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Perusing Package Options For ICs

Feb. 18, 2011
Improvements in semiconductor processing have led to higher levels of integration, allowing multiple analog and digital functions to be packed within a single compact housing.

Make it smaller. That customer request (or demand) can be heard by designers of integrated circuits (ICs) in application areas ranging from automotive to space. Product developers want IC functions that can fit in smaller packages and modules and they want them to be lighter in weight and lower in cost, to boot.

Obviously, not all of these goals are easy to meet simultaneously, but the packaging side of RF and microwave ICs has come a long way in recent years, and system integrators have many highperformance, practical options when it comes to putting an IC die into a package.

Cellular communications applications, such as multimode, multiband cellular telephones, are often given credit for transforming many IC products from single-function devices to multichip modules (MCMs), system-on-chip (SoC) devices, or system in package (SiP) solutions in which multiple chips are combined to form a small subsystem. But over the past several decades, it's actually been military customers who have provided the greatest motivation to integrate more functions in a single module or package, so as to reduce the weight and size of avionics systems, manpack radios, and space-borne satellite- communications (satcom) systems.

By way of example, military tactical radios have largely morphed into softwaredefined radios that can be reconfigured for a variety of operating bands and modulation/ encryption schemes via a user interface. Although populated by a wealth of digital hardware, these radios still rely on RF/microwave electronics for the transmit and receive functions, and this implies multiple chip sets for each band covered. An integrated device such as an MCM or SiP can simplify the task of including required frequency bands. A similar trend has taken place in the design of thirdgeneration (3G) and fourth-generation (4G) cellular handsets or "smartphones," which must support a wide range of cellular frequency bands and standards.

SiP devices are characterized by threedimensional (3D) packaging approaches because chips are often stacked within a small housing. MCMs and SoCs are viewed largely as two-dimensional packaging solutions, with SoCs often a single, highly integrated chip within a surfacemount housing. The motivation for these and other multiple-function packaging concepts is to pack more functions into a smaller volume of available system space, often by combining analog and digital fuctions in the same package. MCMs have actually been in use for more than three decades. In many ways, MCMs and other multichip package solutions continue to integrate device functions where a semiconductor process has left off. Early GaAs monolithic-microwaveintegrated- circuit (MMIC) products from such innovators as Pacific Monolithics (acquired by Richardson Electronics), Sanders Associates (now part of BAE Systems), and TriQuint Semiconductor were simple, single-function devices, such as low-noise amplifiers (LNAs). As semiconductor wafer quality grew more consistent and larger wafers became more commonplace, as semiconductor processes improved, and as modeling tools improved, MMICs have grown in complexity.

Although the majority of its sales are for single-function devices, Triquint has also won sizable contracts for its capability of combining multiple chips within a compact module. Last year, for example, the company was awarded a contract by the US Air Force Research Laboratories (AFRL) to develop new gallium nitride (GaN) amplifier modules for unmanned aerial vehicles (UAVs). The company was tasked with providing 20- and 50-W amplifiers, including matching and bias circuitry, to fit into the same space as existing 1-W transmit amplifiers.

On the commercial side, the firm's model CV100-3A MCM is a complete frequency downconverter in a package measuring just 6 x 6 mm. It contains an on-board local oscillator (LO) and LO amplifier, RF amplifier, and intermediatefrequency (IF) amplifier. Based on GaAs MESFET and InGaP heterojunction-bipolar-transistor (HBT) semiconductor technologies, the MCM is ideal for CDMA, GSM, TDMA, CDMA2000, and WCDMA cellular systems. It has an RF range of 800 to 960 MHz, an LO range of 1000 to 1310 MHz, and an IF range of 200 to 350 MHz.

While MCMs typically resemble advanced components, SoC devices, such as the model CC1111 SoC from Texas Instruments, are actually subsystems that at one time occupied considerably larger housings. The CC1111 contains an RF transceiver, Flash memory, microprocessor, and a Universal Serial Bus (USB) controller, all within a QFN package measuring 6 x 6 mm. It can be supplied in versions for use at 2.4 GHz and at frequencies below 1 GHz (300 to 348 MHz, 391 to 464 MHz, and 782 to 928 MHz) for low-power commercial wireless applications, such as meter reading, building automation, and telemetry.

In some cases, the level of integration for an SoC can be quite high, as in a device developed by the Swiss company CSEM for medical applications (see figure). It integrates a 900-MHz low power RF transceiver, a 32-b digital signal processor (DSP), and static randomaccess memory (SRAM) within a chip capable of running on a 1-V battery while consuming only 3.5 mA in receive mode and only 1.1 A in standby mode.

What differentiates multifunction packaging options such as an SoC and an SiP? An SoC is generally based on a single chip with multiple functions, and its associated matching and bias circuitry within a package, while an SiP is typically multiple ICs within one housing. An SiP may stack chips vertically and use wire bonds to connect them to each other and to the package. The time needed to design a system on a single chip can be extensive compared to combining simpler chips within a common package, but the potential cost of having everything on one chip can be much less than combining several ICs within an SiP. The SiP, especially one with analog and digital functions, requires carefully partitioning to prevent digital signal leakage into RF front-end circuits, while SoCs are generally easier to manufacture.

Although dedicated package houses such as Stratedge and C-MAC Microtechnology offer diverse lines of packages, in some cases suppliers of semiconductor devices, such as Endwave Corp, will also engineer their own high-end MCM packages. Last year, Endwave announced a line of millimeter-wave products available in QFN surface-mount housings. Normally, these packages operate to about 20 to 25 GHz. Endwave modified a standard QFN package to extend the frequency range beyond 50 GHz.

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