The trend towards faster and higher frequency communications is driving the search for high reliability packaging solutions that satisfy ever more demanding performance requirements. However, when utilizing the packaging materials commonly available today, designers often come up against thermal, mechanical and/or cost barriers.

To meet these demands, Osprey Metals, in association with leading international high tech companies, has formulated a range of low cost controlled expansion (CE) alloys to suit a broad range of applications, including RF and microwave packages.

The new materials are binary silicon-aluminum alloys, where the ratio of the two metal constituents can be adjusted to provide coefficients of thermal expansion (CTE) at any pre-selected value, in the range of 7 to 17 ppm/°C, to match or accommodate the thermal expansion of common microelectronic device and substrate materials. These CE materials are true alloys and not composites prepared by powder metallurgy, and are formulated by a patented spray-forming process to achieve uniform, isotropic properties and optimum mechanical characteristics.

Main Attributes

Key characteristics of the materials are low density (less than aluminum), high thermal conductivity, high specific stiffness and excellent thermo-mechanical stability. The alloys are machinable and platable using competitive, standard industry proven methods. As a mixture of silicon and aluminum, they are environmentally friendly, safe to handle and use, and present no disposal problems.

Their development has been driven by the trend towards increased miniaturization and higher power loading, which requires improved heat removal in order to optimize the performance of semiconductor devices. This is a problem with existing materials, as common thermal management materials, such as copper or aluminum, have coefficients of thermal expansion that are much higher than those of semiconductors, resulting in unacceptable stresses and poor reliability. Other highly conductive materials can be prohibitively expensive for most applications (for example, diamond) or present health concerns (beryllium/beryllia). Also, long established controlled expansion materials are limited in other ways, including poor thermal conductivity, weight (too heavy) and imposing fabrication constraints.

The higher silicon-containing CE alloys, such as CE7 and CE9, possess an optimized combination of properties (see Table 1 ). Moreover, there is a range of alloys available with graduated CTE values so that the user can choose the optimum and lowest cost material for the particular application.

Because semiconductors and ceramic circuit boards are relatively fragile, it is desirable to pick the grade of CE material that will place these vulnerable components in slight compression. For example, the CE7 alloy has CTE values that match those of gallium arsenide and alumina fairly closely, as shown in Figure 1 . This means that the set compression on these items will remain throughout the entire service regime. By comparison, some other controlled expansion materials expand on heating at a much higher rate than the principal materials inside the package, notably gallium arsenide and alumina, allowing stresses to build up during duty cycles, which may lead to failure.

As was mentioned earlier, the CE alloys are relatively easy to machine and electroplate, and are suitable for rapid computerized numerical control (CNC) or electronic discharge machining (EDM), both for making prototypes and volume production (see Table 2 ). They are machinable to a fine finish (0.5 µm) with carbide or poly-crystalline diamond (PCD) tools. Machining of features such as recesses, pedestals, holes, slots and tabs is fairly straightforward, and crisp edge-definition, with negligible chip-out, is readily achievable. Conventional tapping can be achieved in almost all of the alloys, although in the CE7 grade the use of EDM tapping or threaded inserts may be desirable. In addition, the materials are readily platable with gold, silver and nickel by a near-standard aluminum plating technology, which has been developed to suit commercial production.

Applications

The CE alloys are suitable for a wide range of applications. Mainstream uses include RF/microwave flanges, carriers and housings, in a wide range of end-uses, encompassing the telecommunications, aerospace and military sectors. There is a growing requirement for these materials in housings for electro-optical modules, in response to the strong growth in optical fiber networks. Another important application area is represented by heat sinks for power modules and discrete devices.

Other less obvious but nevertheless important industrial applications include waveguide components, heating blocks for die bonders, components for inertial systems and injection molding cavities. The beneficial properties of the CE alloys, which are exploited in the applications referred to, are highlighted in Table 2.

Conclusion

A range of controlled expansion materials has been developed to meet application requirements across the electronics industry, and are available for manufacturing into a wide range of products, including microwave and RF packages. These materials provide high reliability, increased functionality-to-weight ratio and are cost competitive.

Osprey Metals Ltd., Neath, Wales, UK, Tel: + 44 (0) 1639 634121; Fax: + 44 (0) 1639 630100; E-mail: spray.form@ospreymetals.co.uk; Web site: www.ospreymetals.com.

US contact: Advanced Packaging Associates, E-mail: stu.weinshanker@advpkggassoc.com.

Circle No. 300