Abrupt Failure Mechanisms of Power Semiconductors

Power semiconductor modules are employed in modern high voltage power system applications such as high voltage direct current transmission systems. Such applications include voltage source converters (VSCs) that are used for the transmission of electrical power over long distances, the interconnection of asynchronous conventional ac power grids and the transmission of off-shore renewable energy wind farms.

This research is concerned with the performance and reliability of power semiconductor modules under both healthy and accelerated aged conditions. The Insulated Gate Bipolar Transistors (IGBTs), are the active component that is commonly used in medium and high power electronics applications, and these will be the focus of this project. We envisage the outcome of the research will lead to more reliable electrical power transmission systems and to benefit other power semiconductor device technologies.

The overall objective of this project is to understand the abrupt electrical failure mechanisms of IGBT modules and identify the key coupling factors of wear-out conditions affecting such failures. Traditionally, the commonly observed wear-out failures of power semiconductor modules include the bond wire lift-off, solder layer fatigue, dielectric failure of silicone gel, etc.

Inhomogeneous temperature distribution of power semiconductors under bond wire failures and electrical treeing breakdown in silicone gel
a-c Inhomogeneous temperature distribution in semiconductor die aggravated by evolving bond wire failures. Uneven current/overstress is inevitable. d Electrical treeing breakdown in self-healing gel.
Power semiconductor modules normally comprise multiple semiconductor dies in parallel and these dies are interconnected using bonding and soldering techniques in a module assembly. These functionally identical dies or part thereof are subject to uneven stresses in practice which is addressed by over-rating and quality control during the design phase. However, any local over stress conditions (e.g. electric field intensity, current density and hot spot) can lead to abrupt destruction.

Common wear-out failures due to the degradation occurring in the wire bonds, solder layers and insulating materials degrade their performance in both functional characteristics and robustness. The exaggerated local stress, especially under extreme conditions such as short-circuit will lead to abrupt failures.

Unfortunately, the lack of standardised aging criteria for the above mentioned wear-out failures has prevented this method being exclusively used. This is because the ultimate breakdown of a power semiconductor module is driven by both the wear-out failure and the abrupt failure modes.

The proposed research will comprise of four correlated work packages as detailed below; three relating to research of the failure mechanisms and the fourth associated with the online condition monitoring and protection techniques.

WP1: To design a non-destructive test circuit for the evaluation of device switching performance under extreme conditions before and near the onset of destructive failures.
WP2: Generate specified wear-out failures using accelerated aging test in order to understand the safe operating area and its safety margin in dependence of wear-out failures.
WP3: Post-aging characterisation test followed by a thorough investigation to understand the wear-out ageing and overstress breakdown mechanisms in the power module structures in order evaluate their switching performance (voltage, current and losses) under various load conditions.
WP4: Devise the condition monitoring method and a health status tracking system that can be integrated in the active gate drivers.
The increasing demand for reliable, cheap and clean energy sources will continue to increase globally as the world economies grow and the need for emissions to fall. Today about 20% of the globally consumed energy is provided by electric power, which relies on power electronics for efficient conversion and control. Cost and security are the challeges preventing the clean energy from being exclusively used.

Power semiconductors are the enabler for the power electronics systems and regarded as the most fragile component as well. As there has already been more than 30 years of development of fast advanced IGBTs and their module assemblies, the performance of the IGBTs has been driven closer to their physical limits. It is now more profitable to improve their performance-to-cost ratio from the viewpoints of reliability and optimum control.