AbstractsMathematics

The principal aim of this PhD program is the development of an optimisation and risk based methodology for reliability and robustness predictions of packaged electronic components. Reliability based design optimisation involves the integration of reduced order modelling, risk analysis and optimisation. The increasing cost of physical prototyping and extensive qualification testing for reliability assessment is making virtual qualification a very attractive alternative for the electronics industry. Given the availability of low cost processing technology and advanced numerical techniques such as finite element analysis, design engineers can now undertake detailed calculations of physical phenomena in electronic packages such as temperature, electromagnetics, and stress. Physics of failure analysis can also be performed using the results from these detailed calculations to predict modes of failure and estimated lifetime of an electronic component. At present the majority of calculations performed using finite element techniques assume that the input parameters are single valued without any variation. Obviously this is not the case as variation in design variables (such as dimensions of the package, operating conditions, etc) can have statistical distributions. The research undertaken in this PhD resulted in the development of software libraries and a toolset which can be used in parallel with finite element analysis to assess the impact of design variable variations on the package reliability and robustness. This resulted in the development of the ROMARA software which now contains a number of best in class reduced order modelling techniques, optimisation algorithms, and stochastic risk assessment procedures. The software has been developed using the C# language and demonstrated for a number of case studies. The case study detailed in this thesis is related to a power electronics IGBT structure and demonstrates the technology for predicting the reliability and robustness of a wirebond interconnect structure that is subjected to electro-thermo-mechanical loads. The design variables investigated in this study included wire-loop ratio, current in the wire, and thickness of the silicon die each represented as input variables with normal distribution. In terms of reliability the damage variable under investigation was the plastic strain at the wire/aluminium pad interface. Using ANSYS for predicting the physics in the package we have demonstrated the ability of the ROMARA code to optimise the design of wirebond, in terms of minimising the induced damage. Other real cases have been investigated using the developed ROMARA software and these are reported in the public domain and briefly detailed in this thesis.