AlSiC Microprocessor Lids Handle the Heat
December 31, 1969 |Estimated reading time: 10 minutes
Microprocessor clock speeds operate at 2 GHz with power levels up to 130 W. This calls for materials, heatsinks and lids that can dissipate heat and reduce thermally induced stresses. AlSiC metal-matrix composite can provide a solution. This article discusses advanced microprocessor lid designs using high-heat-spreading materials.
By Mark A. Occhionero
Aluminum Silicon Carbide (AlSiC) metal-matrix composites not only offer required thermal-management solutions, but also provide device-compatible thermal expansion (unlike copper), and a high thermal conductivity at 200 W/mK. AlSiC microprocessor lids are cast to shape with tight dimensional control, and the metal-matrix composite material is lightweight and high strength, yielding improved shock and vibration tolerance and protection to the microprocessor during assembly and service life.
Forecasts by the semiconductor industry indicate that in 2005, advanced microprocessors will break the 200-W power threshold and operate at 5 GHz. Advancing thermal management needs may not be met by AlSiC material alone. One solution is to incorporate advanced heat-dissipation materials such as thermal pyrolytic graphite* and diamond into the AlSiC composite using a casting process.** These materials have thermal-conductivity values greater than 1,000 W/mK and can dissipate heat rapidly. However, they are costly and require complicated assembly integration. AlSiC provides a means of integrating these materials efficiently, as well as a functional attachment method. In most cases, these materials can be incorporated into existing lid designs without changing current assembly configurations.
Ever-increasing power densities in microprocessor devices have resulted in the need for microprocessor assembly materials to provide improved thermal dissipation for improvements in reliability. These composite materials need to have high thermal conductivities for high thermal dissipation. They also need to have coefficient of thermal expansion (CTE) values compatible with devices and assembly. Without compatible CTE values, wide differences in thermal expansion will result in thermal stresses as a device heats and cools, which can cause failure and decreased reliability.
AlSiC materials have high thermal conductivity values of 200 W/mK and, by varying composition, can be made compatible with lower CTE semiconductor devices or low-CTE dielectrics and higher CTE assembly materials, such as PCBs and LTCC materials. The AlSiC CTE is controlled by the composition of SiC, in particulate form, to the aluminum metal matrix. The material is fabricated by an Al-metal casting into a precision casting mold that contains a porous particulate SiC preform of a defined volume concentration and shape. Both SiC preform and casting molds have the shape of the finished product, which include walls, septums, pedestals, cavities, guides and other complex functional features for cost-effective manufacturing.
The AlSiC composite material has other desirable attributes, such as high strength and stiffness, while remaining lightweight. This provides a robust enclosure for electronic components while reducing shock and vibration issues during service. Composite strength also is important to maintain shape during high-speed automated assembly associated with higher-density materials.
Figure 1. AlSiC lid products, all of which are cast geometries (except thru-holes). Ni- and Ni/Au-plated lids are shown.
As a result of these thermal-management attributes and cost-effective forming capabilities, AlSiC lids have seen increased use in microprocessors and flip-chip applications. Typical lid product examples (Figure 1) were cast to shape requiring no machining,1 and are provided with “as-cast” Al-metal surfaces that support Ni, Ni/Au and other plating or anodization. Beyond current thermal-management capabilities, AlSiC products can be integrated with high-heat dissipation materials such as CVD diamond or pyrolytic graphite* for improved thermal dissipation without significant change to existing product designs. AlSiC materials provide a functional means of integrating these materials into microprocessor systems.
