Well-selected materials and components alone do not make a thermally performant system with high reliability and long lifetime expectancy. They need to match and act jointly, thermally and thermo-mechanically. Proper qualification is essential not only on the material level, but also with assemblies and system.
Discrete devices are basic, most semiconductor-type, components that are packaged in standardized or to-be standardized manner. Common examples are diodes, transistors and rectifiers. Source: jedec.org
Their package design serves the purpose of protecting the device, providing compatibility with a particular mounting technology and ensuring a minimum thermal performance with respect to the device's power losses under specified thermal load.
The most common characteristic and datasheet value for such devices is the thermal junction-to-case resistance (Rth,J-C). This value can be determined by conducting transient thermal tests in different setups.
Thermal impedance spectroscopy
The thermal impedance of a device is the frequency-dependent thermal resistance and describes its heating and cooling behavior. One could describe this function as the "transient thermal DNA" of the device, because it contains all the thermally relevant information to describe the package under any load condition.
From the thermal impedance, applying particular mathematical operations, the thermal time constant spectrum can be calculated which is basis for many further analyses. Most packages' transient thermal behavior can be approximated with three time constants - taking the most characteristic from the spectrum. A continuous spectrum, on the contrary, offers more detailed insights.
Power electronic modules work with high power densities compared to compact discrete packages and dissipate more heat per unit area. Their design is optimized for minimum junction-to-case thermal resistances which is achieved by larger cross-sectional areas, highly performant attach technology and the use of high-end material. Power electronic modules can be found in a wide range of applications, from automotive to power grid to aerospace.
Transient thermal analysis of these modules is often looking for deeper understanding. While the junction-to-case thermal resistance of small packages is usually a sufficient characteristic to know a package's thermal perfomance, it often does not allow to describe a power electronic module's dynamic thermal behavior and assess its lifetime expectancy. The thermal structure function is a numerical model that is derived from the thermal impedance and stands for the physical heat path from source to sink.
Parts of this function can be directly associated with material layers along the heat path. This means that comparing multiple of such curves provides different insights, such as:
- Comparative performance analysis between modules
- Layer-wise comparison of materials, geometries or attach technologies
- Individual quantification of performance variation
- Detection and localization of flaws and failures during reliability testing
- Monitoring of damage evolution and state-of-health
Light-emitting diodes are most prominent and can come at high power densities in several variants, such as in automotive applications or as laser diodes. Because light must be able to leave the source in a non-obstructed manner. This comes with different challenges in package design which has lead to wide variation and many, very application-specific solutions.
The transient thermal characterization of opto-electronic packages needs to cope with not only the optical power which needs to be taken into account for the measurement of dissipated heat, but also non-standardized packages, high-end attach technologies and materials and all of that in very small scales.
These increasing challenges do not stop us from putting a spotlight on package thermal performance and reliability.