U.S. patent application number 14/887304 was filed with the patent office on 2016-10-06 for lead frame with deflecting tie bar for ic package.
This patent application is currently assigned to Freescale Semiconductor, Inc.. The applicant listed for this patent is FREESCALE SEMICONDUCTOR, INC.. Invention is credited to Zhigang Bai, You Ge, Meng Kong Lye, Zhijie Wang.
Application Number | 20160293526 14/887304 |
Document ID | / |
Family ID | 56896239 |
Filed Date | 2016-10-06 |
United States Patent
Application |
20160293526 |
Kind Code |
A1 |
Wang; Zhijie ; et
al. |
October 6, 2016 |
LEAD FRAME WITH DEFLECTING TIE BAR FOR IC PACKAGE
Abstract
A packaged integrated circuit (IC) device having a heatsink
mounted onto an IC die, itself mounted onto a die pad, is assembled
using a lead frame having tie bars that deflect during an
encapsulation phase of the device assembly, which enables the die
pad, the die, and the heatsink to move relative to the lead frame
support structure when compressive force is applied by the molding
tool. This movement results in negligible relative displacement
between the heatsink and the die during encapsulation, which
reduces the probability of physical damage to the die. Each tie bar
has a number of differently angled sections that enable it to
deflect when compressive force is applied to it.
Inventors: |
Wang; Zhijie; (Tianjin,
CN) ; Bai; Zhigang; (Tianjin, CA) ; Ge;
You; (Tianjin, CN) ; Lye; Meng Kong; (Shah
Alam, MY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
FREESCALE SEMICONDUCTOR, INC. |
AUSTIN |
TX |
US |
|
|
Assignee: |
Freescale Semiconductor,
Inc.
Austin
TX
|
Family ID: |
56896239 |
Appl. No.: |
14/887304 |
Filed: |
October 19, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 24/49 20130101;
H01L 2224/2919 20130101; H01L 2224/75981 20130101; H01L 24/75
20130101; H01L 2224/83447 20130101; H01L 24/32 20130101; H01L
2224/32245 20130101; H01L 24/48 20130101; H01L 23/49551 20130101;
H01L 2224/78981 20130101; H01L 23/3107 20130101; H01L 2224/49173
20130101; H01L 24/78 20130101; H01L 2224/48247 20130101; H01L 24/83
20130101; H01L 24/85 20130101; H01L 24/33 20130101; H01L 2224/85447
20130101; H01L 23/36 20130101; H01L 21/565 20130101; H01L
2224/73265 20130101; H01L 24/29 20130101; H01L 2924/00014 20130101;
H01L 23/4334 20130101; H01L 2224/33181 20130101; H01L 2924/00014
20130101; H01L 2224/45099 20130101; H01L 2924/00014 20130101; H01L
2224/45015 20130101; H01L 2924/207 20130101; H01L 2224/2919
20130101; H01L 2924/00014 20130101; H01L 2224/75981 20130101; H01L
2924/00014 20130101; H01L 2224/78981 20130101; H01L 2924/00014
20130101; H01L 2224/83447 20130101; H01L 2924/00014 20130101; H01L
2224/85447 20130101; H01L 2924/00014 20130101; H01L 2224/73265
20130101; H01L 2224/32245 20130101; H01L 2224/48247 20130101; H01L
2924/00 20130101 |
International
Class: |
H01L 23/495 20060101
H01L023/495; H01L 21/56 20060101 H01L021/56; H01L 23/367 20060101
H01L023/367; H01L 23/31 20060101 H01L023/31; H01L 23/00 20060101
H01L023/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 3, 2015 |
CN |
201510297163.5 |
Claims
1. An article of manufacture comprising a lead frame for a packaged
device, the lead frame comprising: a rectangular die pad; and a
plurality of tie bars connected to the die pad at proximal ends of
the tie bars, wherein, during assembly of the packaged device, (i)
distal ends of the tie bars are connected to a dam bar of the lead
frame and (ii) the tie bars are configured to deflect to enable the
die pad to move relative to the dam bar, wherein: the lead frame
comprises four tie bars extending from four corners of the die pad,
each tie bar is perpendicular from an end of the die pad to the dam
bar, the die pad defines a die-pad plane, and each tie bar has one
or more sections that are not parallel to the die-pad plane and
that enable the tie bars to deflect when the die pad moves relative
to the lead frame support structure during the assembly.
