BACKGROUND OF THE INVENTION Heat sinks are used in a variety of electronic devices to dissipate heat from the device, preventing any failure of or damage to the device. Heat sinks are made of a variety of heat conducting materials and are formed in a variety of shapes to adequately conduct the heat from the device. More specifically, heat sinks are also used with integrated circuits to reduce the heat of the semiconductor die. to enable the integrated circuit to function properly.

The purpose of the heat sink is to keep a semiconductor junction, such as a junction of the transistor, below a maximum specified operating temperature. Knowing the power that will be dissipated in a given integrated circuit, it is possible to calculate a junction temperature. Because the transistor life declines rapidly at operating temperatures near a maximum operating temperature, the heat sink is chosen to keep the semiconductor junction below the maximum specified temperature.

All semiconductor packages can use other thermal enhancements, which can range from simple airflow to schemes that can include passive as well as active heat sinks. This is particularly true for the high performance flip chip packages which can be designed to handle in excess of 20 watts with arrangements that take system physical constraints into consideration. The use of lightweight finned external passive heat sinks can be effective for dissipating up to 10 watts in the bigger packages. For moderate power dissipation (e. g. less than 6 watts), the use of passive heat sinks and heat spreaders attached with thermally conductive double-sided tapes or retainers can offer quick thermal solutions in these packages.

Typically, flip chip packages are thermally enhanced ball grid arrays (BGAs) with die facing down. They are offered with exposed metal heat sink at the top. These are considered high-end thermal packages and they lend themselves to the application of external heat sinks (passive or active) for further heat removal efficiency.

However, these types of conductive lid heat sinks, often exhibits the problem of separating from the top of a package. Because the adhesion strength of the adhesive is sometimes not strong enough to hold the conductive lid to the package, the conductive lid can come off of the package.

Accordingly, there is a need for an improved conductive lid functioning as a heat sink and method of employing a conductive lid with an integrated circuit.

A lid for an integrated circuit, commonly made of a metallic or ceramic material, is attached on top of a flip chip package to provide protection to the silicon die and other discrete surface mount components and to increase thermal conductivity of the package. Proper material construction and processing of the lid, the package, and the adhesive is required so that the lid will stay attached to the package during the assembly and long term field usage. Degradation of the adhesive can be caused by many factors, including improper dispensing of adhesive that does not produce full adhesive surface as designed, shear stresses introduced by thermal expansion/shrinkage differences at the interfaces, push/pull stresses introduced by silicon/package/board warpage, adhesive degradation due to chemical attack, etc. Excessive component handling stresses during test, assembly, or rework, such as excessive torque or pressure, can also result in separation of the lid from the package.

SUMMARY OF THE INVENTION Accordingly, there is a need for an improved lid for an integrated circuit and method of securing a lid to an integrated circuit. The embodiments of the present invention relate to adding through-holes to the corners and/or sides of a conductive lid for an integrated circuit.

The holes may be made anywhere along the epoxy dispense area to improve the strength of the bond between the conductive lid and the substrate. The configuration and total number of holes created can vary for each application. The holes preferably extend from the bottom of the conductive lid which is bonded to the package through to the top of the conductive lid. Although straight holes can also be utilized, the holes can be tapered in shape with the top larger in diameter.

In particular, a conductive lid adapted to function as a heat sink for an integrated circuit comprises a recessed portion adapted to receive a die of an integrated circuit and a foot portion having a flat surface adapted to be coupled to a substrate of the integrated circuit. A through-hole or plurality of through-holes are located in the foot portion. The through-hole is adapted to receive an adhesive to secure the conductive lid to the substrate of the integrated circuit. The through-hole could be a straight through-hole, or could be a tapered through-hole, such a conical through-hole or a multi-diameter through- hole or other through-hole where the hole has a plurality of widths. The adhesive is preferably a thermally conductive resin to conduct heat from the integrated circuit.

A method of forming a conductive lid for an integrated circuit is also disclosed. The method generally comprises the steps of forming a recessed portion for receiving a die of an integrated circuit; creating a foot portion around the recessed portion; and providing a plurality of through- holes in the foot portion. The method could be implemented by using a molding process, such as injection molding, or by a stamping process, wherein the through-holes are either punched or drilled in the foot portion of the conductive lid. Methods of securing a conductive lid to a substrate by receiving an adhesive in a through-hole, and using a conductive lid having a through-hole to conduct heat from an integrated circuit are also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a top plan view of an integrated circuit having a conductive lid according to the present invention; Fig. 2 is a cross-sectional view of the integrated circuit having a conductive lid of Fig. 1 according to one embodiment of the present invention; Fig. 3 is a cross-sectional view of the integrated circuit having a conductive lid of Fig. 1 according to an alternate embodiment of the present invention.

