U.S. patent application number 10/630544 was filed with the patent office on 2005-07-07 for apparatus for improving stencil/screen print quality.
This patent application is currently assigned to Micron Technology, Inc.. Invention is credited to Cobbley, Chad A., Jiang, Tongbi, VanNortwick, John.
Application Number | 20050145169 10/630544 |
Document ID | / |
Family ID | 25467587 |
Filed Date | 2005-07-07 |
United States Patent
Application |
20050145169 |
Kind Code |
A1 |
Jiang, Tongbi ; et
al. |
July 7, 2005 |
Apparatus for improving stencil/screen print quality
Abstract
A method and apparatus for improved stencil/screen print quality
is disclosed. The stencil or screen assists in application of a
printable material onto a substrate, such as an adhesive to a
semiconductor die of a semiconductor wafer during a lead-on-chip
(LOC) packaging process. In one embodiment, the stencil includes a
coating applied to at least one surface of a pattern of the stencil
or screen to retard running of the printable material onto the
surface. In another embodiment, the stencil or screen includes a
second coating applied to at least one other surface of the pattern
to promote spreading of the printable material onto the
substrate.
Inventors: |
Jiang, Tongbi; (Boise,
ID) ; Cobbley, Chad A.; (Boise, ID) ;
VanNortwick, John; (Kuna, ID) |
Correspondence
Address: |
SCHWEGMAN, LUNDBERG, WOESSNER & KLUTH, P.A.
P.O. BOX 2938
MINNEAPOLIS
MN
55402-0938
US
|
Assignee: |
Micron Technology, Inc.
|
Family ID: |
25467587 |
Appl. No.: |
10/630544 |
Filed: |
July 29, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10630544 |
Jul 29, 2003 |
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09513628 |
Feb 28, 2000 |
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6599365 |
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09513628 |
Feb 28, 2000 |
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08935745 |
Sep 23, 1997 |
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6669781 |
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Current U.S.
Class: |
118/504 ;
438/945 |
Current CPC
Class: |
B41N 1/242 20130101;
H01L 2224/4826 20130101; H05K 3/1225 20130101; B41C 1/14 20130101;
Y10S 438/945 20130101 |
Class at
Publication: |
118/504 ;
438/945 |
International
Class: |
B05C 011/11 |
Claims
1-8. (canceled)
9. A semiconductor die stencil having a top surface, a bottom
surface, and one or more side surfaces, the bottom surface having a
surface tension less than a surface tension of the top surface and
less than a surface tension of the side surfaces.
10. A semiconductor die stencil having at least a first surface and
a second surface, the first surface having a surface tension less
than a surface tension of the second surface to retard adhesive
running from the second surface onto the first surface.
11. The semiconductor die stencil of claim 10, wherein the first
surface is a bottom surface.
12. The semiconductor die stencil of claim 10, wherein the second
surface is a top surface.
13. The semiconductor die stencil of claim 10, wherein the second
surface is a side surface.
14. A semiconductor die stencil having at least a first surface and
a second surface, the first surface having a surface tension
greater than a surface tension of the second surface to promote
adhesive running onto a semiconductor die
15-39. (canceled)
40. The semiconductor die stencil of claim 9 wherein the top
surface is a coating selected from the group consisting of any one
or more of tungsten, tungsten carbide, tungsten nitride, nickel,
and nickel alloy in any combination
41. The semiconductor die stencil of claim 9 wherein the bottom
surface is a polymeric material.
42. The semiconductor die stencil of claim 12 wherein the top
surface is a coating selected from the group consisting of any one
or more of tungsten, tungsten carbide, tungsten nitride, nickel,
and nickel alloy in any combination
43. The semiconductor die stencil of claim 11 wherein the bottom
surface is a polymeric material.
44. The semiconductor die stencil of claim 14 wherein the second
surface is a polymeric material.
45. The semiconductor die stencil of claim 14 wherein the first
surface is a coating selected from the group consisting of any one
or more of tungsten, tungsten carbide, tungsten nitride, nickel,
and nickel alloy in any combination.