Discussion
Typical materials used in microprocessor heatsink-lid systems are shown in Table 1. The table also compares CTE, thermal conductivity, material density and strength and stiffness. Semiconductor materials, Silicon (Si) and Gallium Arsenide (GaAs) also were included for CTE-value comparisons. From the data given in Table 1, AlSiC-9 is the material best suited for a heatsink lid, based on CTE compatibility with semiconductor devices (less than half that of copper), lightweight (one-third the weight of copper), and having both high strength and stiffness. Higher thermal-conductivity values for copper cannot be fully exploited because of its incompatible CTE value when compared to semiconductor materials. Stress compensating layers must be inserted between copper and the devices to reduce thermally induced stresses. These stress-compensation layers often have high-thermal resistance and reduce thermal-dissipation capabilities of the assembly. It has been demonstrated that equivalent thermal dissipation performance and improved service reliability are achieved with AlSiC for high-power semiconductor devices, when compared with equivalent copper heat sinks.2
AlSiC CTE values are defined by the ratio of the SiC in the Al-metal matrix. Lower CTE AlSiC materials have a higher volume concentration of the low-CTE value SiC particulate (4 ppm/°C). These materials are compatible with attachment to ceramic substrates; low CTE LTCC materials are compatible with direct GaAs devices. Higher CTE values are obtained with lower SiC content, allowing for a greater contribution to the composite CTE value from the higher CTE-value aluminum metal (23 ppm/°C). These materials are compatible with attachment to circuit boards and assemblies using high CTE LTCC materials.
Figure 2. Porous particulate SiC preform (left) and final AlSiC composite lid (right). SiC preform and final product have similar geometric features.
One*** AlSiC material and product fabrication starts with a porous SiC particulate preform that is injection-molded to the shape of the final part. The SiC concentration is measured and controlled in this process, as the resultant concentration defines the CTE of the final composite. The porosity of SiC particulate preform is completely infiltrated by casting molten Al-metal into a precision casting mold that forms the shape of the final part. The result is that both the material and shape of the final product are fabricated in a single process step for a cost-effective manufacturing process. Figure 2 shows an example of the porous SiC preform and the resulting AlSiC microprocessor lid. It is important to note the close dimensional and geometrical similarity between the preform dimensions and the final product. Figure 3 shows an optical micrograph of an AlSiC polished cross section. Note that all the SiC particles are surrounded (porosity completely infiltrated) by a continuous metal-matrix phase of aluminum to form a fully dense and hermetic composite.3
Figure 3. Optical micrograph of polished AlSiC cross section. SiC particulate phase (dark contrast) and Al metal-matrix (white contrast) fill porosity of SiC preform completely to form a fully dense AlSiC-composite microstructure.
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Designs
AlSiC products have been incorporated in many lid-product applications. In most cases, these applications are simple heat sinks with a simple lid, a flip-chip cavity and a surrounding wall, competing with stamped products of Cu and Al of similar, but less-complex, designs. The AlSiC fabrication process allows for more complex geometrical features, including pedestals, chip cavities of varying depth, pin fins and walls. Some of these features are illustrated in Figures 3 and 4.
Figure 4. Cast pin fins on an AlSiC baseplate illustrating the cast product-process capabilities.
Because AlSiC products are cast, they require a draft angle of 3° to 5° on outside product dimensions and on internal features such as walls, cavities or pedestals, to allow removal from the infiltration casting molds. AlSiC dimension-tolerance capability is ±0.005 in. (±0.127 mm) on all dimensions. Critical cavity depths (wall heights) are held at ±0.002 in. Outside and inside corners also require a minimum radius of 0.032 in. (0.81 mm). Flatness values are 0.002 in./in. (0.05 mm/25.4 mm).
Figure 5. Flip-chip lids measuring 41 mm2, integrated with TPG sheets in the foreground. Sheets are 25-mm2 x 0.81-mm thick.
Pin fins, which increase the cooling surface area, also can be incorporated in cast-AlSiC products (Figure 5). The minimum pin diameter is 0.040 in. (1 mm) at the tip; and spacing is 0.040 (1 mm) at pin base. Maximum pin height is 0.2 in. (5 mm). A 7°- to 15°-draft angle is required for pin features to allow better removal from the infiltration mold.
Applications
The design and composition of AlSiC products allow them to serve as unique thermal-management solutions in microprocessor assemblies. AlSiC products assist in increasing functionality and reliability in assembly and service, as well as possessing thermal-management attributes. The high strength and stiffness of AlSiC (about two times greater than copper), and the close dimensional control of cavity depth coupled with cavity flatness, allows for greater application pressure during assembly for a thinner bond-line length. Higher strength and stiffness avoids flexing under higher loads, and avoids putting stress on the fragile chip or chip interconnections. Lightweight AlSiC components have positive influence yields in high-speed automated assemblies, reducing inertial and shock-related forces that can cause interconnect failure.
