Abstract:
A stencil printer for printing viscous material on an electronic substrate includes a stencil having apertures formed therein, and a print head positioned over the stencil and configured to deposit viscous material within the apertures of the stencil. The print head includes a housing defining an elongate chamber, a source port defining a passage having an inlet positioned to allow viscous material to flow into the elongate chamber, a pair of blades defining a slot that provides an outlet from which viscous material can flow out of the elongate chamber, an elongate plunger movable in the elongate chamber to reduce a volume of viscous material within the elongate chamber, and at least one sensor to detect pressure of the viscous material within the elongate chamber.

Description:
BACKGROUND 
     In manufacturing a surface-mount printed circuit board, a stencil printer can be used to print solder paste onto the circuit board. Typically, a circuit board having a pattern of pads or some other conductive surface onto which solder paste will be deposited is automatically fed into the stencil printer; and one or more small holes or marks (known as “fiducials”) on the circuit board are used to properly align the circuit board with the stencil or screen of the stencil printer prior to printing solder paste onto the circuit board. In some systems, an optical alignment system is used to align the circuit board with the stencil. 
     Once the circuit board has been properly aligned with the stencil in the printer, the circuit board is raised to the stencil, solder paste is dispensed onto the stencil, and a wiper blade (or squeegee) traverses the stencil to force the solder paste through apertures in the stencil and onto the board. As the squeegee is moved across the stencil, the solder paste tends to roll in front of the blade, which desirably causes mixing and shearing of the solder paste so as to attain a desired viscosity to facilitate filling of the apertures in the screen or stencil. The solder paste typically is dispensed onto the stencil from a standard cartridge. 
     In some stencil printers, any excess solder paste remaining under the squeegee, after it has fully traversed the stencil, remains on the stencil when the squeegee is returned to its initial position for printing on a second circuit board. In some stencil printers, a second squeegee moves across the stencil in the direction opposite to that of the first squeegee. The first squeegee and the second squeegee are used on alternating boards to continually pass the roll of solder paste over the apertures of a stencil to print on each successive circuit board. In the stencil printers that utilize two squeegees, there is still the problem that at the end of a manufacturing day, or when the stencil is to be changed, which is excess solder paste typically remains on the stencil and must be manually removed. Also, in these known printers, it is difficult to maintain a desirable viscosity because volatile solvents escape from the solder paste, thereby affecting the viscosity of the solder paste. 
     In these stencil printers, the squeegee blades are typically at a predetermined angle with respect to the stencil to apply downward pressure on the solder paste to force the solder paste through the apertures in the stencil as the squeegee is moved across the stencil. The angle of the blade is selected based on the speed at which the blade traverses the stencil and based on the desired downward pressure on the solder paste from the blade. It is desirable to maintain a consistent pressure on the solder paste as the squeegee traverses the stencil; however, in a typical printer, the pressure varies due to variations in paste viscosity throughout a production run and due to variations in the angle of the squeegee caused by deformation of the squeegee due to the pressure applied by the squeegee driving device. 
     Responding to some of the problems, noted above, improved solder paste print heads are described in U.S. Pat. Nos. 5,947,022, 6,324,973, 6,453,810, and 6,955,120, each of which is incorporated herein by reference in their entirety. These patents describe a movable print head having a chamber including ports to which removable cartridges that supply solder paste are coupled. Solder paste is passed from the removable cartridges, into the chamber, then out of a dispensing slot, through a stencil and onto a circuit board in a desired pattern. 
     There is presently a need for a print head having improved pressure control, especially across the entire width of the print head, greater flexibility, reduced maintenance, and increased value to the operator. 
     SUMMARY 
     The improved print head disclosed herein includes a chamber with a direct pressure control system for discharging viscous material from the chamber. A “viscous” material is a material that has a viscosity sufficient under the given configuration and conditions such that the material will not naturally (under simple gravitational principles) flow without other external forces to fill and dispense from a dispensing region within the chamber. A viscous material print head (also known as a “dispensing head”) of this disclosure has a chamber through which viscous material (e.g., solder paste) can be channeled. The print head includes at least one source port that can be coupled with a source of viscous material, such as a solder paste cartridge, and a dispensing slot through which the viscous material can exit the chamber. 
