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Optimal lid design parameters for reducing warpage of flip-chip package
Keywords: Flip-chip package, Lid and stiffener ring, warpage control
Flip-chip packages possess warpage at room temperature after the die attach process due to mismatch in coefficients of thermal expansion of the constituent materials. To reduce the warpage these packages are integrated with either lids or stiffener rings [1-2] which, however, consume considerable substrate space reducing the effective area available for placement of dies and passive components. With chip module size growing in modern semiconductor devices, e.g. to enable heterogeneous integration of different functional dies, there is a need to reduce the substrate area occupied by the lid while reducing the package warpage to an allowable value. However, this requires an accurate understanding of how different variables of the lid affect the warpage of the package. Hence, the goal of this work is to determine effects of the lid design parameters including (i) stiffener ring versus lid, (ii) lid foot width, (iii) lid thickness, (iv) lid material, (v) thickness of thermal interface material (TIM) connecting the die and the lid and (vi) the glue material attaching the lid to the substrate on the warpage of the package and shear stress at the interface between the lid and the TIM. We develop a three-dimensional finite element model using the commercial software, ANSYS, incorporating effects of geometric non-linearity and material addition/removal during the assembly process to predict deformations of the package. The simulations are carried out in two steps. First, the process of attaching the die to the substrate is simulated, and warpage and stresses resulting from differential thermal deformations of the components are computed. Using this deformed shape of the package and residual stresses as initial conditions, the process of attaching the lid to the package is simulated and deformations of the lidded package are computed. Results reveal that whereas the warpage of the package reduces with an increase in the width of the stiffener ring, the warpage of the lidded package is not a monotonic function of the lid foot area. There is an optimal range of the lid foot area for which the warpage is minimum. Furthermore, it is found that the effects of the TIM thickness and the lid material type on the warpage depend on the lid foot width. There exist certain values w1 and w2 of the lid foot width for which the warpage is independent of the TIM thickness and the lid material type, respectively. For a lid having the foot width, w < w1, thicker TIM leads to less warpage whereas for that with w > w1, thinner TIM leads to less warpage. Similarly, for foot widths smaller and larger than w2, stainless steel (SS) and copper lids have opposite effects on the warpage. However, for the range of lid foot width studied, the interfacial shear stress decreases with an increase in the TIM thickness and is less for the copper lid compared to the SS lid. The glue material is not found to have a significant effect on the warpage. Various results presented in this work provide useful insights into interaction among effects of lid design variables on the warpage. Outcomes of this study should provide packaging engineers with guidelines to obtain optimal lid designs that minimize the package warpage while offering noticeable saving in the effective substrate area.
Priyal Shah, Sr. Packaging Engineer
AMD, Inc.
Santa Clara, CA
United States

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