Abstract:
A semiconductor package includes a chip carrier to receive a semiconductor with a dimension generally greater than 22 mm. The chip carrier has a first coefficient of thermal expansion that is larger than the coefficient of thermal expansion of the semiconductor. A heat spreader having parallel channels on opposite sides is attached to the chip carrier along the channels. The heat spreader has a second coefficient of thermal expansion that is smaller than or equal to the coefficient of thermal expansion of the chip carrier. The interplay between the heat spreader and the chip carrier can effectively reduce package warpage and maintain coplanarity within the specification.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates generally to the packaging of semiconductor devices. More particularly, this invention relates to a large semiconductor device package that minimizes the impact of different coefficients of thermal expansion (CTE) associated with the environment in which the package exists. The invention further relates to a specific heat spreader design and installation technique employed for reducing package warpage due to internal thermal stress caused, for example, by differences in coefficients of thermal expansion between different material layers. 
   DESCRIPTION OF THE RELATED ART 
   One important characteristic of microelectronic technology is that integrated circuit geometry becomes smaller and smaller and circuitry density gets higher and higher, resulting in more powerful performance and more complex functionality per chip. This dramatic performance and functionality improvement requires so many more input/output (I/O) connections between a chip and a printed circuit board (PCB) that many of today&#39;s chips have hundreds, or even thousands, of pads serving this interconnection purpose. Even though the chip size keeps growing, the space left for each pad on the chip keeps shrinking. The shrinkage of the chip pad area has reached a degree that exceeds the capability of traditional wire bond technology to make connections from the chip to the package. 
   Flip-chip (FC) technology has proved to be a more attractive approach to solving the pad area limitation problem. In this method, a pattern of solder bumps is formed on one side of a chip that carries the integrated circuits. The chip is flipped upside down, aligned with the same pattern of solder pads formed on a substrate, and the solder bumps are brought into contact with the solder pads. Upon heating, the solder in the bumps and pads flows together; and upon cooling, the solder will form mechanical and electrical joints that connect the chip to the substrate. 
   Substrates made of different materials have different advantages. Ceramic has been chosen for FC packages from the very beginning because of its ease of assembly and high-temperature performance. However, the high cost of ceramic substrates limits their application to high-end products such as a CPU. Due to its low cost, organic material is mote commonly used in a wider spectrum of applications, such as communication and automotive products. For example, Bismaleimide Triazine (BT) is one of the most popular advanced laminates and is widely used in FC packages. 
   The connection between the chip and the substrate is usually formed in a high temperature environment, e.g., 220° C. Because of the CTE difference between the chip and the substrate, thermal-mechanical stress and strain exist when the environmental temperature changes. This problem is more prominent with organic substrates than with ceramic substrates. For instance, while the CTE of the chip, mainly silicon, is only about 3 PPM/° C., and the CTE of ceramic material is about the same, the CTE of the BT substrate could be as high as 17 PPM/° C. in the plane of the substrate. 
     FIG. 1  is a schematic description of the thermomechanical deformation experienced by an organic substrate at different stages in an IC packaging process. As shown in view  10  of  FIG. 1 , substrate  112  contracts more than a chip  114  and bends itself into a convex shape after cooling to room temperature, following formation of solder joints  116  connecting chip  114  to substrate  112  at a high temperature such as 220° C. This contraction difference produces thermal stress and strain concentration on the solder joints between the chip and the substrate, potentially causing early fatigue failure of the system. 
   One approach to solve this stress and strain concentration problem is referred to as the “under-fill” operation, wherein a special liquid epoxy is used to fill completely the open space between the chip and the substrate. View  120  in  FIG. 1  shows the flip-chip bonded device from view  110  after it has been re-heated to a temperature of 70-10 0 ° C. to reduce the thermal strain and then injected with under-fill material  118  in the space between the chip and the substrate. Upon curing, the resulting encapsulation provides appropriate thermal stress relief by spreading the concentrated stress and strain from the deformed solder joints to the under-fill material. 
   Besides stress and strain concentration, the other CTE-related issue is package warpage, wherein the bottom surface of the substrate contracts more freely than the top surface due to solder joint restraints from the chip, resulting in the substrate bowing into a convex shape. This problem remains substantially unsolved after under-fill curing, as shown in view  130  in FIG.  1 . The reason is that the under-fill material primarily deals with shear stress and strain concentration on the joints. 
   The degree of package warpage mainly depends on the size of the chip and the package. When the package size is less than 27 mm 2 , the package warpage is a less critical problem. However, for large flip-chip packages wherein the chip size could be several hundred square millimeters, the package warpage could easily exceed 8 mils, the standard set by the Joint Electron Device Engineering Council (JEDEC). Excessive warpage of the package prevents the attachment of the substrate to the printed circuit board and is one of many factors causing low yields. 
   A heat spreader lid has been proved to be very effective in reducing package warpage in addition to dissipating the heat produced by the semiconductor device. The material chosen for making the heat spreader usually has high CTE that is similar to that of the organic material used for making the substrate. For example, copper has been frequently chosen for making the heat spreader because its CTE is also about 17 PPM/° C. 
