 |
Description  |
|
|
TECHNICAL FIELD OF THE INVENTION
The invention relates to the packaging of integrated circuit (IC)
semiconductor devices, and more particularly to injection transfer
molding.
BACKGROUND OF THE INVENTION
Present plastic packaging techniques involve molding a plastic package
"body" around a semiconductor die. Prior to molding, the die is attached
to a lead frame, or the like, having a plurality of leads. The leads have
inner portions within the package body, and outer portions exiting the
package body for connecting to external circuits, such as by conductors on
a printed circuit board. Various forms of plastic packaged ICs are known,
including DIP (Dual In-line Package), PQFP (Plastic Quad Flat Pack) and
PLCC (Plastic Leaded Chip Carrier).
Microelectronics Packaging Handbook, edited by Tummala and Rymaszewski,
published 1988 by Van Nostrand Reinhold, discloses transfer molding at
pages 578-591. Generally, as described therein, transfer molding has been
and still is the standard workhorse of the electronic packaging industry.
It is an automated version of compression molding in which a preform of
plastic compound is forced from a pot into a hot mold cavity. The molds
are steel, and have top and bottom "halves". Each mold half has a cavity
defining the size, shape and surface finish of a plastic IC package.
"Gates" are small openings into the cavities where the molding compound is
injected. "Vents" are other small openings allowing air to escape the
cavity when molding compound is injected.
The molding compound is typically a polymer that is a solid at room
temperature. It is melted prior to transfer to the cavity. The viscosity
of the melted compound is generally relatively high, and the transfer
occurs at elevated temperatures and pressures--typically at 900 psi
(pounds per square inch) and 175 degrees centigrade.
One problem in the molding process is minimizing "wire sweep". Wire sweep
refers to the displacement and/or distortion of wires attaching the
semiconductor die to the lead frame (or the like) as the molding compound
is injected into the cavity. As noted in Microelectronics Packaging
Handbook, gates are usually placed in the bottom mold half, so that the
"jet" of molding compound is directed away from the bond wires.
Another approach to minimizing wire sweep is to provide gates in both the
top and bottom mold halves, as disclosed for example in copending,
commonly-owned U.S. Patent Application No. 619,107, filed 11/27/90 by
Schneider and Fehr.
FIGS. 1A and 1B show a typical plastic-packaged semiconductor device 100 of
the prior art. A semiconductor die 102 is connected, such as by a number
of bond wires 104, to inner ends of a corresponding number of leads 106.
Other techniques for connecting the die to the inner ends of the leads are
known. A plastic body 108 is formed about the die 102 and the inner ends
of the leads, by any of a number of known molding processes. The exposed
outer portions of the leads 106, exterior the body 108, are bent
downwardly (indicated by the dashed line "C") and outwardly (indicated by
the dashed line "D") to form what is commonly termed a "gull wing"
configuration. Each lead 106 has a width (w) on the order of eight
thousandths of an inch (0.2 mm), and the spacing (s) between adjacent
leads 106 is typically on the order of twenty thousandths of an inch (0.5
mm).
The wires used to interconnect the die to the leadframe are typically made
of pure gold having a diameter on the order of 0.0013 inch. The length of
the bond wires can extend in length up to about 0.160 inch. In each
device, these bond wires may be spaced apart at separations of only 0.005
inch.
FIG. 1B shows a lead form (or frame) 120 having a plurality of leads 106.
The lead frame is formed from a conductive foil having a thickness (t) on
the order of a few thousandths of an inch (e.g. 0.004-0.006 inch). The
material for the leads 106 is typically copper, or "Alloy 42". As shown,
the leads 106 terminate in an outer square ring portion 122 of the lead
frame 120, from which the completed (packaged) device is ultimately
excised, as indicated by the dashed line "A". Of particular note in FIG.
