Yuji ISHINO, and Masaya TAKASAKI. Graduate School of Science and Engineering. Saitama University, JAPAN. INTRODUCTION. Recent developments in the ...
EXPERIMENTAL STUDY ON MICROASSEMBLY BY USING LIQUID SURFACE TENSION Yasuhiro KATO, Takeshi MIZUNO, Hiroshi TAKAGI, Yuji ISHINO, and Masaya TAKASAKI Graduate School of Science and Engineering Saitama University, JAPAN. INTRODUCTION Recent developments in the electronic industries have been promoting the miniaturization of electronic devices, which makes electronic components assembled on the substrate more minute and closely. In manufacturing process of small electronic devices, surface-mounting machines commonly pick up chips by using vacuum (negative pressure). In the future, surface mount devices will be more miniaturizing, which increases difficulty in picking up or manipulating objects. One of the methods of overcoming such a problem is to use liquid for the manipulation of objects. Sato et al. [1] have studied the principle and characteristics of microparts self-alignment using liquid surface tension. Obata et al. [2] have proposed micromanipulation based on capillary force. In this method, the capillary force between a spherical object and the concave probe-tip accomplishes picking it up. However, wetting the stage is necessary for placing the object. The authors have proposed a method of picking up an object by using liquid surface tension in combination with vacuum suction [3]. The feasibility of the proposed method has been demonstrated experimentally. In the experiments, however, the volume of liquid was not controlled, and only one object was treated. This paper clarifies the relationship between the formed drop size and liquid volume stored into the nozzle. In addition, the proposed method is also applied to spherical objects. PRINCIPLES OF PICKING UP The conventional method using vacuum is shown in Figure 1. The object is picked up by using negative pressure generated by vacuum and placed by breaking the vacuum. However, the misalignment between the center of the handling part of the machine causes failing in
picking the object. Figure 2 shows a schematic illustration of the picking up procedure using a drop [3]. This process is explained as follows: (1) the nozzle is positioned above a chip, (2) a drop is formed on the nozzle’s top, (3) the nozzle approaches to the chip for the drop to contact, (4) just after the contact, the chip moves on the surface of the drop to the bottom of the drop due to gravity, (5) the drop is sucked by making the pressure negative in the nozzle so that the chip is fixed on the center of the top of the nozzle. The proposed method can align the center of an object with the center of the nozzle automatically even if there is a deflection between them initially. It is called as self-centering effect [3].
Nozzle
Vacuum Break
Vacuum
Chip Stage
FIGURE 1. Schematic illustration of picking up procedure by conventional method.
(1)
(2)
(3)
(4)
(5)
Liquid
FIGURE 2. Schematic illustration of picking up procedure: (1) Positioning, (2) forming, (3) lowering, (4) picking up, and (5) sucking.
A triaxial stage has a X-Y slider and a vertical positioner. The nozzle unit is attached to the positioner for up-down movement. Objects to be picked up are placed on the X-Y slider. The microscope is used to observe the process of picking up an object and to measure the size of drops or the deflection between the center of an object and the center of the nozzle. Ultrapure water is used as liquid to avoid the ill effects of contaminations. DROP SIZE CONTROL Process of storing water into the nozzle Figure 4 shows the process of storing liquid to the nozzle. First, the nozzle approaches a tank storing water. After the nozzle touches the water surface, water is sucked into the nozzle by decreasing pressure in the nozzle for a preset time. The nozzle is raised from the water surface, and increases pressure for forming a drop on its top. The decreased pressure P and the time TS for sucking water into the nozzle are parameters determining the size of formed drops on its top. Measurement of drop sizes The sizes of formed drops on nozzle’s top were measured. Figure 5 shows the results when TS was set to be 0ms to 60ms and P=-0.5, -1.0 and -1.5 kPa (gauge pressure). In TS < 10 ms, drops with curvature larger than the nozzle’s external diameter are formed as shown in Figure 6 (a). These drops evaporated soon after forming or were blown off due to the pressure fluctuation of inside the nozzle. Thus, these drops are inadequate for picking up with self-centering effect. In 10 < TS < 20 ms, drops were formed in a hemispherical shape on the nozzle’s top as shown in Figure 6 (b). It was observed that the curvatures of the formed drops were constant independently of P or TS, but depended on the
external diameter of the nozzle’s top. The drops kept its sizes due to capillarity in spite of evaporation. In TS > 20 ms, water formed spherical drops on the nozzle’s top as shown in Figure 6 (c). As the P and TS become larger, the curvatures of these drops tend to increase in proportion. The drop was stable on the nozzle’s top, and its form was kept during picking up although the drop evaporated gradually or the pressure fluctuated. Pressure sensor Compressor Air
Triaxial -stage
Controller Ejector unit Microscope Monitor
FIGURE 3. Schematic experimental system.
illustration
Nozzle
of
the
p [kPa] T S [ms]
water -tank (a)
(b)
(c)
(d)
FIGURE 4. Schematic illustration of store/form procedure: (a) lowering, (b) sucking, (c) upping, and (d) forming by applying positive pressure. Average of drop diameter : D [mm]
EXPERIMENTAL APPARATUS Figure 3 shows an experimental system. Compressed air is passed through the ejector unit into the nozzle. The ejector unit controls pressure in the nozzle with three electronic switches. This research pay attention to the liquid volume stored in the nozzle.
