U.S. patent application number 11/067879 was filed with the patent office on 2005-07-28 for method and apparatus for underfilling semiconductor devices.
This patent application is currently assigned to Nordson Corporation. Invention is credited to Fang, Liang, Quinones, Horatio, Ratledge, Thomas Laferl.
Application Number | 20050161846 11/067879 |
Document ID | / |
Family ID | 28794431 |
Filed Date | 2005-07-28 |
United States Patent
Application |
20050161846 |
Kind Code |
A1 |
Quinones, Horatio ; et
al. |
July 28, 2005 |
Method and apparatus for underfilling semiconductor devices
Abstract
A method and apparatus for underfilling a gap between a
multi-sided die and a substrate with an encapsulant material. The
die and/or the substrate is heated non-uniformly by a heat source
to generate a temperature gradient therein. The heated one of the
die and the substrate transfers heat energy in proportion to the
temperature gradient to the encapsulant material moving in the gap.
The differential heat transfer steers, guides or otherwise directs
the movement of the encapsulant material in the gap. The
temperature gradient may be established with heat transferred from
the heat source to the die and/or the substrate by conduction,
convection, or radiation. The temperature gradient may be
dynamically varied as the encapsulant material moves into the
gap.
Inventors: |
Quinones, Horatio;
(Carlsbad, CA) ; Fang, Liang; (San Diego, CA)
; Ratledge, Thomas Laferl; (San Marcos, CA) |
Correspondence
Address: |
WOOD, HERRON & EVANS, LLP (NORDSON)
2700 CAREW TOWER
441 VINE STREET
CINCINNATI
OH
45202
US
|
Assignee: |
Nordson Corporation
Westlake
OH
|
Family ID: |
28794431 |
Appl. No.: |
11/067879 |
Filed: |
February 28, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11067879 |
Feb 28, 2005 |
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10408464 |
Apr 7, 2003 |
|
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6861278 |
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60371826 |
Apr 11, 2002 |
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Current U.S.
Class: |
264/6 ;
257/E21.503; 264/459; 264/494 |
Current CPC
Class: |
H01L 2924/12042
20130101; H01L 2924/01043 20130101; H01L 2924/01006 20130101; H01L
2924/15787 20130101; H01L 2224/16 20130101; H01L 2924/01025
20130101; H01L 2924/00 20130101; H01L 2924/00 20130101; H01L 24/28
20130101; H01L 2224/83102 20130101; H01L 2924/01082 20130101; H01L
2924/01033 20130101; H01L 21/563 20130101; H01L 2924/01065
20130101; H01L 2924/14 20130101; H01L 2924/00 20130101; H01L
2924/0102 20130101; H01L 2224/73203 20130101; H01L 2224/92125
20130101; H01L 2924/15787 20130101; H01L 2924/01042 20130101; H01L
2924/351 20130101; H01L 2924/351 20130101; H01L 2924/3011 20130101;
H01L 2924/01073 20130101; H01L 2924/01052 20130101; H01L 2924/12042
20130101 |
Class at
Publication: |
264/006 ;
264/459; 264/494 |
International
Class: |
B29B 009/00; H01L
021/50; H01L 021/44 |
Claims
We claim:
1. An apparatus for underfilling a gap between a multi-sided die
and a substrate with a dispenser operative for dispensing an
encapsulant material adjacent to at least one side edge of the die
to encapsulate a plurality of electrical connections formed
therebetween, comprising: a heat source operative to non-uniformly
transfer heat energy to one of the die and the substrate so that
said one of the die and the substrate non-uniformly transfers heat
to the encapsulant material moving in the gap between the
multi-sided die and the substrate.
2. The apparatus of claim 1 wherein said heat source is adapted to
transfer heat to the heated one of the die and the substrate with a
plurality of temperature zones distributed to promote non-uniform
heat transfer to the encapsulant material.
3. The apparatus of claim 1 wherein said heat source is configured
to conductively transfer heat energy to the heated one of the die
and the substrate.
4. The apparatus of claim 3 wherein said heat source comprises: a
support block having a surface coupled in thermal communication
with the heated one of the die and the substrate; and at least one
heating element coupled in thermal communication with said support
block, said at least one heating element adapted to transfer heat
energy to said surface.
5. The apparatus of claim 1 wherein said heat source is configured
to convectively transfer heat energy to the heated one of the die
and the substrate.
6. The apparatus of claim 5 wherein said heat source is capable of
directing a flow of a heated gas toward the heated one of the die
and the substrate.
7. The apparatus of claim 6 wherein said heat source comprises: a
first porous element and a second porous element positioned between
said heat source and the heated one of the die and the substrate,
said first and second porous elements having a different porosity
effective to control the flow of the heated gas to the heated one
of the die and the substrate.
8. The apparatus of claim 1 wherein said heat source is configured
to radiatively transfer heat energy to the heated one of the die
and the substrate.
9. The apparatus of claim 8 wherein said heat source is operative
to provide a radiative flux of electromagnetic energy incident on
the heated one of the die and the substrate.
10. The apparatus of claim 9 wherein said heat source comprises: a
mask positioned between said heat source and the heated one of the
die and the substrate, said mask including a pattern of openings
configured to transmit the radiative flux to the heated one of the
die and the substrate.
11. The apparatus of claim 9 wherein said heat source comprises: a
laser providing an area of radiative flux dimensionally smaller
than a surface area of the heated one of the die and the substrate;
and a reflective device for moving the area of radiative flux
relative to the heated one of the die and the substrate in a manner
effective to non-uniformly transfer heat energy.
12. The apparatus of claim 9 wherein said heat source comprises: a
thermal transfer element covering the heated one of the die and the
substrate and operative to absorb said radiative flux, said thermal
transfer element formed of a thermally-conductive material having a
pattern of thicknesses that varies so as to alter a path length for
heat conduction from said thermal transfer element to the heated
one of the die and the substrate.
13. The apparatus of claim 1 further comprising: a dispenser
operative for dispensing the encapsulant material adjacent to at
least one side edge of the die.
14. The apparatus of claim 13 wherein said heat source is adapted
to non-uniformly transfer heat energy to one of the die and the
substrate before said dispenser operates to dispense the
encapsulant material.
15. The apparatus of claim 13 wherein said heat source is adapted
to non-uniformly transfer heat energy to one of the die and the
substrate after said dispenser operates to dispense the encapsulant
material.
