Zoltan Sarkany

Impact of nonlinearities on electronic device transient thermal responses

This paper investigates the influence of nonlinearities on electronic device thermal transient responses. The discussions in the paper are based on practical examples where thermal responses of a power device are recorded in various boundary conditions for different values of dissipated power. Then, the measurement results are analysed using the Network Identification by a Deconvolution method and the differences between particular cases are discussed in detail. The presented experimental results demonstrate that the nonlinearities due to the temperature dependence of thermal model parameters might have important influence on the results, especially when still air cooling is applied. In addition, in selected cases the simulations results obtained with compact thermal models were compared with measurements.

Measurement based compact thermal model creation - accurate approach to neglect inaccurate TIM conductivity data

In this paper two possible ways are investigated to create accurate thermal models without having validated information on the thermal properties of the applied thermal interface materials. One way is the calibration of a detailed numerical thermal model based on the physical information which can be derived from experimental structure functions. In the paper we show a complete calibration procedure using a TO-220 package as an example. Another approach is the generation of dynamic compact models based on real measurements. In order to apply this approach one has to identify the junction-to-case thermal resistance of the tested package using the JEDEC JESD 51-14 standard.

Issues in junction-to-case thermal characterization of power packages with large surface area

There are several ways to define the junction-to-case thermal resistance; however, it is rather challenging to characterize the heat-flow in a package by a single number in an accurate and reproducible way. For many power package families such as TO-type packages the thermal transient testing and the so-called dual interface method can give reliable results. The diverging point of structure functions from dual thermal transients gives a good picture of the material interfaces in such structures. However, the location and nature of the diverging point strongly depends on the shape and direction of the heat-spreading. If the package area is much larger than the dissipating chip the shape of the heat-flow changes when using different interfaces. This causes structure functions corresponding to the two setups deviate much before reaching the case surface. In this paper the origin of this phenomenon is investigated. Measurement and simulation results are compared on different large IGBT modules with several modifications in their structure enabling a detailed analysis of the heat-flow path. A comparison is given between heating only a small fraction of a large module and heating all chips. Some samples went through thermal cycling reliability tests which resulted in cracks below the chips. The effect of the reduced die-attach area is visualized with the help of structure functions.

Characterization method for thermal interface materials imitating an in-situ environment

In this paper the aspects of thermal interface material characterization are discussed from a practical point of view. A novel method based on existing measurement standards is introduced for a quick and repeatable thermal conductivity measurement of nanoparticle-based thermal greases. The effect of the surface roughness of the DUT fixture is evaluated, and a method is introduced for the long-term reliability testing of these nanocomposites.

Temperature change induced degradation of SiC MOSFET devices

Besides the electric parameters of a semiconductor device, the lifetime is a key measure of quality. Overheating is one of the top failure causes in electronic systems. In this paper, a power cycling experiment done on silicon carbide (SiC) MOSFET devices is presented. The experimental setup and measurement conditions are described in detail and a discussion is given on the importance of the electrical setup and the control strategy. The data collected during the power cycling are analyzed, and the primary failure modes are identified. The high-resolution monitoring of the voltage drop on the device, in combination with other monitored parameters, enables the detection of a bond wire lift-off or breakage. With the help of the thermal transient measurement and structure function analysis, the structural changes in the heat flow path can also be identified. Finally, the results of the electric measurements are compared and verified by scanning acoustic microscopy tests.

Simulation based method to eliminate the effect of electrical transients from thermal transient measurements

Thermal transient testing is the industry de-facto test method for the identification of junction temperatures and structural defects inside semiconductor devices. Unfortunately, at the beginning of the thermal transient curve in each case an electric effect can be observed, which appears immediately as the unit power step takes place. This electric effect covers the initial phase of thermal transient, and without knowing it the exact temperature of the device cannot be determined. The current industrial methods make a simple correction by cutting the first stage of the thermal measurement, but it is inaccurate, and the correction requires a manual step, therefore it is uncertain. This article describes a methodology to accurately reproduce the thermal transient curve eliminating the effect of the initial electric transients. We approached the problem by using the combination of thermal transient simulation and measurements, fitting the simulated results to the measured curve knowing that the second half of the measured curve certainly reflects the reality.

Failure prediction of IGBT modules based on power cycling tests

This article describes a possible method to assess the long-time behaviour of IGBT modules using the combination of power cycles to stress the devices and thermal transient testing to monitor possible die-attach degradation. The failure of an IGBT module is a complex phenomenon; it consists of thermal, electrical and thermo-mechanical effects. After a theoretical overview of the possible mechanisms, a detailed description on the structure of selected IGBT module and the power cycling parameters is given. To better understand the temperature distribution on the device and the reason of the failure after the cycling, the module was opened up, inspected visually and an equivalent thermal model was built and calibrated to the physical test results. Failure mechanisms such as die attach resistance increase, wire bond cracking and gate oxide degradation were detected.

Thermal transient analysis of semiconductor device degradation in power cycling reliability tests with variable control strategies

In this article we investigate the different failure mechanisms in IGBT modules as a result of power cycling tests. The power cycling is carried out with different control strategies, such as constant current load, constant power and constant junction temperature. With the continuous monitoring of the tested device voltage, junction temperatures and periodic thermal transient tests, the crack of the wire bonds or even degradation of the die attach layer can be identified. A comparison between the effects of the studied control strategies on the lifetime of the tested device is also presented.

Lifetime estimation of power electronics modules considering the target application

The design of power electronics modules and power packages is heavily influenced by thermal concerns. New substrate materials, thinner and thermally more conductive attachment materials are used to decrease the thermal resistance of a given module. When a new material or technology is applied, its reliability has to be tested thoroughly before the module can be considered for production. The reliability or the expected lifetime of the module can be expressed by a number of temperature or power cycles the system can withstand. These numbers however make more sense if they can be linked to the foreseen lifetime of the application where the power module will be applied. In our changing industrial environment the design of power modules is not aimed at maximum lifetime anymore, but it is determined by the application itself. In this article we will summarize the steps which take the designer from the application requirements (mission profile) to the expected lifetime of the application considering the reliability of the power modules used.

Thermal resistance measurement of discrete capacitors

Not only the active devices are affected by the generated power in electronics but capacitors also suffer from the elevated temperature levels. This paper attempts transferring the concepts of the thermal transient measurement method used in the semiconductor characterization to capacitor components. We show how temperature dependent electrical parameters could be used to measure the temperature of capacitors and also discuss the key requirements against the measurement setup.