0.8
p=-0.5kPa p=-1.0kPa p=-1.5kPa
0.6 0.4 0.2 0 0
10 20 30 40 50 Suction time: TS [ms]
60
FIGURE 5. Relation between the average of drop diameter and the suction time TS. The mark ◆ represents the result when the nozzle raised just after it contact water without negative pressure.
However, when the time TS was too long, a drop was sometimes formed in the side of nozzle as shown in Figure 6 (d). Two types phenomenon were observed in this case. One is that the formed drop moved to side of the nozzle, which must be caused by pressure fluctuation inside the nozzle. The other is that the drop moved during increasing its size, and then the drop was formed. This must be caused by moisture on the nozzle’s side face. These two kinds of phenomenon were often observed for large P and TS. Hemispherical drops are, therefore, used in following experiments to avoid the above problem that the drop is formed in the side of the nozzle.
The lines in Figure 7 are the maximum deflection defined in Figure 9 where a drop contacts with both the chip and the stage at the same time. From the geometrical relation it is given by:
(1)
2
Lmax =
D2 ⎛ D l ⎞ − ⎜ − h⎟ + 4 ⎝2 2 ⎠
.
The parameter l is the depth and the width of the chip in Figure 7 (a) and (b), respectively.
w
Z
t Y
X Misalignment in X -direction [mm]
The results were classified into three patterns as follows shown in Figure 8: Success: Picking up a chip with self-centering effect. Failure 1: With a large misalignment, the drop touched the chip and the stage at the same time. When nozzle was raised, the chip remained on the stage. Failure 2: The drop touched the electrode so that the chip was picked up with gradient.
FIGURE 6. Classification of drop configuration. The case of (d) often occurred when the water stored in the nozzle was more than the condition of (c).
Misalignment in Y -direction [mm]
PICKING UP WITH A HEMISPHERICAL DROP Figure 7 show the results of picking up a chip resistance whose size is 0.4 x 0.2 x 0.1 mm. It is an industrial component called as “0402”. Chips are placed in the stage with a prescribed deflection. Hemispherical drop sizes depend on the nozzle’s external diameter, so three differential nozzles were used. Misalignment was given in the X-direction in Figure 7 (a) and in the longitudinal direction (Y-direction) in Figure 7 (b).
d h
0.4
Electrode w = 0.4 mm d = 0.2 mm h = 0.1 mm t = 0.1 mm Success failure1
(a)
0.3 0.2 0.1 0 0.25
0.30
0.35
0.40
0.45
0.50
Diameter of water drop : D [mm]
0.5
Success Failure1 Failure2 (b)
0.4 0.3 0.2 0.1 0 0.25
0.30 0.35 0.40 0.45 Diameter of water drop : D [mm]
0.50
FIGURE 7. Relation between the diameter of the hemispherical drop and the misalignment for picking up a chip shown above: (a) X-direction; (b) Y-direction.
Misalignment [mm]
0.6 0.5
Success Failure
0.4 0.3 0.2 0.1 0
Failure 1
Success
Failure 2
FIGURE 8. Classification of results for picking up a chip resistance.
0.3 0.4 0.5 0.6 Diameter of spherical object : d [mm]
FIGURE 10. Relation between the sizes of spheres and misalignment.
From the geometrical relation, it is given by:
h
d D:
Lmax =
1 (d + D ), 2
(2)
where d: diameter of the sphere, D: diameter of the drop. A sphere can be successfully picked up with misalignment in less than 95% of Lmax.
l 2
L max
FIGURE 9. Definition of maximum deflection Lmax.
Roughly speaking, a chip can be successfully picked up with misalignment less than 65% of Lmax. Failure 2 was not observed for X-direction misalignment. Because the drop does not touch the electrode part of the chip. PICKING UP SPHERICAL OBJECTS Four kinds of zirconium balls whose diameter are 0.3, 0.4, 0.5 and 0.6 mm were used as objects to be picked up. They are placed in the stage with a prescribed deflection. Figure 10 shows the results of picking up a ball with one of three nozzles. It was observed that the formed drop curvature was almost equal to the external diameter of the nozzle. The results were classified into two patterns that had been observed by the experiment to the chip, Success and Failure 1. The line in Figure 10 is the maximum deflection where a drop contacts with both the ball and the stage at the same time.
CONCLUSION Microassembly using water drop was studied experimentally. The relation between the drop sizes and the water volume stored inside the nozzle was examined. The conditions in which the formed drops become hemispherical have been clarified. In picking up with a hemispherical drop, misalignment allowable for successful picking up was 65% of the geometrical limit Lmax in the case of the chip resistance “0402”, and 95% of Lmax in the case of spherical object. REFERENCES [1] Sato K, Seki T, Hata S, Shimokohbe A. Principle and Characteristics of Microparts Self-alignment using Liquid Surface Tension. Journal of the Japan Society of Precision Engineering 2000; 2: 282-286. [2] Obata K, Motokado T, Saito S, Takahashi K. A scheme for micro-manipulation based on capillary force. J. Fluid Mech. 2004; 498, 113-121. [3] Takagi H, Takasaki M, Ishino Y, Mizuno T. Microassembly using Surface Tension. Dynamics & Design Conference 2007; 424.
