by

Tasos Karahalios

B.S. Economics

Massachusetts Institute of Technology, 1998

Submitted to the Department of

Mechanical Engineering in Partial Fulfillment of the Requirements

for the Degree of

Bachelor of Science in Mechanical Engineering

at the

Massachusetts Institute of Technology

June 1999

©1999 Massachusetts Institute of Technology

All rights reserved

Signature of Author................................................................................................................

Department of Mechanical Engineering

May 7,1999

Certified by............................................................................................................................

Dr. Jung-Hoon Chun

Associate Professor of Mechanical Engineering

Thesis Supervisor

Accepted by...........................................................................................................................

Professor Ernest G. Cravalho

Professor of Mechanical Engineering

Chairman of the Undergraduate Thesis Committee

by

Tasos Karahalios

Submitted to the Department of Mechanical Engineering

On May 7, 1999, in partial fulfillment of the

Requirements for the degree of

Bachelor of Science in Mechanical Engineering

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.

Thesis Supervisor: Dr. Jung-Hoon Chun

Title: Associate Professor of Mechanical Engineering

I would first like to thank Professor Jung-Hoon Chun for his advice and guidance from the moment I was given the opportunity to work as a UROP student, up until the completion of my thesis project. I would also like to thank Professor Taiqing Qiu for taking the time to answer many of my questions.

It has been fun working with all the students in the DBM lab. They are a great group of people to work with because they are all willing to take the time to help whenever there is problem. I would like to in particular thank Ho-Young Kim for all his help with this project. I was fortunate to have been given the benefit of his experience working with the sensor, which saved me a great amount of time and aggravation. I would also like to thank Jeanie Cherng for staying up late and helping me run the experiments, and Juan-Carlos Rocha for answering whatever happened to be my question of the day.

Lastly I would like to thank my parents George and Maria Karahalios and my sister Karrie. Whenever I have a problem, I know they are there for me.

Title Page 1

Abstract 2

Acknowledgments 3

Table of Contents 4

List of Figures 5

List of Tables 6

1 INTRODUCTION 7

1.1 Motivation and Background 7

1.2 Objective of the Investigation 9

1.3 Overview of the Thesis 10

2 SPREADING SENSOR 11

2.1 Sensor Development Background 11

2.2 Sensor Line Resistance 13

2.3.2 Heat Transfer Effect 18

3 EXPERIMENTAL APPARATUS 21

3.2 Sensor Substrate Fixture 22

3.3 Data Acquisition 23

4 DROPLET SPREADING EXPERIMENTS 25

4.1 Sensor Reliability 25

4.1.2 Droplet Diameter Response 27

1.1 Diagram of Uniform Droplet Spray Apparatus 9

2.1 Droplet-Sensor Interaction 12

2.2 Droplet Dimensions and Composition 12

2.3 Typical Sensor Defect 13

2.4 Sensor Defect Resistor Model 14

2.5 Sensor Resistance Temperature Response 17

2.6 Heat Transfer Model 18

3.1 Droplet Collection 21

3.2. Substrate Sensor Fixture 22

3.3 Data Acquisition Set-up 24

4.1 Sensor Voltage Response 26

4.2 Diameter Spreading Response 27

4.3 Top View of Sensor 1 after Droplet Acquisition 29

4.4 Top View of Droplet Deposited on Sensor 1. 29

4.5 SEM Micrograph of Bottom View of Droplet Deposited on Sensor 1. 29

4.6 Heated Sensor Response 31

4.7 Heated Sensor Diameter Response 32

4.8 Top View of Heated Sensor after Droplet Deposit 34

4.9 Top View of Droplet on Heated Sensor 34

4.10 Heated Sensor Surface after Droplet Removal 34

4.11 Room Temperature Sensor after Droplet Removal 34

2.1 Experimental Line Resistance Estimates 15

4.1 Initial Spray Conditions 25

4.2 Impact Conditions 25

4.3 Heated Sensor Initial Spray Conditions 30

4.4 Heated Sensor Impact Conditions 30

4.5 Initial Spreading Velocities 35

This chapter provides an introduction to the project, its objectives, and a brief look at the motivation and background information concerning the UDS process and the spreading sensor. Chapter 2 will provide more extensive information concerning the spreading sensor. It will also describe the steps taken to characterize the sensor in terms of determining its individual line resistance, and its temperature resistance dependence. Chapter 3 will focus on the procedure and apparatus used to perform the experiments. Chapter 4 will present and discuss the experimental results. Finally a summary of the thesis will be presented in Chapter 5.

Figure 2.1: Droplet-Sensor Interaction

Figure 2.2: Sensor Dimensions and Composition

where r is the resistivity, l is the length of an individual line, and A is the cross sectional area of an individual sensing line. For the purposes of this study, two approaches were used to determine this value experimentally.

