Apparatus for dispensing material in a printer

A printer for printing a viscous material at predetermined positions forming a pattern on a substrate. The printer includes a frame, a device having a number of perforations arranged to form the pattern, a support apparatus that supports the substrate in a printing position, and a material dispenser having a substantially cylindrical chamber to contain the viscous material. The chamber has an opening through which the material is dispensed. The material dispenser is positioned over the device and constructed and arranged to dispense the viscous material through the perforations in the device onto the substrate. In embodiments of the present invention, the material dispenser is adapted to receive standard cartridges containing the viscous material, and the printer further includes a pressurized air source for forcing the viscous material from the cartridges into the material dispenser and from the material dispenser through the perforations on the device and onto the substrate.

FIELD OF THE INVENTION 
The present invention relates to an apparatus and process for dispensing 
material, and more specifically to an apparatus and process for dispensing 
solder paste in a screen or stencil printer. 
BACKGROUND OF THE INVENTION 
In typical surface-mount circuit board manufacturing operations, a stencil 
printer is used to print solder paste onto a circuit board. Typically, a 
circuit board having a pattern of pads or some other, usually conductive, 
surface onto which solder paste will be deposited is automatically fed 
into the stencil printer and one or more small holes or marks on the 
circuit board, called fiducials, is used to properly align the circuit 
board with the stencil or screen of the stencil printer prior to the 
printing of solder paste onto the circuit board. In some prior art 
systems, an optical alignment system is used to align the circuit board 
with the stencil. Examples of optical alignment systems for stencil 
printers are described in U.S. Pat. No. 5,060,063, issued Oct. 21, 1991 to 
Freeman, and in U.S. Pat. No. Re. 34,615, issued Jan. 31, 1992, also to 
Freeman, each of which is incorporated herein by reference. 
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 desired viscosity to 
facilitate filling of the apertures in the screen or stencil. The solder 
paste is typically dispensed onto the stencil from a standard cartridge 
such as that manufactured by SEMCO Corporation. 
In some prior art 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 prior art screen printers, a second 
squeegee is used that 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 each successive circuit 
board. In the prior art 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, excess solder paste typically remains on the 
stencil and must be manually removed. Also, in these prior art 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 the prior art stencil printers discussed above, the squeegee blades are 
typically at a predetermined angle with 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 typical prior art printers, 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. 
It is desirable to provide a method and apparatus for dispensing material 
onto a stencil of a printer that overcome the problems discussed above. 
SUMMARY OF THE INVENTION 
Embodiments of the present invention provide methods for dispensing 
material and stencil printers having a dispensing apparatus that overcome 
the problems of the prior art discussed above. 
In a first embodiment of the present invention, a printer for printing a 
viscous material at predetermined positions forming a pattern on a 
substrate is provided. The printer includes a frame, a device, mounted to 
the frame, having a number of perforations arranged to form the pattern, a 
support apparatus, coupled to the frame that supports the substrate in a 
printing position beneath the device, and a material dispenser having a 
substantially cylindrical chamber to contain viscous material to be 
printed on the device. The chamber has an opening through which the 
viscous material is dispensed. The material dispenser is coupled to a 
frame, positioned over the device, and constructed and arranged to 
dispense the viscous material through the perforations in the device and 
onto the substrate. 
Alternate versions of the first embodiment of the present invention include 
a number of different features. In one version, the material dispenser is 
constructed and arranged to be movable along a first axis across the 
device while the viscous material is being dispensed from the chamber. In 
another version, the chamber has a cylindrical axis extending along a 
length of the chamber, and the interior surface of the chamber is coated 
with a coating material having a low coefficient of friction to allow 
mixing of the viscous material within the chamber when the material 
dispenser is moved across the device. In alternate embodiments, different 
coatings having both high and low coefficients of friction may be used on 
the interior surface of the chamber to enhance the laminar flow of 
material in the chamber. 
