Experimental Characterization of Material Properties of 63Sn37Pb 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, wiese @ehmgw I.et.tu-dresden.de ABSTRACT The paper presents new material data of real flip chip solder joints. A testing apparatus was designed to perform reversible shear tests on flip chip joints. In contrast to previous test setups for similar purposes this tester provides an infinite stiffness, a very high precision force (1 mN resolution) and displacement (20 nm resolution) measurement. The experimental program included cyclic shear tests for elastic plastic material data as well as a newly developed reversal creep and relaxation test for time depended material properties. A subsequent FEM simulation was applied to evaluate experimental raw data and to determine the parameters of material models provided by ANSYST5 . A user defined creep model had been added to the source code, in order to receive a convenient solder model for FE-Analysis of SnPb37 flip chip solder joints under thermomechanical stresses in electronic packages. The results of this study shows that the material behavior of flip chip solder joints is much more like that of bulk samples with a comparable micro structure than it is commonly believed up to now, because of the previously published data. INTRODUCTION Although flip chip technology provides many advantages (minimum space, matrix joint design, high number 1/0, excellent electrical properties) it suffered from its extensive costs and themomechanical fatigue of the joints due to different thermal expansion coefficients of silicon chip (CTEs, = 2 ppm/K) and substrate (CTEA120o= 6 ppm/K). The development of a low cost flip chip technolgy demanded the use of a cost effective commonly used substrate material like FR4 (CTE,., 4 = 17 ppm/K), which even increased the thermomechnical fatigue problem. Analytical and numerical simulations on thermomechanical problems in flip chip interconnections pointed out that the use of underfill reduces the fatigue problem significantly [ ]. But the precision of quantitative FEM results (e.g. local strain values) depends above all on the assumed material behavior. The application of material parameters that have been gained in standardised tests with bulk specimens suffers from the fact that a Flip Chip Joint in real applications experiences different loading characteristics, has a much different geometry and consists of a different microstructure. Consequently, a new research method should take into account all characteristics of thermomechanical fatigue of Flip Chip Joints. The simplest idea was to perform shear experiments on real Flip Chip Joints instead of tensile tests on extra micro specimens. However, an evaluation of such experiments could not be carried out with simple analytical calculations, because the specimen geometry as well as the load characteristic are too complex. The key idea was to perform a 3D FE-Analysis of the experiment and to compare the experimental results with simulated results. Adju
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