Electroformed squeegee blade for surface mount screen printing

A novel electroformed squeegee blade for the uniform deposition of printing material such as solder paste and the like onto printed wiring boards is disclosed. A method of fabricating the electroformed squeegee blade is also disclosed. The method of fabrication, which produces an electroformed squeegee blade having smooth, planar, and flat surfaces, involves electroforming at least one uniform layer of metal onto a conductive substrate and removing the substrate. Also disclosed is an apparatus and method that employ the electroformed squeegee blade to uniformly deposit printing material on printed wiring boards.

FIELD OF THE INVENTION 
The present invention relates generally to the deposition of printing 
material on printed circuit/wiring boards and more particularly to a 
method of fabricating an electroformed squeegee blade used for the uniform 
deposition of printing material on printed wiring boards and/or printed 
circuit boards (PWBs or PCBS). 
BACKGROUND 
Screen printing apparatus, which have a wide variety of uses in the 
electronic substrate fabrication and electronic assembly industries, are 
well known as exemplified by U.S. Pat. Nos. 5,478,699; 3,930,455; 
5,387,044; 5,357,856 and 5,323,700. These uses include, but are not 
limited to, the deposition of printing material, such as solder paste, 
conductive epoxies, thermal set plastics, conductive/resistive inks and 
the like, in the preparation of surface mount PCBs, Flip Chip Bumping, 
Co-Fired Via Fill, BGA, conductive/resistive circuit applications and the 
like. 
The printing apparatus typically comprises a screen printer, a stencil, and 
a squeegee blade assembly. Various printing materials may be applied to a 
substrate by the application of printing material through stencil 
apertures on to the substrate. For example, in the preparation of printed 
wiring boards (PWBS) for the placement of surface mounted components, 
solder paste is deposited on the PWB through stencil apertures. The 
stencil apertures match the shape of PWB/PCB pads and are at the same 
locations as the PWB/PCB pad image array to be applied. The stencil images 
are aligned with the PWB/PCB images to be applied, placed in intimate 
contact with the PWB/PCB and held firmly together with the PWB/PCB until 
after the print stroke is completed. An amount of solder paste that forms 
the solder pads is applied to the top surface of the stencil. The squeegee 
blade is then used to draw the solder paste over the entire stencil image 
area, push the solder paste into the corresponding apertures of the 
stencil and form the solder deposition on the PWB pads. The component to 
be mounted is then placed directly into the wet solder paste. 
As the electronics industry has progressed over the past few years, the 
trend has been towards the use of more complex devices and smaller 
circuits. A concern in the soldering of components is to consistently 
provide the precise volume of solder to the PWB/PCB pad sites to form 
effective solder joints. It is imperative that the amount/volume of solder 
paste applied to each PCB/PWB soldering pad is consistent and uniform so 
that the lowest possible defect levels in manufacturing is maintained. An 
inadequate amount of solder due to incomplete aperture filling during the 
squeegee printing stroke can lead to problems in forming consistent, 
uniform and reliable electromechanical solder joints. Excessive solder due 
to the overfilling of stencil apertures during the squeegee printing 
stroke can result in shorting or "bridging" between complex components, 
for example, QFPs, TSOPs, etc., pins or leads, resulting in problems in 
forming consistent, uniform and reliable solder joints. Both insufficient 
and excessive solder paste deposits lead to increased repair and rework 
requirements, high overall manufacturing costs, as well as finished 
product quality/reliability issues due to random variation from documented 
industry standard solder joint specifications. 
Efforts have thus focused on the development of materials that are better 
suited for fine pitch screening processes. One example is the introduction 
of new types of apparatus to aid movement of the solder paste over the 
aperture. For example, U.S. Pat No. 5,254,362 to Shaffer et al. relates to 
a method and apparatus for depositing solder paste on a printed wiring 
board. The method involves the vibration of a squeegee blade assembly by a 
vibrator to produce a slight circular motion in addition to the planar 
motion of the squeegee blade. U.S. Pat. No. 5,044,306 to Erdmann relates 
to a method of using two squeegee blades for wiping solder paste onto a 
printed wiring board. The method involves using a first squeegee to wipe 
solder paste across a stencil in combination with a second squeegee that 
cooperates with a tray to deposit solder paste at the beginning of a 
wiping stroke. The tray also cooperates with the first squeegee to pick up 
excess solder paste at the end of a wiping stroke. 
