Increasing the heat transfer coefficient can be done in several ways. The most straightforward has been to increase air velocity over the surfaces being cooled. This can be done with higher flow/pressure fans and blowers, or by channeling existing airflow to hot components. Other approaches to increasing h include offset fins (Figure 1), which re-introduce airflow boundary layers, and dimpled fins, which introduce boundary layer disturbances.

Conduction-Cooled Modules

Figure 2. Thermal contact resistance between the conduction card and chassis rail.

Q = k⋅A⋅(ΔT/Δx)

where k is thermal conductivity in W/m°K, A is cross-sectional area perpendicular to heat flow, and Δx is the heat path length. Increases in A and decreases in Δx are limited by form factor envelopes, component layout, and component density. The new VPX-REDI specification (VITA 48) helps here by introducing a 1" pitch and allowing card edge retainers on the secondary side of the module.

Thermal conductivity increases can be achieved with different materials; e.g. copper instead of aluminum, although weight constraints typically limit copper’s use to small areas. Composite materials hold promise in being able to increase thermal conductivity without excessive, or in some cases any, increase in weight. One issue with some of these materials such as aluminum-graphites, is their very low “through thickness” conductivities as compared to “in plane” (very high). In some applications, this may be acceptable and still result in overall cooling improvement, while in others no cooling benefit is obtained. An accurate finite element thermal analysis is required to determine this. In addition, characterization of material properties is highly recommended to ensure you get what you pay for.

Very high effective thermal conductivities may be obtained through the use of heat pipes due to their use of liquid-to-vapor phase change. The increased conductivity is only in the axial direction of the heat pipe, and they are orientation-dependent due to body force effects on the condensate. Nevertheless, innovative embedded heat pipe designs have been developed to increase cooling and operate in harsh military environments.