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cl BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to packaging of electronic components using
Tape Automated Bonding (TAB) techniques. More specifically, the present
invention relates to TAB In Board (i.e., TIB) packaging wherein TAB
processed semiconductor dice are embedded into a packaging substrate of a
multi-chip module (MCM), which is cooled by a heatsink.
2. Description of the Prior Art and Related Information
For many years, semiconductor devices like integrated circuits were
connected to the outside world by small wires with diameters the size of a
human hair. Needless to say, this interconnection procedure was difficult
and required sophisticated manufacturing hardware to automate. Also, as
lead counts increased, wire bonding became extremely labor-intensive. That
increased the assembly costs as well as the risk of device malfunctions
due to faulty connections from human error.
Furthermore, as higher speed circuits required more interconnects and as
the surface area of VLSI circuits got smaller, assembly limitations of
wire interconnects became apparent. With lead counts of some devices
surpassing 300, such space restrictions were well beyond the capabilities
of conventional wire bonding processes.
Fortunately, with the advent of Tape Automated Bonding (TAB) packaging
techniques, the opportunities for automated assembly of interconnects for
semiconductor devices improved dramatically.
TAB technology was explored by General Electric back in 1972. It could
easily be adapted to relatively small, multileaded packages with
interconnects that could be handled by automated bonding machines Indeed,
TAB technology is now shifting toward the area of multi-chip TAB, wherein
modules that pack several densely leaded chips into a small area. TAB
technology also facilitates multilayer substrate configurations.
Although there are many variations, the typical TAB packaging process
involves the following steps. A strip of polyimide film ranging from 35 to
105 millimeters wide is needed to begin the process. The polyimide film
resembles photographic film and is commercially identified under the trade
names Tedlar, Kapton, or Mylar. The film, called the carrier tape, has a
number of openings or windows formed along its length. Sprocket holes are
evenly spaced along the edges as in conventional photographic film.
A thin foil of electrically conductive metal is laminated to the surface of
the carrier tape. The metallic foil is usually a copper based material,
made very thin, typically 0.5 to 3 mils in, thickness. Through a
photoresist or photolithographic process, the metal foil is etched away to
form cantilevered leads, more commonly known beam leads. An opening
provided in the metal foil nearly coinciding with the openings in the
carrier tape allow a semiconductor chip or die to pass therethrough. Since
the beam leads overlie the opening, they are situated for electrical
connection to the semiconductor chip when the latter is inserted through
the opening. The semiconductor chip has bonding pads positioned on its
surface to align with the beam leads. A thermocompression bonding process
is then used to attach the beam leads to the semiconductor chip. As is
well known, thermocompression involves using a thermode or heated platen
to compress the beam lead against the pad to form a bond.
Importantly, the beam leads can be bonded on both ends so that the opposite
ends of the leads can extend over other windows or openings in the carrier
tape such that these leads can be joined to other chips, a packaging chip
carrier, or a substrate. In other words, the inner beam leads are attached
to the die (called Inner lead Bonding or ILB) and the outer leads can be
attached to the package or other devices (called Outer Lead Bonding or
OLB). For mass production with automation, the sprocket holes along the
edge of the carrier tape permit incremental feeding of the tape past a
number of operating stations, one of which gang bonds all of the leads by
thermocompression in one stroke.
After the bonds are formed, it is common to encapsulate the
interconnections on the semiconductor chip by coating the structure with a
polymeric material. The polymeric material hardens in place and gives
strength to the interconnections.
But TAB packaging technology still has many limitations. One such
limitation appears when TAB packaging is used in conjunction with
multi-chip module or MCM technology. In general terms, MCM is a technique
whereby several dice are bonded to a silicon substrate. The interconnects
between each die can be achieved with lithographic processes or by TAB
technology.
In one field of application, PC workstations, it is well understood that
MCM technology is becoming a requirement for pushing the capabilities of
processing data to higher levels. By reducing the distances between ICs,
it is possible to increase IC performance, and as a by-product, conserve
precious board space.
Various MCM technologies exist currently. They range from basic FR4 Sister
Boards with ICs on them to exotic, silicon-substrate-based modules.
MCM does have its share of problems, however. For example, one issue to
contend with is when one acquires multiple ICs from different companies to
build a specific MCM, only to find that the various firms do not wish
their wafer-specific data known by anyone who would be considered a
competitor.
Another issue concerns thermal management. When industry requirements push
silicon ICs to faster and faster speeds, more energy is normally required.