Fig. 4 is a cross-sectional view of the integrated circuit having a conductive lid of Fig. 1 according to a further embodiment of the present invention; Fig. 5 is a more detailed cross-sectional view of a packaged integrated circuit having a conductive lid of Fig.

1 according to one embodiment of the present invention; Fig. 6 is a flow chart showing a method of forming a conductive lid by a molding process according to an embodiment of the present invention ; Fig. 7 is a flow chart showing a method of forming a conductive lid by a stamping process according to an embodiment of the present invention; Fig. 8 is a flow chart showing a method of securing a conductive lid in an packaged integrated circuit according to an embodiment of the present invention; Fig. 9 is a flow chart showing a method of using a conductive lid on an integrated circuit according to an embodiment of the present invention; Fig. 10 is a top plan view of a printed circuit board having a plurality of components according to an embodiment of the present invention; Fig. 11 is a perspective view of a component 1104 according to an embodiment of the present invention; Fig. 12 is a partial-sectional view showing an enlarged portion of the component 1104 of Fig. 11; Fig. 13 is a cross-sectional view of component 1104 taken at lines 4-4 of Fig. 11; Fig. 14 is a cross-sectional view of component 1104 taken at lines 5-5 of Fig. 11;

Fig. 15 is a series of figures showing the formation of an integrated circuit according to an embodiment of the present invention; Fig. 16 is a series of figures showing the formation of an integrated circuit according to an alternate embodiment of the present invention; Fig. 17 is a flow chart showing a method of forming a lid according to an embodiment of the present invention; Fig. 18 is a flow chart showing a method of applying a lid to an integrated circuit according to an embodiment of the present invention; Fig. 19 is a flow chart showing a method of securing a lid to an integrated circuit using an adhesive according to an embodiment of the present invention; and Fig. 20 is a flow chart showing a method of securing a lid to an integrated circuit using solder according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS Turning first to Fig. 1, a top plan view of an integrated circuit having a conductive lid according to the present invention is shown. In particular, a conductive lid functioning as a heat sink for a integrated circuit comprises a through-hole 104. A plurality of through-holes 104 could be added to the corners and/or sides of the conductive lid. The configuration and total number of holes created can vary for each application. The through- holes may be made anywhere along the epoxy dispense area to improve the strength of the bond between the conductive lid and the substrate. The through-holes of Fig. 1 are positioned on both the corners and along the sides, and are generally evenly spaced. As shown in the cross-sectional view of Fig. 2 taken at lines A-A of Fig. 1, an integrated circuit 200 comprises through-holes 104 extending from the top 206 of the conductive lid to the bottom of the conductive lid at a foot portion 208, which preferably extends around the periphery of the conductive lid. The conductive lid 102 is positioned on the package 204. The conductive lid could be made of any conductive material, such as copper or aluminum, or any other suitable metal or

alloy. During lid attachment, the epoxy will penetrate into the through-holes and become cured. When the integrated circuit is packaged is assembled, as will be described in more detail in reference to Fig. 5, the lid would then be"anchored"in place by the epoxy.

Turning now to Fig. 3, a cross-sectional view of the integrated circuit having a conductive lid according to an alternate embodiment of the present invention is shown.

While straight holes can be utilized as shown in Fig. 1, the holes can be tapered in shape with the top generally larger in diameter to improve the adhesive affect of the through- hole. In particular, a conducted lid 302 comprises a through-hole 304 which is adjacent to a recess 306. The through-hole 304 preferably extends from the top 308 of the conductive lid to a foot portion 310. The through-hole could be formed by an injection molding, drilling or some other suitable method of forming holes in a heat sink.

Although a conical shaped hole is shown, any other shape could be employed. The larger diameter at the top acts like the head of a screw or nail to prevent separation of the metal lid from the package.

Turning now to Fig. 4, a cross-sectional view of the integrated circuit having a conductive lid according to a further embodiment of the present invention is shown.

According to the embodiment of Fig. 4, a conductive lid 402 comprises an example of a multi-diameter hole 404 having a small hole 406 formed from a foot portion 408 and a large hole 410 formed from the top 412 of the conductive lid.

