Circuit card assembly(Circuit card assembly) cooling
The Circuit card assembly is the basic building block of most electronic systems. The exceptions are usually high-power transmitters(built around traveling-ware tubes) or high-voltage power supplies(built around isolated floating decks, where are lengths and voltage isolation usually require emersion in a dielectric liquid or gas). the thermal designer is therefore most often concerned with removing heat from heat-dissipating components mounted to a Circuit card assembly Radiation is of little use and is used only as a safety factor except in some low power space applications. Heat is removed from the components either by conduction or convection or a combination of the two. Where heat is removed by conduction to the circuit card or a heatsink on the circuit card, methods of removing the heat from the card must be utilized.
Typically in the analysis of Circuit card assemblys, the components are characterized as a one-dimensional heat source and resistance connected to the card(Figure 9.27). The resistance is called , which is a misnomer because refers to remperature defference in most heattranfter texts. However, the term originated in the electronics industry and now is tradition.
This one-dimension characterization is usually adequate, but it must be recognized that it is not in all cases and detailed analysis of the chip and its mounting and conduction to the component case are often necessary(covered in Section 9.5). However, as system-level thermal designer, there is often little knowledge of how the component is constructed; we have to use either the manufacturer's stated or a MIL-SPEC value. The MIL-SPEC values usually give conservative(too high) temperature estimates because they are set at a level to cover all manufacturers'a values, thus requiring a design for the worst-case manufacturer. In an increasingly more commercial environment, it is becoming more and more important.
Figure 9.28
This type of electronic module packaging has been defined in specific sizes called Standard Electronic Modules(SEMs), as illustrated in Figure 9.29. The heatsink is a solid sheet of material, which extends beyond the edges of the circuit card to be clamped into the chassis to provide a good conductive path to the chassis. A common clamping mechanism made by Wedge-lock is shown in Figure 9.30. Resistance through this interface depends on the number of sections of Wedge-Lock used in the mechanism. Values for 3 and 5 piece mechanisms are defined by table 9.6.
Providing a uniform heat distribution is assumed, the conduction heatsink provides a parabolic temperature distribution according to Eq. 9.48 and is illustrated in Figure 9.31.
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