Another example of such efforts is the introduction of new types of 
squeegee blades. See, for example, U.S. Pat. No. 5,078,082 to van Dyk 
Soerewn and Hall, Developing a Low PPM Defect Level Ultra-Fine Pitch 
Process, New Technologies for Perfect Printing, User's Guide to More 
Precise SMT Printing, MPM Corporation, pp. 12-21 (1994). 
In place of the traditional hard rubber blades, the squeegee blades 
available in the market place today are typically manufactured from high 
density polyurethane or stainless steel in a stamping, cutting or etching 
process. These squeegee blades are sometimes treated with alternate 
coatings and processes with the intention of improving the solder paste 
deposition process. 
The use of stainless steel squeegee blades has enabled a more controlled 
and consistent print height across the entire board area. However, as 
disclosed in Freeman, New Technologies for Perfect Printing, User's Guide 
to More Precise SMT Printing, MPM Corporation, pp. 22-29 (1994), "any 
irregularities in the circuit board surface can cause damage to the fine 
webs in the fine pitch areas, since steel is not `forgiving.` In addition, 
steel lacks lubricity, causing greater wear on the stencil." Thus, room 
for improvement remains. 
Polyurethane squeegee blades are designed to be somewhat flexible, thereby 
readily deforming to the stencil surface. The inherent compliance of 
polyurethane squeegee blades is desirable in certain applications, for 
example, step or multi-level stencils. In the use of step or multi-level 
stencils, compliance with the stencil surface has proven advantageous. At 
the same time, however, deformation of the blade edge is disadvantageous 
due to aperture scooping occurring during the printing process. Scooping 
is the result of polyurethane blade material compliance or deformation 
extending beyond the top plane of the stencil into an aperture and 
removing (scooping) solder paste out of the aperture during the squeegee 
print stroke. Scooping is a major cause of insufficient solder joints and 
the associated problems that result from insufficient solder joints. 
Reduced squeegee compliance, deformation and scooping are achieved by using 
a less flexible blade material. The more rigid, less compliant material of 
choice for squeegee blades has been stainless steel. Print quality is 
improved by using metal squeegee blades, thereby yielding a more 
controlled and consistent print height across the entire image area, a 
better definition of the solder paste deposition on the PCB/PWB pads, a 
minimized amount of "scooping" and an overall improved aperture filling. 
Many of the negative effects of polyurethane squeegee blades are thus 
overcome with metal blades. 
Although metal squeegee blades offer improved printing results, they are 
not ideal in all aspects due to sticking or adhesion of printing material 
to the metal blade. Research to combat the inherent incompatibility of the 
metal squeegee blade with metal stencils as well as the tendency of 
printing materials and solder paste to stick to the metal squeegee blade 
has thus been conducted. For example, various alternative and specialty 
coatings as well as secondary plating process steps have been explored to 
minimize the adherence or sticking of solder paste to the squeegee blades. 
The adherence of print material or solder paste to the squeegee blade has 
a profound negative impact on the consistency and quality of the resultant 
deposition on the PWB/PCB. Printing material additives such as tackifiers, 
binders, carriers and fluxes added for improved downstream process 
performance, increase the adherence of print material to the squeegee 
blade. Secondary surface preparation steps such as plating and specialty 
coatings improve the release characteristics of metal squeegee blades to 
some degree. The drawback of current metal squeegee blades that are 
processed through a secondary step is that with repeated use, the 
specialty coating or plating is worn away exposing the underlying bare 
metal and causing the recurrence of solder paste sticking or adhesion 
problems. The exposed bare metal increases the occurrence of insufficient 
or excessive solder paste, which in turn, affects the performance of the 
metal squeegee blade, thereby degrading the consistency, reliability and 
overall quality of the printing process. 
Additional conventional methods of fabricating squeegee blades include 
casting from a mold and machining. These methods may require a finishing 
step to produce the final product. In addition, chemical etching, a 
subtractive fabrication process, wherein characteristic surface features 
are formed when a polished metal surface is etched by suitable reagents, 
has been used. Chemical etching as a manufacturing process has 
characteristics that limit its use in high tolerance, high precision 
applications. 