     The print head can be mounted to a frame, to which a stencil is also mounted. The stencil is positioned between the dispensing slot of the print head and the substrate (e.g., a printed circuit board). Viscous material can accordingly be dispensed from the print head through apertures in the stencil and onto selected locations on the surface of the substrate. 
     Aspects of the present disclosure are directed to a stencil printer for printing viscous material on an electronic substrate. In one embodiment, the stencil printer comprises a stencil having apertures formed therein, and a print head positioned over the stencil and configured to deposit viscous material within the apertures of the stencil. The print head includes a housing defining an elongate chamber, a source port defining a passage having an inlet positioned to allow viscous material to flow into the elongate chamber, a pair of blades defining a slot that provides an outlet from which viscous material can flow out of the elongate chamber, an elongate plunger movable in the elongate chamber to reduce a volume of viscous material within the elongate chamber, and at least one sensor to detect pressure of the viscous material within the elongate chamber. 
     Embodiments of the stencil printer further include a controller coupled with the print head to control the operation of an elongate plunger to maintain the viscous material at a desired pressure. The print head further may include a motor, which is coupled to the housing of the print head and to the controller, to drive the movement of the elongate plunger within the elongate chamber under the control of the controller. The at least one sensor may include a pressure transducer provided along a wall of the elongate chamber. The stencil printer further may comprise a flexible membrane positioned between the elongate plunger and the viscous material disposed within the elongate chamber. The blades may be angled toward one another. The stencil printer further may comprise three sensors positioned along a length of the elongate chamber. 
     Another aspect of the disclosure is directed to a method for printing solder paste comprising: supplying viscous material to an elongate chamber of a print head; applying pressure to the viscous material with an elongate plunger to displace the viscous material onto a substrate; and sensing pressure within the elongate chamber to maintain the viscous material at a desired pressure. 
     Embodiments of the method further include sensing pressure within the elongate chamber by averaging readings taken from multiple pressure transducers positioned within the elongate chamber to control pressure of viscous material within the elongate chamber. The method further may comprise disposing a flexible membrane between the elongate plunger and viscous material disposed within the elongate chamber. The method further may comprise supplying viscous material to the elongate chamber while maintaining the viscous material at the desired pressure. Sensing pressure may occur at a left-hand side of the elongate chamber and at a right-hand side of the elongate chamber. The method further may comprise positively controlling the amount of viscous material discharged from the elongate chamber during a print stroke. The viscous material may be solder paste, and the method further may comprise passing the solder paste through a stencil as the solder paste is discharged from the elongate chamber for selective deposition on the substrate. 
     Another aspect of the disclosure is directed to a print head comprising a housing defining an elongate chamber, a source port defining a passage having an inlet positioned to allow a viscous material to flow into the elongate chamber, a pair of blades defining a slot that provides an outlet from which the viscous material can flow out of the elongate chamber, an elongate plunger movable in the elongate chamber to reduce a volume of viscous material within the elongate chamber, and means for detecting pressure of the viscous material within the elongate chamber. 
     Embodiments of the print head further may comprise a controller coupled with the print head to control the operation of an elongate plunger to maintain the viscous material at a desired pressure. The print head further may comprise a motor, which is coupled to the housing of the print head and to the controller, to drive the movement of the elongate plunger within the elongate chamber under the control of the controller. The means for detecting pressure may include a pressure transducer provided along a wall of the elongate chamber. The print head further may comprise a flexible membrane positioned under the elongate plunger within the elongate chamber. The blades may be are angled toward one another. The means for detecting pressure may include three sensors positioned along a length of the elongate chamber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are not intended to be drawn to scale. In the drawings, each identical or nearly identical component that is illustrated in various figures is represented by a like numeral. For purposes of clarity, not every component may be labeled in every drawing. In the drawings: 
         FIG. 1  is a front perspective view of a stencil printer of an embodiment of the present disclosure; 
         FIG. 2  is a perspective view of a print head supported by a print head gantry system; 
         FIG. 3  is an end view of the print head and print head gantry system shown in  FIG. 2 ; 
         FIG. 4  is a perspective view of the print head taken from one end of the stencil printer; 
         FIG. 5  is a perspective view of the print head shown in  FIG. 4  taken from an opposite end of the stencil printer; 
         FIG. 6A  is a cross-sectional view of the print head having blades at a 30° degree angle; 
         FIG. 6B  is a cross-sectional view of the print head having blades at a 45° degree angle; 
         FIG. 7  is a perspective cross-sectional view of the print head; and 
         FIG. 8  is a cross-sectional view of a print head of another embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     For purposes of illustration, embodiments of the present disclosure are described below, with reference to a stencil printer used to print solder paste onto a circuit board. The apparatus and associated methods may also be used in other applications requiring dispensing of other viscous or printing materials, such as glues, adhesives, and encapsulants on a variety of substrates. For example, the apparatus may be used to print epoxy for use as underfill for chip-scale packages. Print heads of embodiments of this disclosure can be used in stencil printers. In certain embodiments, the stencil printer may include an Accela® or Momentum® series stencil printer platform offered by Speedline Technologies, Inc. of Franklin, Mass. 
