WO2018027678A1

WO2018027678A1 - Accelerating the breakup of fluid bridges for micro-dispensing applications

Info

Publication number: WO2018027678A1
Application number: PCT/CN2016/094405
Authority: WO
Inventors: Ho Cheung Shum; San To CHAN; Hammad Ali FAIZI
Original assignee: The University Of Hong Kong
Priority date: 2016-08-10
Filing date: 2016-08-10
Publication date: 2018-02-15

Abstract

Conductive viscoelastic adhesives are often used to bond silicon dies and printed circuit boards (PCB) together. The deposit of these adhesives on substrates through nozzles may lead to liquid bridges whose breakup causes stringing and the generation of satellite droplets resulting in the contamination of the adjacent droplets. Also the time to break up the bridges may be excessive. In order to overcome these problems the nozzle is rotated and the aspect ratio of the bridges is kept small.

Description

ACCELERATING THE BREAKUP OF FLUID BRIDGES FOR MICRO-DISPENSING APPLICATIONS

Field of the invention:

The current invention generally relates to a method for breaking up complex fluid bridges in micro-dispensing applications for electronic packaging, 3D printing and contemporary food engineering. More specifically, the method accelerates the breakup process of stabilized liquid bridges and alleviates or eliminates the tailing and satellite generation issues. In a nutshell, this method can lead to a fine quality dispensing of viscous and/or non-Newtonian fluids with high throughput for industrial applications.

Background of the invention:

Unlike a water bridge which breaks up rapidly, liquid bridges of viscous and/or non-Newtonian fluids can break up slowly with the bead on a string structure (BOAS) . This structure leads to the formation of even smaller droplets, i.e., satellite droplets that can contaminate the substrates on which the material is dispensed as a drop or dot. In actual micro-dispensing applications, undesired phenomena like tailing and satellite droplets can be observed. These issues pose a serious threat to precision dispensing patterns by contaminating them.

With the advancement of rheology and materials science, functional fluids such as silver epoxy have been widely used in the electronic packaging industry. However, compared to Newtonian fluids like water, they exhibit different rheological behaviors such as viscoelasticity and shear-dependent viscosity. The contact-based dispensing methods, which are used to eject a certain volume of fluid by contact between the dispensing nozzle and the substrate, are no longer effective due to the tailing problem See, W.M. Jones &L.J. Rees, “The Stringiness of Dilute Polymer Solutions, ” J. Non-Newtonian. Fluid Mech., 11, 257 (1982) and C.J.S. Petrie, “Elongational Flows: Aspects of the Behaviour of Model Elasticoviscous Fluids, ” Research Notes in Mathematics 29, Pitman (1979) , which are incorporated herein by reference in their entirety.

Tailing is defined as the falling of the dispensed material from the center of the dot, when a nozzle completes the cycle of dispensing See Fig. 1. Fig. 1A shows a photograph of the dispensing of a material onto a substrate at the left. The right side of Fig. 1A shows an elongation of the material that results in its thinning. Fig. 1B shows the breaking of the material to form a Break Point 1 at the source of the material and a Break Point 2 forming a tail at the dot of material deposited on the substrate. The break also produces satellite drops. This phenomenon can contaminate the adjacent droplets, which can lead to short circuits when the substrate is a circuit board. Occasionally there is a random creation of satellite droplets that can aggravate the situation. The stability of the tails mainly depends on the fluid property, viscosity, interfacial tension, volume and interfacial area.

For any fluid type, Newtonian or Non-Newtonian, the fluid thread would break from a single mass into several fluid blobs in order to minimize its surface energy. It breaks up when two fluids form a free surface. If the fluid system has more surface area than the minimum required to contain the volume of fluid, it would have an excess of free surface energy. Any system would try to move to a lower energy state as shown in Fig. 2. On the left in Fig. 2 the dispensed material is unstable, leading to a thinning of the stream. This instability is relieved as shown on the right side of Fig. 2 where the stream breaks. Likewise for this system the fluid would break up into smaller masses to minimize its energy. This phenomenon is often termed “Capillary Instability. ” See the article, J.B. Bostwick &P.H. Steen, “Stability of Constrained Capillary Surfaces, ” Annu. Rev. Fluid Mech. 47, 539 (2015) , which is incorporated herein by reference in its entirety.