46. A semiconductor die stencil to assist in application of a
printable adhesive in a desired pattern onto a semiconductor die
comprising: a stainless steel sheet which is impervious to a
printable adhesive applied thereto; a plurality of apertures in the
sheet of material defining a desired pattern for application of the
printable adhesive; and a coating applied to at least one top or
one bottom surface of the sheet to retard spreading of the
printable adhesive onto the at least one top or one bottom surface
of the sheet without obstruction of the flow of printable adhesive
through the apertures.
47. The semiconductor die stencil of claim 46, wherein both the
coating and the material have a surface tension, the surface
tension of the coating being less than the surface tension of the
material.
48. The semiconductor die stencil of claim 47, wherein the surface
tension of the coating is at least an order of magnitude less than
the surface tension of the material.
49. The semiconductor die stencil of claim 48, wherein the coating
is a polymeric material.
50. A semiconductor die stencil to assist in application of a
printable adhesive material onto a substrate comprising: a sheet of
material which is impervious to a printable adhesive material
applied thereto; a plurality of apertures in the sheet material
defining a desired pattern; and a coating applied to at least one
surface of the sheet to promote spreading of the printable adhesive
material.
51. The semiconductor die stencil of claim 50, wherein both the
coating and the sheet have a surface tension, the surface tension
of the coating greater than the surface tension of the sheet.
52. The semiconductor die stencil of claim 50, wherein the coating
is selected from the group consisting of tungsten, tungsten
carbide, tungsten nitride, nickel, and nickel alloy.
Description
FIELD OF THE INVENTION
[0001] This invention relates generally to stencil and screen
prints, and particularly to improving the quality of such stencil
and screen prints.
BACKGROUND OF THE INVENTION
[0002] Stencils and screens to apply patterns or printable material
on substrates have been used in many contexts. For example, such a
patterned adhesive layer, as described in Farnworth et al., U.S.
Pat. No. 5,286,697, which is hereby incorporated by reference, may
be used to attach a semiconductor die to a lead frame. Farnworth et
al. describes the use of a patterned screen to deposit an adhesive
layer on the die so that the die can be mounted to the lead frame.
The screen is patterned so that the streets between individual dies
on a wafer are free from adhesive to permit later sawing of the
wafer into separate the dies, and so that the bond pads are also
free from adhesive to permit later connection of the bond wires
between the lead fingers of the lead frame and the bond pads.
[0003] Farnworth et al. specifically teaches a screen printing
process in which the patterned screen is situated between the wafer
and a liquid adhesive nozzle. Once the screen is properly aligned
over the wafer, liquid adhesive is released from the nozzle,
coating the wafer with adhesive in the desired pattern. The screen
is then removed from the wafer, and the dies of the wafer are
attached to a lead frame and separated from one another. The
application of liquid adhesive on a wafer using a screen as
described in Farnworth et al. is similar to a silk screen process
used for printing artwork, such as on T-shirts. This same stencil
and screen printing process similar to the silk screen process used
for printing artwork on T-shirts has also been utilized in other
semiconductor applications. Such semiconductor applications include
printing a patterned nonconductive polyimide barrier for flip
chips, printing conductive adhesive bumps or solder bumps, and
printing conductive ink on flexible printed circuit boards
(PCBs).
[0004] However, it has been found that a shortcoming to such screen
and stencil printings is the migration or overflow of the printable
material (such as the adhesive or the ink) underneath the bottom of
the screen, which causes the substrate to which the printable
material is being applied (such as the wafer or the printed circuit
board) to stick to the screen. This results in a poorly patterned
layer. More significantly, migration underneath the bottom of the
screen shortens screen life, which is defined as the number of
consecutive uses, or prints, of the screen without cleaning of the
screen. Migration underneath the bottom of the screen forces
cleaning of the screen between prints to ensure consecutive clear
prints. Reduced screen life therefore slows down the screen and
stencil printing process, and adds cost to the process.
SUMMARY OF THE INVENTION
[0005] The above-mentioned shortcomings are addressed by the
present invention, which will be understood by reading and studying
the following specification. The invention describes a stencil and
screen used to assist in the application of a patterned printable
material layer on a substrate. For example, the stencil or screen
may be used to apply an adhesive to a semiconductor die, which is
typically a part of a semiconductor wafer. Preferably, the stencil
or screen includes a coating applied to at least one surface of a
patern to retard migration or overflow of the printable material
onto the surface. Alternatively, the stencil or screen includes a
second coating applied to at least one other surface of the pattern
to promote spreading of the printable material onto the substrate
to which it is being applied.
