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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention pertains to a "leads over chip" (LOC) semiconductor die
assembly and, more particularly, to a method and apparatus for reducing
the stress resulting from lodging of filler particles present in plastic
encapsulants between the undersides of the lead frame leads and the active
surface of the die and improved lead locking of the leads in position over
a portion of the active surface of a semiconductor die assembly.
2. State of the Art
The use of LOC semiconductor die assemblies has become relatively common in
the industry in recent years. This style or configuration of semiconductor
device replaces a "traditional" lead frame with a central, integral
support (commonly called a die-attach tab, paddle, or island) to which the
back surface of a semiconductor die is secured, with a lead frame
arrangement wherein the dedicated die-attach support is eliminated and at
least some of the leads extend over the active surface of the die. The die
is then adhered to the lead extensions with an adhesive dielectric layer
of some sort disposed between the undersides of the lead extensions and
the die. Early examples of LOC assemblies are illustrated in U.S. Pat. No.
4,862,245 to Pashby et al. and U.S. Pat. No. 4,984,059 to Kubota et al.
More recent examples of the implementation of LOC technology are disclosed
in U.S. Pat. Nos. 5,184,208; 5,252,853; 5,286,679; 5,304,842; and
5,461,255. In instances known to the inventors, LOC assemblies employ
large quantities or horizontal cross-sectional areas of adhesive to
enhance physical support of the die for handling.
Traditional lead frame die assemblies using a die-attach tab place the
inner ends of the lead frame leads in close lateral proximity to the
periphery of the active die surface where the bond pads are located, wire
bonds then being formed between the lead ends and the bond pads. LOC die
assemblies, by their extension of inner lead ends over the die, permit
physical support of the die from the leads themselves, permit more diverse
(including centralized) placement of the bond pads on the active surface,
and permit the use of the leads for heat transfer from the die. However,
use of LOC die assemblies in combination with plastic packaging of the LOC
die assembly has demonstrated some shortcomings of LOC technology as
presently practiced in the art.
One of the shortcomings of the prior art LOC semiconductor die assemblies
is that the tape used to bond to the lead fingers of the lead frame does
not adequately lock the lead fingers in position for the wire bonding
process. At times, the adhesive on the tape is not strong enough to fix or
lock the lead fingers in position for wire bonding as the lead fingers
pull away from the tape before wire bonding. Alternately, the lead fingers
will pull away from the tape after wire bonding of the semiconductor die
but before encapsulation of the semiconductor die and frame, either
causing shorts between adjacent wire bonds or the wire bonds to pull loose
from either the bond pads on the die or lead fingers of the frame. While
wire bonding fixtures may be used to attempt to overcome these problems,
the fixtures and their use add cost to the fabrication process.
Additionally, if large amounts of tape are used to fix the lead fingers in
place, the reliability performance of the packaged device will be affected
as tape absorbs moisture from the surrounding environment, causing
problems during encapsulation and potential corrosion problems.
After wire bonding the semiconductor die to the lead fingers of the lead
frame, forming an assembly, the most common manner of forming a plastic
package about a die assembly is molding and, more specifically, transfer
molding. In this process (and with specific reference to LOC die
assemblies), a semiconductor die is suspended by its active surface from
the underside of inner lead extensions of a lead frame (typically Cu or
Alloy 42) by a tape, screen print or spin-on dielectric adhesive layer.
The bond pads of the die and the inner lead ends of the frame are then
electrically connected by wire bonds (typically Au, although Al and other
metal alloy wires have also been employed) by means known in the art. The
resulting LOC die assembly, which may comprise the framework of a
dual-in-line package (DIP), zig-zag in-line package (ZIP), small outline
j-lead package (SOJ), quad flat pack (QFP), plastic leaded chip carrier
(PLCC), surface mount device (SMD) or other plastic package configuration
known in the art, is placed in a mold cavity and encapsulated in a
thermosetting polymer which, when heated, reacts irreversibly to form a
highly cross-linked matrix no longer capable of being re-melted.
The thermosetting polymer generally is comprised of three major components:
an epoxy resin, a hardener (including accelerators), and a filler
material. Other additives such as flame retardants, mold release agents
and colorants are also employed in relatively small amounts.