16. The apparatus of claim 13 said heat source is adapted to
non-uniformly transfer heat energy to one of the die and the
substrate when said dispenser operates to dispense the encapsulant
material.
17. An apparatus for underfilling a gap between a die and a
substrate with an encapsulant material, comprising: a heat source
including a plurality of regions each adapted to transfer heat
energy to the heated one of the die and the substrate, at least two
of said regions operative to transfer heat energy to heat
encapsulant material in the gap between the die and the substrate
to different temperatures as the encapsulant material moves within
the gap.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation of application
Ser. No. 10/408,464, filed Apr. 7, 2003, which claims the benefit
of U.S. Provisional Application Ser. No. 60/371,826, filed Apr. 11,
2002, the disclosure of which is hereby incorporated herein by
reference in its entirety.
FIELD OF THE INVENTION
[0002] This invention relates generally to liquid dispensing
methods and apparatus used in semiconductor package manufacturing
and, more particularly, to the underfilling of one or more
semiconductor dies carried by a substrate.
BACKGROUND OF THE INVENTION
[0003] In the microelectronics industry, a die carrying an
integrated circuit is commonly mounted on a package carrier, such
as a substrate, a circuit board or a leadframe, that provides
electrical connections from the die to the exterior of the package.
In one such packaging arrangement called flip chip mounting, the
die includes an area array of electrically-conductive contacts,
known as bond pads, that are electrically connected to
corresponding area array of electrically-conductive contacts on the
package carrier, known as solder balls or bumps. Typically, the
solder bumps are registered with the bond pads and a reflow process
is applied to create electrical connections in the form of solder
joints between the die and the package carrier. The process of flip
chip mounting results in a space or gap between the die and the
package carrier.
[0004] The die and the package carrier are usually formed of
different materials having mismatched coefficients of coefficient
of thermal expansion. As a result, the die and the package carrier
experience significantly different dimension changes when heated
that creates significant thermally-induced stresses in the
electrical connections between the die and the package carrier. If
uncompensated, the disparity in thermal expansion can result in
degradation in the performance of the die, damage to the solder
joints, or package failure. As the size of the die increases, the
effect of a mismatch in the coefficient of thermal expansion
between the die and the substrate becomes more pronounced. In
stacked die packages, the mismatch in coefficient of thermal
expansion between the die laminate and the package may be even
greater than in single die packages. The failure mechanism in
stacked die packages may shift from solder joint damage to die
damage.
[0005] To improve the reliability of the electrical connections in
flip chip package assemblies, it is common in the microelectronics
industry to fill the gap between the die and the package carrier
with an encapsulant material. Underfilling with encapsulant
material increases the fatigue life of the package and improves the
reliability of the electrical connections by reducing the stress
experienced by the electrical connections during thermal cycling or
when the die and the package carrier have a significant temperature
differential. The encapsulant material also isolates the electrical
connections from exposure to the ambient environment by
hermetically sealing the gap and lends mechanical strength to the
package assembly for resisting mechanical shock and bending. The
encapsulant material further provides a conductive path that
removes heat from the die and that operates to reduce any
temperature differential between the die and substrate. As a
result, underfilling with encapsulant material significantly
increases the lifetime of the assembled package.
[0006] Various conventional underfilling methods are used to
introduce the encapsulant material into the gap between the die and
the substrate. One conventional method relies surface tension
wetting or capillary action to induce movement of a low-viscosity
encapsulant material with strong wetting characteristics from a
side edge into the gap. According to this method, encapsulant
material is dispensed as an elongated single line, L-shaped or
U-shaped bead adjacent to one, two or three contiguous side edges
of the die, respectively, and capillary forces operate to attract
the encapsulant material into the gap. Typically, the viscosity of
the encapsulant material is reduced and the flow rate increased by
pre-heating the substrate in the vicinity of the die to a uniform,
steady-state temperature between about 40.degree. and about
90.degree., before the encapsulant material is dispensed onto the
substrate. The underfill material is subsequently cured after the
electrical connections have been fully encapsulated.
[0007] With reference to FIG. 1, a time sequence for a typical
underfilling operation relying on capillary action is shown.
Isochronal contour lines 11 represent the advance of the leading
edge or wave front of the encapsulant material 10 moving into the
gap separating a die 12 from a substrate 14. Initially, the
encapsulant material 10 is dispensed as an L-shaped bead onto the
substrate 14 adjacent to contiguous side edges of the die 12 and is
attracted into the gap by capillary forces. As time progresses, the
wave front of encapsulant material 10 advances substantially
diagonally, as indicated by arrow 16, through the gap. Drag causes
the flow rate to diminish with increasing time as indicated by the
reduced separations between adjacent pairs of contour lines 11 and,
as the underfilling operation nears completion, the advance rate of
the wave front of encapsulant material slows dramatically.
[0008] For larger size dies and smaller gap dimensions, the time
necessary to underfill using conventional capillary underfilling
methods becomes longer because of the longer fluid path of the
liquid encapsulant and shear rates. As a result, throughput
diminishes and underfilling operations become less cost effective.
One way of enhancing the velocity of the encapsulant material is to
perform a forced underfill that relies upon, for example, vacuum
assistance to enhance the fill rate and the quality of filling.
Vacuum-assisted underfilling utilizes a pressure differential
created across a bead of encapsulant material to draw the
encapsulant material into the gap. Regardless of the underfilling
method, it is important that voids are not formed in the
encapsulant material. Voids may result in corrosion and undesirable
thermal stresses that degrade performance or adversely effect the
reliability of the package assembly.
[0009] It would therefore be desirable to provide a manner of
underfilling the gap formed between a die and a package carrier
that prevents the occurrence of voids between the die and the
package carrier and that reduces the time required to perform an
underfilling operation.
SUMMARY OF THE INVENTION
[0010] The present invention overcomes the foregoing and other
shortcomings and drawbacks of underfill apparatus and methods
heretofore known. While the invention will be described in
connection with certain embodiments, it will be understood that the
invention is not limited to these embodiments. On the contrary, the
invention includes all alternatives, modifications and equivalents
as may be included within the spirit and scope of the present
invention.