[0006] For example, the pattern may include a number of apertures.
The side walls of these apertures are side surfaces of the pattern.
During the printable material application process, the stencil or
screen is aligned over the substrate to which the printable
material is to be applied. The bottom surface of the screen or
stencil is coated with a material such as polytetrafloroethylene or
another polymeric material to retard running of the printable
material onto the bottom surface (i.e., between the stencil or
screen and the substrate). The top surface and side surfaces of the
pattern are alternatively coated with a material such as tungsten
to promote spreading of the printable material from the top and
side surfaces to the parts of the object surface exposed
underneath.
[0007] The material coating the bottom surface of the pattern,
which is desirably a polymeric material, in accordance with an
embodiment of the invention, has three advantageous aspects. First,
it acts as a low surface tension layer to minimize printable
material migration in order to provide better print definition.
Second, it acts as a gasket layer to constrain the flow of the
printable material. Third, it acts as a buffer layer to minimize
the stencil height inconsistency caused by defects, such as burr,
on the bottom of the stencil and the top surface of the substrate
onto which the printable material is applied.
[0008] A stencil or screen according to an embodiment of the
invention therefore increases life of the stencil or screen. That
is, consecutive prints can be made using a stencil or screen
according to an embodiment of the invention without cleaning. This
results in faster screen printing and reduces cost of the printing.
Still other and further advantages, aspects and embodiments of the
invention will become apparent by reference to the drawings and by
reading the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a perspective view of a prior art lead frame
suitable for use with a lead-on-chip (LOC) die;
[0010] FIG. 2 is a perspective view of a prior art package
lead-on-chip (LOC) die;
[0011] FIG. 3 is a flow chart of a screen printing method for
coating a wafer and dies with a patterned adhesive layer using a
stencil according to one embodiment of the invention;
[0012] FIG. 4 is a side view of the screen printing method of FIG.
3, illustrating the distinct layers of a stencil according to one
embodiment of the invention in detail;
[0013] FIG. 5(a) is a side view showing adhesive applied to a
surface of a stencil according to one embodiment of the invention
having a high surface tension; and,
[0014] FIG. 5(b) is a side view showing adhesive applied to a
surface of a stencil according to one embodiment of the invention
having a low surface tension.
DETAILED DESCRIPTION OF TH INVENTION
[0015] In the following detailed description of the preferred
embodiments, reference is made to the accompanying drawings which
form a part hereof and in which is shown by way of illustration
specific preferred embodiments in which the inventions may be
practiced. These embodiments are described in sufficient detail to
enable those skilled in the art to practice the invention, and it
is to be understood that other embodiments may be utilized and that
logical, mechanical and electrical changes may be made without
departing from the spirit and scope of the present invention. The
following detailed description is, therefore, not to be taken in a
limiting sense, and the scope of the present invention is defined
only by the appended claims.
[0016] The invention as it is described in conjunction with FIGS.
14, 5(a) and 5(b) is described in relation to the application of an
adhesive layer onto a semiconductor die so that the die may be
attached to a lead frame. Those of ordinary skill within the art,
however, will appreciate that the invention is not limited to such
an application. Any application that may be benefit from improved
stencil or screen printing in accordance with the invention is
amenable to the invention. For example, the printing of artwork on
T-shirts, the printing of patterned nonconductive polyimide
barriers for flip chips, the printing of conductive adhesive bumps
or solder bumps, and the printing of conductive ink on flexible
printed circuit board are also all within the scope of the
invention.
[0017] As those of ordinary skill within the art will recognize,
the terms stencil and screen are generally speaking quite similar
to each other. However, the pattern within a given stencil
generally has larger apertures than the pattern within a given
screen. Furthermore, a screen typically includes an emulsion layer
on both the top and bottom surfaces of its pattern. The invention
is generally described herein relative to a stencil. However, those
of ordinary skill within the art will appreciate that the invention
is equally applicable to screens.
[0018] Referring first to FIG. 1, a representative prior art
lead-on-chip (LOC) die 110 and lead frame 112 are shown The LOC die
110 is thin, flat and generally rectangular in shape and includes a
die face 114, or active side, wherein the integrated circuitry is
formed. A plurality of die wire bonding pads 116 are formed across
the center and side edges of the die 110 in electrical contact with
the integrated circuits formed on the die 110.