While many variations of the three major components are known in the art,
the focus of the present invention resides in the filler materials
employed and its effects on the active die surface and improved lead
locking of the lead fingers of the frame.
Filler materials are usually a form of fused silica, although other
materials such as calcium carbonates, calcium silicates, talc, mica and
clays have been employed for less rigorous applications. Powdered, fused
quartz is currently the primary filler used in encapsulants. Fillers
provide a number of advantages in comparison to unfilled encapsulants. For
example, fillers reinforce the polymer and thus provide additional package
strength, enhance thermal conductivity of the package, provide enhanced
resistance to thermal shock, and greatly reduce the cost of the polymer in
comparison to its unfilled state. Fillers also beneficially reduce the
coefficient of thermal expansion (CTE) of the composite material by about
fifty percent in comparison to the unfilled polymer, resulting in a CTE
much closer to that of the silicon or gallium arsenide die. Filler
materials, however, also present some recognized disadvantages, including
increasing the stiffness of the plastic package, as well as the moisture
permeability of the package.
Another previously unrecognized disadvantage discovered by the inventors
herein is the damage to the active die surface resulting from encapsulant
filler particles becoming lodged or wedged between the underside of the
lead extensions and the active die surface during transfer molding of the
plastic package about the die and the inner lead ends of the LOC die
assembly. The filler particles, which may literally be jammed in position
due to deleterious polymer flow patterns and flow imbalances in the mold
cavity during encapsulation, place the active die surface under residual
stress at the points of contact of the particles. The particles may then
damage the die surface or conductive elements thereon or immediately
thereunder when the package is further stressed (mechanically, thermally,
electrically) during post-encapsulation handling and testing.
While it is possible to employ a lower volume of filler in the
encapsulating polymer to reduce potential for filler particle lodging or
wedging, a drastic reduction in filler volume raises costs of the polymer
to unacceptable levels. More importantly, if the volume of the filler in
the encapsulating polymer is reduced, as more polymer is used, the
reliability of the encapsulated part is affected as the polymer tends to
absorb moisture, and is more permeable to moisture thereby causing a
variety of problems for the encapsulated part during encapsulation and
subsequent use. Currently available filler technology also imposes certain
limitations as to practical beneficial reductions in particle size
(currently in the 75 to 125 micron range, with the larger end of the range
being easier to achieve with consistency) and in the shape of the filler
particles. While it is desirable that particles be of generally spherical
shape, it has thus far proven impossible to eliminate non-spherical flakes
or chips which, in the wrong orientation, maximize stress on the die
surface.
Ongoing advances in design and manufacturing technology provide
increasingly thinner conductive, semiconductive and dielectric layers in
state-of-the-art dice, and the width and pitch of conductors serving
various purposes on the active surface of the die are likewise being
continually reduced. The resulting die structures, while robust and
reliable for their intended uses, must nonetheless become more
stress-susceptible due to the minimal strength provided by the minute
widths, depths and spacings of their constituent elements. The integrity
of active surface die coats such as silicon dioxide, doped silicon
dioxides such as phosphorous silicate glass (PSG) or borophosphorous
silicate glass (BPSG), or silicon nitride, may thus be compromised by
point stresses applied by filler particles, the result being unanticipated
shortening of device life if not immediate, detectable damage or
alteration of performance characteristics.
The aforementioned U.S. Pat. No. 4,984,059 to Kubota et al. does
incidentally disclose several exemplary LOC arrangements which appear to
greatly space the leads over the chip or which do not appear to provide
significant areas for filler particle lodging. However, such structures
may create fabrication and lead spacing and positioning difficulties.
In addition to solving the problems associated with filler particle lodging
and damage, it is desirable to have improved lead locking of the lead
fingers of the frame during operations involving the semiconductor die. If
the gaps between the lead fingers and the semiconductor die are sealed
with an underfill material, the adhesive used to bond the lead fingers to
the semiconductor die is more effective in locking the lead fingers in
position. Furthermore, the use of an underfill material, in addition to
the tape, screen print or spin-on dielectric adhesive layer, provides an
additional stabilizing means to immobilize the lead fingers in position,
thus reducing or eliminating localized stress failures occurring during
the transfer molding process. Previously, improving lead finger locking
has been approached from the perspective of improved adhesives and by
increasing the flexibility of the lead fingers, rather than sealing the
gaps between the leadfingers and the semiconductor die.
