Attachment assembly for integrated circuits

An integrated circuit package has a top die attach area and a bottom heat spreader thermally coupled to the die for conducting heat generated by the die through a thermal interface in the main circuit board to a heat sink or heat pipe mounted underneath the main circuit board. The preferred thermal interface is a thin, thermally conductive slug mounted through an opening formed in the main circuit board. The heat spreader spans the bottom surface of the integrated circuit package substantially parallel to the main circuit board and preferably extends substantially to the inner periphery of the pin arrangement which preferably, although not exclusively, is in a ball grid array. The opening formed in the main circuit board through which the thin, thermally conductive slug is fitted, is preferably substantially flush with the bottom surface of the main circuit board and juxtaposed against the heat spreader. A thermally conductive adhesive is preferably applied to the adjoining surfaces of the heat spreader and the slug.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates generally to integrated circuit packages, and more 
specifically to an integrated circuit package having a top cavity die 
attach area while sinking heat through the bottom of the package. 
2. Description of Related Art 
Packages for integrated circuits have evolved over the years from the 
simplistic single and dual inline packages (SIP) and (DIP) respectively, 
through the more sophisticated and denser pin grid array (PGA), staggered 
pin grid array (SPGA), and ball grid array (BGA). The so-called tape 
carrier package (TCP) is used in small form-factor products such as laptop 
computers, due to its small footprint and low clearance characteristics. 
The TCP is produced through the so-called Tape Automated Bonding (TAB) 
process that is pervasive in portable electronics. An exemplary, but not 
exclusive application of the TCP can be found in the mobile Pentium.RTM. 
processor from the Intel Corporation of Santa Clara, Calif. 
Referring now to FIG. 1, a TCP device 10 (such as the mobile Pentium.RTM. 
processor) is constructed from a flexible tape 12 that is laminated with a 
copper foil on its bottom. The flexible tape 12 has an opening 14 formed 
near its center for attachment of a die 15. The copper foil is 
photo-imaged and etched forming an arrangement of pads around the opening 
14 with gold plated electrical traces extending therefrom to conductive 
fingers 16 formed at the outer borders of the tape 12. The fingers 16 are 
bent in a gull-winged fashion for attachment to a circuit board 18. 
Special "gold bumps" 25 are deposited on standard aluminum bond pads 
formed around the top periphery of the die 15. The gold bumps 25 are then 
attached to the arrangement of pads formed on the bottom of tape 12 
through so-called thermal compression bonding. After the die 15 is 
attached, a blob of encapsulant such as polyimide siloxane 20 is applied 
to the opening 14 to protect and seal the die 15. 
To attach the TCP 10, special tooling presses the gull-winged shaped 
fingers 16 to the circuit board 18 and applies heat to fuse the fingers 16 
to pads formed on the circuit board 18. In its installed position, the 
bottom of the die 15 is directly attached with a thermally conductive 
adhesive 26 to the top of a plurality of thermal vias 22 (i.e. 
metal-plated through holes) formed through circuit board 18. The bottom of 
the thermal vias 22 are coupled to the top of the heat sink 24 through 
thermal plane 27 and thermal grease. The TCP device 10 sinks heat through 
its bottom due to its die 15 being in direct contact with thermal vias 22 
on the circuit board 18, and ultimately through the heat sink 24 disposed 
underneath the circuit board 18. Alternatively, a heat pipe (not shown) 
could be coupled between the thermal plane 27 and a remotely located heat 
sink 24. 
By way of further background, a standard Ball Grid Array (BGA) package (not 
shown) is a piece of "circuit board-like" planar laminate material 
(however much thicker and sturdier than the TCP tape) with its bottom 
(i.e. the side towards the main circuit board in its mounted position) 
having a die attach cavity formed thereon with electrical traces etched 
from the die attach area to an array of solder balls (ergo BGA). After the 
die is affixed to the die attach area, the aluminum bond pads are bonded 
out to the etched traces on the laminate material by well known wire bond 
techniques and a resin encapsulant is applied over the die for protection. 
A thin piece of metal acting as a heat spreader is attached to the top of 
the BGA package and a heat sink is typically attached to the heat spreader 
to further dissipate heat. 
