Investigation On Viscoelastic Characteristics Of Micro-Electronic Flip-Chip Packages

With the development of LSI and ULSI, the I/O numbers of IC packages have been growing, which has become a restriction for the development of IC industry. FCB (Flip Chip Bonding) has given a solution for this problem, with its more excellent electrical characteristics and more I/O number support. It has taken a revolutionary progress of packaging technology. However, FCB also brought a lot of technical problems. In particular, packaging stress and strain are aroused by the mismatch of CTE (coefficient of thermal expansion) of packaging materials in packaging process and application. This is the one of main reasons of the flip-chip package failure. Because lead-free activity in electronic industry has maken a great progress since of the official continuous promotion, lead solders must be substituted and the lead material must be removed. We have to face to more problems resulting by the melting point increasement of lead-free solders. Experiment and/or test are not the most effective way to finding and solving those issues quickly. So the numerical simulation, such as finite element method, is applied more and more.Employment of Different material models will directly influence on numerical simulation results. The early studies usually described the underfill materials as classical linear elasticity in order to get a better computational efficiency. But the underfill shows significiant viscoelasticity actually.In this thesis, the main purpose is to investigate the effects of different material models (especially viscoelastic underfill model) on stresses and/or strains of IC packages. In order to adapt to the needs of the lead-free solder usage, 96.5Sn3.5Ag is used as solder. Anand model of viscoplasticity is used to describe the viscoplastic mechanical behavior of 96.5Sn3.5Ag solder. The classical linear elastic model and the Maxwell viscoelastic model are used to describe the mechanical behavior of underfill separately for comparison. The 3D finite element model of the flip chip package is established directly by ANSYS software. At first, by using the finite element software, the welding and curing processes of packaging, as well as the storing process in a dry environment after packaging are simulated and analyzed. In the anlysis, the creep deformation and stress relaxation behaviors of underfills are investigated especially. Also the influence of the underfill's creep deformation and stress relaxation behaviors on the residual stresses and deformation of solder bumps, the influence of the storing time after packaging on packaging residual stresses and strains of underfills and solder bumps are studied separately.It is found that the difference of material model of underfills can not overthrow the trend and rule that the largest stress appears in the solder bumps at opposite corners of package. The influences of the underfill's viscosity, on the residual stresses and strains in all parts of package and on the warp of package are remarkable. Particularly, the creep deformation and stress relaxation behaviors also affect significantly the stresses and strains of solder bumps, as well as the creep deformation and stress relaxation behaviors of solder bupms. The viscoelastic characteristics of underfills should be properly taken into account for simulations, analysises and estimations of packages and package safety.