Aggravating the thermal management problem is the common practice of
closely packing ICs together to maximize board space utilization, thus
creating even more heat build-up.
Another issue is that of reworkability. When one examines the aggregate
yields of, for example, a typical five-chip MCM, one sees that a certain
number of MCMs require some repair or rework. But if an MCM is not easily
repairable, then a great amount of revenue is lost due to ICs that cannot
be recovered from that MCM.
Still another very important concern for MCMs is testability. The MCM user
must be able to pre-test his devices before MCM assembly can occur so that
a reasonable module yield can be obtained. Too much material cost as well
as assembly cost are lost after the fact when MCM pre-testing is not
available.
One final MCM concern is flexible manufacturability. That is, the
capability of having anyone produce an MCM for a firm without having to
rely on high-cost IC fabrication environments that cannot be duplicated
without massive amounts of capital investment.
As is apparent at this point, a need exists in MCM technology that
addresses all of the foregoing concerns. Accordingly, it is an object of
the present invention to improve MCM technology using existing current
interconnect technologies that are available to anyone. It is another
object of the present invention to ensure that chip-to-chip spacing is
extremely close, resulting in good signal speeds and excellent board area
conservation. It is yet another object of the present invention to provide
component pretestability that is easily achieved without exotic hardware.
Other objects of the present invention include managing thermal build-up
in the package; achieving an intermix capability of different IC vendor
chips; simplifying in-house assembly of the MCMs; and reworking of
defective MCMs with existing tooling.
SUMMARY OF THE INVENTION
The present invention relates to TAB in board packaging having a heatsink
adapted to the package for cooling purposes. According to the present
invention, several semiconductor chips are housed inside an MCM packaging
substrate made of a polyimide or ceramic material. The interconnects
between IC chips or dice are formed by TAB techniques known in the art.
Unlike conventional TAB techniques, however, the present invention
provides that the outer beam leads need not be bent. In fact, the leads
remain straight, nearly parallel to the top face of the MCM substrate when
the dice are installed.
To keep the beam leads relatively straight, the present invention provides
that the die be recessed into the MCM packaging substrate. For that
purpose, the packaging substrate has a cut-out or cavity coinciding with
the shape of the die. The die is then lowered into this cavity so that its
top surface is substantially flush with the top face of the packaging
substrate. In this particular orientation, the outstretched outer beam
leads overlie the top face of the substrate.
In all other respects, the remaining interconnections are conventional.
Signal, power, and ground inner beam leads extending from the die have
already been formed by conventional TAB technology. The outer beam leads
are then bonded to prearranged signal paths formed on the packaging
substrate, which signal paths connect with pins that reach out to the
exterior of the packaging substrate.
For cooling purposes, the present invention provides a heatsink such as a
copper slug that is embedded into the bottom face of the packaging
substrate. Before die installation, the heatsink is inserted up into a
recess located in the underside of the substrate and kept in place by a
friction fit between the recess and the heatsink. Thereafter, the dice are
lowered into their respective cavities from the top face of the substrate.
Since the cavities are in communication with the recess, the bottom of the
die can be lowered to a depth when it engages the top of the heatsink.
Any bonding or joining processes known in the art can be used to attach the
die to the heatsink and form a thermal junction In a preferred embodiment
of the present invention, however, a compliant thermal conduit (CTC)
thermal joining process is used to bond the die to the heatsink. This
process is disclosed in co-pending U.S. patent application Ser. No.
07/589,094. Briefly, the process involves injecting a compliant,
heat-conducting thermoplastic material into the joint. The material cures
and forms a flexible bond that is also conducive to heat transfer.
Primarily through conduction, the heat generated by the die during
operation passes through the thermal junction into the heatsink, which
transfers the heat into the ambient atmosphere. In addition, when the MCM
is assembled to the motherboard, the packaging substrate is inverted Once
inverted after installation, the underside of the packaging substrate
faces up and consequently, so does the heatsink. Hence, the inverter
position clearly is conducive to convective heat flow away from the
heatsink.
Therefore, the present invention has many advances over the prior art.
First, it is possible to isolate the chip vendor so that it need not
deliver a product finished to wafer level; the vendor should then not be
concerned with divulging proprietary secrets involving its chips. Second,
having individual TAB components in tape carriers facilitates
pre-testability. Third, if TAB packaging is used as in the preferred
embodiment, rework of defective chips can be done easily. Fourth,
manufacturing flexibility is possible since basically the same substrate
can be used for many different product designs and environments. Indeed,
the present invention can be modified for use with multiple window TAB
packages. Fifth, the heatsink is easily adaptable to many cooling
configurations when extremely high power dissipation is necessary.