2. (canceled)
3. The article of claim 1, wherein each tie bar comprises: a
proximal section that extends from the proximal end of the tie bar
at a downward angle; an intermediate section connected to the
proximal section, wherein the intermediate section is substantially
parallel to and lower than the die-pad plane; and a distal section
that extends from the intermediate section at an upward angle to
the distal end of the tie bar.
4. The article of claim 3, wherein the distal end of each tie bar
lies above the die-pad plane such that, during the assembly, the
die pad is lower than the dam bar.
5. The article of claim 1, further comprising: a die mounted on the
die pad; bond wires electrically connecting the die to leads of the
lead frame; a heatsink mounted on the die; and molding compound
encapsulating the die, the bond wires, and the die pad.
6. The article of claim 5, wherein the tie bars deflect and the die
pad, the die, and the heatsink move relative to the dam bar during
an encapsulation phase of the assembly process, such that there is
negligible relative displacement between the heatsink and the die
during the encapsulation phase.
7. The article of claim 5, wherein the heatsink is exposed at an
outer surface of the packaged device.
8. The article of claim 5, wherein a section of each tie bar is
exposed at a different outer surface of the packaged device.
9. A method for assembling a packaged device, the method
comprising: mounting a die by a bottom surface thereof on a die pad
of a lead frame, wherein the die pad is rectangular and the lead
frame has a plurality of tie bars connected to the die pad at
proximal ends of the tie bars and to a dam bar at distal ends of
the tie bars, and wherein there are four tie bars extending from
four corners of the die pad; electrically connecting the die to
leads of the lead frame with bond wires; mounting a heatsink onto a
top surface of the die to provide a sub-assembly; and encapsulating
the sub-assembly with a molding compound, wherein the tie bars
deflect and the die pad moves relative to the dam bar during the
encapsulating step, wherein: the die pad defines a die-pad plane;
and each tie bar has one or more sections that are not parallel to
the die-pad plane and that enable the tie bars to deflect when the
die pad moves relative to the dam bar during the assembly.
10. (canceled)
11. The method of claim 9, wherein each tie bar comprises: a
proximal section that extends from the proximal end of the tie bar
at a downward angle; an intermediate section connected to the
proximal section, wherein the intermediate section is substantially
parallel to and lower than the die-pad plane; and a distal section
that extends from the intermediate section at an upward angle to
the distal end of the tie bar.
12. The method of claim 9, wherein the tie bars deflect and the die
pad, the die, and the heatsink move relative to the dam bar during
the encapsulating step such that there is negligible relative
displacement between the heatsink and the die during the
encapsulating step.
13. The method of claim 12, wherein the die pad is not directly
supported from below during the encapsulating step.
14. The method of claim 9, wherein, during the encapsulating step,
a molding tool applies compressive force to the sub-assembly to
inhibit the molding compound from covering an outer surface of the
heatsink, wherein the compressive force causes the tie bars to
deflect and the die pad to move relative to the dam bar.
15. The packaged device assembled using the method of claim 14.
Description
BACKGROUND
[0001] The present invention relates to packaged integrated circuit
(IC) devices and more particularly to packaged IC devices assembled
using lead frames.
[0002] In order to prevent internally generated heat from damaging
an IC die in a packaged IC device, the device may include a
heatsink that is mounted on top of the die, where the top side of
the heatsink is exposed at an outer surface of the packaged device.
During the molding or encapsulation phase of the device assembly
process, the molding tool applies force to the device sub-assembly
to prevent liquid molding compound from seeping between the top
side of the heatsink and the bottom surface of the top mold chase
of the molding tool and thereby covering some or all of the top
side of the heatsink with molding compound, which would inhibit the
packaged device's heat dissipation capabilities. Unfortunately, the
compressive force applied by the molding tool during the
encapsulation phase can result in physical damage, such as
cracking, to the IC die.
BRIEF DESCRIPTION OF THE DRAWINGS
[0003] Embodiments of the invention will become fully apparent from
the following detailed description, the appended claims, and the
accompanying drawings in which like reference numerals identify
similar or identical elements.
[0004] FIGS. 1A and 1B are, respectively, a top plan view and a
cross-sectional side view of a packaged IC device according to one
embodiment of the invention;
[0005] FIGS. 2A and 2B are, respectively, a top plan view and a
cross-sectional side view of a lead frame used to assemble the
packaged IC device of FIG. 1;
[0006] FIGS. 3A and 3B are, respectively, a top plan view and a
cross-sectional side view of the device sub-assembly after the IC
die of FIG. 1 has been mounted onto the die pad and electrically
connected to the lead structures of the lead frame of FIG. 2;
and
[0007] FIG. 4 is a cross-sectional side view of the sub-assembly of
FIG. 3 after the heatsink of FIG. 1 has been mounted onto the IC
die located between the top and bottom mold chases of a molding
tool used during the encapsulation phase of the assembly of the
packaged IC device of FIG. 1.