Small hole 406 and large hole 410 generally form cylindrical portions. The larger diameter hole at the top also acts metal lid from the package when the adhesive material extends into the larger hole 410.

Turning now to Fig. 5, a more detailed side elevation of a packaged integrated circuit having a conductive lid according to one embodiment of the present invention is shown. In particular, a semiconductor die 502 having a plurality of bumps 504 of a flip chip device is coupled to a substrate 506. Underfill 508 and a thermal grease 510, which is thermally conductive, surround the semiconductor die, as is well known in the art. The substrate may

further comprise a plurality of contacts 520. A recess 512 of the conductive lid provides room for the semiconductor die 502 and defines a foot portion 514. As can be seen in Fig. 5, the foot portion is positioned on an adhesive material 516. The adhesive material could be any type of epoxy or resin used for attaching a heat sink to a substrate. The adhesive material is also preferably thermally conductive for better heat dissipation. An example of an appropriate adhesive material is a Modified Cyclo-Olefin Thermoset (MCOT) supplied by Ablestik Laboratories, Rancho Dominguez, California. As can be seen, a portion of the resin 518 extends into the through- hole 304 to adhesive material 516 to secure the conductive lid 302 to the substrate 506.

Turning now to Fig. 6, a flow chart shows a method of forming a conductive lid by a molding process according to an embodiment of the present invention. In particular, a mold for an injection molding process to create a conductive lid for integrated circuit is formed at a step 602. A recessed portion is formed for receiving a die of, an integrated circuit using the mold at a step 604. A foot portion around the recessed portion is formed using the mold at a step 606. Finally, a plurality through-holes are formed in the foot portion using the mold at a step 608.

The mold could comprise any type of mold for any appropriate molding process, such as injection molding.

The resulting conductive lid could be any type of conductive lid having a through-hole for receiving an adhesive material, such as the conductive lids shown for example in Figs. 2-4.

Turning now to Fig. 7, a flow chart shows a method of forming a conductive lid by a stamping process according to an embodiment of the present invention. In particular, a conductive lid for an integrated circuit is created at a step 702. A recessed portion in the conductive lid for receiving a die of the integrated circuit is formed a step 704. A foot portion is formed around the recessed portion of the connective lid at a step 706. A plurality of through-holes are formed in the foot portion in a first step at a step 708, and the plurality of through-holes are

formed in a second step at a step 710. An example of a conductive lid having through-holes formed by steps 708 and 710 can be seen in cross-sectional view of Fig. 4.

Turning now to Fig. 8, a flow chart shows a method of securing a conductive lid in a packaged integrated circuit according to an embodiment of the present invention. A conductive lid for an integrated circuit having through- holes is provided at a step 802. An underfill is provided between a substrate and a die of the integrated circuit at a step 804. A thermal grease is also provided on top of the die of the integrated circuit a step 806. An adhesive material is also applied to the substrate at a step 808.

Finally, the conductive lid is positioned on the adhesive material such that the adhesive extends into the through- hole at a step 810. The method of Fig. 8 could be used to secure any type of conductive lid having a through-hole, such as a conductive lid shown in Figs. 2-4.

Finally, turning to Fig. 9, a flow chart shows a method of using a conductive lid on an integrated circuit according to an embodiment of the present invention. A substrate for receiving a die is provided at a step 902. A thermal grease is provided to the die at a step 904. An adhesive material is applied to the substrate at a step 906, and the conductive lid having a through-hole extending thru the conductive lid to substrate secures the conductive lid to substrate. Finally, heat from the integrated circuit is conducted from the die by way of the conductive lid and the adhesive material in the through-hole at a step 910. The method of Fig. 9 could be used to conduct heat from the die of any integrated circuit having a conductive lid with a through-hole, such as a conductive lid shown in Figs. 2-4.

Turning to Fig. 10, a top plan view of a printed circuit board having a plurality of components according to an embodiment of the present invention is shown. In particular, a printed circuit board 1102 comprises a plurality of components 1104, such as a BGA having a lid according to the present invention. A perspective view of a component 1104 according to an embodiment of the present invention comprises a substrate 1202 and a lid 1204 as

shown in Fig. 11. Also shown in Fig. 11 are the recesses 1206 and edge walls 1208 that are positioned along the four sides of the lid 1204. Additional details of the lid 1204 will be described in more detail in reference to the partial-sectional view showing an enlarged portion of the component 1104 taken at lines 3-3, the cross-sectional view of component 1104 taken at lines 4-4 through an edge wall 1208, and the cross-sectional view of component 1104 taken at lines 5-5 through a recess 1206.

