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Research Papers

# Studies on Size and Lubricant Effects for Fluidic Self-Assembly of Microparts on Patterned Substrate Using Capillary Effect

[+] Author and Article Information
Cheng Lin, Fangang Tseng

Department of Engineering and System Science, National Tsing Hua University, 101, Sec. 2, Kuang Fu Rd., Hsinchu, Taiwan 30043

Ching-Chang Chieng1

Department of Engineering and System Science, National Tsing Hua University, 101, Sec. 2, Kuang Fu Rd., Hsinchu, Taiwan 30043

1

e-mail: cchieng@ess.nthu.edu.tw

J. Electron. Packag 130(2), 021005 (Apr 25, 2008) (9 pages) doi:10.1115/1.2912216 History: Received July 04, 2007; Revised October 03, 2007; Published April 25, 2008

## Abstract

Conventional pick-and-place technology platform in handling microscale component assembly processes has technical limitations in terms of capacity, efficiency, and accuracy. The fluidic self-assembly (FSA) approach employs a lubricant fluid carrying micropart flows over a target wafer patterned with binding sites, which results in part-substrate attachment. This technique transports microparts from one location to another with orientation control and parallel sorting. The present study demonstrates a FSA approach for fast, economic, and precise handling of microscale parts with square (few are in rectangular) shapes. The microparts fabricated from silicon-oxide wafers and ranging in size from $350×350×170μm3to1000×1000×440μm3$ aligned and filled to designated sites in the substrate under water. The effects of micropart sizes and lubricants on the FSA processes are compared. This study provides a fundamental analysis for achieving and optimizing the self-alignment. The polymer or solder adhesion force of the square-patterned micropart immobilized at the larger binding sites were estimated to be $117±15μN$ and $510±50μN$, respectively, which results in higher assembly yield of up to 100% for these samples.

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## Figures

Figure 1

Model system and experimental setup of fluidic microassembly: (a) schematic diagram of the FSA model system, (b) photograph of the FSA experimental system, (c) top view of the FSA experimental system, and (d) side view of the FSA experimental system

Figure 2

Flowchart for conducting FSA as a polymer lubricant is applied: (a) fabrication of the substrate with an array of the binding sites, (b) fabrication of silicon square microparts with Au binding sites, (c) immersion of the substrate and all microparts in the SAM solution, (d) immersion of the substrate with lubricant on the binding sites in water, and (e) conducting FSA

Figure 3

Self-assembly procedure using a low melting alloy: (a) the substrate is dipped in liquid alloy and lifted through dilute HCl, and (b) the parts are directed toward the coated substrate in the hot, dilute HCl

Figure 4

Photographs of the low melting alloy coating square gold sites on a side on a glass substrate: (a) top view (L=1000μm), (b) side view (L=1000μm), and (c) L=1000μm, 500μm, and 350μm squared sizes with solder formed

Figure 5

Experimental setup of the blowing test: (a) syringe pump can provide for controlled flow rate, and (b) side and top views of a water flow were imposed at a micropart by a syringe needle. The flow rate at which detachment of the self-assembled micropart was measured and rough estimated of the drag force can be calculated using Eq. 1.

Figure 6

Photograph of microparts on substrate binding sites after FSA by polymer lubricant for micropart sizes of (a) 1000×1000×440μm3 and (b) 1000×1000×170μm3. Misalignment includes (1) orientational, (2) translationally misaligned, and (3) 2×1 (two parts occupying one site).

Figure 7

Square-silicon microparts on the substrate binding sites after FSA by polymer lubricant for micropart sizes of side lengths L (a) 1000μm, (b) 500μm, (c) 350μm and thickness of 170μm, and (d) filling ratios and precision ratios for these micropart

Figure 8

Adhesion force measurement for microparts of square shape and thickness of 170μm in different side lengths and the comparison of gravitational forces and surface tension using the 2D model simulation

Figure 9

Computed overlap ratio between a moving part and a binding site, for square and rectangular microparts

Figure 10

Solder height (left scale) and adhesion force measurement (right scale) for various sized microparts of square shape with thickness of T=170μm

Figure 11

Photograph of microparts with different thicknesses after FSA by solder: (a) micropart size=1000×1000×440μm3, (b) micropart size=1000×1000×170μm3, and (c) filling ratios and precision ratios for different thicknesses of square microparts

Figure 12

Self-alignments of microparts with thickness (T=170μm) achieved at time t2 through translation and rotation for side length of square microparts of L=1000μm for various initial offset positions at time t0. (a) and (b) illustrate the (a) translation motion and (b) rotation motion, respectively. Microparts are at offset positions at time t0=0s (left photos) and are self-aligned (right photos) in the directions indicated by white colored arrows at t2. (c) illustrates histories of recovery angle versus recovery time for rotational alignment for different sized microparts.

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