Flip interconnect

Interconnection of densely populated multiple chip integrated hybrid circuits (12) in a manner such that heat can be efficiently extracted therefrom. An integrated circuit die (31) is attached to a flip chip interconnect layer by soldering the connection pads thereto. The interconnect layer is slid off its substrate (11) in the manner of a decal. After the circuit has been tested and found acceptable, the other side of the die (31) is permanently bonded to a thermal conduction plate or heat sink (32). The decal interconnect (33) is made of alternating layers of an insulator (41) and a conductor (42) built on top of an erodible sacrifical layer (48) applied to a substrate. The sacrificial layer (48) is dissolved by a suitable solvent to float the multilayer interconnect off the substrate (11).

BACKGROUND 
The present invention relates to mounting and electrically connecting 
integrated circuits and, more particularly, to the packaging of an 
integrated circuit die in a high density multichip package. 
Hybrid circuits involve a technology in which some elements are fabricated 
in film layers directly on a substrate material and other elements are 
discrete add-on-components. A typical hybrid circuit may be hermetically 
sealed within a protective package and may comprise a plurality of densely 
arranged integrated circuit chips. Several different approaches are used 
to make the input/output connections to the connection pads on the face of 
each tiny integrated circuit die. 
One interconnection method is the so-called HDMI, or high density multichip 
interconnection, technique. In one HDMI process, the HDMI interconnect is 
built up layer by layer on a silicon or ceramic substrate, and the densely 
arranged integrated circuit chips are next bonded face-up to the HDMI 
substrate. Finally, the chips are connected electrically to the HDMI 
interconnect by wire bonding or by Tape Automated Bonding (TAB); both well 
known processes. 
Another interconnection method is the so-called flip chip technique. One 
specific example of flip chip bonding is the process that is sometimes 
referred to as the C4 process. A flip chip is an integrated circuit die 
prepared for reflow face bonding by the growth of solder hemispheres on 
the bond pads of the chip. In the flip chip interconnection process, after 
the die has been prepared with solder bumps on its face, it is flipped 
over for attaching to a matching rigid substrate on which interconnecting 
thin films have previously been deposited. All connections are then made 
by applying heat. 
Another technique for face down bonding to the bond pads is referred to as 
flip TRB, or testable ribbon bonding. A conductive connector ribbon having 
a kink for stress relief is used in place of the solder bumps. A die 
having the kinked ribbons is flipped over and attached to a mating rigid 
substrate on which interconnecting thin films have previously been 
deposited. 
In conventional high density multichip packages, an integrated circuit die 
is typically mounted on a substrate made of a material that is not a good 
conductor of heat. Hence, the integrated circuit die is only partially 
cooled from the back side. Typically, the heat escapes by radiation from 
the cover of the package that encloses the chip and thus it is difficult 
to keep the package cool. As the chip density increases, it becomes more 
important to find ways to remove the heat from the integrated circuit 
package to keep the chips as cool as possible. 
Accordingly, it is an objective of the present invention to provide a 
mounting and interconnection method that permits a direct thermal 
connection from an integrated circuit chip to a heat sink in a high 
density multichip package. Another objective of the present invention is 
to interconnect a flip chip die in a high density package without the 
strain of a rigid substrate. 
SUMMARY OF THE INVENTION 
In accordance with these and other objectives and features of the 
invention, there is provided a technique for removing the heat from one 
side of an integrated circuit die and making the electrical connections to 
the other side of the die. In this technique, the die is attached to a 
flip-chip interconnect layer in a conventional manner, and then the 
interconnect layer is slid off its substrate in the manner of a decal. The 
decal may have been previously removed before die attach. This results in 
a thin foil interconnect fastened to one side of the multiple die. After 
the circuit has been electrically tested and found acceptable, the other 
side of the die is bonded to a thermal conduction plate or heat sink. The 
final configuration is a face-up die on a heat sink interconnected by a 
"decal". Heat is then easily conducted away from the die. If desired, the 
heat sink may be thermally bonded to other heat sink materials and 
structures such as heat pipes and liquid grids. Processes other than HDMI 
may also be used to fabricate the "decal" which is essentially a high 
density flexible interconnect (having line widths of less than 3 mils). In 
addition, processes such as "flip TAB" and "flip TRB" processes may be 
used to to attach the chips to the decal. It may also be fabricated by 
laminating multiple layers of interconnects, without the use of a rigid 
substrate.

