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Heat Transfer Handbook part 97. The Heat Transfer Handbook provides succinct hard data, formulas, and specifications for the critical aspects of heat transfer, offering a reliable, hands-on resource for solving day-to-day issues across a variety of applications. | 956 HEAT TRANSFER IN ELECTRONIC EQUIPMENT 13.2 THERMAL RESISTANCES 13.2.1 Introduction To determine the component temperatures and temperature gradients encountered in the flow of heat within electronic systems it is necessary to define the relevant heat transfer mechanisms and establish their governing relations. In a typical system heat removal from active regions of the microcircuit s or chip s may require the use of several mechanisms some operating in series and others in parallel to transport to the coolant or ultimate heat sink the heat generated. Practitioners of the thermal arts and sciences generally deal with four basic thermal transport modes conduction including contact resistance convection phase change and radiation. The expression of the governing heat transfer relations in the form of thermal resistances Kraus 1958 greatly simplifies the first-order thermal analysis of electronic systems. Recognizing that heat flow q is analogous to electrical current and that the temperature drop AT is analogous to a voltage drop it is possible to define a general thermal resistance R as AT R q 13.1 Although strictly speaking this analogy applies only to conduction heat transfer it is possible to generalize this definition to all the modes of thermal transport. Thus following the established rules for resistance networks thermal resistances that occur sequentially can be summed to yield the overall thermal resistance along that heat flow path. Conversely the reciprocal of the overall thermal resistance of several parallel heat transfer paths or mechanisms can be found by summing the reciprocals of the individual resistances. In refining the thermal design of an electronic system primary attention should be devoted to reducing the largest thermal resistances along a specified thermal path and or providing parallel paths for heat removal from a critical area or component. While thermal resistances associated with various paths and thermal transport mechanisms .
