Conductive ink packaging for printed circuit board screen printing operations

A package contains conductive inks or slurries adapted for attachment to a screen printer for applying the inks to a substrate for firing into printed circuit boards. The bag interior containing the ink is formed between a liquid and gaseous impermeable top wall connected along its periphery to a bottom wall formed with openings through which the ink is adapted to migrate towards the print screen under vacuum. A seal member extends around the entire periphery of the bag and is adapted to sealingly mate with a corresponding fastening element attached to the print frame to form an air seal between the bag and print frame in operative position. A removable, peel-away cover normally covers the screen bottom wall to allow the bag to have a long shelf life.

TECHNICAL FIELD 
The present invention relates generally to vacuum screen printing of 
conductive powders and inks onto circuit board substrates and, more 
particularly, to packages connectable to the screen printing equipment and 
containing the inks for supply to the screens. 
BACKGROUND ART 
FIG. 1 is an illustration of a screen printer disclosed in my 
above-identified co-pending application. The printer uses a vacuum 
pull-down method for deflecting a printing screen 502 which is coated with 
viscous ink, into contact with a substrate 100. In the preferred 
embodiment the substrate is a printed circuit board substrate and the ink 
comprises conductive metal particles suspended in a viscous carrier or 
screening agent. It should be understood, however, that other types of 
substrates and inks may be used with this printer (all such materials are 
collectively referred to hereafter as "ink"). 
Printing screen 502, in which a desired printing pattern has been formed by 
conventional means, is supported in a screen holder 504. Holder 504 is 
mounted at either side to frame mount arms 506 and 508 by support members 
510 and 512 and bolts 737 and 738. Frame mount arms 506 and 508 are 
fixably mounted at one end to pivot bar 518. Pivot bar 518 extends through 
a pair of positioning devices 520 and 522, described hereinafter, which 
are used to adjust the position of pivot bar 518 along the Y-axis and 
around the Z-axis (commonly referred to as the "theta axis"). Positioning 
devices 520 and 522 are manually controlled, respectively, by knobs 534 
and 536. Adjusting knob 772 is used to adjust the position of screen 
holder 504 along the X-axis. Thus, by turning knobs 534, 536 and 772, the 
operator can adjust the position of screen holder 504 until screen 502 is 
precisely registered above substrate 100. 
A pair of counterweights 524 and 526 are rigidly attached to pivot bar 518, 
allowing the operator to easily rotate screen holder 504 about pivot bar 
518. Alternatively, air springs may be used for this purpose. The operator 
does so by grasping handle 528 and either lifting it until the screen 
holder is in the fully opened position or pushing it downward until the 
bottom edges of the screen holder contact vacuum seal 678 inserted into 
the upper surface of support frame 530. Support frame 530 has a central 
recess therein, in which substrate 100 is positioned on top of printing 
platen 556. Platen 556 is bonded to a product carrier 550 (illustrated in 
FIG. 2). Product carrier 550 and, hence, platen 556 may be adjusted along 
the Z axis by rotating knobs 708A and 708B, to thereby space substrate 100 
from screen 502 to the desired degree. Platen 556 has regularly spaced 
apertures 558 therein used for substrate hold-down, as described below. 
Non-porous tape 557 (FIG. 2) is used to cover the apertures in platen 556 
not covered by the substrate. 
When screen holder 504 is rotated to the fully down position, the rear 
section thereof depresses a micro-switch 529, which causes a vacuum to 
draw substrate 100 into firm contact with platen 556, thereby preventing 
relative movement between them during the printing operation. In the down 
position the surface of screen 502 is essentially parallel to the surface 
of substrate 100. After ink has been applied to the upper surface of 
screen 502, filling all of the unmasked screen openings, a vacuum print 
system is energized. This system evacuates the region between substrate 
100 and screen 502, causing screen 502 to deflect downward into contact 
with substrate 100 and ink to be transferred through the screen openings 
onto the substrate. 
After ink transfer, the vacuum is released and the operator rotates screen 
holder 504 away from substrate 100, removes the substrate from the printer 
and inserts the next substrate for printing. Concurrent with release of 
the print vacuum, a pilot actuated valve is operated to vent the print 
chamber to atmospheric pressure, thereby avoiding a double strike of ink 
onto the substrate. Upon rotating screen holder 504 away from substrate 
100, micro-switch 529 is opened, shutting off the hold-down vacuum and 
returning the pneumatic hold-down system 900 to the standby mode. Screen 
502 may be removed from holder 504 whenever the operator desires to do so, 
such as at the end of a production run. 