[0011] Generally, the invention relates to a method and apparatus
for underfilling a gap between a multi-sided die, which may be a
semiconductor device, and a package carrier, such as a substrate,
to encapsulate a plurality of electrical connections formed
therebetween. The die may comprise a flip chip package having a
flip chip mounted to a substrate with a plurality of electrical
connections formed in the gap between opposed surfaces of the flip
chip and the substrate. The die and/or the substrate is heated
non-uniformly by the heat source to generate a temperature gradient
in the gap. The heated one of the die and the substrate transfers
heat energy in proportion to the temperature gradient to the
encapsulant material moving in the gap. The differential heat
transfer steers, guides or otherwise directs the movement of the
encapsulant material in the gap.
[0012] According to the principles of the invention, an apparatus
is provided for underfilling a gap between a multi-sided die and a
substrate with a dispenser operative for dispensing an encapsulant
material adjacent to at least one side edge of the die to
encapsulate a plurality of electrical connections formed
therebetween. The apparatus comprises a heat source operative to
non-uniformly transfer heat energy to one of the die and the
substrate so that the one of the die and the substrate
non-uniformly transfers heat to the encapsulant material moving in
the gap between the multi-sided die and the substrate.
[0013] According to the principles of the present invention, an
apparatus is provided for underfilling the gap between the
multi-sided die and the substrate with a dispenser operative for
dispensing an encapsulant material adjacent to at least one side
edge of the die. The apparatus includes a heat source operative to
transfer heat energy to first and second regions of one of the die
and the substrate so that said first and second regions are heated
to respective first and second temperatures. The first temperature
differs from the second temperature so as to non-uniformly transfer
heat to the encapsulant material moving in the gap between the
multi-sided die and the substrate.
[0014] According to the principles of the present invention, a
method is provided for underfilling the gap between the multi-sided
die and the substrate. The method includes heating at least one of
the die and the substrate by either conduction, convection or
radiation to generate a temperature gradient on the heated one of
the die and substrate. An encapsulant material is dispensed
adjacent to at least one side edge of the die and subsequently
moved into the gap for encapsulating the plurality of electrical
interconnections. Heat energy is transferred non-uniformly from the
heated one of the die and substrate to the moving encapsulant
material in a pattern determined by the temperature gradient for
selectively varying the flow rate of the moving encapsulant
material in the gap. In one aspect of the invention, the individual
temperatures of the temperature gradient may be varied dynamically
as the encapsulant material flows into the gap.
[0015] From the foregoing summary and the detailed description to
follow, it will be understood that the invention provides a unique
and effective method and apparatus for underfilling the gap between
a die, such as a flip chip, and a substrate. The invention is
particularly advantageous in applications in which the gap between
the die and the substrate is small and in applications utilizing
relatively large dies with a large space to underfill. In these
situations, differential or non-uniform heating of either the die
and/or the substrate according to the principles of the present
invention augments the capillary action or forced (e.g.,
vacuum-assisted) capillary action normally relied upon to move the
underfill material into the gap for fully encapsulating the
electrical connections with a lower incidence of void formation.
The augmentation provides a more uniform leading edge or wave front
for encapsulant material advancing in the gap by selectively
lowering the viscosity of the material in the regions of differing
temperature so as to vary the flow rate of the material and the
directionality of the material as it moves within the gap.
[0016] The present invention improves the durability and
reliability of electronic components that require an underfill
encapsulant material in the gap between a die mounted on a
substrate. The present invention also reduces the time required to
effectively and reliably underfill encapsulant material within the
gap between the die and the substrate. The present invention
improves upon the overall throughput of underfilling process while
at the same time accommodating the need for flexibility and also
accommodating multiple different chip sizes, reduced gap
dimensions, and the various types of encapsulant material used in
the industry.
[0017] The above and other objects and advantages of the present
invention shall be made apparent from the accompanying drawings and
the description thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The accompanying drawings, which are incorporated in and
constitute a part of this specification, illustrate embodiments of
the invention and, together with a general description of the
invention given above, and the detailed description of the
embodiments given below, serve to explain the principles of the
invention.
[0019] FIG. 1 is a schematic view showing an underfilling operation
performed with capillary action and with the substrate heated to a
uniform temperature in accordance with prior art practices;
[0020] FIG. 2 is a schematic perspective view of a package assembly
of a die, shown in phantom, and substrate during an underfilling
operation;
[0021] FIG. 3 is a schematic side view of a package assembly of a
die and substrate following an underfilling operation;
[0022] FIG. 4 is a diagrammatic view of temperature zones created
in the gap between a die and a substrate according to an embodiment
of the present invention;
[0023] FIG. 5 is a perspective view of an embodiment of a heating
block of the present invention for transferring heat by conduction
to a substrate during an underfilling operation to provide the
temperature zones of FIG. 4;
[0024] FIG. 6 is a perspective view of another embodiment of a
heating block of the present invention for transferring heat by
conduction to a substrate during an underfilling operation to
provide the temperature zones of FIG. 4;
[0025] FIG. 7 is a perspective view of a heating block according to
the present invention for transferring heat by convection to a
substrate during an underfilling operation to provide the
temperature zones of FIG. 4;
[0026] FIG. 8 is a perspective view of a non-contact arrangement to
the present invention for transferring heat by convection to a die
during an underfilling operation to provide the temperature zones
of FIG. 4;
[0027] FIG. 9 is a perspective view of a non-contact arrangement
according to the principles of the present invention for
transferring heat by radiation to a die during an underfilling
operation to provide the temperature zones of FIG. 4;
[0028] FIG. 10 is a perspective view of a non-contact arrangement
to the principles of the present invention for transferring heat by
radiation to a die during an underfilling operation to provide the
temperature zones of FIG. 4;
[0029] FIG. 11 is a perspective view of a non-contact arrangement
to the principles of the present invention for transferring heat by
radiation to a die during an underfilling operation to provide the
temperature zones of FIG. 4;
[0030] FIG. 12 is a perspective view of a non-contact arrangement
according to the principles of the present invention for
transferring heat by convection to a stacked die package during an
underfilling operation to provide the temperature zones of FIG.
