There are three mechanisms by which thermal transport occurs: Through fluid motion, convection, through electromagnetic waves radiation, or through particle collisions within a medium, conduction.
When performing thermal characterisation of the heat transport behaviour within a device, component or bulk materials, thermal conduction is the transport mechanism of most relevance, as the rate of thermal transport across air gaps within components via both radiation and convection is, typically, orders of magnitude less than the rate of heat transfer that occurs between solid components in physical contact.
Acquiring greater insight and understanding of the thermal transport mechanisms is a topic that has recently become subject to heightened interest, due to the ever increasing drive for reduced size and greater power in the electronic components that are ubiquitous in the modern world.
Thermal management is a vital consideration when designing and selecting materials for devices, in order to reduce the possibility of component failure and optimise performance.
Values used to quantify thermal performance include:
Thermal Conductivity The maximum rate at which thermal energy can be transferred through a medium under a steady state temperature differences.
Thermal Diffusivity How quickly a steady state of heat transfer is ahieved after a temperature gradient is induced across a medium.
Thermal Boundary Resistance Also referred to as interfacial thermal resistance, thermal contact resistance, Kapitza resistance and thermal boundary insulance or TBR. Increased thermal resistance that occurs at the interface between two different materials. Practically and conventionally, the measured, or effective, TBR is synonymous with thermal contact resistance. Although strictly speaking thermal boundary resistance/Kapitza resistance refer to the resistance that occurs between two perfectly joined materials at the atomic scale due to misalignment of their electronic band structures. Whereas thermal contact resistance arises due to structural imperfections, such as roughness or airgaps, between two materials.
Quantitative measurement of these values continues to present a significant challenge, with accuracies of ~5% being typical for bulk, homogenous materials. Further complications arise because:
Thermal properties differ at the micro and macro scales, therefore an accurate representation of how a material functions within a device cannot be reliably determined using values measured for a bulk substrate.
Intricacies in thermal behaviour, such as thermal boundary resistance, occur at the junctions between materials. This can be due to roughness, contamination, defects or lattice misalignment.
Mixed materials, such as composites, alloys, thin films and multilayers, are necessary for many applications, and possess completely different properties to the compounds of which they consist.
Thermal behaviour is affected by the environment in which a component or device is used, the most obvious example being the dependency of thermal conductivity on temperature. This has large implications in extreme environments, such as is the case for materials utilised for aerospace, sintered die attach or nuclear applications.
Many of the most widely used techniques for evaluating thermal properties, such as Transient Plane Source and Transient Line Source, are referred to as 'contact techniques' as they rely upon physical contact between sensor and sample.
An alternative, non-contact technique is Laser Flash Analysis, which heats the front-side of a coupon of bulk material, and measures the rate at which the back-side of the coupon warms, in order to determine the thermal diffusivity of the material in question.
These techniques are widely used due to their easy implementation, however are limited by the range of sample environments in which they can be implemented, limited spatial resolution and lack of depth sensitivity. The thermoreflectance techniques offered by TherMap Solutions are able to overcome these obstacles.