Abstract:
A system and method for jetting a viscous material includes an electronic controller and a jetting dispenser operatively coupled with the electronic controller. The jetting dispenser includes an outlet orifice and a piezoelectric actuator operatively coupled with a movable shaft. The jetting dispenser is under control of the electronic controller for causing said piezoelectric actuator to move the shaft and jet an amount of the viscous material from the outlet orifice. The electronic controller sends a waveform to the piezoelectric actuator to optimize control of the jetting operation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to provisional U.S. Patent App. No. 62/165,242, filed May 22, 2015, the entire contents of which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention generally relates to non-contact, jetting dispensers for depositing small droplets of a viscous fluid onto a substrate, and more specifically, to dispensers of this type that are actuated by one or more piezoelectric elements. 
       BACKGROUND 
       [0003]    Non-contact viscous material dispensers are often used to apply minute amounts of viscous materials, e.g., those with a viscosity exceeding fifty centipoise, onto substrates. For example, non-contact viscous material dispensers are used to apply various viscous materials onto electronic substrates like printed circuit boards. Viscous materials applied to electronic substrates include, by way of example and not by limitation, general purpose adhesives, ultraviolet curable adhesives, solder paste, solder flux, solder mask, thermal grease, lid sealant, oil, encapsulants, potting compounds, epoxies, die attach fluids, silicones, RTV, and cyanoacrylates. 
         [0004]    Specific applications abound for dispensing viscous materials from a non-contact jetting dispenser onto a substrate. In semiconductor package assembly, applications exist for underfilling, solder ball reinforcement in ball grid arrays, dam and fill operations, chip encapsulation, underfilling chip scale packages, cavity fill dispensing, die attach dispensing, lid seal dispensing, no flow underfilling, flux jetting, and dispensing thermal compounds, among other uses. For surface-mount technology (SMT) printed circuit board (PCB) production, surface mount adhesives, solder paste, conductive adhesives, and solder mask materials may be dispensed from non-contact dispensers, as well as selective flux jetting. Conformal coatings may also be applied selectively using a non-contact dispenser. Generally, the cured viscous materials protect printed circuit boards and mounted devices thereupon from harm originating from environmental stresses like moisture, fungus, dust, corrosion, and abrasion. The cured viscous materials may also preserve electrical and/or heat conduction properties on specific uncoated areas. Applications also exist in the disk drive industry, in life sciences applications for medical electronics, and in general industrial applications for bonding, sealing, forming gaskets, painting, and lubrication. 
         [0005]    Jetting dispensers generally may have pneumatic or electric actuators for moving a shaft or tappet repeatedly toward a seat while jetting a droplet of viscous material from an outlet orifice of the dispenser. The electrically actuated jetting dispensers can, more specifically, use a piezoelectric actuator. When an input voltage is applied to and/or removed from the piezoelectric actuator, the resulting movement of a mechanical armature and the tappet or shaft can include undesirable action, such as oscillation. For example, oscillation can cause fluid to be pumped from the outlet and cause volume inaccuracies in the dispensed amount, or it can generate air bubbles due to cavitation, or it can pull air into the fluid through the outlet. 
         [0006]    For at least these reasons, it would be desirable to provide a jetting system and method that address these and other issues, and provide for greater control of jet dispensing operations. 
       SUMMARY 
       [0007]    In an illustrative embodiment the invention provides a system for jetting a fluid including a jetting dispenser and an electronic controller. The jetting dispenser includes a jetting dispenser with a movable shaft, an outlet orifice and a piezoelectric actuator operatively coupled with the movable shaft to jet an amount of the fluid from said outlet orifice. The system further includes an electronic controller operatively coupled to the piezoelectric actuator. The electronic controller applies a voltage with a varying rate of change to the piezoelectric actuator to reduce an oscillation amplitude of the movable shaft during movement toward and/or away from the outlet orifice. The system may include various other features. For example, the controller may send a stepped waveform to the piezoelectric actuator including a first voltage and a second voltage, the second voltage being different than the first voltage. The electronic controller may apply the voltage as a waveform having a decreasing and/or increasing rate of change in the voltage. 