FIG. 2 depicts a sectional view of screen holder 504, support frame 530 and 
the hold-down mechanism for a substrate 100. A product carrier 550 is 
positioned in a recess of the support frame. The recess periphery is 
defined by the vertical portions of side members 705 and 707, a front 
member and a rear member (not shown in detail), while the recess bottom is 
defined by support member 531. The lateral dimensions of product carrier 
550 are slightly less than the corresponding dimensions of the recess, 
thereby defining an evacuation channel 552 between product carrier 550 and 
recess. 
Screen holder 504 employs an extruded channel 660 having a U-shaped 
cross-section. In the preferred embodiment, channel 660 is made of 
aluminum. Four sections of such channel stock are welded together into a 
rectangular frame, as shown in FIG. 1. In FIG. 1, screen holder 504 is in 
a partially open position, while in FIG. 2 it is in the fully down 
position. 
A tensioning bar 662, shown in cross-section in FIG. 2, runs the length of 
channel 660 and is secured to the top portion 664 of U-shaped channel 660 
by a plurality of securing bolts 666. Securing bolts 666 are fed through a 
plurality of bores spaced symmetrically about channel 660. A plurality of 
tension adjusting bolts 668 are secured to tensioning bar 662 through 
outside leg 675 of U-shaped channel 660. Again, bolts 668 are spaced 
symmetrically along the length of channel 660. Tensioning bar 662 has an 
offset portion 670 which has two legs 672 forming a U-shaped recess 673 
that accepts a tensioning rod 674 therein. The inner sides of legs 672 are 
tapered inwardly so that the entrance to recess 673 is slightly wider than 
the diameter of rod 674 combined with a single wrap of screen 502, while 
the bottom of recess 673 is slightly narrower than the rod 674/screen 502 
combination, so that when rod 674 is inserted therein it is held snugly. 
In the preferred embodiment tensioning rod 674 is made of stainless steel 
and may be inserted along the entire length of channel 660. 
Printing screen 502 passes over the inside leg 676 of channel 660, over leg 
672 and is secured in recess 673 by tensioning rod 674. Inside leg 676 has 
a rounded bottom 677 and is slightly longer than outside leg 675. Screen 
502 is stretched taut by adjusting bolts 666 around the periphery of 
channel 660. Inside leg 676 is rounded to reduce the likelihood of tearing 
screen 502 where the two come in contact. Support frame 530 has a channel 
680 spaced around the periphery of the recess, as shown more clearly in 
FIG. 7a of my aforesaid copending application. 
Referring again to FIG. 2, the seal 678 (in Figure) is inserted into 
channel 680, so that when U-shaped channel 660 is in the fully down 
position, rounded bottom 677 of inner leg 676 rests on the seal. In the 
preferred embodiment seal 680 is made of very low durometer rubber (e.g., 
50-60 DUR). Since rounded bottom 677 is pulled downward into the seal by 
vacuum pressure during the printing operation, slightly shorter outside 
leg 675 rests against the top surface of support frame 530 when screen 
holder 504 is in the fully-down position. 
As indicated above, screen 502 has been treated in conventional manner 
before insertion into holder 504 so that the openings therein define the 
desired pattern to be printed on substrate 100. When ink of sufficient 
viscosity is applied to the upper surface of screen 502, these openings 
remain covered prior to printing. In the preferred embodiment, the ink has 
a minimum viscosity of approximately 200,000 centipoise. Thus, when screen 
502 is tautly mounted in screen holder 504, as described above, and holder 
504 is in the fully-down position, two parallel air-tight paths are 
defined, one for screen hold-down and the other for substrate hold-down 
during the printing operation. 
Referring again to FIG. 2, the screen hold-down path begins with the region 
555 between screen 502 and platen 556, and continues along evacuation 
channel 552 to evacuation port 604, through fitting 606, hose 608, and a 
fitting (at 613 in my aforesaid copending '049 application at FIG. 8) to 
vacuum hold-down system (at 900, shown in FIG. 12 of my aforesaid 
copending '049 application). When a vacuum is drawn along this path, 
screen 502 is deflected downward into contact with substrate 100, and ink 
on the upper surface of screen 502 is drawn through the openings therein 
into contact with substrate 100. When the vacuum is released, screen 502 
returns to its normal unstretched position, but a quantity of ink is 
transferred to the surface of substrate 100 through a combination of 
vacuum and meniscus forces, thereby defining the desired pattern on 
substrate 100. 
