Making the Matrix
By Kevin Loutfy, Nano Materials International Corp.
It has taken more than a decade to overcome the hurdles on the way to achieving aluminum diamond metal matrix composites that can be reliably produced in large quantities at low cost. However, considering the benefits they can deliver to GaN device, amplifier, and subsystem manufacturers, the effort was well worth it.
To create an MMC, a metal such as aluminum, copper, or silicon is combined with diamond or silicon carbide. In the case of NMIC’s MMCs, an aluminum alloy and low-cost, industrial-grade synthetic diamond particles are employed. Besides having unparalleled thermal conductivity, diamond is also extremely hard. So while the end product must have high thermal conductivity and a CTE close to that of SiC, it must also retain mechanical strength. To achieve this, fine and coarse diamond particles are blended to allow the most diamond (60% by volume) to be incorporated in the mix.
Voids under the active area of a die are the bane of all RF power device manufacturers because they make it possible for hot spots to arise that lead to premature device failure. To meet manufacturer requirements, NMIC’s product has been refined to reduce even extremely small voids between the diamond particles and aluminum alloy to maintain optimum thermal conductivity and thermal stability. It employs an aluminum alloy skin from 0.05 to 0.1 mm thick on the top and bottom surfaces of the part and the skin becomes part of the matrix microstructure (Figure 4). This ensures that a strong, homogenous, smooth surface with surface roughness of less than 1.0 µm Ra is created onto which metallization can be applied (Figure 5).
For use as a heat spreader, an MMC must be able to survive wide swings in temperature encountered in operation, which NMIC solved by creating a process that produces a precise SiC surface layer on the diamond particles. While a conventional SiC coating acts as an additional thermal interface between the SiC layer and diamond particles, NMIC’s SiC surface-conversion layer actually becomes part of the diamond particles themselves, which results in negligible thermal resistance in the metal matrix, high strength and stiffness, and allows thermal conductivity to extend to near the theoretical limit.
NMIC produces the aluminum diamond itself by a process called “squeeze casting,” a manufacturing-proven process widely used in the production of other thermal management materials such as aluminum silicon carbide (AlSiC). It employs high-pressure infiltration of molten aluminum or aluminum alloy into a preform that contains SiC-converted diamond powder.
The pressure achieved by squeeze casting is higher than processes such as gas pressure-assisted infiltration and results in higher thermal conductivity. It also consolidates the matrix in seconds rather than minutes required by other processes, reducing the time aluminum or aluminum alloy is in a molten state. This is important because there is little reaction time available to allow aluminum carbide to form, as aluminum carbide increases thermal resistance at the diamond/aluminum interface, reduces thermal conductivity, and degrades performance during temperature or humidity cycling.
Nickel-gold plating is applied to the MMC to support die attach. Nickel plating and then gold plating are applied as the initial plating, followed by 2 µm of gold, after which chemical resistance and bake tests to determine plating adhesion are conducted to validate the plating process. Customers have also performed these tests to simulate the cleaning that occurs during device fabrication, and no weight loss or blistering of the plating occurred.
Solder die attach, yield, and RF and thermal testing are now routinely obtained by the company’s customers using gold tin solder between the aluminum diamond heat spreader and the die. CTE and thermal conductivity from room temperature to higher than 400° C show that CTE increases up to 400° C and then slowly declines at temperatures much higher than what is required of a GaN heat spreader. In addition, thermal conductivity decreases with increasing temperature but remains more than 400 W/mK well past the operating temperature of GaN devices.
NANO MATERIALS INTERNATIONAL CORP.
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