Issue



Embedded Thermoelectric Coolers for High-performance ICs: Taking the Edge off Hot Spots


03/01/2007







BY BOB CONNER, Nextreme Thermal Solutions

anufacturers and OEMs need a package-level cooling solution that provides cooling in close proximity to the chip, to cool hot spots and smooth out non-uniform power dissipation. Existing thermal management solutions are aimed at uniformly cooling the entire die. Applying uniform chip cooling solutions inefficiently reduces hot spot temperature, because cooling the rest of the chip is unnecessary and requires a larger heatsink. Moreover, bigger heatsinks are not an option in many applications.

An efficient way to smooth out these hot spots is to cool the high-heat flux areas locally, which minimizes over-cooling of the adjacent silicon. Mitigating hot spots through non-uniform heat transfer close to the on-chip heat source is the primary driver for innovation in electronic cooling.

One technology that addresses this problem is an embedded thermoelectric cooler (eTEC) (Figure 1). These devices cool hot spots to increase product performance, reliability, and yield while reducing cost. An eTEC is a miniature, solid-state heat pump fabricated with a nano-structured thin film using semiconductor processing techniques. eTECs operate by the Peltier effect: when an electric current is driven through a circuit containing two dissimilar materials, heat is absorbed at one junction (the cold side) and released at the other junction (the hot side). The thin-film eTEC materials have high electrical conductivity and poor thermal conductivity to maximize current flow and minimize heat flowing back from the hot side to the cold side. Unlike conventional bulk thermoelectric coolers (TECs) that are made with 1- × 1- × 1-mm pellets assembled into a large array between two ceramic substrates, eTECs fabricated with thin films are small, thin, fast, efficient, and reliable. An eTEC cools a hot spot by moving heat from low to high thermally conducting materials. The devices are site-specific; hot spot cooling minimizes total heat transfer, reducing heatsink size.


Figure 1. Embedded thermoelectric cooler (eTEC)
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Minimum input power is needed to cool the hot spot. Small size and weight allow for unobtrusive integration in close proximity to the hot spot. A solid-state design ensures reliability; high-heat flux cools concentrated hot spots. A fast response time promotes rapid cooling and heating to maintain a precise temperature. Active cooling management is achieved by varying input current to the eTEC. Wafer-fabrication scalability enables low-cost, volume production. Additionally, the devices are optimized for non-uniform cooling to complement uniform cooling solutions.

How It Works

The eTEC is located over the hot spot between the backside of the die and the spreader (Figure 2). The active side of the die is face-down towards the substrate. Some of the heat flows through the solder balls to the substrate. Most of the heat flows through the backside of the die, the first thermal interface material (TIM1), the heat spreader (which forms the lid of the package), the second thermal interface material (TIM2), and finally through the heat sink to ambient. The hot side of the eTEC is soldered to the heat spreader, providing an excellent thermal interface. The same TIM1 that is used to interface the die to the heat spreader is used to interface the die to the eTEC’s cold side.


Figure 2. Flip chip IC with eTEC cooling a hot spot
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Reducing the hot spot temperature with a uniform cooling solution requires either a larger heatsink or faster fan speed. In Figure 3, the y-axis depicts the thermal driving force - hot-spot temperature (Thot spot) - ambient temperature (Tambient). This value is increasingly limited to just 50 to 60ºC, because the Thot spot must be kept below approximately 100ºC for CMOS ICs and the Tambient, which is typically ~40ºC, is driven up as increasing integration and shrinking system footprints force designers to dissipate more heat in a smaller volume.


Figure 3. eTEC creates temperature inversion for hot-spot cooling.
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The x-axis in Figure 3 represents the thermal resistance of each component in the thermal stack. The total thermal resistance (Rthermal) to the heat flow, responsible for Thot spot - Tambient temperature rise, is typically broken down as follows: 40-45% from the heat sink thermal resistance (Rsink); 40-45% from the IC package, which is the thermal resistance of die plus TIM1 plus heat spreader (Rdie + RTIM1 + Rspreader); and 10-15% from the TIM2 thermal resistance (RTIM2). The slope of the line is the heat transfer rate, q = (Thot spot -Tambient)/ Rthermal), which is analogous to the flow of electric current through a conductor (I = ΘV/Relectrical), where the temperature difference is analogous to ΘV (voltage difference) and q (heat transfer rate) is analogous to I (current).

Conclusion

An eTEC provides an efficient, non-uniform cooling solution to reduce Thot spot. This technology reduces the temperature at the TIM1-to-heat-spreader interface, creating a temperature inversion. The eTEC serves as a heat pump to move heat from the IC and TIM1, which have low thermal conductivity; and toward the heat spreader, which has a high thermal conductivity. Temperatures inside the IC package are reduced where it matters most, while the temperature of the case and heat sink increase above ambient by a very modest amount. This temperature change is proportional to the fractional increase in total dissipated package power, which is typically 2 to 3%, resulting from the operation of the eTEC used to cool a small hot spot. Spot-cooling only the hot spots reduces the overall amount of heat that must be removed from the chip, since the eTEC does not pump heat from the “background.”

eTECs also work with any other viable cooling solution an IC manufacturer uses, as all require knocking down the top of the hot spot. They fit well into existing packaging methodologies and extend the life of current thermal management solutions such as heat spreaders, thermal interface materials, heat sinks, and fans. They can also be used in conjunction with emerging thermal management solutions, such as liquid microchannel coolers, providing a considerable volume and weight savings.

BOB CONNER, VP, marketing & business development, may be contacted at Nextreme Thermal Solutions, 3040 Cornwallis Road, Mailstop: 13981, Research Triangle Park, NC 27709-3981; 919/485-2774; E-mail: bconner@nextremethermal.com.