to 800 watts per meter kelvin depending on the residual silicon content, with a coefficient of thermal expansion of 2.0 to 3.5 parts per million per kelvin and a density of 3.1 to 3.4 grams per cubic centimeter. The table below summarizes the typical engineering ranges.

In our factory, we measure the thermal conductivity by laser flash and the coefficient of thermal expansion by dilatometer for every batch. The data is provided with the shipment. VETEK also provides the density and the bending strength, so that the designer can compare the three systems on a common basis.

3. How Does Interfacial Bonding Set the Real Performance Ceiling of Each System?

The interface between the diamond particle and the matrix is the single largest factor limiting the thermal conductivity. Diamond does not wet copper, aluminum, or silicon carbide well. If the interface is weak or if there is a gap, the heat cannot cross from the diamond to the matrix, and the high conductivity of the diamond is wasted. Diamond-copper requires a carbide-forming element such as titanium, chromium, or tungsten, or a diamond surface metallization, to form a bond. Diamond-aluminum is sensitive to the formation of aluminum carbide at the interface, which can improve bonding but degrade conductivity if it grows too thick. Diamond-silicon carbide uses a silicon infiltration process, and the residual silicon affects both the conductivity and the expansion. The table below shows the interface strategy and the typical conductivity improvement for each system.

4. Why Do Density and Mechanical Constraints Eliminate Some Options Before Thermal Performance Is Even Considered?

Density is often the first constraint that eliminates a system. Diamond-copper has a density of 6.0 to 6.7 grams per cubic centimeter, which is more than twice the density of aluminum. For an airborne or portable application, this density is unacceptable, and diamond-aluminum becomes the only viable option. Diamond-silicon carbide has a density of 3.1 to 3.4 grams per cubic centimeter, which is slightly higher than diamond-aluminum but still much lower than diamond-copper. Mechanical constraints also matter. Diamond-silicon carbide is brittle and difficult to machine, but it has good corrosion resistance and can be used in direct liquid cooling. Diamond-copper is easier to machine and to solder, which makes it suitable for solderable heat spreaders. Diamond-aluminum is difficult to solder because of the oxide layer, but it can be used with thermal grease or with a nickel plating. In our factory, we help customers screen the three systems against the density, the machinability, and the joining method before we compare the thermal performance.

We also consider the coefficient of thermal expansion relative to the device and the substrate. Diamond-silicon carbide is the closest match to silicon, which makes it suitable for direct die attach. Diamond-copper requires a compliant solder or a graded interlayer to bridge the expansion mismatch. Diamond-aluminum sits between the two and is often used with a nickel-gold plating and a solder that can accommodate the mismatch.

5. How Should Engineers Specify and Verify a Diamond Composite Material for a Given Thermal Application?

The specification should include six items. The first is the target thermal conductivity and the measurement method. The second is the coefficient of thermal expansion and the temperature range. The third is the density limit. The fourth is the diamond volume fraction. The fifth is the interface treatment. The sixth is the relative density, which indicates the porosity. The verification should include four tests. The first is the laser flash measurement of thermal diffusivity, which is converted to conductivity using the specific heat and the density. The second is the dilatometer measurement of the coefficient of thermal expansion. The third is the Archimedes density measurement. The fourth is the metallographic and interface analysis. In our factory, we perform these tests on every development batch and on a sample basis for production batches. VETEK provides the test report with each shipment and can provide a third party verification on request.

We recommend that the designer specify the thermal conductivity at the operating temperature, not at room temperature. The conductivity of diamond-copper and diamond-aluminum decreases as the temperature rises, and the decrease can be 10 to 20 percent between 25 and 150 degrees Celsius. A material that meets the specification at room temperature may not meet it at the device operating temperature.

Summary for Thermal Design Engineers

The differences between Diamond Composite Materials systems are not a matter of one being universally better. Diamond-copper offers the highest thermal conductivity but the highest density and the highest CTE. Diamond-aluminum offers the lowest density at a moderate conductivity. Diamond-silicon carbide offers the closest CTE match to silicon and good corrosion resistance at a moderate conductivity. The real performance of each system is set by the interface engineering, not by the diamond fraction alone. The specification should include the thermal conductivity, the CTE, the density, the diamond fraction, and the interface treatment, and the verification should include laser flash, dilatometer, density, and metallographic tests. WuYi TianYao New Material Tech.Co.,Ltd. has been developing Diamond Composite Materials for over 8 years and provides full comparison and verification support for thermal design engineers.

Need help selecting or verifying a diamond composite material for your thermal application? Contact WuYi TianYao New Material Tech.Co.,Ltd. for a free consultation. We will review your heat flux, CTE match, and weight constraints and recommend the optimal composite system.