Note: Descriptions are shown in the official language in which they were submitted.
S33~3
METHOD AND APPARATUS FOR MULTI POINT DISPENSING
OF VISCOUS MATERIAL
TECHNICAL FIELD
The present invention is directed to a technique
for multi point dispensing of viscous materials and more
particularly, to a method and an apparatus for multi point
dispensing of solder paste for surface mounting of chip
carriers on circuit boards.
BACKGROUND OF THE INVENTION
Over the past few years, the complexity and
capabilities of integrated circuits (It's) have grown while
their dimensions continued to shrink. The rapid advances
in semiconductor technology nearly doubles the number of
circuit functions per semiconductor chip per year. In
order to keep pace with the very large scale integration
technology, the semiconductor chips require higher pin out
packages having small sizes thereby achieving optimal
electronic performance and increased circuit board
densities. Because of the foregoing, the well known dual
in-line packages (Dips) are gradually being replaced by
surface-mounted chip carrier packages.
Coincident with toe higher packing density of
surface-mounted components is the corresponding reduction
in solder joint size and spacing on the printed wiring
board (PUB, as well as the increase in number of solder
joints on each PUB assembly. Several methods of solder
deposition are described in a paper by D. Schoenthaler
entitled "Soldering Circuit Assemblies in the 1980's"
published in Proceedings ox Printed Circuit World
Convention II, Munich, West Germany Vol. 1, pages 131-140,
June 9-12, 1981. When using screen or stencil printing of
solder on a PUB, the volume of solder deposited on the PUB
must be sufficient to prevent joint opens due to poor
component lead plenarily, PUB war page, inaccurate component
placement and misregistration ox the solder mask with
respect to the bond sites on the PUB On the other hand,
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in view of the reduced spacing between neighboring solder
joints on the PUB, the quantity ox solder deposited should
not be excessive to avoid an electrical short between two
adjacent solder joints.
As mentioned in the above reference by D.
Schoenthaler, an alternative method for depositing solder
paste on Pubs is a technique using a multi point dispensing
tool comprising a hollow chamber having a multi orifice
nozzle attached at one end thereof. The nozzle orifices
are spaced to match the PUB footprint pattern and the
corresponding Chip carrier lead geometry Applicants
observed that the implementation of multi point dispensing
techniques in a manufacturing environment encountered
various problems affecting the reliability and
reproducibility of the solder joints. Some of these
problems include the poor control of the volume and
geometry of the dispensed solder paste, clogging of the
orifices of the nozzle, variations in the paste viscosity,
and difficulty in achieving uniform paste distribution
among the orifices due to paste crust and/or void
formations within the dispensing tool.
Therefore, there exists a need for an improved
solder paste multi point dispensing technique capable of
being reliably and reproducibly implemented in a semi-
conductor manufacturing environment.
SUMMARY OF TOE INVENTION
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In accordance with an aspect of the invention there is provided a method for loading solder paste into a
solder dispensing tool comprising the steps of: placing a
supply container of solder paste in transfer relationship
with the top portion of the dispensing tool; applying
vacuum to two regions of the dispensing tool thereby
pulling solder paste out of the supply container into the
dispensing tool; vibrating the dispensing tool for a first
predetermined time period substantially along its axial
direction while the vacuum is applied; and removing the
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vacuum while maintaining the vibration of the dispensing
tool for a second predetermined time period.
In accordance with another aspect of the invention
there is provided an apparatus for Loading solder paste
from a supply container into a multi orifice solder
dispensing tool comprising: a vibratable table; a
supporting body attached to the table and adapted to
receive a lower portion of the dispensing tool; a plate
member adapted to be coupled on one side to a top portion
of the dispensing tool and on the other side to the supply
container; first means for applying vacuum to the top
portion of the dispensing tool via the plate member; and
second means for applying vacuum to the lower portion of
the dispensing tool via the supporting body.
