Recently, one industry analyst noted that server farms in Seattle are dissipating as much power as the rest of the Seattle metro area, including the Boeing plants in Everett. Also, 25% of the total cost of their operation is for power and air conditioning. The idea of electronics being a low-cost, environmentally friendly industry is being challenged by the reality that this equipment may be one of the drivers for the rolling West Coast blackouts. This power consumption is transformed to heat, forcing designers to address thermal issues in addition to the other difficulties of creating new designs for ICs and equipment.
Similarly, power densities for other parts of our environment are as follows: A typical house uses from 0.1 to 1 W/ft2, with a peak power density of about 10 W/ft2. An office consumes from 5 to10 W/ft2 with a peak of about 25 W/ft2. The above-mentioned server farm falls in the range of 1 to 10 kW/ft2. All of these figures assume a constant power load with peaks only when restarting equipment. These power densities will continue to increase in all areas and cause concerns about thermal management from the end-user to the designer, as consumers demand solutions to their heat problems.
The power dissipated in a 2- by 2.5-ft rack can be 10 kW or higher. This is the heat equivalent of 100 100-W light bulbs inside a space of less than half a typical home refrigerator. Using existing methods to provide sufficient cooling to one such rack isn't particularly difficult, but cooling hundreds of these racks in a densely packed area requires a whole different approach (Fig. 1).
Beyond the heating and cooling problems, thermal management is concerned with classical heat issues. The reliability of ICs and systems is inversely related to the temperature relationship: the higher the operating temperature, the lower the reliability. As electronics equipment heats up, the operating conditions for the components cause changes in its operation. This affects timing, noise performance, and in the analog sections, parameter drift.
System Level Concerns: Because the systems use multiple ICs, the total system power is becoming a major problem. The systems are shrinking with more highly integrated chips on each board. But also the environment—noise, external ambient temperature, etc.—is growing more restrictive. These drivers increase the total power density within the board and the system.
Smaller sizes make adding extra cooling more difficult. Alternatives for shedding the heat can become very complex. Also, the costs of acquisition and ownership limit the acceptable options for removing or reducing the heat.
Heat can transfer from its source to the ambient through radiation, convection, and conduction. Radiation is fairly limited and hard to enhance because its characteristics are a function of surface properties, the best surface being a black sphere. On the other hand, convection and conduction are easier to change. Ways to improve local cooling through convection include forced-air cooling and heatsinks. Conduction occurs via the package leads and body, so pc-board construction and heat pipes are good thermal enhancements.
To address these issues, designers need accurate data on power consumption. The data must address thermal matters such as temperature gradients across the box. Sometimes the solution for high-temperature problems is to move the devices around to place the hot components in a cooling air flow. Or, it may be to allow more space for heatsinks. But moving components to solve thermal problems usually conflicts with the requirement to maintain short electrical distances between the fast components.
Tools For Analysis: Every design with thermal issues needs to go through iterations because each design change has to be analyzed for effectiveness. Thermal analysis tools implement computational fluid dynamics (CFD) to model the heat generation and removal functions. Most of the tools create a mesh around the various components to form the various boundary layers. This mesh usually isn't constant, but it alters resolution, depending on the components.
For example, a heatsink may have a fairly coarse mesh that models the thermal radiation characteristics of the cooler at some volume of air flow. But an individual component would require fairly fine detail to model local hot spots, the conduction paths through the leads, and the multiple thermal paths available for heat distribution and removal through the packaging.
FLO/MCAD from Flomerics enables parts and assemblies from any mechanical computer-aided design (MCAD) software—such as Pro/Engineer, I-DEAS, and Solid Designer—to be transferred easily and rapidly to and from Flotherm for thermal analysis. The interface program intelligently filters the geometrical data for a particular part or assembly and creates a simplified "thermal equivalent" for analysis purposes.
This data reduction step is critical, because production-quality MCAD solid models contain a vast amount of thermally insignificant geometric detail. Simply importing the geometry from the MCAD system into Flotherm will create a thermal analysis problem so complex that it will take weeks to solve.
A wiser approach is to simplify the geometry to a level that matches the thermal importance of the part, e.g., little or no simplification for thermally critical geometry, a lot of simplification for small or passive geometry. A few moments spent simplifying the problem can save days or weeks later.
The CFD process starts with preprocessing in building and analyzing a flow model. It includes building the model (or importing one from a CAD package), applying a mesh, and entering the data. After preprocessing, the CFD solver performs the calculations and produces the results. Postprocessing is the final step in CFD analysis, and it involves organization and interpretation of the data and images.
Avijit Goswaami, a director at Applied Thermal Technologies, says that the key to good thermal performance is to start the thermal design early. Thermal design is no longer a function that can be performed as an afterthought, because too many issues and parameters are thermally constrained within systems today. There's no other way than up from the start. Otherwise, the design is bound for trouble.