         [0008]    The invention also provides a method for jetting a viscous material from a dispenser including a piezoelectric actuator operatively coupled to a movable shaft and a fluid body with an outlet orifice. The method comprises applying a voltage with a varying rate of change to the piezoelectric actuator. The piezoelectric actuator is moved under the applied voltage. The shaft is moved with the piezoelectric actuator. An amount of the viscous material is jetted from said outlet orifice using the moving shaft. 
         [0009]    Applying the voltage waveform may further comprise applying a stepped waveform to the piezoelectric actuator including a first voltage and a second voltage, the second voltage being different than the first voltage. Applying the voltage waveform may comprise applying the voltage with a decreasing and/or increasing rate of change. 
         [0010]    Various additional features and advantages of the invention will become more apparent to those of ordinary skill in the art upon review of the following detailed description of the illustrative embodiments taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a perspective view of a jetting dispenser system according to an illustrative embodiment of the invention. 
           [0012]      FIG. 2  is a cross sectional view taken along line  2 - 2  of  FIG. 1 . 
           [0013]      FIG. 2A  is an enlarged cross sectional view of the tappet assembly and fluid body taken from  FIG. 2 , and showing the tappet in an open condition. 
           [0014]      FIG. 2B  is a cross sectional view similar to  FIG. 2A , but showing the tappet in a closed position after jetting a droplet of fluid. 
           [0015]      FIG. 3  is a partially exploded perspective view of a piezoelectric actuator of the dispenser. 
           [0016]      FIG. 4  is a perspective view of the piezoelectric jetting dispenser with certain elements shown in dashed lines to better show inner details. 
           [0017]      FIG. 5  is a side elevational view of a lower portion of the actuator illustrating a lever amplification mechanism. 
           [0018]      FIG. 6  is a graphical illustration showing a typical signal output having a trapezoidal waveform. 
           [0019]      FIG. 7  is a graphical illustration overlaying the typical trapezoidal waveform of  FIG. 6  with the resulting, oscillating movement of the mechanical output associated with a piezoelectric jetting dispenser. 
           [0020]      FIG. 8  is a graphical illustration showing a stepped reduction in the input signal voltage in accordance with an illustrative embodiment of the invention. 
           [0021]      FIG. 9  is a graphical illustration of the stepped reduction in actual voltage corresponding to in  FIG. 8 , but overlaid with a graphical illustration of the resulting movement of the mechanical output of the piezoelectric jetting dispenser. 
           [0022]      FIG. 10  is a graphical illustration of another alternative embodiment showing both a stepped reduction in the input signal voltage, and a stepped application of the signal voltage during a jetting dispensing cycle. 
           [0023]      FIG. 11  is a graphical illustration of the stepped reduction and application in actual voltage corresponding to  FIG. 10 , but overlaid with a graphical illustration of the resulting movement of the mechanical output of the piezoelectric jetting dispenser. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    Referring to  FIGS. 1 through 4 , a jetting system  10  in accordance with an embodiment of the invention generally comprises a jetting dispenser  12  coupled with a main electronic control  14 . The jetting dispenser  12  includes a fluid body  16  coupled to an actuator housing  18 . More specifically, the fluid body  16  is held within a fluid body housing  19 , which may include one or more heaters (not shown), depending on the needs of the application. The fluid body  16  receives fluid under pressure from a suitable fluid supply  20 , such as a syringe barrel (not shown). A tappet or valve assembly  22  is coupled to the housing  18  and extends into the fluid body  16 . A mechanical amplifier (e.g., a lever  24 ) is coupled between a piezoelectric actuator  26  and the tappet or valve assembly  22 , as will be described further below. 