[0019] The lead frame 112 is formed from metal sheet and includes
side rails 118 and 120 formed with indexing holes 122. The indexing
holes 122 facilitate transport and indexing of the lead frame 112
by automated packaging machinery. The lead frame 112 also includes
side bars 124 and 126 for increased rigidity and to limit the flow
of encapsulating material during the encapsulation process. The
side rails 118 and 120 and side bars 124 and 126 are trimmed away
during a trim and form operation.
[0020] Furthermore, the lead frame 112 includes a plurality of
generally parallel and spaced lead fingers 128. During the
packaging process the lead fingers 128 are connected to thin bond
wires 130 (as shown in FIG. 2) which also connect to the wire bond
pads 116. The lead fingers 128 thus function to electrically
connect the integrated circuitry formed on the LOC die 110 to
external circuitry (e.g., a printed circuit board). In addition,
the lead fingers 128 function to support the LOC die 110 during the
packing process and to facilitate heat transfer away from LOC die
110. The lead frame 112 also includes bus bars 132 for making
multiple connections to the wire bond pads 116.
[0021] Referring next to FIG. 2, the packaged prior art LOC die 110
or semiconductor package 134 is shown The semiconductor package 134
includes an encapsulating material 136 which encapsulates the chip
110 and all but a terminal portion 138 of the lead fingers 128.
Kinks 140 in the lead fingers 128 help to rigidify the assembly.
The encapsulating material 136 is typically formed of an insulative
plastic material. The LOC die 110 is located in the center of the
package 134 with the wire bond pads 116 of the die 110 connected to
the thin bond wires 130, which in turn connect to the lead fingers
128.
[0022] In addition to being attached to the thin bond wires 130,
the lead fingers 128 also attach to the front side 114 of the die
110, or to an alpha barrier (not shown) attached to the die 110.
Prior to encapsulation, the lead fingers 128 provide the only
physical connection between the lead frame 112 and the LOC die 110.
As has been described in the background, a process for attaching
the lead fingers 128 to the die 110 is to apply adhesive to the die
110 so that the die can then be affixed to the lead fingers 128.
The process by which adhesive is applied to the die 110 so that the
lead fingers 128 can be attached thereto is shown by reference to
FIG. 3 and FIG. 4.
[0023] Referring first to FIG. 3, a flowchart of a screen printing
method for coating a wafer and dies with a patterned adhesive layer
using a stencil, according to one embodiment of the invention, is
shown. The process of FIG. 3 is a hot screen printing process
similar to a silk screening process used for printing artwork. This
is an exemplary process by which a stencil according to an
embodiment of the invention may be utilized. The invention is not
so limited, however, and other processes, such as a cold screen
printing, are amenable to application of embodiments of the
invention.
[0024] In a first step 210 of a hot screen printing process, a
stencil is aligned over a wafer, which is one type of substrate
onto which printable material may be applied. Other types of
substrate include printed circuit boards (PCBs), T-shirts, silk,
paper, die, and ceramics; the invention is not limited to any
particular type of substrate, however. The wafer is patterned to
correspond to a desired adhesive pattern on the wafer. The pattern
of the screen is such that the streets between individual dies on
the wafer are free from adhesive to permit later sawing of the
wafer to separate the dies, and so that the bond pads of the dies
are also free from adhesive to permit later connection of the bond
wires between the lead fingers of the lead frame and the bond
pads.
[0025] In step 212, adhesive is sprayed over the stencil The
adhesive is one type of printable material which can be applied to
a substrate. Other types of printable material include ink, polymer
resins, diluted or soluble polymers, composite materials, and
solder paste; the invention is not limited to any particular type
of printable material, however. Adhesive flows through the
apertures of the stencil, and onto the wafer. The hot screen
printing process is preferably performed at an elevated temperature
of about 100.degree. C. to about 500.degree. C., to facilitate the
flowability of the adhesive.
[0026] In step 214, the adhesive-coated wafer is removed from
underneath the stencil, and the packaging process as has been
already generally described continues with respect to that wafer.
