Integrating passives: to save real estate and enhance reliability, even the DoD is embedding parts

Circuits Assembly, August, 2007

While commercial electronics manufacturers typically drive technology trends, the Department of Defense is actively pursuing methods to reduce the size, weight and cost of its high-reliability electronics, particularly in mobile RF applications. Complex RF modules may require a high number of top-performing passives. These critical passives may in turn consume considerable real estate. Significant size and weight reduction and reliability improvements can be realized by using integrated passives.

Integrated passives typically are composed of some combination of resistors, capacitors, inductors and filters. Integrated passive with HDI is typically used for digital applications. This involves embedding resistors, inductors and capacitors between layers of organic PCBs. Devices are connected using the board's innerlayer metalizations. Using embedded passives with HDI reduces the total part count required by replacing surface-mounted discretes with components internal to the board (Figure 1).

[FIGURE 1 OMITTED]

The thin-film components' range of values, precision and functional density achieved by thin-film technology are well-suited for RF functions. For integrated passives with thin-film technology, individual thin-film devices are deposited onto a substrate (such as silicon) and interconnected through metalized 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 devices are available as separate components. Thin-film devices may reduce parts count by replacing a network of individual discretes with a single device. Placing, soldering and inspecting one component is much faster than processing multiple SMT components.

The reduced parts count will result in greater manufacturing throughput and lower inventory. In volume applications, if the assembly process is properly managed, the integrated passive device cost will be lower than the cost of processing discretes. Also, the parasitic effects of the solder joints are mitigated.

Applications

Integrated passive devices (IPDs) can be used in digital and RF applications. The high tolerances and performance characteristics of IPDs typically most benefit demanding RF applications. These applications like cellphones, 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 IPDs and embedded components in military programs to decrease size, reduce parts counts and increase reliability. Integrated inductors, in conjunction with system on chip (SoC) architecture, are used to reduce the cost and weight of phased array antenna systems for DDG class shipboard electronics systems. Embedded capacitors and resistors are used in the redesign of the Air Radio Set (ARS-6). Embeddeds aid in the ability to produce an open architecture design that easily permits upgrades such as sophisticated GPS.

Many of the applications require the improved inductance characteristics and reduced EMI that IPD networks provide. Reduced EMI, combined with improved reliability, reduced parts count, and decreased module final product assembly time, makes IPDs excellent choices for any application requiring precision and dependability.

Resistors in a thin-film IPD network can use 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 permits the dielectric to be tuned to make the devices meet a range of capacitance. Silicon nitride, aluminum oxide and tantalum oxide films are used as capacitor materials. Material selection depends 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 inductor properties are a direct function of materials used and the frequency of interest. IPD inductors have a significant advantage over inductors used in SMT-based configurations. Thin-film processing permits edge coupling of the inductors, which creates less stray capacitance than broadside coupled inductors. IPD inductors also have fewer package-related parasitics.

The American Competitiveness Institute (aciusa.org) is a scientific research corporation dedicated to the advancement of electronics manufacturing processes and materials for the Department of Defense and industry. This column appears monthly.