Here is the abstract you requested from the Thermal_2009 technical program page. This is the original abstract submitted by the author. Any changes to the technical content of the final manuscript published by IMAPS or the presentation that is given during the event is done by the author, not IMAPS.
|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 substrate’s 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 Si’s CTE (2.6 µm•m−1•K−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