         [0025]    For purposes of cooling the piezoelectric actuator  26 , air may be introduced from a source  27  into an inlet port  28  and out from an exhaust port  30 . Alternatively, depending on the cooling needs, both of the ports  28 ,  30  may receive cooling air from the source  27  as shown in  FIG. 2 . In such a case, one or more other exhaust ports (not shown) would be provided in the housing  18 . A temperature and cycle control  36  is provided for cycling the actuator  26  during a jetting operation and for controlling one or more heaters (not shown) carried by the dispenser  12  for maintaining the dispensed fluids to a required temperature. As another option, this control  36 , or another control, may control the cooling needs of the actuator  26  in a closed loop manner. As further shown in  FIG. 4 , the piezoelectric actuator  26  further comprises a stack  40  of piezoelectric elements. This stack  40  is maintained in compression by respective flat, compression spring elements  42 ,  44  coupled on opposite sides of the stack  40 . More specifically, upper and lower pins  46 ,  48  are provided and hold the flat spring elements  42 ,  44  to one another with the stack  40  of piezoelectric elements therebetween. The upper pin  46  is held within an upper actuator portion  26   a  of the actuator  26 , while a lower pin  48  directly or indirectly engages a lower end of the stack  40 . The upper actuator portion  26   a  securely contains the stack  40  of piezoelectric elements so that the stack  40  is stabilized against any sideward motion. In this embodiment, the lower pin  48  is coupled to a lower actuator portion  26   b  and, more specifically, to a mechanical armature  50  ( FIG. 2 ). 
         [0026]    An upper surface  50   a  of the mechanical armature  50  bears against the lower end of the piezoelectric stack  40 . The springs  42 ,  44  are stretched between the pins  46 ,  48  such that the springs  42 ,  44  apply constant compression to the stack  40  as shown by the arrows  53  in  FIG. 4 . The flat springs  42 ,  44  may, more specifically, be formed from a wire EDM process. The upper end of the piezoelectric element stack  40  is retained against an internal surface of the upper actuator portion  26   a.  The upper pin  46  is therefore stationary while the lower pin  48  floats or moves with the springs  42 ,  44  and with the mechanical armature  50 , as will be described herein. 
         [0027]    When voltage is applied to the piezoelectric stack  40 , the stack  40  expands or lengthens and this moves the armature  50  downward against the force of the springs  42 ,  44 . The stack  40  will change length proportional to the amount of applied voltage. 
         [0028]    As further shown in  FIG. 2 , the mechanical armature  50  is operatively coupled to the mechanical amplifier which, in this illustrative embodiment, is formed as the lever  24  coupled to the armature  50  generally near a first end  24   a  and coupled to a push rod  68  at a second end  24   b.  The lever  24  may be integrally formed from the lower actuator portion  26   b  through, for example, an EDM process that also forms a series of slots  56  between the mechanical armature  50  and the lever  24 . As will be further discussed below, the lever  24  or other type of mechanical amplifier amplifies the distance that the stack  40  expands or lengthens by a desired amount. For example, in this embodiment, downward movement of the stack  40  and the mechanical armature  50  is amplified by about eight times at the second end  24   b  of the lever  24 . 
         [0029]    Now referring more specifically to  FIGS. 2, 2A, 2B, and 5 , a flexural portion  60  couples the lever  24  to the mechanical armature  50 . As shown best in  FIG. 5 , the lever  24  pivots about a pivot point  62  that is approximately at the same horizontal level as the second end  24   b  of the lever  24 . This position of the pivot point  62  serves to minimize the effect of the arc through which the lever  24  rotates. The series of slots  56  is formed in the lower actuator portion  26   b  forming the flexural portion  60 . When the piezoelectric stack  40  lengthens under the application of a voltage by the main control  14  as shown by the arrow  66  in  FIG. 5 , the lever  24  rotates clockwise generally about the pivot point  62  as the stack  40  pushes downward on the mechanical armature  50 . The slight rotation of the lever  24  takes place against a resilient bias applied by the flexural portion  60 . As the second end  24   b  is rotating slightly clockwise about the pivot point  62 , it moves downward and likewise moves an attached push rod  68  downward ( FIG. 2 ) as indicated by the arrow  67  in  FIG. 5 . 