From the foregoing, the prior art has neither provided for improved locking
of the lead fingers to the semiconductor die, nor recognized the stress
phenomenon attendant to transfer molding and the use of filled
encapsulants, nor provided an LOC structure which beneficially
accommodates this phenomenon.
SUMMARY OF THE INVENTION
The present invention provides a lead-supported die assembly for an LOC
arrangement that substantially reduces the stress that may otherwise
potentially form between the leads and the active die surface due to the
presence of filler particles of the polymer encapsulant and improved lead
locking of the leads in position over a portion of the active surface of a
semiconductor die assembly. Accordingly, an underfill material is
introduced in the gap between each lead finger and semiconductor die,
between the bonding location of the die and the edge of the die, to
underfill and seal the gap. After the underfill material is cured, the
compound filler is prevented from flowing into the gaps. Accordingly, a
stacking of filler particles in which the filler particles try to force
the lead away from the die thus causing stress in the connection between
the lead and the die is prevented or reduced. Moreover, the underfill
material substantially immobilizes the lead fingers and reduces the stress
created during the transfer molding process as well as other processes.
The resulting exclusion of filler material from the gap will effectively
eliminate or reduce the residual stress typically experienced by the
active die surface in conventional LOC assemblies. This lessened residual
stress is carried forward in the encapsulated package after cure,
permitting the package to better withstand the stresses of
post-encapsulation handling and testing, including the elevated potentials
and temperatures experienced during burn-in, without adverse effects. The
resulting lead stability also improves lead finger locking to the tape as
less force is transferred to the tape from the lead fingers, which force
may cause the lead fingers to become dislodged therefrom prior to the wire
bonding operations or, subsequently, during encapsulation of the assembly.
The LOC apparatus of the present invention comprises a lead frame to which
the active surface of a die is adhered by a LOC tape, permitting the lead
frame to physically support the die during pre-encapsulation handling and
processing, such as wire bonding. The gap found between the lead finger
and the semiconductor die is sealed with an underfill material. With such
an arrangement, intrusion of filler particles between the inner lead ends
and the active surface of the die during the encapsulation process is
effectively prevented.
Stated in more specific terms and on the scale of an individual lead and
the underlying active surface of the die, an underfill material is
introduced onto the semiconductor die at a location near the lead finger.
More specifically, the underfill material may be introduced nearby the
lead axis between the location of the dielectric adhesive (such as LOC
tape, screen print or spin-on, as known in the art) disposed on the
underside of a lead and the edge of the die. The underfill material will
migrate into and fill the gap by way of capillary action. The underfill
prevents filler particles from flowing into the gaps so as to
substantially eliminate the stress created by filler particles wedged or
lodged between the lead finger and the die. The underfill also enhances
the stability of the free end of the lead finger, so as to immobilize the
lead finger during the die assembly molding process.
Although the die assembly and method of assembly of the present invention
have been described in relation to several preferred embodiments, it is
believed that major advantages of the assembly and method according to the
invention are sealing the gaps between the lead fingers and the
semiconductor die in order to prevent the lodging of damaging filler
particles, and immobilizing the free ends of the lead fingers so as to
eliminate localized stress failures resulting during the encapsulation
process and during post-encapsulation handling and testing. These and
other features of the present invention will become apparent from the
following detailed description taken in conjunction with the accompanying
drawings, and as defined by the appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 comprises a flow chart of an exemplary process sequence for plastic
package molding;
FIGS. 2A and 2B are side schematic views of a typical transfer molding,
showing pre-molding and post-molding encapsulant positions;
FIG. 3 shows a top schematic view of one side of a transfer mold of FIGS.
