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By Alaa A. Elmoursi
Figure 1. Thermal conductivity of electronic packaging materials as a function of their thermal expansion coefficients
It is well known that composite materials, which in the present context means two or more materials consolidated together, can possess a wide range of physical properties. Their use in electronic packaging is not new: the printed circuit board familiar to all of us is an example of a polymer matrix composite (PMC). Metal-matrix composites (MMC) are fabricated using a high thermal conductivity metal matrix such as aluminum or copper, with a low a material added to reduce the overall a of the composite. One such material that has undergone considerable commercial development is the aluminum/silicon carbide composite, represented by the dashed lines in Figure 1. By proper adjustment of the relative composition of the composite, the a can approach that of silicon and insulating materials while maintaining high thermal conductivity. The use of kinetic spray to fabricate MMC's addresses two important areas: further improvement of the material properties of MMC's for thermal management, and advanced manufacturing techniques that will allow cost-effective MMC fabrication on a production scale.
Kinetic spray processing: MMC's are typically made by squeeze casting molten metal into a ceramic perform. Because of the high temperature involved, undesirable compounds can be formed between the molten metal and the ceramic. In addition, this process cannot be employed to make MMC coatings. In 1994, Alkhimov and collaborators suggested a process whereby particles less than 50 microns in diameter could be sprayed to form a coating, with particles impinging on the substrate at about room temperature or even below. Delphi Research Labs (DRL) researchers discovered how to breech the 50 micron diameter barrier in making coatings with "cold" particles via an innovative spray gun design, producing coatings with particles as large as 200 microns. They named their new process kinetic spray because, for their particles to stick to the substrate, all of the incident particle's kinetic energy must be converted to heat or strain energy via plastic deformation upon impacting the substrate. Moreover, the DRL researchers found that they could kinetic spray mixtures of ceramic and metal particles, thereby forming MMC coatings without the requirement of melting the metal.
Figure 2. Schematic diagram of the kiunetic spray process
A schematic diagram of the kinetic spray nozzle we use at DRL is shown in Figure 2. The main gas flow is delivered to the premixing chamber located downstream of the flow straightener at a pressure of about 2 MPa (300 psi) and a temperature that is controllable between 100 and 500 °C. Simultaneously, powder is delivered to the premixing chamber at a pressure of about 2.4 MPa (350 psi). The nozzle inlet has a restricted zone with a selected diameter such that the gas achieves supersonic velocity upon exiting the nozzle. The powder particles accelerate due to drag effects with the gas. These high velocities cause the powder particles to plastically deform as they impact the substrate and form a coating. However, the mechanism for solid particles colliding with a substrate to form a coating is not completely understood and requires further research.
Aluminum composites have been formed by kinetic spraying of powder mixtures. The Al/Diamond and Al/SiC mixtures had a composition of 70% Al by volume and the balance diamond and SiC, respectively. The Al/AlN and Al/W mixtures had a composition of 50% Al by volume and the balance AlN or W. All composites were sprayed on brass, stainless steel, aluminum and alumina substrates.
In order to facilitate a better understanding of the properties of Al MMC's we have studied the properties of kinetically sprayed pure Al. The as-deposited coating has a room temperature thermal conductivity of only about half of that of bulk aluminum (114 W•m-1•oC-1 vs 240 W• m-1•°C-1), but after annealing at 550 °C the conductivity rises to approximately 168 W•m-1•°C-1 (70% that of bulk).
Table 1. Qualitative adhesion of the Al-based composites and pure aluminum to brass stainless steel, aluminum and silver coated alumina. "Perp" indicates thermal conductivity perpendicular to the spray direction; "par" indicates parallel to the spray direction.
The powder mixtures of the Al-based composites were kinetic sprayed on several substrates. A summary of the coating thickness deposited on an aluminum substrate is shown in Table 1. The thickness of the composites is about one half of that of the pure aluminum, suggesting that the hard material in the matrix is eroding away the aluminum during spraying. However, as can be seen in the cross-sectional scanning electron microscope (SEM) images, the hard material also becomes lodged in the aluminum matrix. With the exception of the Al/AlN composite, all other composites could be deposited to several millimeters of thickness.
Results for adhesion of the Al-based composites to several substrates are shown in Table 1. The "Yes" and "No" in the table refer to whether the sprayed composite powder formed a uniform coating on the substrate or not. Table 1 also displays thermal conductivity results on the as-deposited coatings as well as that after annealing at 550 °C. The pure aluminum and the aluminum/SiC composite coatings were thick enough to measure the thermal conductivity both perpendicular and parallel to the spray direction. The other composite coatings were not thick enough to perform measurements in the direction parallel to the spray.
Figure 3. Cross-sectional SEM image of the Al/diamond composite on a brass substrate (bottom). The diamond particles are the dark contrast in upper layer.
Al/Diamond composite: Figure 3 is a cross-sectional SEM image showing the interface between the Al/diamond composite and the brass substrate, which was sand blasted before applying the coating. The interface shows no sign of delamination and the aluminum is clearly providing the bonding to the brass substrate. The diamond is uniformly dispersed in the matrix. EDX results indicate 66% C and 34% Al by weight (56% C by volume, measured without standards), indicating a high content of diamond in the matrix.
The thermal conductivity at room temperature of the as-deposited Al/diamond MMC perpendicular to the spray direction is only 168 W•m-1•°C-1, but rises to 202 W•m-1•°C-1 after annealing. As far as I know, this is the first time an Al/diamond composite has been successfully formed by any method. Johnson and Sonuparlak reported the formation of a diamond/Al composite by pressureless infiltration, but in this case it was necessary to coat the diamond particles with SiC prior to infiltration to prevent the formation of Al4C3. A distinct advantage of the kinetic spray process is that it allows the formation of Al/diamond at low temperatures, thus obviating the need for any additives or reaction inhibitors.
In conclusion, the work at Delphi has shown that the kinetic spray method can be successfully used to fabricate composites of an aluminum matrix and hard materials such as SiC, AlN, diamond and tungsten. Kinetic spray lends itself to low-cost and high-volume processing. In addition unique composites can be fabricated.
Alaa Elmoursi is the Group Leader-Coating Materials in the Manufacturing Dept. of Delphi Research Labs in Shelby Twp., MI.
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