Fraunhofer-Institut für Zuverlässigkeit und Mikrointegration IZM

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  • Publication
    Thermo-mechanical reliability of flip chip structures used in DCA and CSP
    ( 1998)
    Schubert, A.
    ;
    Dudek, R.
    ;
    Vogel, D.
    ;
    Michel, B.
    ;
    Reichl, H.
    The continuing demand towards high-density and low profile integrated circuit packaging has accelerated the development of flip chip structures as used in direct chip attach (DCA) technology and chip size packages (CSP). The advantages in density, cost and electrical performance are obvious. Solder joints, the most widely used flip chip interconnects, have a relatively low structural compliance due to the large thermal expansion mismatch between silicon die and the organic substrate. This causes high thermally induced creep strain on the interconnects during temperature cycling and leads to early failure of the solder connections. The reliability of flip chip structures can be enhanced by applying an epoxy-based underfill between the chip and the substrate, encapsulating the solder joints. However, over ranges of design, process, and material parameters, different failure modes are observed with significant dependence on material properties and geometry. Nonlinear finite element analysis for flip chip structures is carried out to investigate the reliability impact due to a number of selected design and material parameters. Especially two fundamental issues are addressed, namely, the optimization of thermomechanical properties of underfill materials and manufacturing process-induced defects.
  • Publication
    Reliability investigations of flip chip interconnects in FCOB and FCOG applications by FEA
    ( 1998)
    Schubert, A.
    ;
    Dudek, R.
    ;
    Döring, R.
    ;
    Michel, B.
    One major concern over thermally induced mechanical stress is that it causes reliability problems in electronic device packaging and interconnects. IC packaging has accelerated development of flip chip structures as used in flip chip on board (FCOB) or flip chip on glass (FCOG) technology. Much testing is usually required to meet the reliability needs of an assembly or to optimize its design. Finite element analysis (FEA) is used to understand the reasons for failure and the critical parameters which may be varied; however, use of FEA generates difficulties concerning the geometrical description and constitutive modeling of the materials used. Solder joints, the most widely used FCOB interconnects, have relatively low structural compliance due to the large CTE mismatch between die and organic substrate. This causes high thermally induced creep strain on interconnects during temperature cycling and leads to early failure. Flip chip reliability can be enhanced by applying an epoxy-based underfill between chip and substrate. However, over ranges of design, process and material parameters, different failure modes are observed with significant dependence on material properties and geometry. Nonlinear FEA of flip chip structures is carried out to study the reliability impact of selected design and material parameters. Two fundamental issues are addressed: optimized manufacturing process-induced defects and underfill material thermo-mechanical properties. Anisotropic conductive films (ACF) are widely used for FCOG packaging. Nonlinear FEA simulations are conducted to investigate stress development and relaxation in ACF joints.
  • Publication
    Materials mechanics and mechanical reliability of flip chip assemblies on organics substrates
    ( 1997)
    Schubert, A.
    ;
    Dudek, R.
    ;
    Michel, B.
    ;
    Reichl, H.
    ;
    Jiang, H.
    This paper demonstrates a combined approach of numerical analysis and experimental investigations to study the mechanical reliability of flip chip solder joints. The effect of various design parameters like bump geometry, "soft" and "hard" underfill, and used solder mask on the thermal fatigue life of solder joints is discussed. Since special attention has been directed towards Flip Chip on Board (FCOB) assemblies, constitutive properties of polymeric and solder materials are discussed in detail. The solder is modeled using a nonlinear constitutive law with time dependent (creep) and time independent plastic strains. Furthermore, material testing shows that the underfill and solder mask materials might be considered as linear viscoelastic with temperature time shift properties. Thermal mismatch between the materials assembled is often the main reason for thermally induced stresses. Thermal cycling (125 degrees C...-55 degrees C...125 degrees C) is therefore the load generally used in t he 3D non-linear finite element analysis. Calculation results of the solder bump deformation due to temperature changes are accompanied by experimental deformation analysis. The used MicroDAC method is based on algorithms of local object tracking in images obtained from electron scanning microscopy. The measured deformation fields were utilized for proper materials selection and processing, as well as for verification of finite element analysis.
  • Publication
    Materials mechanics and mechanical reliability of flip chip assemblies on organics substrates
    ( 1997)
    Schubert, A.
    ;
    Dudek, R.
    ;
    Vogel, D.
    ;
    Michel, B.
    ;
    Reichl, H.