U.S. patent application number 10/630310 was filed with the patent office on 2004-04-15 for method and apparatus for filling a mask with solder paste.
Invention is credited to Mackay, John T..
Application Number | 20040069840 10/630310 |
Document ID | / |
Family ID | 25505314 |
Filed Date | 2004-04-15 |
United States Patent
Application |
20040069840 |
Kind Code |
A1 |
Mackay, John T. |
April 15, 2004 |
Method and apparatus for filling a mask with solder paste
Abstract
Method and apparatus for filling (printing) cells of a mask with
solder paste. A first (flood) blade is disposed a distance above a
front surface of the mask, moves a glob of solder paste across the
surface of the mask. The first blade is followed by a second
(cleaning) blade which is in contact with the surface of the mask
and which advances across the surface of the mask to remove
residual solder paste from the surface of the mask. The distance
between the first blade and the mask is on the order of a few (2-5)
times the average particle size in the solder paste. The first
blade may be plastic. The second blade is preferably non-compliant,
such as metal. Profiles for the two blades are disclosed. A method
printing multiple masks with multiple sets of blades is
disclosed.
Inventors: |
Mackay, John T.; (San Jose,
CA) |
Correspondence
Address: |
Gerald E. Linden
12925 LaRochelle Cr.
Palm Beach Gardens
FL
33410
US
|
Family ID: |
25505314 |
Appl. No.: |
10/630310 |
Filed: |
July 30, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10630310 |
Jul 30, 2003 |
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09962007 |
Sep 24, 2001 |
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6609652 |
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Current U.S.
Class: |
228/248.1 ;
228/215; 257/E21.508 |
Current CPC
Class: |
H05K 2203/1581 20130101;
H01L 2924/01082 20130101; H05K 2203/043 20130101; B23K 35/0244
20130101; B23K 3/0623 20130101; H01L 2924/01006 20130101; H01L
2924/01029 20130101; H01L 2924/01039 20130101; H05K 3/3485
20200801; H01L 2224/11003 20130101; H05K 2203/0557 20130101; B23K
2101/42 20180801; H01L 2924/14 20130101; B23K 35/224 20130101; B23K
1/0016 20130101; B23K 3/0638 20130101; H01L 21/4853 20130101; H01L
2924/01005 20130101; H01L 24/11 20130101; H01L 2924/01027 20130101;
H01L 2224/13099 20130101; H01L 2924/01013 20130101; H05K 2201/10477
20130101; H05K 2203/085 20130101; H01L 2924/01015 20130101; H01L
2924/00014 20130101; H05K 2203/0278 20130101; H01L 2224/05573
20130101; B23K 3/087 20130101; H01L 2924/01033 20130101; H01L
2924/01075 20130101; H05K 2203/0113 20130101; H01L 2924/01074
20130101; H01L 2924/01322 20130101; H01L 2924/014 20130101; H01L
2924/01042 20130101; B23K 2101/40 20180801; H01L 2924/351 20130101;
H05K 2203/0338 20130101; B23K 35/0222 20130101; H01L 2924/351
20130101; H01L 2924/00 20130101; H01L 2924/00014 20130101; H01L
2224/05599 20130101 |
Class at
Publication: |
228/248.1 ;
228/215 |
International
Class: |
B23K 031/02 |
Claims
What is claimed is:
1. Method of filling cells of a mask with a viscous material,
comprising: with a first blade disposed a distance above a front
surface of the mask, and with a glob of viscous material in front
of the first blade, advancing the first blade across the surface of
the mask; and with a second blade in contact with the surface of
the mask, advancing the second blade across the surface of the mask
to remove residual viscous material from the surface of the
mask.
2. Method, according to claim 1, wherein the viscous material is
solder paste.
3. Method, according to claim 1, wherein: the viscous material
comprises particles having an average particle size; and the
distance is equal to a few average particle sizes.
4. Method, according to claim 1, wherein: the first blade is made
of a plastic material.
5. Method, according to claim 1, wherein: the second blade is moved
in unison with the first blade, across the mask.
6. A set of blades for filling cells of a mask with solder paste,
comprising: a first, flood blade which is generally rectangular in
cross-section, having a leading surface, a trailing which is
generally parallel to the leading surface, and a side edge which is
generally perpendicular to the leading and trailing surfaces and
which, in use, is disposed opposing the mask, wherein the side edge
is chamfered (beveled) so as to present a sloping surface for
pushing the solder paste down into the cells of the mask when the
first blade is moved across the mask; and a cleaning blade for
clearing residual solder paste from the mask.