In one embodiment of the invention, a method
for multi point dispensing of viscous material onto a
board comprises positioning a viscous material dispenser
having a plurality of orifices at a predetermined distance
from the board; during a first time interval, increasing
the presume within the dispenser to a predetermined
value thereby forcing viscous material out of the
dispenser orifices onto the board; moving the
dispenser, at the end of the first time interval,
relative to the board at a substantially constant
velocity while maintaining the dispensing pressure
substantially at said predetermined value for a
predetermined second time interval; an removing the
pressure within the dispenser at the end of the second time
interval.
Another embodiment of the invention comprises the
steps of placing a supply container of solder paste in
transfer relationship with the top portion of the
dispensing tool; applying vacuum to two regions of the
dispensing tool thereby pulling solder paste out of the
supply container into the dispensing tool; vibrating the
dispensing tool for a first predetermined time period
substantially along its axial direction while the vacuum is
applied; and removing the vacuum while maintaining the
vibration of the dispensing tool for a second predetermined
time period.
A further embodiment of the invention relates to a
method for handling solder paste within a solder paste
dispensing tool which comprises covering the top surface of
the paste within the tool with a liquid seal for preventing
crust layer formation therein.
In accordance with another embodiment of the
invention, a method for handling solder paste within a
multi orifice paste dispensing tool comprises placing the
orifice portion of the dispensing tool, when not in use,
in a container including a liquid seal material for
preventing the paste in the orifices from drying.
A still further embodiment of the invention is
directed to a method for controlling the viscosity of
solder paste within a multi orifice solder dispensing tool
comprising the steps of monitoring the temperature within
the tool; and circulating a flow of heating fluid proximate
to the exterior surface of the dispensing tool in response
to the temperature monitoring step.
BRIEF DESCRIPTION OF TEE DRAWINGS
FIG. 1 shows a known multi point solder paste
dispenser;
FIG. 2 is a perspective schematic representation
of the interior of a multi point dispensing nozzle in
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accordance with an embodiment of the invention;
FIGS. PA and 3B show time diagrams of the
dispensing parameters in accordance with one embodiment of
the invention;
FIG. 4 illustrates a solder paste loading
technique in accordance with another embodiment of the
invention;
FIG. 5 schematically shows a technique for
preventing paste crust formation in accordance with a
further embodiment of the invention;
FIG. 6 illustrates the effect of temperature on
solder paste viscosity; and
FIG. 7 illustrates a technique for controlling the
viscosity of the solder paste according to a still further
embodiment of the invention.
DETAILED DESCRIPTION
Although the following description relates to
multi point dispensing of solder paste, it is well within
the spirit and scope of the present invention to apply the
inventive concepts disclosed herein to dispensing other
types of viscous materials. Viscous materials of a type
having a dispense viscosity ranging from about 2x105 cups
to about 2X106 cups at l sea 1 shear rate may be
dispensed using the teachings described herein. In other
words, the various principles to be described hereafter are
readily applicable to materials such as adhesives (e.g.,
conductive as well as nonconductive epoxies), silicones
(e.g., Room Temperature Vulcanizing (REV) Silicone),
rubbers, and/or other viscous materials widely used in the
electronic industry.
In FIG. 1, reference numeral 10 indicates a multi-
point solder paste dispenser of the type described in the
above-referenced paper by Do Schoenthaler (see FIG. 5
thereof). The dispenser 10 comprises a generally Solon-
Dracula hollow cartridge 11 adapted to receive solder paste to be dispensed on a printed wiring board (PUB) 12. A
nozzle 13 having a plurality of openings 14 is attached at
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one end 15 of the cartridge 11. As shown in FIG. I the
openings 14 are located along the periphery of the nozzle
13 and are spaced and shaped to substantially Match a
plurality of bond sites 16 on the PUB 12. At the other end
17 of the cartridge 11, pressurized air is supplied thereto
to force solder paste out of the openings I of the nozzle
13 onto the bond sites 16 of the PUB 12.