The substrate hold-down path begins with apertures 558 in platen 556, and 
runs through channels 554 and manifold grooves in product carrier 550, 
hollow shafts 584 and 586 in vertical adjusting bolt 564, bore 588 in 
vertical adjustment mount bracket 567, fitting 578 and hose 591 to vacuum 
system 900. The vacuum drawn through this path holds substrate 100 firmly 
in contact with platen 556 during the printing operation so that it does 
not move in response to meniscus forces when screen 502 returns to its 
unstretched rest position. 
It is important to maintain the viscosity of the conductive ink at 
predetermined levels to ensure adequate flow through the screen 502 during 
printing. If, however, suitable conductive inks or slurries are improperly 
stored prior to use, solvents or binders within the inks may be subjected 
to evaporation which dries the ink and changes the viscosity to possibly 
unacceptable levels. Additionally, if the inks are stored in an oxygen 
containing environment, oxidation of the conductive metals in the ink is 
likely to occur which may disadvantageously prevent complete fusing of the 
ink to the substrate at controlled temperature. 
It is accordingly one object of the present invention to provide packaging 
for conductive slurries or inks which allow for long shelf life of the 
inks prior to use. 
Another object is to maintain the viscosity of the ink at predetermined 
levels to ensure adequate flow through a screen during printing by 
preventing evaporation of volatiles and other components in the ink. 
Still another object is to store the ink out of contact with oxygen to 
prevent oxidation of conductive metals in the ink. 
A further object is to store ink in packaging which is easily attachable to 
a screen print frame for easy and immediate use. 
SUMMARY OF THE INVENTION 
A package for containing an ink or slurry having metal particles to be 
deposited onto a substrate for subsequent firing into electrical 
conductors, comprises a bag for containing the ink or slurry in a bag 
interior formed between first and second side walls. The first side wall 
is preferably formed of a material isolating the ink or slurry from the 
external environment and the second side wall is formed with openings of 
predetermined size to allow for flow of the ink or slurry from the bag 
through the openings. A cover is attached to the bag for normally covering 
the second side wall openings to prevent flow of ink from the bag interior 
and to isolate the ink, with the first side wall, from the external 
environment. Removal of the cover from the second side wall exposes the 
ink to an operating environment for flow through the openings. 
Attaching means, disposed around the periphery of the bag, is preferably 
provided for attaching the bag to the periphery of a print frame of a 
screen printer. The attaching means, in the preferred embodiment, 
sealingly attaches the bag to the print frame with the first side wall 
facing upward and the second side wall facing downwards toward the print 
screen. The resulting seal and upward orientation of the first side wall 
serves to isolate the second side wall and ink from the external 
environment. 
In accordance with the invention, the attaching means includes one of a 
male or female member extending around the entire periphery of the first 
side wall and the other of the male or female member is attached to 
project inwardly from the print frame. Upon mating engagement of the male 
and female members, the bag is sealingly secured to the print frame around 
its periphery. 
The male and female attachment members may be of different forms as will 
occur to one of ordinary skill in the art from a review of this 
application. However, a preferred form is "zip-lock" male and female 
attachment members that are easily releasably attachable to each other to 
provide a reliable seal. 
The cover is preferably made of a gas and liquid impermeable material. 
Adhesive is disposed on one face of the cover for adhesive contact with 
substantially the entire surface area of the second side wall. 
Advantageously, the cover is peelable from the second side wall without 
leaving adhesive residue on the second wall which may otherwise obscure 
the openings and disrupt the flow of ink down to the screen. 
The second side wall is preferably a screen material and the openings 
therein are of at least one predetermined mesh size. 
The first side wall is preferably formed of flexible material to allow the 
bag to collapse as the ink is supplied to the print screen and the volume 
in the bag reduces. 
The ink or slurry is normally contained in the bag under a vacuum or in a 
gaseous environment preventing oxidation of the metal particles. 
Still other objects and advantages of the present invention will become 
readily apparent to those skilled in this art from the following detailed 
description, wherein only the preferred embodiments of the invention are 
shown and described, simply by way of illustration of the best mode 
contemplated of carrying out the invention. As will be realized, the 
invention is capable of other and different embodiments, and its several 
details are capable of modifications in various obvious respects, all 
without departing from the invention. Accordingly, the drawing and 
description are to be regarded as illustrative in nature, and not as 
restrictive.

