U.S. patent application number 15/061807 was filed with the patent office on 2016-06-30 for methods for vacuum assisted underfilling.
The applicant listed for this patent is NORDSON CORPORATION. Invention is credited to Alec J. Babiarz, Horatio Quinones, Thomas L. Ratledge, Jiangang Zhao.
Application Number | 20160189984 15/061807 |
Document ID | / |
Family ID | 47175221 |
Filed Date | 2016-06-30 |
United States Patent
Application |
20160189984 |
Kind Code |
A1 |
Babiarz; Alec J. ; et
al. |
June 30, 2016 |
METHODS FOR VACUUM ASSISTED UNDERFILLING
Abstract
Methods for applying an underfill with vacuum assistance. The
method includes receiving a substrate with the surface at least
partially covered by a glass-like film that has a top surface of
reduced roughness relative to the surface of the substrate,
providing the underfill on the top surface of the glass-like film
along an exterior edge of the electronic device, evacuating the
space to provide a vacuum condition in the open portion of the
space, and heating the underfill to cause flow of the underfill
toward the exterior edge and into the open portion of the space,
wherein the glass-like film reduces trapping of gas under the
underfill.
Inventors: |
Babiarz; Alec J.;
(Encinitas, CA) ; Quinones; Horatio; (San Marcos,
CA) ; Ratledge; Thomas L.; (San Marcos, CA) ;
Zhao; Jiangang; (Concord, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NORDSON CORPORATION |
WESTLAKE |
OH |
US |
|
|
Family ID: |
47175221 |
Appl. No.: |
15/061807 |
Filed: |
March 4, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14215559 |
Mar 17, 2014 |
9314882 |
|
|
15061807 |
|
|
|
|
13548965 |
Jul 13, 2012 |
8796075 |
|
|
14215559 |
|
|
|
|
13004198 |
Jan 11, 2011 |
|
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13548965 |
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Current U.S.
Class: |
438/127 |
Current CPC
Class: |
H01L 2224/92125
20130101; H01L 2924/01029 20130101; H01L 2224/32225 20130101; H01L
24/27 20130101; H01L 21/324 20130101; B23K 37/00 20130101; H01L
2224/73204 20130101; H01L 2224/73204 20130101; H01L 2924/3512
20130101; H01L 2224/73204 20130101; H01L 2924/00 20130101; H01L
2924/00 20130101; H01L 2224/16225 20130101; H01L 2224/16225
20130101; H01L 2224/32225 20130101; H01L 2224/32225 20130101; H01L
2924/00 20130101; H01L 2224/92125 20130101; H01L 2224/16225
20130101; H05K 13/0465 20130101; H01L 21/563 20130101 |
International
Class: |
H01L 21/56 20060101
H01L021/56; H01L 21/324 20060101 H01L021/324 |
Claims
1. A method of providing an underfill on a substrate having a
surface to which an electronic device is mounted by electrically
conductive joints and is separated from the substrate by a space,
the space having an open portion that is unoccupied by the
conductive joints, the method comprising: receiving the substrate
with the surface at least partially covered by a glass-like film
that has a top surface of reduced roughness relative to the surface
of the substrate; providing the underfill on the top surface of the
glass-like film along an exterior edge of the electronic device;
evacuating the space to provide a vacuum condition in the open
portion of the space; after evacuating the space to provide the
vacuum condition, heating the underfill to cause flow of the
underfill toward the exterior edge and into the open portion of the
space, wherein the glass-like film reduces trapping of gas under
the underfill.
2. The method of claim 1 wherein obtaining the substrate with the
surface at least partially covered by a glass-like film further
comprises: depositing the glass-like film on the substrate.
3. The method of claim 2 further comprising: after depositing the
glass-like film on the substrate, plasma activating a surface of
the glass-like film.
4. The method of claim 1 further comprising: after depositing the
glass-like film on the substrate, mounting the electronic device by
the electrically conductive joints on the surface of the
substrate.
5. The method of claim 1 providing the underfill on the top surface
of the glass-like film further comprises: dispensing the underfill
onto the top surface of the glass-like film.
6. The method of claim 1 wherein providing the underfill on the top
surface of the glass-like film further comprises: placing a solid
underfill onto the top surface of the glass-like film.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 14/215,559, filed Mar. 17, 2014, which is a continuation
of U.S. patent application Ser. No. 13/548,965, filed Jul. 13,
2012, now U.S. Pat. No. 8,796,075, issued Aug. 5, 2014, which is a
continuation-in part application of U.S. patent application Ser.
No. 13/004,198, filed Jan. 11, 2011, the disclosures of which are
incorporated by reference herein in their entirety.
BACKGROUND
[0002] The invention relates generally to methods for applying an
underfill between an electronic device and a substrate.
[0003] It is typical for an electronic device, such as a flip chip,
chip scale package (CSP), ball grid array (BGA) or package on
package assembly (PoP) to include a pattern of solder bumps that,
during mounting, are registered with pads on a substrate, or joined
using another type of interconnect technology such as copper
pillars or other types of thermal compression bonding
interconnects. The substrate can be a printed circuit board,
electronic chip or wafer, for example. The solder is reflowed by
heating and, following solidification, solder joints connect the
electronic device and the substrate. Underfill, which may be an
epoxy, may be used to fill the open space between the electronic
device and the substrate that remains between the reflowed solder
balls. The underfill protects the solder joints against various
adverse environmental factors, redistributes mechanical stresses
due to shock, and prevents the solder joints from moving under
strain during thermal cycles.
