A flexible metal squeegee blade (500) includes a plurality of grooved portions (502) etched on a rectangular metal blade plane (504) to provide for greater flexibility at the grooved portions (502) of the blade.

CROSS REFERENCE TO RELATED APPLICATIONS 
This application is related to U.S. application Ser. No. 08/645,404 by Tan, 
et al., entitled "A DIFFERENTIAL THICKNESS STENCIL," U.S. application Ser. 
No. 08/645,402 by Aun, et al., entitled "A STENCIL SHIFTER" U.S. 
application Ser. No. 08/645,405, by Tan, et al., entitled "A PASTE 
RETAINER," all related to corresponding applications previously filed in 
Malaysia, assigned to Motorola, Inc. and filed concurrently herewith. 
TECHNICAL FIELD 
This invention relates in general to squeegee blades and in particular to 
squeegee blades for printing paste on a substrate. 
BACKGROUND 
Screen printing on substrates, such as ceramic thick-film hybrid 
microcircuits and printed circuit boards (PCBs) for Surface Mounting 
Technology (SMT) is well-known. 
Referring to FIGS. 1 and 2, films or paste of various materials, such as 
solder, can be deposited by screen or stencil printing. The screen printer 
is a precision apparatus that provides for mounting or setting-up a screen 
or stencil 14, positioning the screen relative to a substrate 32 in x, y, 
and z directions, and a rolling angle, positioning a squeegee 16 with a 
blade 18 attached relative to the screen, positioning and moving a 
nestplate 42 that supports the substrate 32, and moving the squeegee 16 
with a controlled speed. The stencil 14 is typically constructed of a 
stamped stainless steel and etched with a through-hole pattern in the 
regions in which a paste is to be printed through the etched cavities or 
holes 142 of the stencil 14. For reproducible quality printing, the screen 
or stencil tension and surface must be uniform and held down by an 
aluminum frame 146. However, in reality, the top surface of stencils 14 
are often not perfectly flat. 
The reservoir of paste 12 is deposited on the stencil 14 by a paste 
dispenser when the nestplate is in its original unraised position. During 
printing, when the nestplate 42 is raised, the squeegee blade 18 is 
deflected in the deformed area of the stencil 14 outside of the nestplate 
42 and substrate 32. The nestplate 42 is raised to position the supported 
substrate 32 in direct contact with and below the forced-up center portion 
of the stencil 14, so that the center portion of the stencil 14 is brought 
in contact with the substrate 32 in the area under the squeegee 16 stroke 
and the paste 12 is forced through the cavities 142 of the stencil 14 by 
the blade 18. Preferably, the nestplate 42 will be raised a distance 22 
higher than the stencil 14 to obtain good printing results. 
After the squeegee blade sweep during which the paste comes in contact with 
the substrate, the paste wets the substrate. As the squeegee passes, the 
nestplate 42 supporting the substrate 32 is lowered to its original 
position away from the original substantially flat stencil 144, leaving 
the paste that was in the holes of the stencil deposited on the substrate 
in the desired pattern. 
The reason why the nestplate 42 has to travel a little higher then the 
stencil 14 is because the bottom surface of the substrate covering portion 
of the stencil has be mated tightly to the top surface of the substrate or 
PCB 32 to ensure that the paste 12 being printed to the surface of the PCB 
is limited to the area of the hole 142 on the stencil. In other words, the 
printed paste must follow the shape of the hole 142 such that paste will 
not be allowed into any gaps between the two surfaces of the PCB and the 
stencil 14 to create the smear paste situation. However, this level 
difference of the stencil 14 surface requires the squeegee blade to be 
able to be deflected more over the substrate portion in order for the 
blade and substrate to be constantly in contact with the stencil surface. 
Conventionally, the current squeegee design uses a single piece of rigid 
metal blade. This conventional uniform long squeegee blade must form a 
parallel seal with the stencil during the dynamic printing stroke as well 
as the static original position when the nestplate has not been raised 
yet, and maintain a constant angle of attack with the stencil so that the 
force exerted on the paste is constant. As the nestplate surface becomes 
larger to accommodate printing more substrates simultaneously, the blade 
length has to be increased also. However, as the blade length increases, 
maintaining a parallel aligned blade across the length of the nestplate 
becomes that much more difficult. Usually, set-up time is quite 
substantial for a person to align the blade such that it will be parallel 
to the stencil during both static and dynamic conditions. 
Moreover, as the substrate size and line definition of the pattern usually 
called pitch become smaller, with the advance of microelectronics, 
controlling the quality of the printing process becomes more complex. The 
quality problems includes paste smearing, uneven thickness of paste 
deposition, paste escape from the reservoir outside the nestplate area 
which resulted in paste volume variation, and the mixing of escaped "dry" 
paste from the reservoir with the "fresh" paste applied by the squeegee 
blade. 