Advanced Solutions
There are applications and designs with thermal dissipation needs beyond AlSiC. For these applications, AlSiC can be combined with high-heat-dissipation materials such as CVD diamond and pyrolytic graphite.* CVD-diamond materials have thermal conductivity values of 1,200 to 1,800 W/mK, dependent upon grade. Pyrolytic graphite* has anisotropic thermal conductivity with a value of 1,350 W/mK in the X-Y plane, and a low value of 20 W/mK in the Z plane. Despite this anisotropy, designs with this material can take advantage of high-heat-spreading inserts to distribute heat to areas where is can be better managed.
These materials can be integrated in the AlSiC-forming process for a hybrid-composite structure of AlSiC and high-heat-spreading material. Integration is accomplished by assembling both the SiC preform and the high-heat-dissipation material in the same infiltration tooling. The result is a hybrid composite of both materials after infiltration in a casting process** that provides an advantage in integrating materials in electronics assemblies. The process allows high-heat dissipation inserts to be strategically placed in the area of need, economically. The AlSiC envelope also provides a means of functional attachment for these brittle and difficult-to-integrate materials. This process** also forms a thermal interface between the AlSiC and high-heat-dissipation materials that have rough surfaces in their raw form.
Thermal composite performance depends on the orientation, volume fraction and thermal paths of product designs. These systems have been shown to reduce junction temperatures by 10° to 30°C, compared to systems without high-heat-dissipation materials.
This shows that high-heat-spreading materials can be incorporated easily into existing AlSiC product designs without the need to change the microprocessor assembly design.
Figure 6. Cross-section showing high-thermal conductivity TPG insert (dark) in AlSiC composite.
Figure 6 shows an interface that is formed between the AlSiC microstructure and the thermal material. From the thermal interface between the device and the lid surface, thermal energy moves through the isotropic 200-W/mK AlSiC and graphite material. The heat energy then is rapidly dissipated in the X-Y plane and dissipated through the AlSiC at the edges of the graphite material,* where it is either dissipated or meets other heat-sinking materials. This configuration is suitable for rapid lateral-heat dissipation (from a spot). Other configurations are available for through conductivity, and are similar in construction and concept.
Conclusion
As microprocessors increase power densities, heat-sink and flip-chip-lid materials that dissipate heat and expand at similar rates during thermal-power cycling are required. AlSiC metal-matrix composites accomplish this by combining physical properties of Al metal and SiC to achieve desired thermal-management attributes. Attributes of strength and stiffness, low density and good dimensional control also increase assembly reliability and thermal-dissipation performance. The fact that AlSiC is a cast product allows for more complex functional geometries produced at low costs and high volumes. The casting process also can be used to integrate high-heat-dissipation materials in AlSiC for improved thermal dissipation. These inserts often can be incorporated within existing AlSiC product designs and allow for transparent substitution in current assemblies for improved thermal dissipation.
* Thermal Pyrolytic Graphite (TPG), GE Advanced Ceramic Corp., Strongsville, Ohio.** Concurrent integration casting process, CPS, Chartley, Mass.*** AlSiC materials, CPS, Chartley, Mass.
REFERENCES
- T. Schuetze, H. Berg, O. Schilling “The New 6.5-kV IGBT Module: A Reliable Device for Medium Voltage Applications” PCIM Power Electronic Systems, p. 39-46, Vol 27, No 9, September 2001.
- Mark A. Occhionero, Robert A. Hay, Richard W. Adams, and Kevin P. Fennessy, “Aluminum Silicon Carbide (AlSiC) Microprocessor Lids and Heat Sinks for Integrated Thermal Management Solutions” presented at the 2000 HDI Conference in Denver, Colo., April 25-28, 2000.
- Mark A. Occhionero, Robert A. Hay, Richard W. Adams, and Kevin P. Fennessy, “Aluminum Silicon Carbide (AlSiC) For Cost-Effective Thermal Management And Functional Microelectronic Packaging Design Solutions” 12th European Microelectronics and Packaging Conference, June 7- 9, 1999, S10-04.
Mark A. Occhionero, Ph.D., thermal management materials specialist, senior research scientist, Ceramic Process Systems (CPS), may be contacted at (508) 222-0614, ext. 42; e-mail: marko@alsic.com.