DETAILED DESCRIPTION 
Referring now to FIG. 1 of the drawings, there is shown a conventional 
hybrid 10 of the type that is presently used in many applications. The 
hybrid 10 comprises a substrate 11 which is made of a material such as 
silicon, or ceramic, or the like. A high density multichip interconnection 
(HDMI) interconnect 12 is built up on top of the substrate 11 for the 
purpose of making electrical connections. A plurality of densely disposed 
integrated circuit chips 13 are bonded face up on top of the HDMI 
interconnect 12. The chips 13 are electrically connected to the HDMI 
interconnect 12 by tab connections 14. The density employed for integrated 
circuit chips on hybrid circuit boards is increasing. For example, one 
application in a well-known super computer is known to have 240 integrated 
circuit chips on a single CPU board. Each chip has 284 pins on 0.011 inch 
centers. Collectively the 240 chips on the CPU board contain about 2.7 
million gates. 
Referring now to FIG. 2, there is shown an enlarged cross-sectional view of 
the connection method illustrated in FIG. 1. FIG. 2 shows only a portion 
of the hybrid 10 comprising the substrate 11 having the HDMI interconnect 
12 built on top thereof, with one of the integrated circuit chips 13 
bonded thereto. The chip 13 is electrically connected to the HDMI 
interconnect 12 by a tab connection 14. It will be understood that FIG. 2 
is not drawn to scale. For example, the entire HDMI interconnect 12 is on 
the order of 0.0025 inches thick. As may be seen in FIG. 2, the HDMI 
interconnect 12 is a multilayer structure comprising an insulator 20 made 
of a suitable material such as polyimide, or the like. Other layers are 
conductors 21, made of a suitable conductive material such as copper, or 
the like. A ground plane 22 is provided at the very bottom of the HDMI 
interconnect 12 adjacent the substrate 11. The HDMI interconnect 12 shown 
in FIG. 2 is also provided with a power plane 23, a first signal layer 24, 
and a second signal layer 25. In addition, the HDMI interconnect 12 
includes many vias (not shown). It will be understood that the presently 
used HDMI interconnect 12 illustrated in FIGS. 1 and 2 is built up layer 
by layer using well known processes and techniques. 
As was pointed out hereinbefore, in conventional high density multichip 
packages, an integrated circuit die is typically bonded to a material that 
is not a good conductor of heat. Hence, the integrated circuit die 
typically is only partially cooled from the back side. Instead, heat 
generally escapes by radiation from the cover of the package that encloses 
the chip. As chip density increases, it becomes more important to find 
ways to remove heat from integrated circuit packages and keep the chips as 
cool as possible. Typically, in conventional hybrids, there are many 
thermal interfaces between the chip and a heat sink. It is a feature of 
the present invention to eliminate unnecessary thermal interfaces and to 
efficiently remove heat from hybrid packages. 
Referring now to FIG. 3, there is shown a flip interconnect 30 made in 
accordance with the principles of the present invention. As shown in FIG. 
3, the flip interconnect 30 has at least one face-up integrated circuit 
die 31 on a heat sink 32 interconnected by a decal interconnect 33. It 
will be understood that the dimensions are not to scale in the figure. For 
example, the decal interconnect 33 may be on the order of 0.0025 inch 
thick. 
The flip interconnect 30 provides means for removing heat from one side of 
the integrated circuit die 31 and making the electrical connections to the 
other side of the die 31. In accordance with the principles of the present 
invention, the die 31 is attached to a flip-chip interconnect layer in a 
conventional manner, and then the interconnect layer is slid off its 
substrate in the manner of a decal. In addition to conventional soldering 
processes, processes such as "flip TAB" and "flip TRB" processes may be 
used to to attach the chips to the decal. This results in a thin foil 
decal interconnect 33 fastened to one side of the die 31. After the die 31 
has been electrically tested and found acceptable, the other side of the 
die 31 is permanently bonded or pressed to the heat sink 32. The final 
configuration is a face-up die 31 on a heat sink 32 interconnected by a 
"decal" interconnect 33. Heat is then easily conducted away from the die 
31. If desired, the heat sink 32 may be thermally bonded to other heat 
sink materials and structures such as heat pipes and liquid grids. 
The decal interconnect 33 may be a standard HDMI interconnect. The HDMI 
interconnect may be built up in the normal way on a silicon or ceramic 
substrate and floated off as a "decal". Referring now to FIG. 4, there is 
shown an examplary embodiment of a decal interconnect 33 in accordance 
with the invention built up on a substrate 40 made of a material such as 
silicon or ceramic, or the like. The decal interconnect 33 comprises one 
or more layers of an insulator 41 made of polyimide, or the like, and one 
or more layers of a conductor 42 made of aluminum or copper, or the like. 
In the exemplary embodiment illustrated in FIG. 4, there is provided a 
ground plane 43, a power plane 44, a first signal layer 45, and a second 
signal layer 46. The decal interconnect 33 may also include many vias (not 
shown). A plurality of conductive pads 47 are provided for making 
connection to a flip chip semiconductor die. 