Viewed under a microscope, one can observe the development of perturbations on the free surface of the fluid. These perturbations can be attributed to several factors like the vibration of the fluid container, density mismatches, non-uniformity in the shear stresses or environmental acoustical waves. As the free surface of the fluid deforms, there is a change in the internal pressure of the fluid thread. This is induced by the capillary pressure which is the difference in pressure across an interface of two immiscible fluids. According to the Young-Laplace equation, the capillary pressure is inversely proportional to the mean curvature of the interface. In other words, the pressure is a function of the two radii of curvature. In the thinned area, the first radius of curvature is smaller than in thickened area. This means that the pressure is higher in the thinned area, leading to a pressure gradient. This would lead to a flow of material from the thinned area to thickened area. For some other arbitary wavelength, the second radius of curvature would drive fluid from the thickened area to thinned area. However, for most of the other wavelengths, the fluid flow for the second radius of curvature is from thinned to thickened area, escalating or accelerating the break up. See Fig. 3, which shows fluid thread break up by Capillary Instability.

The current approach in surface mounting technology to the breaking of liquid bridges is to create capillary instability by elongation as reported. See S.R. Marongelli, D. Dixon, S. Porcari, et al. “Practical production uses of SMT adhesives” . Proceedings of the IEEE/CPMT International Electronic Manufacturing Technology (IEMT) Sympo: 147-155, 1998, which is incorporated herein by reference in its entirety. When the aspect ratio, the bridge length divided by the nozzle diameter, is increased, breakup can be sped up due to the capillary thinning effect as shown in Fig. 1 Fig. 4 and Fig. 5. In particular, Fig. 4A shows on the left a material thread undergoing elongation and on the right the resulting dispensed dot of material. Fig. 4B shows a series of dots in which the tails extend toward, may even touch, one another. Thus, capillary instability creates the random falling of liquid tail and satellite droplets, which affects the precision of dispensing and thereby contaminates the substrate.

Apparatus for carrying out this prior art process of capillary instability is shown in Fig. 5. The apparatus includes a nozzle 14 from which the materials is dispensed. The nozzle terminates in a standoff 15 which is wider in diameter than the nozzle and which sets the size (diameter) of the dot 10 that is dispensed. The adhesive dot diameter is approximately two times the inside diameter of the nozzle. The standoff 15 also holds the nozzle a set distance away (e.g., 2.29 mm) from the substrate upon which the dot is deposited. The deposition beings with the standoff in contact with the substrate and the material being dispensed. When nearly complete with the deposition, the nozzle and standoff are lifted off of the substrate. This causes a tail 12 to form because of the surface tension of the adhesive with respect to the substrate. The more the nozzle moves away from the substrate, the thinner the tail becomes. Eventually the tail breaks and the part connected to the dot settles back into the dot.

As mentioned above, industries utilize Non-Newtonian adhesives to meet their product formation and packaging demands. In particular, in electronic packaging applications, conductive viscoelastic adhesives are often used to bond silicon dies and printed circuit boards together. Usually, these adhesives contain high molecular weight polymers whose relaxation time becomes important in the breakup dynamics. The difficulties with capillary instability are due to the stabilization provided by the polymer additives in the adhesives. On top of that, most polymers are elastic in nature, which means they can be stretched to store energy. Instead of breaking up into droplets, such fluids remain joined by threads, which delay the break up. See, Goldin M, Yerushalmi J, Pfeffer R and Shinnar R, “Breakup of laminar capillary jet of a viscoelastic fluid, ” J. Fluid Mech. 38 689, 1969, which is incorporated herein by reference in its entirety. The polymers in the fluids are stretched in the extensional flow. Due to this, they build up extensional stresses that stabilize the fluids. In other words, threads under tension are stable. This property of extensional viscosity can be functional on both the rate of deformation and the total strain accumulated, Due to these reasons, the idea, as reported in the Marongelli article, fails.

Other than the elongation technique to dispense adhesives, no other technique has been reported to dispense microdots of fluids for surface mounting technology albeit the pervasive issue. Only publications related to basic fundamental study of the capillary/liquid bridge and extensional viscosity have been widely reported.

