In today’s world of 30 gigabyte movie viewable IPODs, handheld computers, and wireless global positioning system (GPS) data phones, there is a strong market-driven demand  to increase the functionality of internal electronics while drastically reducing the total area and profile. While commercial electronics manufacturers typically drive the technology trends in electronics, the Department of Defense is actively pursuing methods to reduce the size, weight, and cost of its high-reliability electronics, particularly in mobile radio frequency (RF) applications.

Complex RF modules may require a high number of top performing passive components in order to function properly. These critical passive components may take up considerable real estate surrounding active devices. Reducing the overall size of the RF module while maintaining or even increasing the number of functions the component module can perform, is a goal of many commercial and military programs. A significant size and weight reduction and an increase in reliability can be realized by using integrated passive devices (IPDs).

Types of IPDs
IPDs typically are composed of some combination of resistors, capacitors, inductors, and filters. They can be classified into two categories:

•     Thin-film and ceramic technology
•     High Density Interconnect (HDI) or other circuit board technology

Integrated passive device HDI technology is typically used for digital applications. This technology involves embedding passive resistors, inductors, and capacitors between the layers of multilayer organic printed circuit boards (PCBs). The devices are connected using the circuit board’s inner layer metallizations. Using embedded passive device HDI technology reduces the total part count required by replacing surface mounted discrete components with components internal to the circuit board (Figure 6-1).

The range of values of the thin-film components, the level of precision, and functional density that can be achieved by thin film technology are well suited for RF functions. In IPDs using thin-film technology, the individual thin-film devices are deposited onto a substrate (such as silicon) and interconnected through metallized transmission lines to form a network. The passive network can then be packaged and attached as part of a circuit or module. These thin-film IPD devices are available as separate components. Thin-film IPDs may reduce part count by replacing an entire network of individual discrete components with a single device. Placing, soldering, and inspecting one component is much faster than processing multiple SMT components.

The reduced part count will result in increased manufacturing throughput and decreased inventory. When the assembling process in volume manufacturing is properly managed, the cost of using IPDs will be lower than the cost of processing discrete components.

Improvements in reliability are achieved by reducing the number of solder joints (and the likelihood of joint failure) used to attach the resistors, capacitors, and inductors.  Additionally, the parasitic effects of the solder joints are mitigated.

Applications
Integrated passive devices can be used in both digital and RF applications.  The high tolerances and high performance characteristics of IPDs typically have the most benefit for
demanding RF applications. Some of these applications include cell phones, personal digital assistants (PDAs), wireless computer networks, commercial and military radar systems, and phased array antennas (PAAs). IPDs function in these systems as:

•     RF Front end modules
•     RF power amplifier couplers
•     Pass band filters (low pass, high pass)
•     Functional interposers
•     Multi-band transceivers

The EMPF has implemented the use of IPD devices and embedded components in military programs as a method of decreasing size, reducing part count, and increasing reliability. Integrated inductors, in conjunction with system on chip (SoC) architecture, are used to reduce the cost and weight of phased array antenna systems that will be used in DDG class shipboard electronic systems. Embedded capacitors and resistors are utilized in the redesign of the Air Radio Set (ARS-6). The embedded passives aid in the ability to produce an open architecture design that easily allows for upgrades such as including a sophisticated global positioning system (GPS).

Many of the applications require the improved inductance characteristics and reduced electromagnetic interference (EMI) that integrated passive device networks provide. EMI is decreased by reducing the line length for coupling. The EMI reduction combined with the improved reliability, reduced part count, and decreased module final product assembly time make IPDs excellent choices for any application requiring high precision and high dependability.

Thin-film IPD components
The resistors in a thin-film IPD network can utilize simple linear forms in the case of low to mid-power requirements (<1000W) or more complex serpentine geometries with large line widths for higher power applications.  Tantalum nitride, tantalum silicide, and ruthenium oxide are common resistor materials, although the materials may be changed to suit the resistivity and power handling requirements,  temperature coefficient of resistance (TCR), and processing capabilities.

Capacitors used in IPD networks can be a metal-insulator-metal (MIM) design or inter-digitated fingers. The MIM structure allows the dielectric to be tuned to make the devices  meet a wide range of capacitance. Silicon nitride, aluminum oxide, and tantalum oxide films are used as capacitor materials. The material selection is dependent on the capacitance density required and the temperature coefficient of capacitance (TCC).

Single layer and stacked spiral inductor component configurations are available to IPD networks. Many of the properties of the inductor are a direct function of the materials used and the frequency of interest. IPD inductors have a significant advantage over inductors used in SMT-based configurations. The thin film processing allows for edge coupling of the inductors which creates less stray capacitance than broadside coupled inductors. IPD inductors also have reduced package related parasitics.

Thin-film Substrates and Packaging
Thin-film integrated passive components can be processed using many different types of substrates:

•     Traditional ceramic (alumina, beryllium oxide, etc.)
•     Low-temperature co-fired ceramic
•     High-temperature co-fired ceramic
•     High-resistivity silicon wafers
•     Standard oxide isolated silicon wafers
•     Leucine aminopeptidase on porous (LAP) glass

The co-fired ceramic substrates offer a significant cost advantage over the other substrate technologies. However, the low tolerances obtained using the ceramic substrates make them unlikely IPD substrate candidates for high performance RF applications. Because of their smooth surfaces and high tolerances, the LAP and wafer substrates produce higher quality devices than can be obtained on ceramic. Also, the wafer and LAP processes use existing high-volume wafer production processes, which  offset the higher material cost of the wafer substrate.

IPD packaging can be categorized as either stand alone chip scale package IPD devices or integratable IPD modules. Chip-scale IPD packages contain the entire IPD network in a single system in package (SIP) structure. This single package is designed to replace a surface mount passive component network. It is common to see these single packaged networks in ball grid array (BGA), quad flat no lead QFN, and flip-chip packages. The grid array packages help take full advantage of the size reduction achieved by using IPD technology.

The integratable modular chip IPD structure is designed to integrate active devices onto the IPD substrate. The active devices are assembled onto the IPD substrate using die attach chip-on-board (COB) technology or may be soldered to the IPD substrate using flip-chip or BGA technology. In either case, a solderable or wire bondable gold finish is typically applied to the pads on the IPD substrate. The integrated IPD module can then be connected to a conventional lead-frame or attached with other conventional methods such as land grid or ball grid arrays.

Conclusion
Using integrated passive devices increases the functionality, reduces the cost, increases overall reliability, reduces size and weight, lowers component profile, and increases the performance characteristics for networks requiring passive components.  These improvements are particularly beneficial for devices using precise RF networks where there are a significant number of passive components such as cell phones, wireless networking hardware, and military transmit and receive equipment. To take full advantage of this technology, the properties of the resistors, capacitors, inductors, and substrate must be properly integrated together.

References:

Arbuckle, Brian, Elizabeth Logan, and David Pedder. “Processing Technology for integrated passive devices.” Solid State Technology November, 2000.
Coombs, Clyde. Printed Circuits Handbook. New York: McGraw-Hill, 2001.
CSMP: An Integrated Passive Device Technology. <http://www.statschippac.com/en-US/STATSChipPAC/IntegratedServices/Packaging/SiP/csmp.htm>.
Young, James L. “Integrated Passive Devices at the Wafer-Level”. Chip Scale Packaging. May-June 1999.