DETAILED DESCRIPTION
[0008] Detailed illustrative embodiments of the present invention
are disclosed herein. However, specific structural and functional
details disclosed herein are merely representative for purposes of
describing example embodiments of the present invention. The
present invention may be embodied in many alternate forms and
should not be construed as limited to only the embodiments set
forth herein. Further, the terminology used herein is for the
purpose of describing particular embodiments only and is not
intended to be limiting of example embodiments of the
invention.
[0009] As used herein, the singular forms "a," "an," and "the," are
intended to include the plural forms as well, unless the context
clearly indicates otherwise. It further will be understood that the
terms "comprises," "comprising," "includes," and/or "including,"
specify the presence of stated features, steps, or components, but
do not preclude the presence or addition of one or more other
features, steps, or components. It also should be noted that in
some alternative implementations, the functions/acts noted may
occur out of the order noted in the figures. For example, two
figures shown in succession may in fact be executed substantially
concurrently or may sometimes be executed in the reverse order,
depending upon the functionality/acts involved.
[0010] In one embodiment, the present invention is an article of
manufacture that comprises a lead frame for a package device. The
lead frame comprises a die pad and a plurality of tie bars
connected to the die pad at proximal ends of the tie bars, wherein,
during assembly of the packaged device, (i) distal ends of the tie
bars are connected to lead frame support structure and (ii) the tie
bars deflect to enable the die pad to move relative to the lead
frame support structure.
[0011] In another embodiment, the present invention provides a
method for assembling a packaged device. A die is mounted onto a
die pad of a lead frame further comprising a plurality of tie bars
connected to the die pad at proximal ends of the tie bars and to
lead frame support structure at distal ends of the tie bars. The
die is wire bonded to leads of the lead frame using bond wires. A
heatsink is mounted onto the die to provide a sub-assembly. An
encapsulation phase is performed on the sub-assembly, wherein the
tie bars deflect and the die pad moves relative to the lead frame
support structure during the encapsulation phase.
[0012] Referring now to FIGS. 1A and 1B, respectively, a top plan
view and a cross-sectional side view (along the cut line AA of FIG.
1A) of a packaged IC device 100 according to one embodiment of the
invention are shown. The packaged IC device 100 has as a top
surface 102, an opposing bottom surface 104, and a peripheral
surface 106. A heatsink 110 having a top surface 112, which may be
patterned with features 114, is mounted on an IC die 120, which is
itself mounted on a die pad 130. The device 100 also has a
plurality of tie bars 140 connected at proximal ends 141 to the die
pad 130. Each of the tie bars 140 has a proximal end 141, a
proximal section 143, an intermediate section 145, a distal section
147 and a distal end 149. A plurality of outer leads 150, which are
electrically connected to bond pads (not shown) on an active
surface of the die 120 (such as with bond wires), extend out from
the package body and allow the die 120 to be connected to other
electronic components or circuitry. A molding compound 160 covers
the die 120, the die pad 130, and the bond wires as well as
portions of the heatsink 110, the tie bars 140, and the outer leads
150.
[0013] The top surface 112 of the heatsink 110 preferably is
patterned with the features 114 in order to increase the total
effective surface area of the top surface 112 and thereby increase
the heat dissipation capabilities of the heatsink 110. Preferably
the entire top surface 112 of the heatsink 110 is exposed at the
top outer surface 102 of the packaged device 100. Note also that
the intermediate section 145 of each tie bar 140 is exposed at the
bottom outer surface 104 of the packaged device 100 and a distal
end 149 of each tie bar 140 is exposed at the peripheral surface
106 of the packaged device 100. These exposed portions 145, 149 of
the tie bar 140 provide additional heat dissipation paths for the
packaged device 100.
[0014] As shown in FIG. 1B, the proximal section 143 of each tie
bar connects the proximal end 141 (at which the tie bar 140 is
connected to the die pad 130) to the intermediate section 145, and
the distal section 147 connects the intermediate section 145 to the
distal end 149.