Industries are also keen to have an integrated approach to tackle all the aforementioned issues for two reasons. Firstly they want to avoid extra mechanical modules into their systems as this would complicate mechanical designs. Secondly, industries are unlikely willing to adopt new systems that further increases the current cost of the dispensers. Therefore there exists a need of a method, simple enough, yet able to address all the issues without complicating existing dispenser design.

Summary of the Invention:

The present invention is directed to a method for reducing tackle stringing, satellite drop generation and long breakup times in micro-dispensing applications. The method includes the use of centrifugal force to break up stabilized liquid bridges in such a way that there is a reduction in tailing and satellite drop generation issues. The bridge is formed by rotating a material dispensing nozzle while material is dispensed onto a stationary substrate. The bridge is limited to a small aspect ratio.

Because most current industrial fluids are viscoelastic and shear thinning in nature, centrifugal forces on liquid bridges with low aspect ratios help to suppress the effects of high extensional and shear viscosity. Unlike other methods, centrifugal forces also preserve the rotational symmetry.

There is a need to keep a small aspect ratio in order to suppress extensional viscosity. Therefore, with a low aspect ratio and centrifugal force, faster breakup of a liquid bridge can be achieved. The low aspect ratio suppresses the length of the liquid tail after the breakup, ensuring high precision in the deposition of the material on the substrate.

The invention can be used in general in micro-dispenser applications for surface mounting technologies, such as back-end processing in the semiconductor industry and the print heads of 3D Printers.

Brief Description of the Drawings:

Many aspects of the present invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present embodiments. Moreover, in the drawings all the views are schematic and like reference numerals designate corresponding parts throughout the several views, and wherein:

Fig. 1A shows a photograph of the dispensing of a material onto a substrate at the left and the elongation of the material that results in its thinning on the right, while. Fig. 1B shows the breaking of the material to form a Break Point 1, a Break Point 2 and satellite drops；

Fig. 2 is a schematic illustration of capillary instability showing an unstable dispensed stream of material on the left and a stable system on the right where the stream has been broken；

Fig. 3 is a schematic illustration of fluid thread break up by capillary instability；

Fig. 4A is a photograph showing a material thread undergoing elongation on the left and the resulting dispensed dot of material on the right, while Fig. 4B shows a series of dispensed dots in which the tails extend toward one another；

Fig. 5 is a schematic illustration of an apparatus and method for dispensing dots using capillary instability according to the prior art；

Fig. 6A is a schematic showing the basic principle of the rotation method for slender capillary bridges undergoing elongation according to the present invention, while Fig. 6B is an enlarged view of the area in the square of Fig. 6B showing the vector forces；

Fig. 7A is a schematic showing the basic principle of the rotation method for a stable rotund capillary bridges according to the present invention, while Fig. 7B is an enlarged view of the area in the square of Fig. 7B showing the vector forces.

Fig. 8 is a photograph showing the details of the accelerated breakup process of liquid bridges from thinning to break according to the present invention；

Fig. 9 is a schematic illustration of apparatus used for the rotation process for the accelerated breakup of liquid bridges according to the present invention；

Fig. 10 is a series of photographs that demonstrate the effect of rotation on liquid bridges formed by 60kcSt silicone oil in which Fig. 10A shows the breakup process without applying rotation and Fig. 10B shows the breakup process with rotation according to the present invention；

Fig. 11 is a graph showing the breakup time of liquid bridges formed by 60kcSt silicone oil as a function of the rotational speed of the nozzle and width of the bridge；

Fig. 12 is a series of photographs demonstrating the effect of rotation on liquid bridges formed by the adhesive HYSOL FP5201 in which Fig. 12A shows the breakup process without applying rotation and Fig. 12B shows the breakup process with rotation；

Fig. 13 is a graph showing the breakup time of liquid bridges formed by the adhesive HYSOL FP5201 as a function of the rotational speed of the nozzle； and

Fig. 14 is a series of photographs that demonstrate the effect of rotation on a rotund liquid bridge.

Detailed Description of Illustrative Embodiments of the Invention

The present inventors have discovered that if slender and rotund liquid bridges are rotated under a certain range of rotational speed (e.g., 0-330 Hz) and aspect ratio (1.0-2.0) , the bridges breakup without any stringing and satellites issues.