The wafer is separated into individual dies, which are then each
attached to lead fingers of a lead frame via the adhesive coating
on the dies. The dies are ultimately encapsulated in a protective
package as has been described in conjunction with FIG. 2. After
encapsulation, the resulting integrated circuit chips are tested
for proper performance, after which they are ready for inclusion
into an electronic device such as a computer.
[0027] If there are no more wafers to which adhesive needs to be
applied, control proceeds from step 216 to step 218, and the hot
screen printing process is finished. Otherwise, the stencil is
checked for cleanliness in step 220 by visual inspection. The
adhesive, when sprayed over the stencil for application onto the
wafer, has a tendency to migrate between the stencil and the wafer,
and stick to the underside of the stencil. An advantage of the
inventive stencil described herein is that it does not always
require cleaning between immediately subsequent uses. Thus, if the
stencil is still clean, control proceeds from step 220 back to step
210, where it is aligned over a new wafer, and the hot screen
printing process of FIG. 3 starts over. Otherwise, the stencil is
first cleaned in step 222 before the process starts over at 210
with respect to a new wafer.
[0028] Referring next to FIG. 4, a side view illustrating the
screen printing method of FIG. 3 is shown. In a hot screen printing
process, semiconductor wafer 10, which usually includes a plurality
of semiconductor dies, is coated with a liquid adhesive layer 12
dispensed from a nozzle 14. A stencil 16 having a pattern is
situated between the wafer 10 and the nozzle 14 to provide the
desired pattern. As shown in FIG. 4, stencil 16 includes apertures
corresponding to the desired adhesive layer pattern. Only one
aperture is shown for simplicity and clarity of description and
illustration. What is referred to as side surfaces of pattern 16
includes, but is not necessarily limited to, the side walls of the
apertures.
[0029] A dockering roller 18 (or blade), moveably mounted for
motion across the stencil 16 as indicated by arrow 20, can be used
to aid in the distribution of the adhesive. The adhesive is a
suitable thermoplastic or thermoset adhesive. Such suitable
thermoplastic adhesive materials include those sold under the trade
name Ablestik XR-041395, available from Ablestik, Inc., and which
is a polyimide. Such suitable thermoset adhesives include thos sold
under the trade name Ablestik 8176T, also available from Ablestik,
Inc, and which is a phenolic resin.
[0030] The pattern of stencil 16 has one or more surfaces modified
in accordance to an embodiment of the invention. For example, it
has been found that adhesive tends to migrate onto the bottom
surface of the pattern, between stencil 16 and wafer 10, during the
application process. Therefore, preferably the bottom surface has a
coating 24 applied thereto to retard running of the adhesive onto
the surface. Furthermore, preferably side surfaces of the pattern
(i.e., between which adhesive 12 is deposited), as well as the top
surface of the pattern, have a coating (or plating) 26 applied
thereto to promote spreading of the adhesive onto the wafer.
[0031] The stencil 16 having its surfaces so coated has an extended
life, and therefore can be used for consecutive prints without
cleaning. Therefore, in an exemplary method of the invention, the
stencil is aligned over a first semiconductor wafer, adhesive is
sprayed over the stencil to form a patterned adhesive layer on the
wafer, and then the stencil is aligned over a second wafer, and
adhesive is sprayed a second time over the stencil to form an
adhesive layer on the second wafer. The stencil does not require
cleaning after the spraying of the adhesive over the stencil when
it is aligned over the first wafer, and before the aligning of the
stencil over the second wafer.
[0032] With respect to a preferred construction of pattern 16, the
pattern itself is preferably constructed from metals or metal
alloys. One such metal alloy is stainless steel. Stainless steel
stencils are commonly available commercially, (e.g., from Photo
Stencil, Inc., of Colorado Sprigs, Colo., and from UTZ Engineering,
Inc., of Clifton, N.J.). Stainless steel stencils in particular
have a surface tension of 1384 dyne/cm at melting point.
[0033] Coating 24 is applied underneath the bottom surface of
pattern 16 to retard running of the adhesive onto the bottom
surface. Preferably, coating 24 has a surface tension less than the
surface tension of pattern 16. One such coating is
polytetrafloroethylene, which is a polymeric material available
under the trade name DuPont Teflon, available from DuPont, Inc.
