12 June 2012
Smart cooling solution boosts processor performance
Heat is not distributed evenly across a circuit board; hot spots occur near the processors and chipsets, since these components generate the most heat. For this reason, processors feature integrated mechanisms to protect against overheating and consequent damage.
However, users want to take advantage of the full performance potential; downclocking the cpu or shutting the processor down can only be an emergency solution. Existing cooling solutions have reached their limits, while the trend towards more performance continues unabated. This means new cooling concepts are needed to allow users to exploit the available computing power.
The modular COM Express concept can pave the way for future performance growth, with newer and more powerful modules mounted on a customer specific carrier board. While this scalable design solution helps customers to quickly and inexpensively create a variety of applications, full performance depends on the processor staying cool.
The classic COM cooling design resembles a sandwich with the different functions layered on top of each other. A copper or aluminium block is mounted on the chip to absorb heat. An optional phase change material can be placed between the chip and copper or aluminium block to mitigate the effects of thermal peaks. To account for different component heights and manufacturing tolerances, the next layer is a height balancing, thermally conductive material, the so called gap filler. The last layer is a heat spreader, an approximately 3mm thick aluminium or copper plate. All heat generated by the module is distributed across the complete heat spreader.
Module dimensions and interfaces are defined by the COM Express specification and, while this standardisation guarantees compatibility, size specifications may mean the heat sink cannot always be as large as desired. As a consequence, this cooling structure is only suitable for modules with a maximum power dissipation of 35W.
Modern COM Express modules, such as the conga-BM67, feature an Intel Core i7 or i5. The power dissipation of these processors is significantly more than 35W and hot spots around the processor and chipset become a real problem. An improved cooling concept is needed to lower cpu temperature – crucial when using second generation Turbo Boost technology for performance and energy efficiency. As a result, the processor can operate in excess of the maximum permitted thermal design power levels.
For the best heat dissipation results, a perfect thermal connection to the cooling system is required, but the thermal conductivity of the gap filler material is limited. When power losses are high, the gap filler layer inevitably gets thinner. Thin gap filler layers have lower mechanical tolerances so, to compensate for differences in component height, more pressure must be applied. At a certain pressure, the pcb will bend, leading to mechanical damage to the connections and vias and bga solder joints breaking.
The cooling capacity depends, to a large degree, on the amount of heat absorbing material used and the heat dispensing surface area. Copper is expensive; large heat sinks are heavy and require space that is generally not available. Simply increasing the size of the heat sink is therefore not a viable long term solution.
Heat pipes: a suitable alternative?
In laptops, heat pipes transport much more heat than an equivalent pipe made of solid copper. The secret lies in the fact that energy is absorbed during evaporation and released during condensation. The heat pipe is connected both to a hot and cold interface and filled with a working fluid. This fluid evaporates at the hot end and condenses at the cold end, with the condensate returning to the hot interface by capillary action. Since the heat pipe contains a vacuum, the working fluid can evaporate at low temperatures.
The capillary forces depend on the structure of the heat pipe; geometry and location influence how fast the working fluid is transferred, hence the cooling performance. Bend radius, heat pipe diameter and mounting position also need to be considered. A laptop provides a comparatively large space to accommodate a heat pipe solution. By contrast, COM modules must always be connected to the cooling solution at the same geometrical position, because the modules are interchangeable.
Classic cooling meets heat pipe
Fast spot cooling, good thermal connection, elimination of mechanical stress and greater cooling performance while retaining geometric dimensions – achieving all these requirements sounds impossible. However, congatec has mastered the challenge by combining the classical solution with a structurally modified heat pipe. Unlike the classical design, a flattened heat pipe is used to transfer heat from the chip to the heat spreader plate. The heat pipe is attached directly to the cooling blocks on the chip and the heat spreader plate. As a result, more heat is transported from the processor environment to the heat spreader, hot spots are cooled more quickly and the processor is cooled more optimally. Spiral springs with defined spring tension, as well as the heat pipe itself with its flexible height, put optimum pressure on the processor chip.
Manufacturing tolerances in the soldering process or height differences of the chips can be balanced in every direction, making a gap filler layer unnecessary. This is another advantage because when gap filler materials are heated, they can leak silicone oil, with consequences elsewhere in the system. Recesses in the heat spreader accommodate the flattened heat pipe, thereby maintaining the height. At the hot interface, the heat pipe rests freely in a recess; at the condensation end, it is placed in a wide groove on the heat spreader plate. This ensures there is plenty of room for the pipe to deflect while guaranteeing good thermal connection at both ends.
congatec's cooling solution provides scope for innovative customer ideas. For example, the heat pipe can be designed in such a way that it can be connected to a customer specific heat sink. Fanless designs are possible, provided the casing is equipped with appropriately sized cooling fins. Ultimately, the design depends on the specific application. The key features of the concept are equally applicable to other electronic circuits; hot spots also occur in power modules and semiconductor circuits in rectifiers and inverters, for instance, could benefit from this effective, inexpensive, small scale cooling solution.
The cooling solution is also suitable for systems with low power dissipation. The modules have a higher thermal reserve, which increases their life span and reliability. Average reductions in temperature of only 5K can double the statistical life span – a convincing argument when considering the total cost over the lifetime of a system.
Heat pipe advantages:
• Rapid spot cooling for full performance
• Elimination of gap filler layer
• Elimination of mechanical stress leads to higher quality
• Better cooling extends the life span of the module
• Heat pipe principle enables innovative customer specific cooling concepts
Konrad Pfaffinger develops computer modules for congatec.