BEST MODE FOR CARRYING OUT THE INVENTION 
Ink package 10, with reference to FIGS. 4-6, comprises a bag 12 having a 
gas and liquid impermeable flexible top wall 14 and a bottom wall 16 
sealed along their entire edges to each other (not shown) or to an 
attachment member 18 as depicted in FIG. 5 to define a bag interior 20 
adapted to contain an ink or slurry 22. This ink or slurry 22 preferably 
contains metallic particles of one or more particle size (e.g., 70 microns 
or less) which may be suspended in solvents or binders to form an ink or 
paste. 
The bag 12 formed between the top and bottom walls 14, 16 may extend over 
the entire area formed between the print frame 660 to overlie the screen 
502 as best depicted in FIG. 4. The openings or meshes 24 in the bottom 
wall 16 are preferably coextensive with the working area of the screen 502 
so that an appropriate amount of ink is supplied through the bottom wall 
openings 24 to the screen 502 under vacuum. 
To sealingly secure the ink package 10 to the entire periphery of print 
frame 660 and maintain vacuum conditions in the region of print screen 
502, a sealing connection 25 is provided between the package and print 
frame. In the preferred embodiment depicted in FIG. 5, the seal 25 may 
take the form of a male closure strip 28 extending along the entire edges 
of each top and bottom wall 14, 16. The closure strip 28 may typically be 
formed from thermoplastic material which can be heat fused or otherwise 
sealingly secured to the top and bottom wall edges 14, 16. The closure 
strip 28 may include a downwardly depending male interlocking element 30 
(which may be coextruded with the strip) adapted to sealingly engage and 
mate with an upwardly directed interlocking channel 32 formed in a female 
closure strip 34 attached to and projecting inwardly from the print frame 
660. By depressing the entirety of the bag edges after removing the 
peelable cover 38 and positioning the bag on top of screen 502 with the 
screen bottom wall 16 facing downwardly, a complete air seal (which may 
also be a liquid seal) is obtained between the bag periphery with the 
print frame 660. 
Representative interlocking closure strips are disclosed, for example, in 
U.S. Pat. Nos. 4,363,345; 5,012,561; 4,829,641; 4,285,105; 4,578,813; 
3,338,285; and 4,964,739. From a review of this specification, one of 
ordinary skill in the art will easily understand the manner in which the 
closure strips 28, 34 may be secured to both the bag 10 and print frame 
660 to carry out the objects of the present invention. 
The top wall 14 of the bag 10, as mentioned above, is preferably made of 
flexible, liquid and gas impermeable material which isolates the ink or 
slurry 22 in the bag interior 20 from the external environment to prevent 
oxidation and drying out of the ink. The top wall 14 may be formed of a 
thermoplastic material. 
The bottom wall 16 may be formed of any suitable material which is capable 
of being formed with suitably sized mesh openings 24 allowing the ink or 
slurry 22 to migrate across the bottom wall through the openings to load 
the screen 502 with ink upon demand (e.g., vacuum being applied to the 
screen in the aforesaid manner. In one embodiment, the thin wettable 
screen 16 may be a wire mesh sealingly secured to the periphery of the top 
wall 14 and having approximately 30 lines per inch. 
When not in use, the bottom wall screen 16 is isolated from the external 
environment with cover 38 which is coextensive with at least the entire 
area of the screen openings 24. With reference to FIG. 6, this cover 38 is 
preferably coated with an adhesive 40 so as to effect adhesive contact 
with the entire surface area of the bottom surface of the bottom wall 
screen. Upon peeling of the cover 38 in the direction of arrow A in FIG. 
6, the cover is entirely removed from the bottom wall screen 16 so that 
the bag may then be sealed in place to the print frame 660 in the manner 
described above. A suitable adhesive 40 will occur to one of ordinary 
skill in the art as a result of the above disclosure. This adhesive 
preferably has the characteristic of allowing the cover 38 to be peeled 
from the screen bottom wall 16 without leaving any adhesive residue on the 
bottom wall which may otherwise clog the openings 24 and affect the flow 
of ink to the print screen 502. 
It will be readily seen by one of ordinary skill in the art that the 
present invention fulfills all of the objects set forth above. After 
reading the foregoing specification, one of ordinary skill will be able to 
effect various changes, substitutions of equivalents and various other 
aspects of the invention as broadly disclosed herein. It is therefore 
intended that the protection granted hereon be limited only by the 
definition contained in the appended claims and equivalents thereof.