Fracture mechanical analysis of viscoelastic materials in electronic components

Bei der Umhüllung mikroelektronischer Bauelemente treten aufgrund der unterschiedlichen thermischen Ausdehnungskoeffizienten der verwendeten Werkstoffe mechanische Belastungen auf, die das Risiko der Rissentstehung und -ausbreitung erhöhen. Da ein solches Versagen in der Umhüllung zum Ausfall des gesamten Systems führen kann, ist es während der Entwicklungsphase nötig, dieses Risiko abzuschätzen, um es zielgerichtet minimieren zu können. Somit wird im Rahmen der vorliegenden Arbeit eine Methodik entwickelt, die es ermöglicht derartige Risse bruchmechanisch beurteilen zu können. Die verwendeten Verkapselungswerkstoffe sind häufig Polymere, die viskoelastisches Werkstoffverhalten aufweisen. An einem Beispiel wird die Charakterisierung dieser Eigenschaften zur Beschreibung multi-axialer Spannungszustände demonstriert. In diesem Zusammenhang stellt sich die Frage nach einem passenden Bruchkriterium, und es werden verschiedene bereits vorhandene Kriterien vorgestellt und verglichen. Sie werden insbesondere in Bezug auf die Anwendbarkeit bei thermomechanischen Belastungen beurteilt. Anschließend werden die gewonnenen Ergebnisse auf einen Demonstrationsfall übertragen, an dem Simulation und Experiment verglichen werden und eine gute Korrelation demonstriert wird.Cracks inside polymeric packaging materials which are used for protection and isolation of electronic components, can lead to failure of the whole system. Therefore the understanding of crack initiation and propagation becomes vital for the design of reliable microsystems and a methodology is developed that enables the fracture mechanical analysis of cracks. As polymeric packaging materials show viscoelastic material behaviour, a characterisation method is being developed to describe the multi-axial stress-strain relationship. In this context the question arises about an adequate fracture criterion. Therefore different fracture criteria for viscoelastic materials are reviewed and compared. They are evaluated with respect to thermo-mechanical loading. After that the results of this analysis are applied to a demonstration case. Simulation and experiment are compared and a good correlation is demonstrated

Eco-reliability - a combined approach to balance environment with reliability

Over the past years a new concept has evolved to combine eco-optimization with reliability aspects. As a short phrase we have coined the word eco-reliability, which we would like to explain and explore in the following paper

Establishing ecoreliability of electronic devices in manufacturing environments

EcoReliability describes the inclusion of reliability aspects into the environmentally conscious design of electronic systems to address the originally separated domains from one mutual perspective. This paper motivates the importance of such an approach for the case of electronic products and in particular embedded electronics. Environmental analysis of electronics has often been narrowed down to energy use, but the total resource use is now seen as equally important. Using technical examples from promising applications in robust electronics for manufacturing equipment, measures in system design aiming at an increase of sustainability through determination of a truly balanced degree of reliability are presented. The first case is taken from the field of power electronics with demanding requirements towards robustness. Measures to increase the allowable number of thermal cycles during operation are compared towards shifts in environmental attributes. For the second case, miniaturized sensors are introduced that face issues of obsolescence when applied to machine tool environments in long-term scenarios

Fracture mechanics in new designed power module under thermo-mechanical loads

Thermo-mechanically induced failure is a major reliability issue in the microelectronic industry. On this account, a new type of Assembly Interconnected Technology used to connect MOSFETs in power modules has been developed. The reliability is increased by using a copper clip soldered on the top side of the chip, avoiding the use of aluminium wire bonds, often responsible for the failure of the device. Thus the new designed MOSFET package does not follow the same failure mechanisms as standard modules. Thermal and power cycling tests were performed on these new packages and resulting failures were analyzed. Thermo-mechanical simulations including cracks in the aluminium metallization and intermetallics (IMC) were performed using Finite Element Analysis in order to better understand crack propagation and module behaviour

