Study On Viscoplastic Behaviors And Failure Of Electronic Packages Under Therm-mechanical Loading

The electronic industry has been developed rapidly in recent years and becomes one of the most potential industries. Many package materials and structures have been developed rapidly, and Pb-free solders which have replaced SnPb solders are turning into a necessary trend synchronously. The surface mounted technology that is represented by ball grid array technology with the advantages of low cost, high integration, light weight and powerful function, is been penetrating into all kinds of manufactures in whole electronic industry. On the other hand, the statistical data are shown that thermal fatigues and vibration fatigues of solders are the main failure forms in electronic packages. Therefore it is necessary urgently for the new package materials and structures to discuss thoroughly their responses under therm-mechanical loads, and furthermore to find out the thermal visco-elastic-plastic behaviors, mechanisms of their failure and to afford the essential theoretical basis and analysis methods for improving the better reliability of package structures. In this paper, based on realizing roundly the research status in test study, theoretical analysis and numerical simulation of behaviors and failure in the package materials and structures, the SnAgCu Pb-free solder and BGA structure is selected for developing this task to investigate in this paper. The main contents are:1. The uniaxial tensile tests of Sn4Ag0.5Cu (in short words, SAC405) solder are completed under different temperatures and load rates. The rate-dependent mechanical properties of this solder are analyzed, and the effects on load conditions on solder performances are discussed. The applicability and material parameters in two visco-plastic constitutive models named Anand unified model and partitioned model are determined, and the numerical simulations are both agreed with the test data.2. The tensile-compress fatigue tests of SAC405 solder are accomplished under different temperatures, strain ranges and load frequencies, which are shown that the main factor on low-cycle fatigue life of this solder is load strain range, and the effects of temperature and load frequency are a little. According to the test data, the factor variations are concluded that a modified Manson-Coffin law is proposed with a power form to frequency and an index form to temperature. This law form is simple and coincident with tests.3. An anisotropic damage scalar is introduced to describe the cyclic softening of SAC405 solder based Lemaitre's damage theory. The rate-dependent damage evolution parameter ? is influenced mainly by load strain ranges. The parameter ? is enhanced obviously with strain-range increasing, added feebly with load frequency increment and affected less with temperature. The evolution parameter equation with double power form is set up according to the test data. Considering partitioned damage visco-plastical constitutive equations, isotropic strengthened model and unload linear law, the numerical simulative curves of low-cycle fatigue behaviors of SAC405 solder under tensile-compress cyclic loads are accorded with experiment results.4. With homogenization theory and higher order discrete-layer laminate model, the double-scale thermal elastic-viscoplastic analytic model is completed for BGA package structure with non-perfect periodic microstructures filled with underfill. The the macro and meso Rayleigh-Ritz methods are adopted to solve problems, in which meso domain is simplified to simple and effient axisymmetric problem and macro displacement function is used the manner of power function combined with series function that have better solving precision and convergence rate. The displacements, stress and strain in the interface between solder joint and underfill have local changes obviously under thermal loads. Although the local displacements are lower order of magnitude, but the corresponding strain and stress are the same order of magnitude with macros'. This difficulty can be solved by double scale analysis in which the local changes of displacements and stress could be respected at the bi-material interface in this paper.5. The experimental study and numerical simulation of dynamic properties for M12PP-02(S5)main control board with BGA package structure are completed, in which metallographical method and dyeing penetrant testing technology are used to observe the crack shape and growth in solder joints. The stress distribution of BGA and single solder joint's stress and strain are obtained through the FEM simulation analyses to BGA package. The fatigue life of BGA solder joints predicted with Engelmaier fatigue model and the application scope of this model is discussed in BGA electronic package.