MULTI POINT DISPENSING PROCESS
Shown in FIG. 2 is an inside perspective view of a
nozzle 20 according to an illustrative embodiment of the
invention. The nozzle 20 has a flat bottom face 21 with a
plurality of openings 22 arranged in a predetermined
pattern. The openings 22 are dimensioned and spaced to
substantially correspond to the metallized terminations of
a chip carrier (not shown) to be mounted on a board 23.
Also shown on the board 23 is a plurality of bond sites 2
having a pattern substantially identical to that of the
nozzle openings 22. Alignment of the openings 22 with the
bond sites 24 may be achieved by using any well known
alignment technique, such as that described in U.S. Patent
4,295,596.
The nozzle 20 is adapted to be connected at one
end of a cylindrical solder paste cartridge 25 comparable
to the cartridge 11 of FIG. 1. the solder paste may be
selected from known commercially available types commonly
used in the electronics industry. Positioned within the
nozzle 20 is a pyramid shaped diverter 26 for directing the
paste flow towards the openings 22. The diverter 26
prevents stagnation of the paste in the central portion of
the nozzle 20 while, at the same time, reducing the volume
of paste needed to fill the nozzle 20 and the cartridge
25.
A solder paste dispensing sequence will be
described with reference to FIGS. 2, PA and 3B. At a first
stage of the sequence, the nozzle 20 and the board 23 are
moved relative to each other, and are positioned at a
predetermined initial distance, I, from each other.
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Although the initial distance, I, is shown in FIG. 2 as
being different from zero, the present teachings are also
applicable when the surface 21 of the nozzle 20 and the
board 23 are initially in contact with each other. During
a time interval, to pressurized air is supplied to the
dispensing tool (i.e. r to the cartridge 25 and the nozzle
20 attached thereto) forcing paste out of the orifices 22
onto, and in adherence with, the bond sites 24 of the board
23 thus closing the initial gap, if any, between the bottom
face 21 of the nozzle 20 and the board 23. At time t
the dispensing tool (including the nozzle 20 and the
cartridge 25) is moved upwards and away from the surface of
the board 23 at a predetermined substantially constant
velocity, V. Alternatively, if the dispensing tool is
fixed, then at time to the board 23 is moved downwards
and away from the tool at the constant velocity, V. While
the dispensing tool is moving away from the board 23, the
dispense air pressure Pry remains substantially constant
and is terminated at time to. The dispensing nozzle 20
continues to move upwards causing the paste adhering to the
board 23 to separate from the nozzle openings 22 thus
leaving deposits on the bond sites 24 of board 23.
The foregoing dispensing sequence achieved
reliable and reproducible multi point dispensing of solder
paste when predetermined process parameters were accurately
monitored. In accordance with an embodiment of the
invention, these process parameters include the initial
gap, I; the dispense pressure P, the tool velocity V and
the time periods to and to. In a preferred embodiment
of the invention, the initial gap is of the order of 0.0
to 0.25 mm, the higher end of such range allowing for board
war page; the dispense pressure P ranges between 0.07 and
0.7 kg/cm2; the range of the tool velocity V is between
12.7 and 127 mm/minute; the delay time to is of the order
of 0.1 to 0.5 sec.; and the dispense time to is typically
between 0.2 and 1.0 sec. When selecting a short delay time
if of about 0.1 sea, -the preferred corresponding dispense
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time to is selected in a range between 0.2 and 0.4 sec.
If the delay time to is a longer one, e.g., of the order
of 0.5 sec., the preferred dispense time to is set
between 0.65 and 1.00 sec. One condition to be maintained
for the selection of the two time periods to and to is
that the latter is always larger than the former in order
to permit the paste to be fed out of the nozzle as the
latter retracts.
Swallower PASTE LOADING
An important aspect of solder paste dispensing is
the method used to load solder paste into the dispensing
cartridge and nozzle assembly. Poor dispensing usually
results from anomalies such as semirigid paste, crust,
and/or voids. Large volumes of crust or semirigid paste
will clog the small openings of the dispensing nozzle.
moreover, voids near the nozzle orifices tend to cause
preferential paste flow around such voids resulting in
skips it no deposit) similar to physically clogged
openings.
