Abstract:
A jetting dispenser includes an actuator with a piezoelectric unit that lengthens by a first distance in response to an applied voltage, and an amplifier operatively coupled to the piezoelectric unit. The amplifier includes first and second ends and the second end moves through a second distance, larger than the first distance under the applied voltage. First and second springs are positioned on opposite sides of the piezoelectric unit. The springs are coupled to the piezoelectric unit in a manner that maintains the piezoelectric unit under constant compression. A fluid body includes a movable shaft operatively coupled with the second end of the amplifier and includes a fluid bore and an outlet orifice. The movable shaft is moved by the second end of the amplifier under the applied voltage and jets an amount of fluid from the fluid bore through the outlet orifice.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to provisional U.S. Patent App. No. 62/165,244, 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. Piezo stacks are very accurate and extremely fast reacting ceramic devices. A property of the piezo stack is that when a voltage is applied the ceramic material will perform a displacement in one direction. One main drawback is that the piezo stack produces a very small displacement. For example, a 7 mm×7 mm×36 mm long stack produces about  36  microns of movement. This displacement is not enough for proper jetting of fluids. It is known to form an actuator with a piezo stack and an amplification mechanism. Space limitations and life expectancy are also considerations when designing an actuator that includes a piezo material. Life expectancy is severely shortened when the stack is placed in a tension condition. The piezo stack needs to be able to operate at a frequency of 1000 Hz continuous, and needs to apply sufficient force to reliably and accurately jet a small amount of fluid. There are a wide variety of methods of achieving the amplification that is necessary for this application, however achieving long life cycles 10 9  can be challenging. The two main methods of rocker arm or lever amplification are pivot and flexural. The pivot method is susceptible to wear which will reduce the overall displacement, and the flexural method is prone to breakage around areas of high stress. 
         [0006]    A piezo stack produces a significant amount of heat during operation. The amount of heat generated by the actuator is dependent on several factors such as heater body temperature, piezo frequency, and duty cycle. This heat is transferred to the surrounding metal in the actuator. This results in a change in the position of the lever or rocker arm and can negatively affect the intended stroke of the jetting device. 
         [0007]    For at least these reasons, it would be desirable to provide a jetting system and method that addresses these and other issues. 
       SUMMARY 
       [0008]    In a first illustrative embodiment the invention provides a jetting dispenser including an actuator and a fluid body. The actuator includes a piezoelectric unit that lengthens by a first distance in response to an applied voltage, and an amplifier operatively coupled to the piezoelectric unit. The actuator further comprises a pair of springs positioned on opposite sides of the piezoelectric unit. The springs are coupled to the piezoelectric unit in a manner that maintains the piezoelectric unit under constant compression. The dispenser further includes a fluid body with a movable shaft operatively coupled with the amplifier and including a fluid bore and an outlet orifice. The movable shaft is moved by the amplifier when voltage is applied to and removed from the piezoelectric unit and thereby moving the movable shaft to jet an amount of fluid from the fluid bore through the outlet orifice. 
         [0009]    The dispenser may include additional or alternative aspects in various embodiments. For example, the pair of springs further comprise first and second flat springs. The actuator may further comprise an upper actuator portion, and the first and second flat springs each include a first end and a second end. The first ends are fixed to the upper actuator portion and the second ends are fixed for movement with the amplifier under the applied voltage. The second ends are fixed to an armature coupled to the amplifier. The armature is moved by the piezoelectric unit as the voltage is applied and removed. 
         [0010]    In another embodiment, the invention provides a jetting dispenser comprising an actuator including a piezoelectric unit that lengthens by a first distance in response to an applied voltage, an upper actuator portion containing the piezoelectric unit, and a lower actuator portion including an amplifier operatively coupled to the piezoelectric unit. The amplifier includes first and second ends and the second end moves through a second distance, larger than the first distance under the applied voltage. The upper actuator portion is formed from a first material having a first coefficient of thermal expansion and the lower actuating portion is formed from a second material having a second coefficient of thermal expansion. The first coefficient of thermal expansion is lower than the second coefficient of thermal expansion. The jetting dispenser further includes a fluid body with a movable shaft operatively coupled with the second end of the amplifier and including a fluid bore and an outlet orifice. The movable shaft is moved by the second end of the amplifier under the applied voltage and thereby jets an amount of fluid from the fluid bore through the outlet orifice. As examples, the ratio of the first coefficient of thermal expansion to the second coefficient of thermal expansion may be at least 1:5 or at least 1:10. In one embodiment the material forming the lower actuator portion comprises stainless steel and the material forming the upper actuating portion comprises an alloy. The alloy may further comprise a nickel-iron alloy. 
         [0011]    In another embodiment, the invention provides a jetting dispenser comprising an actuator including a piezoelectric unit that lengthens by a first distance in response to an applied voltage, an upper actuator portion containing the piezoelectric unit, and a lower actuator portion including an amplifier operatively coupled to the piezoelectric unit. The amplifier includes first and second ends and the second end moves through a second distance, larger than the first distance under the applied voltage. The amplifier is formed integrally with the lower actuator portion and includes a flexural portion formed by a series of slots in the lower actuator portion. The jetting dispenser further comprises a fluid body including a movable shaft operatively coupled with the second end of the amplifier and including a fluid bore and an outlet orifice. The movable shaft is moved by the second end of the amplifier and thereby jets an amount of fluid from the fluid bore through the outlet orifice. 
