Crack Propagation Experiments on Flip Chip Solder Joints
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S. Wiese, F. Feustel, S. Rzepka, E. Meusel, Dept.of Electrical Engineering, Dresden University of Technology, Germany TU Dresden, IHM, D-01062 Dresden, [email protected] ABSTRACT The paper presents crack propagation experiments on real flip chip specimens applied to reversible shear loading. Two specially designed micro testers will be introduced. The first tester provides very precise measurements of the force displacement hysteresis. The achieved resolutions have been I mN for force and 20 nm for displacement. The second micro tester works similar to the first one, but is designed for in-situ experiments inside the SEM. Since it needs to be very small in size it reaches only resolutions of 10 mN and 100nm, which is sufficient to achieve equivalence to the first tester. A cyclic triangular strain wave is used as load profile for the crack propagation experiment. The experiment was done with both machines applying equivalent specimens and load. The force displacement curve was recorded using the first micro mechanical tester. From those hysteresis, the force amplitude has been determined for every cycle. All force amplitudes are plotted versus the number of cycles in order to quantify the crack length. With the second tester, images were taken at every 10th ... 100th cycle in order to locate the crack propagation. Finally both results have been linked together for a combined quatitive and spatial description of the crack propagation in flip chip solder joints. INTRODUCTION Flip chip packages fail very often due to the thermomechanical fatigue of solder joints. Numerous FEM simulations have been carried out in order to characterize mechanical stresses in flip chip packages. Most known simulations have used data that had been acquired from tensile tests of bulk specimens. However, regarding the failure aspect of flip chip joints three significant characteristics need to be considered - micro structure, joint geometry and load profile. The grain size of a Flip Chip solder joint is usually, due to its lower thermal mass, smaller than that of a bulk specimen. The real joint shape is very inhomogeneous, which leads to stress concentration points in the flip chip joint body. It is very likely that those stress concentrations cause an immediate crack nucleation, which signifies, that the failure of flip chip solder joints depends mainly on the crack propagation. Since the flip chip joints are very small, the plastic deformation zone around the crack tip is limited by the joint dimensions, which may lead to a crack propagation behavior different to that of bulk specimen, where the plastic deformation zone can spread out freely and cover only a small fraction of the total volume. The typical load condition can be described as reversal shear loading. In order to receive relevant data it seemed to be advantageous to perform crack propagation tests (mode II) on real flip chip joints. SPECIMENS The ideal specimen would consist of only one single solder joint. This however is very difficult to realize. The minimum spe
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