1B are "dambars" 124 bridging adjacent leads 106 at a position indicated
by the dashed line "B" (closely adjacent or immediately exterior to the
body 108. The dambars 124 are formed from the conductive material forming
the leads 106, and hence are of the same thickness as the leads 106. These
dambars 124 aid in maintaining alignment between the inner ends of the
leads, although a die attach pad (not shown) formed from the foil is
typically employed and will serve the same purpose. More importantly,
however, the dambars 124 are critical in the molding process, discussed
hereinbelow. (Since the leads 106 create a gap between the clamshell
halves of the mold, the dambars 124 prevent plastic from "flashing"
between the leads 106 exterior the body 108. After the die is packaged in
the plastic body, the dambars 124 are excised, and any residual plastic
flash between the outer portions of leads 106 is cleaned out in a
"dejunking" step. )
FIG. 1C shows, generally, a tape-mounted semiconductor device assembly 10,
as described in copending, commonly-owned U.S. Patent Application No.
454,752, entitled HEAT SINK FOR SEMICONDUCTOR DEVICE ASSEMBLY, filed Dec.
19, 1989 by Long, Schneider and Patil. The semiconductor device assembly
10 includes an upper, segmented plastic film layer 14, formed of segments
14A, 14B, 14C and 14D), a lower plastic film layer 16, metallic leads 18
sandwiched between the two plastic layers 14 and 16, a metallic
(preferably copper) die attach pad 20 supported between the two plastic
layers 14 and 16, a semiconductor device 22 mounted on the die attach pad
20 and bond leads 24 connecting the semiconductor device 22 to the leads
18. In lieu of employing bond wires 24, conductive bumps may be employed
to provide a conductive path from the device 22 to the leads 18 in a tape
automated bonding (TAB) process.
The upper plastic layer 14 does not form a continuous surface, but rather
is segmented to include an inner ring portion 14A, one or more
intermediate ring portions 14B and 14C disposed outside of the inner ring
portion, and an exterior ring portion 14D disposed outside of the
intermediate ring portions. The upper plastic layer 14 is formed of a
plastic tape, such as KAPTON, and forms a thin, insulating supportive
structure for the leads 18. The inside periphery of the inner ring portion
14A supports the outside periphery of the die attach pad 20, and the
outside periphery of the inner ring portion 14A supports the innermost
ends of the leads 18, in essence forming a "bridge" between the die attach
pad and the leads. A layer-like quantity of silicone gel 28, such as Dow
Corning Q1-4939, having a 1 to 10 mixing ratio of curing agent to base,
encapsulates the leads 24. A body 30, formed of molding compound
(described hereinafter), is formed around the device 22, leaving outer
portions of the leads 18 exposed, exterior the body. The silicone gel 28
acts as a moisture barrier and a stress relief for the leads 24 during
body molding, as well as prevents molding compound from contacting the
semiconductor die. Surface tension between the silicone gel and the leads
24 keeps the silicone gel in place around the leads during assembly of the
semiconductor device assembly. The lower plastic layer 16 covers the
bottom of the die attach pad 20, and extends generally over the entire
area described by the intermediate ring portion 14C, on the opposite side
of the leads 18 and die attach pad 20. The lower plastic layer 16 is
formed of a plastic tape material, such as KAPTON.
A "surrogate" lead frame (edge ring) 12 is provided for handling the
semiconductor device assembly during manufacture thereof, and shorts the
outer ends of the leads 18 to facilitate electroplating. After molding the
body about the device, the semiconductor device assembly is excised from
the lead frame 12 and exterior ring portion 14D, neither of which properly
form any part of the ultimate semiconductor device assembly 10.
FIGS. 2A, 2B and 2C show transfer molding apparatus of the prior art.
Transfer molding is an automated version of compression molding in which
hot, liquid molding compound is forced from a reservoir, or pot, into mold
cavities.
Molding compounds are typically resins, such as advanced B-stage compounds.
In general purpose applications, wood-flour-filled phenolics, for
instance, are fairly popular due to their excellent moldability and low
cost. As powders and granules, they are also easily shaped into pellets by
automatic preformers. The main drawback with phenolics is their limited
colorability. When coloring is a major design factor, melamine, polyester,
or urea are usually selected because there is a wider selection of shades
and colors. For electronic packaging, the preferred resin is epoxy.