From a rheological point of view, in a Newtonian fluid like water, liquid bridges break up into droplets under axial shear stresses. However, for viscoelastic fluids under high axial shear stresses, the ligaments of the bridges undergo large stretching before they break up, which leaves long tails of material and generate satellite drops of material. The ability of these materials to stabilize the liquid bridge is only due to the molecular orientation that arises when they are stretched. The shear stresses lead to fluid flow on the surface, resulting in strain hardening in the bridge. This leads to the large extensional viscosity of the fluid. The strain hardening can introduce abnormally high extensional viscosity, as high as 1000 times that of shear viscosity. This explains the amount of resistance to the breakup of liquid bridges and the consequent filament (tail) formation when a viscoelastic fluid is used. Therefore, to counter high extensional and shear viscosity, as well as the associated large break up time and tailing/satellite issues, there are two important things to be considered.

First, there is a need to keep a small aspect ratio of 1.0-2.0, i.e., the ratio of the length to the width of the bridge. In the current practice, industries utilize an aspect ratio of 7.0-8.0 which results in tailing and satellite formation. However according to the present invention, a small aspect ratio results in less stretching of the polymeric additives in the fluid, hence reducing the tailing issue. Second, when centrifugal force or torsion is applied to the viscoelastic ridge, the extensional viscosity does not come into play.

From a fluid mechanics point of view, a liquid with an axisymmetric gyrostatic shape in a uniformly rotating cylinder is considered under the influence of surface tension. Fig. 6A shows a cylindrical slender capillary bridge formed with silicone oil, e.g., Brookfield’s silicone oil, which is inexpensive and has shear thinning ability and a larger storage modulus than its loss modulus compared to adhesives. In Fig. 6A the upper part of the cylinder is being rotated by a rotating dispensing nozzle while it is being stretched due to the upward movement of the nozzle. The result is the thinning of the cylinder at its mid-section. The vector components of the centrifugal force Fc (shown in the enlargement of the square area in Fig. 6A) are shown in Figs. 6B. These vector forces induce a surface flow in a particular direction as shown by the large arrow on the fluid surface in Fig. 6B. The basic idea is to introduce a rotational nozzle, which induces high speed rotational shear in the fluid that may decrease the viscosity of the shear-thinning fluid. The associated centrifugal force acts on the fluid surface and can induce surface flow. As long as the rotation is centered along the nozzle axis of symmetry, the centrifugal force will have rotation symmetry, thus the induced surface flow and bridge breakup. This concept of fluid flow in liquid bridges may be applied under the influence of centrifugal force on either a slender or a round shaped geometrical configuration of a low aspect ratio, and will result in the breakup of the liquid bridges without tailing or satellite issues.

Fig. 7A shows a stable rotund capillary bridge for an adhesive such as Hysol FP5201 or Albestik R4. This bridge is generally stable. The centrifugal force vectors are shown in Fig. 7B. As can be seen they produce a surface fluid flow in the direction shown by the large arrow on the fluid surface in Fig. 7B. It should be noted that this arrow is in the opposite direction from the arrow in Fig. 6B. The breaking of the stable adhesive bridge establishes due to the centrifugal forces instead of Capillary Instability.

As noted, the current invention is a method to accelerate the breakup process of liquid bridges based on the application of centrifugal force. According to the method a liquid bridge formed by viscoelastic fluids, e.g. silicone oil, between a rotatable nozzle and a fixed substrate is subjected to centrifugal force onto the surface of the liquid bridge. Since under shear displacement, the viscosity of the fluid will decrease, the rotation of the nozzle, which exerts shear on the liquid bridge, decreases its viscosity and induces surface flow such that the breakup process of the liquid bridge is accelerated.

The rotational method of the present invention applies angular displacement so as to induce symmetric surface forces, and as a result the surface flow is radially symmetric. In particular, centrifugal force acting on the surface has the same magnitude over the entire circumference of the liquid bridge. This imposes rotational symmetry in the entire breakup process, enhancing the precision of dispensing the material.

In general, the breakup process of liquid bridges with low aspect ratios is slow. However, as mentioned above, there is a need to keep a small aspect ratio to suppress extensional viscosity. Therefore, with a low aspect ratio and centrifugal force, faster breakup of a liquid bridge can be achieved. This suppresses the length of the liquid tail after the breakup, ensuring high precision in the deposition of the material.