[0004] In the process of underfilling, voids may be formed due to
the following reasons, but not limited to, uneven surface
topography in the gap between the electronic device and substrate,
fluid flow rate race conditions as underfill flows around a solder
connection, different wetability conditions on the substrate, air
in the underfill, or air induced from the dispensing process.
Because the voids are unfilled by underfill, unsupported solder
joints adjacent to voids may not be adequately protected against
cold flow when exposed to strain from thermal expansion during
operation or to mechanical shock caused by dropping the assembled
end product, such as a cell phone, that includes the underfilled
electronic device. Voids at solder joints prevent the solder bump
from being in held in a state of hydrostatic compression and strain
restraint, which may increase solder joint fatigue and thereby
increase the probability of solder joint cracking.
[0005] Therefore, improved methods are needed for applying an
underfill that reduces the probability of forming voids in the
underfill.
SUMMARY
[0006] In one embodiment, a method is provided for distributing an
underfill into the space between the reflowed solder balls which
connect an electronic device to a substrate. The method includes
providing the underfill onto the substrate near to at least one
exterior edge of the electronic device with at least one gap in the
underfill, providing an air path to the space between the
electronic device and the substrate and then evacuating that space
through the gap, or gaps, to provide a vacuum condition in the
space. After evacuating the space, the underfill is heated above
room temperature to cause capillary flow of the underfill to the
exterior edge, or edges, and into the space between the electronic
device and substrate and around the reflowed solder balls. The
underfill can be provided as a material which is solid at room
temperature and is positioned by pick and place equipment onto the
substrate, and thereafter becomes liquid at elevated temperatures,
or as a liquid material that can be dispensed onto the substrate
by, for example, a valve or dispenser.
[0007] Another embodiment of the invention is directed to a method
of providing an underfill on a substrate upon which electronic
device is mounted by electrically conductive joints and is
separated from the substrate by a space. The space has an open
portion that is unoccupied by the conductive joints. The method
includes providing the underfill onto the substrate proximate to at
least one exterior edge of the electronic device, and evacuating
the space to provide a vacuum condition in the open portion of the
space between the underfill and external edges of the electronic
device. After evacuating the space to a vacuum condition, the
underfill is heated to a temperature above room temperature to
cause flow of the underfill to at least one exterior edge and into
the open portion of the space, thereby allowing any air trapped
under the underfill itself to vent before reaching the external
edge of the electrical device and the gap between the electrical
device and substrate.
[0008] Other embodiments of the invention are directed to methods
of blocking air that has been trapped under the underfill from
flowing under the electronic device. In one such method, an
obstacle is placed between the edge of the electronic device and
the underfill prior to applying the vacuum. After applying the
vacuum condition, the underfill is heated to a temperature above
room temperature to cause flow of the underfill over the obstacle
and from at least one exterior edge into the open portion of the
space. Forcing the underfill to flow over an obstacle, helps block
the air trapped under the underfill from flowing under the
electronic device and allows the trapped air to vent prior to
reaching the gap under the electrical device
[0009] Yet another embodiment of the invention is directed to a
method of exposing a surface of the substrate to a plasma so as to
change the wettability of the substrate prior to providing the
underfill onto the substrate proximate to at least one exterior
edge of the electronic device. This plasma treatment reduces the
opportunity for air to be trapped under the underfill. The method
further includes evacuating the space to provide a vacuum
condition, in the open portion of the space. After evacuating the
space to a vacuum condition, the underfill is heated to cause flow
of the underfill toward at least one exterior edge and into the
open portion of the space. Since the plasma treatment of the
substrate reduces the entrapment of air under the underfill, an
amount of air trapped under the electronic device during the
underfill operation may also be reduced.
[0010] Similar to the plasma treatment method, a glass-like film
may be deposited on the substrate so as to provide a more perfectly
smooth and flat surface. This flat surface has fewer depressions or
imperfections in which air can be trapped when the underfill is
positioned on top of the glass-like film. As entrapment of air
under the underfill is reduced, an amount of air trapped under the
electronic device during the underfill operation may also be
reduced.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0011] The accompanying drawings, which are incorporated in and
constitute a part of this specification, illustrate exemplary
embodiments of the invention and, together with a general
description of embodiments of the invention given above, and the
detailed description given below, serve to explain the principles
of the embodiments of the invention.
[0012] FIG. 1 is a side view of an electronic device mounted to a
substrate by an array of solder balls and with underfill provided
along a side edge of the electronic device.
[0013] FIG. 1A is a side view similar to FIG. 1 in which the
underfill has moved into the open space between the electronic
device and substrate that is unoccupied by the solder balls.
[0014] FIG. 2 is a flow chart of a procedure for vacuum
underfilling in accordance with an embodiment of the invention.