7. A set of blades, according to claim 6, wherein: the side edge
has a first area which is flat and perpendicular to the trailing
surface, followed by a second area which forms approximately a
45-degree angle with the first area, followed by a third area which
forms a steeper, approximately 60-degree angle with the first
area.
8. A set of blades, according to claim 6, wherein: the side edge is
beveled to direct and force solder paste into the cells of the
mask.
9. A set of blades, according to claim 6, wherein the first blade
has an overall thickness and the sides edge comprises: a flat area
and at least one beveled area, and the flat area comprises
approximately 75% of the overall blade thickness.
10. A set of blades, according to claim 6, wherein: the cleaning
blade comprises a relatively non-compliant material, such as
metal.
11. A set of blades, according to claim 6, wherein: an end portion
of the cleaning blade forms an angle of 30-60 degrees with the
surface of the mask.
12. A set of blades, according to claim 6, wherein: the cleaning
blade comprises an end portion which extends from a base portion;
and the base portion extends substantially parallel to the first,
flood blade.
13. A set of blades, according to claim 6, wherein: the cleaning
blade comprises an end portion which extends from a base portion;
and the base portion is sufficiently long to prevent chatter.
14. Method of printing a sequence of masks with solder paste,
comprising: positioning a first mask between a first print landing
areas and a second print landing area; with a first set of blades
parked at the first landing area, disposing a glob of solder paste
in front of the first set of blades; advancing the first set of
blades advances across the towards the second print landing area to
fill the cells of the first mask with solder paste; continuing to
advance the first set of blades until it is entirely beyond the
mask, and residual solder paste that is being pushed forward is on
the second print landing area; then, retracting the first set of
blades; removing the first mask, and positioning a second mask
between the two print landing areas; with a second set of blades,
starting from the second print landing area, pushing the solder
paste entirely across the second mask to fill the cells of the
second mask, until residual solder paste that is being pushed
forward is on the first print landing area.
15. Method of filling cells of a mask with a viscous material,
comprising: disposing a quantity of the viscous material on a
surface of the mask; bringing a first blade to a distance of a few
mils from the surface of the mask; contacting the mask with a
cleaning blade; advancing the print blade across the surface of the
mask to fill the cells; and advancing the cleaning blade across the
surface of the mask, behind the print blade.
16. Method, according to claim 15, wherein: the viscous material is
solder paste.
17. Method for forming solder balls on a substrate having a
plurality of pads on a surface thereof, comprising: providing a
mask having a plurality of cells; filling the cells of the mask
with solder paste by filling the cells of the mask with a flood
blade spaced a distance from the surface of the mask and moving
across the mask followed by a cleaning blade in contact with the
surface of the mask and moving across the mask; with the mask
disposed on the surface of the substrate, reflowing the solder
material; and separating the substrate from the mask.
18. Method, according to claim 17, wherein: the cells extend
entirely through the mask.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application is a continuation-in-part of U.S. Ser. No.
09/962,007 filed Sep. 24, 2001 (status: Issue Fee paid Jun. 9,
2003, hereinafter "Parent Application").
TECHNICAL FIELD OF THE INVENTION
[0002] The invention relates to techniques for filling (printing)
cells (openings) of masks (stencils) with solder paste, such as
masks which are used in conjunction with techniques for forming
solder balls (bumps) on substrates which are electronic components,
such as dies, wafers, packages, etc.
BACKGROUND OF THE INVENTION
[0003] A number of patents describe techniques for forming solder
balls (also referred to as "bumps") on substrates which are
electronic components, such as forming arrays of solder balls on
pads of integrated circuit (IC) chips which are individual
components or one of several components on a semiconductor wafer.
For ICs, the solder balls tend to be relatively small, such as 4
mils. Another example of an electronic component having solder
balls is a ball grid array (BGA) package. For BGA packages, the
solder balls tend to be relatively large, such as 25 mils. In some
cases, solder balls may be formed on an interconnection substrate
in addition to or rather than on the electronic component being
attached thereto.
[0004] As used herein, the term "solder ball" refers to a
substantially spherical or hemispherical mass (bump) of solder
(e.g., lead-tin solder) resident on a substrate (e.g., electronic
component), suitable for being re-flowed to join the electronic
component to another electronic component. A "large solder ball" is
a solder ball having a diameter of greater than 20 mils (>0.020
inches). A "small solder ball" is a solder ball having a diameter
of up to 20 mils (<=0.20 inches).
[0005] As used herein, the term "electronic component" includes any
circuitized substrate, typically having "pads", including but not
limited to integrated circuit (IC) chips (including prior to or
after singulation from a semiconductor wafer), printed circuit
boards, polyimide interconnection elements, ceramic substrates, and
the like. As used herein, a "substrate" is an electronic component
having a nominally flat surface upon which it is desirable to form
solder balls to effect electrical connections to another electronic
component.