4;
[0031] FIG. 13 is a graphical representation of dynamic variation
of the temperatures in the temperature zones of FIG. 4 according to
the cumulative time for completing an underfilling operation;
and
[0032] FIG. 14 is a graphical representation of dynamic variation
of the temperatures in the temperature zones of FIG. 4 according to
the completion percentage of an underfilling operation.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0033] With reference to FIGS. 2 and 3, a semiconductor device
package 30 consisting of a die 32 mounted on a package carrier or
substrate 34 in a flip chip mounting arrangement is shown. As those
skilled in the art will appreciate, substrate 34 may comprise an
organic or ceramic substrate material such as a printed circuit
board, a flip chip multi-chip module or a flip chip carrier. The
die 32 is electrically and mechanically connected to the substrate
34 through an area array of solder bumps 36 on the underside of the
die 32 that are registered or aligned with a corresponding area
array of solder pads 38 on the substrate 34. Upon heating, the
solder pads 38 on the substrate reflow and physically connect with
the solder bumps 36 of die 32 to provide mechanical, thermal and
electrical coupling therebetween in the form of solder joints. With
this mounting arrangement, a gap 40 is formed between a contact
side 41 of the die 32 and a top surface 42 of the substrate 34.
[0034] The gap 40 is filled with an encapsulant material 44, such
as a liquid epoxy, according to the principles of the present
invention. Various different encapsulant materials are suitable for
use in the invention, including but not limited to a line of
encapsulants commercially available under the HYSOL.RTM. tradename
from Loctite Corp (Rocky Hill, Conn.). As illustrated in FIG. 2,
encapsulant material 44 is provided from an underfill dispenser 35
as an L-shaped bead 45 dispensed onto the surface of the substrate
proximate to the gap 40 and on two contiguous sides of the die 32.
Although the present invention is described for use with an
L-shaped bead, the principles of the invention are applicable to
any bead shape, including a single line of encapsulant material 44
disposed along one side edge of die 32, a U-shaped bead of
encapsulant material 44 disposed along three side edges of die 32,
or other dispensing patterns. The amount of encapsulant material 44
in bead 45 depends upon the desired fillet volume and the under-die
volume determined by the size of die 32 and the height tolerances
of the solder junctions created between bumps 36 and pads 38.
[0035] The underfill dispenser 35 may take any form readily known
in the art for dispensing liquid encapsulant or underfill material
in a desired pattern relative to the die 32. One suitable underfill
dispenser 35 is the DP-3000 pump commercially available from
Nordson Asymtek (Carlsbad, Calif.).
[0036] With continued reference to FIGS. 2 and 3, the encapsulant
material 44 flows or moves in the gap 40, as indicated generally by
arrows 46, under capillary action or with forced assistance. After
flow ceases (FIG. 3), the encapsulant material 44 fully
encapsulates all of the electrical interconnections provided by the
solder junctions and a fillet 47 is formed along the side edges of
the die 32. The encapsulant material 44 is cured after the
conclusion of the underfilling operation.
[0037] According to the principles of the present invention, a
temperature gradient is established in the die 32, the substrate
34, or both the die 32 and substrate 34 for transferring heat to
the encapsulant material moving into the gap 40 between the
underside 41 of the die 32 and the top surface 42 of the substrate
34. To establish the temperature gradient, heat, also referred to
herein as heat energy, may be transferred in a spatially
non-uniform, non-equal or otherwise inhomogeneous manner from a
heat source to the die 32 and/or the substrate 34 by contact
heating or by non-contact heating. Heat from the die 32 and/or the
substrate 34 is subsequently transferred by conduction to the
encapsulant material advancing through or moving in the gap 40. The
transferred heat elevates the temperature of the encapsulant
material 44 in the gap 40 so as to reduce the temperature-dependent
viscosity and to thereby increase the uniformity of the leading
edge or wave front of the advancing encapsulant material 44. The
non-uniform heat transfer varies the flow of the encapsulant
material 44 by altering the flow rate and the directionality of the
movement in the gap 40. The principles of the present invention may
be incorporated into any conventional underfill dispensing system,
such as the M-2020, the X-1020, M-620 and C-720 underfill
dispensing systems commercially available from Nordson Asymtek
(Carlsbad, Calif.).
[0038] With reference to FIG. 4 and in which like reference
numerals refer to like features in FIGS. 2 and 3, the temperature
gradient may include a plurality of, for example, four
spatially-distributed temperature zones T.sub.A, T.sub.B, T.sub.C
and T.sub.D for promoting the flow of encapsulant material 44 from
an L-shaped elongated bead dispensed onto the substrate 34 adjacent
to two side edges of the die 32. In each of the temperature zones
T.sub.A, T.sub.B, T.sub.C and T.sub.D, heat is transferred to the
encapsulant material 44 in the gap 40 (FIG. 3) between the die 32
and the substrate 34 in an amount sufficient to raise the
temperature of the encapsulant material 44 to a characteristic
temperature corresponding to the heat energy transferred within
each zone. The characteristic temperature of each of the
temperature zones T.sub.A, T.sub.B, T.sub.C and T.sub.D may be
constant or steady-state, as in FIGS. 5-12, or may be modulated or
non-steady-state, as in FIGS. 13-14. The characteristic temperature
in each of the temperature zones T.sub.A, T.sub.B, T.sub.C and
T.sub.D may be quantified by a single value or by a distribution of
values having a mean or arithmetic average equal to the
corresponding characteristic temperature.
[0039] The amount of heat that must be transferred by conduction
from either the die 32 or the substrate 34 to the encapsulant
material 44 in each of the temperature zones T.sub.A, T.sub.B,
T.sub.C and T.sub.D to establish the associated temperature in
material 44 depends upon the product of mass, specific heat, and
the required temperature rise of material 44. For example, the
desired temperature for volumes of the encapsulant material 44 in
temperature zone T.sub.A is less than the desired temperature for
other volumes of material 44 in temperature zone T.sub.D as the
flow resistance or impedance in the portion of the gap associated
with zone T.sub.A is less than the impedance in the portion of the
gap 40 associated with zone T.sub.D. The arrangement of temperature
zones T.sub.A, T.sub.B, T.sub.C and T.sub.D may have a mirror
symmetry, as illustrated in FIG. 4, or any other arrangement or
configuration, including the number of zones, without limitation as
required by the specific underfilling operation.