[0015] FIGS. 3A-C are diagrammatic top views illustrating a
sequence for vacuum underfilling beneath an electronic device
mounted on a substrate in accordance with an embodiment of the
invention.
[0016] FIGS. 4A-C are diagrammatic top views similar to FIGS. 3A-C
in accordance with another embodiment of the invention.
[0017] FIGS. 5A-C are diagrammatic top views similar to FIGS. 3A-C
in accordance with yet another embodiment of the invention.
[0018] FIGS. 5D, 5E and 5F are diagrammatic top views similar to
FIG. 5A in which the underfill is provided on the substrate with,
respectively, an L pattern, a U pattern, and an I pattern.
[0019] FIG. 5G is a diagrammatic top view similar to FIG. 5A in
which the underfill is provided on the substrate with no gaps.
[0020] FIG. 6A is a diagrammatic top view similar to FIG. 5A in
which a dam is positioned between at least one side edge of the
electronic device and the underfill.
[0021] FIG. 6B is a side view of the dam positioned between at
least one side edge of the electronic device and the underfill.
[0022] FIGS. 7A and 7B are side views of the underfill flowing over
the dam at different time sequences.
[0023] FIG. 8A is a diagrammatic top view similar to FIG. 5A in
which a channel is positioned between at least one the side edge of
the electronic device and the underfill.
[0024] FIG. 8B is a side view of the channel positioned between the
side edge of the electronic device and the underfill.
[0025] FIGS. 9A and 9B are side views of the underfill flowing over
the channel at different time sequences.
[0026] FIG. 10 is a cross-sectional view of a plasma-treated
substrate according to an embodiment of the invention.
[0027] FIG. 11 is a cross-sectional view of a plasma-treated
substrate according to another embodiment of the invention.
[0028] FIG. 12 is a schematic representation of a vacuum
underfilling system in accordance with an embodiment of the
invention.
DETAILED DESCRIPTION
[0029] Generally, the embodiments of the invention are directed to
a vacuum-assisted process for underfilling an electronic device
mounted on a substrate by an array of solder balls. Underfill is
dispensed or otherwise provided (e.g., in either a liquid or solid
form) in one or more lines around the edges of an unheated
electronic device, which is mounted to an unheated substrate by
means of an array of reflowed solder balls. Preferably, at least
one gap is left in the one or more lines of underfill and,
preferably, if the space between the electronic device and
substrate is very small, there is a space between the underfill and
the exterior edges of the electrical device. The substrate is
transported into a vacuum chamber, before significant capillary
underfilling (and air or gas entrapment) occurs, and a vacuum is
applied to evacuate the space. While the vacuum is being applied,
the gap, or gaps, in the one or more lines of underfill allows air
to flow out from under the device through the gap(s), to establish
a vacuum condition (i.e., a pressure less than atmospheric
pressure) under the electronic device between the electronic device
and the substrate. An alternative, less preferred process, is to
provide no gap in the underfill and to rely upon the air trapped
under the device to bubble through the underfill when the device is
placed under vacuum. Under either process, while the vacuum
condition is being maintained, the electronic device and substrate
are heated to cause the underfill to completely flow under the
electronic device into the spaces between the reflowed solder
balls. Underfilling in the presence of the vacuum condition means
any void entrapped in the underfill will be partially evacuated of
gases commensurate with the level of the applied vacuum. The vacuum
pressure applied must not be lower than the vapor pressure of the
underfill, otherwise the underfill will boil and the process will
become less stable. The vacuum chamber is then vented. Any voids
present in the underfill will now collapse because of the evacuated
condition and become filled with underfill. The underfilled
electronic device and the substrate are then moved out of the
vacuum chamber.
[0030] The embodiments of the invention also apply to other
interconnect technologies, in addition to solder bumps, for
creating conductive joints between the electronic device and the
substrate, such as copper pillars and other thermal compression
bonding interconnect technologies.
[0031] With reference to FIG. 1, an assembly 10 includes a
substrate 12, such as a printed circuit board, and an electronic
device 14 that is mounted to a surface 16 of the substrate 12. In
representative embodiments, the electronic device 14 may be a flip
chip, chip scale package (CSP), ball grid array (BGA) or package on
package assembly (PoP), for example. Likewise, the substrate 12 may
be a printed circuit board (PCB), electronic chip or wafer, for
example, or any substrate or interposer used in semiconductor
packaging of electronic devices
[0032] With reference to FIGS. 1, 1A, and 3A, the electronic device
14 has a footprint on the substrate 12 such that the substrate 12
is exposed adjacent to each of the side or exterior edges 18, 20,
22, 24 of the electronic device 14. Solder joints 26 mechanically
and electrically connect the electronic device 14 with the
substrate 12. A space 28 is defined between the electronic device
14 and substrate 12 and a portion of the space 28 is open (i.e.,
unoccupied) and unfilled by the solder joints 26 that may have a
representative form of solder balls. At each of the exterior edges
18, 20, 22, 24, a gap 27 is defined between the electronic device
14 and the substrate 12. The gap 27 communicates with the space 28.
Preferably, for small gaps 24 (e.g., less than 200 microns), a
space 43 on the surface of substrate 12 exists between the
underfill 30 and corresponding device edges 18, 20, 22 and 24.