Another conventional technique is electropolishing. This technique starts 
with a chemically etched squeegee blade as described earlier, and adds an 
electrochemical etching process that, in theory, smooths the surface of 
the squeegee blade. The electropolished surface does help solder paste 
release to a certain degree when compared to a non-electropolished 
surface. However, a typical electropolished squeegee blade still has most 
of the drawbacks associated with a chemically etched squeegee blade. Thus, 
although electropolishing minimizes the imperfections introduced by 
chemical etching, it is unable to completely compensate for them and the 
drawbacks associated with chemical etching are carried forward to the 
finished product even after electropolishing. 
Difficulties observed during the use of conventional squeegee blades 
include negative wear characteristics, incompatibility with stencil 
materials, incompatibility with printing materials and ineffectiveness of 
material transfer. 
Negative wear characteristics and incompatibility with stencil materials 
associated with the use of conventional metal squeegee blades include, but 
are not limited to, the deformation of the stencil foil material due to 
the unforgiving nature of steel. Also, "coining" or streaking of the base 
stencil foil material is more prevalent with conventional metal squeegee 
blades due to the friction that occurs between the metal squeegee blade 
and the stencil. Stencil fatigue occurs at an earlier point in life cycle 
testing and is most easily noticed as printing material deposition 
definition becomes less well-defined. Increased stencil wear, shorter 
stencil life, damage to fine aperture areas, and coining and streaking of 
the stencil are the result of a lower lubricity and an incompatibility 
between stencil materials and conventional metal squeegee blades. 
Incompatibility with printing materials and ineffective material transfer 
of conventional squeegee blades is most evident in the observed high 
volume of printing material sticking to the squeegee blade, incomplete 
aperture filling, redistribution of printing material away from the 
stencil image area, inconsistent printing deposition volumes and piling of 
solder paste along the sides of the squeegee print stroke. Ineffective 
material transfer is best illustrated by the pulling of printing material 
or solder paste out of stencil apertures by the trailing edge of the 
squeegee blade during the print stroke. As the squeegee blade passes over 
an aperture, solder paste adhering or sticking to the blade causes the 
solder paste to be pulled out of the previously filled aperture, thereby 
showing signs of lifting or "tailing" of printing material out of the 
aperture on the trailing edge of the print stroke. Incompatibility between 
metal squeegee blade and printing material, lower solder paste shearing 
forces, lower lubricity and a tendency toward solder paste sticking 
introduce significant inconsistency into the printing process. 
Control of the solder paste can become a problem depending on the ratio of 
the aperture width and thickness of the stencil. This is because up to 70% 
of the printing material or solder paste adheres to the squeegee blade 
rather than depositing on the board. Another problem is stencil offset, 
which is caused by friction between the squeegee blade and the stencil. 
During the printing cycle, the stencil can be offset in the direction of 
the squeegee blade print stroke, thus offsetting the stencil aperture 
image relative to the PCB/PWB pads with which they are intended to 
register. 
FIG. 1 depicts a prior art application of solder paste 1 by a solder paste 
application machine 2. The solder paste application machine 2 can be a 
Model SP 200 screen printer by MPM Corporation (Franklin, Mass.) or an 
equivalent. FIG. 1 depicts the machine 2, which is connected to and 
controls squeegee blade assembly 3 including squeegee blade holder 4 and 
squeegee blade 5 made of stainless steel, plated stainless steel, 
propriety specialty coated stainless steel or polyurethane. Squeegee blade 
assembly 3 may be either a floating or non-floating head assembly. The 
squeegee blade 5 is shown in position, after having been drawn across the 
stencil 6. An amount of solder paste 1 is shown still adjacent to the 
squeegee blade 5. This solder paste 1 is in excess of the amount deposited 
and has been drawn across the stencil 6 in the direction indicated by the 
arrow A.sub.1. 
The stencil aperture 7 is filled with solder paste 1. At the leading edge 
7a (i.e., the first edge of an aperture to be contacted by the squeegee 
blade 5) of the stencil aperture 7, the amount of solder paste 1 is much 
less than the amount of solder paste 1 at the trailing edge 7b (i.e., the 
last edge of an aperture to be contacted by the squeegee blade 5) of the 
stencil aperture 7. As a result, surface mounted components (not shown), 
which are mounted to a printed wiring board (not shown) upon which solder 
paste 1 has been deposited may move, losing effective electrical contact 
with the desired circuit(s) or even becoming disconnected from solder 
paste 1 due to uneven application of solder paste 1. Therefore, such 
application of solder paste 1 is inappropriate to securely and effectively 
fasten surface mounted components to printed wiring boards. 
