In just a handful of years, the packaging and interconnect technology known as low-temperature cofired ceramics (LTCCs) has entered the mainstream of electronics design. LTCCs, which up until about five years ago were mostly relegated to the realm of low-volume military applications, are now being applied in high-volume consumer designs, especially in the wireless arena. And while wireless designs represent a growing market for LTCCs, other areas such as automotive, medical, and instrumentation also are making use of the technology.
LTCCs provide a modular approach to building highly integrated subsystems that are capable of operation at microwave frequencies. An LTCC module consists of a multilayer substrate constructed from layers of a dielectric foil (Fig. 1). RF passive components may be embedded within the substrate, along with the interconnect. Passive and active components both may be mounted or formed on the top-most layer.
RF design is aided further by the use of low-loss dielectrics and high-conductivity metals, which make it possible to construct high-Q resonators and filters. It's also possible to build fully shielded structures within the LTCC stack. For interconnect between the module and the pc board, wirebonds or solder balls may be used. In addition to high-frequency performance and circuit integration, LTCC modules offer superior thermal performance, hermeticity, and high reliability.
Construction of an LTCC substrate starts with the dielectric material. This material is a glass ceramic cast tape known as Green Tape, which is DuPont's name for its series of LTCC material systems. First, pieces of the tape are cut to the desired-size foils. Next, these foils are punched or drilled to create vias. Then, using screenprinting or photo-imaging methods, metallization for via fills and conductor traces are added (Fig. 2).
Passive Components Possible
Resistors and capacitors (thick film) and inductors and cavities can be embedded into the foils. The metals used for the substrate's inner layers, via fills, and wire-bond pads are typically gold or silver. Alloys of platinum and gold or palladium and silver may be used for solder assembly. The number of foils or layers required will depend on the complexity of the design. Substrates with over 50 layers have been demonstrated. Stacks with up to 50 layers are considered production-worthy.
Once the individual layers of tape are formed, they are collated, stacked, and laminated to create a "green" multilayer structure. That structure is then fired at approximately 850°C to 900°C to harden the material. After firing, the stack is cut to size. The top layer is then completed with the mounting of passive and active components. Bare die may be assembled using flip-chip techniques. Passive components may be formed by using thick-film techniques or by mounting surface-mount discretes on the top layer.
The process may be likened to techniques used to construct thick-film hybrids. Yet the LTCC process is more cost-effective because it relies on parallel processing of layers rather than the sequential buildup of layers in a conventional thick-film device. Another distinction is that conventional thick-film hybrids require multiple firing steps, while LTCCs are fired in one stage—hence, the label "co-fired," which reflects the fact that in an LTCC, the dielectric layers and conductors are fired together.
Among substrate materials, LTCCs face varying degrees of competition from other ceramic materials and from organic pc-board materials. In the ceramic domain, there are high-temperature cofired ceramics (HTCCs), which are multilayer structures based on dielectrics such as 92% alumina. Unfortunately for HTCCs, their high firing temperature (approximately 1500°C) precludes the use of highly conductive metals, which are required in high-frequency work. Typically, HTCC metals are tungsten or molybdenum (refractory metals) that are plated with nickel for solderability and gold to promote wirebonding and prevent corrosion of the nickel. On the other hand, the 850°C to 900°C firing temperature of LTCCs allows the use of highly conductive silver and gold traces.
Among organic materials, the LTCC weighs in against traditional board materials like FR4 and high-performance materials such as PTFE (teflon). LTCC either matches or exceeds the high-frequency and thermal characteristics of these materials.
Samuel Horowitz, marketing manager at DuPont Microcircuit Materials, Research Triangle Park, N.C., notes that the first generation of LTCC dielectric materials was better than FR4 in terms of high-frequency performance. But they weren't as good as the more-expensive PTFE. The current second-generation LTCC materials, however, now have the same dielectric properties as PTFE. Furthermore, the use of LTCC materials allows passive components to be embedded in the multilayer stack, while PTFE does not.
Vern Stygar, product manager for LTCC materials at Ferro Corp., Vista, Calif., cites two market factors that are driving the adoption of LTCC solutions—rising operating frequencies and rising power-dissipation levels. In the past, when the big wireless applications were running at about a gigahertz, demands on the dielectric materials weren't so great and traditional organic pc-board materials were sufficient.