[0006] Various technique for ball bumping substrates include using
a mask (stencil) having cells (apertures, openings), filling the
cells with solder paste, then reflowing the solder paste.
Typically, the stencil is placed on a substrate having pads, with
the filled mask openings aligned with the pads, then reflowing the
solder paste. After reflow, the mask is removed.
[0007] The mask may be filled either after being placed on the
substrate, or before (off-line). Typically, the filling process
involves dispensing a relatively large amount (much more than is
needed to fill the cells of the mask), of solder paste onto the
stencil, then moving (advancing) a screening blade (sometimes
called a "doctor blade") across the mask surface in a manner to
force solder paste into the cell openings. For purposes of the
present discussion, solder paste is considered to be a "viscous
material".
[0008] Conventional solder paste typically contains tiny particles
of solder material (lead/tin), in a matrix of flux, and comprises
about 30% (by volume) solid material. A typical solder paste
contains particles of lead/tin solder, in a matrix of flux, with
the following proportions: 80% (by weight) solid material (e.g.,
particles of lead/tin solder), and 20% (by weight) flux (including
volatiles). In terms of relative volume percentages, the same
typical solder paste may contain approximately 55% (by volume) of
solid material (metal) and 45% (by volume) of flux. It should be
understood that the present invention is not limited to a
particular solder paste formulation.
[0009] U.S. Pat. No. 5,988,487 describes captured-cell solder
printing and reflow methods. A screening stencil is laid over the
surface of the substrate and solder paste material is deposited
into the stencil's apertures with a screening blade. The stencil is
placed in such a manner that each of its apertures is positioned
over a substrate pad upon which a solder bump is to be formed.
Next, a flat pressure plate is laid over the exposed top surface of
the stencil, which creates a fully enclosed (or "captured") cell of
solder material within each stencil aperture. Then, with the
stencil and plate remaining in place on top of the substrate, the
substrate is heated to a temperature sufficient to reflow the
solder material. After reflow, the substrate is cooled, and the
pressure plate and stencil are thereafter removed, leaving solder
bumps on the substrate pads.
[0010] U.S. Pat. No. 6,293,456 describes various methods for
forming solder balls on substrates. Mask configurations, methods or
mounting the masks, and solder material compositions are
described.
[0011] Parent application Ser. No. 09/962,007 filed Sep. 24, 2001
(status: issue fee paid) describes techniques for ball bumping
substrates, particularly wafers. A number of masks (e.g., 110) are
shown therein.
[0012] The present invention is particularly useful in conjunction
with the ball-bumping techniques described above. The following
patents are cited as techniques which may also benefit from the
present invention. It should be understood that the present
invention is not limited to the specific exemplary ball-bumping
techniques mentioned herein.
[0013] U.S. Pat. No. 5,539,153 ("Hewlett Packard") discloses method
of bumping substrates by contained paste deposition. The solder is
applied through stencil/mask and paste method; the mask, however,
remains attached to the substrate during reflow.
[0014] U.S. Pat. No. 5,492,266 ("IBM-1") discloses fine pitch
solder deposits on printed circuit board process and product.
[0015] U.S. Pat. No. 5,658,827 ("IBM-2") discloses method for
forming solder balls on a substrate. Solder balls are formed by
squeegeeing solder paste through apertures in a fixture into
contact with pads on a substrate, and heating the fixture, paste
and substrate to reflow the solder paste into solder balls.
[0016] Consistency in the size (e.g., height/volume) of the solder
ball contacts is a critical factor. For a given solder paste, the
size of the solder ball is determined principally by the size of
the mask cells, and how well the cells are filled. Desirably, each
and every cell is equally filled to full capacity, with no voids.
This requires a technique for consistently filling the cells of a
mask. A solder ball from a partially filled cell will be smaller
than a solder ball from a fully filled cell. If one or more of the
resulting solder balls are significantly shorter than others
(usually due to an insufficient amount of solder paste deposited on
one or more conductive pads prior to contact formation) it is
likely that these smaller (shorter) contacts may completely miss
their mating contact pads (e.g., on the circuit board) and will
fail to form an electrical connection between the ball-bumped
electronic component and the underlying interconnection substrate
(e.g., printed circuit board). Hence, quality control is critical,
since proper electrical connections between the electronic
component and the underlying substrate to which it is assembled are
formed only if each and every one of the solder ball contacts
reflows correctly and wets its associated conductive pad on the
underlying substrate. It can be very expensive to inspect all the
bumps (solder balls), and time consuming. Defective assemblies of
electronic components to interconnection substrates can be
difficult or impossible to repair after assembly if connections are
not properly formed. Even prior to assembly, the correction of
improperly formed solder balls on the exterior of an electronic
component such as a BGA package can be very difficult and involves,
initially, careful quality control inspection of the ball bumps
prior to assembly of the packaged device to a substrate.
