RESEARCHER GETS AGGRESSIVE WITH PASSIVES

FAYETTEVILLE, Ark. - Although they may not know it, most consumers want integrated passives (IPs), according to University of Arkansas researcher Rick Ulrich. IPs can reduce the weight and size of consumer electronics such as cell phones and portable computers while increasing their reliability and performance.

Ulrich, professor of chemical engineering, and his co-authors conduct research on IPs at the U of A High Density Electronics Center (HiDEC). Details of their work appear in a recent issue of IEEE Circuits and Devices.

"The driving forces for using IPs are the same as those driving microelectronics in general," said Ulrich. "The consumer wants them because they reduce size and weight in portable applications such as cell phones, pagers and electronic personal assistants."

On a circuit board, passives - resistors, capacitors and inductors - occupy the majority of the physical space. Although microchip size has decreased dramatically in the past decade, the size of passives has changed very little, while the number that each board requires has increased.

Before the size of electronic devices can decrease, the size of their circuit boards must decrease. This is a particular problem because cell phones, audio/video equipment and the increasing array of wireless devices all use analog signals. In these devices, circuit layouts depend more on impedance levels and signal path properties, which are controlled by passives.

"Passive component size and density are a major hurdle in the miniaturization of most electronic systems, especially in the consumer sector," Ulrich explained. "An average circuit board in a cell phone has 380 separate components, including 322 passives. More than a trillion passive devices will be placed in electronic systems this year."

Boards are now dominated by discrete passives, according to Ulrich. These small, rectangular surface-mounted components have solder joints at each end. The physical difficulty of attaching these passives limits how small they can become. In addition, solder joints decrease board reliability.

Unlike discrete passives, IPs are built into the surface of the circuit board as it is being fabricated. Microchips can then be placed directly on top of the IPs, greatly reducing both the length of the signal pathways and the physical size of the board. Because IPs do not have any casing or packaging of their own, they do not add to the weight of the device, and eliminating the solder joints increases system reliability.

IPs have been used successfully with ceramic boards, called thick films, for over 50 years and much of the fabrication has been automated. However, these old, thick-film technologies cannot be applied to the flexible substrates in most circuit boards because of the high temperatures required during fabrication.

Ulrich has been developing thin-film IPs at HiDEC for nearly a decade. Although there are some short-term issues remaining, he sees lack of industry infrastructure as a primary problem for IP use. Because they are not in widespread use, it is difficult to find vendors that can supply the quantity of materials necessary for large-scale production.

Currently the driving force in IP development is high-performance users like the military, aerospace and supercomputer industries. Besides increasing reliability and decreasing size, IPs can solve some of the problems associated with very-high-speed processing.

"Market penetration will probably follow the traditional path from critical-requirement applications to consumer products," said Ulrich. "The broad application of IPs is a certainty, and their development is underway at almost all major microelectronics manufacturers."

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