         [0012]    In another embodiment, the invention provides an actuator comprising a piezoelectric unit and a fluid body. The piezoelectric unit lengthens by a first distance in response to an applied voltage. An upper actuator portion contains the piezoelectric unit, and a lower actuator portion includes an amplifier operatively coupled to the piezoelectric unit. The amplifier includes first and second ends. The second end moves through a second distance, larger than the first distance under the applied voltage. The amplifier is formed integrally with the lower actuator portion and includes a flexural portion. The fluid body includes a movable shaft operatively coupled with the second end of the amplifier and further includes a fluid bore and an outlet orifice. The movable shaft is moved by the second end of the amplifier and thereby jets an amount of fluid from the fluid bore through the outlet orifice. 
         [0013]    In another embodiment, the invention provides an amplifier for producing a mechanically amplified movement of a piezoelectric actuator from a first distance to a second distance under an applied voltage. More specifically, a portion of the amplifier moves a second distance, which is larger than the first distance, when a voltage is applied to the piezoelectric actuator. The amplifier includes an armature adapted to be coupled for movement with the piezoelectric actuator under the applied voltage. The armature is formed integrally with a flexural portion including a plurality of slots for providing flexure when the piezoelectric actuator moves through the first distance. The amplifier further includes a lever having first and second ends. The second end moves through the second distance, larger than the first distance, under the applied voltage. 
         [0014]    In another embodiment, a method of jetting fluid using an actuator including a piezoelectric unit, an amplifier coupled to the piezoelectric unit, and a pair of springs positioned on opposite sides of the piezoelectric unit includes maintaining a constant compression of the piezoelectric unit with the pair of springs. A voltage is applied to the piezoelectric unit to cause the piezoelectric unit to lengthen by a first distance caused by the lengthening of the piezoelectric unit. The amplifier is actuated by a second distance, second distance larger than the first distance. Caused by the actuation of the amplifier, a movable shaft of a fluid body is moved to jet an amount of the fluid from a fluid bore in the fluid body through an outlet orifice. 
         [0015]    The above method may include additional or alternative aspects. For example, the pair of springs may further comprise first and second flat springs. In an aspect, the actuator may further comprise an upper actuator portion, and the first and second flat springs each may include a first end and a second end, the first ends being fixed to the upper actuator portion, and the method may further comprise moving the second ends with the amplifier under the applied voltage to the piezoelectric unit. In another aspect, the method may further comprise moving an armature, coupled to the amplifier and the second ends of the first and second flat springs, as the voltage is applied and removed. In another aspect, the actuator may further comprise a lower actuator portion including the amplifier, the upper actuator portion formed from a first material having a first coefficient of thermal expansion and the lower actuator portion formed from a second material having a second coefficient of thermal expansion, wherein the first coefficient of thermal expansion is lower than the second coefficient of thermal expansion. In yet another aspect, the amplifier may include a first end operatively coupled with the armature and a second end operatively coupled with the movable shaft and the method may further comprise moving the second end of the amplifier through the second distance. 
         [0016]    In another embodiment, a method of mechanically amplifying a movement of a piezoelectric actuator through a first distance includes, under a voltage applied to the piezoelectric actuator, moving a first end of a lever through the first distance, the first end of the lever operatively coupled with the piezoelectric actuator. The method further includes moving a second end of the lever through a second distance, larger than the first distance, under the applied voltage. 
         [0017]    The above method may include additional or alternative aspects. For example, the method may further comprise, under the voltage applied to the piezoelectric actuator, moving an armature coupled to the piezoelectric actuator; and flexing a flexural portion when the piezoelectric actuator moves through the first distance, the flexural portion formed integrally with the armature and operatively coupled with the lever. In another aspect, the flexural portion may include a plurality of slots for providing the flexing of the flexural portion 
         [0018]    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 
         [0019]      FIG. 1  is a perspective view of a jetting dispenser system according to an illustrative embodiment of the invention. 
           [0020]      FIG. 2  is a cross sectional view taken along line  2 - 2  of  FIG. 1 . 
           [0021]      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. 
           [0022]      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. 
           [0023]      FIG. 3  is a partially exploded perspective view of a piezoelectric actuator of the dispenser. 
           [0024]      FIG. 4  is a perspective view of the piezoelectric jetting dispenser with certain elements shown in dashed lines to better show inner details. 
           [0025]      FIG. 5  is a side elevational view of a lower portion of the actuator illustrating a lever amplification mechanism. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    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. 
         [0027]    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 ). 
         [0028]    An upper surface  50   a  of the mechanical armature  50  bears against the lower end of the piezoelectric stack  40 . The spring elements  42 ,  44  are stretched between the pins  46 ,  48  such that the spring elements  42 ,  44  apply constant compression to the stack  40  as shown by the arrows  53  in  FIG. 4 . The flat spring elements  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 spring elements  42 ,  44  and with the mechanical armature  50  as will be described. 
         [0029]    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 spring elements  42 ,  44 . The stack  40  will change length proportional to the amount of applied voltage. 
         [0030]    As further shown in  FIG. 2 , the mechanical armature  50  is operatively coupled with a 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 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 . 
         [0031]    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  form 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 . 
         [0032]    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 bushings  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  by a pressfit 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 . 
         [0033]    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 spring elements  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. 
         [0034]    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 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 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. 
         [0035]    It will be appreciated that the piezoelectric actuator  26  may be utilized in reverse to jet droplets. In this case, the various mechanical actuation structures including the lever  24  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 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 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 . 
         [0036]    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. Specifically, this temperature range allows the actuator  26  to be run at higher frequencies and with more aggressive waveforms. 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. 
         [0037]    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.