The mold set 200 has two halves, a top half 202 and a bottom half 204, each
of which is provided with a recess 206 and 208, respectively. The recesses
face each other when the mold is closed, forming a cavity 210 defining the
size, shape and surface finish of a molded body (e.g., 108 of FIG. 1A). As
shown in FIG. 2C, the mold halves close around the lead frame (e.g., 122
of FIG. 1B; or surrogate lead frame 12 of FIG. 1C), that close about open
to receive lead frames and are closed (as shown), so that the
semiconductor device (e.g., 102 of FIG. 1B) is contained within the cavity
210.
The bottom half 204 of the mold set is typically provided with a primary
"runner" 212 receiving molten molding compound from a pot 214. One or more
secondary runners 214 extend from the primary runner 212 to the cavity
210, in the bottom half 204 of the mold set. At the interface between the
secondary runner 214 and the cavity 208 is a "gate" 216. "Gates" are small
openings into the cavity 210 where the liquid molding compound is
injected, and are normally found only in the bottom mold half ("chase")
under the plane of the chip and the wires to minimize wire sweep. Typical
gate dimensions are 60-100 mils wide (at the cavity interface) by 20-30
mils deep (from the secondary runner to the cavity). Air vent slots (not
shown) are located opposite each gate to prevent partial fill and voids in
the finished part.
In the case of a molding press provided with multiple mold sets (hence,
multiple cavities), the layout of the runner system is balanced to provide
for an even distribution of molding compound to each cavity. The object is
to fill each cavity with compound of uniform density so that parts located
next to a pressurized input (not shown) will have identical properties to
those located at the other locations along the primary runner.
It has been noted that the flow of molten compound (plastic) into the
bottom mold half causes not only wire sweep, but also can cause distortion
of the die attach pad (e.g., 20, FIG. IC). Large die attach pads are
forced upward in the process, destroying wire bonds or disorienting the
die. The result is often reject parts, which represent waste and decreased
throughput. Further, as lead count increases (lead pitch gets finer) each
lead becomes increasingly more delicate, exacerbating the aforementioned
problems.
These problems are particularly evident when the semiconductor device (die)
is mounted to a tape, in what is termed a "tape automated bonding" (TAB)
process (See e.g., FIG. 1C). Because the supporting TAB structure (e.g.,
14, 16, 18, FIG. 1C) is flimsier than lead frame counterparts (e.g., 122,
FIG. 1B), extra care in handling during the molding process is required.
Proposed solutions include positioning delicate metal inserts (not shown)
within the mold to aid in supporting the tape, or modifying the mold (not
shown) to clamp down on only a predetermined portion of the tape. In the
former, an additional time lag is introduced into the molding cycle. In
the latter, accurate tape indexing and operator monitoring would be
required. Another option is to use a stronger tape, such as TapePac
(trademark of National Semiconductor). However, the TapePac tape has a
relatively low number of leads, and presents a restrictive sourcing
requirement.
U.S. Pat. No. 4,987,473 discloses a leadframe (50) wherein one series of
lead tips (50a, 50c, 50e..) are bent upwards, and another series of lead
tips (50b, 50d, 50f..) are bent downwards to form a leadframe with
multi-tier leads, for the purpose of allowing denser packaging. This
patent is cited as an example of lead frames.
U.S. Pat. No. 4,994,895 discloses a hybrid integrated circuit package. By
way of example, a circuit substrate (2) is mounted to a lead frame (9)
having upwardly deformed inner leads (31) and lowered stages (100), and is
directed to relieving stress on the substrate.
U.S. Pat. No. 4,556,896 discloses a lead frame structure. IN FIG. 12, for
example, we see two mold halves (60 and 70), and an opening (50) formed in
a portion (30) of the lead frame straddling the lateral edge of the cavity
(61). In other words, part of the lead frame structure is modified to act
as a gate.
U.S. Pat. No. 4,788,583 discloses a semiconductor element (1) mounted on a
stage (2; also known as "die attach pad"), and stage bars (3 and 4; also
known as "tie bars") extending from both sides of the stage for supporting
the stage during the production process. In FIG. 2A therein, the stage
bars (15 and 16) are shortened, and end inside the resin package (17). A
two step molding process is disclosed. As shown in FIG. 5 therein, after a
first, inner package molding process, portions of the stage bars (26 and
27) in the vicinity of the frames (28 and 29) are cut off. Thereafter, an
outer resin package portion (19) is formed.