Turning now to Fig. 12, a partial-sectional view shows an enlarged portion of the component 1104 taken at lines 3- 3. In particular, the recesses 1206 can be seen in more detail. According to one embodiment of the present invention, recesses 1206 comprise semi-circles. As shown in Fig. 12, the semi-circle recesses have beveled edges 1302 which comprise partial conical surfaces. The recesses 1206 may be formed, for example, by stamping, etching, molding design, injection molding or milling/drilling, all of which are well-known methods in the art of forming lids for integrated circuits. As will be described in more detail in reference to the remaining figures, excess bonding agent between the lid 1204 and the substrate 1202 forms a bond post 1304 for locking the lid to the substrate. Advantageously, the recesses expose the bonding agent to allow, for example, visual inspection of the bond between the lid and the substrate. Also shown is a corner 1306 of the lid between two end recesses on adjacent sides of the lid. Although the recesses are formed as partial conical structures at the edges of the lid, the recesses may be cylindrical or conical holes extending through the foot of the lid, as shown above.

Turning now to Fig. 13, a cross-sectional view of component 1104 taken at lines 4-4 is shown. In particular, component 1104 comprises a BGA having a flip chip 1402 coupled to the substrate 1202 by a plurality of solder bumps 1404. The flip chip 1402 and any other components are positioned within a recessed portion 1405 of the lid 1204. The lid may be a square having an approximate thickness between 0.5mm and 3mm, although other shapes and/or thicknesses may be used. Also shown in the cross

sectional view is an underfill layer 1406 between the flip chip 1402 and the substrate 1202. A bonding agent 1408 enables a foot portion 1409 of the lid to be secured to the substrate 1202. Because the cross section is taken through an edge wall 1208, a foot portion 1409 extends from the recess 1405 to the end of the substrate 1202. Finally, a thermal gel 1410 can be used to secure the lid to the flip chip 1402. The thermal gel may be, for example, lid attach epoxy from AI Technology, Inc. , Princeton Junction, NJ.

The thermal gel is generally thermally conductive and a soft adhesive.

As can be seen in the cross-sectional view of component 1104 taken at lines 5-5, the reduced foot portion 1502 remaining after the formation of the recess 1206 provides a bond 1504 between the lid 1204 and the substrate 1202. The bonding feature of bond post 1304 can also be seen in more detail. By applying a bonding agent according to any embodiment of the present invention, the enhanced bonding strength provides long term stability and allows the footprint width requirement to be significantly reduced. For example, the width of the foot portion 1502 can be reduced from a width of approximately 4mm width to approximately 2mm. This is beneficial for enabling packages with large die and/or multiple discrete components to be accommodated within a lid. Also, although a flip chip is shown in the cross sections of Figs. 13 and 14, any other element may be employed in the component 1104. For example, a chip lead component or any type of discrete component may be employed according to various embodiments of the present invention.

Turning now to Fig. 15, a series of figures having cross-sectional views shows the formation of an integrated circuit according to an embodiment of the present invention. In particular, a flip chip 1402 having a plurality of solder bumps 1404 are coupled to a substrate 1202 as shown in Fig. 15-A. An underfill layer 1406 is then applied as shown in Fig. 15-B. The underfill layer may be any material well known in the art of semiconductor manufacturing. A bonding agent is then applied to the substrate. The bonding agent is preferably provided around

the perimeter of the substrate. A portion of the bonding agent is shown as an adhesive layer 1602 in Fig. 15-C. The adhesive may be an epoxy adhesive from Bondline Electronic Adhesive, Inc. , San Jose, CA, for example, or any other adhesive well know in the art of semiconductor manufacturing. A thermal gel 1410 may then be applied between the lid and the die as shown in Fig. 15-D.

Finally, the lid 1204 is attached and solder balls 1412 are applied to the substrate as shown in Fig. 15-E. A portion of the adhesive extends over the beveled edge 1302 to form an adhesive bond post 1604. Also, an adhesive bond 1606 is provided to the reduced foot portion 1502.

Turning now to Fig. 16, a series of figures having cross-sectional views shows the formation of an integrated circuit according to an alternate embodiment of the present invention. The lid of the present invention can be attached to the package by soldering or surface mount solder reflow, which is a reliable method for attaching electronic components. The solder bonding can provide stronger and more stable and reliable adhesion. In particular, a flip chip 1402 having a plurality of solder bumps 1404 are coupled to a substrate 1202 as shown in Fig.