In another version of the first embodiment, the printer further includes a 
heater to heat the viscous material or a cooler to cool the material, and 
the material dispenser includes at least one port to receive pressurized 
paste to increase the paste pressure in the chamber to force viscous 
material from the chamber. 
In yet another version of the first embodiment, the material dispenser 
further includes a pressure sensor that senses pressure within the 
chamber, and the printer further includes a controller, coupled to the 
pressure sensor, that senses the pressure within the chamber and maintains 
the pressure at a desired value. 
In another version of the first embodiment, the material dispenser is 
adapted to receive a removable cartridge, and in some versions, the 
removable cartridge is a standard SEMCO cartridge. 
In still another version of the first embodiment, the material dispenser 
includes a pair of inwardly facing blades with side dams that contact the 
device during printing to prevent excess material from accumulating on the 
device. 
In another version of the first embodiment, the printer further includes a 
solder gathering squeegee arm that collects excess viscous material 
remaining on the device when the material dispenser is lifted off of the 
device. 
In a second embodiment of the present invention, a printer for printing a 
viscous material at predetermined positions forming a pattern on a 
substrate is provided. The printer includes a frame, a device, mounted to 
the frame, having a number of perforations arranged to form a pattern, a 
support apparatus that supports the substrate in a printing position 
beneath the device, and a material dispenser having a chamber to contain 
the viscous material to be printed on the substrate. The chamber has an 
opening through which the viscous material is dispensed. The material 
dispenser is positioned over the device, constructed and arranged to 
dispense the viscous material through the perforations in the device and 
onto the substrate, and adapted to receive a removable cartridge that 
supplies the viscous material to the chamber. The chamber has an inlet 
adapted to receive the viscous material from the removable cartridge. 
Alternate versions of the second embodiment of the present invention may 
include one or more of the features of versions of the first embodiment 
discussed above. 
A third embodiment of the present invention provides a material dispenser 
for dispensing a viscous material onto a stencil of a printer. The 
material dispenser includes a substantially cylindrical chamber to contain 
the viscous material. The chamber has an opening through which the viscous 
material is dispensed. The material dispenser also includes a pair of 
inwardly facing blades and side dams mounted on the material dispenser in 
close proximity to the opening. The blades are adapted to contact the 
stencil during printing to prevent excess material from remaining on the 
device. 
In one version of the material dispenser, an interior surface of the 
chamber is coated with a coating material having a low coefficient of 
friction to allow mixing of the viscous material within the chamber when 
the material dispenser is moved across the device.

DETAILED DESCRIPTION 
For purposes of illustration, embodiments of the present invention will now 
be described with reference to a stencil printer used to print solder 
paste onto a circuit board. One skilled in the art will appreciate, 
however, that embodiments of the present invention are not limited to 
stencil printers that print solder paste onto circuit boards, but rather, 
may be used in other applications requiring dispensing of other viscous 
materials such as glues and encapsulents. Further, stencil printers in 
accordance with embodiments of the present invention are not limited to 
those that print solder paste on circuit boards, but rather, include those 
used for printing other materials on a variety of substrates. Also, the 
terms screen and stencil may be used interchangeably herein to describe a 
device in a printer that defines a pattern to be printed onto a substrate. 
FIG. 1 shows a front view of a stencil printer 10 in accordance with one 
embodiment of the present invention. The stencil printer 10 includes a 
frame 12 that supports components of the stencil printer including a 
controller 14, a stencil 16, and a dispensing head 100 having a dispensing 
slot 102 from which solder paste may be dispensed. 
The dispensing head 100 is coupled to a first plate 18 using two 
thumbscrews 22. The first plate 18 is coupled to a second plate 20 which 
is coupled to the frame 12 of the stencil printer 10. The first plate 18 
is coupled to the second plate 20 in such a manner that the first plate 
can be moved with respect to the second plate along a z axis, the z axis 
being defined by the coordinate axis system 23 shown in FIG. 1. The first 
plate is moved by motors under the control of the controller 14. 
