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7-9 October 2009, Leuven, Belgium Crack Tip Localization of Sub-critical Crack Growth by Means of IR-Imaging and Pulse Excitation D. May 1 , B. Wunderle 1,3 , R. Schacht 1,2 , B. Michel 1 1 MicroMaterials Center Berlin, Fraunhofer IZM, Berlin, Germany Email: daniel.may@izm.fraunhofer.de 2 Hochschule Lausitz (FH), Senftenberg, Germany 3 TU Chemnitz, Germany Abstract- Taking the advantage of the thermo-elastic effect, while using an infrared camera system, the mechanical stress could be made visible. The stress concentrations at the tip of a subcritical crack growth could clearly be detected. Through the observation during the periodic loading of a CT-specimen the crack growth rate can be determined. For this purpose, a specially developed loading stage will be presented. It is now possible to have further investigation in material class of polymers, which is very important in the field of system integration. First promising results will be presented. I. INTRODUCTION Today’s need for fast and reliable technology development including further miniaturization, functionality and complexity at lower cost is a trend that will continue [1]. To assure reliability for advanced packages presupposes the understanding and description of failure mechanisms and to be able to apply them to large models [2]. For e.g. power packages delamination of the die-attach or encapsulant is known to be a problem which needs optimization [3]. Then for e.g. System-in-Package solutions, the number of interfaces and thus possible delamination sites has increased a situation where numerical lifetime prediction methods also face the challenge of availability of critical parameters for materials and material pairings. These parameters in general are given as energy release rates for fracture-mechanical treatment and depending on process conditions, moisture and temperature. These values are usually had to be measured because they are not readily available from manufacturer’s datasheets. Modern Finite Element (FE) tools allow the programming of routines to calculate energy release rates as failure criterion for a possible crack in a rather short time [4]. However, the bottle neck is a rapid and yet accurate determination of critical and undercritical data for crack growth. In the past, many different setups have been proposed to study crack growth ([5,6]). Most of them need complicated methods to determine the crack length under external loading conditions in respect to the specimen geometry and the real process conditions. As for most designs the precise crack length is needed for an energy release rate and phase angle determination from finite element simulations, here a new method is proposed using the transient thermal response of the specimen under different loading conditions by infrared thermography [7]. The IR thermography is becoming increasingly important for non-destructive testing of microelectronic components and structures on the chip, package and board level. Besides the use of pulse thermography for determining the temperature of thermal conductivity according to Parker [8] or for detection of delaminations in micro-electronic thin-film assemblies, and the degradation of electrical vias in printed circuit boards [9,10] the lock-in thermography is also used to detect shorts in integrated circuits. Müller [11] has shown that the lock-in thermography can be used in investigation of fracture and cracking behavior of steel components using the thermo-elastic effect [12]. To investigate in crack propagation in solids, among others, a so called Compact Tension (CT) specimen is used. If the subcritical crack growth is examined the sample with strain amplitude less than the critical has to be loaded periodically. During the experiment the length of the growing crack must be measured by appropriate methods. These experiments are carried out, in specially designed testing equipment. Due to the high costs often only a few of such machines are available. Therefore long experimental periods are being developed for a material such as to be characterized under different temperatures and loads. To gain even more quickly usable material parameters, e.g. at an early product development phase and to make a choice of materials a design of a novel strain apparatus should be as simple and yet as precise as necessary to carry out several parallel investigations. This work focuses on the adaptation of the stress analysis on materials of micro-electronics (e.g. in the system integration very important material class of polymers). A. Theoretical Basics In the early 19th Century, J. Gough [13] observed that the temperature of a polluted materials changes. The correlation between mechanical stress and temperature change has J. Thomson [12] in 1853 first recognized in theory and published. According to Stanley Chan and Biot [14, 15] the link between material parameters, state of stress of the sample and the temperature change can be described as follows: ∆ · ··∆ (1) where T is the absolute temperature, α is the coefficient of thermal expansion, ρ the density, c p , heat capacity (at constant pressure) and Δ (σ1 + σ2 + σ3) the change in the sum of principal stress (hydrostatic stress). Here with positive values of the principal stresses (tensile stress) a cooling and with negative values (compressive load) a warming take place in the material. With the development of modern infrared measurement in the ©<strong>EDA</strong> <strong>Publishing</strong>/THERMINIC 2009 91 ISBN: 978-2-35500-010-2
7-9 October 2009, Leuven, Belgium 1960s it became possible to measure temperature changes C. Experimental Setup contactless and without feedback on the specimen. With currently To investigate the stress field near the crack tips a suitable available IR detectors high geometric (
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International Workshop on THERMal I
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