U.S. Pat. No. 5,018,003 discloses a lead frame (8), wherein an outer frame
portion (2) is disposed in a gate portion of mold halves for the purpose
of splitting the mold compound flow evenly into the top and bottom
portions of the mold. Evidently mold design would be affected by whether
or not the disclosed lead frame were to be used.
U.S. Pat. No. 4,043,027 discloses a process for encapsulating electronic
components, showing bottom gating.
U.S. Pat. No. 4,894,704 discloses a lead frame for resin molding. A
projection or extended portion (16) is formed at the edge of the inner
lead part (15) nearest to a gate part (13). A resin flow passes through
the mold and collides with the projection near an inlet of the cavity. The
air at corners near the cavity inlet is purged away and, consequently,
pressure transfer efficiency at the corners is enhanced. Notably, the
projections (16) are coplanar with the lead frame.
While the patents cited above show various modifications to a lead frame,
they neither disclose nor suggest the lead frame structure of the present
invention.
DISCLOSURE OF THE INVENTION
It is therefore an object of the present invention to provide a technique
for molding 1C packages that alleviates the problems of wire sweep and die
attach pad distortion.
It is a further object of the present invention to provide a process for
molding 1C packages that decreases waste and increases throughput.
It is further object of the present invention to provide a molding
technique for molding package bodies about tape mounted (TAB) devices,
which avoids the use of metal inserts, modified mold clamping
configurations, or the use of a particular mold design.
It is a further object of the present invention to provide a mold set for
molding plastic 1C packages without exerting unbalanced forces on the die
attach pad and/or die.
It is further object of the present invention to provide a plastic 1C
package formed by the disclosed techniques.
According to the invention, a plastic-packaged semiconductor device has a
die mounted to a substrate, such as a lead frame or a tape carrier. The
substrate has a plurality of leads, and the die is connected, such as by
bond wires, to the leads. Preferably, the die is mounted to a die attach
pad (paddle).
The die attach pad is supported by tie bars to an outer ring of the lead
frame.
In one embodiment of the invention, one of the four corner tie bars, namely
the tie bar adjacent a gate, is bent (kinked) upward, to deflect molding
compound upward in the mold cavity, thereby alleviating wire wash. (The
bond wires are atop the die).
In another embodiment, the tiebar is cut (rather than kinked), and the
remaining "stub" is bent upward to act as the deflector.
In both embodiments, the tiebar is modified to be out-of-plane with the
remaining elements of the lead frame, and the modifications are preferably
performed as closely to the ultimate mold gate location as possible,
without impairing the volume of compound flow through the gate. By
extending upward, in front of the gate, the tiebar modification acts as a
baffle to deflect a jet of incoming molding compound from directly
impacting the bond wires.
A molded plastic body is formed about the die, and inner portions of the
leads. Outer portions of the leads exit the body, for connecting the
completed device to other circuits.
The body is molded in a press, by placing the mounted and connected die
within a cavity created by recesses in two mold halves. One or both of the
mold halves are provided with a "runner" receiving liquid molding compound
from a reservoir, and at least one "gate" leading from the runner to the
cavity.
A completed, packaged semiconductor device with increased reliability is
thereby created.
The invention provides a technique for dispersing the force of the stream
of molten molding compound plastic from directly impinging onto bond wires
during the transfer molding process, without modifications to existing
molds.
The modified tiebar of the present invention prevents molding compound from
moving bond wires connecting semiconductor dies to leadframes.
Modifications such as those shown are low cost, and have no adverse impact
on device reliability. Furthermore, longer bond wires can be used, which
affords a degree of design flexibility hitherto unknown.
Top gating (versus bottom gating) affords certain advantages, but results
in certain disadvantages. Without the tiebar modification of the present
invention, a top-gated mold would result in the top section being filled
at a faster rate than the bottom section. Also, the molding compound would
flow directly onto the wires, causing sagging and bending. The resulting
loss of yield due to these problems would be significant. By modifying the
tiebar as shown herein, the cavity can be filled through the top gate
without damage to the wires, by ensuring a balanced fill rate between the
top and bottom sections of the package. This will improve yield.