     The print head of embodiments of the present disclosure are designed to provide controlled application of printing material for automated stencil printing operations. As mentioned above, the printing material may be solder paste, adhesives, or other printing materials. The material is protected from the ambient environment and is applied to the stencil under a regulated, controlled pressure. The printing material is enclosed in an elongate housing whose only opening is on the bottom, where it is held close against the stencil. The print head includes a mechanical elongate plunger on a top of the housing, which provides the means for applying controlled pressure to the material onto the stencil. Several pressure sensors are provided along a length of the housing to measure internal material pressure within the chamber. During operation, there are two forces to consider, the first being a force of the physical housing pushing against the stencil, and the second being the pressure applied to the material inside the chamber of the housing against the stencil. The first force is controlled by a print head z-axis control and the second force is controlled by the housing plunger. 
     Referring now to the drawings, and more particularly to  FIG. 1 , there is generally indicated at  10  a stencil printer of an embodiment of the disclosure. As shown, the stencil printer  10  includes a frame  12  that supports components of the stencil printer. The components of the stencil printer may include, in part, a controller  14 , a display  16 , a stencil  18 , and a print head assembly or print head, generally indicated at  20 , which is configured to apply the solder paste in a manner described in greater detail below. 
     As shown in  FIG. 1  and described below, the stencil and the print head may be suitably coupled or otherwise connected to the frame  12 . In one embodiment, the print head  20  may be mounted on a print head gantry  22 , which may be mounted on the frame  12 . The print head gantry  22  enables the print head  20  to be moved in the y-axis direction under the control of the controller  14  and to apply pressure on the print head as it engages the stencil  18 . As described below in further detail, the print head  20  may be placed over the stencil  18  and may be lowered in the z-axis direction into contact and sealingly engage the stencil. 
     The stencil printer  10  may also include a conveyor system having rails  24 ,  26  for transporting a printed circuit board (sometimes referred to as a “printed wiring board,” “substrate,” or “electronic substrate” herein) to a print position in the stencil printer. The rails  24 ,  26  sometimes may be referred to herein as a “tractor feed mechanism,” which is configured to feed, load or otherwise deliver circuit boards to the working area of the stencil printer, which may be referred to herein as a “print nest,” and to unload circuit boards from the print nest. 
     The stencil printer  10  has a support assembly  28  to support the circuit board, which raises and secures the circuit board so that it is stable during a print operation. In certain embodiments, the substrate support assembly  28  may further include a particular substrate support system, e.g., a solid support, a plurality of pins or flexible tooling, which is positioned beneath the circuit board when the circuit board is in the print position. The substrate support system may be used, in part, to support the interior regions of the circuit board to prevent flexing or warping of the circuit board during the print operation. 
     In one embodiment, as will be described in greater detail below, the print head  20  may be configured to receive solder paste from a source, such as a dispenser, e.g., a solder paste cartridge, that provides solder paste to the print head during the print operation. Other methods of supplying solder paste may be employed in place of the cartridge. For example, solder paste may be manually deposited between the blades or from an external source. Additionally, in a certain embodiment, the controller  14  may be configured to use a personal computer having a suitable operating system, such as a Microsoft DOS or Windows XP operating system, with application specific software to control the operation of the stencil printer  10 . The controller  14  may be networked with a master controller that is used to control a production line for fabricating circuit boards. 