Finally, the present invention also permits stacking of the packaging
substrates so that Multiple Package capabilities are possible. As a
result, the MCM can expand as the user's requirements grow to maximize its
hardware purchase.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view of a die with outer and inner beam leads formed by a
TAB process.
FIG. 2(a) is a side view of the die shown in FIG. 1 produced according to
the present invention with the outer beam leads extending parallel to the
top surface of the die.
FIG. 2(b) is a side view of a prior art die wherein the TAB beam leads are
bent to facilitate assembly.
FIG. 3 is a plan view of the present invention multi-chip module with the
TAB packaged dice embedded in the cavities of a packaging substrate.
FIG. 4 is a sectional view of the multi-chip module of FIG. 3 taken along
line 4--4 to reveal the contact between the dice and the heatsink.
DETAILED DESCRIPTION OF THE INVENTION
In the following description, numerous details such as specific materials
and configurations are set forth in order to provide a more complete
understanding of the present invention. But it is understood by those
skilled in the art that the present invention can be practiced without
those specific details. In other instances, well-known elements are not
described explicitly so as not to obscure the present invention.
The present invention relates to an apparatus and process for using Tape
Automated Bonding (TAB) techniques to prepare semiconductor IC chips that
are eventually embedded into cavities of a multi-chip module (MCM)
packaging substrate. In a preferred embodiment, the cavities are in
communication with a heatsink that is inserted into a recess located in
the underside of the substrate. Thus, when the chips are lowered into the
cavities, their bottom surfaces engage the heatsink. In this manner, heat
transfer from the chips is efficient, and effective cooling of the MCM is
possible. This technique has been coined with the acronym TIB, which
stands for TAB In Board.
In the preferred embodiment, the present invention provides an IC chip or
die 10 as shown in a plan view in FIG. 1. Through a TAB procedure known in
the art, inner beam leads 14 and outer beam leads 16 are formed from the
TAB polyimide carrier tape or film. According to the TAB process, an
opening or window 18 is cut into the carrier tape so that the die 10 can
pass through. Subsequent to formation of the window 18 and the beam leads
14 and 16, some carrier tape is left over. This surplus tape is called the
tape body 12.
After the die 10 is inserted through the window 18, the inner beam leads 14
are thermocompression bonded to electrical contact pads (not shown)
located on the top of the die 10. So in this configuration, the inner beam
leads 14 connect with the die 10 while the outer beam leads 16 connect
with other devices. Signal paths (not shown) are provided on the surface
of the tape body 12 when necessary to interconnect specific inner beam
leads 14 with specific outer beam leads 16. Taken collectively, the die
10, the tape body 12, and the beam leads 14, 16 are loosely termed the TAB
component 22.
It is important to pause and note here that the present invention, by
utilizing individual TAB components in carriers, achieves pre-testability
of components before assembly. Also by using TAB as the preferred
interconnect, the present invention solves the MCM rework issue.
A unique aspect of the present invention is in the trimmed-out TAB
component 22. According to the present invention, there is no mechanical
shaping of the beam leads 14 and 16. As is shown in FIG. 2(a), the outer
beam leads 16 are trimmed horizontally without need for bending. This
reduces the lead length to an absolute minimum for a TAB chip on tape
package.
By contrast, a prior art TAB component, shown in FIG. 2(b), requires
bending or crimping the outer beam leads to facilitate assembly to the
packaging substrate. But bending induces mechanical stresses into the
delicate leads thus increasing the risk of breakage or impairing lead
conductivity.
According to the present invention, after the TAB component 22 has been
excised, it is installed into a packaging substrate 20, illustrated in a
plan view in FIG. 3. FIG. 4 presents a sectional view taken along line
4--4 of FIG. 3. It is apparent here that the TAB component 22 is not
simply placed on top of the substrate 20; rather, the TAB component 22 is
embedded into the substrate 20. For each die 10 installed, the substrate
20 has a well or cavity 24 that holds a specific chip 10 therein. Laid
into the surface of the substrate 20 are signal paths (not shown).
Consequently, when the IC chip 10 is lowered into the cavity 24, its outer
beam leads 16 can be bonded to these signal paths, or they can be
connected to other devices. The signal paths connect the chips 10 with the
outside world and function conventionally as a signal or power conductor,
or as a ground.
