A new substrate technology has been developed that addresses the dense component placement, high heat dissipation, harsh environments and high frequencies associated with high performance devices such as microwave amplifiers for base stations, broadband amplifiers designed into multi-media networks, and high power microwave power combiners with on-board terminations. The new technology - low temperature co-fired ceramic on metal (LTCC-M) - utilizes a patented process that bonds ceramic to metal to deliver improved performance and reliability at lower cost. An example of LTCC-M is shown in Figure 1.
The LTCC-M process shares some similarities to LTCC. However, LTCC-M bonds a multi-layer "green tape" board to a Kovar or CuMoCu metal base using special materials and a 900°C firing process. The addition of the bonded metal base provides several advantages. For example, open cavities that extend down to the metal base can be created with LTCC-M to allow direct component die mounting on the base (or one or two layers above). This eliminates the need to attach a fully packaged high power component on a large substrate-mounted heat sink, which requires machining of both the substrate and sink in conventional LTCC processing. The metal core in an LTCC-M substrate allows heat to be carried away from hundreds of watts of power dissipation with a thermal conductivity of 170 W/m°C. Furthermore, high frequency loss as low as 0.5 dB per inch of transmission line at 40 GHz is possible.
There are a number of other benefits associated with LTCC-M. Shrinkage in the x-y plane during firing is typically 0.1 percent, a significant improvement over LTCC, which has a typical shrinkage of 12.7 to 14.8 percent. LTCC-M boards can be as large as 16" x 16" and have greater component density than LTCC boards, which are limited to 6" x 6" wafer sizes. The larger wafers allow multiple populated circuits to be created in one pass through the furnace, which can then be cut from the finished wafer. This capability lowers production costs because it reduces parts handling and other repetitive steps.
Production costs can also be saved in other ways using LTCC-M. To protect against environmental exposure, LTCC substrates often are placed in costly leaded Kovar packages. With LTCC-M, a hermetic package with lid can be produced for a savings of as much as 50 percent. LTCC-M also allows a variety of optical and electrical connection options that simplify integration and reduce assembly costs.
All of this is made possible by an LTCC-M process that greatly reduces the number of production steps compared to standard LTCC. The first part of the process, production of a populated substrate, is similar to the one used for LTCC parts, except that LTCC-M parts have a bonded metal base. Both LTCC and LTCC-M start with a roll of "green tape" however LTCC-M bonds that tape to Kovar or CuMoCu. The populated multiplayer board is formed in a similar fashion to LTCC:
- Cutting the dielectric substrate into blanks for each layer
- Punching via holes for connections between layers
- Applying metallization, insulation and resistive pastes to each layer
- Bonding the layers
- Co-firing the "green" multiplayer assembly and metal base
Although both substrates share many of the same processing steps, the LTCC-M technology has advantages. One is that up to 24, 0.004" thick layers for traces and buried passives are possible at a price as low as $2 per square inch per layer. For a given circuit design, such layering allows shrinkage in overall package dimensions. This combination of characteristics cannot be duplicated with conventional LTCC technology. A complete comparison of LTCC-M to LTCC and other substrates can be seen in Table 1.
High performance systems pose difficult design problems, including circuit and system packaging. Conventional substrate technologies have been shown to either lack the performance or price point to meet the trend towards higher bandwidth and more feature-laden products in smaller packages. The new LTCC-M technology serves as a solution for these high performance circuits and systems.
Westampton, NJ 800-808-LTCC (5822), www.laminaceramics.com.
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