To popularize silicon/aluminum(Si/Al) alloys in the microwave module packaging,the performances and manufacturing techniques of Si/Al alloy are described.As one of the most desired electronic packaging material,silicon aluminum alloy has been used in the fields of aerospace and microwave electronic packages.Furthermore,with the development of manufacturing and processing techniques in our country,the homemade Al-Si alloy will be widely used in RF/microwave packaging.
These alloys are available in the composition range 70Si-30Al to 12Si-88Al,which enable the coefficient of expansion to be adjusted between 7ppm/℃ and 20 ppm/℃，so as to compatible with many common microwave devices and substrates.These alloys are typically between three and six times lighter than established packaging and baseplate materials used in RF and microwave products such as Kovar, copper-molybdenum(Cu-Mo) and copper-tungsten(Cu-W). Si/Al alloy is readily platable, using industry proven methods. Being composed of silicon and aluminum, the alloys are environmentally friendly, safe to handle and use, and present no disposal problems.
AlSi42, AlSi60 and AlSi70(also called CE Alloys in foreign countries) are being widely used for microwave packages for aerospace and space applications at operating frequencies as high as the Ka band (with an upper limit of 36 GHz), taking advantage of their reasonably close CTE match to circuit boards and components, their high thermal conductivity, low density, hermeticity, dimensional stability and ease of manufacture.While the CTE of AlSi50(Al-50%Si) alloy is noticeably higher than that of alumina, which is widely used as substrates for microwave and RF circuits, it is far lower than that of aluminum. More pertinently, it is sufficiently close in its CTE to alumina for moderately compliant conducting adhesives to take up mismatch stresses without compromising the adhesion or fracturing the alumina substrates.
Si-40%Al and Si-30%Al alloys are best suited for RF and microwave packaging for aerospace applications involving operational frequencies as high as 40GHz,because of their close CTE match to circuit board,high thermal conductivity, low density,dimensional stability, and ease of machining.AlSi50(Al-50%Si) alloy is best suited for substrates for RF and microwave circuits and currently is widely used for the housing of a transmit and receive module operating in the 3~30GHz range.AlSi70(Si-30%Al) alloy is best suited in the fabrication of high-speed MEMS switching device,comprising an array of these modules widely used in aerospace applications. AlSi70 provides low coefficient of thermal expansion(CTE),reduction in weight,enhanced stiffness and excellent dimensional stability.
Major emphasis will be placed on material properties,which might have significant impact on RF performance under harsh operating environments.Parameters such as mechanical strength,strength-to-weight ratio,stiffness factor,thermal shock resistance,dimensional stability and structural integrity must be given serious consideration,if reliable operation over extended periods is the principal requirement.Properties of soft, hard and metallic substrates best suited for MEMS application will be summarized with emphasis on fabrication cost,electrical performance, and device reliability under harsh operating conditions.Substrates best suited for RF/microwave MEMS devices will be identified and their important characteristics will be summarized with emphasis on insertion and dispersion losses.Electrical,mechanical and thermal properties of substrates best suited for microwave MEMS devices and operating in mm-wave regions will be investigated as a function of environmental parameters.