Author: Manfred Schmitz, CTO MEN Mikro Elektronik
Good thermal management is a decisive factor for the reliability of computers. By a simplified formula, a computer's life is halved by every temperature step of 10°K. In CompactPCI systems, two basic types of cooling are common. The most simple technique is convection cooling, or natural convection. Since warm air is lighter than cold air, warm air rises. The flow speed, however, is relatively low. For this reason, you require sufficiently large ventilation slots and large heat sinks. If you want to operate a CPU board with a power dissipation of 25 watts at 85°C, for example, you need a heat sink that covers the space of several slots in the system.
A smaller heat sink is possible if you accelerate the air using fans. (Forced Air Cooling). CPU fans directly attached to the boards need a lot of maintenance and are not an option because of their lack of reliability. Instead, the card cages are ventilated as a whole. This works well as long as the heat dissipated per slot does not exceed a certain extent, and as long as sufficient air flow, which must also be cool enough, can be guaranteed. The fan tray level below the card cage in turn needs maintenance.
For even more demanding applications, for instance on board an airplane, the so-called conduction cooling technique is common. Strictly speaking, the term does not even mean the cooling of the boards but only the transfer of the boards' heat out of the enclosure. To do this, the conductive material is directly connected to the heat source. Usually the board is framed by a tailor-made aluminum block for this purpose, which takes off the heat and directly transfers it on to the massive enclosure wall over special tensioning wedges called "wedge locks", which also consist of thermally conductive material. This allows to make the enclosure hermetically tight, and to give it particular mechanical stability. Of course, the heat must then be transferred away at the outside of the housing through further measures (e.g., by a fan). Liquid coolers are also suited to do this.
The IEEE standard on which the CompactPCI specification is based defines a special geometry for boards with conduction cooling, which does not allow cooling a board both by convection and by conduction cooling without further efforts. As quantities of conduction-cooled boards are rather low, such boards are especially expensive.
This is why the CompactPCI Serial specification takes another path. Any convection-cooled card can be converted into a conduction-cooled card using an individual aluminum block. Saving development costs for special conduction-cooled boards allows CompactPCI Serial to employ a technology that used to be expensive on a broad basis. CompactPCI Serial – PICMG CPCI-S.0 – stands for modular, robust computers in a wide range of application fields with optimized costs.