     In one configuration, the stencil printer  10  operates as follows. A circuit board is loaded into the stencil printer  10  using the conveyor rails  24 ,  26 . The support assembly  28  raises and secures the circuit board to a print position. The print head  20  is then lowered in the z-axis direction until the blades of the print head contact the stencil  18  at a desired pressure. The print head  20  is then moved in the y-axis direction across the stencil  18  by the print head gantry  22 . The print head  20  deposits solder paste through apertures in the stencil  18  and onto the circuit board. Once the print head has fully traversed the stencil  18  across the apertures, the print head is lifted off the stencil and the circuit board is lowered back onto the conveyor rails  24 ,  26 . The circuit board is released and transported from the stencil printer  10  so that a second circuit board may be loaded into the stencil printer. To print on the second circuit board, the print head is lowered in the z-axis direction into contact with the stencil and moved across the stencil  18  in the direction opposite to that used for the first circuit board. 
     Still referring to  FIG. 1 , an imaging system  30  may be provided for the purposes of aligning the stencil  18  with the circuit board prior to printing and to inspect the circuit board after printing. In one embodiment, the imaging system  30  may be disposed between the stencil  18  and the support assembly  28  upon which a circuit board is supported. The imaging system  30  is coupled to an imaging gantry  32  to move the imaging system. In one embodiment, the imaging gantry  32  may be coupled to the frame  12 , and includes a beam that extends between side rails of the frame  12  to provide back and forth movement of the imaging system  30  over the circuit board in a y-axis direction. The imaging gantry  32  further may include a carriage device, which houses the imaging system  30 , and is configured to move along the length of the beam in an x-axis direction. The construction of the imaging gantry  32  used to move the imaging system  30  is well known in the art of solder paste printing. The arrangement is such that the imaging system  30  may be located at any position below the stencil  18  and above the circuit board to capture an image of predefined areas of the circuit board or the stencil, respectively. In other embodiments, when positioning the imaging system outside the print position the imaging system may be located above or below the stencil and the circuit board. 
     Referring to  FIGS. 2 and 3 , the print head  20  and the print head gantry  22  are illustrated. As shown, the print head gantry  22  rides along a pair of rails  34 ,  36  to achieve movement of the print head gantry and the print head  20  in the y-axis direction. A ball screw  38  coupled to a drive motor (not shown) is provided to drive the movement of the print head gantry  22  along the rails in the y-axis direction. In another embodiment, a belt and pulley mechanism may be provided in place of the ball screw drive mechanism to achieve the movement of the print head gantry  22  along the rails. A drive assembly  40  is provided to move the print head  20  in a z-axis direction with respect to the print head gantry  22 . The arrangement is such that the ball screw  38 , upon command of the controller  14 , moves the print head gantry  22  and the print head  20  to perform a print stroke. As will be described in greater detail below, the print head  20  is configured to move in the z-axis direction to engage the stencil  18  for depositing viscous material (e.g., solder paste) within the apertures of the stencil during the print stroke as the print head  20  moves in the y-axis direction. 
     The print head gantry  22  supports the working components of the print head  20 . As shown, the print head  20  includes a support bracket  42  that is suitably secured to the print head gantry  22  so that the print head is centrally positioned on the print head gantry. The print head  20  further includes an elongate chamber housing  44 , which is secured to the support bracket  42 , a plunger  46 , and a plunger actuator assembly  48  designed to move the plunger vertically with respect to the chamber housing. 
     Referring to  FIGS. 3-5 , the chamber housing  44  includes a rectangular body  50  having a top wall  52  and two side walls  54 ,  56 . The ends of the body  50  chamber housing  44  are enclosed by a pair of end caps  58 ,  60 , which support respective material supply cartridges  62 ,  64 . Specifically, the left-hand supply cartridge  62  as shown in  FIG. 3  is connected to and supported by a fitting  66  that is secured to the end cap  58  to deliver viscous material to a left-hand side of the chamber housing  44 . Similarly, the right-hand supply cartridge  64  is connected to and supported by a fitting  68  that is secured to the end cap  60  to deliver viscous material to a right-hand side of the chamber housing  44 . The arrangement is such that viscous material is delivered to both ends of the chamber housing  44  to ensure that a suitable amount of viscous material is disposed in the chamber housing during a dispense operation. One or more additional supply cartridges may be suitably provided along the length of the chamber housing  44  to deliver viscous material to the interior regions of the chamber housing for longer print heads. 
