Thermal management of rugged COTS (commercial off-the-shelf) cards destined for defense applications has always been a challenge due to the high operating temperatures and other harsh environmental requirements. Add to this the demands for ever increasing functional density, achieved by using the latest processors and dense packaging, and the abilities of the incumbent cooling approaches (air and conduction) are pushed to the brink. Fortunately, the increasing challenges continue to be met with air and conduction cooling innovations. For power dissipations or densities beyond these limits, liquid flow-through (LFT) module designs offer a large step increase in cooling capability with plenty of headroom for the foreseeable future.

Thermal Trends

Figure 1. An air-cooled heat sink with offset fins.

As somewhat of a counter-balance, lower core voltages and the recent trend to multiple processing cores reduce heat and heat density, respectively. Also, Intel has announced the use of new transistor materials on their next-generation processors that promise to substantially reduce leakage power. Finally, power management techniques are being architected into processors for even further power decreases. It remains to be seen whether these power reduction approaches will be enough to alleviate the traditional power increases seen to date.

At the plug-in module level, trends show increasing power and heat loads, particularly for high-processing-density DSP (digital signal processor) modules, which have seen an exponential increase in power over the last decade. Power increases are likely to continue with the introduction of new high-speed serial technologies via the new VPX specification (VITA 46). VITA 46 allows up to 768 Watts of power to be brought onto a 6U x 160-mm module. This is a huge increase over the 90 Watts allowed on VME cards, and poses a substantial cooling challenge to thermal engineers. In addition, the new VPX-REDI specification (VITA 48) allows a 1" pitch with increased space available for taller, and typically hotter, components.

Cooling Solutions at the Plug-In Module Level

Cooling military COTS electronics has typically been done through either conduction or air cooling, or a combination of both; for example, avionics chassis with forced-air-cooled sidewalls cooling conduction modules inside the chassis. Airflow over electronic components is also used for cooling; however, the modules are not as rugged under military conditions like shock and vibration as conduction modules.

For air-cooled modules, the simple convection equation (Newton’s law of cooling) can be used to determine where effort should be focused for improvements in cooling.

Q = h⋅A⋅ΔT

Where Q is heat load in Watts, h is the heat transfer coefficient in W/m²K, A is area exposed to the air flow, and ΔT is the temperature difference between ambient and the surface being cooled.

In most cases, ΔT is fixed because there is a maximum component junction temperature and a maximum ambient temperature. To increase the amount of heat that can be cooled, either h or A, or both, need to be increased. Increases in A usually take the form of finned heat sinks placed in contact with hot components. This introduces conduction heat transfer as a factor between the die and the heat sink surfaces. The various resistances in this conduction path need to be accurately determined and modeled for the computational fluid dynamics (CFD) thermal analysis. In particular, any thermal interface materials (TIMs) need to be characterized because datasheet thermal conductivity or resistance values have been found to be as much as an order of magnitude too optimistic compared to independently measured values.