Localized Cooling for Data Centers
01/01/2009
For the past 50 years, the thermal management industry has offered only heatsinks, fans, and thermal grease as methods for electronics thermal management. While these techniques have been refined and improved over the years, nothing new has been introduced to address the exponential growth of thermal issues in modern-day electronics.
The electronics industry has reached a breaking point — a sort of thermal overload. As components, packages, and systems continue to shrink in size, we are simultaneously adding functionality. The heat generated in these dense electronic systems can be quite large and in-turn lead to significant increases in temperatures that cause component-, device-, and system-level failures.
The answer to these problems has always been to use a larger fan or larger heatsink to move all of the heat out of the electronic package and into the system environment. That’s easiest but it costs the most to manage.
The EPA estimates that by 2011, energy consumption by U.S.-based data centers could top more than 100B kwh, representing an annual cost of at least $7.4B. According to a recent study by Emerson Network Power, 50% of the power consumed in data centers goes toward battling heat with air conditioning.
The most efficient thermal management system involves embedding thermal management functionality at the source of the heat to remove excess heat before passing the remaining heat on to the next level. The cost of implementing thermal management solutions can be compared to the level in which the solution is introduced. Implementing heatsinks, fans, and large-scale cooling creates an energy savings potential. Introducing localized cooling in the overall thermal management design translates to a greater cost savings potential at the rack and data center levels.
Localized thermal management solutions have been introduced∗ that work deep inside electronic components using thin-film thermoelectric structures known as thermal bumps. The thermal bump is made from a thin-film thermally active material that is embedded into flip chip interconnects (in particular copper pillar solder bumps) for use in electronics packaging.
Thermal bumps act as solid-state heat pumps and pull heat from one side of the device and then transfer it to the other as current is passed through the thermoelectric material. Thermal bumps today are already extremely small — 238??m in diameter by 60??m high — and have the capability to be scaled to different sizes for different applications.
Thermal bumps can be introduced into a system at the chip or board level using discrete modules. Integration possibilities include:
Chip Cooling: Thermal bumps can be integrated for heat removal from the back or front-side of the die and even laterally. Backside cooling can be enhanced by the introduction of thermal bumps either into the heatsink to form an active heatsink, or into the heat spreader. For a 3-D chip stack, the lateral heat-removal concept can be combined with an interposer through which heat can be removed.
Board Cooling: Hot spots on PCBs can be cooled by the introduction of discrete modules strategically placed near the source of the heat. Metal traces, which can be several micrometers high; can be stacked or interdigitated to provide highly conductive pathways for collecting heat from the underlying circuit and funneling that heat to the thermal bump. Additionally, adding thermal vias (e.g., copper filled vias) can provide excellent pathways for the rejected heat.
Summary
The use of thermal bumps and discrete devices in a thermal management solution does not obviate the need for system-level cooling or for a reasonable means of rejecting heat out of the system. Rather, it introduces a fundamentally new methodology: cool only what you need to cool and nothing else. Manage that heat appropriately by channeling it to an environment where it can easily and inexpensively dissipate. Do not just move it away from the source and introduce it to an environment where its removal becomes more costly. Designing thermal management systems where the cheapest solutions are used at the chip level has led to the introduction of the most expensive systems at the next level where the environment is at the same time most sensitive to higher temperatures.
*Nextreme’s UPF OptoCooler
PAUL MAGILL, Ph.D.,VP marketing and business development, may be contacted at Nextreme Thermal Solutions, 3908 Patriot Drive Suite 140, Durham, NC 27703; 919-597-7300; E-mail: [email protected]