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Improved Composite Metal for High Performance Semiconductor Applications

An improved version of the popular copper tungsten composite with thermal and electrical properties approaching the product's theoretical maximum

February 1, 1999
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Improved Composite Metal for High Performance Semiconductor Applications

Polese Co.
San Diego, CA

As performance demands on silicon and GaAs devices continue to increase, so do thermal management concerns and the need for cost-effective packaging alternatives. In addition to heat dissipation, concerns such as material compatibility must be considered in order to optimize performance and reliability. With recent advances in powder metallurgy and processing technology, traditional materials are attracting renewed interest due to their ability to provide cost-effective solutions in increasingly demanding applications. Metal-matrix composite materials such as copper tungsten (CuW) are continuing to find favor in microwave and RF applications.

Solutions to Common Problems

In order to maximize the performance of a semiconductor device, its operating temperature should be controlled. Dissipation of heat generated in a device is commonly achieved with the use of a thermally enhanced mounting component. Unfortunately, materials with good heat transfer properties such as aluminum or copper have coefficients of thermal expansions (CTE) much greater than that of the device, causing reliability concerns. Thus, in high duty cycle or surge conditions, today's very thin GaAs devices are of particular concern. On the other hand, controlled expansion options such as the use of Kovar® offer little in the way of heat dissipation. By combining the two technologies, an acceptable, cost-effective compromise can be reached.

In addition to thermal expansion mismatch, other critical mechanical characteristics of packaging materials also must be considered. Device and substrate mounting materials should be sufficiently strong to mechanically isolate them from their environment to assure that these typically brittle devices do not fail in assembly or during normal handling. As typical device and substrate materials are particularly susceptible to tensile stresses, brittle device and substrate materials should be placed in slight compression. This compression is achieved by attaching the device to a material with a greater CTE where the differential cooling shrinkage after assembly will provide the desired effect. The magnitude of this stress is a function of the size of the device/substrate, mechanical properties of the materials involved, assembly temperature and operating conditions.

Enhanced Material Properties

As the theoretical thermal and electrical properties of the materials typically exceed those required in most applications, compromises in material fabrication processes are typically made to minimize cost. However, improvements in semiconductor fabrication technologies have resulted in lower cost, higher density devices, fueling the growing need for improved characteristics without jumping to exotic, significantly more expensive products.

The improved version of the popular CuW composite features thermal and electrical properties approaching the product's theoretical maximum. Figure 1 shows the thermal expansion properties of one of the new ehanced 85 percent tungsten/15 percent copper (85W/15Cu) materials. Table 1 lists the material's enhanced thermal and electrical properties vs. its standard performance. This breakthrough enables the development of new cost-effective, high performance devices. While most properties of composite materials are a function of their parent materials and fabrication processes, the thermal expansion of many of these materials can be tailored for almost any microelectronic application by varying the volume ratio of the mixture.

Table I
Typical Properties of 85W/15Cu Composite

Characteristic

Standard

Enhanced

Thermal Conductivity (W/mK)

170

207

Electrical Conductivity (%IACS)

31

43

Young's Modulus (GPa)

274

298

Thermal Capacity (J/cc K)

2.95

2.78

Applications

Typical microwave and RF applications can be broken down into three categories that cover a wide range of commercial, aerospace and military electronic uses. The most common products are flanges or housings, which serve as both the primary package component and heat transfer medium. Insulators such as ceramic, glass and even plastic are used to mechanically and electrically isolate leads from the composite material. Both devices and substrates are mounted directly onto the package base. Several CuW flanges fabricated for use with RF power devices and circuits are shown in Figure 2 .

Table II
Compositions Typically Used for Common Packaging Materials

Device/Substrate Material

Typical Grades

GaAs MMIC-mount/heat spreader

85W/15Cu
87W/13Cu
90W/10Cu

Alumina ceramic carrier/flange

85W/15Cu
87W/13Cu

Beryllia ceramic carrier/flange

82W/18Cu

Also very common are carriers onto which circuits are built for inclusion into a larger system. In this case, the package is typically made from a material that is thermally and/or mechanically incompatible with the next-level assembly. Figure 3 shows CuW carriers for use in advanced microwave applications.

Additional products include shims (typically 0.020" to 0.007" thick), which are used not only as a heat spreader, but more importantly to mechanically isolate the very fragile device from the next-level packaging materials. The need for this product will continue to increase as devices grow in size and power density and decrease in thickness.

When fabricated with the new CuW composite materials, all of these products benefit from the materials' enhanced properties. However, several trade-offs must be considered, depending on the specifics of each application. Table 2 lists material compositions typically used for common packaging materials. In all cases, flat, uniform surfaces with high integrity finishes and precision features greatly enhance both the fabrication and performance of the final product.

Conclusion

High performance semiconductor packages may now be fabricated from an enhanced version of CuW composite materials that features improved thermal and mechanical properties without the high cost of exotic material solutions.

Polese Co.,
San Diego, CA
(619) 271-1993.

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