         [0030]    The second end  24   b  of the lever  24  is fixed to the push rod  68  using suitable threaded fasteners  70 ,  72 . The push rod  68  has a lower head portion  68   a  that travels within a guide bushing  74  and bears against an upper head portion  76   a  of a tappet or valve element  76  associated with the tappet or valve assembly  22 . The guide bushing  74  is held in the housing  18  with a pin  75  as best seen in  FIGS. 2A and 2B . The assembly of the push rod  68 , guide bushing  74 , and pin  75  allows for some “give” to ensure proper movement of the push rod  68  during operation. In addition, the push rod  68  is made of a material that will slightly bend sideward, in an elastic manner, during its reciprocating movement with the tappet or valve element  76  and lever  24 . The tappet assembly further comprises a coil spring  78  which is mounted within a lower portion of the housing  18  using an annular element  80 . The tappet or valve assembly  22  further comprises an insert  82  retained in the fluid body  16  by an O-ring  84 . The annular element  80  and the insert  82  comprise an integral element, i.e., a cartridge body in this embodiment. A cross-drilled weep hole  85  is approximately in line with the lower end of the spring  78  to allow any liquid that leaks past the O-ring  86  to escape. An additional O-ring  86  seals the tappet or valve element  76  such that pressurized fluid contained in a fluid bore  88  of the fluid body  16  does not leak out. Fluid is supplied to the fluid bore  88  from the fluid supply  20  through an inlet  90  of the fluid body  16  and passages  92 ,  94 . The O-ring  84  seals the outside of the cartridge body formed by the annular element  80  and insert  82  from the pressurized liquid in bore  88  and passage  94 . The fluid passages  92 ,  94  are sealed by a plug member  96  threaded into the fluid body  16 . The plug member  96  may be removed to allow access for cleaning the internal passage  94 . 
         [0031]    The operation of the system  10  to jet droplets or small amounts of fluid will be best understood by reviewing  FIGS. 2-4  in conjunction with  FIGS. 2A and 2B .  FIG. 2A  illustrates the tappet or valve element  76  raised to an open condition when the voltage to the piezoelectric stack  40  has been sufficiently removed. This causes the stack  40  to contract. As the stack  40  contracts, the flat springs  42 ,  44  pull the armature  50  upward and this raises the second end  24   b  of the lever  24 , and also raises the push rod  68 . Thus, the coil spring  78  of the tappet or valve assembly  22  can then push upward on the upper head portion  76   a  of the tappet or valve element  76  and raise a distal end  76   b  of the tappet or valve element  76  off a valve seat  100  affixed to the fluid body  16 . In this position, the fluid bore  88  and the area beneath the distal end  76   b  of the tappet or valve element  76  fills with additional fluid to “charge” the jetting dispenser  12  and prepare the jetting dispenser  12  for the next jetting cycle. 
         [0032]    When the piezoelectric stack  40  is activated, i.e., when voltage is applied to the piezoelectric stack  40  by the main electronic control  14  ( FIG. 1 ), the stack  40  expands and pushes against the mechanical armature  50 . This rotates the lever  24  clockwise and moves the second end  24   b  downward, also moving the push rod  68  downward. The lower head portion  68   a  of the push rod  68  pushes down on the upper head portion  76   a  of the tappet or valve element  76  as shown in  FIG. 2B  and the tappet or valve element  76  moves quickly downward against the force of the coil spring  78  until the distal end  76   b  engages against the valve seat  100 . In the process of movement, the distal end  76   b  of the tappet or valve element  76  forces a droplet  102  of fluid from a discharge outlet  104 . Voltage is then removed from the piezoelectric stack  40  and this reverses the movements of each of these components to raise the tappet or valve element  76  for the next jetting cycle. 
