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Research Papers

The Effects of Wetting and Surface Roughness on Liquid Metal Droplet Bouncing

[+] Author and Article Information
Wen-Kai Hsiao, Jung-Hoon Chun, Nannaji Saka

Laboratory for Manufacturing and Productivity, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge MA 02139

J. Manuf. Sci. Eng 131(2), 021010 (Mar 27, 2009) (8 pages) doi:10.1115/1.3090884 History: Received November 27, 2007; Revised January 05, 2009; Published March 27, 2009

In droplet-based manufacturing processes, such as dropwise rapid prototyping, solder bumping, and spray forming, the quality of the deposit is adversely affected by bouncing of liquid droplets off the target surfaces. This study investigates the effects of wetting and surface roughness on the bouncing phenomenon. An analytical model, based on the conservation of energy during deposition, was developed to correlate wetting and surface roughness to a dimensionless droplet bouncing potential. In addition, experiments were conducted to image the deposition behavior of Sn-37wt% Pb solder droplets, averaging 280μm in diameter, on prepared substrates with a wide range of wetting properties and roughness levels. The high-speed image data correlate well with the model prediction that droplets are likely to bounce as a target surface becomes less wetting or is roughened.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

Stages of droplet deposition with schematic for bouncing potential formulation

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Figure 2

Contact angle dynamics during deposition

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Figure 3

Effect of contact angle on bouncing potential

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Figure 4

Void formation at the interfaces between solder splats and Au-plated surfaces

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Figure 5

Surface roughness model schematics

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Figure 6

Force balance of liquid penetrating into a notch

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Figure 7

Effect of notch angle on liquid penetration into surface notches

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Figure 8

Effect of Fa on bouncing potential

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Figure 9

Schematic of the experimental setup

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Figure 10

(a) Droplet exhibiting sticking behavior on a Pd-plated surface (Sn-37 wt % Pb solder, di=287 μm, ui=3.6 m s−1), (b) droplet exhibiting partial bouncing behavior on a Rh-plated surface (Sn-37 wt % Pb solder, di=287 μm, and ui=3.9 m s−1), (c) droplet exhibiting bouncing behavior on a Sn (oxide) surface (Sn-37 wt % Pb solder, di=283 μm, and ui=3.8 m s−1), (d) droplet exhibiting sticking behavior on an Au-plated, 27 μmAl2O3 sandblasted surface (Sn-37 wt % Pb solder, di=273 μm, and ui=3.9 m s−1), and (e) droplet exhibiting necking behavior on an Au-plated, 180 μmAl2O3 sandblasted surface (Sn-37 wt % Pb solder, di=280 μm, and ui=4 m s−1)

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Figure 11

Transition from sticking to bouncing: smooth and rough surfaces

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