[0040] The rate of heat flow throughout the volume of encapsulant
material 44 in each of the temperature zones T.sub.A, T.sub.B,
T.sub.C and T.sub.D, until equilibrated, will depend upon the
thermal conductivity, the temperature difference among different
portions of the encapsulant material 44, and the length and
cross-sectional area of the various heat flow paths. Typically,
encapsulant material 44 entering one of the temperature zones
T.sub.A, T.sub.B, T.sub.C and T.sub.D will flow for a short
distance in that zone before equilibrating thermally with other
portions of material 44 equilibrated at the associated temperature
of that temperature zone. It is appreciated by those of ordinary
skill in the art that one or all of the die 32, the substrate 34,
and the encapsulant material 44 may be preheated before the
underfilling operation to reduce the time required to establish the
temperature gradient. It is also appreciated by those of ordinary
skill in the art that the temperature change across the boundaries
between adjacent ones of the temperature zones T.sub.A, T.sub.B,
T.sub.C and T.sub.D may be abrupt and well-defined, as depicted in
FIG. 4, or less sharply-delineated so as to provide transition
regions over which the temperature changes continuously.
[0041] According to the principles of the present invention, the
increased flow rate and the altered directionality of the movement
of the encapsulant material 44, as tailored by the non-uniform or
unequal heating, enhances the throughput of the underfilling
operation and reduces the occurrence of voids so as to improve the
quality of the underfill. To that end, the different temperature
zones T.sub.A, T.sub.B, T.sub.C and T.sub.D provide regions of
differing temperature in which the non-uniform heat transfer
guides, steers or otherwise directs the encapsulant material in the
gap. It is appreciated that the non-uniform heating to provide
temperature zones T.sub.A, T.sub.B, T.sub.C and T.sub.D may occur
before dispensing the encapsulant material 44 onto the substrate
34, after dispensing the encapsulant material 44 onto the substrate
34, or the two events may be simultaneous.
[0042] With reference to FIG. 5, a support plate or block 48 is
shown that operates to transfer heat by conduction to the substrate
34 and, subsequently, from the substrate 34 to the encapsulant
material 44 (FIGS. 2, 3) moving into the gap 40 (FIG. 3) during an
underfilling operation. The support block 48 includes a plurality
of, for example, four heating elements 50a-d each disposed in a
corresponding one of a plurality of suitably-shaped cavities 51a-d
formed in block 48. The heating elements 50a-d are arranged in the
support block 48 so as to permit the establishment of the
temperature zones T.sub.A, T.sub.B, T.sub.C and T.sub.D (FIG. 4).
The heating elements 50a-d may be any structure capable of
resistively converting electrical energy into heat energy and
transferring the heat energy to the support block 48. Suitable
heating elements 50a-d include a line of cartridge heaters
commercially available under the FIREROD.RTM. tradename from Watlow
Electric Manufacturing Company (St. Louis, Mo.).
[0043] An upper surface 52 of the support block 48 has rectangular
dimensions similar to the rectangular dimensions of the contact
surface 41 of die 32. The support block 48 is positioned so that
upper surface 52 is coupled in thermal communication with a bottom
surface 43 (FIG. 3) of substrate 34 and so that the upper surface
52 substantially underlies or mirrors the perimeter of the contact
surface 41 of die 32. Heat is transferred by conduction from the
upper surface 52 of support block 48 in areas of physical,
surface-to-surface contact between with the bottom surface 43 of
substrate 34. Recognizing that surfaces are not perfectly flat or
smooth, it is appreciated the total surface area of contact between
surfaces 43 and 52 should be significantly greater than the total
surface area of non-contacting areas between surfaces 43 and 52 and
adequate to provide the desired temperatures for the encapsulant
material 44 in each of the temperature zones T.sub.A, T.sub.B,
T.sub.C and T.sub.D. The amount of heat transferred in each of the
temperature zones T.sub.A, T.sub.B, T.sub.C and T.sub.D is
sufficient to provide the corresponding characteristic temperature
for the encapsulant material 44 moving in gap 40.
[0044] Each of the heating elements 50a-d is coupled electrically
with a corresponding one of a plurality of temperature controllers
54a-d. The temperature controllers 54a-d control the electrical
energy supplied to each of the heating elements 50a-d to heat
corresponding portions of the support block 48 to achieve the
corresponding temperature in each of the temperature zones T.sub.A,
T.sub.B, T.sub.C and T.sub.D (FIG. 4). The temperature controllers
54a-d are any conventional device familiar to those of ordinary
skill in the art that is operative to supply electrical energy to a
resistive heating element.
[0045] With reference to FIG. 6 and in accordance with another
embodiment of the invention, a support plate or block 56 is shown
that operates to transfer heat by conduction to the substrate 34
(not shown) and, subsequently, from the substrate 34 to the
encapsulant material 44 (FIGS. 2, 3) moving into the gap 40 (FIG.
3) during an underfilling operation. The support block 56 is in
good thermal contact with a heating element 58, which is controlled
by a suitable temperature controller 60. A temperature sensor 61,
such as a thermocouple, provides temperature information as
feedback to the temperature controller 60 for use in controlling
the temperature of the support block 56. Heating element 58 is
conventional and suitable heating elements 58 include various thick
film heaters and cast-in heaters commercially available, for
example, from Watlow Electric Manufacturing Company (St. Louis,
Mo.). Sufficient heat energy is transferred from the heating
element 58 by conduction to the support block 56 to heat block 56
to a substantially uniform temperature.
[0046] An upper surface 62 of the support block 56 has rectangular
dimensions similar to the rectangular dimensions of the contact
surface 41 of die 32. The support block 56 is positioned so that
upper surface 62 is coupled generally in thermal communication with
the bottom surface 43 (FIG. 3) of substrate 34 and so that the
upper surface 62 substantially underlies the perimeter of the
contact surface 41 of die 32. Heat energy is selectively
transferred from the upper surface 52 of support block 48 primarily
by conduction in areas of physical contact with the bottom surface
43 of substrate 34.
[0047] According to the principles of the present invention, the
upper surface 52 of support block 56 is modified so that the amount
of heat energy transferred from upper surface 52 to bottom surface
43 of substrate 34 creates the temperature zones T.sub.A, T.sub.B,
T.sub.C and T.sub.D (FIG. 4). To that end, upper surface 52
includes a plurality of four portions 64a-d each corresponding to
one of temperature zones T.sub.A, T.sub.B, T.sub.C and T.sub.D.