FIG. 4 schematically illustrates a method and an
apparatus for loading paste into the dispenser cartridge
and nozzle in accordance with an embodiment of the
invention. Solder paste is placed in a supply container 40
where it is warmed by being mixed vigorously without
I introducing additional air or changing the character of the
paste itself. Heat is applied if necessary to elevate the
paste temperature to about 38-49C. The supply container
40 is coupled -to one side of an adaptor plate 41 which in
turn is located on top of a dispensing cartridge 42 to be
loaded or filled with solder paste.
A first seal 43 is located around a neck portion
44 of the adaptor plate 41 to prevent leaks between the
supply container 40 and the adaptor plate 41. A second
seal 46 is positioned between the top portion of the
dispensing cartridge 42 and the adaptor plate 41. A wire
screen 45 is positioned proximate to the bottom of the
supply container 40 for filtering semirigid paste and crust
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particles out ox the paste prior to its transfer into the
dispensing cartridge 42. A small gap 47, formed between
the top of the neck portion 44 of the adaptor plate I and
the wire screen 45, reduces the amount of paste wasted each
time a filled supply container is attached to the plate
41.
The neck portion 44 of the adaptor plate 41
comprises at least one hole 48 for allowing paste to be
transferred from the supply container 40 into the
dispensing cartridge 42. Preferably, an array of small
diameter holes (such as holes 48) is formed in the neck
portion 44 of the adaptor plate 41. The latter further
comprises an elongated chamber 49 for coupling a vacuum
source (not shown) to the top portion of the dispensing
cartridge 42.
The bottom portion of the dispensing cartridge 42
comprises a multi-orifice nozzle 51 attached thereto. The
nozzle 51 is of a type described above in connection with
FIG. 2. The dispensing tool including the cartridge 42 and
the nozzle 51 is placed on a loading station 52 which
comprises a loading table 53 and a supporting body 54.
Body 54 comprises, in a recessed portion thereof, a seal 56
adapted to mate with the periphery of the nozzle 51. Also
comprised within a central opening 57 of the body 54 is a
filtering arrangement comprising a filter member 58
supported by a porous member 59, such as a metal screen or
a rigid plate having a plurality of holes there through.
The filter member 58 may be, for example, a sheet of filter
paper. Preferably, the filter member 58 includes a rough
surfaced rubber pad positioned within the opening 57 of the
supporting body 54 such that its top rough surface is in
slight contact with the bottom flat face of the nozzle 51.
The supporting body 54 further comprises an elongated
conduit 60 for coupling a source of vacuum (not shown) to
its central opening 57.
In order to load paste into the dispensing
cartridge 42, the paste is pulled by vacuum (applied via
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the chamber 49) through the wire screen I and the array of
holes 48 from the supply container I into the cartridge
I Also, vacuum is pulled (via conduit 60 and central
opening 57) from beneath the nozzle 51 through the
filtering arrangement 58,59. Furthermore, heat is applied
to the entire loading apparatus, as schematically
illustrated by reference numeral 61 in FIG. 4, such that
the paste and all nozzle and cartridge surfaces are
maintained within a predetermined range of about 38-49C
dependent upon the type of paste used. Heating of the
paste enables it to maintain its viscosity in a suitable
range resulting in good flow characteristics.
The vacuums applied via chamber 49 and conduit 60
are held constant while paste is pulled into the dispensing
cartridge 42. When the paste is fully loaded into the
cartridge/nozzle assembly, the vacuums are maintained while
the loading table 53 is vibrated up and down in the axial
direct on of the cartridge 42 as illustrated by
bidirectional arrow 62. Typically, the table 53 would be
vibrating during an initial period of about 20 seconds with
vacuums applied. Next, with the vacuums turned off, the
vibration of the table 53 is continued for an additional
period of about 10 seconds. The use of a rough-surfaced
rubber pad as a filter member 58 is advantageous in that
grooves in the rough surface permit each nozzle opening to
be filled by the vacuum pulled, through conduit 60, from
under the screen 59. The foregoing causes a complete
filling of the nozzle 51. The combination of elevated
temperature, mechanical vibration and application of vacuum
above as well as beneath the paste within the
cartridge/nozzle assembly, tends to extract any entrapped
air and cause the paste to settle in the cartridge without
forming voids which would subsequently fill with air
resulting in the drying of portions of the paste and voids
or skips in the deposits.
