Stacked module systems

The present invention stacks integrated circuit packages into circuit modules. In a preferred embodiment, solder paste and primary adhesive respectively are applied to selected locations on the flex circuitry. Supplemental adhesive is applied to additional locations on the flex circuitry, CSP, or other component. The flex circuitry and the CSP are brought into proximity with each other. During solder reflow operation, a force is applied and the CSP collapses toward the flex circuitry, displacing the primary adhesive and the supplemental adhesive. The supplemental adhesive establishes a bond providing additional support to the flex circuitry. In another embodiment, CSPs or other integrated circuit packages are bonded to each other or to other components with a combination of adhesives. A rapid bond adhesive maintains alignment of the bonded packages and/or components during assembly, and a structural bond adhesive provides additional strength and/or structural integrity to the bond.

TECHNICAL FIELD

The present invention relates to aggregating integrated circuits and, in particular, to stacking integrated circuits using flex circuitry.

BACKGROUND

A variety of techniques are used to stack packaged integrated circuits. Some methods require special packages, while other techniques stack conventional packages.

The predominant package configuration employed during the past two decades has encapsulated an integrated circuit in a plastic surround typically having a rectangular configuration. The enveloped integrated circuit is connected to the application environment through leads emergent from the edge periphery of the plastic encapsulation. Such “leaded packages” have been the constituent elements most commonly employed by techniques for stacking packaged integrated circuits.

Leaded packages play an important role in electronics, but efforts to miniaturize electronic components and assemblies have driven development of technologies that preserve circuit board surface area. Because leaded packages have leads emergent from peripheral sides of the package, leaded packages occupy more than a minimal amount of circuit board surface area. Consequently, alternatives to leaded packages known as chip scale packages or “CSPs” have recently gained market share.

A commonly used style of CSP provides connection to a packaged integrated circuit through a set of contacts (often embodied as “bumps” or “balls”) arrayed across a major surface of the package. Instead of leads emergent from a peripheral side of the package, contacts are placed on a major surface and typically emerge from the bottom surface of the package.

The absence of “leads” on package sides renders most of the conventional stacking techniques devised for leaded packages inapplicable for CSP stacking. Frequently, CSP stacking provides one or more flex circuits interconnecting the contacts of respective CSPs. Also, CSP stacking may more often dispose one CSP bonded to another CSP of the stack. Conventional stacking techniques devised for leaded packages also often are inadequate for stacking integrated circuits packaged in different forms, such as a stack comprising both CSPs and leaded packages.

A variety of previous techniques for stacking CSPs and mixed integrated circuit packages may present complex assembly problems. Therefore, a technique and system is needed for stacking CSPs that provides a thermally-efficient, reliable structure allowing efficient production at reasonable cost with readily understood and managed materials and methods.

SUMMARY

The present invention stacks integrated circuit packages into modules that conserve PWB or other board surface area and prepares units containing integrated circuit packages for such stacking. Although the present invention is applied most frequently to CSPs that contain one die, it may be employed with CSPs and other integrated circuit packages that include more than one integrated circuit die. Multiple numbers of integrated circuit packages may be stacked in accordance with the present invention. The integrated circuit packages employed in stacked modules devised in accordance with the present invention are connected with flex circuitry that may exhibit one or two or more conductive layers.

In accordance with a preferred embodiment, a combination comprising a form standard and a CSP is attached to flex circuitry. Solder paste is applied to first selected locations on the flex circuitry and primary adhesive is applied to second selected locations on the flex circuitry. Supplemental adhesive is applied to additional locations on the flex circuitry, CSP, form standard, or other component of the combination. The flex circuitry and the combination of the form standard and CSP are brought into proximity with each other. During solder reflow operation, a force is applied that tends to bring the combination and flex circuitry closer together. As the heat of solder reflow melts the contacts of the CSP, the combination collapses toward the flex circuitry displacing the primary adhesive and the supplemental adhesive as the solder paste and contacts merge into solder joints. In a preferred embodiment, the form standard will be devised of heat transference material, a metal, for example, such as copper would be preferred, to improve thermal performance. In other preferred embodiments, a CSP without a form standard is attached to flex circuitry. The supplemental adhesive establishes a bond providing additional support to the flex circuitry.

In another embodiment, CSPs or other integrated circuit packages are bonded to each other or to other components with a combination of adhesives. A rapid bond adhesive maintains alignment of the bonded packages and/or components during assembly, and a structural bond adhesive provides additional strength and/or structural integrity to the bond.

DESCRIPTION OF PREFERRED EMBODIMENTS

Although several embodiments are described herein, the present invention can be used to advantage with CSPs or leaded packages of a variety of sizes and configurations ranging from larger packaged base elements having many dozens of contacts to smaller packages including, for example, packages approaching the size of the die such as die-sized ball grid array packages. Although the present invention is applied most frequently to packages that contain one die, it may be employed with packages that include more than one integrated circuit die.

FIGS. 1A-1Cdepict the construction of an exemplary unit58in accordance with a preferred embodiment for stacking in a circuit module. Form standard35is devised to be employed with a CSP in the disclosed embodiment to provide a standard form for flex circuitry. Form standard35is attached to the upper major surface18of CSP12with adhesive37and partially wraps around lateral edges of CSP to form a primary combination50. In respective preferred embodiments, adhesive37is a thermoset adhesive or epoxy that will not soften during subsequent reflow operations such as exposure to 200-250 degrees Centigrade, for example. Unit58in the depicted embodiment comprises primary combination50and flex circuitry30.

