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The thermo-structural behavior of an underfilled flip-chip package was evaluated in this study using the structural FEM code NIKEDP,
whereby an axisymmetric model of a typical flip-chip structure was
employed. Numerical simulations were conducted for underfill materials
of different thermo-structural properties and two solder bump heights. The parametric sensitivity of the thermal strain in the solder joints and the axial, as well as shear, stress in the underfill material
were then analyzed. The Coffin-Manson relation was used to relate the
higher number of cycles to failure to the decrease in solder strain associated with the
application of undefilling. Finite Element thermo-structural simulations
confirmed the ability of underfill material to significantly lower flip chip solder joint deformation and shear strain.
However, simulations have failed to come up with a theoretical basis for
the common assumption that 'hydrostatic' compressive pressure applied by
the underfill on the solder joints plays an important role in decreasing solder fatigue in an underfilled flip-chip. The
life expectancy of an underfilled flip-chip is highly dependent on, and
may be limited by, the adhesion strength of the underfill material to
the chip and substrate. |
