Nikkei Electronics Asia - January 2006
Columns:Pins & Vias
Integrated Passives Go Mainstream

For the last decade or so, research institutes and manufacturers have worked hard to find methods of integrating discrete passive devices using a variety of techniques. They have been driven by the knowledge that the steep rise in operating speed and circuit density of digital communications systems would be accompanied by a similar increase in the number of passive components needed next to active ICs to avoid parasitic inductance effects. Some of these methods are now becoming mainstream technologies in the manufacture of portable, high-functionality communication applications.

The basic problem with separately mounted discrete passives is that, as circuit densities and system speeds increase, the electrical impact of the resistance and impedance of a multilayer PCB's power distribution planes can reduce the amount of voltage available to the board's components. At the same time, the very high data bandwidth needed by high-speed digital systems between major components produces increased switching noise. This creates immediate, large demands on current, which in turn produce voltage gradients across power and ground planes.

The solution to these voltage gradients is to place capacitors, usually discretes, next to the active components that require high-speed current. Discretes work well at lower frequencies. But in high numbers, and at high frequencies, those capacitors can themselves become the source of parasitic inductance and resistance.

Integrating Passives

High-speed communications systems, particularly wireless designs, require very high ratios of passive to active components. Designers of these systems are therefore turning to techniques that eliminate separate discrete passives mounted on the surface of the IC module or PCB, and instead either embed them within the substrate or combine them via thin-film techniques into integrated passive devices (IPD).

In particular, IPDs have come down in price as volumes have risen. Not only are IPD device prices lower, but since they replace multiple discretes they are relatively less expensive to assemble, as well as to procure and store. IPD usage has also tracked the increase in the use of chip-scale packaging (CSP) and wafer-level packaging (WLP) techniques, and many are available in those package types. Compared to discrete passives or plastic packages, some CSPs can reduce footprint by 90%.

Another method for integrating passives, especially capacitors, has been to embed them in the PCB substrate. Embedded capacitance can be achieved by using the power and ground planes of the circuit board to provide a distributed capacitor, which performs better as frequency increases because the planes have very low inductance. But standard laminate substrates are too thick to provide the needed capacitance. In addition, as operating frequencies rise and supply voltage decreases, the electrical characteristics of the substrate material become increasingly important.

LTCC, PZT Perovskites

In the past, all three types of passive which are candidates for integration - capacitors, resistors and inductors - have been fabricated with different materials. The search is therefore on for new low-resistance, low dielectric constant substrate materials that can accommodate the varied electrical functions of capacitors, resistors and inductors. Low-temperature cofired ceramic (LTCC) materials, along with some advanced organic laminates and thin films on substrates, promise to successfully accommodate the electrical characteristics of all three.

In particular, LTCC has one of the widest ranges of dielectric properties of any packaging material, enabling substrates with multiple dielectric constants. One method that's been used successfully for integrating passives into RF applications is LTCC system-in-package (SiP) technology. Another combines CSP with integrated passives-on-chip (IPOC) substrates.

A recent breakthrough by STMicroelectronics significantly increases the junction capacitance density in thin-film passive integration. The new technology is based on a class of materials called PZT Perovskites, and offers a 50-fold improvement over existing competitive technologies employing materials such as oxides or nitrides of silicon or tantalum.

Perovskites are compounds that contain lead, zirconium, titanium and oxygen, and which vary according to the ratio of zirconium and titanium. These compounds' extremely high dielectric constant is over two hundred times greater than that of silicon dioxide.

by Ann R. Thryft

(January 2006 Issue, Nikkei Electronics Asia)

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