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WORKING PRINCIPLE :

transporting the components to the assembly unit; a jet pump delivers the unassembled components upward into a narrow fluid channel (5 mm inner diameter tubing). Originally we installed a mechanical pump between the bottom of the chamber and the dispensing head to circulate the components, however, this lead to mechanical damage to the components and the pump. To prevent this damage, we use an indirect circulation approach coupling a mechanical pump (QV variable speed pump, Fluid metering, Inc., NY) with the customized jet pump as shown in Fig. 1(B). The jet pump requires a smaller diameter nozzle (1.48 mm2) to accelerate the carrier fluid into a desired direction. Typical velocity of carrier fluid in the upward fluid channel (20 mm2 tubing) is 25 cm/s to lift up the dense silicon components (density of 2.33 g/cm) to the dispensing head. (ii) dispensing the transported components on the substrate; the components are dispensed on top of the substrate by gravity. Gentle introduction of components to the receptor is important to reduce the effect of liquid flow which induces additional drag. The dispensing head is located 5 cm away from the polyimide web (50 μm thick, 5 cm wide) pointing not directly at the web. Since the components leave the narrow dispensing head and meet large volume of liquid, the velocity decreases and the components fall downward following a vertical path to the web by gravity. (iii) assembling the components on the substrate; the dispensed components are self-assembled on the advancing substrate. Fig. 1(C) shows the detailed attachment process. The substrates contain multiple pre-defined solder-coated receptors which are formed using a customized dip-coating of a low melting point solder on copper clad polyimide (50 μm thick, Pyralux LF series, DuPont, NC) films patterned using a microfabrication technique. Procedure to fabricate the substrate is described in the experimental section. The self-assembly process is accomplished in water at 80 °C where the solder (Indalloy #117, MP. 47 °C, Indium Corp., NY) is molten. An immersion heater is used to maintain constant temperature. The surface of the molten solder based receptor wets the gold contact of the component and the component is assembled on a stable, aligned, and electrically connected position. This assembly process is powered by minimization of the free surface area of the molten solder (400 mJ/m2 in water). Successful self-assembly process requires removal of metal oxides from the surface of the molten solder since metal oxides impede the wetting of metal contact with the solder. This removal is accomplished by adding a few drops of hydrochloric acid to the water (pH 2.0). A web moving velocity with single dispensing head is currently 10 m/hour. As the components reach at the angled web, a uniform distribution of components is required. The described process is predominantly based on gravitational sedimentation resulting sliding and tumbling of components on the angled web. In addition, the customized vibrator can agitate the web at controlled vibration conditions through up-and-down motion of the lower roller. This additional agitation helps the distribution of the components over the web and the removal of excess components at the web turning point. The operation parameters such as the angle of web, vibration amplitude and frequency will be discussed in the results section. (iv) collecting and recycling the unassembled components; the excess components drop down to the bottom of the chamber by gravity and are collected the T-section of the jet pump. After this point, the excess components are transported upward again and previous steps are repeated.