     Referring now particularly to  FIGS. 4 and 5 , the elongate plunger  46  is designed to sealingly fit within an elongate chamber provided in the chamber housing  44  and accessible through an opening  70  formed in the top wall  52  of the chamber housing. As shown, the elongate plunger  46  includes a narrow body portion  72  that reciprocally moves within the elongate chamber, and a top flange portion  74 , which is wider than the body portion. The plunger actuator assembly  48  includes a plunger actuator  76  that is secured to the top flange portion  74  of the elongate plunger  46 , a pair of ball screws  78 ,  80  to enable z-axis movement of the plunger actuator with respect to the support bracket  42 , a pair of pulleys  82 ,  84 , which are driven by a belt  86  and a single stepper motor  88  mounted on the support bracket. The stepper motor  88  is shown mounted on a top surface of the support bracket  42 ; however, in another embodiment, the stepper motor may be mounted below the support bracket in a space defined by the plunger actuator  76  to save space. A circuit board  90 , coupled to the controller  14 , is provided to control the operation of the stepper motor  88  as well as the other components of the print head  20 . The arrangement is such that upon operation of the stepper motor  88 , the belt  86  and the pulleys  82 ,  84  rotate their respective ball screws  78 ,  80  to move the plunger actuator  76  in the z-axis direction. When operated, the body portion  72  of the elongate plunger  46  enters into the chamber of the chamber housing  44  to apply pressure on the viscous material. 
     Referring to  FIGS. 6A and 6B , with additional reference to  FIG. 7 , the chamber housing  44  of the print head  20  further includes a chamber  92  formed in the body  50  of the chamber housing. As shown, the chamber  92  is accessible through the opening  70  and configured to receive viscous material from a port (e.g., port  94  shown in  FIGS. 6A and 6B ) in fluid communication with its respective supply cartridge  62  or  64 . The viscous material in the chamber  92  is pressurized by the elongate plunger  46  when the plunger is moved downwardly in the z-axis direction by the plunger actuator assembly  48 . 
     The print head  20  further includes a pair of blades  96 ,  98 , which are pivotally attached to respective end caps (end cap  58  shown in  FIGS. 6A and 6B ) by pivot mechanisms  100 ,  102 . As shown, each blade  96 ,  98  is angled in a direction toward one another to define a slot  104  through which viscous material is dispensed. During operation, the print head  20  is lowered to engage the stencil  18  by the drive assembly  40  so that the blades  96 ,  98  engage the stencil at a predetermined pressure. Once sufficiently engaged, the viscous material within the chamber  92 , which is pressurized by the elongate plunger  46 , enters the apertures formed in the stencil to deposit viscous material on the electronic substrate.  FIG. 6A  illustrates blades  96 ,  98  angled at 30° with respect to a horizontal plane.  FIG. 6B  illustrates blades  96 ,  98  angled 45° with respect to a horizontal plane. The angle of the blades  96 ,  98  may be selected based on the type of viscous material deposited on the electronic substrate as well as other operating parameters. The provision of end caps  58 ,  60  having pivot mechanisms  100 ,  102  connected to the blades  96 ,  98  enable the quick release and attachment of blades having different angles. 
     Referring back to  FIG. 5 , with additional reference to  FIGS. 6A and 6B , to achieve a regulated pressure of viscous material within the chamber  92 , the back side  56  of the elongate housing  44  is provided with several pressure transducers, each indicated at  106 . Together, the pressure transducers  106  are configured to measure pressure within the chamber  92  of the chamber housing  44  by, for example, averaging the pressure within the chamber with the controller  14  and the circuit board  90 . Although four pressure transducers  106  are shown in  FIG. 5 , any number of pressure transducers may be provided, e.g., three pressure transducers, to measure the pressure of viscous material within the chamber  92 . 
