BY JIM LAWSON
Packaging markets have come full circle over the past 40 years. In the 1960s, thick- and thin-film hybrid technology addressed, in part, limitations of integrated circuit (IC) technology.
Over the next two decades, thin- and thick-film feature improvements occurred less frequently than advances in silicon and printed circuit board (PCB) technologies. IC packages also had progressed rapidly, challenging the PCB industry to improve its own feature sets. Thin- and thick-film ceramic technologies had lost their competitive edge as advances in silicon integration and laminate interconnect were achieved.
The microcircuit industry's response was to devise a new term — multichip module (MCM) — for the exact packaging methods the industry had been using for years.
SoC, SiP
In the 1990s, the silicon industry continued progressing at the speed of Moore's Law and another viable packaging concept, acronym and buzz word emerged — “SoC” or system-on-a-chip — that essentially is a “system” contained on a single piece of silicon.
While it is possible to integrate a significant amount of functionality in one silicon device, the cost of doing so may be prohibitive. If the increased cost can be justified, then SoC is a viable option.
Packaging of mixed discrete semiconductor technologies was one compelling competency that launched the hybrid industry years ago. Today, packaging technologies continue to provide the option of splitting a system into multiple dice, expanding performance while providing significant cost advantages.
Broadly speaking, any packaging solution that uses multiple chips and provides a complete function can be called a “system-in-a-package.” While an SiP is similar conceptually to an MCM, today's SiP typically has a lower cost, a higher volume potential and interconnect structures that accommodate a wide variety of functions.
Consumer demand for transportability, no-cost reliability and adaptable products remains omnipresent and, as such, packaging continues to be an important element in the supply chain.
LTCC Packaging
Modern packaging must address system requirements beyond the structural role of the past. An ideal package model would provide interconnect, support multiple assembly techniques, support multiple I/O, provide protection and integrate circuit components.
It has been the practice in the PCB and IC industries to measure densities in terms of in/in2 or metric equivalents. These density figures describe the efficiency of the 2-D real estate used to create an IC or to fabricate a PCB without regard to the key to efficient packaging — the third or “Z” dimension. The notion of a 2-D measurement has spawned the term “SoP” or system-on-a-package. This contrasts with system-in-a-package, whereby the package becomes part of the system by providing efficient and innovative use of the Z dimension.
Low-temperature cofired ceramic (LTCC) uses the Z dimension for interconnect layers, embedded circuit elements, and integral features such as shelves and cavities. LTCC is a mixture of thick-film and ceramic technologies, with the thick film contributing electrical interconnect and passive circuit elements, and ceramic contributing dielectric properties as well as the foundation for physical features, from vias to complex stepped cavities and cutouts.
These characteristics allow the designer to: 1) effectively use the system's Z dimension to realize embedded circuitry and interconnect, and 2) reuse the X-Y real estate for active and additional passive circuit elements on the top layer. Furthermore, thick-film resistor and capacitor elements may be adjusted by means of YAG-laser trimming during test, allowing precise functions to be realized with less costly, wider tolerance add-on circuit elements.
The Future
The LTCC infrastructure has developed and is supplying the communications, automotive, and military and aerospace markets with high value-added packaging solutions. Making use of a cubic configuration to integrate the package, the interconnect and passive elements into one structure, LTCC SiP is poised to serve a multi-billion dollar market in the near term and enjoy double-digit growth in the years to come.
Jim Lawson, Global Business Development manager, may be contacted at C-MAC MicroTechnology, a Div. of Solectron MicroSystems; (919) 474-3555; Fax: (919) 941-0530; E-mail: [email protected].