         [0033]    It will be appreciated that the piezoelectric actuator  26  may be utilized in reverse to jet droplets. In this case, the various mechanical actuation structure including the lever  24  or other type of mechanical amplifier would be designed differently such that when the voltage is removed from the piezoelectric stack  40 , the resulting contraction of the stack  40  will cause movement of the tappet or valve element  76  toward the valve seat  100  and the discharge outlet  104  to discharge a droplet  102  of fluid. Then, upon application of the voltage to the stack  40 , the amplification system and other actuation components would raise the tappet or valve element  76  in order to charge the fluid bore  88  with additional fluid for the next jetting operation. In this embodiment, the tappet or valve element  76  would be normally closed, that is, it would be engaging the valve seat  100  when there is no voltage applied to the piezoelectric stack  40 . 
         [0034]    As further shown in  FIG. 2 , the upper actuator portion  26   a  is separate from the lower actuator portion  26   b  and these respective portions  26   a ,  26   b  are formed from different materials. Specifically, the upper actuator portion  26   a  is formed from a material having a lower coefficient of thermal expansion than the material forming the lower actuator portion  26   b.  Each of the actuator portions  26   a,    26   b  is securely fastened together using threaded fasteners (not shown) extending from the lower actuator portion  26   b  into the upper actuator portion  26   a.  The assembly of the upper and lower actuator portions  26   a,    26   b  is then fastened to the housing by a plurality of bolts  110 . More specifically, the lower actuator portion  26   b  may be formed from PH17-4 stainless steel, while the upper actuator portion  26   a  may be formed from a nickel-iron alloy, such as Invar. 17-4 PH stainless steel has a very high endurance limit, or fatigue strength, which increases the life of flexural portion  60 . The coefficient of thermal expansion of this stainless steel is about 10 μm/m-C, while the coefficient of thermal expansion of Invar is about 1 μm/m-C. The ratio of the thermal expansions may be higher or lower than the approximate 10:1 ratio of these materials. The coefficients of thermal expansion associated with the upper and lower actuator portions  26   a,    26   b  effectively provide offsetting characteristics to each other. The differing coefficients of thermal expansion of the upper and lower actuator portions  26   a,    26   b  thereby allow the actuator  26  to operate consistently across a wider temperature range. Also, piezo stacks, when operated at a high duty cycle, can generate significant heat. Use of Invar provides for more absolute positioning of the end of the actuator  26 , and hence more accurate and useable stroke. 
         [0035]    Referring now to  FIGS. 6 and 7 ,  FIG. 6  illustrates a digital signal  116  generated by the main control  14  ( FIG. 1 ) for directing a single dispense cycle, and without application of the principles of the present invention. In this example, the digital signal voltage is dropped abruptly and at a constant rate to 0 volts and held there for a very short period of time. After that short period of time, the signal voltage is then raised at a constant rate to the level used to activate the piezoelectric stack. As previously explained, the application of voltage will lengthen the piezoelectric stack and may be used to close the valve element during a jet dispensing operation. The resulting analog voltage waveform is shown by the dashed line  117  in  FIG. 7 . The movement of the mechanical actuation components in response to the abrupt removal and reapplication of voltage is shown by the solid line  118  in  FIG. 7 . Because the piezoelectric actuator moves much more quickly than the mechanical components coupled to it, such as a mechanical armature, amplifier, and jetting valve, the result is that these mechanical components will oscillate back and forth after the removal of voltage as shown by the oscillating solid line  118  in  FIG. 7 . This oscillation has negative effects such as those discussed in the Background section above. 
         [0036]      FIGS. 8 and 9  illustrate waveforms  120 ,  122  of the digital input signal voltage ( FIG. 8 ), and the resulting analog voltage applied to and removed from the piezoelectric stack ( FIG. 9 ). More specifically, in this embodiment the input signal voltage waveform  120  and the resulting input analog voltage waveform  122  show that the voltage to the stack  40  ( FIG. 4 ) is removed at a varying or discontinuous rate. More generally, the electronic controller applies a voltage in a waveform having a varying rate of change. In this example, the voltage is removed from the piezoelectric stack in a stepped manner including a first reduction in voltage whereupon the voltage is reduced and then maintained at a first voltage level  124  for a period of time. Then the voltage is reduced to a second voltage level lower than the first voltage level  124  and maintained at the second voltage level  126  for a period of time. The second voltage level  126 , in this example, is 0 volts. However, it will be understood that the varying rate of change in voltage may include more than one step in the reduction, or may include other manners of effecting a varying rate of change when removing the voltage. Moreover, for additional control, the rate of voltage reduction may vary between the steps. That is, the voltage reduction represented by a first portion  130  of the waveform is at a different rate of reduction than the voltage reduction represented by a second portion  132  of the waveform. 
