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Material Trade-offs for Advanced Electronic Packages
By Iris Labadie, Market Development Specialist, Dr. Jerry Aguirre, and Paul Garland, Kyocera America, Inc., San Diego, CA
In the electronics industry's headlong rush to shrink everything, everything is smaller, even though the components and circuits are far more complex than ever. The overall trend has been an increase in functionality and a decrease in size. Improvements in radar and communications technology have also provided impetus for advanced package design.
However, the evolution of electronic packaging would not be possible without a wider selection of improved packaging materials. These material choices, which are available at varying price points, provide the circuit designer with the ability to achieve higher performance while aligning product life cycles with cost targets for the application.
Moore's Law seems to govern everything, and that has certainly been true of advanced electronic packages and the design engineering that goes into them. The advent of compound semiconductor devices such as GaN, SiC, and GaAs, and the increasing number of consumer, industrial, and aerospace applications that require higher frequency, higher power, and more bandwidth are all enter into these design considerations. These complex packages routinely include features such as multiple ground/power planes, low loss interconnects, RF shielding, multiple cavities, embedded passives, laminate waveguide filters and other structures.
Advanced packages are often needed for aerospace, harsh environments, and non-commercial applications. The external conditions in which they must operate may require the package to be hermetic, dimensionally stable, mechanically robust, and dielectrically stable over a wide temperature range. Often these conditions favor the use of ceramic over plastic or organic laminate solutions for long-term reliability.
In some recent broadband satellite applications where RF performance must be maintained in a space environment, advanced packaging features developed for transmit/receive modules in phased array radar systems were utilized to create a series of frequency converter modules operating at Ku (12-18GHz) and Ka band (26.5-40GHz). These modules were engineered to operate over a broad frequency range and include embedded passive components such as filters and power dividers.
Multilayer Low Temperature Co-fired Ceramic (LTCC) with gold metallization was the material set chosen for this application. This choice was made because of its low loss characteristics, ability to braze on seal rings, connectors and heat sinks, and the ability to make a much smaller and lighter weight module. The improved module design — at less than 10-in.
, was an order of magnitude reduction in size from the previous 150 cubic inch module made from a combination of duroid and aluminum.
Multilayer co-fired ceramics are typically made by first punching out cavities and vias in the unfired or green ceramic tape layers. Metal paste is used to fill the vias and print circuit traces on the tape surface. The green tape layers are then stacked together, laminated and fired in a well controlled furnace. The densification of the ceramic and metallization occurs at the same time during firing. This firing may be categorized into the two broad groups of High Temperature Co-fired Ceramic (HTCC) and Low Temperature Co-fired Ceramic (LTCC). LTCC package materials generally have lower flexural strength and lower thermal conductivity but also have a lower dielectric constant and can readily facilitate high frequency designs up to 100GHz. With the exception of LTCC with precious metal metallization, both post-fired and co-fired ceramic packages have a final gold-plated layer to protect against corrosion and facilitate end-user attachment of devices and other components. HTCC packages tend to be less expensive than LTCC because a major contributor to the cost is the metal paste used for the vias and circuit traces.
Paste used for HTCC metallization is typically made using a refractory metal such as tungsten or molybdenum and is much lower in cost than the gold or silver metallization used in LTCC material sets. In a recent cost analysis performed for a long-term high-reliability program, several LTCC modules were evaluated for conversion to HTCC. The cost of the HTCC tungsten paste was far less per gram than LTCC with gold paste. Because gold is so costly and the price keeps escalating, it's important to have an alternate strategy to prevent cost overruns, and this naturally involves possible alternative material sets.
However, in some high frequency applications where electrical performance requirements are very stringent, an insertion loss difference of only 0.05dB between equivalent transmission lines may result in having to choose the gold option. Silver paste for LTCC is also available at approximately 1/15 the cost of gold paste. Copper paste for LTCC packages is even lower at less than 1/3 the cost of silver paste. An advantage of LTCC materials is the ability to fabricate high Q filters as well as transmission lines with very low insertion loss. The ability to braze on external metal components such as heatsinks, seal rings, and miniature 50Ω coaxial connectors, and other types of metal alloy leads to any side of the package is an advantage for HTCC and some LTCC materials sets. Kyocera's copper LTCC has limited brazing, but is generating interest for millimeter-wave applications. In general when comparing the total cost of the various package materials for similar designs, an HTCC package costs less than half that of an LTCC package with gold metallization. An equivalent copper LTCC package is approximately 30 percent less expensive than gold LTCC.
Many other varieties of ceramic materials for device packaging are also available. Ceramics which are metallized after firing commonly include beryllium oxide, alumina, aluminum nitride or other less familiar materials such as mullite or forsterite. The metallization is also made from refractory metals and is applied after the ceramic has already been fired in a high-temperature furnace. After the metallization is applied to the substrate, it is fired again at high temperatures to adhere the metal traces to the ceramic. There are also customizable ceramic materials for packages such as HITCE
where precise matching of the thermal expansion of the underlying printed circuit board to the package is required. These packages can sometimes cross the boundary between ceramic and organic packages in router, MPU power, and RF applications.
Historically, inexpensive plastic packages have dominated consumer and portable electronics where there is always pressure to reduce the package price. Improvements in device operating parameters such as the signal-to-noise ratio have allowed the expanded use of plastic packages.
Organic laminate packages normally manufactured by adding layers to a PCB core such as Kyocera's HDBU have advanced by increasing the routing density and decreasing the lines and spaces (< 1 mil) to accommodate fabrication of thousands of I/Os for array area packages. Packages made from these materials are well suited for network and graphics processors and optical interface cards. The relatively low cost in volume production also makes organic laminate packages a viable option for mobile phones and digital cameras. While there is no single best package material for all applications, material selection for advanced packages should be based on the electrical requirements, space constraints, manufacturability, and budget. A balanced approach to design will result in an optimal package for the best performance and price.
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