Other objects, features and advantages of the invention will become
apparent in light of the following description thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a perspective view of a completely formed plastic-packaged
semiconductor device of the prior art.
FIG. 1B is a plan view, partially cutaway, of the semiconductor device of
FIG. 1A, at an intermediate step in the manufacture thereof, showing a
lead frame.
FIG. 1C is a partial cross-sectional view of a tape-mounted semiconductor
device, similar in various respects to the devices shown in FIGS. 1A and
1B.
FIG. 2A is schematic diagram of a transfer mold press of the prior art.
FIG. 2B is a cross-sectional detail view of a portion of a prior art bottom
mold for the transfer mold press of FIG. 2A, taken on a line A--A through
FIG. 2A.
FIG. 2C is a cross-sectional view of a prior art mold set, with
semiconductor device contained within its cavity, for the transfer mold
press of FIG. 2A, taken on a line B--B through FIG. 2A.
FIG. 3 is a partial plan view of a corner of an unmodified lead frame.
FIG. 4 is a cross-sectional view of a lead frame similar to that of FIG. 3,
in a mold set. The inflow of molding compound is illustrated.
FIG. 5 is a cross-sectional view of a lead frame with a modified tie bar,
according to the present invention, in a mold set. The inflow of molding
compound is illustrated.
FIG. 6 is a cross-sectional view of a lead frame with modified tie bar,
according to another embodiment of the present invention, in a mold set.
FIG. 7 is a cross-sectional view of a lead frame with modified tie bar,
according to yet another embodiment of the present invention, in a mold
set.
DETAILED DESCRIPTION OF THE INVENTION
FIGS. 1A and 1B illustrate a plastic-packaged semiconductor device 100 of
the prior art, and are discussed hereinabove. The techniques for creating
such packages are well known, including techniques for mounting a die to a
lead frame, molding a plastic body about the die and inner portions of
leads, excising the lead frame, and "dejunking" (removing) superfluous
plastic "flash" from between the leads.
FIG. 1C illustrates a packaging technique for tape mounted semiconductor
devices, and is discussed above.
FIGS. 2A, 2B and 2C illustrate the transfer molding process for forming
bodies around semiconductor devices with a mold set, and are discussed
above.
FIG. 3 shows a standard leadframe 300, a major component of which is a
patterned, planar metal layer (foil) 302. The layer 302 includes a square
die paddle (or die attach pad) 304, an outer generally square ring 306, a
tie bar 308 extending from a corner of the paddle 304 to the outer ring
306, and a plurality of conductive leads 310 (only a few of many shown,
for clarity) extending from adjacent the paddle 304 to a support ring 312
interior the outer ring 306. A semiconductor die 314 is mounted to the
paddle, and bond wires 316 connect circuit elements on the top surface of
the die to the inner ends of the leads 310. For additional support of the
delicate leads 310, a square plastic film layer ring 318 may be disposed
inward of the support ring 312. Additionally, the underside of the lead
frame may be provided with another plastic film layer, for additional
support of the leads.
Ultimately, the leadframe with the die mounted and the bond wires attached
is placed between the two halves of a mold cavity, so as to be suspended
in the cavity, and molding compound is injected through a gate into the
cavity.
Typically, a single gate is located at a single corner of the cavity, and
vents (not shown) are located at the other three corners. In FIG. 3, the
location of the gate is indicated by the label "G", along the tiebar 308
and just inside of the support ring 312 and outside of the plastic ring
318. (The support ring will be excised after molding, since it shorts the
leads together.) Gates are necessarily small, on the order of 0.080 by
0.030 inches, limited by the space available between leads and to allow
for easy breaking off of flash at the termination of the molding process.
FIG. 4 illustrates the flow of molding compound, an a transfer molding
process with a "standard" leadframe arrangement. In the example shown, the
die attach pad (304; FIG. 3) is depressed below the plane of the lead
frame. A mold set 400 comprises a top mold half 402 and a bottom mold half
404, together which form a cavity 406. A leadframe 300', similar to the
leadframe 300 of FIG. 3, is supported between the mold halves 402 and 404,
in a conventional manner. The top mold half 402 is provided with a runner
408, and a gate 410 at the interface of the runner 408 and the cavity 406.
