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20-23 April 2008International audienceThis study introduces a computation method to assess damage in electronic solder joints under random vibration. It addresses full 3D dynamic behaviour of electronic board. Finite Element Modelling (FEM) of electronic BGA (Ball Grid Array) and CGA (Column Grid Array) packages assemblies are developped and adjusted with experimental modal identification of the test board. Vibration FEM simulations are performed to calculate stress transfer functions of critical solder joints. Then, solder joint time-stress responses due to an input random excitation of the board are generated. Stress range distributions are established from rainflow counting. Finally, linear damage computation is done and compared with experimental results by using an empirical damage law. The potential and the limitation of this method are discussed

eads.net, tel: +33 (0) 1 46 97 36 74 AbstractNumerical methods have become a useful mean to predict the thermo-mechanical reliability of solder interconnects in electronics, at least comparatively. In principle, each calculation finally rests on a creep fatigue model or criterion. The article proposes to review some important failure models of solder found in literature, to discuss their benefits and drawbacks, and finally to compare them on the example of a semi-analytical assembly model. Cyclic fatigue criteria constitute the classical approach in creep fatigue modelling. Strain range, strain range partitioning, inelastic energy and energy partitioning, all these approaches have lead to the successful development of failure models for solder in electronics. Beside cyclic criteria, continuous damage mechanical models have been developed recently in different publications. They are basically extended sets of constitutive equations including a new variable called damage. In such approaches, damage may be integrated over any cyclic or non-cyclic solicitation.A semi-analytical model of a flip chip PBGA (Plastic Ball Grid Array) assembly has been developed in order to implement a continuous damage mechanical approach. Then it is used for comparisons with some cyclic fatigue criteria on the same thermal cycle. The assembly model is based on the theory of bi-material thermostats and on a continuous damage mechanical model found in a recent publication. Only the outline of the model is described. Easy parametrisation and short calculation delays allow to run the model over high number of thermal cycles and to study the damage evolution. Results haven't been correlated with experimental data, but are in a correct order of magnitude, whatever failure criterion is applied. NotationsAbbreviations and acronyms FEM Finite Element Method PBGA Plastic Ball Grid Array PCB Printed Circuit Board Latin symbols D damage (dimensionless) e pitch [mm] E Young's modulus [GPa] h thickness or height [mm] C half package length [mm] Nd50%) median cyclic fatigue life (number) P accumulated inelastic strain (dimensionless) R isotropic hardening [MPa] T temperature [K] U displacement [pm] U energy density [MPa] W accumulated inelastic damage (dimensionless) W X kinematic hardening variable [MPa] -Y Greek symbols a a,, kinematic hardening tensor (dimensionless) Y y[ fatigue ductility coefficient (dimensionless) Ax variation range of x E,, strain tensor (dimensionless) P other damage variable (dimensionless) V Poisson ratio (dimensionless) o,, stress tensor [MPa] Subscripts and superscripts C creep e elastic f failure in inelastic P plastic ref reference IntroductionThe liability to temperature variations is a general issue for soldered electronic assemblies. Actually, thermal expansion mismatches between constitutive parts lead to internal shear efforts and to irreversible strain in solder joints. After numerous repetitions, low cycle fatigue and creep damage occur. Creep-fatigue is a limit state obtained by damage accumulation after repeated l...

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