The second plate 20 is movably coupled to the frame 12 such that the second 
plate 20 can move with respect to the frame 12 along an x axis, the x axis 
also being defined by the coordinate axis system 23. As described below in 
further detail, the movements of the first and second plates allow the 
dispensing head 100 to be placed over the stencil 16 and moved across the 
stencil to allow printing of solder paste onto a circuit board. 
Stencil printer 10 also includes a conveyor system having rails 24 for 
transporting a circuit board 26 to a printing position in the stencil 
printer. The stencil printer has a number of pins 28, positioned beneath 
the circuit board when the circuit board is in the dispensing position. 
The pins are used to raise the circuit board 26 off of the rails 24 to 
place the circuit board in contact with, or in close proximity to, the 
stencil 16 when printing is to occur. 
The dispensing head 100 is configured to receive two standard SEMCO three 
ounce or six ounce solder paste cartridges 104 that provide solder paste 
to the dispensing head during a printing operation. Each of the solder 
paste cartridges 104 is coupled to one end of a pneumatic air hose 30. As 
readily understood by those skilled in the art, the dispensing head could 
be adapted to receive other standard, or non-standard, cartridges. The 
other end of each of the pneumatic air hoses is attached to a compressor 
that under the control of the controller 14 provides pressurized air to 
the cartridges to force solder paste to flow from the cartridges into the 
dispense head 100 and onto the screen 16. Mechanical devices, such as a 
piston, may be used in addition to, or in place of, air pressure to force 
the solder paste from the SEMCO cartridges into the dispensing head. 
In one embodiment of the present invention, the controller 14 is 
implemented using a personal computer using the Microsoft DOS or 
Windows.RTM. NT operating system with application specific software to 
control the operation of the stencil printer as described herein. 
The stencil printer 10 operates as follows. A circuit board 26 is loaded 
into the stencil printer using the conveyor rails 24. The dispensing head 
is then lowered in the z direction until it is in contact with the stencil 
16. Pressurized air is provided to the cartridges 104 while the dispensing 
head is moved in the x direction across the stencil 16. The pressurized 
air forces solder paste out the cartridges and creates pressure on the 
solder paste in the dispensing head forcing solder paste from the 
dispensing slot of the dispensing head through apertures in the stencil 16 
and onto the circuit board 26. Once the dispensing head 100 has fully 
traversed the stencil 16, the circuit board is lowered back onto the 
conveyor rails 24 and transported from the printer so that a second 
circuit board may be loaded into the printer. To print on the second 
circuit board, the dispensing head is moved across the stencil in the 
direction opposite to that used for the first circuit board. 
Alternatively, a squeegee arm (as described below) could swing in to 
contain the solder paste in the dispenser, and the dispenser can then be 
lifted in the z direction and moved back to its original position to 
prepare to print on the second circuit board using a similar direction 
stroke. Further description of the operation and construction of the 
dispensing head 100 are provided below with reference to FIGS. 2-5. 
The dispensing head 100 includes a housing 105 coupled to two supporting 
arms 106. At one end of each of the supporting arms 106 are thumbscrews 22 
that provide for easy removal and installation of the dispensing head 100. 
The housing 105 has two ports 108 that are adapted to receive a standard 
SEMCO solder paste cartridge. In the illustrative embodiment described 
herein, the dispensing head is adapted to receive two SEMCO cartridges, 
however, the dispensing head may be adapted to include more or less 
cartridges than the two shown herein. The number of cartridges used is 
selected based on the length L of the dispensing head and the capacity of 
the cartridge used. The length L is determined in part based on the width 
of the circuit boards to be printed upon. If the size of the circuit board 
changes, then the dispensing head may be replaced by a new dispensing head 
having a length L sized for the new circuit boards. The effective length 
of the slot 102 may also be reduced to accommodate smaller circuit boards 
by partially covering a portion of the slot. 