[0017] Standard print heads for filling cells of masks with solder
paste typically use hydraulic pressure to force solder paste into
the mask cells, then a print blade scrapes off excess solder paste.
Some of the problems associated with this procedure are:
[0018] a. When filling a mask which is on a substrate (electronic
component), such as has been described above, if a poor seal exists
between the mask and the substrate (due to irregular surface
topology), solder can be pumped (or leak) under the mask, onto an
adjacent area of the substrate.
[0019] b. A resilient blade, such as of Permalex.TM., may be used,
and the blade (the thickness or profile of which is typically
larger than the cross-dimension of the cell) may form a seal with
the cell, thereby trapping air (rather than solder paste) in the
cell.
[0020] c. It has been found that printing a mask normally takes
multiple passes of the blade to ensure complete cell filling,
without "gouging".
[0021] d. It is important that the surface of the mask be clean
(free of excess solder paste) after printing.
[0022] In conventional mask filling techniques, the solder paste
wave or extra paste at printing is pushed out in front of the print
blade. This flood causes the paste to wet the top surface of the
cell sealing the air inside and creates "printing voids". This
problem is exacerbated with the more fluid (e.g., <85% weight %)
pastes. Permalex.TM. blades are known, and they typically avoid
gouging. Nevertheless, often, it takes 6 or more passes to fill
small cells. When multiple passes are made, it is believed that the
solder paste wets the mask surfaces and removes some of the trapped
air each time filling the cell just a bit more each print
stroke.
BRIEF DISCLOSURE (SUMMARY) OF THE INVENTION
[0023] It is an object of the invention to provide an improved
mask-filling technique, such as in conjunction with forming solder
balls on electronic components.
[0024] According to the invention, a method is provided for filling
(printing) cells of a mask with a solder paste as a prelude to ball
bumping an electronic component. Generally, with a first (flood)
blade disposed a distance above a top surface of the mask, and with
a glob of solder paste (viscous) material in front of the first
blade, the first blade is advanced across the surface of the mask.
This is followed by a second (cleaning) blade which is in contact
with the surface of the mask. The second blade advances across the
surface of the mask to remove residual solder paste from the
surface of the mask. The distance between the first blade and the
mask is on the order of a few (2-5) times the average particle size
in the solder paste. The first blade may be plastic. The second
blade is preferably non-compliant, such as metal.
[0025] According to a feature of the invention, the first blade is
generally rectangular in cross-section, having a leading surface, a
trailing which is generally parallel to the leading surface, and a
side edge which is generally perpendicular to the leading and
trailing surfaces and which, in use, is disposed opposing the mask,
wherein the side edge is chamfered (beveled) so as to present a
sloping surface for pushing the solder paste down into the cells of
the mask when the first blade is moved across the mask.
[0026] According to a feature of the invention, a sequence of masks
(two or more masks, in sequence) can be printed by positioning a
first mask between a first print landing areas and a second print
landing area. With a first set of blades, starting at the first
landing area, moving across the mask to the second parking area to
fill the cells of the first mask, then retracting the first set of
blades. Then, with a second set of blades, starting at the second
landing area, moving across the mask to the first landing area to
fill the cells of a second mask.
[0027] Benefits of the present invention include:
[0028] a. the problem of solder being pumped under the mask can be
avoided.
[0029] b. the problem of trapping air in the cell can be
avoided.
[0030] c. a mask can successfully be printed in a single (one)
pass.
[0031] d. the surface of the printed mask will be free of excess
solder paste after printing.
[0032] The "off-wafer" printing of the present invention mainly
used to ensure that the cell volume is substantially exactly the
same for each wafer. Normally, as wafer topography changes it will
hang the mask up and allow leakage under the mask, or at least
change cell volume. For 100 micron balls a 30% change can be
present. The off-wafer printing of the present invention can
achieve <2 micron variation. The largest print volume variation
comes from gouging paste out of the cell at print. This can result
in a 40% loss in wet paste and is a primary concern this is the
main reason that the invention use two blades--one to overfill the
cells and the other (which may be Permalex) to remove excess solder
paste (scrape the cell even with mask top surface.) Also very
important with cell soldering is to have the top surface of the
mask cleared of solder paste residue for "capturing "of the cell
(see, e.g., U.S. Pat. No. 5,988,487 mentioned above).
