Wireless Systems Design

LTCC-Based Packaging Enables 3D Functionality

Today's high-bandwidth, high-speed networks demand packaging that is electrically and optically transparent. Yet that packaging also must serve as the prime method of mechanical and environmental protection. Fortunately, a way exists to satisfy these divergent needs. By designing and manufacturing products using low-temperature co-fired ceramic (LTCC) technology, it's possible to address these requirements in a cost-effective, performance-enhancing way.

LTCC is a three-dimensional (3D) ceramic technology. It utilizes the Z dimension for interconnect layers, embedded circuit elements, and integral features like shelves and cavities. LTCC is actually a mixture of thick-film and ceramic technologies. The thick film contributes electrical-interconnect and passive-circuit elements, such as resistors, inductors, and capacitors. The ceramic provides dielectric properties and the foundation for physical features ranging from vias to complex stepped cavities, cutouts, and buried channels (FIG. 1).

The three-dimensional attributes of a fully utilized LTCC structure are unequaled by any other current and affordable technology. For designers, such a structure provides the following benefits:

  • The designer can effectively use the Z dimension of the system to realize embedded circuitry and interconnect.
  • He or she can reuse the X-Y real estate for active and additional passive-circuit elements on the top layer.
  • The designer can use the same structure as the package, including the required I/O.
  • The thick-film resistor and capacitor elements may be adjusted by means of YAG-Laser trimming during test. This capability allows precise functions to be realized with less costly, wider-tolerance add-on circuit elements.

With LTCC, up to 75 layers of ceramic tape are stacked and laminated to form a homogeneous hermetic package. Each layer has individual via and track patterns. Because both the formulation and processing of ceramics can be realized in house, the electrical and physical properties are customized to suit particular applications. For example, a low-loss dielectric formulation can be used to reduce signal power loss and distortion, maintaining signal integrity even at OC768 (40-Gbps) data rates and above. LTCC also is suitable for the harsh environments found in automotive applications, such as engine and gearbox management.

To achieve electrical signal routing in optoelectronic modules, it is critical to be able to maintain impedance-matched interconnect. This is done by tightly controlling the physical dimensions of the micro-strip and strip-line transmission lines implemented within the substrate. Those dimensions must then be matched against the thickness of the dielectric substrate layers. To minimize the discrete-component requirement, lumped element inductors, resistors, and capacitors can be embedded (FIG. 2). For higher frequencies, distributed passive components also can be integrated throughout the layers of the structure, giving functionality in three dimensions.

By printing passive components in this way, low-temperature co-fired ceramic technology allows manufacturers to minimize the number of discrete passives. It can therefore provide potentially significant reductions in size and weight. In addition, integrated printed components within the layered structure can be configured into functional blocks, such as couplers, impedance transformers, and filters. This ability to package complete, modular LTCC-based solutions simplifies system assembly while reducing complexity.

In optical-module design, the use of bare semiconductor die provides some advantages: reduced size and cost due to the elimination of individual component packaging and a direct electrical connection via short controlled-impedance wirebonds or solder balls with flip-chip mounting. Because LTCC is rigid, it enhances ultrasonic wirebonding. At the same time, it provides a stable base for fragile gallium-arsenide and indium-phosphide compound semiconductors, either on the substrate surface or in cavities within the ceramic structure.

Of course, LTCC isn't the only alternative to traditional substrates. Among the other alternatives are high-frequency laminates like duroid, GETEKR, and PTFE. Laminate interconnect, like the semiconductor industry, benefits from an established, broad infrastructure. The technology is widely understood, broadly deployed, and constantly improved upon to address electrical interconnections.

From a competitive technology viewpoint, the ceramic versus laminate comparisons have changed very little over time. Yesterday's arguments and perceptions form today's differentiators. One can still argue that ceramic increasingly outperforms laminate structures as frequency rises above 1 GHz, providing higher Q, lower loss, and lower Tf. The laminate options tend to be more expensive, however. They also have additional tradeoffs that need to be considered. Because they have a tendency to absorb moisture, laminate-based hermetic structures aren't directly achievable. In addition, placement within a hermetic package can trap absorbed moisture within the cavity.

Potential reliability issues arise with bare die or wirebonding. And with no established process for embedding passives, individually packaged components must be used with inherent cost and size implications. High-frequency laminates also have hit both cost and performance barriers. In contrast, LTCC continues to be cost effective while increasing its functionality. It also flaunts dimensional consistency, repeatability, and superior tolerances on dielectric loss and constant.

Compared to standard thick-film-on-fired-ceramic processes, LTCC offers further advantages. Whereas thick film has a practical limit of four to six conductor layers, LTCC supports three-dimensional structures with over 75 tape layers. Each layer has a range of available thicknesses, thereby providing greater versatility. Plus, low-temperature co-fired ceramic can be configured as a complete cavity package. It can therefore support surface-mount-technology (SMT) components, bare wirebonded die, flip-chip, and optical devices, as well as embedded capacitors, inductors, and resistors. In wireless and optical applications, LTCC's dielectric properties allow size and weight to be reduced while maintaining signal integrity.

Ideally, system-level integration is approached by a combination of integration (system-on-a-chip) and partitioning (system-in-a-package). LTCC-based system-in-a-package is a cost-effective, adaptable, and scalable solution for demanding packaging applications. Now that it is established, the LTCC infrastructure is supplying the communications, automotive, and military/aerospace markets with high-value-add packaging solutions. The LTCC-based system-in-a-package goes one step further by using a cubic configuration to integrate the package, interconnect, and passive elements into one structure. As a result, that system-in-a-package is poised to serve a multitude of markets and applications now and in the years to come.

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