Each of the portions 64a-d has a heat flow path with a distinct
total surface area contacting a portion of bottom surface 43 that
is effective to provide the characteristic temperature for material
44 desired in each of temperature zones T.sub.A, T.sub.B, T.sub.C
and T.sub.D. In particular and as illustrated in FIG. 6, portion
64b has the largest total surface area and will therefore provide
the greatest heat transfer of the four portions 64a-d. Portion 64d
has the least total surface area and will therefore provide the
smallest heat transfer among the four portions 64a-d. The surface
area of each of the portions 64a,c-d is defined by the respective
collective surface area of the topmost surfaces of a plurality of
projections 66a,c-d, respectively. Although portion 64b does not
include projections and presents a continuous planar surface, the
present invention is not so limited in that portion 64b may also
include a set of projections.
[0048] As illustrated in FIG. 6, the projections 66a,c-d may have
the form of a rectangular grid of ribs separated by a plurality of
corresponding rectangular depressions or recesses. The surface area
of each of the uppermost surfaces of projections 66a,c-d and the
spacing between adjacent ones of the projections 66a,c-d will
determine the respective total surface areas in portions 64a,c-d of
support block 48. The projections 66a,c-d may be formed by any
suitable process, such as by wet chemical etching. It is
appreciated that the projections 66a,c-d may assume different
forms, such as a plurality of non-interconnected mesas, or any
other form apparent to persons of ordinary skill in the art. It is
further appreciated that the cross-sectional area of the
projections 66a,c-d may be varied along the length of the
respective flow paths, such as tapering, to change the associated
conductive heat transfer.
[0049] While FIGS. 5 and 6 depict non-uniform conductive heat
transfer to the substrate 34, it is contemplated by the invention
that heat may be conducted non-uniformly to the die 32 to provide
the same advantages.
[0050] With reference to FIG. 7 and in accordance with another
embodiment of the invention, a support plate or block 70 is shown
that operates to transfer heat by convection to the substrate 34
(not shown) and, subsequently, from the substrate 34 to the
encapsulant material 44 (FIGS. 2, 3) moving into the gap 40 (FIG.
3) during an underfilling operation. The support block 70 includes
four portions 72a-d each of which corresponds to one of the
temperature zones T.sub.A, T.sub.B, T.sub.C and T.sub.D. Extending
from a lower surface 76 to an upper surface 78 of the support block
70 in each of the four portions 72a-d is a corresponding set of
through holes or perforations 74a-d. The perforations 74a-d are
drilled or machined in the support block 70 by laser drilling or
conventional drilling, or may be formed by other processes,
including selective chemical or plasma etching.
[0051] To that end, adjacent ones in each set of perforations 74a-d
are arranged with a spaced-apart relationship to provide an ordered
arrangement, such as a grid or array, or may be arranged in a
random pattern. Uniform heat transfer within the temperature zones
T.sub.A, T.sub.B, T.sub.C and T.sub.D is typically desired and
would likely result from ordered arrangements. Each portion 72a-d
of the support block 70 is characterized by a porosity given by the
ratio of the total cross-sectional area of the respective set of
perforations 74a-d to surface area of the remaining unperforated
part of the support block 70. The porosity of each portion 72a-d of
the support block 70 is characterized by, among other parameters,
the number of perforations 74a-d, the pattern of perforations
74a-d, the geometrical shape of each perforation 74a-d, and the
average pore diameter of each perforation 74a-d. Typically, the
ratio of the total cross-sectional area of the perforations 74a-d
to the surface area of the remaining unperforated part of the
corresponding portion 72a-d ranges from 10% to about 90%. The
perforations 74a-d may have a cylindrical configuration with a
circular cross-sectional profile or other cross-sectional profiles,
such as polygonal, elliptical or slotted. The perforations 74a-d
may have a single, uniform cross-sectional area or may have a
distribution of cross-sectional areas.
[0052] A heated gas source 80 provides a forced flow of heated gas,
represented by arrows 82, directed toward the lower surface 76 of
the support plate 70. The flow 82 of heated gas has a spatially
uniform temperature and a spatially uniform volumetric flow rate,
although the present invention is not so limited, over the entire
surface area of the lower surface 76. The heated gas source 80 may
comprise, for example, a heating element and a blower operative to
direct gas past the heating element generate a flow of heated gas.
The porosity of the various portions 72a-d of support block 70 is
operative to regulate the convective fluid communication between
the forced flow 82 of heated gas from heated gas source 80 to the
lower surface 43 of substrate 34, wherein the upper surface 78 of
support block 70 either supports substrate 34 as shown or is spaced
a short distance from lower surface 43 of substrate 34.
Specifically, the differing porosities of the portions 72a-d of
support block 70 determine the passage of the flow of heated gas
and, as a result, the convective transfer of heat energy that
elevates the temperature of the substrate 34. Portions of support
block 70 having greater porosity will transfer or transmit heated
gas in a distributed flow with a flow rate effective to cause a
greater rise in temperature in corresponding portions of substrate
34. As illustrated in FIG. 7, for example, the porosity of portion
74b is larger than the porosity of portion 74d so that more heat
energy will be convectively transferred by the flow of heated gas
through portion 74b than the flow of heated gas through portion
74d. The porosity in each of portions 74a-d is effective to provide
the corresponding characteristic temperature of encapsulant
material 44 in each of the associated temperature zones T.sub.A,
T.sub.B, T.sub.C and T.sub.D.
[0053] With reference to FIG. 8 and in accordance with another
embodiment of the invention, a plurality of, for example, four heat
nozzles 84a-d are mounted to be able to deliver individual flows of
heated gas for convectively heating respective regions of die 32 to
provide heat energy for subsequent transfer from the die 32 to the
encapsulant material 44 (FIGS. 2, 3) moving into the gap 40 (FIG.
3) during an underfilling operation. Each of the heat nozzles 84a-d
is continuously supplied a flow of a heated gas, such as heated
air, from a respective heated gas source 86a-d. The respective
flows of heated gas from heat nozzles 84a-d impinge the upper
surface 39 in a manner effective to generate the temperature zones
T.sub.A, T.sub.B, T.sub.C and T.sub.D (FIG. 4). Substrate 34 is
supported on a support block 88, which may be heated to a uniform
temperature to supplement the heating provided by the heat nozzles
84a-d.