A tiebar 308' supports the die paddle 304' in a manner similar to that
shown in FIG. 3, and exits the cavity 406 at the gate 410.
The flow of molten molding compound is illustrated by arrows
".fwdarw..fwdarw..fwdarw.". As is evident, with the molding compound
entering the cavity directly atop the lead frame, there is a direct path
to the bond wires 316. As discussed hereinbefore, the pressurized "jet" of
molding compound, entering directly atop the lead frame and inplane
therewith, will cause wire wash. This effect is somewhat exacerbated when
the die paddle is depressed (as shown).
In transferring the molding compound into the mold cavity, the pressure
results in a "jet" of compound impinging directly onto the bond wires.
This causes the wires to move, and can cause some of the wires to touch
adjacent wires--resulting in shorts and reject devices. This problem is
exacerbated when the mold gate is on the same side (top half) of the
cavity as the die and wires.
FIG. 5 illustrates an embodiment 500 of the present invention wherein the
tiebar adjacent the gate is kinked to act as a baffle in the stream of
incoming molding compound. The tiebar is designated "502". Remaining
elements are similar to those shown in FIG. 4.
The tiebar 502 adjacent the gate 410 is modified by bending (kinking) so as
to form a baffle within the cavity 406 directly in front of and closely
adjacent to the gate 410. The tiebar is bent upward, out of the plane of
the lead frame, approximately 0.050 inches, and is spaced inward from the
gate (wall of the cavity) approximately 0.050 inches.
As illustrated, the modified tiebar 502 deflects the jet of molding
compound ".fwdarw..fwdarw..fwdarw." away from the bond wires 316, thereby
alleviating the effect of wire wash.
The kinked portion of the tiebar, located just inside of the mold gate
serves as a baffle in the path of the flow of molding compound,
effectively slowing down the velocity of the molding compound during the
transfer process. The wires are thus shielded from the high pressure jet
of molding compound. The shaping of the tiebar is done with a depressing
tool (die) during the manufacture of the leadframe, in a manner akin to
that which is commonly used to depress the die paddle. The modified
(kinked) section of tiebar also can be shaped such that the molding
compound is forced to flow onto the lower (non-gated) section of the
cavity. (In the example of FIGS. 4 and 5, the top mold half is gated.)
FIG. 6 illustrates an alternate embodiment 600 of the invention wherein the
tiebar 502' adjacent the gate 410 is cut and bent to act as a baffle in
the stream of incoming molding compound. In this example, the tiebar 502'
is cut at a position corresponding to the wall of the cavity 406, and the
remaining inboard portion of the tiebar 502' is bent upwards to form a
baffle in front of the gate 410. This arrangement will deflect the jet of
incoming molding compound as described with the embodiment of FIG. 5. The
arrows ".fwdarw..fwdarw..fwdarw." are omitted in this view, for clarity.
FIG. 7 illustrates yet another embodiment 700 of the invention wherein the
tiebar 502" adjacent the gate 410 is cut and bent to act as a baffle in
the stream of incoming molding compound. In this example, the tiebar 502"
is cut closer to the die paddle 304', and is bent upward at a position
corresponding to the wall of the cavity 406, thereby forming a baffle in
front of the gate 410. This arrangement will deflect the jet of incoming
molding compound as described with the embodiment of FIG. 5. The arrows
".fwdarw..fwdarw..fwdarw." are omitted in this view, for clarity.
Alternative shapes for the tiebar modification, other than those
specifically illustrated in FIGS. 5-7 are contemplated, so long as the
modification produces an out-of-plane obstacle directly in front of the
gate to prevent direct impingement of the jet of molding compound onto the
bond wires.
FIG. 7 illustrates another embodiment of the invention, similar to that
shown in FIG. 6, wherein the tiebar adjacent the gate is cur and bent the
other way to act as a baffle in the stream of incoming molding compound.
* * * * *
|
|
 |
|
 |
Description |
|
