# Resistance calculator

#### Introduction

The thermal resistance calculator is a free tool on ColdStream to calculate the thermal resistance of a default heat sink design. This tool finds its use in giving the user a clear indication of how some of the boundary conditions affect the thermal resistance of this standard heat sink.

The thermal resistance calculator tool has 3 refinement levels. The following chapters delve deeper into each of these refinement levels.

#### Minimal amount of boundary information

Input symbol Explanation Units
$$T_{amb}$$ This is the ambient temperature Kelvin [K]
$$T_{j}$$ This is the maximum juntion temperature Kelvin [K]
$$P$$ This represents the heat dissipated from the heat source Watts [W]

The resistance to heat flow from the junction of the heat generating component, through the case and the heat sink to the environment will be automatically calculated using the following equation: $R_{heat\space sink}=\frac{T_j - T_{amb}}{P}$

#### Thermal interface material refinement

Input symbol Explanation Units
$$t_{interface}$$ This is the thickness of the thermal interface material Meters [m]
$$\kappa_{interface}$$ This is the thermal conductivity of the thermal interface material [$$\frac{W}{m*K}$$]
$$W_s$$ This is the width of the heat source Meters [m]
$$L_s$$ This is the length of the heat source Meters [m]

The resistance to heat flow from the junction of the heat generating component, through the case and the heat sink to the environment will be automatically calculated using the following equation: $R_{heat\space sink}=\frac{T_j - T_{amb}}{P}-R_{interface}$$R_{interface}=\frac{t_{interface}}{L_s*W_s*\kappa_{interface}}$

#### Junction information refinement

Input symbol Explanation Units
$$R_{th-jc}$$ This is the thermal resistance between the junction and the case [$$\frac{K}{W}$$]

The resistance to heat flow from the junction of the heat generating component, through the case and the heat sink to the environment will be automatically calculated using the following equation: $R_{heat\space sink}=\frac{T_j - T_{amb}}{P}-R_{th-jc}$