2A and 2B, depicting encapsulant flow and venting of the primary mold
runner and the mold cavities wherein the die assemblies are contained;
FIGS. 4A, 4B and 4C depict encapsulant flow scenarios for a mold cavity;
FIGS. 5A and 5B depict cross-sectional side views of prior art packaged SOJ
semiconductor devices;
FIGS. 6A and 6B depict cross-sectional side views of a an embodiment of a
packaged SOJ semiconductor device according to the present invention;
FIGS. 7A and 7B depict cross-sectional side views of another embodiment of
a packaged SOJ semiconductor device according to the present invention;
FIGS. 8A and 8B depict top views of a lead frame according to the present
invention;
FIG. 9A depicts a partial cross-sectional end view of a semiconductor die
assembly wherein a first method of dispensing underfill is used;
FIG. 9B depicts a partial cross-sectional end view of a semiconductor die
assembly wherein a second method of dispensing underfill is used;
FIG. 10 depicts a partial cross-sectional side view of the packaged SOJ
semiconductor device of FIG. 7A;
FIG. 11 depicts a partial cross-sectional side view of the embodiment of a
packaged SOJ semiconductor device of FIG. 7A;
FIG. 12 depicts a partial cross-sectional side view of an embodiment of a
packaged SOJ semiconductor device of FIG. 7A; and
FIG. 13 depicts a partial cross-sectional side view of another embodiment
of a packaged SOJ semiconductor device of FIG. 7A.
DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT
So that the reader may more fully understand the present invention in the
context of the prior art, it seems appropriate to provide a brief
description of a transfer apparatus and method for forming a plastic
package about an LOC die assembly. The term "transfer" molding is
descriptive of this process as the molding compound, once melted, is
transferred under pressure to a plurality of remotely-located mold
cavities containing die assemblies to be encapsulated.
FIG. 1 is a flow chart of a typical process sequence for plastic package
molding. It should be noted that the solder dip/plate operation has been
shown as one step for brevity; normally plating would occur prior to trim
and form.
FIGS. 2A and 2B show pre-molding and post-molding positions of encapsulant
during a transfer molding operation using a typical mold apparatus
comprising upper and lower mold halves 10 and 12, each mold half including
a platen 14 or 16 with its associated chase 18 or 20. Heating elements 22
are employed in the platens to maintain an elevated and relatively uniform
temperature in the runners and mold cavities during the molding operation.
FIG. 3 shows a top view of one side of the transfer mold apparatus of
FIGS. 2A and 2B. In the transfer mold apparatus shown, the encapsulant
flows into each mold cavity 44 through the short end thereof.
In operation, a heated pellet of resin mold compound 30 is disposed beneath
ram or plunger 32 in pot 34. The plunger descends, melting the pellet and
forcing the melted encapsulant down through sprue 36 and into primary
runner 38, from which it travels to transversely-oriented secondary
runners 40 and across gates 42 into and through the mold cavities 44
through the short side thereof flowing across the die assemblies 100,
wherein die assemblies 100 comprising dies 102 with attached lead frames
104 are disposed (usually in strips so that a strip of six lead frames,
for example, would be cut and placed in and across the six cavities 44
shown in FIG. 3). Air in the runners 38 and 40 and mold cavities 44 is
vented to the atmosphere through vents 46 and 48. At the end of the
molding operation, the encapsulant is "packed" by application of a higher
pressure to eliminate voids and reduce non-uniformities of the encapsulant
in the mold cavities 44. After molding, the encapsulated die assemblies
are ejected from the cavities 44 by ejector pins 50, after which they are
post-cured at an elevated temperature to complete cross-linking of the
resin, followed by other operations as known in the art and set forth in
FIG. 1 by way of example. It will be appreciated that other transfer
molding apparatus configurations, as well as variations in the details of
the described method, are known in the art. However, none of such are
pertinent to the invention, and so will not be discussed herein.
Encapsulant flow in the mold cavities 44 is demonstrably non-uniform. The
presence of the die assembly 100 comprising a die 102 with lead frame 104
disposed across the mid-section of a cavity 44 splits the viscous
encapsulant flow front 106 into upper 108 and lower 110 components.
Further, the presence of the (relatively) large die 102 with its
relatively lower temperature in the middle of a cavity 44 permits the flow
front on each side of the die 102 to advance ahead of the flow front which
passes over and under the die 102. FIGS. 4A and 4B show two mold cavity
encapsulant flow scenarios where, respectively, the lower flow front 110
and the upper flow front 108 lead the overall encapsulant flow front 106
in the cavity 44 containing the die assembly 100. FIG. 4C depicts the
advance of a flow front 106 from above, before and after a die 102 is
encountered, the flow being depicted as time-separated, instantaneous flow
fronts 106a, 106b, 106c, 106d, 106e and 106f.