   As shown in view  140  of  FIG. 1 , after attaching a heat spreader lid  119  to substrate  112 , the substrate warpage has been substantially corrected so that the electronic package remains flat. The reason is that the heat spreader is attached to the substrate via adhesive material, such as epoxy, in a high temperature environment. As the environment cools down, the heat spreader contracts in a similar fashion as the substrate wherein the top surface of the heat spreader contracts more quickly than the bottom surface. In other words, the heat spreader tends to bend itself upwards, producing normal stress at the contact area between the heat spreader and the substrate to pull back the substrate from bending downwards. If the heat spreader parameter is selected appropriately, the moment produced by the normal stress between the heat spreader and the substrate can effectively balance the moment produced by the shear stress between the chip and the substrate, maintaining the flatness of the package. 
   Mechanically speaking, the effectiveness of the heat spreader in terms of reducing package warpage depends on the size of the contacting area between the heat spreader and the substrate. Larger contacting area means more normal stress and more moment exerted on the substrate. Therefore, the more space is left for mounting the heat spreader, the better anti-warpage effect it can achieve.  FIGS. 2A ,  2 B, and  2 C show a 4-side heat spreader  200  and how it works in suppressing package warpage.  FIG. 2A  is a top perspective view of heat spreader  200  with a lid  210 .  FIG. 2B  is a bottom perspective view of heat spreader  200  with four contact edges  220 . As shown in  FIG. 2C , heat spreader  200  exerts normal stress on the substrate through the four edges to bend substrate  230  into a concave shape and therefore minimizes package warpage in every direction. When the chip size is relatively small and there is enough free space on the substrate for attaching the heat spreader lid, the 4-side heat spreader has been proved most effective in suppressing package warpage. 
   On the other hand, IC manufacturers continually try to increase the size of the chip to achieve more complex function on a single chip and meanwhile reduce the package size relative to the chip size to reduce the system&#39;s dimensions. This development trend implies that the die-to-package ratio is getting larger and larger. When the difference between the die size and the package size is smaller than 7 mm in one dimension, a flip-chip ball grid array (BGA) is defined as having an ultra large die-to-package ratio (ULDPR) FC BGA. 
   In a package like ULDPR FC BGA, there is not enough room left for attaching a 4-side heat spreader. Instead, a 4-corner post heat spreader has been used.  FIGS. 3A ,  3 B, and  3 C show a 4-corner heat spreader  300  and how it works in suppressing package warpage.  FIG. 3A  is a top perspective view of heat spreader  300  with a lid  310  and corner posts  320 .  FIG. 3B  is a bottom perspective view of heat spreader  300  with lid  310  and four posts  320 . Similar to the 4-side heat spreader, the 4-corner heat spreader  300  exerts normal stress on the four corner of substrate  330  to attenuate the package warpage, as shown in FIG.  3 C. Since the anti-warpage effect is approximately proportional to the size of the contact area between the heat spreader and the substrate, the 4-corner heat spreader is less effective compared with the 4-side heat spreader and the result is less homogeneous since the stress concentrates on the four corners. 
   In view of the foregoing discussion, it is highly desirable to provide an improved technique for mounting a heat spreader that can leverage the remaining free space on the substrate to the maximum so as to minimize the impact of material mismatches in the coefficients of thermal expansion. A need also exists for a packaging strategy that allows a large chip to be reliably mounted to a substrate while maintaining the coplanarity of the substrate within the JEDEC specification. Moreover, a need exists for a method of packaging integrated circuits that can be reliably connected to a motherboard containing other electrical circuits. 
   SUMMARY OF THE INVENTION 
   The present invention relates to an improved packaging technique that can maintain the flatness of a package like ULDPR FC BGA. More specifically, a chip is usually rectangular and a substrate square in shape. Therefore, there is more free space on two opposite sides of the substrate than the other two. The present invention is a heat spreader having parallel channels on opposite sides that takes advantage of this shape difference and attaches to the two sides with more space so as to substantially reduce the package warpage caused by the CTE difference between the chip and the substrate. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a schematic representation showing the interaction between a flip chip and a substrate at different stages in an IC packaging process due to the difference in coefficients of thermal expansion; 
       FIGS. 2A ,  2 B, and  2 C are three perspective views of a prior art 4-side heat spreader; 
       FIGS. 3A ,  3 B, and  3 C are three perspective views of a prior art 4-corner heat spreader; 
       FIGS. 4A ,  4 B, and  4 C are three perspective views of an illustrative channel heat spreader of the present invention; and 
       FIG. 5  illustrates a semiconductor package in which a channel heat spreader is attached to the substrate via adhesive material in accordance with an embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the prior art, a 4-side heat spreader has been proved very effective in suppressing package warpage only when the die-to-package ratio is relatively small and there is enough room on the substrate to attach the aside heat spreader. A 4-corner heat spreader is less effective in suppressing package warpage since the stress concentrates only on the four corners of the substrate. Since the die is usually in the shape of rectangle and the substrate in the shape of a square, there is more space on two opposite sides of the substrate than the other two sides. 