As illustrated in the cross-sectional view in FIG. 4, the significance of
straight leads can be seen. Each TAB component 22 is lowered into the
cavity 24 to a depth where the top surface of the TAB component 22 stands
nearly flush with the top face 26 of the substrate 20. The outer beam
leads 16 reach outward, parallel to and overlying the top face 26 for
connection with the signal paths. Therefore, these straight outer beam
leads 16 permit easy and convenient bonding to the multi-chip module
substrate 20.
Of course the outer and inner beam leads 16 and 14 need not be configured
as shown in FIG. 1. A person skilled in the art can easily devise other
beam lead designs to match the particular purpose. Furthermore.,
arrangement of the TAB components 22 in the multi-chip module substrate 20
can be altered to satisfy design needs that depart from the six-chip
layout shown in FIG. 3.
Each cavity 24 is a passage that leads vertically through the substrate 20
into a heatsink 28. In actual assembly steps, the heatsink 28 is installed
in the substrate 20 before the TAB components 22. The heatsink 28 is
incorporated into a recess in the bottom of the substrate 20, as is shown
in FIG. 4. Typically, the heatsink 28 is forced fit into the recess. Thus,
the interference fit between the two structures prevents unexpected
disassembly.
The heatsink 28 in a preferred embodiment is made from a copper slug.
Copper is a good thermal conductor and its large surface area, exposed to
the air under the substrate 20, is good for radiating heat. Other thermal
conducting materials known in the art are suitable. Also, the heatsink 28
can be either drilled and tapped for fitment with other cooling devices
known in the art. It is also possible to mount a threaded post on the
heatsink 28. This type of direct die to heatsink contact is unique to MCM
approaches. It allows a very efficient thermal path to remove heat
build-up. If additional cooling is required, it is very easily managed by
adding another heatsink or heat pipe to the heatsink already built in the
substrate.
The heat comes from the dice 10 when they are powered up for heavy use.
Consequently, the interface between the dice 10 and the heatsink 28 is
critical for proper cooling. A poor connection between the bottom of the
dice 10 and the top surface of the heatsink 28 impedes heat transfer out
of the dice 10.
A special process is necessary to ensure proper heat flow. The present
invention provides that the thermal junction between each die 10 and the
heatsink 28 be formed by a process called Compliant Thermal Conduit For
Printed Circuit Boards, which is disclosed in co-pending U.S. application
Ser. No. 07/589,094, filed Sept. 27, 1990, assigned to Sun Microsystems,
Inc.
Briefly, this die attach process involves applying a quantity of heat
conductive thermoplastic material to join the die 10 to the heatsink 28.
At first, the thermoplastic material is fluid and flows to fill in any
voids in the joint. After curing, the bond stiffens insofar as no further
flow occurs, but the joint remains flexible.
In a preferred embodiment, the thermoplastic material should be hexagonal
boron nitride; other materials known in the art such as caulk or RTV are
suitable. Importantly, the compliant nature of the thermoplastic material
maintains a solid thermal bond even when the substrate 20 undergoes
flexing during assembly. Another benefit is that a thermal joint produced
by this process remains free from voids that might impair the thermal
transfer rate between the die 10 and the substrate 20.
In an alternate embodiment (not shown), the present invention provides that
the packaging substrate be stacked onto other substrates prepared in a
similar manner. Blind vias are formed in the substrates to interconnect
them vertically. Moreover, by alternating every 180 degrees for the
electrical contacts, it is possible to slip in cooling devices that
radiate out to the sides of the multi-chip module to manage heat build-up.
The present invention easily adapts to a Land Grid Array interconnect
approach to the motherboard, known in the art. In the present invention,
the contacts to the motherboard are on the same side as the TAB components
22. After MCM assembly, the substrate 20 is inverted during installation
to the motherboard. This exposes the backside (i.e., bottom surface) of
the heatsink 28 in the MCM so that its orientation facilitates a correct
thermal path upward for the heat to flow.
This also protects the TAB IC's from any potential handling damage. If
additional cooling is required, the assembly allows a generous choice in
cooling techniques to be attached to the exposed heatsink.
In summary, the present invention provides an MCM approach which can be
utilized nicely with multiple window TAB carrier tapes. That is, the TAB
carrier tape can be made with multiple IC chips being installed. This
technique is useful in attaining very close coupling between two IC chips
that need the minimum signal path distance. The present invention allows
this form of tape design to be employed while still allowing rework and
providing good thermal management.
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