AFT's Growing Capabilities In Metal Matrix Composites By Dr. Birol Sonuparlak As demand from our customers for materials with high thermal conductivity, tailored coefficient expansion, reduced weight and high stiffness increases, the Composites Business Unit continues to penetrate new markets and add new capabilities into their existing core capabilities. Current core capabilities include: - Specialized Joining Techniques
- --Solderability at as high as 400oC
- --AlSiC to Metals (Ti., Ta, Mo etc)
- Specialized Plating and Coating
- --Aluminum skin
- --Ni,Au,Ag,Cd Plating
- --Cu base Thermal Spray Coating.
- --Anodizing
- Reinforced Metals
- --AlSiC
- --Graphite/Al
- --Diamond/Al
- Net Shape Metal Reinforced Components --Mainly AlSiC (silicon carbide aluminum)
- --Plates with tailored bow and flatness
- --Plates as thin as 0.010" (250 µm)
--3-D electronic packages
We have recently started to produce graphite-reinforced aluminum and diamond-reinforced aluminum in limited quantities for applications where very high heat dissipation is required. We will report properties of these new materials in future months, as data becomes more available. In this article, we would like to focus on two of our core capabilities which were developed as our innovative solutions to solve our customer problems: AlSiC base plates less than 0.040" thick and weldable AlSiC electronic packages. Thinner Base Plates Than Previously Thought Possible AlSiC base plates 0.050"-0.2: thick and as large as 11"x7" have been in production for awhile. Recently, our Composites Business Unit has started to see increasing demand for AlSiC base plates thinner than 0.040" for improved thermal performance. AlSiC's superior stiffness (225 GPa) allows designers to reduce AlSiC plate thickness to as thin as 0.010", whereas copper (135 GPa) and aluminum (70 GPa) base plates cannot be used at the same thickness in the same application. The net-shape thin plate casting ability is one of the fundamental advantages of AlSiC base plate manufacturing process at PCC-AFT. Net-shape parts are produced by infiltrating preforms that contain all the details of the finished part within a casting mold system. The casting molds are designed not only to contain the preforms, but to form the desired thickness of aluminum skin and all the dimensions of the final part. As-cast parts can easily meet +/- 0.003" in length and width. Weldable Electronic Packages
Weldable
electronic packages are another application where we demonstrated our innovative
approach to provide solutions to our customers. In responding to a customer
demand for a hermetically sealable AlSiC electronic package, PCC-AFT's Composite
division produced electronic packages by joining AlSiC composites to titanium
seal rings in-situ in the pressure infiltration casting process. Due to the
matched CTE of AlSiC and titanium, Ti seal
ring can be joined to AlSiC composites with minimum post fabrication stress. A
Ti lid can then be welded to the Ti seal ring utilizing laser welding. Also,
due to the high thermal conductivity
of AlSiC, the interface temperature remains below the melting temperature
of aluminum alloy during laser welding process. As a result, there is no
change expected in this interface during the welding process.
Laser sealed AlSiC packages produced with Ti seal rings maintain the hermetic integrity after 1,100 thermal cycles between -55 and 125°C. The aluminum alloy in the joint is similar in chemical composition to the AlSiC matrix and the aluminum skin covering the composite. There is no discontinuity such as cracks or porosity at the aluminum/AlSiC interface and the aluminum/Ti interface. The reaction interface is kept under 25 µm by controlling the time the molten aluminum is in contact with the seal ring. There is no detectable change in this microstructure and the integrity of the joint after the AlSiC housings is thermally cycled at 1,100 times between -55 and 125°C. The aluminum alloy used in the infiltration process joins Ti seal ring to AlSiC electronic package. Aluminum alloy thickness at the joint is about 75µm and is void free. Finally, A Few Words About Finishing Metal skin on finished materials is characteristic of PCC-AFT AlSiC products. The pure metal skin allows the part to be plated or finished as though it was an aluminum metal part. Cast parts fabricated using PIC process can be plated using traditional aluminum plating techniques and coated with various specialized coatings. Ni and Ni/Au plating is utilized when AlSiC parts are solder joint to other electronic components. Cu base thermal spray coating is also suitable for specific solder alloys. Chemical film coatings are suitable for epoxy bonding. Since the coatings cost less than the Ni plating, these coatings easily replace Ni plating in applications where the parts will not be exposed to corrosive environments--or exposed only to mild corrosive environments that will not harm the parts.
For more information about Metal Matrix Composites, please e-mail birols@pcc-aft.com or call 303.833.6140.
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