[0033] Other objects, features and advantages of the invention will
become apparent in light of the following description thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] Reference will be made in detail to preferred embodiments of
the invention, examples of which are illustrated in the
accompanying drawings. The drawings are intended to be
illustrative, not limiting. The cross-sectional views, if any,
presented herein may be in the form of "slices", or "near-sighted"
cross-sectional views, omitting certain background lines which
would otherwise be visible in a true cross-sectional view, for
illustrative clarity.
[0035] FIG. 1 is an exploded cross-sectional view of a method and
apparatus for forming solder balls on substrates, such as is
disclosed in U.S. Pat. No. 5,988,487.
[0036] FIG. 1A is an enlarged (magnified) view of the substrate
(102) shown in FIG. 1, after completion of ball bumping.
[0037] FIG. 2 is a side, cross-sectional view of a technique for
applying solder paste to cells in a mask, according to the
invention. This drawing corresponds to FIG. 12 of the Parent
Application.
[0038] FIG. 3 is a side, cross-sectional view of a set of blades,
such as those shown in FIG. 2, according to the invention.
[0039] FIG. 4 is a schematic side view of two sets of blades, such
as those shown in FIG. 2, according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0040] FIG. 1 illustrates a technique 100 for forming solder balls
on a surface of a substrate 102, such as is set forth in U.S. Pat.
No. 5,988,487. The substrate 102 has number of pads 104 on its top
(as viewed) surface. The pads 104 are typically arranged in an
array, having a pitch (center-to-center spacing from one another).
The substrate 102 is disposed atop a heater stage 106. A mask
(stencil) 110 is provided. The mask 110 is a thin planar sheet of
relatively stiff material, such as molybdenum, having a plurality
of openings (cells) 112, each corresponding to a pad 104 whereupon
it is desired to form a solder ball on the substrate 102. The mask
110 is placed on the top (as viewed) surface of the substrate 102
with the cells 112 aligned over the pads 104. The cells 112 in the
mask 110 are filled with solder material 114. This is done in any
suitable manner such as by smearing solder material on the top (as
viewed) surface of the mask 110 and squeegeeing the solder material
114 into the cells 112 of the mask 110. Squeegeeing is typically a
multi-pass process.
[0041] The cells 112 in the mask 110 may be filled with solder
paste prior to placing the mask 110 on the top surface of the
substrate, in which case the solder-paste-filled cells 112 would be
aligned over the pads 104.
[0042] A pressure plate 120 is disposed onto the top (as viewed)
surface of the mask 110. This holds the mask 110 down onto the
substrate 102, and the substrate 102 down onto the heater stage
106. This also closes off the cells 112 ("captured cell"). The
heater stage 106 is heated up, typically gradually, to a
temperature sufficient to cause the solder material in the cells
112 to melt (reflow). When the solder material melts, the
individual solder particles will merge (flow) together and, due to
surface tension, will try to form (and, typically, will form) a
sphere. When the solder material resolidifies, it assumes a general
spherical or hemispherical shape. The mask 110 is then removed from
the substrate 102.
[0043] FIG. 1A is an enlarged (magnified) view of the substrate 102
shown in FIG. 1, after completion of ball bumping. Herein it can be
observed that the solder balls 130 are generally spherical, have a
diameter "D" and have a height "H".
[0044] When printing, for example, on the surface of an integrated
circuit wafer, it must be appreciated that the surface of the wafer
is often not very flat, topologically speaking. And this irregular
topology can lead to variations in the effective overall volume of
a cell being filled with solder paste. Also, as mentioned above,
when printing on an irregular surface, solder paste can ooze out
under the mask, creating subsequent problems during reflow. As a
general proposition, any variations in the process, from
cell-to-cell, are simply not desirable. Hence, printing on a known
flat surface (KFS)--such as the surface of the heater stage (e.g.,
106)--is preferred. Also, by printing "off-line", the wafer is
spared from the sometimes excessive forces required to get a good
print (effective cell filling). we didn't mention the excessive
force problem above.
[0045] According to an aspect of the invention, it is generally
preferred to print "off-line"--in other words, with the mask on a
smooth surface without irregularities, rather than on the surface
of an electronic component (e.g., substrate 102). This is for
purposes of (i) uniformity and (ii) to avoid damaging an underlying
component.
[0046] Off-wafer printing is good for three reasons:
[0047] 1. Low force
[0048] 2. Excellent cell volume control
[0049] 3. Finished solder void control--when the solder paste wets
to the pad of a part to be bumped this flux can be trapped during
the reflow (solder voids). With off wafer printing the solder paste
is not wetted to the pad, the ball is sphereized in the mask and
only contacts the pad after liquefied this avoids flux trapped
voids. No other process offers this void avoidance.
