Thermal resistance is the temperature difference across a structure when a unit of heat energy flows through it in unit time. It is the reciprocal of thermal conductance. The SI units of thermal resistance are kelvin per watt. Shortcut: WP:CU Marking articles for cleanup This page is undergoing a transition to an easier-to-maintain format. ... This Manual of Style has the simple purpose of making things easy to read by following a consistent format — it is a style guide. ... Fig. ... In physics, heat, symbolized by Q, is defined as energy in transit. ... A pocket watch, a device used to measure time Two distinct views exist on the meaning of time. ... In physics, thermal conductivity, λ, is the quantity of heat transmitted, due to unit temperature gradient, in unit time under steady conditions in a direction normal to a surface of unit area, when the heat transfer is dependent only on the temperature gradient thermal conductivity = heat flow rate / (distance × temperature... Cover of brochure The International System of Units. ... The Kelvin scale is a thermodynamic (absolute) temperature scale where absolute zero—the lowest possible temperature where nothing could be colder and no heat energy remains in a substance—is defined as zero kelvin (0 K). ... The watt (symbol: W) is the SI derived unit of power, equal to one joule per second. ...

The thermal resistance of materials is of great interest to electronic engineers, because most electrical components generate heat and need to be cooled. Some electronic components malfunction when they overheat, while others are permanently damaged.

Consider a component such as a silicon transistor that is bolted to the metal frame of a piece of equipment. The transistor's manufacturer will specify parameters in the datasheet called the thermal resistance from junction to case (symbol: R_θJC), and the maximum allowable temperature of the semiconductor junction (symbol: T_JMAX). The specification for the design should include a maximum temperature at which the circuit should function correctly. Finally, the designer should consider how the heat from the transistor will escape to the environment: this might be by convection into the air, with or without the aid of a heat sink, or by conduction through the printed circuit board. For simplicity, let us assume that the designer decides to bolt the transistor to a metal surface (or heat sink) that is guaranteed to be less than ΔT_HS above the ambient temperature. This article or section does not cite its references or sources. ... Close-up photo of one side of a motherboard PCB, showing conductive traces, vias and solder points for through-hole components on the opposite side. ... This article or section does not cite its references or sources. ...

Given all this information, the designer can construct a model of the heat flow from the semiconductor junction, where the heat is generated, to the outside world. In our example, the heat has to flow from the junction to the case of the transistor, then from the case to the metalwork. We do not need to consider where the heat goes after that, because we are told that the metalwork will conduct heat fast enough to keep the temperature less than ΔT_HS above ambient: this is all we need to know.

Suppose the engineer wishes to know how much power he can put into the transistor before it overheats. The calculations are as follows.

Total thermal resistance from junction to ambient = R_θJC + R_θB

where R_θB is the thermal resistance of the bond between the transistor's case and the metalwork. This figure depends on the nature of the bond - for example, a thermal bonding pad or thermal transfer grease might be used to reduce the thermal resistance.

Maximum temperature drop from junction to ambient = T_JMAX − (T_AMB + ΔT_HS).

We use the general principle that the temperature drop ΔT across a given thermal resistance R_θ with a given heat flow Q through it is:

.

Substituting our own symbols into this formula gives:

,

and, rearranging,

The designer now knows Q_MAX, the maximum power that the transistor can be allowed to dissipate, so he can design the circuit to limit the temperature of the transistor to a safe level.

Let us plug in some sample numbers:

(typical for a silicon transistor)

(a typical specification for commercial equipment)

(arbitrary figure)

R_θJC = 1.5 K/W (for a typical TO-220 package)

R_θB = 0.1 K/W (a typical value for an elastomer heat-transfer pad for a TO-220 package)

The result is then: The TO-220 is a style of electronic component package, commonly used for transistors, silicon-controlled rectifiers, and integrated circuits. ... The term elastomer is often used interchangeably with the term rubber, and is preferred when referring to vulcanisates. ...

watts

= 28.125 watts

This means that the transistor can dissipate about 28 watts before it overheats. A cautious designer would operate the transistor at a lower power level to increase its reliability. Reliability concerns quality or consistency. ...

This method can be generalised to include any number of layers of heat-conducting materials, simply by adding together the thermal resistances of the layers and the temperature drops across the layers.