A perceived drawback to conduction-cooled modules is the thermal contact resistance that arises between the module’s card edges and the chassis rails, which they contact (Figure 2). Values between 0.3 to 0.5°C/W have been used for 160-mm deep cards (per card edge), meaning that temperature deltas of 15 to 25°C for a 100-W module are lost at this interface. Experimental characterization of various surface finishes and properties has shown that contact resistance values can be reduced substantially from the above values.

Figure 3. A liquid flow-through (LFT) module capable of cooling 650+ W.

What’s Next?

Air cooling and conduction cooling will continue to have incremental gains; however, there is a need for a large step increase in cooling ability for future designs and to provide cooling headroom. Liquid flow-through (LFT) cooling at the circuit-card level has been proven to be able to cool at least 650 W (PAO coolant, 55°C inlet, <15 psi drop across module) with four 100- 150 W heat loads on a 0.85" pitch, VITA 48.3 module (Figure 3). This kind of performance qualifies LFT as the next step in cooling for military COTS modules.

This article was written by Ivan Straznicky, Principal Mechanical Engineer at Curtiss-Wright Controls Embedded Computing, Kanata, Ontario, Canada. For more information, click here.