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Thermomechanical Optimization of a Novel High Density Interconnect (HDI) Substrate
Keywords: High density MCM, Hybrid silicon packaging, Chip scale packaging
We examine the thermomechanical tradeoffs of a novel design for high density interconnect (HDI) substrates. Fabricated from silicon (Si) wafers with planar cavities of highly-filled composite encapsulant, the technology leverages established Si photolithography but offers improved mechanical properties. However, the correct balance and layout of encapsulant regions across the Si wafer to minimize thermomechanical stresses while increasing the substrates mechanical strength and flexibility have not been established. We analyze these effects with finite element (FE) modeling and experimental demonstration. Filling the encapsulant 90% by weight with silicon dioxide (SiO2) particles dramatically lowers its coefficient of thermal expansion (CTE) but matching Sis CTE (2.6 mm−1K−1) is not possible. Instead, the cavities are distributed across the wafer such that the thermomechanical stress never exceeds the fracture strength of either material. Fillets in the cavity corners and Si ribs can also reduce the overall and maximum stress in some designs. The cavity designs are represented by FE models of a unit cell on the wafer a parameterized silicon frame with encapsulant. Experimentally measured fracture strength and modulus of each material serves as FE model input and validation. We further generalize the study to a range of candidate CTE encapsulants, nondimensionalize geometric variables such as fillet radius and cavity edge length, and examine the validity of the results for varying substrate thicknesses. The unit cell is also modified to include buried Si die to highlight their role in stress relief. Finally, a larger model of a heterogeneous array of cells captures the consequence of non-uniform expansion across the wafer. These reduced-order studies serve to establish the design rules for hybrid Si-encapsulant substrates.
Brian Smith, Senior Member of the Technical Staff
Draper Laboratory
Cambridge, MA

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