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At the outset of the project, SiC-reinforced aluminium alloys were evaluated but found to suffer from the extreme hardness of the SiC which made them impossible to machine with conventional tooling and this constituent rendered them unplatable. Instead, novel light-weight high silicon aluminium-silicon (Al-Si) alloys specifically tailored for electronics packaging applications have been produced using a spray forming technique. The alloys can be produced with tailored CTE by adjusting the ration of aluminium to silicon. Attention has been focused on alloys between 55wt% Si (CTE 11ppm/C) to 70wt% Si (CTE 6.5 ppm/C).

The novel materials have been assessed for ease of machining and processing cost using a number of alternative techniques and the results to date indicate that the omission of SiC (a necessary constituent of conventional MMC) from the alloy greatly facilitates machining, allowing milling to be carried out using carbide tooling, without the need for expensive diamond-faced tools. Machined finishes have routinely achieved surface roughness of approximately 1µm or less. The ability to bolt down packages of these materials has been demonstrated.

Plating procedures for applying silver and gold finishes to packages of the Al-Si alloys have been successfully developed so that the metallisations can sustain temperature cycling appropriate to joining and other processing steps without blistering. Having evaluated a range of processes for joining high silicon Al-Si alloys to themselves and to other materials used in electronics packages, techniques have been developed for bonding Kovar sidewalls to bases of Al-Si, lid-sealing and Al-Si packages and hermetically soldering-in feedthroughs. Comparative trials involving different solders have led to the selection of lead- and gold-based solders for bonding the feedthroughs. Lid-sealing trials using laser and resistance welding have produced encouraging results, although not sufficiently satisfactory for immediate application. Diffusion soldering has been identified as the preferred method of bonding kovar side-walls to bases of the high silicon Al-Si alloys.

Prototype MIC packages both of 'homogeneous' (i e all Al-Si alloy package) and 'heterogenous' construction (Al-Si base and kovar side walls) have been fabricated using the Al-55/70Si alloys machined by EDM (electro-discharge machining) and electroplated with silver and gold. Prototype packages, utilising conventional kovar side walls containing feed-throughs, and Al-Si alloy bases ('heterogeneous' packages) have also been fabricated. For 'homogeneous' MIC packages, the Al-Si alloys were shaped by EDM and electroplated with a gold finish. Other types of components have also been produced using a combination of machining processes, namely carriers for gallium arsenide power MMICs and iris components used in microwave filters.
Aluminium based Metal Matrix Composites (MMCs) offer a combination of materials properties unattainable alloys. These include high thermal conductivity,high modulus of elasticity,low density and low thermal coefficient of expansion. This combination of properties is ideal for electronic packaging where it offers major benefits,namely,lower weight,better thermal performance and higher reliability of electronics systems. Commercially available,structural grades,of MMCare not optimised for this purpose and they are difficult to fabricate into the complex,high tolerance,shapes required. This project will therefore concentrate on developing new MMC materials specifically optimised for electronic packaging and on devising new,innovative,methods for shaping,coating and joining MMC in a cost effective manner.

Because of the weight saving offered it is believed that the earliest applications will be in the aerospace and avionics industries. Since the inherent,materials,cost of MMCs are low, the development of new materials and fabrication technologies in this project should facilitate the use of MMCs in fields as diverse as machine tools,airframe construction and the automotive industry.

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GEC Marconi Ltd
Elstree way
WD6 1RX Borehamwood
United Kingdom

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