[0033] An underfill 30 is used to fill the space 28 between the
electronic device 14 and the substrate 12, as shown in FIG. 1A. In
one example, the underfill 30 is a curable non-conductive silicon
dioxide particle filled epoxy that is fluid when applied to the
substrate 12 and flows by capillary action. Other types of
underfill can be used including those that are solid at room
temperature or are frozen. Underfills are typically filled with
small particles of glass, for example to provide the desired
properties in the cured underfill. When cured and hardened, the
underfill forms a strongly bonded, cohesive mass.
[0034] With reference to FIG. 2, a procedure for vacuum
underfilling in accordance with an embodiment of the invention is
described. In the FIG. 2 embodiment, a liquid underfill is
dispensed onto the substrate. Instead of dispensing the underfill
30 in a liquid form, the underfill 30 could be applied in a solid
form in position to buy a pick and place machine, for example, as
mentioned above. In block 52, liquid underfill 30 is dispensed onto
the substrate 12. The underfill 30 may be applied as one or more
continuous lines (FIG. 3A) proximate to one or more exterior edges
18, 20, 22, 24 of the electronic device 14. Preferably, underfill
30 does not touch edges 18, 20, 22, or 24 so that surface 43 is not
covered by the underfill 30 until full vacuum is applied.
Typically, the dispensed amount of underfill 30 is equal to the
volume of the open space 28 under the electronic device 14 plus the
fillet 31 (FIG. 1A) that forms along the perimeter of the device 14
after the underfill operation has been completed. The substrate 12
is unheated when the underfill 30 is applied and a gap 42 (FIG. 3A)
is preferably present in the underfill 30 so that an air path to
the open portion of space 28 through the gap 42 is maintained. As
discussed above, the less preferred method is not to leave a gap 42
or open space 43, and to rely on air trapped under the electronic
device 14 to bubble through the underfill 30.
[0035] The underfill 30 may be applied to the substrate 12 using
multiple different types of dispensers and in multiple different
ways. For example and although the invention is not so limited, a
series of droplets of underfill 30 may be dispensed onto the
surface 16 of the substrate 12 from a moving jetting dispenser that
is flying above the surface 16.
[0036] In block 54, the underfill 30 is cooled when dispensed onto
the substrate 12. In one embodiment, the substrate 12 is cooled,
for example, by one or more thermoelectric coolers to a temperature
below room temperature and the underfill 30 cools shortly after
application to approximately the temperature of the substrate 12.
Alternatively or in addition to cooling the substrate 12, the
underfill 30 may be cooled in the dispenser before being dispensed
onto the substrate 12. In one embodiment, the underfill 30 is
cooled to a temperature in the range of 0.degree. C. to 10.degree.
C. Cooling increases the viscosity of the underfill 30, which
further prevents or reduces capillary flow into the open portion of
the space 28 between the electronic device 14 and the substrate 12
before vacuum is applied
[0037] In block 56, the unfilled portion of space 28 is evacuated
to a sub-atmospheric pressure through the gap 42 in the underfill
30 or space 43 to establish a vacuum condition (i.e., a pressure
less than atmospheric pressure) in space 28. Or, if no gap has been
provided, or if the open space 43 is not maintained, the gas will
bubble through the underfill 30. To create the vacuum, in one
embodiment, the substrate 12, which carries the electronic device
14 and the underfill 30, is moved into a vacuum chamber, sealed
inside the chamber, and the vacuum chamber is evacuated to a
sub-atmospheric pressure. In one embodiment, a suitable
sub-atmospheric pressure for the vacuum is greater than or equal to
25 inches of Hg (about 95 Torr) to 26 inches of Hg (about 100
Torr). In any event, the sub-atmospheric pressure is limited such
that the physical properties of the underfill are not significantly
or detrimentally modified.
[0038] Any suitable technique may be used for moving the substrate
12 into and out of the vacuum chamber, and conventional vacuum
systems are familiar to a person having ordinary skill in the art.
The substrate 12 is preferably transferred into the vacuum chamber
before the occurrence of capillary underfilling (and air or gas
entrapment) or before the underfill 30 is allowed to touch any of
surfaces 18, 20, 22, 24 thereby maintaining surface 43 to be
uncovered with underfill 30.
[0039] In block 58, after the vacuum chamber is evacuated, and
while the vacuum condition is being maintained, the underfill 30 is
heated to a temperature in excess of room temperature, for example
to a temperature in a range of 30.degree. C. to 120.degree. C. The
underfill 30 may be heated by heating the substrate 12, the
electronic device 14 or both and in any desired sequence to direct
flow. In response to the heating, the underfill 30 flows by
capillary action through the narrow gap 27 from each of the
exterior edges 18, 20, 22, 24 into the space 28 and around the
reflowed solder balls. Because the open portion of the space 28 is
evacuated, the underfill 30 can flow across the space 28 such that
any void entrapped in the underfill 30 will be evacuated of gases
to the vacuum level.
