KEEPING COOL: MODELING MICROCHIPS INCREASES RELIABILITY

FAYETTEVILLE, Ark. — Responding to demands, the microelectronics industry is producing devices that weigh less and do more, faster. But the faster microchips in these devices produce more heat, which can cause the chip to fail. University of Arkansas wind engineering expert Panneer Selvam, a professor of civil engineering, has developed a method to accurately identify areas of chip overheating, the most common cause of chip failure.

Selvam’s work began with a request from the Air Force, which uses high-power electronics in many applications. Although it is not the best substance for cooling, the Air Force wanted to be able to use air as a coolant in microelectronics devices.

"Although air is a poor heat transfer fluid, it has many advantages," explained Selvam. "There are no logistics issues, no inventory problems, no environmental concerns, no safety issues involved and no special training associated with the use of air."

Selvam accurately modeled the performance of a tiny microchip cooling device called a Micro-Jet Array (MJA), which sits directly below the chip. This is the first time that anyone has been able to precisely investigate the air flow in an MJA. That model has been validated and optimized and can make microelectronics devices cheaper and more reliable.

Selvam will present his results on Wednesday, October 10 at the IMAPS 2001 Intenational Symposiun on Microelectronics in Baltimore.

To handle the large volume of data and highly complex graphics, microchips must run very fast. While a 50 MHz chip speed was common five years ago, today’s standard computer processor chip speed is 800 MHz. In August, 2001, Intel nearly doubled that speed when it introduced the 1.5 GHz chip. But higher speeds cause problems, particularly with heat build-up.

"In some ways a microchip is like a human," Selvam explained. "When it runs faster, it becomes hotter. If it is not cooled down, it will stop running altogether. Too much heat can cause permanent damage, and one of the most efficient ways to cool both humans and microchips is by passing air over the surface."

Most computers and other devices have fans that circulate air and cool the chip to a certain degree. However, because air from a fan is directional, it cools only part of the chip, resulting in hot spots that can cause the chip to fail.

Selvam’s team worked in conjunction with the Air Force Research Laboratory that developed the MJA. The microchip/MJA package was fabricated by Simon Ang, co-director of the High-Density Electronics Center (HiDEC) at the University of Arkansas.

This MJA is smaller than a microchip and sits directly underneath it. Its gridded system of holes forces air across the chip at approximately 72 km/hr (45 mph). Although the basic principles have been used in many engineering applications, the Air Force device is far smaller than any previously used.

Because a microchip/MJA package is around 2.5 cm (1 inch) square and thinner than a dime, no instruments exist that are capable of actually measuring the temperature inside the layers of the chip. Early experimental work found an unexpected flow pattern, but they could not measure it or explain its cause. However, Selvam’s computer model accurately explained precisely what was happening at various points and identified the flow problem as re-circulation.