Material Characterization, Modelling and Simulation of Epoxy Moulding Compounds under High Temperature Storage and Temperature Cycling Thermal Ageing
Lion, Alexander, Univ.-Prof. Dr.-Ing. habil.
Lion, Alexander, Univ.-Prof. Dr.-Ing. habil. ; Petriková, Iva Prof. Dr. Ing. Ph.D.
Date oral examination:
Molding compound Characterization; Material modelling; Simulation; Thermal Ageing
Due to time-to-market law, the semiconductor industry needs to evaluate the reliability of the electronic product in a short time. The experimental method is not only time consuming but also costly. Therefore, the virtual prototyping method is generally used by combining the Finite element Method (FEM) and properly modelling methods. As a report in , around 65% of failure modes in electronic packages is coming from thermo-mechanical related issues. Among them, the Epoxy Moulding Compounds (EMCs) plays an essential role in the reliability of the electronic package as encapsulation material. In order to use the FEM to predict and understand the thermo-mechanical behaviour of electronic packages under different loading conditions, the material properties of EMCs should be carefully characterised and properly modelled.
Currently, more and more electronic packages are exposed to severe environments, such as high temperature, high moisture, etc. Among them, high-temperature conditions can lead to irreversible changes in EMCs since it is polymer-based material. These changes can be attributed to chemical processes such as thermo-oxidation and can lead to degradation of the applied epoxy resin, which we refer to here as thermal ageing. As a result, the thermo-mechanical properties of the EMCs change severely due to thermal ageing. Due to ongoing changes in the ageing of EMC of a package, the stress and strain distribution in the package change significantly concerning ageing time, while embrittlement also affects the fracture strength. As a consequence, the long-term reliability of a package is severely affected.
The experiment showed that a very thin layer generates on the EMCs surface during the thermal ageing process; the thickness of this skin layer appears to depend on both ageing temperature and ageing time. As a result, the overall properties of the EMC (with skin layer) changes with the storage time. Since an appropriate constitutive representation of the material properties of the slowly growing oxidation layers in EMC is not available, it is cumbersome to predict the reliability of a real package for long-term applications. Due to this limitation, this thesis focuses on the experimental characterization as well as on the numerical modelling of ageing of EMCs at high-temperature storage (HTS) and Temperature Cycling (TC). In the end, the thermo-mechanical properties change of EMCs during thermal ageing can be appropriately modelled by numerical method. Besides that, thermo-mechanical properties change of the EMCs under HTS and TC loading conditions is studied. The relationship between HTS and TC impacts on material properties change of the EMCS is also established.
Based on the above descriptions, an appropriate modelling method, simple and efficient, needs to be developed to simulate thermal ageing effects of EMCs and later can be used to evaluate the long-term reliability of the electronic package. First of all, for the thermomechanical modelling, the material properties should be adequately characterised, such as elastic modulus and thermal expansion. At the same time, a proper constitutive model needs to be found or established to describe or represent the mechanical behaviour of the interested materials under a defined loading condition. Second, to demonstrate that the constitutive equation and the characterised material parameters are correct, a proper verification test needs to be built and performed to prove that. In the end, the characterised material parameters and the modelling method can be then used in the simulation of a real product.
In this thesis, the thermo-mechanical properties of the EMCs before thermal ageing is systematic characterisation firstly, such as modulus, coefficient of thermal expansion and curing shrinkage. Secondly, based on the measurement data, the responded material constative equation and material parameters are established for modelling. Furthermore, several verification tests are also build up and performed to check the characterised material parameters as well as the modelling method. Finally, the simulation results compare with the verification test.
However, the thermo-mechanical properties of EMCs are changing significantly during the thermal ageing process. The material properties change not only depend on the ageing time but also on ageing temperature. In order to model this thermo-mechanical properties change, a two-layer modelling method is proposed with an equivalent thickness of an oxidation layer concept. It is assumed that the mechanical properties of partly aged EMC can be modelled with sufficient accuracy, by modelling a fully aged equivalent layer and unaged core. This proposal is proved by a series of specially designed experiments. According to this modelling method, the material properties of the unaged and fully aged EMC need to be fully and accurately characterised. Moreover, the equivalent thickness of fully aged layer as a function of ageing time is established by combining experimental results and numerical analysis of properly chosen samples. Combining the two-layer modelling method, established equivalent thickness and characterised material properties of the unaged core and fully aged layer, the overall material properties of a partly aged EMC can be predicted at any time. In the end, the modelling method and characterisation parameters of unaged and fully aged EMC are verified by designed bi-material samples.
Due to time-to-market law, the semiconductor industry needs to evaluate the reliability of the electronic product in a short time. The experimental method is not only time consuming but also costly. Therefore, the virtual prototyping method is generally used by combining the Finite element Method (FEM) and properly modelling methods. As a report in , around 65% of failure modes in electronic packages is coming from thermo-mechanical related issues. Among them, the Epoxy Moulding Compounds (EM... »