[0040] In block 60, after sufficient time has been provided for
complete capillary flow to have occurred, then the vacuum condition
is removed and atmospheric pressure is restored. For example, the
vacuum chamber may be vented to provide the atmospheric pressure
condition. Under the influence of atmospheric pressure, any voids
present in the underfill 30 will collapse because of their
evacuated state of sub-atmospheric pressure and become filled with
underfill 30 (FIG. 3C). The substrate 12 is then transferred from
the vacuum chamber to a curing oven and the underfill 30 is
cured.
[0041] With reference to FIGS. 4A-4C and in alternative
embodiments, the underfill 30 may be applied proximate to the
exterior edges 18, 20, 22, 24 of the electronic device 14 as a
series of disconnected regions (FIG. 4A) with multiple gaps 61. In
FIG. 4B, the gaps 61 disappear as the underfill 30 is heated after
evacuating the open portion of spaces 28 and 43 to a vacuum
condition. In FIG. 4C, the underfill 30 flows beneath the device
14.
[0042] With reference to FIGS. 5A-5E and in alternative
embodiments, the underfill 30 may be applied proximate to one or
more of the exterior edges 18, 20, 22, 24 of the electronic device
14 in one or more passes. In this case, FIG. 5A shows a line of
underfill applied along each of the four edges of the device, with
a gap 62 and space 43 present at each corner between each pair of
exterior edges 18, 20, 22, 24. In FIG. 5B, the underfill 30 is
heated after evacuating the space 28 through the gaps 62 to a
vacuum condition. In FIG. 5C, the underfill 30, in the heated
state, flows beneath the device 14.
[0043] In an alternative embodiment and as shown in FIG. 5D, the
underfill 30 could be provided as lines using an L pass along
exterior edges 18 and 24 of the electronic device 14, preferably
providing space 43. In this case, a gap is present along the
exterior edges 20 and 22. In another alternative embodiment and as
shown in FIG. 5E, the underfill 30 could be provided as lines using
a U pass along exterior edges 18, 20, 22 of the electronic device
14 preferably providing space 43, but not along exterior edge 24 of
the electronic device 14. In another alternative embodiment and as
shown in FIG. 5F, the underfill 30 could be provided as a line
using an I pass along exterior edge 20 of the electronic device 14,
preferably providing space 43, but not along exterior edges 18, 22,
and 24. As probably the least preferred alternative embodiment and
as shown in FIG. 5G, the underfill 30 could be applied as lines
along all four edges 18, 20, 22 and 24 and in an overlapping manner
with no gaps defined at the corners. In this case, the air, or gas,
trapped under the electronic device 14 will bubble through the
underfill 30 when the vacuum is applied.
[0044] The lines of underfill, in addition to being applied in the
preferred method from a non-contact jetting valve, such as the DJ
9000 sold by Nordson ASYMTEK of Carlsbad, Calif., could
alternatively be applied as solid pre-forms of epoxy. The solid
pre-forms are placed on the substrate 12 and then melted upon the
application of heat. The solid pre-forms could be placed into
position by a pick and place machine or mechanism.
[0045] Gas or air 66 can be trapped under the underfill 30 when the
underfill is provided on the substrate. Air that is trapped under
the underfill 30 when the underfill 30 is applied or laid along the
edge of the electronic device 14 may vent underneath the electronic
device 14 after the vacuum is applied and the underfill 30 is
heated it induce capillary flow. The vented air may become trapped
under the electronic device 14 as air pockets, which may lead to
the formation of voids in the underfill 30. Ensuring space 43 is
maintained until the full vacuum is applied mitigates this trapped
air from venting under the electronic device 14.
[0046] In accordance with alternative embodiments of the invention,
the substrate 12 may include an obstacle positioned on the surface
16 proximate to at least one exterior edge 18, 20, 22, 24 of the
electronic device 14. In a representative embodiment, the obstacle
may be formed as a linear body. The obstacle is located between the
location of the dispensed underfill 30 and the adjacent exterior
edge 18, 20, 22, 24 of the electronic device 14.
[0047] The obstacle serves as an impediment over which the
underfill 30 must flow before flowing toward the exterior edge 18,
20, 22, 24 of the electronic device 14 and into the open portion of
the space 28, during the procedure for vacuum underfilling shown in
FIG. 2 thereby maintaining space 43. The liquid underfill 30 (or a
majority thereof) is able to flow over the obstacle, and the
obstacle has only a negligible or minor effect on the flow and flow
rate of the underfill liquid. However, air or gas pockets are
generally incapable of surmounting the obstacle or are vented as
the underfill 30 flows over space 43 before reaching the gap 27. As
such, the obstacle removes air or gas pockets from the underfill.
Therefore, this embodiment helps reduce or eliminate trapped gas
under the electronic device 14 during the vacuum-assisted
underfilling operation.
