Life-time simulation of copper wire bonds using multi-physics effect

Abstract

In this era of electric vehicle replacing combustion engine, the demand of automotive integrated circuit (IC) package has increased significantly. Copper wire interconnect has become the default option due to superior electrical and mechanical properties, and lower cost compared to the conventional gold wire interconnect. The reliability of copper wire interconnect is not fully understood, that is why the qualification of components with copper wire interconnects has extended stress test requirement to ensure robustness of the components. The motivation is to investigate the failure mechanisms of copper wire interconnect corrosion using multi-physics effect to enable more accurate life-time simulation. As there were many qualitative investigations based on AEC-Q006 requirement, the results were reported as pass or fail without detailed explanation of the rate of degradation. There were also literatures focused on the detailed degradation of copper wire interconnect due to intermetallic formation, over-acceleration of highly accelerated temperature and humidity stress test (HAST) using green moulding compound, corrosion of the copper interconnect caused by the presence of sulphur and chlorine in the moulding compound. However, there is limited research focused on rate of degradation of copper wire bond in high temperature and humidity stress condition. Hallberg-Peck equation is the reference model for high temperature and humidity stress test, but its relevance to the copper interconnect is not validated. Besides, there are weakness in Hallberg-Peck equation as it uses temperature and relative humidity terms, the equation is not valid when the use condition is dry or has relative humidity higher than 85%. This thesis proposed a novel acceleration model using energy concept. Energy is the universal form that can be used to compare different terms such as thermal, humidity, electrical, mechanical, and chemical energy. As the topic is very wide, this thesis focused on thermal, humidity and electrical energy. In this Thesis, the degradation of copper wire under the multi-physics effect of temperature, humidity, voltage, and current is investigated. Initial characterization work was conducted to establish the test vehicle and methodology for the experiments. Simulation using finite element method (FEM) was also conducted to determine the limit of applied current with respect to joule heating and heat transfer to the surrounding, and the relationship of delta resistance to the reduction of cross-sectional area of the wire. The established test methodology to investigate the rate of degradation of copper wire bond under high temperature and humidity stress test is novel. Accelerated stress tests were conducted with both in-situ and ex-situ electrical resistance measurement of the daisy-chain test structures. The stress conditions were selected according to enthalpy-based water-vapour energy and their relevance to the industrial standards. Further analyses such as electrical, physical and electrochemical characterisations were conducted. Applied current lead to joule heating of the wire without other degradation observed. Oxidation of copper at the wire span contributed to the increase in the electrical resistance, while the oxidation at stitch bond interface led to open contact failure. The relative degradation at the wire span is directly proportional to the ratio of the enthalpy-based water-vapour energy. The delta resistance of wire bonds after 110°C 85%RH 960h referenced to 85°C 85%RH 960h is 2.4×, which correlates to the ratio of the enthalpy-based water-vapour energy of stress conditions. However, the calculation using Hallberg-Peck equation gives over-estimated acceleration of 5.4× instead. Electrochemical characterisation was conducted on different surface material composition to compare their corrosion potentials and corrosion rates. The corrosion rate of all copper electrodes in 15ppm chlorine solution is higher than in deionised (DI) water. This thesis proposed a novel energy-concept acceleration model for package reliability focusing on copper wire bond degradation due to corrosion. This model enables comparison of multi-disciplinary factors in one universal energy form. The results from the series of accelerated stress tests designed based on novel enthalpy-based water-vapour energy prove that energy-concept model is more accurate than the conventional Hallberg-Peck equation considering wide temperature and relative humidity range. Accurate prediction of degradation will create greater confidence for the industry to reduce unnecessary stress tests duration for new package qualification to enable faster time-to-market and cost saving. The established methodology for this investigation and the energy-concept acceleration model also provide a good platform and fundamental for further investigation considering electrochemical reactions as well as mechanical stress in view of package reliability.