The BGA device is attached to the main circuit board by placing it so that 
its solder balls are aligned with attachment pads formed on the main 
circuit board and passing the entire circuit board through an oven that 
re-flows the solder balls. The BGA package, which has a small footprint, 
low clearance, and is easier to install than the TCP, sinks heat through 
the top mounted heat spreader in contact with a top mounted heat sink--as 
is conventional with most integrated circuit packages. Consequently, a 
drawback with switching a design with a part having a TCP to a BGA package 
in a tight form-factor product (e.g. a laptop computer), is that it 
requires extensive mechanical and thermal changes to the internals of the 
product (e.g. case size, circuit board mounts, and heat sink/heat pipe 
designs). 
Another drawback with the TCP is that it requires special tooling to: cut 
the assembly from the tape, form the leads into a gull wing like shape, 
and to place and solder the assembly onto the circuit board. Moreover, the 
thin flexible tape used as the substrate in a TCP is subject to warp under 
heat. Consequently, an attempt to adapt a TCP into a BGA arrangement is 
undesirable since standard re-flow soldering techniques would most likely 
warp the tape substrate thus skewing the solder balls off of the 
attachment pads--making attachment unreliable. Moreover, the attachment 
process becomes more complex and ostensibly more expensive if stiffeners 
are added to the tape substrate after the part is already formed and cut 
with the special TCP tooling. 
Another drawback with TCP is the die is required to have special gold bumps 
deposited on its aluminum pads for thermal compression bonding--adding 
complexity and cost to the product. 
Yet another drawback with the TCP is that the yield rates for successful 
circuit board installation are typically much lower as compared to more 
conventional packages such as the BGA since, among other things, the die 
which is brittle, is directly attached to the main circuit board which may 
be subject to twisting and shock. 
Yet another drawback, particularly with devices which have high power 
consumption requirements such as a processor, is that heat is dissipated 
through thermal vias which are formed in the circuit board--potentially 
weakening or causing damage to surrounding components. 
Accordingly, it can be seen from the foregoing, that there is a need for an 
alternative integrated circuit package that minimizes mechanical and 
thermal changes to systems originally designed for low profile TCP 
devices. 
SUMMARY OF THE INVENTION 
To overcome the limitations of the prior art described above, and to 
overcome other limitations that will become apparent upon reading and 
understanding the present specification, the present invention discloses 
an integrated circuit package having a top die attach area and a bottom 
heat spreader thermally coupled to the die, for conducting heat through 
the bottom of the die, through the main circuit board (preferably through 
a thin, thermally conductive slug mounted in an opening formed in the main 
circuit board), and ultimately through a heat sink or heat pipe to heat 
sink. The heat spreader spans the bottom surface of the package 
substantially parallel to the main circuit board and preferably extends 
substantially to the inner periphery of the pin arrangement which 
preferably, although not exclusively, is in a ball grid array. 
The preferred embodiment has an opening formed in the main circuit board 
through which the thin, thermally conductive slug is fitted substantially 
flush with the bottom surface of the circuit board and juxtaposed against 
the heat spreader after the device is installed. A thermally conductive 
adhesive is preferably applied to adjoining surfaces of the heat spreader 
and the slug and between adjoining surfaces of the slug and the heat sink, 
respectively. The slug provides a thermal interface between the heat 
spreader and the heat sink or heat pipe mounted underneath the circuit 
board without substantially transferring any heat to the main circuit 
board. 
A feature of the present invention is maintaining mechanical and thermal 
compatibility with exiting designs for TCP circuit layouts excepting or 
course, changes to the circuit board for the pin arrangements and 
preferably for the opening formed in the main circuit board to accommodate 
the slug. 
Another feature of the present invention is the integrated circuit package 
of the present invention is easy to install and only requires standard BGA 
manufacturing equipment and processes. 
Another feature of the present invention is that printed circuit board 
assembly yield rates are typically much higher for BGA devices than for 
TCP devices since the die is not directly affixed to the main circuit 
board. 