The first approach relied on the fact that under high-powered (50x) magnification, defects due to the sensor fabrication process became apparent. Although all the sensing lines were intended to be isolated from each other, in actuality, due to fabrication defects, a small number of them were connected. A picture of a typical defect was recorded in Figure 2.3 using a Scanning Electron Microscope.

Figure 2.4: Sensor Defect Resistor Model

where N is the number of defects on the sensor.

Table 2.1: Experimental Line Resistance Estimates

From the results, the individual line resistance was determined to be 2.6 kW . In order to verify this value, a second method was also used to estimate the line resistance. A wafer, four inches in diameter, was prepared using the same materials and in the same manner as was used to produce the sensors. Using a four-point probe, a line resistance estimate equal to 2.7 kW was obtained (Qiu, 1999).

Equation 1 can be solved in terms of the droplet diameter to show that the diameter is directly dependent on the line resistance. Thus any error in the line resistance measurement produces a directly proportional error in the diameter measurement. Fortunately, both methods used for measuring the line resistance produced values that varied by less than 4%. For this analysis, the original value of 2.6 kW was used as the line resistance.

Figure 2.5: Sensor Resistance Temperature Response

Figure 2.6: Heat Transfer Model

Figure 2.6 shows the case of a droplet at an elevated temperature, T_d, in contact with a pair of connected sensing lines. For an upper bound estimate, the region of the sensing line beneath the droplet, B, is assumed to be at T_d as well. The rest of the sensor line, A, is assumed to be at its original temperature, T_o. Therefore the sensing line can be modeled as two temperature dependant resistors in series whose values are labeled accordingly in Figure 2.6 as R_B(T_d) and R_A(T_o). The original resistance of the sensing line, R_l, and the resistance including the heat transfer effect from the droplet, R, are described as:

From equations 4a and 4b, an expression for the percent change in the resistance due to the heat transfer from the droplet can be shown as:

The next ratio to consider, shown in Equation 7, is that of the resistance of the heated region of the sensor line to the original sensor line resistance. This ratio will scale as the ratio of the final droplet diameter, D_f, to the overall sensor line length, L_o.

Now using Equations 5,6 and 7 the final expression, describing the percentage change in the measured resistance due to the heat transfer from the molten droplet to the sensor, is shown as:

Figure 3.1: Droplet Collection

A mask made of thin sheet metal covers the substrate fixture. The mask has a 1-mm diameter hole positioned directly above the sensor. In addition to the mask shown in Figure 3.2, it was discovered that aluminum foil covering the sides of the fixture was necessary to prevent stray droplets from interfering with the sensor. When rotating the fixture through the droplet generator spray, a single droplet is desired to pass through the mask. For a given flight distance, the charging voltage on the UDS generator is adjusted so that the edge of the spray cone barely reaches the position of the hole on the mask. This procedure greatly increases the probability of only a single droplet reaching the sensor.

As shown in Figure 3.2, two sensors were placed in the substrate in order to run experiments. The sensor positioned under the hole has two copper leads attached to it. These leads are supported by a ceramic spacer to insulate them from each other and to allow for the sensor resistance to be measured. A second sacrificial sensor is positioned next to the first sensor. This second sensor is attached to a thermocouple and is intended only to monitor the temperature of the sensor surface. As previously mentioned, since the sensors are all of the same material and thickness it is assumed that the temperature of both sensor surfaces will be the same.

Figure 3.3: Data Acquisition Set-Up

In order to set the acquisition rate and duration of the oscilloscope, a characteristic spreading time, t_s, was used based on the droplet diameter, D, and the impact droplet velocity, V_i .

Table 4.1: Initial Spray Conditions

Next, the droplet temperature, velocity, and diameter at impact were determined by numerical simulation (Passow, 1992). These models were verified by Abel (1993), Chen (1996), and Kim (1996). The results of the simulations are shown in Table 4.2.

Table 4.2: Impact Conditions

The droplet spreading diameter response can be calculated using the voltage response according to Equation 10. This equation is a rewritten form of Equation 1.

Figure 4.2: Diameter Spreading Response

Figure 4.2 shows that the droplets reach their maximum diameter within the first

200 m s of spreading. The droplets then seem to undergo a contact diameter reduction for approximately 600 m s. Once again, as mentioned earlier Sensor 1 data was only recorded for the first 450 m s, but a final value was also recorded after a few seconds and is shown in the figure. The mechanism involved in the diameter reduction may be a combination of the droplet retracting due to surface tension effects, as well as contact area reduction due to a peel-back effect at the outer regions of the droplet as it cools.