Ideally, printing material or solder paste should be deposited evenly 
across the entire stencil image area. If the solder paste deposition is 
not uniform, components mounted in such an arrangement may shift, move or 
become dislodged with subsequent PWB processing. 
It is thus highly desirable to select or specify an improved means for 
consistently applying printing material evenly to a PWB through the 
apertures of a stencil. 
All references cited herein are incorporated herein by reference in their 
entireties. 
SUMMARY OF THE INVENTION 
Embodiments of the invention are directed to a novel electroformed squeegee 
blade suitable for the deposition of printing material such as solder 
paste and the like on a printed wiring board. 
Other embodiments of the invention are directed to a method of fabricating 
an electroformed squeegee blade suitable for screen printing and other 
applications in which a smooth squeegee blade is suitable. The method for 
preparing the electroformed squeegee blade according to the invention 
involves electroforming at least one uniform layer of metal. The invention 
also encompasses electroformed squeegee blades prepared utilizing multiple 
layer fabrication techniques, which have additional benefits and 
advantages. For example, a variety of blade edge designs and various blade 
edge thicknesses that allow for user definable printing characteristics 
could be prepared. Thus, electroformed squeegee blades permit the 
manufacture and use of multiple layer designs and a wide variety of blade 
edges that are custom fit to the printing application and printing 
materials being used. 
The method according to the invention eliminates any requirement for 
secondary processing, additional finishing, specialty coating or plating 
steps. The method thus produces smooth, planar, and flat surfaces without 
additional lapping, grinding, forming, or machining to obtain flatness or 
planarity. 
The electroformed squeegee blade of the invention is more compatible with 
stencil materials due to increased lubricity and decreased friction 
between the electroformed squeegee blade and the stencil. 
Further embodiments of the invention include a method and apparatus for 
uniformly depositing solder paste on a printed wiring board using the 
electroformed squeegee blade. 
Other features and advantages of the invention will become more fully 
apparent from the following detailed description of preferred embodiments, 
the appended claims, and the accompanying drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Electroformed squeegee blades are used in the printing operation of the 
electronics packaging industry, and more particularly in surface mount 
manufacturing. Unlike conventional methods of producing squeegee blades, 
squeegee blades according to the invention are grown atom by atom with an 
electroforming process that utilizes specialized chemistries and process 
conditions that render the squeegee blade with improved properties and 
characteristics related to the squeegee blade application. 
Print tests using the electroformed squeegee blade according to the 
invention show a minimum 75% reduction in the amount of residual print 
material left on the stencil after each print cycle. The result is a 
reduction in the build-up of dried material clogging stencil apertures and 
improved print definition. The enhanced printing consistency that results 
from the use of an electroformed squeegee blade according to the invention 
reduces the occurrence of bridging, bleed-out and insufficient aperture 
filling, key causes of manufacturing defects. By reducing the number of 
defects associated with stencil printing, a reduction in manufacturing 
costs is achieved. 
Due to the enhanced material compatibility between the stencil and the 
electroformed squeegee blade, stencil wear normally associated with the 
printing operation is reduced. Printing tests performed with current 
technology metal squeegee blades show stencil coining and wear by visual 
inspection, while electroformed squeegee blades exhibit little or no wear 
at all after an equal number of printing cycles on a test stencil. 
A major cause of the coining and wear seen for the current technology metal 
squeegee blades introduced during the manufacturing process is a lack of 
blade edge planarity. The electroformed squeegee blade according to the 
invention provides enhanced squeegee blade edge planarity that has been 
unachievable under current chemical etching, laser cutting, molding or 
secondary processing steps. As a result of the reduced wear, the life 
expectancy of the stencil is anticipated to increase by 200%. By 
increasing the number of print cycles due to decreased stencil wear, 
manufacturing quality remains consistent for an extended period of time, 
thus improving overall manufacturing yields. In comparative testing, after 
an equal number of print cycles with an electroformed versus a 
conventional metal squeegee blade, significant wear and coining can be 
observed with the conventional squeegee blade, while no wear can be 
detected with the electroformed squeegee blade. 