     As shown in  FIGS. 6A and 6B , when the plunger  46  is moved in the z-axis direction into the elongate chamber  92  by the plunger actuator  76  of the plunger actuator assembly  48 , the viscous material within the chamber is pressurized. In certain embodiment, a desired or preset pressure is provided for optimal dispensing of material from the print head  20 . Each pressure transducer  106  is coupled to the controller  14  and/or the circuit board  90  at  108 , with the controller being configured to manipulate the plunger actuator to achieve the preset pressure. Specifically, the controller  14  is configured to respond to readings taken from the pressure transducers  106  to control the operation of the plunger  46  with the plunger actuator assembly  48  and the supply of viscous material provided to the chamber  92  by the supply cartridges  62 ,  64 . This arrangement may be referred to as a “closed loop” system in that the supply of material and the pressurization of the material are automatically performed by the controller  14  (and/or the circuit board  90 ) to control this function of the print head  20 . 
     Referring to  FIG. 8 , in one embodiment, a flexible membrane  110  is positioned between the elongate plunger  46  and the viscous material disposed within the elongate chamber  92 . The flexible membrane  110  is provided to separate the elongate plunger  46  from the viscous material to better control the pressure of viscous material within the chamber  92 . 
     In a certain embodiment, the stepper motor  88  that drives the movement of the elongate plunger  46  within the elongate chamber  92  may be programmed by the controller  14  (and/or the circuit board  90 ) to cause the vibration of the viscous material (e.g., solder paste) within the chamber. The vibration of the plunger  46  aides in the transfer of viscous material from the print head  20  to the stencil  18  when filling empty apertures provided in the stencil. In other embodiments, another device, such as a piezo electric device, may be used to vibrate the elongate plunger  46 , especially at higher frequencies. Such a device may be independent of the stepper motor  88 , the elongate plunger  46 , and plunger actuator assembly  48 . 
     During operation, several modes of operation of the print head  20  can be achieved. In a “ready to print” mode, an adequate fill of material is delivered to the elongate chamber  92  of the print head  20 , and the blades  96 ,  98  touch the stencil  18  with zero housing force and zero material pressure. 
     In a “print” mode, the blades  96 ,  98  engage or are otherwise moved against the stencil  18  with the prescribed force with viscous material held in the elongate chamber  92  being forced by the elongate plunger  46  while the print head  20  is driven across the stencil and board at a prescribed distance and velocity by the print head gantry  22 . While in “print” mode, the internal material pressure of the elongate chamber  92  is monitored and the elongate plunger  46  position is continually adjusted to maintain a prescribed pressure by the controller  14  (and/or the circuit board  90 ). Also, an oscillatory or vibrational motion can be applied to the plunger  46  to aid in transferring and packing the viscous material evenly into the apertures of the stencil  18 . This motion would also be under the control of the controller  14  (and/or the circuit board  90 ). 
     In a “filling material” mode, the blades  96 ,  98  of the print head  20  are held against the supported stencil  18  or flat surface with a prescribed force. Initially, the elongate plunger  46  is at or below the lower end of the print operational end of travel or touching the top of the existing fill of material. As viscous material is fed into the elongate chamber  92  through the port  94  by the respective supply cartridges  62 ,  64 , internal material pressure is monitored and the elongate plunger  46  is raised so as to maintain a prescribed fill pressure. Thus, the movement of the elongate plunger  46  is coordinated with the filling material by the controller  14  (and/or the circuit board  90 ). The filling continues until the elongate plunger  46  reaches the top of the print operational range of travel. While material filling is in progress, an oscillatory motion as described above can be applied to the plunger  46  to aid in distributing the material evenly throughout the housing chamber  92 . 
     In a “cleaning” mode, the elongate plunger  46  can be raised out of the chamber  92  of the elongate housing  44  to aid in cleaning the plunger and the housing. 
     In a “raised” mode, the elongate housing  44  can be raised off of the stencil  18  to aid in cleaning or to change stencils. The “raised” mode can utilize a z-axis motion routine to relieve the adhesion between the print head  20  and the stencil  18 . 
