A New Substrate for High-performance Requirements
BY JOE MAZZOCHETTE
Designers of today's high-performance components, modules, circuit boards and systems with higher operating frequencies and thermal management requirements face difficult design problems, many of them associated with circuit and system packaging. These new designs require high-circuit densities, low-electrical loss and excellent dimensional control.
Many products now require more and more functions without increasing package size. In some cases, the package size needs to be decreased. The cost structure of some products must be improved by incorporating more devices operating at higher power on a larger circuit board. Such boards require a physically robust substrate with high heat dissipation and low-loss characteristics.
Even though device technology is available for advanced product features, better packaging systems are needed to improve both performance and cost.
The standard FR-4 fiberglass multilayer substrate may have too much loss for high-frequency signals, is too limited in heat dissipation capabilities and has significant physical constraints. Polytetrafluoroethylene (PTFE) substrate material has extended the range of radio frequency (RF) applications, but it has its own limitations, including higher cost. It also is a poor thermal conductor, outgases, has a limited operating and processing temperature range, is a thermal mismatch with semiconductor materials, and still limits circuit and packaging designs.
High-temperature co-fired ceramic (HTCC) substrates offer better performance than PTFE but they too are expensive for many applications and limited in design flexibility. Because HTCC parts are fired at 1,400° to 1,500°C, they must use refractory metals for circuit traces, which results in high electrical resistance compared to noble metals. This poor conductivity often has a detrimental effect on circuit performance. Metal finishing of HTCC packages is required.
Because low-temperature co-fired ceramic (LTCC) parts are fired at about 950°C, silver and gold conductors can be used. Also, a wide variety of resistive and dielectric materials can be applied before firing to form passive components. Moreover, multiple layers with buried components can be formed, and active components with large I/O counts can be connected with wire bonding, surface mount or flip chip techniques. These techniques allow unpackaged semiconductor device mounting, which further reduces board real estate for a given circuit configuration. Side benefits of buried components on a populated circuit board include fewer connections on the surface, lower assembly costs and higher reliability. LTCC parts have excellent heat dissipation and RF transmission characteristics.
LTCC technology would seem to be the ideal substrate, but may not be optimal for some high-performance products. For example, certain high-power devices must be surface mounted with large heat sinks to limit temperature rise. Several discrete processing steps are required to produce a finished part. Also, the firing process that creates the LTCC substrate results in a certain amount of shrinkage, which at times is difficult to predict, and may require trial-and-error experimentation before arriving at a final design.
 Figure 1. Multilayer LTCC-M board construction. |
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A new ceramic technology — low-temperature co-fired ceramic on metal (LTCC-M) — bonds ceramic to metal. Circuit boards created with LTCC-M (Figure 1) can provide numerous advantages.
Open cavities that extend down to the metal base can be created with the LTCC-M process to allow direct component die mounting on the base (or one or two levels above). This eliminates the need to attach a fully packaged high-power component on a large substrate-mounted heat sink. The metal core in an LTCC-M substrate allows heat to be carried away through a material with a thermal conductivity of 170 W/m°C.
Another benefit of the bonded metal base is that shrinkage in the X-Y plane during firing typically is 0.1 percent. (Traditional LTCC has a typical shrinkage of 12.7 to 14.8 percent.) The virtually nonexistent shrinkage allows for quick, reliable prototyping and production of large (up to 16 x 16″) substrates.
LTCC-M provides benefits in the size of the package as well. With LTCC-M, up to 24 layers measuring between 0.002 and 0.010″ (0.004″ standard) in thickness can be used for traces and buried passives. The ability to process multiple layers with LTCC-M allows for the production of large circuits or many small circuits on a single large substrate. This overall combination of multiple layers and large substrate size cannot be duplicated with conventional LTCC technology.
Large populated LTCC-M boards (up to 16 x 16″) can be created with higher component density, trace density and flexural strength compared to standard LTCC processes. Because the LTCC-M process allows larger wafers, multiple populated circuits can be created in a single firing, which are then cut from the finished wafer.
 Figure 2. RF-optical integrated package produced with LTCC-M technology. |
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With LTCC-M, a hermetic package with a lid can be produced at a lower cost than if an LTCC substrate is placed in a leaded Kovar package to protect against environmental exposure.
Designers using high-performance substrates can integrate LTCC-M as a subcircuit without making wholesale changes in their circuitry. Moreover, LTCC-M offers a straightforward migration path to the next generation of products.
Conclusion
Traditional substrates such as HTCC, PTFE, LTCC and FR-4 fiberglass all have performance or cost concerns in meeting the design requirements for today's high-performance components, modules, circuit boards and systems with higher operating frequencies and thermal management requirements. Design engineers faced with the challenge of delivering a smaller package with more performance for less money are finding that LTCC-M can serve as a solution to their design requirements.
Joe Mazzochette, vice president of engineering, may be contacted at Lamina Ceramics Inc., (800) 808-5822; Fax: (609) 265-9905; E-mail: [email protected]; Web site: www.laminaceramics.com.