Another benefit of the electroformed squeegee blade according to the 
invention is that as the surface tension attributed to the squeegee blade 
is reduced, the tendency of the printing material to stick to the squeegee 
blade is minimized. This is a problem most noticeable during the use of 
two squeegee blades as the direction of squeegee blade travel is changed 
and a switch from front to rear or rear to front squeegee blades is made. 
The result is often a smearing of the printing material, presenting an 
opportunity for the introduction of printing defects. With an 
electroformed squeegee blade, the printing material has a tendency to roll 
into a uniform cylindrical mass ahead of the squeegee blade during 
printing. Because of the lowered surface tension associated with the 
electroformed squeegee blade, the cylindrical mass of printing material is 
left on the stencil, precisely aligned for the next print stroke. As a 
result, 30% less printing material is required to be applied to the 
stencil and waste is reduced. This more efficient use of printing material 
or solder paste reduces waste and improves the overall print deposition 
quality, uniformity and consistency. 
During printing trials, an electroformed squeegee blade in accordance with 
the invention required a reduced amount of solder paste printing material 
relative to that of conventional squeegee blades. This is due to a 
minimization of sticking of the printing material to the squeegee blade, 
an elimination of accumulating or piling of printing material along the 
side of the stencil image outside of the squeegee's reach, and an overall 
cleaner stencil surface. In addition, improvement is achieved due to a 
consistent volume of paste being available across the whole stencil image 
area, which further enhances printing performance. 
The advantages of the invention extend beyond the actual printing process. 
After printing is completed and the clean-up process begins, a significant 
savings is realized in time, materials, and toxic material disposal costs. 
After each test printing run with an electroformed squeegee blade, 
clean-up has been reduced to less than 5 minutes, while printing with 
competitive squeegee blades including stainless steel, plated stainless 
steel, propriety specialty coated steel, and 80, 85 and 90 durometer 
polyurethane squeegee blades, requires anywhere from 15 to 30 minutes for 
clean-up. The number of clean-up wipes required after use of an 
electroformed squeegee blade is reduced to one from the fifteen to twenty 
required for competitive products. More efficient use of printing 
materials, reduced waste, and overall reduction in clean-up requirements, 
results in a dramatic reduction in toxic waste disposal costs. 
The method of the invention, which is a variation of the method disclosed 
in U.S. Pat. No. 5,478,699 to Blessington deceased, et al., involves 
electrodepositing metal on a conductive substrate. 
Typical substrate materials include stainless steel, iron plated with 
chromium or nickel, nickel, copper, titanium, aluminum, aluminum plated 
with chromium or nickel, titanium palladium alloys, nickel-copper alloys 
such as Inconels.RTM. 600 and Invar.RTM. (available from Inco, Saddle 
Brook, N.J., and the like. Non-metallic substrates can also be used if 
they have been made conductive, for example, by being appropriately 
metallized using metallization techniques known to the art, such as 
electroless metallization, vapor deposition, and the like. A conductive 
substrate is first cleaned by methods well known to those of skill in the 
art. The sequence of cleaning steps can include washing with isopropyl 
alcohol, vapor degreasing in trichloroethylene, electrocleaning, rinsing 
in distilled water, washing in nitric acid, and final rinsing in distilled 
water. 
The process takes place within an electroforming zone comprising an anode, 
a cathode, and an electroforming bath. The bath may be composed of: ions 
or salts of ions of the layer-forming material, the concentration of which 
can range from trace to saturation, which ions can be in the form of 
anions or cations; a solvent; a buffering agent, the concentration of 
which can range from zero to saturation; an anode corrosion agent, the 
concentration of which can range from zero to saturation; and, optionally, 
grain refiners, levelers, catalysts, surfactants, and other additives 
known in the art. The preferred concentration ranges may readily be 
established by those of skill in the art without undue experimentation. 
