Combined Experimental-Numerical Investigation of Microstructure and Thermal Conduction in Squeezed Thermal Interface Materials

Rajath Kantharaj, Purdue University

Abstract

Thermal management of electronics is one of the biggest engineering challenges of this decade, as billions of transistors are put in each microprocessor and the increasing density leads to increased temperatures. Advances in transistor technology enable fabrication of transistors with dimensions on the order of 1 nm. Due to the enormous number of transistors, a higher frequency of operations is achieved. This directly translates to more Joule heating and microprocessor chip power density exceeding 100 W cm−2 . In addition, hotspots with heat fluxes in excess of 1 kW cm−2locally increase the temperature leading to non-uniform chip temperatures. If the heat is not dissipated efficiently, the internal temperature of the chip rises, thereby reducing the efficiency and decreasing the lifetime. At the package level, various cooling technologies have been developed — air and liquid cooling, heat pipes, and vapor chambers — to effectively dissipate the heat. But interfaces between the different components of an electronics packaging arise during its assembly due to surface imperfections. These interfaces impede heat conduction from the chip to the heat sink leading to high temperatures. To overcome this issue, thermal interface materials (TIMs) have been developed. The main goal of a TIM is to provide high effective thermal conductivity and minimize contact thermal resistance at a minimal thickness of the material after application. TIMs generally consist of high thermal conductivity filler particles (e.g., ceramic, metallic, or carbon-based) in a polymer matrix that provides the mechanical conformability in a packaged electronic device. They are generally applied inside the package at the microprocessor chip-metallic lid (or integrated heat spreader) interface or outside the package at the metallic lid-heat sink interface. On an industrial assembly line, the TIM is generally dispensed over the substrate at a controlled flow rate and/or quantity and then squeezed to a final pressure to form the desired bond line thickness. The effective conductivity of the TIM itself depends on the filler particle size, morphology, and particle organization i.e., when the particles form chains or networks of high conductivity pathways across the polymer matrix, the TIM can efficiently conduct heat in the “percolation” regime. There are only a handful of published works that discuss the TIM assembly process-induced modifications of the particle networks within the material and there is a lack of quantitative understanding of particle rearrangements in the TIM induced by its assembly process, and the effect on its thermal performance during operation. The objective of this thesis is to fundamentally understand the impact of the particle redistribution within the TIM during squeezing and its impact on the thermal conductivity using combined experimental and computational approaches. Automated procedures for dispensing and constant velocity squeezing are developed in this work to investigate constant velocity squeezing of isolated line dispense patterns of TIMs consisting of large spherical filler particles. In both the dispensed and squeezed/cured states, three-dimensional (3D) X-ray micro computed tomography (XRCT) identifies individual particle diameters and locations and quantifies metrics of the TIM microstructure such as the bulk and local particle volume fraction, coordination number, and radial distribution function (RDF).

Degree

Ph.D.

Advisors

Marconnet, Purdue University.

Subject Area

Materials science|Polymer chemistry|Thermodynamics|Water Resources Management

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