Aluminum silicon carbide provides thermal management solutions for electronic packaging
INTRODUCTION
Designing low-cost, highly reliable microwave electronics, microelectronics, optoelectronics, and power semiconductor systems using state-of-the-art materials is impractical. To ensure the reliability of such devices, electronic packaging and substrate thermal management solutions are required, so engineers need materials that can provide thermal management characteristics while achieving optimal power density in smaller designs. Cost-effective production of such materials requires robust molding processes that meet the functional requirements of the package design.
Aluminum silicon carbide (AlSiC) metal matrix composites provide a highly reliable and cost effective thermal management solution for electronic packaging. It offers high thermal conductivity (~200 W/mK) and a tunable low coefficient of thermal expansion (CTE). The low density, high strength and stiffness of AlSiC give it advantages over traditional high-density materials for applications that require weight reduction and need to withstand shock and vibration.
Table 1 AISiC material properties
AlSiC can be manufactured in net-shape or near-net-shape at low cost. Examples of net-shape or near-net-shape fabricated AlSiC products are shown in Figure 1. In addition, the AlSiC forming process allows for economical integration with high heat dissipation materials such as diamond and high thermal conductivity graphite, making it ideal for applications requiring high heat dissipation capabilities. Because AlSiC provides the required thermal stability and temperature uniformity requirements. In addition, it is a preferred material for high-power transistors and insulated gate bipolar transistors (IGBTs), providing good thermal cycling reliability.
Aluminum Silicon Carbide Manufacturing Process
As a unique manufacturing process, AlSiC first produces porous, low-CTE silicon carbide (SiC) particles and then dissolves and infiltrates high-CTE aluminum metal into the mold. The AlSiC manufacturing process is cost effective because both the preform and the infiltrated mold cavity can be designed to match the end product shape. As a result, the cast composite product requires no further processing (net-shape manufacturing) or very little processing (near-net-shape manufacturing).AlSiC thermal conductivity values range from 180 W/m/K to 200 W/m/K, depending on the SiC/Al ratio.
Figure 1 AISiC products manufactured by net forming and near net forming
Figure 2 AlSiC prefabrication
AlSiC flip chip lid
AlSiC materials are primarily used for flip chip lids. AlSiC is an ideal material for this application because its CTE is compatible with dielectric substrates, ceramic ball arrays (BGAs), low temperature sintered ceramic (LTCC) materials, and printed circuit boards, and also has high thermal conductivity values (refer to Table 1 for AlSiC material grades corresponding to specific systems and component types). At the same time, the high strength and stiffness of AlSiC also provide protection for IC devices during the assembly process. The low density of these materials also improves reliability when the device is subjected to shock or vibration. For example, in highly automated assembly machines, where high-speed acceleration and deceleration movements between different step operations can cause inertial shocks and vibrations, utilizing AlSiC products can increase throughput.
AlSiC can be fabricated with complex profiles, thus enabling the manufacture of complex flip-chip packages at low cost. Figure 3 shows an example of a product profile with multiple cavities to accommodate electronics, pillars to provide IC device connections, holes for material fill, and different flange designs. the surface of AlSiC castings also supports different marking methods including laser marking, paint, ink, and screen printing, as well as electroplating, anodizing, and other surface metal treatments suitable for aluminum.
AlSiC Photovoltaic Packages
The geometric profile of an optoelectronic package is more complex than that of a flip-chip cover, and therefore requires more precise dimensional control for the optically aligned pattern. Figure 4 is an example of an AlSiC optoelectronic package. All packages in the figure are die-cast, and no additional machining is required for the critical optical alignment part. Therefore the cost is lower compared to conventional packages.
Thermal management in optoelectronic devices is also very important. Devices typically operate near room temperature, which requires materials with good thermal performance to maintain temperature uniformity and optimize cooler performance. adjustable matched CTE values for AlSiC ensure alignment of sensitive optics during operation, while also eliminating residual stresses that may be introduced during soldering or brazing assembly.
Materials such as alloy 48, iron-nickel-cobalt alloy (Kovar) and stainless steel can also be integrated to facilitate laser attachment of optics. This integration can be accomplished economically during the AlSiC forming process, where inserts can be inserted into the SiC preform prior to dissolution infiltration.CPS’s AlSiC manufacturing process allows for the simultaneous integration (Concurrent IntegraTIon) of these materials during the dissolution infiltration process.
Figure 3AlSiC Flip Chip Package
Figure 4 AlSiC optoelectronic package
Figure 5 AlSiC microprocessor cover and TPG board embedded with TPG material
AlSiC Power Semiconductor Substrates and IGBT Substrates
AlSiC has been used in power amplifier substrate applications since 1994. The coefficient of thermal expansion (CTE) of AlSiC can be adjusted for a specific application, specifically the ratio of aluminum metal to silicon carbide particles is typically modified to match the CTE value of the die or substrate. This eliminates the need for thermal interface stacking, which increases thermal resistance, and ensures that the IGBT substrate is compatible with the connected ceramic substrate in high power applications. the AlSiC forming process also allows for the net forming geometry discussed earlier.
AlSiC is also used for IGBT substrates in high power and high reliability systems. High power IGBTs are typically mounted on aluminum nitride substrates. The substrate material must match the CTE value of the aluminum nitride, thus preventing voiding or peel failure. AlSiC substrates have been shown to have excellent reliability for copper substrate systems, withstanding thousands of thermal cycles without failure. Although AlSiC and copper have similar thermal performance, the reliability of copper does not reach 1,000 thermal cycles. Figure 4 shows examples of AlSiC power device substrates as well as AlSiC IGBT substrates ranging in size from 45 mm × 85 mm to 140 mm × 190 mm.
AlSiC Rapid Thermal Solutions
The AlSiC molding process can incorporate materials that dissipate heat quickly during the metal-aluminum dissolution infiltration process. For example, materials such as pyrolytic graphite (TPG)2 and CVD diamond substrates can be integrated directly into AlSiC. These materials are more costly and difficult to integrate during assembly because they are fragile or require specially treated surfaces to adhere. AlSiC materials enhance the functionality of these materials and provide a useful method for direct integration. During AlSiC integration, these materials can be placed where they are most needed, enabling more economical use of these expensive materials. By and large, these applications are limited to high-performance and military systems. In fact, AlSiC flip-flop systems embedded with fast heat dissipation materials are being tested and evaluated. Figure 5 shows an AlSiC microprocessor cover embedded with TPG material and the TPG board in front.
Conclusion
The properties of AlSiC materials make them a reliable and cost-effective solution for electronic thermal management and packaging applications. Its large thermal conductivity and CTE matching capabilities eliminate thermal interfaces and prevent field failures, thus providing the reliability needed for different microwave electronics, microelectronic devices, optoelectronic devices and power semiconductor systems. In optoelectronic packaging applications, AlSiC can produce complex geometries that are critical for optical alignment. In addition, its thermal properties provide higher reliability than other material systems for power substrate applications.
The AlSiC forming process allows for the easy addition of additional functional accessories that can be used to meet custom design requirements. AlSiC is cost effective for any size application, and its net shape manufacturing capability allows for the fabrication of low cost AlSiC heat sink parts that meet the requirements of various system engineers.