[0015] As shown in FIG. 1B, the proximal section 143 of each tie
bar connects the proximal end 141 (at which the tie bar 140 is
connected to the die pad 130) to the intermediate section 145, and
the distal section 147 connects the intermediate section 145 to the
distal end 149.
[0016] FIGS. 2-4 represent different stages in the assembly of the
packaged IC device 100 of FIG. 1.
[0017] FIGS. 2A and 2B are, respectively, a top plan view and a
cross-sectional side view (along the cut line AA of FIG. 2A) of a
metal (e.g., copper) lead frame 200 used to assemble the packaged
IC device 100 of FIG. 1. The lead frame 200 may be formed either
individually or in a strip as is known in the art. The lead frame
200 includes the die pad 130, four tie bars 140, and outer leads
150, all of which are shown in FIG. 1. In addition, the lead frame
200 includes inner leads 152 and a rectangular dam bar 154, which
holds all of the other elements of the lead frame 200 together
during device assembly. The inner and outer leads 152, 150 project
inwardly and outwardly from the dam bar 270. The inner leads 152
are electrically connected to the die 120 with the bond wires and
covered with the molding compound 160, while the outer leads 150
project outwardly from the molding compound 160. The dam bar 154 is
removed during the assembly process, in this case preferably by
punching.
[0018] The distal end 149 of each tie bar 140 connects the tie bar
140 to the dam bar 154. As shown in FIG. 2B, (i) the proximal
section 143 of each tie bar 140 extends at a downward angle from
the proximal end 141 to the intermediate section 145, (ii) the
intermediate section 145 is parallel to the plane defined by the
die pad 130, and (iii) the distal section 147 extends at an upward
angle from the intermediate section 145 to the distal end 149.
Because of the relative lengths and angles of these different
sections, the elevation level of the die pad 130 is below the
elevation level of the dam bar 154.
[0019] FIGS. 3A and 3B are, respectively, a top plan view and a
cross-sectional side view (along the cut line AA of FIG. 3A) of a
device sub-assembly 300 after the IC die 120 has been mounted and
attached to the die pad 130 (using a suitable adhesive) and
electrically connected with bond wires 380 to the inner leads 152.
FIG. 3A shows the bond wires 380, while in FIG. 3B, the bond wires
380 are omitted for simplicity. Note that, during the die-placement
and wire-bonding phases of the assembly process, the die pad 130 is
directly supported from below by assembly-phase tooling (not
shown).
[0020] FIG. 4 is a cross-sectional side view of the sub-assembly
300 of FIG. 3 after the heatsink 110 has been mounted on the IC die
120 using a suitable adhesive, such as a thermally conductive
adhesive. The resulting sub-assembly is located in a molding tool
between a top mold chase 410 and a bottom mold chase 420 so that an
encapsulation phase of the assembly may be performed. A status gap
430 is provided to indicate that the top and bottom mold chases 410
and 420 are properly seated. As shown in FIG. 4, a bottom surface
412 of the top mold chase 410 abuts the top surface 112 of the
heatsink 110. During the encapsulation phase, the liquid molding
compound 160 (not shown in FIG. 4) is injected into the mold formed
by the top and bottom mold chases 410 and 420 through injection
gaps (not shown).
[0021] In order to prevent the molding compound 160 from covering
the top surface 112 of the heatsink 110, sufficient compressive
force must be applied to the sub-assembly by the top and bottom
mold chases 410 and 420, and it is this compressive force that can
result in physical damage to the IC die 120.
[0022] However, according to the present invention, due to the
design of the tie bars 140, the tie bars 140 will deflect and allow
the die pad 130 (along with the die 120 and the heatsink 110) to
move (vertically down in the view of FIG. 4) with respect to the
lead frame dam bar 154 in response to the application of
compressive force by the top and bottom mold chases 410 and 420.
Because the die pad 130 is able to move vertically, there is
negligible relative displacement between the heatsink 110 and the
die 120 when the molding tool applies compressive force during the
encapsulation phase.
[0023] During the encapsulation phase, the intermediate sections
145 of the tie bars 140 are directly supported on the top surface
422 of the bottom mold chase 420, but the bottom mold chase 420
does not directly support the die pad 130, thereby enabling the die
pad 130 to move vertically with respect to the tie bar intermediate
sections 145 in response to the applied compressive force. In an
ideal lead frame design, the proximal sections 143 of the tie bars
140 will deflect enough to prevent the applied compressive force
from damaging the IC die 120, but still be stiff enough to ensure
that no molding compound 160 will seep onto the top surface 112 of
the heatsink 110. After the liquid molding compound 160 is injected
into the mold, it is cured, e.g., by heating, to encapsulate the
sub-assembly.