     In one embodiment, the print head includes source ports at each end configured to receive standard three-ounce or six-ounce solder paste cartridges that provide solder paste to the print head during a printing operation. Each of the cartridges is coupled to one end of a pneumatic air hose with the other end of the air hose being coupled with an air compressor that forces solder paste from the cartridge into the print head. As is readily understood by those skilled in the art, the print head can be adapted to receive other standard, or non-standard, cartridges or other sources of solder paste. A mechanical device, such as a piston, may be used in addition to, or in place of, air pressure to force the solder paste from the cartridges into the print head. 
     Any number of source ports can be provided on the print head to couple more or fewer cartridges with the print head. The number of cartridges is selected based on the length of the print head and the capacity of the cartridge used. The length of the print head is determined, in part, based on the width of the electronic substrates (e.g., circuit boards) to be printed upon. If the size of the circuit board changes, the print head may be replaced by a new print head having a length sized for the new circuit board. The effective length of the dispensing slot (where the length is measured along an axis perpendicular to the plane of the illustrated section) may also be reduced to accommodate smaller circuit boards by partially covering a portion of the slot. In a certain embodiment, the dispensing slot is about 20 cm to about 60 cm (8-24 inches) in length. 
     As discussed, the print head includes a housing that defines an elongate chamber. In one embodiment, the chamber is segregated by a flexible membrane ( FIG. 8 ) into an activation region and a dispensing region. In particular embodiments, the dispensing region has a length of 20-60 cm and a diameter/width of about 1 cm. The membrane can be formed from a flexible polymer, such as latex or silicone. 
     As further discussed, when the print head is in the lowered printing position so that it is in contact with the stencil, the stencil printer operates by feeding solder paste from the cartridge into the dispensing region. With the dispensing region filled with solder paste, the compressed gas source pumps gas into the activation region, thereby exerting pressure against the flexible membrane. The flexible membrane is displaced toward the slot as a result of this pressure, and solder paste is thereby driven from the dispensing region through the slot and out of the chamber. The dispensed solder paste then flows onto a stencil positioned over a substrate, such as a printed circuit board, the solder flowing through apertures in the stencil onto the printed circuit board in a pre-defined pattern. 
     Between the two blades, the solder paste is pressurized. To prevent the pressurized solder paste from breaking the seal formed at the interface of the blades and the stencil, force is applied to the print head to press it against the stencil. The force can be applied via a pneumatic actuator. At the end of a stroke across the stencil by the print head, the blade that was previously the trailing blade becomes the leading blade as the print head is passed back across the stencil in the opposite direction. 
     In a certain embodiment, a print head coupled with a controller having a computer-readable storage medium coupled with a processor for executing software code stored on the storage medium. 
     Use of the displacement mechanism (with or without the flexible membrane) allows for a print head design where the displacement region of the chamber can have a very small cross-section in planes perpendicular to the long axis of the chamber, yet the solder paste can still be dispensed substantially evenly through the slot along its length. Use of a comparatively small dispensing region and the even flow of solder paste out of the chamber help to reduce or delay the onset of solder-paste compaction. Moreover, the displacement mechanism enables highly responsive governance over the flow of solder paste through the slot. 
     In other embodiments, viscous materials other than solder paste are printed using the apparatus and methods described above. In one embodiment, liquid epoxy is printed from the print head. Liquid epoxy can be used as an underfill for chip-scale packages, where the epoxy is deposited onto an area about 1 cm 2  on a printed circuit board, and the chip is then mounted onto the epoxy. 
     Thus, it should be observed that the print head of embodiments of the present disclosure is capable of precisely controlling pressure within a chamber of the print head during a print operation. The precise pressure control achieved by the print head improves the quality of the print operation. Specifically, the print head is capable of maintaining a consistent pressure of viscous material within the chamber to improve the integrity of the print deposit across a width of the entire electronic substrate. 
     Embodiments are not limited in their application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. Also, the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having,” “containing,” “involving,” and variations thereof herein, is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. 
     Having thus described several aspects of at least one embodiment, it is to be appreciated various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the scope of the invention. Accordingly, the foregoing description and drawings are by way of example only.