In order to float the decal interconnect 33 off the substrate 40, there is 
provided a sacrificial layer 48 disposed therebetween. Starting with the 
substrate 40 made of silicon or ceramic, or the like, there is first 
applied a sacrificial layer 48 made of a material such as titanium or 
tungsten, or the like. Then the aluminum/polyimide multilayer decal 
interconnect 33 is built on top of that foundation. A flip chip die is 
bonded to the decal interconnect 33 and tested electrically. After 
testing, the device assembly is immersed in a liquid to dissolve the 
sacrificial layer--in this case a 30% hydrogen peroxide mixture, and 
soaked until the sacrificial layer 48 of titanium or tungsten is 
dissolved. The hydrogen peroxide solvent does not affect the polyimide or 
the semiconductor device. After the decal interconnect 33 has been floated 
off the substrate 40, the semiconductor die is bonded or pressed to a heat 
sink as described in connection with FIG. 3, and then the device assembly 
is packaged. 
The sacrificial layer 48 may be made of materials other than titanium or 
tungsten. It is only necessary that the sacrificial layer 48 be an 
erodible foundation that may be dissolved without adversely affecting the 
interconnect 33 or the semiconductor devices. If desired, the sacrificial 
layer 48 may be made of an organic thermoplastic. After the decal 
interconnect 33 has been built up, the assembly may be heated to melt the 
thermoplastic, and then the decal interconnect 33 may be peeled off the 
substrate 40. If desired, an organic soluble sacrificial layer 48 may be 
used with a solvent. For example, a silicon oxide coating may be used for 
the sacrificial layer 48 and it may be dissolved with hydrofluoric acid. 
The acid attacks the silicon oxide but does not affect the aluminum used 
for the conductors 42. The sacrificial layer 48 may also be made of sodium 
carbonate, if desired. Then the sacrificial layer 48 may be dissolved in 
water. 
Processes other than HDMI may also be used to fabricate the "decal" which 
is essentially a high density flexible interconnect having line widths of 
less than 3 mils. It could be fabricated by lamination of layers of 
interconnects, without being processed on a rigid substrate. 
Referring again to FIG. 3, one method of attaching the flip interconnect 30 
is to flip chip the die 31 onto the decal interconnect 33 and later to 
detach the decal interconnect 33 and die 31 from the HDMI substrate. A 
second method is to detach the decal interconnect 33 first, and then flip 
chip the die 31. This second method might be used if the decal 
interconnect 33 changes size upon detachment from the HDMI substrate. 
The decal interconnect 33 may be held in a frame to make it easier to 
handle after removal from the substrate. Typically, this is accomplished 
by using a vacuum. Connection between the decal interconnect 33 and the 
next level of interconnect may be made by a variety of methods including 
wire bonding or by using the so-called "gold dot" process to make a 
separable connection. The next level of interconnect might be, for 
example, to a printed circuit board or to a ring frame of a hybrid circuit 
package. 
It is a feature of the invention that the decal interconnect 33 
interconnects the "flip chip" die 31 with high density and without the 
strain of a rigid substrate. It allows full electrical pretesting prior to 
committing the structure to the heat sink 32 or to the final package. It 
is a principal of the present invention to bond the die 31 to the flexible 
complex high density interconnect 33 by a flip chip process, and support 
this structure by a heat sink 32. The die 31 may be held to the heat sink 
32 by adhesive or pressure. 
The flip interconnect 30 of the present invention has all the advantages of 
conventional flip chip structures: namely high density and low inductance. 
It preserves investment in the HDMI standard process. The stresses induced 
in flip chip bonds by thermal mismatch are relieved by the compliance and 
thermal coefficient of expansion (TCE) match of the flexible interconnect 
33. The heat is directly transferred to the heat sink 32, without the 
interconnect 33 being a thermal barrier. 
The flip interconnect 30 of the present invention is ideal for high speed, 
high power devices due to close packing, low dielectric constant, short 
line interconnects and the availability of a wide variety of heat sink 
materials and structures such as heat pipes, liquid grids, etc. Compared 
to a process which builds the interconnect on top of the die, sometimes 
called an "overlay" process, the present invention provides: easier 
rework, more options for processing the "decal" since the die does not 
have to be protected during the process, and simpler alignment (one die at 
a time). Also, the present invention provides excellent compatibility with 
chip on board applications since both die and substrate can be passivated 
or coated with a resistant material prior to package assembly to provide 
environmental stability. 
Thermal design is enhanced and simplified by making the thermal structure 
planar and possibly rigid, and the electrical interconnect between one die 
and another is conforming. Normally the electrical interface is simplified 
by being planar and the thermal interface must conform, e.g. using gels or 
pistons, for example. The heat sink need not be planar--it could conform, 
for example to a fuel tank in an aircraft. 
Thus there has been described a new and improved method of packaging an 
integrated circuit die for high density. It is to be understood that the 
above-described embodiment is merely illustrative of some of the many 
specific embodiments which represent applications of the principles of the 
present invention. Clearly, numerous other arrangements can be readily 
devised by those skilled in the art without departing from the scope of 
the invention.