         [0037]    The solid line  140  in  FIG. 9  illustrates the resulting reduction or damping of oscillation in the mechanical components, including the lever  24 , push rod  68  and tappet or valve element  76  as shown by the reduced amplitude of oscillation (see  FIG. 2 ). During the first reduction in voltage to level  124 , the armature  50  begins moving upwardly as the piezoelectric stack  40  ( FIG. 4 ) contracts. Holding this voltage at the first step or level  124  allows the armature  50  to more slowly build momentum toward its final upper position. After a short period of time, the voltage is further reduced to its final value of zero at the second level  126  and held there for a short time. By the time the voltage is further reduced to this second level  126 , the momentum of the armature  50  should at least be significantly reduced, such that it will not be accelerating into its final position. It is this acceleration that causes oscillation as the armature  50  rebounds from its final position. The piezoelectric stack  40  and other mechanical components, including those coupled with the armature  50 , are designed such that the armature  50  should stop without significant oscillation at its final upper position. Another advantage of the stepped waveform  122 , or other manners of varying the rate of change in voltage, is that the dispense cycle rate can be increased or quickened, because there is less oscillation between dispense cycles. 
         [0038]      FIGS. 10 and 11  are similar to  FIGS. 8 and 9 , except that the stepped waveform is not only used when removing the voltage as discussed with regard to  FIGS. 8 and 9 , and shown by the first halves  150   a,    152   a  of the waveforms  150 ,  152  in  FIGS. 10 and 11 , but is also used when re-applying the voltage (e.g., at a third voltage level) as shown by the second halves  150   b ,  152   b  of the waveforms  150 ,  152  in  FIGS. 10 and 11 . Again, this stepped re-application of voltage may be substituted with another manner of varying the rate of change in voltage as the voltage is reapplied to the piezoelectric stack, in this example, to close the jetting tappet or valve element  76  and dispense a small amount or droplet of fluid as shown in  FIG. 2B . Also, different waveform portions may have different rates of change in the voltage, for purposes of allowing further control of the valve element closure and jetting process. The use of a stepped application of voltage while moving the jetting tappet or valve element  76  toward the valve seat  100  ( FIG. 2B ) has several possible advantages. For example, the force at which the tappet or valve element  76  impacts the valve seat  100  may be better controlled (e.g., reduced in order to promote longer valve seat life). The amount of energy transferred to the fluid by the tappet or valve element  76  may also be controlled by varying the rate of change in voltage used for jetting the fluid. This aspect may be used to control jetting performance. Also, oscillation of the tappet or valve element  76  during its movement toward the valve seat  100  may be damped and result in higher quality dispense operation and more accurate dispensed fluid volume control. It will also be understood that for certain applications, the rate of change in voltage (either removing voltage or applying voltage) to the piezoelectric actuator  26  may occur: 1) only while raising the jetting tappet or valve element  76  from the valve seat  100  as discussed with regard to  FIGS. 8 and 9 ; 2) both while raising and lowering the jetting tappet or valve element  76  as discussed with regard to  FIGS. 10 and 11 ; or 3) only while moving the jetting valve element toward the outlet during a jetting dispense operation. 
         [0039]    While the present invention has been illustrated by the description of specific embodiments thereof, and while the embodiments have been described in considerable detail, it is not intended to restrict or in any way limit the scope of the appended claims to such detail. The various features discussed herein may be used alone or in any combination. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and methods and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the scope or spirit of the general inventive concept.