[0054] An outlet opening or mouth of each of the heat nozzles 84a-d
is oriented so that heated gas impinges a different region of the
upper surface 39 of die 32 in which each different portion is
correlated with one of the temperature zones T.sub.A, T.sub.B,
T.sub.C and T.sub.D. The amount of heat transferred by the heated
gas flow of each of heat nozzles 84a-d may be precisely controlled
by regulating one or more of the air pressure, the volumetric flow
rate, the duration of impingement, the gas temperature, the
distance from the mouth of each nozzle 84a-d to the upper surface
39, the lateral position of each nozzle 84a-d relative to upper
surface 39, the field of impingement, and the impingement angle of
the gas flow relative to a surface normal of upper surface 39. The
impingement angle, for example, may be any angle effective to
provide convective heat transfer and, generally ranges from about
25.degree. to about 750 with an impingement angle of about
45.degree. being typical, assuming other variable are fixed. In
other embodiments of the invention that convectively transfer heat
energy to the die 32, a single heat nozzle may be provided that has
a plurality of outlets spaced and dimensioned to direct multiple
parallel streams of air toward the upper surface 39 of die 32 in a
pattern that provides the respective temperature zones T.sub.A,
T.sub.B, T.sub.C and T.sub.D.
[0055] With reference to FIG. 9 and in accordance with another
embodiment of the invention, a radiation source, such as a laser
90, is utilized to transfer electromagnetic radiation, represented
diagrammatically by reference numeral 91, for heating respective
regions of die 32 to provide heat energy for subsequent transfer
from the die 32 to the encapsulant material 44 (FIGS. 2, 3) moving
into the gap 40 (FIG. 3) during an underfilling operation. Laser 90
is operative to emit radiation 91, typically having a wavelength or
range of wavelengths in at least one of the infrared, visible, or
ultraviolet portions of the electromagnetic spectrum.
[0056] Radiation 91 from laser 90 is reflected by a scanning mirror
92 to irradiate the upper surface 39 of die 32 through a mask 94
interposed in the optical path between the mirror 92 and the die
32. The scanning mirror 92 includes a reflective surface operative
to redirect the radiation 91. The scanning mirror 92 is
positionable to change the angular relationship between the optical
path of radiation 91 from laser 90 to the mirror 92 and the surface
normal of mirror 92 so that the beam of radiation 91 can be scanned
or rastered laterally in a pattern located within the perimeter of
the mask 94 and die 32. The mask 94 allows selective radiation of
the upper surface 39 of die 32 by blocking radiation in certain
opaque areas and transmitting radiation in other open areas. An
image corresponding to the open and opaque areas of the mask 94 is
projected onto the upper surface 39 of die 32. The scanning of
radiation 91 is programmed and the pattern of opaque and open areas
in mask 94 is controlled so as to transfer heat energy to die 32 in
a manner effective to provide temperature zones T.sub.A, T.sub.B,
T.sub.C and T.sub.D. The amount of heat transferred by radiation 91
can be controlled, aside from the selective transmission afforded
by the mask 94, by varying, among other variables, the scan pattern
and the scan rate. It is appreciated that the simplified optical
system shown in FIG. 9 may include other conventional optical
elements (not shown). In an alternative mask-less embodiment of the
invention, laser 90 may be digitally controlled by a conventional
digital imaging technique for moving or rastering the radiation 91
laterally across the upper surface 39 of die 32 with dwell times
appropriated to provide temperature zones T.sub.A, T.sub.B, T.sub.C
and T.sub.D.
[0057] With reference to FIG. 10 and in accordance with another
embodiment of the present invention, electromagnetic radiation,
represented diagrammatically by reference numeral 98, originating
from a radiation source, such as a lamp 100, is directed in an
optical path through an optical coupling, such as light guide 102,
to the die 32 and allowed to irradiate upper surface 39. The light
guide 102 had a light-emitting outlet suspended in a fixed position
at a given distance above upper surface 39 of die 32. The light
guide 102 also includes a focusing element 104 and a mask 106 that
allows selective radiation of the upper surface 39 of die 32 by
blocking radiation from lamp 100 in certain opaque areas and
transmitting radiation from lamp 100 in other open areas.
[0058] An image 108 of mask 106 is projected onto the upper surface
39 that is effective to radiatively transfer heat energy to provide
temperature zones T.sub.A, T.sub.B, T.sub.C and T.sub.D. In
addition to the selective transmission afforded by the mask 106,
the intensity of the radiation 98 from lamp 100 can varied for
controlling the transfer of heat energy. Typically, the intensity
of the radiation 98 is spatially-uniform before acted upon by the
mask 106 but the invention is not so limited. It is appreciated
that other radiation source arrangement, such as an array of lamps,
may be substituted for lamp 100 without departing from the spirit
and scope of the present invention. The wavelength of the
electromagnetic radiation 98 is typically in the infrared range of
the electromagnetic spectrum but the present invention is not so
limited in that a variety of radiation-emitting sources can be used
in the present invention. It is appreciated that the simplified
optical system shown in FIG. 10 may include other conventional
optical elements (not shown).
[0059] In an alternative embodiment and with reference to FIG. 11
in which like reference numerals refer to like features in FIG. 10,
a thermal transfer element 106a may be positioned directly on the
upper surface 39 of die 32 and exposed to a spatially uniform flux
of radiation 98a originating from the lamp 100. Thermal transfer
element 106a may be substituted for mask 106 relied upon in the
embodiment of the present invention described with regard to FIG.
10, as is illustrated in FIG. 11, or may replace the mask 94 relied
upon in the embodiment of the present invention described with
regard to FIG. 9.
[0060] Thermal transfer element 106a is operative for absorbing
radiation 98a in the uniformly-distributed image 108a originating
from lamp 100 and converting the radiative energy into heat energy
that is subsequently transferred by conduction from element 106a to
the die 32 and, thereafter, to the encapsulant material 44 moving
into gap 40. To that end, thermal transfer element 106a is formed
of a thermally-conductive material having a pattern of thicknesses
that varies so as to alter the path length for heat conduction.
Different portions of thermal transfer element 106a have a
thickness appropriate to retard thermal conduction so as to provide
temperature zones T.sub.A, T.sub.B, T.sub.C and T.sub.D. A
thermally-conductive material suitable for use in forming thermal
transfer element 106a is available commercially under the
SIL-PAD.RTM. tradename from the Bergquist Company (Chanhassen,
Mn).
[0061] While FIGS. 9, 10 and 11 depict non-uniform radiative heat
transfer to the die 32, it is contemplated by the invention that
heat may be radiated non-uniformly to the substrate 34 to provide
the same advantages.