[0048] As the distance between the dispensed underfill 30 and the
exterior edge 18, 20, 22, 24 of the electronic device 14 increases,
the ability of trapped gas 66 under the underfill 30 to reach the
gap 27 decreases. If air 66 is trapped under the underfill 30 and
the underfill 30 is laid adjacent to the exterior edges 18, 20, 22,
24 of the electronic device 14 (i.e., in contact with the
electronic device 14), the air 66 trapped under the underfill 30
may be vented under the electronic device 14 when the vacuum is
applied and the underfill 30 is heated. Air that vents under the
electronic device 14 may become trapped under the electronic device
14. Therefore, the underfill 30 should be positioned on the
substrate 12 far enough away from the exterior edge 18, 20, 22, 24
of the electronic device 14 so as to avoid venting under the
electronic device 14. When the underfill 30 is positioned far away
from the exterior edge 18, 20, 22, 24 of the electronic device 14,
the substrate 12 may be tilted so as to help induce the underfill
30 to flow toward the exterior edge 18, 20, 22, 24 and under the
electronic device 14 when the underfill 30 is heated. The overall
purpose is to prevent air 66 trapped under the underfill 30 from
venting under the electronic device 14, with the underfill 30 then
flowing around the air so as to form a bubble under the electronic
device 14. The use of the obstacle, as in the present embodiment,
effectively achieves the same result, as the underfill 30 is spaced
apart from the exterior edges 18, 20, 22, 24 of the electronic
device 14 by a distance required for the placement of the
obstacle.
[0049] With reference to FIGS. 6A-7B in which like reference
numerals refer to like features in FIGS. 1-5G and in accordance
with an alternative embodiment, the obstacle may be a dam 68 formed
on the surface 16 of the substrate 12. The dam 68 may have a top
wall 72 raised above the surface 16 of the substrate 12 and side
walls 70 ascending from the surface 16 to the top wall 72. As
discussed above, the surface 16 receives the dispensed underfill
30. Consequently, the dam 68 is located on the same surface 16 that
receives the dispensed underfill 30 and on which the electronic
device 14 is mounted and between underfill 30 and the exterior
edges of 18, 20, 22, 24. A height of the dam 68 is sufficiently low
so that the underfill 30 may flow over the dam 68 when the assembly
10 is heated to a given temperature. The height of the dam 68 is
low enough not to impede the underfill flow after heating. Although
the underfill 30 may flow over the dam 68, the air 66 is unable to
surmount the wall 70 or the air vents through the underfill as it
flows over space 32 toward external edges 18, 20, 22, 24 and,
therefore, the air does not flow under the electronic device
14.
[0050] The dam 68 may be formed of a legend ink, such as that which
is typically used on PC boards for visible markings or letters.
Alternatively, a damming material such as that which is typically
used for dam and fill operations could be employed. More generally,
the damming material could be any thixotropic material, meaning any
material that does not flow once it is deposited on the surface 16
of the substrate 12.
[0051] Although the side walls 70 and the top wall 72 of the dam 68
form two right angles in the representative embodiment, the side
walls 70 and/or the top wall 72 may be inclined, contoured, and/or
curved. Alternatively, the two side walls 70 may converge at an
angle, such that the top of the dam 68 forms a peak or a crest
rather than a wall that is parallel to the surface 16 of the
substrate 12. Moreover, a width of the dam 68, including the
dimensions of the side walls 70 or the top wall 72, may vary.
[0052] With reference to FIGS. 8A-9B in which like reference
numerals refer to like features in FIGS. 6A-7B and in accordance
with an alternative embodiment, the obstacle may be a channel 74
formed in the substrate 12 and recessed below the surface 16 of the
substrate 12. The channel 74 may be formed by a router, for
example. As discussed above, the surface 16 receives the dispensed
underfill 30. Consequently, the channel 74 is located on the same
surface 16 that receives the dispensed underfill 30 and on which
the electronic device 14 is mounted. The channel 74 may have a base
78 positioned at a distance below a level of the surface 16 and
side walls 76 descending from the surface 16 to the base 78. The
channel 74 may obstruct or impede the underfill 30 from flowing to
external edges 18, 20, 22, 24, prior to heating the underfill. As
shown in FIGS. 9A and 9B, after vacuum is applied and the underfill
30 is heated, the underfill 30 flows into and/or over the channel
74 before flowing toward the at least one exterior edge 18, 20, 22,
24 of the electronic device 14 and into the open portion of the
space 28. However, the air 66 trapped under the underfill 30 is
trapped in the channel 64; once the air 66 flows into the channel
64, it is unable to surmount the sidewalls 76. Any remaining air
vents through the underfill 30 before the underfill 30 reaches the
gap 27. In this way, the channel 74 helps to prevent the air 66
from flowing under the electronic device 14. The depth of the
channel 74 should be sufficiently shallow so that substantially all
of the liquid underfill 30 may flow through or over the channel 74.
However, the depth of the channel 74, and thus the heights of the
side walls 76, may vary.
[0053] Although the side walls 76 and the base 78 of the channel 74
form two right angles in the representative embodiment, the walls
76 and/or base 78 may be inclined, contoured, and/or curved.
Alternatively, the two side walls 76 may converge at an angle, such
that the channel 74 lacks a planar base. Moreover, a width of the
channel 74, including the dimensions of the side walls 76 or the
base 78, may vary.
[0054] In an alternative embodiment, the obstacle may include
combined features of the dam 68 and the channel 74. For example,
the dam 68 may be immediately followed by the channel 74 on the
substrate 12, such that the underfill 30 flows over the dam 68 and
through the channel 74 before flowing toward the exterior edge 18,
20, 22, 24 of the electronic device 14.