SOLDER PASTE HANDLING
The solder paste used for dispensing dries slowly
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such that electronic devices can be easily placed into the
solder paste deposits within a one hour time frame after
deposition. Aster about two hours, the surface of the
deposited paste becomes noticeably dry, and after 24 hours
the deposits are rigid. Thus, a solder paste dispenser
remaining idle for about one hour will become clogged and
useless as paste dries at the nozzle orifices and on the
surface internal to the dispensing cartridge.
FIG. 5 schematically illustrates a technique for
preventing the paste from drying and forming a crust layer
in accordance with another illustrative embodiment of the
invention. A dispensing cartridge 70, with a multi-orifice
nozzle 71 attached thereto, contains a volume of solder
paste 72 to be dispensed. As already mentioned above,
pressurized air is applied to the top portion of the
cartridge 70 to force the paste 72 out of the orifices of
the nozzle 71.
In order lo evenly transmit the applied air
pressure to the body of the paste 72, a floating piston 73
is positioned within the cartridge 70 above the paste 72.
The piston 73 is preferably tapered and sized to influence
self-centering and prevent an air blow through in sections
of lower paste viscosity. In accordance with a preferred
embodiment of the invention, a liquid 74 is applied around
the floating piston 73 and acts as a liquid seal.
Moreover, after using the dispensing assembly 70,71 and if
thy latter would not be operated again within about 15
minutes, the nozzle 71 is preferably lowered into a small
dish 76 containing a liquid seal 77 thereby preventing
paste in the orifices of the nozzle 71 prom drying.
Liquids such as commercially available wave soldering oils
and/or fluorinated hydrocarbon materials may be used for
the liquid seal 74 and the liquid 77 contained in the
nozzle dish 76.
DISPENSING VISCOSITY CONTROL
As already mentioned above, the temperature of the
paste during dispensing is one of the variables affecting
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the dispensing results. Indeed, most commercial solder
pastes exhibit a substantial reduction in viscosity with a
relatively small increase in temperature (ego, between
-9 and -7C) as illustrated in FIG. 6. According to
another embodiment of the invention, consistent dispensing
results can be achieved by adequately controlling the
viscosity of the paste by monitoring and controlling the
paste temperature and maintaining it at a control
temperature, To, above the usual ambient temperature
range
FIG. 7 schematically shows an arrangement for
controlling the viscosity, and hence the temperature, of
the paste 80 within a dispensing cartridge it. A jacket
sleeve 82 is attached to a dispensing assembly support
plate 83 and shaped such that a gap 84, formed between the
sleeve 82 and the cartridge 81, surrounds the dispensing
cartridge 81. A temperature sensitive control device 86 is
attached to, and through, the support plate 83 to enable
the monitoring of the temperature within the dispensing
cartridge 81. A conduit 87, within the plate 83, connects
a source of warm fluid snot shown) to the gap 84 between
the sleeve 82 and the dispensing cartridge 81. The warm
fluid exits the gap I at the bottom of the sleeve 82
thereby keeping the paste 80 within a predetermined
temperature range in response to the temperature sensitive
control device 86. In a preferred embodiment, a low flow
of warm air, supplied to the fluid conduit 87, keeps the
paste 80 at a desired temperature.
The heating mechanism in FIG. 4, which is
symbolically represented by reference numeral 61, may
include a sleeve and warming fluid arrangement of the type
described in FIG. 7. In particular, a warm air flow around
the dispensing cartridge 42 of FIG. 4 would maintain the
solder paste within prescribed temperatures during the
US cartridge loading operation.
