The dramatic rise in the use of microwave technologies, generated by wireless communications growth, has created challenges for interconnect and packaging techniques. Though most wireless equipment has used conventional pc-board technologies, with some extension in their capabilities to handle the frequencies required, it is becoming clear that such technologies do not address all the technical and commercial needs of the market.
Specific concerns relate to the RF performance of circuits due to limitations in polymer materials and the cost of the circuits due to the high number of passive components required. The good news is that there are ceramic technology solutions that enhance performance and cut cost. It highlights the ability of ceramic technology to perform at the higher frequencies required for evolving wide-bandwidth personal communication systems and estimates the potential saving for handsets if 25 percent of the passive components are embedded.
Responding to new market requirements, the Ceramic Interconnect Initiative (CII) of the International Microelectronics and Packaging Society was formed to educate handset manufacturers about the advantage of ceramics for RF circuits. The group initially was a joint effort of six significant suppliers of technology to the ceramic packaging and interconnect industry-CTS Microelectronics, Heraeus Cermalloy, Kyocera, Motorola, National Semiconductor and DuPont Microcircuit Materials. Now it represents over 50 companies from all levels of the ceramic circuit supply chain. To accomplish its mission the CII focuses on seminars, workshops, a home page (www.imaps.org) and developing and maintaining a technology road map.
Chief among the CII's messages is that printed-circuit board technology has a number of limitations for packaging and interconnects in wireless applications. Specifically, the dimensional stability of polymer materials is poor, with the temperature coefficient of expansion (TCE) on the order of 20 ppm/ degrees C or greater and poor stability in a humid environment, which leads to moisture absorption and changes in board dimension and electrical properties. The dielectric loss associated with such materials results in high attenuation in microwave components, leading to poor component performance and diminished battery life. Such problems become more significant as frequencies increase. What is needed is a technology offering low loss, and mechanically and electrically stable components and circuits.
A further problem, which is particularly important in wireless handsets, is the need for many passive components for impedance matching and filtering. While integration has reduced the semiconductor count, similar integration has not occurred for passive components, which occupy a major fraction of board area and contribute significantly to component and assembly costs. A technology that integrates passive components into the substrate, providing a single interconnect and package entity with controlled impedance elements and fewer solder joints, would give designers new tools to decrease size, lower cost and improve reliability.
The features and the benefits of ceramic solutions to the OEM designing and making handsets for the portable wireless market include a high Q and a low temperature coefficient (at resonant frequency). Those features of many ceramic materials contribute to enhanced RF performance. Lower losses mean reduced power consumption and extended battery life, critical for portable devices.
Ceramic solutions provide an excellent platform to mix analog, digital and RF technologies, creating unique opportunities for design innovation, integration and decreased cost. Ceramic systems are hermetic, providing the designer with options to create integral hermetic packages as part of the basic interconnect structure. Incorporating photo patterning enhances the capability to define very precise features improving impedance control and increasing packaging density.
Ceramic solutions are well-established in the military aerospace and automotive markets and are often the material of choice for harsh environment applications requiring excellent performance and lifetime. The low TCE of ceramics contributes to minimum variations in performance with changes in temperature. The excellent TCE match to silicon, gallium arsenide and silicon germanium facilitates bare chip attach, in many cases without the added expense of underfill.
Ceramic offers outstanding feature size and overall physical property stability, helping to make for easier design, shorter time-to- market, robustness in manufacturing and lower cost. Functional trimming imparts the ability to dynamically adjust circuit performance. Ceramic solutions generally have superior thermal conductivity (100 times greater than FR-4 PWB materials), which helps simplify design, reduce size and lower costs. Integrating passive components into the interconnect decreases cost, reduces size and improves reliability.
Many view ceramic as an expensive technology. Those perceptions trace to another era when ceramic was known for its use in performance-driven military and aerospace applications. Today, new materials and processes, driven by the needs of cost-conscious wireless communications and automotive applications, combine silver or copper metallurgy with larger panels along with processes with fewer steps to provide new, less-expensive answers. In the sections that follow we will see examples of how the benefits of ceramic are being incorporated into new wireless applications.
A major part of the ceramic packaging solution is low-temperature co-fired ceramic (LTCC), a technology that enables embedded thick-film passive components between dimensionally stable ceramic layers. LTCC multilayer interconnects are a glass ceramic composite; thick-film technology used to create passive components such as capacitors, resistors and inductors can be embedded between the layers. Conductors can be gold- or silver-based and allow the direct attachment of silicon and GaAs integrated circuits. The use of those films with photo-patterned materials gives fine-line capability and high performance.
The LTCC process is a parallel processing technique; that is, all layers are produced separately and co-fired in a single process.
Photo-patterned thick film developed for both alumina and LTCC substrates has high resolution with superior line definition. High-frequency characterization studies have shown the performance qualities of photo-defined and -etched thick-film interconnects within LTCCs and have compared these with sputter thin film on alumina. At frequencies from 1 to 16 GHz, the loss tangent (or signal attenuation) is smaller for single-layer alumina or LTCC (DuPont's Green Tape) than it is for printed-circuit board materials (FR-4 epoxy).
New applications-such as integrated receivers for satellite handsets-are moving from development to volume production, taking advantage of the ability of ceramic technology to use 3-D designs to integrate passive components into the substrate.
The capabilities of LTCC ceramic technology are reducing the size of components for portable wireless devices. A power amplifier module redesigned from a 200-mm2 printed-wiring board was reduced to a 50-mm2 LTCC ceramic with integrated inductors and capacitors. In fact, LTCC structures with integrated passives provide leading-edge component densities.
And cost remains a key driver. But integrating 25 percent of the passives in a typical portable phone handset (with 800 of them) could save $4 per unit. Increased panel size and automated processes also contribute to lower manufacturing costs.