THERMAL DESIGNTHERMAL MANAGEMENT OF ELECTRONICS, HEAT EXCHANGERS GUIDES, FREE SOFTWARE AND CALCULATORS 

Let's begin by defining the main terms. The goal of thermal management is to control the temperatures of the electronic devices. But what is temperature? Technically speaking, on a microscopic level it is a measure of the average molecular kinetic energy in the matter. The normal flow of kinetic energy is from a higher temperature region (or an object) toward a lower temperature region (or an object). This flow is referred to as heat transfer. 



In general, there are three types of heat transfer: conduction, convection, and radiation. Conduction is the collisional transfer of energy between atoms, which occurs in solids. Convection is the motion of molecules in air or fluids. Radiation is the energy flow by electromagnetic waves. In practical electronics primarily only the first two types are noticeable. Electric currents and alternating electromagnetic fields cause power dissipation in all electronic parts, which results in increase of their temperatures. This in turn affects the reliability and life expectancy of these components. Failure rate and its inverse, mean time between failures (MTBF), are measures of the electronic system reliability. According to Arrhenius model, each 10°C rise increases the failure rate by 50%. When designing a heat sink for a semiconductor cooling, first of all you need to choose the component's maximum operating junction temperature Tjmax (typically, 105120 ^{o}C for commercial parts). Then for convection cooling the required heatsink thermal resistance should be R_{thhs}<(TjmaxTa)/P(R_{thjc}+R_{thchs}) ^{o}C/W, where Ta ambient, R_{thjc}  thermal resistance between junction and the case from the datasheet (typically 0.52.5 ^{o}C/W for conventional discrete power packages), R_{thchs } thermal resistance between the device's case and the heatsink, P power dissipated by the device in watts. Technically, there is another path for the heat from the case of the device to the air. This path is defined by junction to ambient thermal resistance R_{thjamb}. However, you can usually disregard it because the main heat transfer goes via the heatsink. 

See thermal circuit diagram to the left for an illustration. Once you found the required R_{thhs}, you can pick a stamped or extruded heat sink with equal or lower value of thermal resistance. Note that if you use an insulator between the device and the heatsink, you need to take it into account as well. For offtheshelf parts, Rthhs is normally specified in the datasheet. Harry Lythall found empirically a "rule of thumb" calculation formula for R_{thhs} of home made Ushape folded aluminum sheet. Based on his equation, the required surface area in sq.cm is A=(50/Rthhs)^{2}. In general, the main optimization criteria are to maximize the exposed heat exchanger's surface area, and to minimize its weight and the mean distance of the exposed surface from the component to be cooled. Below you will find free calculators and useful information on the thermal design. 
HEAT AND THERMODYNAMICS BASICS  THERMAL MODELING AND ANALYSIS SOFTWARE  THERMAL DESIGN GUIDES, ANALYSIS & APPLICATION NOTES  
Heat conduction formulas A detailed heat transfer textbook for engineering students (conduction, convection, radiation) The First Law of thermodynamics PCB TRACE CALCULATOR FOR TEMPERATURE RISE

Heat sink calculation, design and analysis Thermal conductivity units conversion Temperature units conversion Duct flow velocity calculator Thermal resistance and fin efficiency online calculators THERMAL DATA OF ELECTRONIC COMPONENTS
Basic thermal management of semiconductor devicesThermal conductivity in cal/sec and W/m*K for various materials 
The design of fan speed control What you need to know about cooling fans HEATSINK PROPERTIES AND DESIGN NOTES
How to select
an extruded heat sinkAnalytical model for simulating electronic systems' thermal behavior Heatsink design guide 
