Review on Percolating and Neck-Based Underfills for Three-Dimensional Chip Stacks

Abstract

Heat dissipation from three-dimensional (3D) chip stacks can cause large thermal gradients due to the accumulation of dissipated heat and thermal interfaces from each integrated die. To reduce the overall thermal resistance and thereby the thermal gradients, this publication will provide an overview of several studies on the formation of sequential thermal underfills that result in percolation and quasi-areal thermal contacts between the filler particles in the composite material. The quasi-areal contacts are formed from nanoparticles self-assembled by capillary bridging, so-called necks. Thermal conductivities of up to 2.5 W/m K and 2.8 W/m K were demonstrated experimentally for the percolating and the neck-based underfills, respectively. This is a substantial improvement with respect to a state-of-the-art capillary thermal underfill (0.7 W/m K). Critical parameters in the formation of sequential thermal underfills will be discussed, such as the material choice and refinement, as well as the characteristics and limitations of the individual process steps. Guidelines are provided on dry versus wet filling of filler particles, the optimal bimodal nanosuspension formulation and matrix material feed, and the over-pressure cure to mitigate voids in the underfill during backfilling. Finally, the sequential filling process is successfully applied on microprocessor demonstrator modules, without any detectable sign of degradation after 1500 thermal cycles, as well as to a two-die chip stack. The morphology and performance of the novel underfills are further discussed, ranging from particle arrangements in the filler particle bed, to cracks formed in the necks. The thermal and mechanical performance is benchmarked with respect to the capillary thermal and mechanical underfills. Finally, the thermal improvements within a chip stack are discussed. An 8 - or 16-die chip stack can dissipate 46% and 65% more power with the optimized neck-based thermal underfill than with a state-of-the-artcapillary thermal underfill.

Study of non-isothermal liquid evaporation in synthetic micro-pore structures with hybrid lattice Boltzmann model

“…Evaporative drying is a phenomenon present in many scientific and engineering processes, for example, design of new materials and structures, food preserving, and production of ceramics and paper. Recently, the self-assembly of nanoparticles induced by controlled evaporation of colloidal suspensions has attracted a lot of attention as a method for the fabrication of complex materials (Brunschwiler et al 2016;Hamon et al 2012;Boles et al 2016). Zurcher et al (2016) and Städler et al (2017) have been working on nanoparticle self-assembly by non-isothermal evaporation to create underfills with high thermal conductivity applied as materials for electronic packaging.…”

Tricoupled hybrid lattice Boltzmann model for nonisothermal drying of colloidal suspensions in micropore structures

“…Drying of colloidal suspensions is an interesting phenomenon observable in everyday life, for example, in the "coffee ring" [1,2] formed after drying of a coffee drop, and utilized commonly in industry, as with ink-jet printing [3]. This phenomenon has sparked the attention of researchers for a variety of applications such as the design of unique types of computer chips with high heat removal capacity [4][5][6] and innovative functional materials with better optical, magnetic, and electronic properties [7,8]. The accurate modeling of the drying of colloidal suspensions, a complex process involving coupled heat and mass transport and phase change, is very challenging.…”

Lattice Boltzmann modeling of heat conduction enhancement by colloidal nanoparticle deposition in microporous structures

“…PTSs are fabricated in a similar way omitting the third step described above. In the experimental study of [9], PTSs and NTSs are reported to increase the heat conduction two-to threefold, compared to state-of-the-art capillary thermal structures. However, an in-depth understanding of the mechanisms of colloidal liquid drying, nanoparticle deposition in the filler particle configuration, and the influence of necks on the heat conduction of NTS is still lacking and needs to be explored systematically.…”

“…Colloidal nanoparticle deposition on intended specific surfaces is also applied as ink-jet printing [5,6], or surface coating [7,8]. Recently, nanoparticle deposition has been used to enhance vertical heat conduction in innovative three-dimensional (3D) chip stacks to remove the heat generated in computer chips more efficiently [9][10][11]. In a 3D integrated chip stack, two or more layers of active electronic components are integrated vertically, as shown in Fig.…”

“…In the work of Refs. [9][10][11], percolating thermal structures [PTSs, Fig. 1(a)] and neck-based thermal structures [NTSs, Fig.…”