In the breakup process driven by the current invention, it is observed that the centrifugal force will transform the liquid bridge into two cones, with the tips touching each other. The breakup occurs at the tips of the cones. This ensures a short tail length and thus high precision. The detailed breakup process of a liquid bridge is shown in Fig. 8 where the two cones are shown in the rightmost view.

The current invention imposes no constraints on the viscoelastic properties or shapes of the liquid bridges. In principle the breakup process of any kind of liquid bridge can be accelerated. Also, this approach works for different kinds of viscoelastic fluids.

For a highly viscoelastic adhesive fluid, e.g. HYSOL FP5201 or Albestik R4 adhesives, experiments have verified that the liquid bridges formed with low aspect ratio are stable and can remain for more than 10 hours. The effectiveness of the current invention is illustrated by applying rotation to such liquid bridges and measuring the breakup time.

EXAMPLES

According to the current invention, the liquid bridge is typically formed with highly viscoelastic fluids, e.g. silicone oil (60kcSt) and HYSOL FP5201 adhesive. Due to the rotating nozzle, centrifugal force is exerted on the surface of the liquid bridge, inducing surface flow. Eventually the flow drives the liquid bridge into two cones to such an extent that a breakup occurs.

Fig. 9 shows the apparatus for carrying out an exemplary embodiment of the invention. A rotating hollow shaft 50 has the nozzle 15 at its end. The shaft is rotated by a motor 52 which receives power from a power supply 54. The viscoelastic fluid travels from a reservoir 56 through the shaft to the nozzle, which dispenses it onto a stationary substrate 20.When a series of dots is to be placed on the substrate, the substrate can be moved after each dot is placed so a new dot can be formed next to it. As an alternative, the rotating nozzle can be moved over the substrate.

For the purpose of seeing the effect of centrifugal force on bridge breaking, a high speed camera 60 focuses on the liquid bridge during breaking. The result is photos such as those in Fig. 8.

Example 1

In Example 1 the liquid bridge is formed by 60kcSt silicone oil. As shown in Figs. 8 and 9, the rotating nozzle 15 exerts centrifugal force onto the surface of the liquid bridge. Due to the induced surface flow, the shape of the liquid bridge is transformed from a catenoid to two cones with tips touching each other. Eventually the liquid bridge breakups into two parts when the two tips separate.

Fig. 10 is a series of photographs that demonstrate the effect of rotation on liquid bridges formed by 60kcSt silicone oil. In Fig. 10A there is shown the breakup process without applying rotation and in which the nozzle is moved upwardly to elongate and eventually break the bridge. It takes 15 seconds for the bridge to break in Fig. 10A. Fig. 10B shows the breakup process with rotation according to the present invention. As can be seen the breakup time is 0.5 seconds.

The breakup time of the liquid bridge as a function of rotational speed of the nozzle is plotted in Fig. 11, showing the dependence of the breakup time on the speed. The time decreases with an increase in rotational speed. Further, it decreases generally linearly with a 4.9 mm width bridge and exponentially with a 2.5 mm wide bridge.

Example 2

In Example 2 the HYSOL FP5201 fluid is used to form the liquid bridge. The breakup process is shown in Figs. 12A and 12B. In particular, Fig. 12 is a series of photographs demonstrating the effect of rotation on liquid bridges formed by HYSOL FP5201 in which Fig. 12A shows the breakup process without applying rotation and Fig. 12B shows the breakup process with rotation. As shown in Fig. 12A at 15 seconds the bridge is still not broken. In Fig. 12B the same amount of thinning is provided with rotation of the present invention at 0.5 seconds as provided at 15 seconds without rotation as shown in Fig. 12A. The breakup time is plotted as a function of rotational speed in Fig. 13. It exponentially decreases as speed increases. The lower the speed the less the effect of the invention. However, speeds above about 350 Hz can induce radial instability in the fluid that would prevent the formation of well formed dots.

Example 3

In Example 3 a rotund liquid bridge formed by 60kcSt silicone oil is considered as shown in Fig. 14. In general, rotund liquid bridges are stable because the surface tension has no destabilizing effects on the rotund surface. The photos in Fig. 14 demonstrate that the current invention is applicable to such stable liquid bridges. In particular, as the photos progress from left to right, the bridge at the rotating nozzle (upper dark column) thins towards breaking.

