ABSTRACT
 
 

Droplet-Based Manufacturing (DBM) relies upon the ability to accurately deposit droplets in a particular pattern in order to produce a desired part. Important to such a process is the ability to control the uniformity of the droplets and an understanding of the spreading and solidification behavior of a droplet as it impacts a substrate.

In this project, the spreading behavior of molten tin droplets was investigated using a microsensor developed at MIT (Kim et al, 1996). The sensor consists of a fine array of resistive lines (Au) on top of a non-conductive silicon base. As the metal droplet spreads across the resistive lines, the sensor resistance is measured. This changing resistance is then correlated to the droplet diameter. In order to determine the diameter, the resistance of a single line must be known. This value was experimentally determined to be 2.6 kW .

Previous experimental work with the sensor had shown that external electrical noise could present a problem by falsely triggering a response. Therefore, a new experimental apparatus for positioning the sensor was designed and fabricated in order to reduce the problems due to noise, and to test the sensor's ability to produce repeatable results. The new apparatus was also designed to allow for testing the spreading behavior response at elevated substrate sensor temperatures.

Molten tin droplets of approximately 400-m m diameter were deposited on the microsensor. The problems associated with noise were reduced, and the sensor produced consistent results for repeated trials under similar conditions. For deposits onto a low temperature sensor (27° C), data reflecting an initial droplet spreading region, and a region of droplet contact area retraction were observed. The average initial droplet spreading velocity was measured to be 5.7 times the impact velocity. This value is comparable to the value of 2.4, determined experimentally in previous work (Kim, 1996). The average maximum diameter recorded during spreading was 815 m m and was achieved in approximately 150 m s. After approximately 800 m s the recorded contact diameter reduced to an average of 337 m m. The experiment was then repeated with a sensor maintained at 100° C. The droplet reached its final diameter in 100 m s and showed no signs of contact area retraction. Micrographs taken of the underside of the droplets and the sensor surfaces support the sensor responses. The results are very encouraging, as they support the sensor's ability to measure droplet spreading behavior as well as changes due to elevating the substrate temperature.
 
 
 

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