Electronics Cooling and Thermal Management: PCB Part 2

IC Thermal management

By now, you have identified the components, generated a schematic, placed the components and resolved routing. So what comes next?

Thermal Cooling

iStock_000013680212XSmallProper identification of  heat-sink requirements needed in electronics thermal management is usually relegated to this stage of design, but some design may be done during PCB routing.  It may occur earlier in design cycle if it is a critical parameter, and one can argue that this should always be kept as an important consideration throughout the whole process.  The mechanical engineer performs heat transfer analysis, usually by the FEA method, finite element analysis, as part of the design to facilitate IC thermal managment.  The finite element analysis is a type of computer modeling in which the heat transfer, temperatures, and heat flux are calculated.

Heat sinks are the most common form of thermal cooling used in electronics cooling.  Heat is transferred to an aluminum heat sink by thermal conductivity, usually through a thermal paste between the heatsink and the PCB.  Then the heat sink cools by convection, either free convection or forced convection.

A power rating of the component, needed when sorting out power planes and traces, provides information on the amount of heat generated by the part. Barring special cases, almost all of the electrical power going into the component turns up as waste heat. Moving a photo album from a smartphone to a disk-drive on a server results in a certain amount of heat generation. Flipping those storage bits far away on a remote server can and should be done more efficiently in the future, but we have to deal with the waste energy now.

One must check the component temperature rating in order to identify the maximum allowed operating temperatures.  There are internal component temperature differences which may lead to the distinction between a junction and a case temperature, which is very important in IC thermal management.  The junction temperature is a chip temperature for all practical purposes, while the case temperature is just that, the temperature of the exterior of the component, and the exact location where this temperature is defined would depend on the heat sink mechanical attachment point (if such is needed).  A board or a system designer usually is concerned with this case temperature, and a component manufacturer usually provides this value.

Finite element analysis (FEA) in Electronics cooling and thermal management

The heat flux in electronics thermal management and electronics cooling can be estimated by computer modeling with the finite element method. A heat flux travels in the direction of decreasing temperature, from a high temperature point to a low temperature point.The thermal conductivity of the solid material determines how much heat travels through a solid given a certain temperature difference for a fixed area.

Similarly, a designer is given some idea on the thermal resistance values for the components.  A thermal resistance describes the magnitude of a heat current being pushed out per unit temperature difference.  For low-power components all of the heat can be dissipated by the either surrounding air or through the board to which it mounts. Heat dissipation through the air is modeled as free convection, or in the case of a cooling fan, forced convection.  At elevated temperatures, radiation cooling becomes relevant.

The FEA model uses thermal conductivity, convective heat transfer, and radiation in its calculations, similar to electrical resistance, capacitance and inductance that electrical engineers use in circuit modeling.

Higher power dissipation is the subject for our next entry.


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