[0050] Printing off-line is illustrated, for example, in FIG. 4 of
the aforementioned Parent Application which is a schematic diagram
of a machine for ball bumping substrates including a print station
414, which may be a flat, non-wettable surface for off-wafer
filling of the cells of the mask with solder material.
[0051] The flat surface is non-wettable from the solder material's
perspective. Suitable materials are Teflon.TM. coatings and chrome.
The flat surface should not only be free from surface topology and
defects such as scratches or dings and dents, but will remain flat
during heating at high rates. Heat differences coupled with the
materials expansion properties may result in warpage during
heating.
[0052] FIG. 2 illustrates an embodiment of the mask-filling
technique of the present invention.
[0053] It should be understood that the technique is not limited to
filling masks for the purpose of ball bumping electronic
components, and has more general applicability to any number of
printing (mask filling) processes, whether ball bumping or
otherwise. It should therefore also be understood that the present
invention is not limited to filling masks with any particular
solder paste or, for that matter, with solder paste at all. The
technique is well-suited to filling the cells of the mask with any
material having a viscosity in the range of 20 kcps-300 kcps
(kilocentipoise).
[0054] As shown in the figure, a quantity (blob, glob, mass) of
solder paste 202 is disposed on the surface of the mask 210
(compare 110). The mask 210 is shown as being disposed on a
suitable support surface 208 (compare 106, or 414 of Parent
Application). The support surface 208 may be a wafer, for printing
with the mask 210 already disposed on a wafer (compare 102), if so
desired. Or, the support surface 208 may be any non-wettable
surface for off-line filling of the mask.
[0055] The mask 210 has a plurality of cells 234 (compare 112)
which may be arranged in an array. The cells 234 may be round,
square or the like. The mask has a thickness, typically 3 mils. The
cells are preferably, but not necessarily, uniform in size, hence
volume. For example, a square cell may have a cross-dimension of 6
mils.
[0056] A first "print" (or "flood") blade 220, such as a rubber
blade made of 90 durometer ULON.TM., is brought to a distance of a
few mils (e.g., 5-7 mils) from (above) the surface of the mask 210.
The blade 220 is advanced in the direction of the arrow 222. As the
blade 220 advances, the cells 234 become filled with solder paste
202 (compare 114). It is preferred that the blade 220 not contact
the mask, and not drag across the mask. Because the blade 220 is
spaced from the mask 210, there will inevitably be an amount of
excess solder paste on the surface of the mask behind (to the left
of, as illustrated) the blade 220.
[0057] Since the blade 220 is not in contact with the mask 210, the
contact pressure is essentially zero. This can be important when
the mask 210 is supported on a delicate electronic component that
might be adversely affected by pressure.
[0058] The gap (spacing) between the blade 220 and the surface of
the mask 210 is generally dependent upon the size of particles (not
illustrated) in the solder paste 202. Typically, the gap is 2-5
times the average particle size.
[0059] The blade 220 suitably has a thickness of approximately
0.250 inches, is spaced approximately 5-7 mils from the surface of
the mask 210, and is suitably formed of a material ranging from a
very hard material such as stainless steel to a relatively soft
material such as 60 Shore A rubber. A suitable material is
Ulon.TM..
[0060] Since the principal purpose of the flood blade 220 is simply
to push solder paste into the cells, its composition and
end-profile (e.g., dull versus pointy) do not matter very much.
[0061] Preferably, the flood blade 220 is inclined in the direction
of travel, rather than straight up and down (as illustrated)--for
example at an angle of 75 degrees (rather than 90 degrees, as
illustrated) with respect to the surface of the mask.
[0062] A second, "cleaning" blade 230, such as a Permalex.TM. blade
by Transition Automation SPK-PLX-1.5-9, is disposed so as to
contact the mask 210, and advances in the direction of the arrow
222. In essence, the cleaning blade 230 follows a suitable distance
behind the flood blade 220, and performs "clean up" duty. By way of
example, the distance between the two blades 220 and 230 is
approximately 1" (one inch) which is quite suitable for printing a
mask for a 6 or 8 inch wafer. This distance between the blades 220
and 230 should be sufficient to allow room for the accumulation of
paste left behind by the flood blade 220.
[0063] Since the cleaning blade 230 need not perform a cell-filling
function, it can have a low contact force (e.g., 2500 grams) with
the surface of the mask 210. As discussed above, a high contact
force can be undesirable. And the non-compliance of the blade 230
allows it to clean the surface of the mask without gouging
(removing solder paste from) the already-filled cells.
