# Thermal resistance

Thermal resistance is a heat property and a measurement of a temperature difference by which an object or material resists a heat flow. Thermal resistance is the reciprocal of thermal conductance.

• (Absolute) thermal resistance R in kelvins per watt (K/W) is a property of a particular component. For example, a characteristic of a heat sink.
• Specific thermal resistance or thermal resistivity Rλ in kelvin–metres per watt (K⋅m/W), is a material constant.
• Thermal insulance has the units square metre kelvin per watt (m2⋅K/W) in SI units or square foot degree Fahrenheithours per British thermal unit (ft2⋅°F⋅h/Btu) in imperial units. It is the thermal resistance of unit area of a material. In terms of insulation, it is measured by the R-value.

## Absolute thermal resistance

Absolute 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 unit of absolute thermal resistance is kelvins per watt (K/W) or the equivalent degrees Celsius per watt (°C/W) – the two are the same since the intervals are equal: ΔT = 1 K = 1 °C.

The thermal resistance of materials is of great interest to electronic engineers because most electrical components generate heat and need to be cooled. Electronic components malfunction or fail if they overheat, and some parts routinely need measures taken in the design stage to prevent this.

## Analogies and nomenclature

Electrical engineers are familiar with Ohm's law and so often use it as an analogy when doing calculations involving thermal resistance. Mechanical and structural engineers are more familiar with Hooke's law and so often use it as an analogy when doing calculations involving thermal resistance.

type structural analogy hydraulic analogy thermal electrical analogy
quantity impulse $J$ [N·s] volume $V$ [m3] heat $Q$ [J] charge $q$ [C]
potential displacement $X$ [m] pressure $P$ [N/m2] temperature $T$ [K] potential $V$ [V = J/C]
flux load or force $F$ [N] flow rate $Q$ [m3/s] heat transfer rate ${\dot {Q}}$ [W = J/s] current $I$ [A = C/s]
flux density stress $\sigma$ [Pa = N/m2] velocity $\mathbf {v}$ [m/s] heat flux $\mathbf {q}$ [W/m2] current density $\mathbf {j}$ [C/(m2·s) = A/m2]
resistance flexibility (rheology defined) [1/Pa] fluid resistance $R$ [...] thermal resistance $R$ [K/W] electrical resistance $R$ [Ω]
conductance ... $...$ [Pa] fluid conductance $G$ [...] thermal conductance $G$ [W/K] electrical conductance $G$ [S]
resistivity flexibility $1/k$ [m/N] fluid resistivity thermal resistivity [(m·K)/W] electrical resistivity $\rho$ [Ω·m]
conductivity stiffness $k$ [N/m] fluid conductivity thermal conductivity $k$ [W/(m·K)] electrical conductivity $\sigma$ [S/m]
lumped element linear model Hooke's law $\Delta X=F/k$ Hagen–Poiseuille equation $\Delta P=QR$ Newton's law of cooling $\Delta T={\dot {Q}}R$ Ohm's law $\Delta V=IR$ distributed linear model ... $...$ ... $...$ Fourier's law $\mathbf {q} =-k{\boldsymbol {\nabla }}T$ Ohm's law $\mathbf {J} =\sigma \mathbf {E} =-\sigma {\boldsymbol {\nabla }}V$ ### Explanation from an electronics point of view

#### Equivalent thermal circuits The diagram shows an equivalent thermal circuit for a semiconductor device with a heat sink:
• ${\dot {Q}}$ is the power dissipated by the device.
• $T_{\rm {J}}$ is the junction temperature in the device.
• $T_{\rm {C}}$ is the temperature at its case.
• $T_{\rm {H}}$ is the temperature where the heat sink is attached.
• $T_{\rm {amb}}$ is the ambient air temperature.
• $R_{\theta {\rm {JC}}}$ is the device's absolute thermal resistance from junction to case.
• $R_{\theta {\rm {CH}}}$ is the absolute thermal resistance from the case to the heatsink.
• $R_{\theta {\rm {HA}}}$ is the absolute thermal resistance of the heat sink.

The heat flow can be modelled by analogy to an electrical circuit where heat flow is represented by current, temperatures are represented by voltages, heat sources are represented by constant current sources, absolute thermal resistances are represented by resistors and thermal capacitances by capacitors.

The diagram shows an equivalent thermal circuit for a semiconductor device with a heat sink.

### Derived from Fourier's law for heat conduction

From Fourier's Law for heat conduction, the following equation can be derived, and is valid as long as all of the parameters (x and k) are constant throughout the sample.

$R_{\theta }={\frac {\Delta x}{Ak}}={\frac {\Delta xr}{A}}$ where:

• $R_{\theta }$ is the absolute thermal resistance (K/W) across the thickness of the sample
• $\Delta x$ is the thickness (m) of the sample (measured on a path parallel to the heat flow)
• $k$ is the thermal conductivity (W/(K·m)) of the sample
• $r$ is the thermal resistivity (K·m/W) of the sample
• $A$ is the cross-sectional area (m2) perpendicular to the path of heat flow.

In terms of the temperature gradient across the sample and heat flux through the sample, the relationship is:

$R_{\theta }={\frac {\Delta x}{A\phi _{q}}}{\frac {\Delta T}{\Delta x}}={\frac {\Delta T}{q}}$ where:

• $R_{\theta }$ is the absolute thermal resistance (K/W) across the thickness of the sample,
• $\Delta x$ is the thickness (m) of the sample (measured on a path parallel to the heat flow),
• $\phi _{q}$ is the heat flux through the sample (W·m−2),
• ${\frac {\Delta T}{\Delta x}}$ is the temperature gradient (K·m−1) across the sample,
• $A$ is the cross-sectional area (m2) perpendicular to the path of heat flow through the sample,
• $\Delta T$ is the temperature difference (K) across the sample,
• $q$ is the rate of heat flow (W) through the sample.