[0055] In the representative embodiment, a single obstacle is shown
extending around an entire periphery of the electronic device 14.
However, in alternative embodiments, one or more obstacles may
extend along any combination of the one or more exterior edges 18,
20, 22, 24. Moreover, the obstacles may be longer or shorter than
the lengths of the one or more exterior edges 18, 20, 22, 24.
Preferably, a positioning of the one or more obstacles will
correspond to the positioning of the dispensed underfill 30, such
that all of the underfill 30 must flow over the obstacles in order
to reach the exterior edges 18, 20, 22, 24 of the electronic device
14.
[0056] With reference to FIGS. 10 and 11 and in accordance with an
alternative embodiment, the surface 16 of the substrate 12 having
an original composition and wettability may be modified to mitigate
the trapping of air under the underfill 30 is originally dispensed.
In this embodiment, the substrate 12 may be plasma treated so as to
change the wettability of the surface on which the underfill 30 is
dispensed. The plasma treatment process may be activated by methods
known to those of ordinary skill.
[0057] With particular reference to FIG. 10, the substrate 12 may
also be plasma treated so as to activate a surface layer 94 of the
substrate 12. Such activation may alter a chemical composition,
and, thus, physical characteristics of the surface layer 94 of the
substrate 12 so as to change its wettability. The surface layer 94
of the substrate 12 has a thickness t.sub.1. The plasma activation
does not add a layer to the substrate 12; rather, it modifies the
layer 94 with thickness t1 of the preexisting substrate 12.
[0058] In an embodiment, the plasma treatment decreases the
wettability of the layer 94 of the substrate 12. By rendering the
surface layer 94 of the substrate 12 less wettable, less air may be
trapped and air that is trapped under the underfill 30, when the
underfill 30 is positioned on the substrate 12, may more easily
escape from beneath the underfill 30 when the vacuum is applied.
The surface layer 94 with decreased wettability may have more
surface imperfections through which the air may escape than the
original surface 16 of the substrate 14. In this way, the trapping
of air under the electronic device 14 during the vacuum-assisted
underfill operation may be reduced by the reduction of trapped air
66 under the underfill 30.
[0059] In another embodiment, the plasma treatment increases the
wettability of the layer 94 of the substrate 12. Less air is
entrapped under underfill 30 deposited on the plasma-treated
surface having an increased wettability than on a non-plasma
treated surface because air may be more easily displaced as the
underfill 30 is applied. By reducing the initial trapping of air
under the underfill 30, the trapping of air under the electronic
device 14 during the vacuum-assisted underfill operation may also
be reduced.
[0060] With particular reference to FIG. 11, plasma deposition may
be used to deposit a very thin, glass-like layer 90 or film on the
surface 16 of the substrate 12. The layer 90 has a thickness t2,
and, thus, a height of the plasma-treated substrate is increased
(as compared to a height of the original substrate 12) by height
t.sub.2. The plasma-treated surface may be so smooth and flat that
there are fewer surface imperfections, such as depressions, in
which air can be trapped. As such, conducting vacuum-assisted
underfilling on the plasma deposited layer 90 helps prevent air or
gas from being trapped under the underfill 30. By reducing the
initial trapping of air under the underfill 30, the trapping of air
under the electronic device 14 during the vacuum-assisted underfill
operation may also be reduced. In an embodiment, a combined plasma
treatment method may be employed, in which the glass-like layer 90
is deposited on the substrate 12 and then is activated so as to
further increase wettability.
[0061] In an embodiment, a combination of the methods provided
above may be employed to help prevent the entrapment of air bubbles
under the electronic device 14. For example, the top surface 16 of
the substrate 12 may be plasma treated so as to increase the
wettability of the top surface 16 and/or to deposit a glass-like
layer 90 on the substrate 12. Such plasma treatment will help
prevent air from being trapped under the underfill 30 when it is
provided on the substrate 12. In addition, an obstacle, such as a
dam 68 or a channel 74, may be provided on the plasma-treated
substrate 12 so as to block any air 66 that may have been trapped
under the underfill 30 from flowing under the electronic device 14
during the vacuum-assisted underfill operation or to prevent the
underfill 30 from flowing over space 43 prior to heating the
underfill 30 in the vacuum.
[0062] With reference to FIG. 12, a system 110 for use in vacuum
underfilling is configured to dispense amounts of the underfill 30
on the substrate 12 upon which the electronic device 14 is mounted
by reflowed solder balls, or another interconnect technology, and
is separated from the substrate 12 by the space 28. The space 28
has an open portion that is not occupied by the conductive joints
26, which in this case are in the form of reflowed solder
balls.
[0063] A controller 120, which is electrically coupled with a
motion controller 118 and a dispenser controller 116, coordinates
the overall control for the system 110. Each of the controllers
116, 118, 120 may include a programmable logic controller (PLC), a
digital signal processor (DSP), or another microprocessor-based
controller with a central processing unit capable of executing
software stored in a memory and carrying out the functions
described herein, as will be understood by those of ordinary skill
in the art.