16-A. An underfill layer 1406 is then applied as shown in Fig. 16-B. A solder layer 1702 is then preferably provided around the perimeter of the substrate as shown in Fig. 16- C. The solder layer is preferably on top of a receiving surface on the substrate with a suitable metal surface such as a nickel/gold (Ni/Au) plated copper (Cu) pad, or an organic surface protection (OSP) protected copper surface.

Alternatively, solder paste from Senju Metal Industry Co. of Japan or any other solder well know in the art of semiconductor manufacturing may be used. A thermal gel 1410 may then be applied as shown in Fig. 16-D. Finally, the lid is attached and solder balls 1412 are attached as shown in Fig. 16-E. A portion of the solder extends over the beveled edge 1302 to form a solder bond post 1704.

Also, a solder bond 1706 is provided to the reduced foot portion 1502.

The use of solder in providing a mechanical bond according to the embodiment of Fig. 16 also provides other

benefits. For example, the package/device ground net can be electrically connected to the lid, if the lid is made of a conductive material, thus providing good ground shielding of the component. A further advantage of solder bonding is that solder is generally more immune to chemical attack, such as a by-product of cleaning during assembly and surface mount, than an adhesive bond such as epoxy. Solder is also generally more robust against moisture and temperature induced weakening of bonds.

Turning now to Fig. 17, a flow chart shows a method of forming a lid according to an embodiment of the present invention. A lid having a recessed portion for receiving a die of an integrated circuit is formed at a step 1802. A foot portion is created around the recessed portion at a step 1804. A plurality of recesses is provided at the edges of the foot portion at a step 1806. The recesses on the lid may be, for example, the recesses shown in the embodiment of Figs. 10-12. Finally, the lid is secured to the substrate by way of the foot portion and the recesses at a step 1808. That is, a portion of the adhesive preferably extends over an edge of the recess to form a bond post. The lid may be secured to the substrate according to any of the bonding methods described in reference to Figs. 15 and 16, for example.

Turning now to Fig. 18, a flow chart shows a method of applying a lid to an integrated circuit according to an embodiment of the present invention. A lid having a plurality of recesses at the edge of a foot portion, such as the lid described in reference to Figs. 10-12, is provided at a step 1902. A bonding agent is applied to a substrate of an integrated circuit at a step 1904. The foot portion of the lid is positioned on the adhesive applied to the substrate at a step 1906. Finally, the lid is secured to the substrate by way of the bonding agent within the recesses of the foot portion at a step 1908.

Turning now to Fig. 19, a flow chart shows a method of securing a lid to an integrated circuit using an adhesive according to an alternate embodiment of the present invention. A lid having a plurality of recesses at the edge of a foot portion is provided at a step 2002. An

adhesive is applied to a substrate of an integrated circuit at a step 2004. An adhesive is then optionally applied between. The foot portion of the lid is positioned on the adhesive applied to the substrate at a step 2008. Finally, the lid is secured to the substrate by way of the adhesive below the foot portion and within recesses at a step 2010.

The materials and/or steps for securing the lid to an integrated circuit may be, for example, the materials and steps described in reference to Fig. 15.

Turning now to Fig. 20, a flow chart shows a method of securing a lid to an integrated circuit using solder according to an alternate embodiment of the present invention. A lid having a plurality of recesses at the edge of a foot portion is provided at a step 2102. Solder is applied to a substrate of an integrated circuit at a step 2104. An adhesive is then optionally applied between a die of the integrated circuit and the lid at a step 2106.

The foot portion of the lid is then positioned on the solder applied to the substrate at a step 2108. Finally, the lid is secured to the substrate by way of the solder below the foot portion and within recesses of the foot portion at a step 2110. The lid may be attached to the package by, for example, soldering or surface mount solder reflow. The materials and/or steps for securing the lid to an integrated circuit may be, for example, the materials and steps described in reference to Fig. 16.

It can therefore be appreciated that the new and novel heat sink and lid for an integrated circuit and method of forming and attaching a heat sink and/or a lid to an integrated circuit has been described. It will be appreciated by those skilled in the art that numerous alternatives and equivalents will be seen to exist which incorporate the disclosed invention. As a result, the invention is not to be limited by the foregoing embodiments, but only by the following claims.