To achieve a cleaner sweep of the squeegee blade over the substrate, the 
squeegee printing force is increased. As the the vertical force from the 
squeegee to the stencil or printing force increase, the squeegee blade 
will be deflected more, therefore increasing the chances that the blade 
contacts fully to the surface of the stencil. However, the increased 
squeegee force itself also causes paste smearing. 
Furthermore, because the printing surface area of the substrate or the 
stencil could be uneven, a paste deposition height variation can occur to 
reduce perfect parallelism. FIGS. 2-4 illustrate this uneven stencil 
printing problem. Because the current squeegee blade is rigid along its 
entire length, it cannot maintain close contact between the blade and the 
uneven stencil surface at the low force pressure needed to prevent paste 
smearing. Consequently, the paste will go through the gaps 302 between the 
noncontacted areas of the bumped stencil 148 and create residue paste 202. 
Because of differentials in normal reaction forces applied during printing 
at the four sides outside the substrate area or nestplate, excessive paste 
can escape there as seen in FIG. 2. This excaped paste inturn causes paste 
volume variation and the mixing of "dry" and "fresh" paste. 
Accordingly, a need exists to compensate for the unevenness of the 
non-parallel printing components and of the different forces to ease the 
blade parallel set-up process and to reduce the printing quality problems 
of the prior-art blade. 
SUMMARY OF THE INVENTION 
Briefly, according to the invention, there is provided a flexible squeegee 
blade for a surface mounting technology printer. The outer surface of the 
blade is indented in at least one portion to form a more flexible squeegee 
blade having a variable stiffness characteristic in order that the blade 
does not have to be set-up perfectly parallel with the stencil as it will 
be compensated by its flexible characteristics.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIGS. 2 and 3, a squeegee blade assembly 16 is shown for 
printing solder paste on a substrate, such as a blank printed circuit 
board (PCB) 32 for later surface placement of electronic components on the 
solder pasted PCB, as part of the Surface Mounting Technology (SMT). A 
squeegee 16 laterally moves and serves as the movement controller for 
wiping a blade across the stencil covered substrate underneath. The 
conventional printer squeegee blade assembly is much like the common 
T-shaped tool with a mounted blade of rubber for wiping or scraping liquid 
off a car window surface, except that the blade is made of a solid piece 
of rigid metal. As the blade's thickness and high stiffness coefficient is 
uniform along its length, the deflection along the length of the blade 
will be limited by the uniform high stiffness coefficient. This uniform 
high stiffness coefficient reduces the chances of having a good mating 
contact between the blade and the stench surface. 
Referring to FIGS. 5 and 6, a flexible metal squeegee blade 500 attached to 
a squeegee 16 for a solder paste printer, in accordance with the teachings 
of the present invention, shows a plurality of grooved portions 502 etched 
on the rectangular metal blade plane 504 to provide for greater 
flexibility at the grooved portions 502 of the blade. 
The flexible metal squeegee blade 500 is made-up of a plurality of blade 
portions 506 having a first stiffness coefficient. At least one blade 
connector portion, in the form of a groove 502 having a second stiffness 
coefficient, integrally joins the plurality of blade portions 506 with the 
at least one blade connector portion 502 for forming a panel 504 having a 
horizontal end 508. A straight or contact edge 608 integrally joins the 
panel 504 at the horizontal, paste sweeping, or contact end 508 to provide 
flexibility at portions of this contact edge that are closest to the 
grooved portions 502 of the blade. A border 515 on the opposite end 516 is 
mountably attached to the squeegee 16 of the printer. In this manner, the 
overall effect is an integral blade made-up of a few different smaller 
blades aligned together to allow individual variable flexibility. 
Meanwhile, the smaller or shorter length blades are still joined together 
without any discontinuous gaps to give the blade sufficient strength. 
As one possible connector shape out of many possible shapes and sizes 
depending on the requirement of the stencil, the blade connector portion 
502 is a vertical grooved indentation of the original uniform blade to 
maintain top-down rigidity. For more flexibility, more vertical grooved 
indentations can be etched or grinded in between the each of the blade 
portions, periodically or at various desired locations along the blade to 
form an accordion-like panel. The more grooves there are, the more 
flexible the blade is, and calibration and paste control will be that much 
easier. 
Preferably, the connector groove portions 502 are each 10 mm long and 2 mm 
wide. These grooves having a depth of 0.1 mm are formed on the original 
blade having a thickness of 0.3 mm. Thus the first stiffness coefficient 
of the blade portion is necessary higher than the second stiffness 
coefficient of the connector portion because the connector portion is 
basically a part of the original blade portion that is etched away and the 
stiffness of the overall blade panel will vary and flex like an accordion. 
The smaller stiffness coefficient of the grooves separates the original 
blade into smaller pieces for the pieces to align or deflect individually 
to the surface of the bump 148 on the stencil. 
For manufacturing simplicity if the straight edge 608 is part of the 
original blade, than it too will have the same original first stiffness 
coefficient but will follow the alignment of the stencil surface 148. 
Hence, the gap 302' between the blade and stencil surfaces will be reduced 
due to this self-alignment brought about by the grooves 502.