[0062] With reference to FIG. 12, a stacked die package 110 is
illustrated that consists of a plurality of, for example, three
individual dies 112a-c mounted in a vertical arrangement to a
substrate 114. Present between dies 112a and 112b and between dies
112b and 112c are corresponding gaps 116a and 116b created by
electrical interconnections. Another gap 116c, created by
electrical connections, is present between die 112c and substrate
114. A bead of encapsulant material 118 adjacent to at least one
side edge of the stacked die package 110 and is subsequently moved
into the gaps 116a-c. According to the principles of the present
invention, the movement of the encapsulant material 118 into gaps
116a-c can be controlled by transferring heat energy to an upper
surface 120 of die 112a in an amount effective to create the
temperature zones T.sub.A, T.sub.B, T.sub.C and T.sub.D (FIG. 4).
Heat is transferred convectively using a plurality of heat nozzles
122a-d and in a manner described above with regard to FIG. 8.
However, it is appreciated that the transfer of heat may be
accomplished in accordance with any of the various specific
embodiments of the present invention, including those described
with regard to FIGS. 5-7 and 9-10, without departing from the
spirit and scope of the present invention.
[0063] With reference to FIGS. 13 and 14 and in accordance with
another aspect of the present invention, the temperatures of each
of the temperature zones T.sub.A, T.sub.B, T.sub.C and T.sub.D
(FIG. 4) may be dynamically varied or modulated during an
underfilling operation as a function of time or percentage of
completion. It is appreciated that the temperature variations are
tailored according to the need of an individual underfilling
operation and are not limited by the specific embodiments of the
present invention illustrated in FIGS. 13 and 14.
[0064] FIG. 13 depicts one specific embodiment in which the
temperature in each of the temperature zones T.sub.A, T.sub.B,
T.sub.C and T.sub.D is modulated as a function of time. It is
apparent that the temperature zones T.sub.A, T.sub.B, T.sub.C and
T.sub.D are initially at a uniform temperature. As the underfilling
operation initiates, heat is transferred to increase the
temperature of the underfill material in each of the corresponding
temperature zones T.sub.A, T.sub.B, T.sub.C and T.sub.D. Early in
the underfilling operation, the heat flux is greatest in
temperature zones T.sub.A and T.sub.C that correspond to regions in
the gap near the corners of the gap for which stagnation would
otherwise be observed. After the corners are substantially
underfilled as the underfilling operation progresses, the heat flux
in temperature zones T.sub.A and T.sub.C is reduced so that the
temperature of the encapsulant material drops in those zones. The
smallest heat flux is transferred in temperature zone T.sub.D in
which the temperature of the encapsulant material increases with a
modest ramp rate as the underfilling operation progresses toward
completion. The heat flux provided to temperature zone T.sub.B
increases with a relative large ramp rate so that, as the
underfilling operation nears completion, the temperature of the
encapsulant material in the corresponding region of the gap is
significantly hotter than in regions corresponding to others of the
zones.
[0065] FIG. 14 depicts another specific embodiment in which the
temperature in each of the temperature zones T.sub.A, T.sub.B,
T.sub.C and T.sub.D is varied as a function of percentage of
completion of the underfilling operation. It is apparent that the
temperature of each of the temperature zones T.sub.A, T.sub.B,
T.sub.C and T.sub.D, and the encapsulant material therein, is
ramped upwardly as the underfilling operation progresses toward
completion. To indicate the progress of the underfilling operation,
a sensor or sensors 130 (FIG. 10) is provided for detecting the
position of the wave front of the encapsulant material 44 (FIG. 2)
through the gap 40 between the die 32 and the substrate 34. The
sensor 130 is any suitable device known to persons of ordinary
skill in the art operative for detecting the position of the wave
front and may include capacitive sensors. The sensor 130 provides a
feedback control signal to a process controller (not shown)
regulating the transfer of heat energy to the encapsulant material
44.
[0066] The transfer of additional heat energy as the underfilling
operation proceeds, as depicted in FIGS. 13 and 14, allows the
temperature in each of the temperature zones T.sub.A, T.sub.B,
T.sub.C and T.sub.D to be increased without concern of gelling or
clogging to which the underfilling operation would otherwise be
susceptible if high heat transfers were applied during the entire
underfilling operation.
[0067] In use to perform an underfilling operation, a heat source
operative to provide a heat gradient for encapsulant material 44
entering gap 40 is provided as illustrated by one of the various
embodiments of the present invention shown in FIGS. 5-13. A bead of
encapsulant material 44 is dispensed from underfill dispenser 35
onto the substrate 34 adjacent to one or more side edges of the die
32. A pressure differential may be created across the bead of
encapsulant material 44 to assist capillary action for moving
material 44 into the gap 40. A heat source, such as those
illustrated in FIGS. 5-13, is employed to heat one of the die 32
and the substrate 34 to establish the temperature gradient of
temperature zones T.sub.A, T.sub.B, T.sub.C and T.sub.D. As the
encapsulant material 44 moves into the gap 40, material 44 in
regions of the gap 40 corresponding to each of the temperature
zones T.sub.A, T.sub.B, T.sub.C and T.sub.D absorbs heat energy
transferred by conduction from either the die 32 or the substrate,
depending upon which is heated by the heat source, and the
temperature rises proportionate to the amount of transferred heat
in each zone. Heating the encapsulant material 44 in regions of the
gap 40 to establish the characteristic temperature corresponding to
each of the temperature zones T.sub.A, T.sub.B, T.sub.C and T.sub.D
is effective to provide a more uniform wave front as the material
44 fills the gap 40. The uniformity of the wave front of
encapsulant material 44 afforded by the principles of the present
invention promotes the rapid completion of the underfilling
operation and also significantly reduces or prevents the occurrence
of voids.
[0068] While the present invention has been illustrated by a
description of various embodiments and while these embodiments have
been described in considerable detail, it is not the intention of
the applicants 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 method, and
illustrative example shown and described. For example, while the
various non-uniform heating methods are illustrated as being used
individually, it will be appreciated that certain of the heating
methods may be combined and used simultaneously to non-uniformly
transfer heat to the encapsulant material during an underfilling
operation. Accordingly, departures may be made from such details
without departing from the spirit or scope of applicants' general
inventive concept.
* * * * *