[0024] Those skilled in the art will understand that the packaged
IC device 100 of FIG. 1 is typically assembled in parallel with
multiple other instances of the device 100 using a strip of the
lead frames 200 of FIG. 2, where the lead frame dam bar 154
functions as the support structure for adjacent lead frames 200 in
the strip. In such case, after the encapsulation phase of FIG. 4,
the resulting encapsulated sub-assemblies are separated by punching
away the dam bar, thereby electrically isolating the outer leads
150 and the die pads 130 of the packaged IC devices 100. The outer
leads 150 may then be trimmed and formed to complete the assembly
of each instance of the package IC device 100 of FIG. 1.
[0025] Note that the exposed top portion of the heatsink 110 is
wider than the bottom portion that mates with the die 120. This
enables greater heat dissipation while still providing access to
the top surface of the die 120 for the bond wires 380.
[0026] Although the invention has been described in the context of
the lead frame 200 of FIG. 2 having four mutually parallel tie bars
140, other embodiments of the invention may have other numbers of
tie bars and/or tie bars that are not all mutually parallel. For
example, the tie bars could be four diagonal tie bars that connect
the four corners of a rectangular die pad to the four corners of a
rectangular support structure.
[0027] Although the invention has been described in the context of
the lead frame 200 of FIG. 2 in which the die pad 130 elevation
level is below the dam bar 154 elevation level, in other
embodiments of the invention, the die pad elevation level is the
same as or above the support structure elevation level.
[0028] Although the invention has been described in the context of
the lead frame 200 of FIG. 2 in which each tie bar 140 has the
proximal, intermediate, and distal sections 143, 145, and 147,
those skilled in the art will understand that there are other
geometry designs for tie bars that will deflect sufficiently during
the encapsulation phase of device assembly to prevent or at least
inhibit physical damage to the IC die.
[0029] Although the invention has been described in the context of
the lead frame 200 which has resilient tie bars 140 that may act
like springs, in general, it is sufficient for tie bars of the
invention to deflect in one direction during the encapsulation
phase. It is not necessary for the tie bars to be able to resume
their original shape after the compressive force of the
encapsulation phase is removed.
[0030] Although the invention has been described in the context of
the packaged IC device 100 of FIG. 1 having the single IC die 120,
other embodiments of the invention may have multiple IC dies
mounted either side-by-side and/or one top of one another and/or
additional elements such as, without limitation, additional
heatsinks and/or interposers.
[0031] A lead frame is a collection of metal leads and possibly
other elements (e.g., power bars, die pads also known as die
paddles and die flags) that is used in semiconductor packaging for
assembling one or more integrated circuit (IC) dies into a single
packaged IC device. Prior to assembly into a packaged device, a
lead frame may have support structures (e.g., a dam bar and tie
bars) that keep those elements in place. During the assembly
process, the support structures may be removed. As used herein, the
term "lead frame" may be used to refer to the collection of
elements before assembly or after assembly, regardless of the
presence or absence of those support structures.
[0032] It will be further understood that various changes in the
details, materials, and arrangements of the parts which have been
described and illustrated in order to explain embodiments of this
invention may be made by those skilled in the art without departing
from embodiments of the invention encompassed by the following
claims.
[0033] In this specification including any claims, the term "each"
may be used to refer to one or more specified characteristics of a
plurality of previously recited elements or steps. When used with
the open-ended term "comprising," the recitation of the term "each"
does not exclude additional, unrecited elements or steps. Thus, it
will be understood that an article of manufacture may have
additional, unrecited elements and a method may have additional,
unrecited steps, where the additional, unrecited elements or steps
do not have the one or more specified characteristics.
[0034] Reference herein to "one embodiment" or "an embodiment"
means that a particular feature, structure, or characteristic
described in connection with the embodiment can be included in at
least one embodiment of the invention. The appearances of the
phrase "in one embodiment" in various places in the specification
are not necessarily all referring to the same embodiment, nor are
separate or alternative embodiments necessarily mutually exclusive
of other embodiments. The same applies to the term
"implementation."
[0035] The embodiments covered by the claims in this application
are limited to embodiments that (1) are enabled by this
specification and (2) correspond to statutory subject matter.
Non-enabled embodiments and embodiments that correspond to
non-statutory subject matter are explicitly disclaimed even if they
fall within the scope of the claims.
* * * * *