The housing 105 includes a cylindrical chamber 110 in which solder paste 
received from the SEMCO cartridges is stored prior to being dispensed onto 
the stencil. In one embodiment, the interior walls of the chamber 110 are 
coated with a material having a low coefficient of friction. Other 
materials, having a wide range of coefficients of friction, are used for 
the coating to create a desired laminar flow of solder paste in the 
chamber. In one embodiment, a number of different coatings having a 
variety of coefficients of friction may be used to create the desired 
laminar flow. At the slot 102, the housing has two inwardly facing blades 
112 and 114. Each of the inwardly facing blades has a length approximately 
equal to the length L of the slot, a width equal to approximately 0.138 
inches, and a thickness of approximately 0.004 to 0.010 inches. Each of 
the blades 112 and 114, in one embodiment of the present invention, is 
made from spring steel. At each end of the blades is a side dam 103. While 
other materials such as plastic could be used to make the blades, the use 
of spring steel provides long life with continued resiliency. Each of the 
blades is arranged to provide an angle of approximately 15 degrees between 
the blade and the top surface of the stencil. Depending in part on the 
material used to make the blades and the speed at which the material 
dispenser traverses the stencil, the blades could be oriented at angles 
other than 15 degrees. 
The dispensing head 100, in one embodiment of the present invention, 
further includes a pressure sensor 116 and a temperature controller 120. 
Incorporated within the temperature controller is a thermoelectric device 
and a temperature sensor. The pressure sensor 116 and the temperature 
controller 120 are coupled to the controller 14 through cables 118 and 122 
respectively. The controller 14 is configured to measure the pressure and 
the temperature of the solder paste within the chamber 110 based on 
signals received from the sensors, and the controller controls the 
pressure and temperature by adjusting the force applied to the solder 
paste in the cartridges and by controlling the thermoelectric device 
contained in the temperature controller. In one embodiment, the pressure 
of the solder paste is maintained in the chamber by applying pressurized 
air at approximately 3-15 pounds per square inch to each of the cartridges 
104. The temperature of the solder paste in the chamber is maintained 
during the printing operation at approximately 68 degrees Fahrenheit. In 
one embodiment, the pressure sensor is implemented using a pressure 
transducer with a programmable regulator and the temperature controller is 
implemented using a temperature probe with a thermoelectric device that 
can provide cooling as well as heating of the solder paste. 
As discussed above, when the dispensing head is in the lowered printing 
position so that it is in contact with the stencil, the stencil printer 10 
operates by forcing solder paste from the dispensing head 100 onto the 
stencil using air pressure applied to each of the SEMCO cartridges as the 
dispensing head moves across the stencil. In the printing position, the 
blades 112 and 114 contact the top surface of the stencil. For each 
direction that the dispensing head moves across the stencil, one of the 
blades 112 and 114 will be a trailing blade and will scrape any excess 
solder paste off the stencil. For example, when the dispensing head 100 
moves in the direction of arrow 122 in FIG. 3, blade 112 will be the 
trailing blade removing any excess solder paste from the stencil. The 
orientation of the blades and the use of a cylindrical chamber in the 
dispensing head causes the solder paste within the chamber 110 to roll or 
flow in the direction of arrow 124 when the dispensing head is moved in 
the direction of arrow 122. The movement of the solder paste within the 
chamber causes mixing and shearing of the solder paste which helps to 
maintain the desired temperature and viscosity of the solder paste. The 
coating for the internal walls of the chamber 110 allows the solder paste 
to move freely within the chamber. 
At the conclusion of printing, when it is desired to lift the dispensing 
head off of the stencil, the controller 14 turns off the pressurized air 
source, prior to lifting the dispensing head. This causes the solder paste 
to remain in the chamber 110 when the dispensing head is lifted and 
effectively reduces the amount of solder paste that is left on the stencil 
when printing is complete. 