A preferred electroforming bath to plate nickel on a substrate comprises 
about 70-90 mg/ml of nickel ion in solution, about 20-40 mg/ml of H.sub.3 
BO.sub.3, about 3.0 mg/ml of NiCl.sub.2.6H.sub.2 O and about 4.0-6.0 
ml/liter of sodium lauryl sulfate. Other suitable electroforming bath 
compositions include, but are not limited to, Watts nickel: about 68-88 
mg/ml of nickel ion, about 50-70 mg/ml of NiCl.sub.2.6H.sub.2 O and about 
20-40 mg/ml of H.sub.3 BO.sub.3 ; chloride sulfate: about 70-100 mg/ml of 
nickel ion, about 145-170 mg/ml of NiCl.sub.2.6H.sub.2 O and about 30-45 
mg/ml H.sub.3 BO.sub.3 ; and concentrated sulfamate: about 100-120 mg/ml 
of nickel ion, about 3-10 mg/ml of NiCl.sub.2.6H.sub.2 O and about 30-45 
mg/ml of H.sub.3 BO.sub.3. Electroless baths such as electroless nickel 
baths can also be employed. Various types are available depending upon the 
properties required in the electroform deposition. These electroless baths 
are well known to those skilled in the art. 
Examples of metals that can be electroformed onto the surface of a 
substrate include, but are not limited to, nickel, copper, gold, silver, 
palladium, tin, lead, chromium, zinc, cobalt, iron, and alloys thereof. 
Preferred metals are nickel and copper. Any suitable conductor or material 
that can be electrochemically deposited can be used, such as conductive 
polymers, plastics, and electroless nickel deposits. Examples of suitable 
auto-catalytic electroless nickel deposits include, but are not limited 
to, nickel-phosphorus, nickel-boron, poly-alloys, such as copper-nickel 
phosphorus, nickel-polytetrafluoroethylene, composite coatings, and the 
like. Methods of preparing electroless nickel deposits employed within the 
scope of this invention are well known to those skilled in the art of 
electroforming. 
The electrolytic bath is energized using a suitable electrical source. 
Layer-forming ions from the solution are electroformed onto the conductive 
surface of the substrate. The process is allowed to proceed until a single 
layer has deposited on the substrate preferably ranging in thickness from 
about 0.200 to about 0.385 mm. After the single layer of metal is 
electroformed onto the surface of the substrate, the substrate is removed 
from the solution. 
In addition, a squeegee blade having desired features can be prepared using 
a pattern of resist having a design complementary to the desired features 
in accordance with the process disclosed in U.S. Pat. No. 5,478,699. 
The electroformed squeegee blade according to the present invention can be 
removed from the substrate by standard methods that include, but are not 
limited to, mechanical separation, thermal shock, dissolution and the 
like. These methods are well known to those of skill in the art. 
Referring to FIGS. 2 and 3, the electroformed squeegee blade 5 of the 
invention causes the solder paste 1 to be applied to a uniform vertical 
height in the stencil apertures 7 regardless of their orientation. As a 
result of the uniform deposition of the solder paste into each of the 
apertures 7, the solder pads 8 are uniformly deposited on the printed 
wiring board 9 of FIG. 2. The printed wiring board 9 is firmly held in 
place beneath the stencil 6 by, e.g., suction, tooling holes, adjustable 
mechanical fixturing or dedicated mechanical fixturing and after the 
deposition of the solder paste 1 is then released to the next operation in 
the printed wiring board manufacturing process. 
As a result, the printed wiring board 9 with solder pads 8 produced 
according to the invention will more firmly hold surface mounted 
integrated circuits and/or components to the printed wiring board 9 for 
subsequent mounting and soldering flow operations. 
Referring to FIGS. 3 and 4, 80, 85 and 90 durometer polyurethane squeegee 
blades have been used as comparison materials against the electroformed 
squeegee blade of the invention. As can be seen, the electroformed 
squeegee blade of the invention, which has an HK.sub.500 hardness 
preferably about 200 to 600, and more preferably of about 390 to 450, 
permits a more uniform deposition of solder paste on the printed wiring 
board. As a result, the number of printed wiring boards that must be 
reworked for solder pad faults and integrated circuit mountings is 
minimal. 
While this invention has been described in conjunction with specific 
embodiments thereof, it is evident that many alternatives, modifications 
and variations will be apparent to those skilled in the art. Accordingly, 
the preferred embodiments of the invention as set forth herein are 
intended to be illustrative, not limiting. Various changes may be made 
without departing from the spirit and scope of the invention as defined 
above.