Polytetrafloroethylene has a critical surface tension of 23.9
dyne/cm at room temperature. Other polymers may also be used since
surface tension of most polymers is one order less than that of
metals or ceramics. Because its surface tension is markedly less
than that of pattern 16, the running property of the adhesive onto
the bottom surface of pattern 16 is controlled. Pattern 16 is
initially constructed from a metal or metal alloy having a high
surface tension to spread the adhesive onto the wafer.
[0034] Furthermore, to promote the spreading of the adhesive on the
semiconductor wafer, plating 26 is applied to at least the side
surfaces or the top surface of pattern 16. As shown in FIG. 4,
plating 26 is applied to both the top surface and the side surfaces
of pattern 16. Plating 26 is a coating having a surface tension
greater than the surface tension of pattern 16. One such plating is
tungsten, which has a surface tension of 2500 dyne/cm at melting
point. Other platings having similarly high surface tensions
include tungsten carbide, and tungsten nitride. Still other
platings having high surface tensions include nickel and nickel
alloy. Because the surface tension of plating 26 is greater than
that of pattern 16, the spreading of the adhesive onto the
semiconductor wafer is promoted.
[0035] Those skilled within the art will recognize that the
embodiment shown in FIG. 4 is only a preferred embodiment of the
invention. Other embodiments conforming to the basic principles of
the invention are also contemplated. For example, a stencil having
a pattern with a coating on the bottom surface of the pattern to
retard running of adhesive onto the surface, but not necessarily
having a plating or second coating on the top or side surfaces of
the pattern to promote spreading of the adhesive onto the
semiconductor die, is within the scope of the invention. For
further example, a stencil having a pattern with a plating or
coating on at least either the top or side surfaces of the pattern
to promote spreading of adhesive onto the die, but without a
coating on the bottom surface of the pattern to retard running of
the adhesive onto the bottom surface, is also within the scope of
the invention.
[0036] As has been described, preferably coating 24 has a surface
tension less than that of pattern 16, and plating 26 has a surface
tension greater than that of pattern 16. This permits the coating
to retard running of the adhesive, and the plating to promote
spreading of the adhesive onto the semiconductor wafer. The reason
why the coating and the plating perform these functions in the
preferred embodiment is shown by reference to FIG. 5(a) and FIG.
5(b). Coating 24, plating 26 as applied to the top surface of the
pattern, and pattern 16, can be referred to as separate layers of
the semiconductor die stencil shown in FIG. 4.
[0037] Referring first to FIG. 5(a), surface 28 is a surface having
a high surface tension, such as that of plating 26 of FIG. 4.
Because the surface tension is high, surface 28 tends to pull
adhesive 30 across the entire surface. The greater the surface
tension, the more the adhesive 30 is spread across surface 28.
Therefore, while pattern 16 of FIG. 4 preferably has a sufficient
high surface tension (1384 dyne/cm) to spread the adhesive, the
addition of plating 26, which has an even greater surface tension
(2500 dyne/cm), promotes the spreading of the adhesive even
more.
[0038] Referring next to FIG. 5(b), surface 32 is a surface having
a low surface tension, such as that of coating 24 of FIG. 4.
Because the surface tension is low, surface 32 does not tend to
pull adhesive 34 across the entire surface. The adhesive remains
where it first is located, and does not migrate or run across the
surface. The lower the surface tension, the less the adhesive 34
tends to be pulled across surface 32. Thus, because preferably
coating 24 of FIG. 4 has a much lower surface tension than pattern
16 (23.9 dyne/cm compared to 1384 dyne/cm), when the adhesive
reaches an edge of coating 24, coating 24 retards the adhesive from
running or migrating any farther.
[0039] A surface-modified stencil has been described. Although
specific embodiments have been illustrated and described herein, it
will be appreciated by those of ordinary skill in the art that any
arrangement which is calculated to achieve the same purpose may be
substituted for the specific embodiments shown. This application is
intended to cover any adaptations or variations of the present
invention. Furthermore, while the invention has been described
specifically in the context of the application of adhesive to a
semiconductor die, the invention is not limited to such an end use,
and the invention is amenable to other semiconductor and
non-semiconductor uses as well. Therefore, it is manifestly
intended that this invention be limited only by the following
claims and equivalents thereof.
* * * * *