[0064] The blade 230 is suitably spring steel or the like, then the
tip or printing edge is coated with a polyimide coating, then a
final metal coating is deposited. This as claimed by the
manufacturer is the common ground between hard steel (no compliance
requiring high pressures to obtain complete contact) and soft
rubber that deflects into cell volume and gouges (conforms too
well)
[0065] The blade 230 suitably has a thickness of 0.010 inches, is
in contact with the surface of the mask 210, and is suitably formed
of a material ranging from a very hard material such as stainless
steel_to a relatively hard material such as spring steel. The end
of the blade 230 in contact with the mask 210 and is specially
coated to ensuring good cleaning of the mask surface without
gouging solder paste out of the cells.
[0066] The flood blade 220 and the cleaning blade 230 may move in
unison, or independently from one another. The may both be inclined
in the direction of travel. The flood blade 220 is suitably of a
plastic material, and is spaced a distance equivalent to a few
(e.g., 2-5) average solder paste particle sizes from the surface of
the mask 210. The cleaning blade 230 is suitably of a metal
material, and is preferably thicker than the cross-dimension of a
cell 234. The flood blade 220 and the cleaning blade 230 are shown
out-of-scale (not to scale), vis-a-vis the mask 210, for
illustrative clarity.
[0067] Therefore, the invention can generally be characterized as
comprising using two dissimilar blades to fill cells of a mask
(210) with solder paste (202). The first blade (220) is not in
contact with the mask, and therefore "overfills" the cells. The
second blade (230) follows behind (after, later) the first blade
(220) and removes excess solder paste from the surface of the mask.
The first blade (220) exerts no direct pressure on the mask. The
second blade (230) exerts very little pressure on the surface of
the mask. The first blade (220) is of a wide range of materials.
The second blade (230) is preferably of a non-compliant
material.
[0068] A person having ordinary skill in the art to which this
invention most nearly pertains will recognize that any suitable
mechanical mechanism (e.g., actuators, etc.) can be used to control
the movement of the blades (220, 230) across the surface of the
mask (210), and that they can be moved in unison with one another,
or independently from one another.
[0069] The two blades (220, 230), herein considered to be a "set"
of blades, can be moved in unison, as discussed above, with the
second blade (230) trailing the first (230) and moving in the same
direction as the first (220). The technique of the present
invention has been found to be reliable for fully filling the cells
of a mask, in only one pass. Alternatively, the second blade (230)
can be independently moved across the surface of the mask,
including in a different direction than the first blade (220),
including making more than one pass across the mask to ensure that
the surface of mask is clean.
[0070] FIG. 3 illustrates an embodiment of a set of blades
comprising a first blade 320 (compare 220) and second blade 330
(compare 230) for printing a mask 310 (compare 210). Profiles for
the two blades 320 and 330 are described. The flood blade 320 is
generally rectangular in cross-section, having a leading edge
(surface) 322, a trailing edge (surface) 324 which is generally
parallel to the leading edge, and a side edge (surface) 326
(comprising 326a,b,c) which is generally perpendicular to the
leading and trailing edges. In use, the side edge 326 is disposed
opposing (facing) the mask 310, but is not in contact with the
surface of the mask. (A non-wettable support surface, compare 208,
is omitted, for illustrative clarity.)
[0071] The side edge 326 is chamfered (beveled) so as to present a
sloping surface for pushing the solder paste (202) down into the
cells of the mask when the blade 320 is moved (left-to-right in the
illustration) across the mask 310. For example, from the trailing
edge 324, the side edge 326 has a first area 326a which is flat and
perpendicular to the trailing edge 324 (and parallel to the mask
310), followed by a second area 326b which forms approximately a
45-degree angle with the first area 326a, followed by a third area
326c which forms a steeper, approximately 60-degree angle with the
first area (or, a shallow, approximately 30-degree angle with
respect to the leading edge 322). This "business end" of this blade
320 is shown with a flat area 326a and compound bevel 326b,c at the
junction of the side edge 326 and the leading edge 322. The flat
area 326a is preferably approximately 75% of the overall blade
thickness.
[0072] When the blade 320 is moved across a mask, with a glob of
solder paste in from of it (see, e.g., FIG. 2), the 60-degree area
326c is the first to encounter the older paste (see FIG. 2) as the
blade moves across the mask. This angle, being less than
90-degrees, starts to push the solder paste down as the blade 330
moves, exerting a mild downward force on the solder paste. (It
should be understood that a similar result could be obtained by
tilting the entire blade 230 of FIG. 2 forward 30-degrees from
vertical.) The next, 45-degree area 326b further helps to push the
solder paste down into the cell. (With a 30-degree tilted blade,
this area would be 15-degrees steeper.) Finally, the flat area 326a
forces the solder paste into the cell.