   This invention is a channel heat spreader wherein there are two parallel channels on the heat spreader that are attached to the two sides on the substrate with more space.  FIG. 4A  is a top perspective view, depicting a channel heat spreader  400  comprising a lid  410  and two parallel channels  420 .  FIG. 4B  is a bottom perspective view illustrating that each channel has a flat surface  422 , which is substantially the same width as substrate  430 . When heat spreader  400  is attached to substrate  430 , as shown in  FIG. 4C , the two channels  420  of heat spreader  400  contact two sides of the same surface of substrate  440  and heat spreader  400  exerts a significant amount of normal stress as well as the corresponding moment to pull back substrate  430  from bending into a convex shape. Unlike 4-corner heat spreader, the contact area in the present invention covers two complete sides. Therefore, there is much less stress concentration and much more homogeneous anti-warpage effect. Unlike 4-side heat spreader, the channel heat spreader leverages the free space on the substrate extensively and has been proved very effective when used with a package like ULDPR FC BGA. 
     FIG. 5  illustrates a semiconductor package with a channel heat spreader mounted on a substrate in accordance with an embodiment of the invention. 
   The package houses a large semiconductor device  510  (i.e., a semiconductor with at least one dimension typically larger than 22 mm). The semiconductor device  510  is made of material with a CTE less than 3 PPM/° C. The semiconductor device is mounted on a substrate  520 . The substrate  520  is made of organic material with a CTE between approximately 16 and 18 PPM/° C. 
   Solder bumps  530  interface the semiconductor device  510  to the organic substrate  520 . There are internal traces inside the substrate  520  (not shown in the figure) connecting the top surface of the substrate to the bottom surface of the substrate. These traces, together with solder balls  580 , provide electrical connection from the semiconductor device  510  to the printed circuit board (not shown in the figure), which also hosts other electronic circuits. 
   The semiconductor device  510  and the substrate  520  are assembled together in a high temperature environment of about 220° C. Due to the CTE difference, there is thermal stress between the semiconductor device  510  and the substrate  520  upon cooldown. 
   In order to reduce the impact of the CTE difference, under-fill resin  540  is injected into the empty space between the semiconductor device  510 , the substrate  520 , and the solder bumps  530 . The under-fill resin  540  surrounding the solder bumps can significantly relieve the thermal stress concentration on the solder bumps  530  and improve the reliability and longevity of the solder bump connections between the semiconductor device  510  and the substrate  520 . 
   The semiconductor device  510  is usually in the shape of a rectangle having two long edges and two short edges. In contrast, the substrate  520  is usually in a square shape having four edges of equal dimension. Therefore, when the semiconductor device  510  is positioned in the central region of the upper surface of substrate  520  with the two long edges of the device  510  parallel to two opposite edges of the substrate  520 , there is more open space on the substrate along these two opposite edges than the other two opposite edges. 
   The channel heat spreader  550  is usually made of metal material, such as copper having CTE equal to 17 PPM/° C., and is engaged with the substrate in a similar high temperature environment of about 150° C. The two channels of a channel heat spreader are attached by adhesive  560 , such as epoxy, to substrate  520  in the direction where there is more open space. 
   A thermal compound  570  (e.g., a thermal grease) is used between semiconductor device  510  and channel heat spreader  550 . When the package cools down to room temperature, heat spreader  550  contracts more than semiconductor device  510 . As a result, the two channels on heat spreader  550  exert normal stress as well as the corresponding moment on the substrate  520  to prevent it from bending into a convex shape. 
   In a preferred embodiment of the invention, the substrate is made of core material and build-up material. Core material is bismalesmide triazine (BT) and build-up material is Ajinomoto ABF. The solder bump is made of either 63Sn37Pb or 95Pb5Sn and its diameter is 100 um. The substrate thickness is about 1.3 mm in which the core occupies either 0.8 mm or 1.0 mm. The dimensions of channel heat spreader  550  are about 27×27 mm 2 . The dimensions of the channels are about 27×2×0.6 mm 3 . The thickness of the heat spreader lid is about 0.5 mm. Thus, the thickness of the heat spreader as measured at the channels is about 1.1 mm. The average package warpage was reduced to 3.98 mils, in contrast with 7 mils average package warpage from a same-size 4-corner heat spreader, or a 43% improvement. The maximum package warpage was reduced to 5.28 mils, compared against 9.8 mils from a same-size 4-corner heat spreader, or 54% improvement. 
   The invention disclosed herein provides methods of package construction that allow for the packaging of a first part having low CTE characteristics assembled to a structure of a second part having high CTE characteristics and then a third part having similarly high CTE characteristics assembled to the second part. The construction package and techniques of the invention provide for mechanical stress alleviation and low warpage through a layered structure with different coefficients of thermal expansion. 
   The final package assembly can then be mounted to a motherboard or similar construction that has a CTE which closely matches the CTE of the second part. The methods disclosed herein may apply to chips of various sizes, including chips having a form factor that is large in comparison to the overall substrate size. 
   The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. Thus, the foregoing disclosure is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible in view of the above teachings. 
   It is intended that the scope of the invention be defined by the following claims and their equivalents.