As higher performance systems are implemented, making the choice between 3U VPX and 6U VPX becomes a matter of what functionality can be packaged on the smaller card vs. the larger. As processors and FPGAs enable more capability, the 3U VPX form factor is favored for its reduced size and weight.
That concentration of power in a smaller board complicates the choice to implement systems using a 3U card. Power density and the ability to dissipate heat becomes an issue. Purely convection or conduction solutions are pushed to their limits with existing cooling techniques defined by the standard.
While OpenVPX offers significant improvements in field-deployed system signal integrity, speed and capability, it has created new challenges in space-constrained installations. Embedded sub-systems must sometimes be packaged to fit existing tight spaces in aircraft, ground vehicles, submarines, spacecraft, etc. As these systems have been implemented, the resulting increased heat has heavily impacted chassis and backplane designs, which in turn calls for new cooling options.