These and various other objects, features, and advantages of novelty which 
characterize the invention are pointed out with particularity in the 
claims annexed hereto and forming a part hereof. However, for a better 
understanding of the invention, its advantages, and the objects obtained 
by its use, reference should be made to the drawings which form a further 
part hereof, and to the accompanying descriptive matter, in which there is 
illustrated and described a specific example of an integrated circuit 
package in accordance with the principles of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Reference is now made to the accompanying drawings which form a part 
hereof, and in which is shown by way of illustration specific embodiments 
in which the invention may be practiced. It is to be understood that other 
embodiments may be utilized and structural changes may be made without 
departing from the scope of the present invention. 
Reference is now made to FIG. 2 which depicts a cross sectional view of an 
integrated circuit package 28 practiced in accordance with the principles 
of the present invention. A top cavity 30 for holding an integrated 
circuit die 32 is formed in a thin "circuit-board like" planar laminate 
material 29. The integrated circuit die 32 is bonded out with bond wires 
34 to electrical traces (not shown) on top of the laminate material 29 and 
coupled through electrical vias (not shown) formed in the laminate 
material 29 to pint embodied solder balls 36 forming a ball grid array 
(BGA) on the bottom of the material 29. Depending on the pin density, the 
laminate material 29 may be multi-layered with traces running between 
layers for coupling the die 32 to solder balls 36. Those skilled in the 
art with the aid of the present disclosure, will recognize other 
configurations for the connections (pins) and (pin) arrangement other than 
BGA (e.g. PGA and SPGA) without departing from the scope of the present 
invention. 
A heat spreader 38 preferably made of a thin piece of metal, is thermally 
coupled to the integrated circuit die 32 using thermally conductive 
adhesive 42 for dissipating heat through the bottom of the package 28. The 
heat spreader 38 spans the bottom surface of the planar material 29 
substantially parallel to the main circuit board 18 and preferably extends 
substantially to the inner periphery of the BGA pin arrangement. The top 
cavity 30, the integrated circuit die 32, and the bond wires 34 are 
encapsulated with a resin 33 (e.g. polyimide siloxane) for protection. 
The package 28 is placed on the circuit board 18 and passed through an oven 
to heat and re-flow the solder balls 36 for attachment to pads coupled to 
electrical traces formed on the circuit board 18. In the preferred 
embodiment, a thermally conductive slug 40 is then fitted through an 
opening 44 (FIGS. 3 and 4) formed in the circuit board 18 and coupled 
between the heat spreader 38 and the heat sink 24. A thin layer of 
thermally conductive adhesive 42 is preferably applied between the heat 
spreader 38 and the slug 40 and between the slug 40 and the heat sink 24. 
It should be understood that plated through thermal vias typically used 
with a TCP device, may be used in lieu of the thermally conductive slug 40 
and the formation of the opening 44, without departing from the scope of 
the present invention. 
Reference is now made to FIGS. 3 and 4 which depict a cross sectional 
exploded view of FIG. 2 and a perspective exploded view respectively, of 
the top cavity die attach area integrated circuit package 28 installed in 
a circuit board 18. While opening 44 in circuit board 18 may have a form 
factor larger than slug 40, it is preferably formed such that slug 40 
snugly fits therein and so that its bottom is substantially planar and 
parallel with the top or bottom surfaces of circuit board 18. Heat 
spreader 38, which is thermally and mechanically coupled to the top of 
slug 40, preferably has a thickness that substantially spans to the top of 
the slug 40 after the solder balls 36 have been heated and attached to the 
electrical traces on circuit board 18. 
Reference is now made to FIG. 5 which depicts a side view of a typical, 
although not exclusive, application for the present invention. A portable 
(laptop) computer 46 includes a flat screen display 47, a keyboard 49, and 
a motherboard 48 having, inter alia, a processor 50 mounted on it in a 
package practiced in accordance with the principles of the present 
invention. While FIG. 5 depicts a heat sink 52 mounted below the 
motherboard 48, those skilled in the art with the aid of the present 
disclosure, will recognize other configurations (e.g. conducting the heat 
with a heat pipe to a remotely mounted heat sink 52, or reversing the 
orientation of the motherboard 48 so that heat sink 52 is below the 
keyboard 49), without departing from the scope of the present invention. 
Although the Detailed Description of the invention has been directed to 
certain exemplary embodiments, various modifications of these embodiments, 
as well as alternative embodiments, will be suggested to those skilled in 
the art. The invention encompasses any modifications or alternative 
embodiments that fall within the scope of the claims.