[0064] The system 110 preferably includes a cooling device 133 and
a cooling device 135 that is coupled with the dispenser 132. The
cooling device 133 is configured to cool the substrate 12 such that
the underfill 30 cools when dispensed onto the substrate 12. The
cooling device 135 is configured to cool the underfill 30 such that
the underfill 30 is cooled before dispensing onto the substrate 12.
The cooling devices 133, 135 are preferred, and optional, and may
be respectively operated by a temperature controller 139 under the
control of controller 120 to reduce the temperature of the
substrate 12 to below room temperature and/or to reduce the
temperature of a portion of the dispenser 132 to below room
temperature.
[0065] The system 110 includes a dispenser 132, which may be a
jetting dispenser, used to dispense the amounts of the underfill.
Downstream from the dispenser 132, the system 110 further includes
a vacuum chamber 154 configured to permit access for inserting and
removing each assembly 10 and configured to provide a sealed
condition in which an interior space of the vacuum chamber 154 is
isolated from the surrounding atmospheric-pressure environment. A
vacuum pump 160 is coupled with the interior space of the vacuum
chamber and is configured to evacuate the interior space as
operated by the controller 120. A vent 174 is used under the
control of the controller 120 to admit gas to the interior space to
raise the chamber pressure. The controller 120 supplies motion
instructions to the motion controller 118 to operate a transfer
device 122 used to move the substrate 12, which is carrying the
underfill 30, into the vacuum chamber 154.
[0066] A heater 166 is disposed inside the vacuum chamber 154 and
is configured to be powered by a temperature controller 169 linked
with the controller 120. Heat is transferred from the heater 166 to
each substrate 12. In one embodiment, the temperature of the
substrate 12 and underfill on the substrate ranges from 30.degree.
C. to 120.degree. C.
[0067] In use, the substrate 10 is moved to a location beneath the
dispenser 132 and underfill is dispensed or otherwise applied. In
the representative embodiment, the controller 120 sends commands to
the motion controller 118 to cause the transfer device 122 to move
the dispenser 32 and the controller 120 sends commands to the
dispenser controller 116 to cause the dispenser 32 to dispense the
underfill in one or more lines around the exterior edges 18, 20,
22, 24 of the electronic device 14. The substrate 12 is not heated
during the dispensing operation. Preferably, at least one gap is
left in the one or more lines of underfill 32 and preferably the
underfill 30 is not in contact with the exterior edge 18, 20, 22
24. For a jetting dispenser 132, the dispenser controller 16
triggers the jetting of droplets at appropriate times during the
movement such that the droplets will impact at a desired location
on the substrate 12. Each dispensed droplet contains a small volume
of the underfill, which is typically controlled with high precision
by the dispenser controller 16.
[0068] In one embodiment, the cooling device 133 may be used to
cool the substrate 12 so that the underfill 30 cools to a
temperature below room temperature upon contact with the substrate
12. Alternatively, the cooling device 135 coupled with the
dispenser 132 may be used to cool the underfill 30 before
dispensing.
[0069] After the dispensing operation is completed and before
significant capillary underfilling (and air or gas entrapment)
occurs, the controller 120 sends commands to the motion controller
118 to cause the transfer device 122 to transport the assembly 10
and dispensed underfill 30 on the substrate 12 into the vacuum
chamber 54. Once the assembly 10 and dispensed underfill 30 on the
substrate 12 are isolated inside the vacuum chamber 54 from the
ambient environment, the controller 120 causes the vacuum pump 160
to evacuate the interior space inside the vacuum chamber 154. While
the vacuum is being applied, each gap allows a vacuum condition
(i.e., a pressure less than atmospheric pressure) to be established
under the electronic device 14 between the electronic device 14 and
the substrate 12 or, if there is no gap, then the gas bubbles
through the underfill to create a vacuum condition under the
electronic device 14.
[0070] When a suitable vacuum pressure exists inside the vacuum
chamber 154 and with the vacuum condition being maintained, the
controller 120 causes the temperature controller 169 to operate the
heater 166, which heats the substrate 12, electronic device 14, and
the underfill 30. The elevated temperature encourages the underfill
30 to flow over the substrate space 43 and into the open portion of
the space beneath the electronic device 14. The underfill 30
completely flows under the electronic device 14 and into the spaces
between the reflowed solder balls. Underfilling in the presence of
the vacuum condition means any void entrapped in the underfill will
be partially evacuated of gases. After flow ends, the controller
120 sends commands to the motion controller 118 to cause the vent
174 to admit gas to the vacuum chamber 154 so that the pressure
inside the vacuum chamber 154 is returned to atmospheric pressure.
Any voids present in the underfill 30 collapse because of the
evacuated condition and become filled with underfill 30. The
substrate 12 with the underfilled electronic device 14 is
transferred out of the vacuum chamber 154 to, for example, a curing
oven (not shown).
[0071] While the invention has been illustrated by the description
of one or more embodiments thereof, and while the embodiments have
been described in considerable detail, they are not intended to
restrict or in any way limit the scope of the appended claims to
such detail. Additional advantages and modifications will readily
appear to those skilled in the art. The invention in its broader
aspects is therefore not limited to the specific details,
representative apparatus and methods and illustrative examples
shown and described. Accordingly, departures may be made from such
details without departing from the scope or spirit of Applicant's
general inventive concept.
* * * * *