To further reduce the amount of residual solder paste on the stencil, a 
solder gathering squeegee arm may be used to remove excess solder paste. 
One example of a solder gathering squeegee arm that could be used in 
embodiments of the present invention is described in U.S. Pat. No. 
5,044,306, which is incorporated herein by reference. FIG. 6 shows a 
solder gathering arm 130 incorporated into the dispensing head 100. The 
solder gathering arm includes a squeegee 131. The solder gathering arm is 
mounted to a bracket 132 which in turn is mounted to one or both of the 
supporting arms 106. The solder gathering arm is mounted to the bracket 
132 using a rotatable joint 134, and the bracket 132 is movable with 
respect to the supporting arms 106 in the direction shown by arrow 122. 
The solder gathering arm operates as follows. When the dispensing head 100 
is lifted slightly off of the stencil, the bracket 132 is moved in the 
direction of arrow 122, and the solder gathering arm is rotated in the 
direction shown by arrow 136 causing the squeegee 131 to scrape along the 
stencil and remove any excess solder on the stencil. The rotation of the 
solder gathering arm and the movement of the bracket 132 is accomplished 
using motors or actuators under the control of the controller 14. 
In embodiments of the present invention, instead of using the solder 
gathering arm, the blades 112 and 114 may be attached to motors or 
actuators that under the control of the controller cause the blades to 
move towards each other, scraping any excess solder paste from the screen 
and completely closing the slot 102. Alternatively, a pair of movable 
blades, in addition to blades 112 and 114, may be mounted to the 
dispensing head and be movable towards each other to scrape excess solder 
paste from the stencil. 
U.S. patent application Ser. No. 08/598,288, filed on Feb. 8, 1996, 
assigned in part to the assignee of the present application, and 
incorporated herein by reference, discloses a screen printer having a 
solder applicator that applies solder paste to a stencil. In one 
embodiment, the solder applicator includes a rotatable member that mixes 
solder paste in the solder applicator. 
Embodiments of the present invention described above may also be 
incorporated in a dual track stencil printer such as those described in 
U.S. patent application Ser. No. 08/802,934, filed Feb. 21, 1997, which is 
incorporated herein by reference. 
Embodiments of the present invention described above include standard SEMCO 
cartridges for loading solder paste into the dispensing head. Other 
removable cartridges may be used in place of the SEMCO cartridges. 
However, it is desirable to use a standard replaceable cartridge. 
The chamber of dispensing heads in embodiments of the present invention 
have been described as being cylindrical. The dispensing head need not be 
cylindrical, but rather, other shapes having a substantially circular 
cross-section to allow rolling of the solder paste within the chamber to 
create a substantially laminar flow may also be used. In addition, in one 
embodiment, the chamber is generally kidney-shaped, and the specific shape 
is programmable for controlling the flow of solder paste within the 
chamber based on the specific solder paste material used, the speed of the 
dispensing head across the stencil and based on any other factors. 
The dispensing head of embodiments of the present invention is described as 
being used with a stencil printer. Stencil printers such as the Ultraprint 
3000 stencil printer and the AP Series stencil printer, both available 
from MPM Corporation of Franklin, Mass, can readily be adapted for use 
with embodiments of the present invention. 
Embodiments of the present invention overcome problems associated with 
prior art stencil printers by providing dispensing heads and stencil 
printers that effectively remove excess solder paste from the stencil. In 
addition, the pressure applied to the solder paste to force it through 
apertures in the stencil is precisely controlled using a closed loop 
feedback system. 
Having thus described at least one illustrative embodiment of the 
invention, various alterations, modifications and improvements will 
readily occur to those skilled in the art. Such alterations, modifications 
and improvements are intended to be within the scope and spirit of the 
invention. Accordingly, the foregoing description is by way of example 
only and is not intended as limiting. The invention's limit is defined 
only in the following claims and the equivalents thereto.