To further investigate the performance of the sensor, photographs of: Sensor 1 after droplet acquisition, the droplet surface, and the droplet underside were taken. They are included as Figures 4.3,4.4,and 4.5, respectively. The maximum splat diameter in Figure 4.4 was determined to be 942 m m using a Nikon Measurescope. In Figure 4.5, an imprint of the sensing lines from the sensor can be seen on the droplet underside. An approximate diameter, referring to the area where these sensing lines are concentrated, has been marked on the figure. This diameter is approximated as 400 m m. The fact that the sensing line imprints appear in this region suggests that this region of the droplet was in contact with the sensor surface as it solidified. The outer regions of the droplet in Figure 4.5 do not show the sensing line imprints, suggesting that this part of the droplet may not have been in contact with the sensor as it solidified. Comparing the diameter of the sensing line region to the final diameter of approximately 290 m m recorded for Sensor 1 in Figure 4.2, they appear to correlate reasonably well.

Table 4.3: Heated Sensor Initial Conditions

Table 4.4: Heated Sensor Droplet Impact Conditions

Figure 4.6: Heated Sensor Response

Figure 4.7: Heated Sensor Diameter Spreading Response

Two things are immediately obvious from the previous plot. The first is that the droplet reaches its maximum diameter slightly faster than in the previous experiments, but it does not exhibit any signs of diameter retraction. Possible explanations for this phenomenon are that at the higher sensor temperature, either the wettability between the droplet and the sensor increases, or the increase in the contact area is due to the higher sensor temperature allowing the droplet to solidify at a slower rate thereby improving its adhesion.

The second issue with the response is that it predicts a maximum diameter of approximately 500 m m. This value is significantly smaller than the maximum diameters predicted in the previous experiments. The measured value of the diameter was determined to be 782 m m with a Nikon Measurescope. A possibility for this lower diameter could be due to the laminar jet of the UDS system being interrupted temporarily causing a droplet of a different size than expected to strike the sensor. Additional experiments should be performed to address this issue.

Pictures taken of the sensor after droplet deposition and of the droplet surface are included as Figures 4.8 and 4.9, respectively. Attempts were made to remove the droplet in order to photograph its underside, but those attempts proved to be unsuccessful due to destruction of the splats as a result of the large amount of force required to remove them from the sensor. In comparison, splats made with sensors which were at lower temperatures were extremely easy to remove. These splats were touched with double sided tape and they readily adhered. The droplets attached to the heated sensor required scraping with a razorblade, which unfortunately did not leave them in any condition to photograph. However, photographs were taken of the heated sensor surface after the droplet was forcibly removed. This photograph is shown in Figure 4.10 and can be compared to Figure 4.11. Figure 4.11 shows the surface beneath the splat deposited on a cooler (27° C) sensor.

Figure 4.10: Heated Sensor Surface after Figure 4.11: Room Temperature Sensor

In this study the performance of a previously developed spreading sensor (Kim, 1996) was investigated. First, the value of the line resistance of the sensor was experimentally determined to be 2.6 kW . A linear relationship between the sensor resistance and its temperature was also experimentally determined. This relation was used to derive an expression and to produce an estimate of approximately 6% error due to heat transfer effects. This error due to heat transfer would have the effect of increasing the recorded spreading diameter by approximately 6% as well. Also, an average value of 5.7 was determined for the ratio of the spreading velocity to the impact velocity for the experiments performed.

A new fixture, capable of controlling the sensor temperature while positioning it to acquire a single droplet, was designed. This fixture was used to test the reliability and the repeatability of the sensor. A series of three experiments with similar initial conditions were performed. The results of these three experiments were very similar, supporting the sensor's ability to record spreading behavior. From the results, numerous values can be calculated. These values include the time required to achieve maximum diameter spreading, the time associated with contact area retraction, and the spreading velocity.

The sensor's ability to record spreading behavior changes due to an elevated substrate temperature was also investigated. The results were significantly different from those performed at lower sensor temperatures. For the heated sensor case, the results showed that the droplet reached and maintained a maximum diameter until it solidified without signs of contact area retraction. Pictures were taken of the droplet undersides and sensor surfaces for both the heated and unheated experiments and were used to compare the sensor responses. Future work could now be performed with droplets of a different diameter, temperature, and material, on sensors over a wide range of substrate (sensor) temperatures.

Abel, G. K., 1993, "Characterization of Droplet Flight Path and Mass Flux in Droplet

Chen, C.-A., 1996, "Droplet Solidification and Its Effect on Deposit Microstructure in the

Kim, Ho-Young, 1996, "Microsensor Development for the Study of Droplet Spreading,"

Passow, C. H., 1992, "A Study of Spray Forming Using Uniform Droplet Sprays," S.M.

Private conversation Taiqing Qiu, Assistant Professor of Mechanical Engineering at

Rayleigh, Lord., 1878, "On the Instability of Jets," Proceedings of the London