Create reliable products using thermal management – Part 1

Article By : Ramanan J.V.

Learn about design considerations with respect to thermal management in embedded systems.

Thermal management is a study and implementation of mechanisms to maintain a body in a specific state or envelope, wherein the operations of the body are within acceptable boundary conditions. Thermal Management is a loosely coined term when used in generic discussions, but has an incredible depth when applied to electronics, products and systems built with electronics.

The physics of thermal management, takes us to analytical and statistical methods where models of bodies or systems are characterized based on their material composition, their state in defined conditions inclusive of their physical characteristics, their inherent conductive or resistive (electrical) characteristics, their energy states, etc. The models created are used for analysis of the behaviour of the said bodies or systems, with a definition of boundary condition of operations.

The result of such an analysis received through computer simulations, leads to a theoretical understanding of the operation of the body or system before they are taken to the drawing table and progressively to the production floor. The whole process viz., defining the boundary of operations or environmental conditions, characterisation of the body or system and its behaviour, mathematically modelling the phenomenon inclusive of the environmental conditions and the system characteristics, is more commonly referred to as Thermal Analysis in the embedded world.

Thermal analysis is therefore an essential exercise (if not mandatory) for the creation of predictable, reliable and successful products and systems.

Design considerations for thermal management

The approach to Thermal Management begins right at the conceptual stage of the product and systems based on its end application. For example, while designing for military applications, it is important to ensure the system can withstand harsh environmental conditions. Military specifications often require operations at extreme temperatures ranging from -55° to +85°C, in high humidity conditions, corrosive atmospheres, high altitude conditions, electrically noisy and high radiation shock environment while undergoing extreme shock and vibration. Thus, out of the many considerations, some vital points that play a major role in the design of a product and/or system, with respect to thermal management are:

1. End Product/system size, weight and power requirements definition
2. Product/system use case definition
3. Platform or environmental condition definitions
4. Standards implementation
5. Prototyping and Design for Manufacturing definitions
6. Infrastructure planning for successful Thermal Management implementation.

The above set of definitions is only a part of the exhaustive definitions which are essential for successful planning and implementation of thermal management in products and systems.

To address these points, the information which need to be available are:

1. Bill of materials of the components which formulate the major functionality
2. Power consumption and heat dissipation requirement of each of the devices to be used or the heat loads
3. Material characteristics of the PCB to be used and the enclosure with which the system is to be built
4. Boundary conditions like weight, volumetric information and structural definitions of the hardware board and the System
5. Temperature envelope definitions of the Environment
6. Definitions of the environment under which the system is to be used where heat transfer mechanisms to be exercised would be
   a. Natural convection
   b. Forced air convection
   c. Conductance with thermal shunts
   d. Conductance with heat pipes
   e. Forced air convection with Conductance
   f. Conductance with fluid flow

With information derived from the above, a profile of the electronic load (at the board and the system level) is created. To this profile, the heat transfer mechanisms are added at the board level as well as at the system level. The profile thus generated is subjected to computer simulations with boundary conditions of operation as static inputs, which produce a contour of the temperature distribution on the bodies (the board and the system). This thermal analysis leads to the information on the energy concentration/distribution within a specified area, which would now require to be transferred.

The methods to be applied for the transfer of the energy distribution (convection, conductance, or a combination of both) is now realised. The method so designed based on the analysis carried out is modelled and provided as an input to the simulation tool. Multiple iterations of the simulations are carried out by providing varied ambient conditions, and the model behaviour is generated as temperature contours. These contours throw light on the static temperature conditions on the boards and the system, which are then verified with the device operations envelope.

Multiple iterations of the above simulation exercise leads to the generation of information which allows the board and system design to be implemented within the desired envelope and ensure board and system performance with a high reliability factor.

In the next series of this article, we will go into details on thermal management implementation at board level.

– Ramanan J.V. is VP Engineering (Defense) at Mistral Solutions.