There are two prevailing doctrines for cooling electronics today: 1) pack your electronic components sparsely so you can cool them economically with air (convection), or 2) pack your electronics densely so you can use a more efficient coolant like liquids. Both have their place. But as each new generation of processors and memories come and go, power budgets keep escalating, and convection cooling comes closer to obsolescence for many of its present-day applications.
In the past 20 years, there have been two variations of air-cooling: convection and conduction. Early electronics chassis would have openings at the top and bottom of a card cage and allow air to simply rise through the chassis past the electronics on the circuit boards and cool by the chimney effect of the air movement. When the electronics got hotter, fans were added to force more air over the hot components. Ultimately, we can only cool about 1 watt per square inch with convection processes, even with fans blowing high volumes of air across printed circuit boards.
When you air-cool your electronics with fans, the lowest Mean Time Between Failure (MTBF) devices in the system are the moving parts. Consequently, the fans lower the overall reliability of the entire system. As the fans move more air, they create a severe noise problem in many environments, making some computer rooms sound like a wind tunnel. Most of the electronics today are convection cooled because it is less expensive.
In many military applications, the noise and low MTBF of fan-driven cooling systems is unacceptable. Therefore, the VME community and the military users developed the technique of conduction cooling. Just think of the entire electronics cabinet as a giant heat sink. Conduction cooling moves the heat from the printed board through a heat transfer surface to the chassis walls and structure where the air circulating nearby removes the heat. But conduction cooling can only cool about half the heat density of convection or forced-air cooling, or about 1/2 watt per square inch.
Consider than many processors today are dissipating more than 20 watts, or about 5 watts per square inch. The higher-end CPUs are dissipating more than 100 watts from their brick packages. One processor chip I have seen is 3 inches x 3 inches (9 square inches) and dissipates 850 watts, or about 95 watts per square inch. As we get above 5 watts per square inch, it becomes obvious that convection and conduction cooling techniques will no longer work.
The VITA Standards Organization (VSO) has been working for two years on these cooling problems. The answer to cooling heat densities above 1 watt per square inch is liquid cooling. Liquid is a much more efficient heat transfer agent than air because it is denser.
There are several ways to use liquid as a coolant. The most obvious is to circulate the liquid through the hot components and remove the heat with a radiator-type device. That is called circulated liquid cooling. The liquid
can be routed through the hot electronics with pipes or it can be pumped into a large cavity called a coldplate, which sits on top of the hot chips. The 850-watt processor mentioned above uses a coldplate to remove the heat and keep the chip running cool (around 70°C). When you are considering cooling VME boards in a rack, the first thing you have to do is put two dripless valves on the ends of the boards (next to the DIN connectors). The top one is for liquid-in and the bottom one is for liquid-out. If you are using water as a coolant, those valves must be dripless because water is a conductor.
This brings us to the topic of coolant type. Water has been used for cooling electronics for many years, but it has its problems. You must use distilled water in such cooling systems because regular water contains salts and other impurities that can cause system failures. All connections in a water-cooling system must be completely drip proof. If you use chilled water (a refrigeration-type cooling system), the room in which it operates must be moisture-controlled. If there is too much moisture in the air, it will condense on the cold water-cooling pipes and drip onto the electronics. Consequently, water may be efficient and economical as a coolant, but it causes a lot of problems.
Several chemical companies have developed coolants that are totally inert and nonconductive. This allows electronic circuit boards to be completely submerged in these liquids. But emersion of electronic circuit boards into a liquid solution is not the most efficient method of heat transfer.
The most efficient heat transfer technique is evaporation. Evaporation is a cooling process and is more efficient than circulating liquids or emersion of PCBs. Several coolants, such as FC-7 and Fluronert, can be manufactured with a specific boiling point. Therefore, a vaporized liquid with a boiling point of 70°C will evaporate when the droplets hit a chip operating at that temperature. The vapor is pumped off and compressed, causing it to condense. Condensation is a heating process and the heat can be removed from the liquid with a heat exchanger (radiator tubes and fans). These liquids can also be used in a system of heat pipes. The tubes have a wick in them that is saturated with the low boiling-point liquid. The heat will cause the liquid on the wick to evaporate. The vapor is pumped out and compressed back into a liquid and the heat removed, similar to a spray-cooling technique.
The first spray-cooled VME systems are now being tested in the U.S. Marine Corps Advanced Amphibious Assault Vehicle (AAAV) (see Figure 1). These tests have been running for more than a year and the concept of liquid spraycooling of electronics has been proven.
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The U.S. Navy funded and is working with VITA to develop liquid cooling techniques for the VITA-34 board specifications. But the definitions and component standards developed under VITA-34 can be utilized on existing VME and other electronic circuit boards. If the electronics in an aircraft can be densely packed and liquid cooled, that will reduce the weight of the aircraft and allow it to carry more ordnance.
However, the most startling benefit of liquid cooling is reliability. It is a generally accepted rule that every time you drop the operating temperature of your electronics by 10°C, the Mean Time Between Failures (MTBF) doubles. If we could drop the operating temperature of military systems to 50ºC, the MTBF on those systems could be five to seven years. No one would need to touch those systems for five to seven years, and that would tremendously reduce maintenance costs while drastically increasing reliability.
Just think of what liquid cooling could do for other industries. In telecom, an entire 100,000 square foot Central Office could be electronically packed into four 19-inch cabinets and stored in a broom closet if it were liquid cooled. The telecom company could sell the huge building, turn off their giant air conditioning system, stop paying $10,000 per month utility bills, and would never have to service the equipment for repairs for five to seven years.
When you start to look at the power curves of next-generation processors and semiconductors and if you can see the benefits of liquid cooling, particularly in reliability, it all starts to make sense. We have already seen spray-cooled VME boards. In the future, you will see heat pipes and coldplates on VME cards. You will see liquid cooling on many of the new fabric-based architectures. It is clear that liquid cooling makes sense for high-density, high-performance embedded computer boards. We must move to liquid cooling techniques for our future high-performance VME systems.