In preferred embodiments the nozzle size can range from 200 microns to 1000 microns. Also the speed of rotation of the nozzle is preferably 150 to 180 Hz when silicone oil is used. When an adhesive such as Henkel Hysol Fp5201 with a fluid viscosity of 21k cst and a thixotropic indices of 3.5, the rotational speed of the nozzle is preferably in the range of 300 to 320 Hz.

The current invention provides fast, reliable and reproducible results. It can be employed in industrial processes that require viscoelastic fluids to be dispensed, e.g. micro-dispensing pumps in semiconductor packaging and the formation of products using 3D printing. Modern 3D printers can easily generate 3D polymeric prototypes from computer drawings and designs. Sophisticated designs can be materialized within a significantly reduced production time. However, there is a problem of stringing when viscous non Newtonian fluids are used. This can be overcome through the use of rotating nozzles in the print heads of 3D printers according to the present invention.

The cost of implementing the current invention in industry is low compared to other possible methods. It only requires a small motor and a power supply, which can be a simple battery. Additional industrial costs come from the fabrication of the rotatable nozzle and the gears required for rotation. No large scale equipment is required.

Although the features and elements of the present invention have been shown and described as embodiments in particular combinations, it should be understood that each feature or element can be used alone or in other various combinations within the principles of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. Further, various changes in form and details may be made therein without departing from the spirit and scope of the invention.

Claims

A method to accelerate a breakup of a liquid bridge with reduced stringing or tailing of material, or without the generation of satellite drops, comprising the steps of:

rotating a nozzle；

delivering a liquid from the rotating nozzle to a stationary substrate so as to form a fluid dot on the substrate and form the liquid bridge between the fluid dot on the substrate and the nozzle； and

rotating the nozzle at such a speed that the breakup is caused by centrifugal force.
The method according to claim 1 wherein the liquid is a viscoelastic fluid.
The method according to claim 2 wherein the viscoelastic fluid is an adhesive.
The method according to claim 1 wherein the aspect ratio of the liquid bridge ranges from 1.0 to 2.0.
The method of claim 4 wherein the liquid bridge is a rotund capillary.
The method of claim 2, wherein the viscoelastic fluid is selected from the group of Brookfield Silicone Oil, Henkel Hysol Fp5201 and Albestik R4.
The method of claim 6, wherein the Brookfield Silicone Oil has shear viscosity ranging from 1k cst to 60k cst.
The method of claim 1 wherein the size of the nozzle for delivering the liquid ranges from 200 microns to 1000 microns.
The method of claim 6, wherein the nozzle diameter is about 1,000 microns, the viscoelastic fluid is about 60k cst Silicone Oil and the rotational speed of the nozzle ranges from 150-180 Hz, whereby fast breakup of the liquid bridge is achieved without any radial instability and rotational symmetry is preserved.
The method of claim 6 wherein the Henkel Hysol Fp5201 has a fluid viscosity of 21k cst and a thixotropic index of 3.5.
The method of claim 10 wherein, the nozzle diameter is about 200 microns and the rotational speed of the nozzle ranges from 300-320 Hz, whereby fast breakup of the liquid bridge is achieved without any radial instability and rotational symmetry is preserved.
The method of claim 4, wherein the nozzle is rotated using a dc power supply.
The method of claim 1, wherein the liquid bridge has a slender capillary shape.
Apparatus for depositing a series of dots of a viscoelastic fluid on a substrate with reduced stringing or tailing of fluid material, or without the generation of satellite drops, comprising:

a reservoir of the viscoelastic fluid；

a hollow shaft extending from the reservoir toward the substrate； and

a motor for rotating the shaft about the axis of the shaft,

wherein the shaft comprises a nozzle at the end of the shaft adjacent the substrate, said nozzle delivering the viscoelastic fluid to the substrate so as to form a dot of material on the substrate.
Apparatus according to claim 14 further including:

a standoff surrounding the nozzle, said standoff keeping the nozzle from approaching the standoff by less than a fixed distance from the substrate, the interior diameter of the standoff being greater than the diameter of the nozzle and defining the diameter of the dot,

whereby a separation is caused between the fluid dot on the substrate and a liquid bridge connecting the fluid dot to the nozzle, which bridge is rapidly broken because of the rotation of the shaft.