Encapsulant filler particles, as noted above, become lodged between lead
ends and the underlying die surfaces. The non-uniform flow characteristics
of the viscous encapsulant flow, as described above, may cause (in
addition to other phenomena, such as wire sweep, which are not relevant to
the invention) particles to be more forcefully driven between the leads
and the die and wedged or jammed in place in low-clearance areas. As the
encapsulant flow front advances and the mold operation is completed by
packing the cavities, pressure in substantially all portions of the
cavities reaches hydrostatic. With prior art lead and adhesive LOC
arrangements, the relative inflexibility of the tightly-constrained
(adhered) lead ends maintains the point stresses of the particles trapped
under the leads. These residual stresses are carried forward in the
fabrication process to post-cure and beyond. When mechanical, thermal or
electrical stresses attendant to post-encapsulation processing are added
to the residual point stresses associated with the lodged filler
particles, cracking or perforation of the die coat may occur, with the
adverse effects previously noted. It has been observed that filler
particle-induced damage occurs more frequently in close proximity to the
adhesive, where lead flexure potential is at its minimum.
To graphically illustrate the above description of particle lodging, FIG.
5A depicts a prior art packaged LOC assembly wherein a single lead 112
extends over a die 102, with a segment of dielectric adhesive 114, in this
instance a piece of Kapton.TM. polyamide tape, adhered to both the lead
112 and the active surface 116 of the die. As better illustrated in FIG.
5B (DETAIL A), filler particle 130, which is part of the packaging
material 123, is lodged between lead 112 and die active surface 116. It is
clear that the lead end 122 is tightly constrained from movement by the
inflexibility of the attachment of the lead end 122 to the die 102 by the
adhesive 114. Moreover, the relative closeness of the lead 112 to the die
active surface 116 and the inability of the lead 112 to flex or relax to
reduce stress occasioned by the presence of the filler particle 130 may
continue even after the encapsulant has reached hydrostatic balance such
that the filler particle 130 may become tightly lodged between the lead
112 and the die active surface 116.
FIG. 6A, and in better detail FIG. 6B, depicts, in contrast to the prior
art, a packaged LOC arrangement according to the present invention,
wherein a single lead end 122 also extends over die 102. In addition to
disposing an adhesive layer 114 between the underside of the lead 112 and
the active surface 116 of the die 102, an underfill material 117 is
applied between the underside of the lead 112 and the active surface 116
of the die 102 and between the adhesive 114 and the side 115 of the die
102. As more fully set forth below (see discussion of FIGS. 9, 10, and
11), various methods of underfilling may be utilized to seal the gap. The
underfill 117 fills and seals the gap found between the underside of lead
112 and the active surface 116 of the die 102. Thus, a filler particle
130, of the same size and shape as that shown with respect to the prior
art, is prevented from entering and becoming lodged between the lead 112
and the active surface 116. Moreover, the stacking of such particles 130
to create a similar lodging effect, also as seen in the prior art, is
likewise prevented.
Additionally, because the underfill 117 (after curing) forms a
substantially rigid link between a longitudinal length of the lead 112 and
the corresponding active surface 116 of the die 102, the free end 121 of
the lead 112 is substantially immobilized and prevented from flexing,
twisting, or bending away from the active surface 116. Thus, in addition
to eliminating point stresses caused by trapped particles, the resulting
relatively inflexible and tightly-constrained lead 112 reaches a steady
state position before being subjected to non-uniform flow characteristics,
which can create additional stresses such as wire sweep, during the
encapsulation or molding procedure. The added lead stability afforded by
the underfill 117 also reduces mechanical, thermal, and electrical
stresses attendant to post-encapsulation processing. Furthermore, the
incorporation of the underfill 117 results in less force being transferred
to the adhesive 114 from the lead end 122, said force potentially causing
a dislodgement of a lead 112 from the adhesive 114 prior to the wire
bonding operation or during the encapsulation process.
FIGS. 7A and 7B depict an alternative arrangement according to the present
invention, wherein the adhesive 114 disposed between the underside of the
lead 112 and the active surface 116 covers a shorter longitudinal portion
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