Thermal Management of Microelectronic Equipment
11 Direct Air Cooling and Fans
Simplicity and easy maintenance make direct air cooling a most attractive approach in cooling of electronics. Direct air cooling can take an active form, represented by forced convection; or a passive form, as free or natural convection. Passive cooling has always been the preferred choice. However, it is generally limited to systems with very low heat dissipation because of the poor heat transfer coefficients being characteristic of free convection.
Forced-air cooling over printed circuit boards (PCBs) is often encountered in electronic equipment. Variations in component size and spacing, cause flow separation over components and recirculation between components. Analysis of the flow field is a fully three-dimensional (3D) problem and extremely complex. Heat transfer involves convection from components and the PCB to the airstream, conduction from components to the PCB, and heat spreading along the PCB, and also radiation exchange among components and from components to surroundings.
Theoretically, the only possible way to analyze components over a PCB is by means of a 3D computational fluid dynamics (CFD) program. The thermal model with this approach may become very large, because of the large number of components involved in a typical board. The number of components on a PCB can be over one hundred. A simple analysis approach, utilizing a general-purpose thermal analyzer, is sometimes used to analyze the entire board. The most difficult task in using such a program is estimating the heat transfer coefficients over the components and the board, which are required as input, because of the complicated flow field.