Research Papers

Hybrid Nanocomposite Thermal Interface Materials: The Thermal Conductivity and the Packing Density

[+] Author and Article Information
Tingting Zhang, Bahgat G. Sammakia

Department of Mechanical Engineering,
Institute for Materials Research,
Binghamton University,
State University of New York,
Binghamton, NY 13902

Zhihao Yang

School of Materials Science and
Energy Engineering,
Foshan University,
Foshan 528000, Guangdong, China

Howard Wang

Department of Mechanical Engineering,
Institute for Materials Research,
Binghamton University,
State University of New York,
Binghamton, NY 13902;
Department of Materials
Science and Engineering,
University of Maryland,
College Park, MD 20742
e-mails: wangh@umd.edu; pvtech@qq.com

1Corresponding authors.

Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received October 15, 2017; final manuscript received April 9, 2018; published online June 11, 2018. Assoc. Editor: Eric Wong.

J. Electron. Packag 140(3), 031006 (Jun 11, 2018) (8 pages) Paper No: EP-17-1110; doi: 10.1115/1.4040204 History: Received October 15, 2017; Revised April 09, 2018

We have investigated a novel hybrid nanocomposite thermal interface material (TIM) that consists of silver nanoparticles (AgNPs), silver nanoflakes (AgNFs), and copper microparticles (CuMPs). Continuous metallic network form while AgNPs and AgNFs fuse to join bigger CuMPs upon hot compression, resulting in superior thermal and mechanical performances. The assembly temperature is as low as 125 °C due to the size effect of silver nanoparticulates. The thermal conductivity, k, of the hybrid nanocomposite TIMs is found to be in the range of 15–140 W/mK, exceeding best-performing commercial thermal greases, while comparable to high-end solder TIMs. The dependence of k on the solid packing density and the volume fraction of voids is discussed through comparing to model predictions.

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Grahic Jump Location
Fig. 1

SEM images of hybrid TIM pastes sintered at 200 °C for 20 min of CuMPs and AgNPs with a mass ratio of 7:3 (a) and (b), of CuMPs and AgNFs with a mass ratio of 6:4 (c) and (d), of CuMPs, AgNFs, and AgNPs with a mass ratio of 6:2:2 + 2% P4VP by weight (e) and (f). Figures (b), (d), and (f) are the magnified images responding to (a), (c), and (f), showing the contiguous networks connected through fused AgNPs and AgNFs.

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Fig. 2

SEM and EDX analysis of sintered TIMs consist of CuMPs, AgNFs, and AgNPs with a mass ratio of 6:2:2. The EDX data show that copper is dominant at the big particles, and silver is dominant at the interval of big particles, which helps to confirm the distribution of metal particles within paste.

Grahic Jump Location
Fig. 3

XRD patterns for a sintered TIM paste consist of CuMPs and AgNPs with a mass ratio of 7:3, and sintered at 200 °C for 20 min. The TIM spectrum composes of both neat Cu and Ag peaks, and there is no appearance of new peaks, implying no formation of other significant phases.

Grahic Jump Location
Fig. 4

The thermal resistance as a function of the thickness of the TIMs for 3 μm CuMPs/AgNFs = 6:4 by weight (open triangle symbol) and for 3 μm CuMPs/AgNFs/AgNPs = 6:2:2, with 2% P4VP by weight (solid round symbol). The symbols are experimental data and both lines through the symbols are the best linear fitting, yielding the intercept as the contact resistance of both interfaces with the substrates, while the inverse of the slope is the bulk k.

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Fig. 5

The mass density of TIMs as a function of the volume fraction of fillers and voids. The symbols are experimental data, and lines through symbols are the best linear fitting. The packing mass density increase linearly with the filler volume fraction, while inversely linear with the void volume fraction.

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Fig. 6

Thermal conductivity as a function of void volume fraction of TIMs. The symbols are experimental data, and the curve through the symbol the best power-law fit with power exponent of −1.5. Data show the inverse correlation of thermal conductivity with the void volume fraction.

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Fig. 7

Comparison of three analytical models and experimental data, where the symbols are experimental data from this study, and curves are model predictions. Three models using the thermal conductivity values of bulk materials yield similar prediction for low volume filling, i.e., at vf < 0.2, above which, their differences become prominent. BSM overestimates the k, while both M-G model and BAM models underestimate the effective k of the TIMs, even assuming Rb = 0 (α = 0) for no interfacial contribution. The shadow centered at the dotted curve is modified BSM calculation using effective bulk kf ∼ 100 − 220 W/mK for best match with the experimental data.

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Fig. 8

Comparative mapping of thermal properties (k and Rc) of various TIMs. The novel hybrid TIMs in this study is show in the upper-right corner, with k's comparable to those of high-end solder TIMs, and Rc's as low as those of the best thermal greases.



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