Cooling The Core Components Via Thermal Module
In order to keep the temperatures down for core components, including the P4 Prescott CPU, which generates above-average heat levels, as well as the graphics card and the chipset, the BTX specification provides for a Thermal Module. This contains an extensive heatsink (Intel's technology sample uses radial fins) and an airflow shaft, including an opening at the back of the case for expelling air. In combination with the linear layout, this solution makes sense.
Because all of the components are placed within the airstream of the Thermal Module, cooling fans for the graphics chip and the Northbridge can be done away with in the future. The BTX specification mentions two sizes for the Thermal Module, the smaller of which is meant for MicroBTX.
Depending on which BTX version is involved, either the Thermal Module Type I or II is used. They differ in height by around 2.5 cm.