### Problems with electrical resistance analogy

A 2008 review paper written by Philips researcher Clemens J. M. Lasance notes that: "Although there is an analogy between heat flow by conduction (Fourier's law) and the flow of an electric current (Ohm’s law), the corresponding physical properties of thermal conductivity and electrical conductivity conspire to make the behavior of heat flow quite unlike the flow of electricity in normal situations. [...] Unfortunately, although the electrical and thermal differential equations are analogous, it is erroneous to conclude that there is any practical analogy between electrical and thermal resistance. This is because a material that is considered an insulator in electrical terms is about 20 orders of magnitude less conductive than a material that is considered a conductor, while, in thermal terms, the difference between an "insulator" and a "conductor" is only about three orders of magnitude. The entire range of thermal conductivity is then equivalent to the difference in electrical conductivity of high-doped and low-doped silicon."

## Measurement standards

The junction-to-air thermal resistance can vary greatly depending on the ambient conditions. (A more sophisticated way of expressing the same fact is saying that junction-to-ambient thermal resistance is not Boundary-Condition Independent (BCI).) JEDEC has a standard (number JESD51-2) for measuring the junction-to-air thermal resistance of electronics packages under natural convection and another standard (number JESD51-6) for measurement under forced convection.

A JEDEC standard for measuring the junction-to-board thermal resistance (relevant for surface-mount technology) has been published as JESD51-8.

A JEDEC standard for measuring the junction-to-case thermal resistance (JESD51-14) is relatively newcomer, having been published in late 2010; it concerns only packages having a single heat flow and an exposed cooling surface.

## Resistance in composite wall

### Resistances in series

When resistances are in series, the total resistance is the sum of the resistances:

$R_{\rm {tot}}=R_{A}+R_{B}+R_{C}+...$ ### Parallel thermal resistance

Similarly to electrical circuits, the total thermal resistance for steady state conditions can be calculated as follows.

The total thermal resistance

${{1 \over R_{\rm {tot}}}={1 \over R_{B}}+{1 \over R_{C}}}$ (1)


Simplifying the equation, we get

${R_{\rm {tot}}={R_{B}R_{C} \over R_{B}+R_{C}}}$ (2)


With terms for the thermal resistance for conduction, we get

${R_{t,{\rm {cond}}}={L \over (k_{b}+k_{c})A}}$ (3)


### Resistance in series and parallel

It is often suitable to assume one-dimensional conditions, although the heat flow is multidimensional. Now, two different circuits may be used for this case. For case (a) (shown in picture), we presume isothermal surfaces for those normal to the x- direction, whereas for case (b) we presume adiabatic surfaces parallel to the x- direction. We may obtain different results for the total resistance ${R_{tot}}$ and the actual corresponding values of the heat transfer are bracketed by ${q}$ . When the multidimensional effects becomes more significant, these differences are increased with increasing ${|k_{f}-k_{g}|}$ .

Spherical and cylindrical systems may be treated as one-dimensional, due to the temperature gradients in the radial direction. The standard method can be used for analyzing radial systems under steady state conditions, starting with the appropriate form of the heat equation, or the alternative method, starting with the appropriate form of Fourier's law. For a hollow cylinder in steady state conditions with no heat generation, the appropriate form of heat equation is 

${{1 \over r}{d \over dr}\left(kr{dT \over dr}\right)=0}$ (4)


Where ${k}$ is treated as a variable. Considering the appropriate form of Fourier's law, the physical significance of treating ${k}$ as a variable becomes evident when the rate at which energy is conducted across a cylindrical surface, this is represented as

${q_{r}=-kA{dT \over dr}=-k(2\pi rL){dT \over dr}}$ (5)


Where ${A=2\pi rL}$ is the area that is normal to the direction of where the heat transfer occurs. Equation 1 implies that the quantity ${kr(dT/dr)}$ is not dependent of the radius ${r}$ , it follows from equation 5 that the heat transfer rate, ${q_{r}}$ is a constant in the radial direction.

In order to determine the temperature distribution in the cylinder, equation 4 can be solved applying the appropriate boundary conditions. With the assumption that ${k}$ is constant

${T(r)=C_{1}\ln r+C_{2}}$ (6)


Using the following boundary conditions, the constants ${C_{1}}$ and ${C_{2}}$ can be computed

${T(r_{1})=T_{s,1}}$ and          ${T(r_{2})=T_{s,2}}$ The general solution gives us

${T_{s,1}=C_{1}\ln r_{1}+C_{2}}$ and          ${T_{s,2}=C_{1}\ln r_{2}+C_{2}}$ Solving for ${C_{1}}$ and ${C_{2}}$ and substituting into the general solution, we obtain

${T(r)={T_{s,1}-T_{s,2} \over {\ln(r_{1}/r_{2})}}\ln \left({r \over r_{2}}\right)+T_{s,2}}$ (7)


The logarithmic distribution of the temperature is sketched in the inset of the thumbnail figure. Assuming that the temperature distribution, equation 7, is used with Fourier's law in equation 5, the heat transfer rate can be expressed in the following form

${{\dot {Q}}_{r}={2\pi Lk(T_{s,1}-T_{s,2}) \over \ln(r_{2}/r_{1})}}$ Finally, for radial conduction in a cylindrical wall, the thermal resistance is of the form

${R_{t,\mathrm {cond} }={\ln(r_{2}/r_{1}) \over 2\pi Lk}}$ such that ${r_{2}>r_{1}}$ 