A critical review of corrosion phenomena in microelectronic systems

In general corrosion is one of the most critical aspects for long term reliability. Most of the accelerated tests are not taken this aspect into account. Especially applications in the power electronics field imply some of the harshest environmental conditons for electrical components. To protect these systems typically polymers are being used for the packaging. This is not a hermetic sealing method, but it provides a sufficient housing for most of the applications being a small sized and very low cost package. Polymeric materials are prone to leakage and permeation of moisture and corrosive gases into package which could damage the dies, wires, bond pads, lead frames and solder joints. Therefore, corrosion is a long-term issue in microelectronic packages and is related to the whole system. This paper gives an overview of corrosion induced degradation of microelectronic packages and presents a measurement principle to characterize the protection efficiency of encapsulation layers by an electrochemical measuremnt

Fracture mechanics in new designed power module under thermo-mechanical loads

Thermo-mechanically induced failure is a major reliability issue in the microelectronic industry. On this account, a new type of Assembly Interconnected Technology used to connect MOSFETs in power modules has been developed. The reliability is increased by using a copper clip soldered on the top side of the chip, avoiding the use of aluminium wire bonds, often responsible for the failure of the device. Thus the new designed MOSFET package does not follow the same failure mechanisms as standard modules. Thermal and power cycling tests were performed on these new packages and resulting failures were analyzed. Thermo-mechanical simulations including cracks in the aluminium metallization and intermetallics (IMC) were performed using Finite Element Analysis in order to better understand crack propagation and module behaviour

Modelling the lifetime of aluminum heavy wire bond joints with a crack propagation law

In this study, an approach for enhanced lifetime modelling of wire bonds has been investigated. Numerical simulations of a 3D wire bond model have been used to acquire a suitable damage parameter. For the lifetime model, a modified Paris law for calculating crack growth per cycle has been employed, to consider the gradual area degradation due to thermo-mechanical loads. With the knowledge of the crack propagation rates, the acquired lifetime model can easily be transferred to different wire bond geometries without repeating the experiments necessary to fit the crack growth parameters

Fracture mechanics in new designed power module under thermo-mechanical loads

Thermo-mechanically induced failure is a major reliability issue in the microelectronic industry. On this account, a new type of Assembly Interconnected Technology used to connect MOSFETs in power modules has been developed. The reliability is increased by using a copper clip soldered on the top side of the chip, avoiding the use of aluminium wire bonds, often responsible for the failure of the device. Thus the new designed MOSFET package does not follow the same failure mechanisms as standard modules. Thermal and power cycling tests were performed on these new packages and resulting failures were analyzed. Thermo-mechanical simulations including cracks in the aluminium metallization and intermetallics (IMC) were performed using Finite Element Analysis in order to better understand crack propagation and module behaviour

Analysis of mechanical properties of thermal cycled Cu Plated-Through Holes (PTH)

The aim of thermo-mechanical reliability assessment in microelectronic packages is life time prediction under different thermal and mechanical induced stress loads. The analysis of long time stability of thermally loaded Plated- Through-Holes (PTH) in Printed Circuit Board (PCB) also requires an accurate determination of material data. This leads to application of different test and measurement methods, which are allowed to measure mechanical materials properties at micro- and/or nanostructural scale. This paper focuses on application of instrumented nanoindentation measurement technique for analysis of mechanical properties of microelectronic relevant electroplating copper. Nanoindentation method has been widely used for characterization of mechanical behaviour of devices in small volume (especially for PTH) and determined typically elastic mechanical properties (reduced modulus and hardness). In combination of modified Finite-Element (FE) simulation models and nanoindentation test results elastic and plastic material properties of copper in small scale were obtained. It was dimensionless functions for determination of presentable stresses developed, which allows to indicate the stress-strain curve of bulk materials. It is a precondition to implementation of this function that the indentation depth is out of indentation size effect. The presentation of calculated stress-strain curves by using of dimensionless function and the influence of thermal cycling of material behaviour of PTH are subject of this paper