Simulation and evaluation of thermal mechanical reliability of 3D-TSV stack with viscoelastic underfill

This paper studies how the underfill with viscoelastic properties impacts the thermal mechanical reliability of a 3D-TSV package system. The Williams-Landel-Ferry equation and the relaxation modulus function in Prony series are used to characterize the viscoelasticity that is both temperature-dependent and time-dependent. The finite element model subjected to a thermal cycle consists of a four-layer chip stack, silicon interposer, solder balls and PCB board. Silicon chips with TSVs are vertically stacked by micro-bumps of Cu-Sn. It is revealed that the chip deformation is increased while the reliability of the micro-bumps are decreased. For applications undergoing heavy thermal cycles, it is not suggested to include underfill in the 3D-TSV stack. The necessity of definition of viscoelasticity properties rather than simplified elasticity is also affirmed by comparison.