[0073] In any case, the flood blade 320 has at least one area which
first encounters the solder paste at an angle between flat
(parallel to the mask surface) and vertical (perpendicular to the
mask surface), to start pushing (directing) the solder paste down
into the cells, followed by a substantially flat (parallel to the
mask surface) area for finally pushing (forcing) the solder paste
into the cells. The point is to fill (in this case, overfill) the
cells of the mask in one pass, without requiring exerting a lot of
pressure on the mask (particularly if the mask were atop a delicate
electronic component).
[0074] FIG. 3 also illustrates an embodiment of a cleaning blade
330 (compare 230). As mentioned above, the cleaning blade 330 is
preferably formed of a relatively non-compliant material, such as
metal. The cleaning blade 330 comes into contact with the mask 310.
An end portion 332 of the cleaning blade 320 preferably forms an
approximately 45-degree angle with the surface of the mask 310. For
example, between 30-degrees and 60-degrees, preferably
approximately 45-degrees. The cleaning blade 330 could simply be
one flat sheet of metal inclined at said approximately 45-degrees.
However, in a set of blades moving in unison across a mask, the
cleaning blade 330 needs to "fit" behind the flood blade 320.
Therefore, the cleaning blade 330 is suitably bent (folded) so that
the angled end portion 332 extends from a base portion 334 which
extends substantially parallel to the flood blade 320
(perpendicular to the mask 310). (Here, the end portion 332 is
shown at 45 degrees to the surface of the mask. The end portion 332
should be between 30-60 degrees to the surface of the mask. The end
portion 332 forms an obtuse angle with the base portion 324.) It
has been found that the base portion 334 should be at least 2" (two
inches, 5 centimeters) in length for filling a "normal" mask for
ball bumping a 6-8 inch wafer. This dimension was determined
empirically. The "long" (quasi-cantilevered, compliantly-mounted)
mounting of the cleaning blade performs well. It is believed that
it creates a bit of compliance, avoiding "chatter" during the
process of scraping excess solder paste off of the surface of the
mask.
[0075] FIG. 4 illustrates an arrangement wherein two sets of blades
are used to expedite automatic mask printing. The drawing is merely
illustrative, and is not to scale. A first set of blades comprises
a flood blade 420 (compare 320) and a cleaning blade 430 (compare
330). A second set of blades comprises a flood blade 440 (compare
320) and a cleaning blade 450 (compare 330). A mask 410 (compare
310) is disposed between two print landing areas 450 and 460. (A
non-wettable support surface, compare 208, is omitted, for
illustrative clarity.)
[0076] The first set of blades 420/430 is "parked" on the first
print landing area 460. A glob of solder paste (compare 202) is
disposed in front of the flood blade 420, on the first print
landing area 460.
[0077] The first set of blades 420/430 then advances across the
mask 410 (from left-to-right, as illustrated), towards the second
print landing area 470, to fill the cells of the mask (to "print"
the mask). The first set of blades continues to print, until it is
entirely beyond the mask, and until the residual solder paste (that
portion of the solder paste which did not make it into the cells)
that is being pushed forward is on the second print landing area
470. Then the first set of blades 420/430 can be retracted, and
repositioned on the first print landing area 460. Meanwhile, the
printed mask is taken away, and another, subsequent mask is
positioned between the two print landing areas 460 and 470 to be
printed. The second set of blades 440/450 is the "mirror image" of
the first set of blades, and prints the subsequent mask by pushing
the residual solder paste across the mask, from right-to-left (as
illustrated). When finished, the residual solder paste that has
been pushed forward (to the left) by the second set of blades will
be on the first print landing area 460, and the second set of
blades will return to its starting position. A subsequent mask can
then be printed by the first set of blades pushing this residual
solder paste over the subsequent mask onto the second print landing
area, etc, so long as there is an adequate supply of residual
solder paste. In this manner, certain efficiencies of operation can
be achieved.
[0078] Although the invention has been illustrated and described in
detail in the drawings and foregoing description, the same is to be
considered as illustrative and not restrictive in character--it
being understood that only preferred embodiments have been shown
and described, and that all changes and modifications that come
within the spirit of the invention are desired to be protected.
Undoubtedly, many other "variations" on the "themes" set forth
hereinabove will occur to one having ordinary skill in the art to
which the present invention most nearly pertains, and such
variations are intended to be within the scope of the invention, as
disclosed herein.
* * * * *