Silicon Carbide Reinforced Aluminum for Performance Electronic Packages
By: Dr. Birol Sonuparlak, M.D. Lehigh and M.A. Robbins
PCC Advanced Forming Technology
Metal Matrix Composites Business Unit
AlSiC™ (silicon carbide reinforced aluminum), a composite material composed of an aluminum metal matrix reinforced by silicon carbide particles, is a relatively new material for the electronics industry, although it is quickly becoming the material of choice in many new system designs where low weight, high thermal conductivity, tailored coefficient of thermal expansion and high stiffness are key design parameters.
The superior combination of properties specific to
AlSiC
electronic packages yields performance advantages in physical, mechanical and thermal properties over competing electronic material systems. Like Kovar, Cu/W and Cu/Mo, ceramic substrates and semiconductor dies can be directly attached to AlSiC components. However, with AlSiC, users typically achieve a 60% reduction in module size and/or weight. It is also 10 times better at thermal conduction than Kovar, while its casting process permits custom and cost effective solutions to replace aluminum housings.
Electronic Package Fabrication Process
Not surprisingly, AlSiC electronic packages were first tested in the military electronics market. Recently, demand has begun to rapidly increase for AlSiC metal matrix composites in commercial electronics applications. To respond to this demand,
AFT's Composite Division
has aggressively increased its production capacity. Development of a flexible, high-volume manufacturing process has helped decrease manufacturing costs and lead times. As more experience is gained in high volume production, the costs associated with manufacturing will continue to go down.
The first step in AlSiC fabrication is the preparation of SiC pre-forms, followed by the infiltration of aluminum. After the infiltration process, the AlSiC electronic package is plated and connectors are soldered into the package to create the final product. The process is monitored and controlled throughout each step to assure the product conforms to customer requirements. Composite heat spreaders and heat sinks are produced using the same AlSiC production process except that they can be soldered to electronic packages and substrates without any connector attachment.
Pre-Form Fabrication
SiC pre-forms can be produced by any one of the traditional ceramic processing routes, such as uniaxial pressing, isostatic pressing, freeze casting, tape casting or injection molding. The injection molding process is the preferred fabrication technique to produce electronic packages and heat sinks with complicated shapes and tight tolerances. Fully automated injection molding machines are capable of producing a pre-form in less than a minute. Pre-form fabrication using the injection molding process requires fabrication of pre-form molds and a SiC/binder mixture. Typically, SiC particles occupy between 50% and 72% of the total pre-form volume.
The molds for the injection molding process are prepared by using state-of-the-art CNC milling and EDM machines; pre-forms are produced by injecting proprietary SiC/binder mixtures into these molds. Green pre-forms are then placed in a furnace and heated to remove the binder, leaving a porous pre-form in the shape of the final part.
Casting
The casting process is conducted in a pressure vessel using the PIC process. The PIC process involves pressure infiltration of aluminum alloy into casting molds which each contain a SiC preform. The casting molds are designed to also include seal rings and ceramic substrates for connector attachment. Net shape casting ability is one of the advantages of the pressure infiltration process. Finished AlSiC parts contain all the details of the machined mold cavity. In addition, the PIC process allows for the fabrication of features Au-Sn eutectic solder in the first approach. Alloy 48 is the preferred ring frame material because its CTE matches closely with AlSiC. This approach allows the customer to use their established process for joining connectors to the package walls and the metal lid to the ring frame. This process is known to maintain the typical hermeticity requirement of 1x10-8 mbar-l/sec.
The preferred hermetic electronic package fabrication approach eliminates joining the Alloy 48 ring frame to the
AlSiC
base because the whole housing is produced as a net shape single cast piece. The final cast piece contains slots for multi-pin connectors and planar feed-thrus or pure aluminum plugs which can be drilled or tapped using conventional machining techniques to fit single pin connectors. Slots for connectors are usually cast-in. Glass/metal connectors, such as compression sealed multi-pin connectors and matched seal RF connectors, or other ceramic/metal connectors are then hermetically soldered to the AlSiC housing using Au/Sn or Au/Ge eutectic solders after Ni/Au plating.
Characteristics of AlSiC Electronic Packages
High thermal conductivity, tailored CTE, high stiffness and the significant weight reduction of AlSiC composites have been shown as the desired properties for many electronic applications. This capability to predict component performance encouraged large electronic system manufacturers to test AlSiC electronic packages in various military electronic applications.
In order to replace existing materials in many electronic applications, AlSiC composites need not only perform better and cost less, but system houses also prefer using existing processes with minimum modification. In order to give this flexibility to manufacturers of system houses, AlSiC electronic packages have been produced by joining AlSiC composites to materials such as seal rings, connectors or substrates that are commonly used in the electronic industry. Ti is preferred as a seal ring material because of its compatibility with AlSiC. Alloy 48 and other low CTE FeNi and FeNiCo alloys have also been used with limited success as seal ring materials. Flexibility offered in tailoring the CTE of AlSiC composites provides an opportunity to match the CTE of the selected seal ring materials, Ti and Alloy 48. As a result, seal ring materials can be joined to AlSiC composites with minimum post fabrication stress.
One important microstructural feature of
AlSiC
composites produced at
AFT
is an aluminum skin on the finished part. The skin not only permits the composite to be handled as though it were pure aluminum, it also is another parameter used to modify characteristics of the composite produced. Furthermore, it provides a strong bonding interface between AlSiC and materials in-situ bonded to AlSiC. When AlSiC composites are in-situ bonded to ceramics, such as BeO, Al2O3, AlN or Low Temperature Cofired Ceramic (LTCC), this aluminum skin can be formed as an electrically isolated metallic layer on the ceramic substrate. It has been demonstrated that a 0.020 - 0.030" separation between adjacent aluminum metallization pads allows 1700 volt electrical isolation.
This core capability of
AFT's Composite Division
provides an excellent opportunity to incorporate substrate metallization in a single processing step in the manufacturing of many electronic packages.
|