The depicted configuration of form standard35is just one of many that can provide a standard form about which flex circuitry may be disposed. Use of a form standard allows a connective design implemented in flex circuitry to be used with CSPs of a variety of designs and configurations. Form standard35may also provide thermal advantages particularly when devised from metallic materials such as copper and copper alloys for example. Other configurations of form standard35may be employed with the present invention including but not limited to those that extend across the bottom surface20of CSP12. Further, some form standards may not extend beyond the perimeter of CSP12. Still other embodiments of the invention may affix flex circuitry to CSP bodies without employing a form standard, and the flex circuitry may partially wrap about a lateral edge of the CSP as shown for example inFIGS. 5 and 6.

The flex circuitry in this embodiment comprises a contiguous flex circuit30, but other embodiments may use two or several flex circuits. In addition, the flex circuitry may be flexible throughout or flexible in some areas and rigid in other areas. Flex circuitry in various embodiments may have one or two or more conductive layers, and also may have one or more outer layers and/or intermediate layers. Flex circuitry has solder paste41applied at selected sites. In the illustrated embodiments, primary adhesive44is also applied at selected sites on the flex circuitry that are proximal to the lateral edge of lower major surface20of CSP12.

InFIG. 1B, the primary combination50and the flex circuitry have been disposed in proximity to each other. Typically, there will be contact between contacts26and solder paste41, but a large gap “G” between flex circuitry and form standard35will be exhibited because primary combination50is suspended above flex circuit30by primary adhesive44and the uncompressed height of contacts26and solder paste41. Weight52is disposed above CSP12on primary combination50while flex circuit30is supported from beneath by work support54. Work support54is preferably a carrier that is in motion through an assembly process or may be stationary. Primary combination50and the flex circuitry are subjected to a solder reflow operation, examples of which are well known to those of skill in the art.

With primary combination50and flex circuit30under force F which tends to move them closer together, primary combination50collapses toward the flex circuitry as contacts26melt in the solder reflow operation and merge with the solder paste on flex circuit30to form solder joints56as primary adhesive44is compressed as well. As a result of such compression, primary adhesive44comes in contact with form standard35and disposes form standard adhesive bonds on form standard35comprising primary adhesive44along a line approximately parallel to the lateral edges of lower major surface20. In respective preferred embodiments, primary adhesive44cures after the solder has melted.

After appreciating this specification, those of skill will recognize that force F may be applied by several methods and apparatus including weights and fixtures that apply force F during the reflow operation that melts contacts26. For example, an alternate system using a fixture40to apply force F is shown inFIG. 1C. These processes are amenable to implementation in standard pick and place operations known in the art.

As shown inFIGS. 1A-1C, portions of flex circuitry30may be partially wrapped about form standard35and, preferably, bonded by primary adhesive to the upper surface of form standard35. In such wrapped configuration, flex circuitry30also becomes partially wrapped about lateral sides of lower major surface20of CSP12. Flex circuitry30disposed in such wrapped configuration may express contacts to connect with contacts26of another unit58or contacts of another CSP or leaded package integrated circuit. Details of such wrapping and connecting methods and structures are not repeated, but are disclosed, for example, by U.S. Pat. No. 6,576,992 B1, No. 6,914,324 B2, and No. 6,940,729 B2, each of which is incorporated herein by reference.

FIG. 1Ddepicts a unit58devised in accordance the described methods and comprising CSP12, form standard35, and flex circuitry30.FIG. 1Dillustrates an adhesive failure60at the bond between flex circuitry30and form standard35. Adhesive failures may be caused by handling and manufacturing variances during the production process. An adhesive failure60may allow relaxation of flex circuitry30and distorted solder joints61, which in turn may cause undesirable deviation from coplanarity of a circuit module or its components.

The use of supplemental adhesive provides additional support to the flex circuitry configuration. Various advantages may result from such additional support, such as an increase in the integrity of component alignment in case of an adhesive failure.

FIG. 2depicts a preferred embodiment in which supplemental adhesive46is disposed between CSP12and flex circuitry30and between sets of contacts26so that the supplemental adhesive bonds formed by supplemental adhesive46are distal from and between the primary adhesive bonds formed by primary adhesive44. In the illustrated embodiment, supplemental adhesive is disposed along a line approximately parallel to the lateral edges of lower major surface20. Supplemental adhesive46can be the same adhesive used for primary adhesive44or a different adhesive, but preferably will allow solder reflow without interfering with the geometry of solder joints56, for example by curing after solder joints56have formed and stabilized.FIG. 3depicts unit58of such embodiment fully assembled for stacking.

Depending on the configuration of integrated circuit package12, contacts26, flex circuitry30, and other components that may be comprised in unit58, those of skill in the art will appreciate that the quantity of supplemental adhesive46can be varied according to the desired volume of the supplemental bond with the flex circuit. For example,FIG. 4Adepicts the use of a lesser quantity of supplemental adhesive46to fill the smaller gap between the overmold disposed on lower major surface20of CSP12and the flex circuitry30of the illustrated embodiment, compared withFIG. 4Bthat depicts the use of a greater quantity of supplemental adhesive46to fill the larger gap between the planar lower major surface20of CSP12and the flex circuitry30of that illustrated embodiment. For further example, similar variations in the quantity of supplemental adhesive46may be appropriate to accommodate variations in the heights of solder joints56. Those of skill in the art further will appreciate that other or additional locations may be available at which the use of varying amounts and types of supplemental adhesive in accordance with the invention disclosed herein may increases the integrity of component alignment in case of an adhesive failure.

As depicted for example inFIG. 5, stiffeners51may be used for embodiments that do not deploy a form standard such as form standard35ofFIGS. 1-4. As shown inFIGS. 5 and 6, the flex circuitry in the disclosed embodiments partially wraps around lateral edges of lower major surface20of CSP12, and primary adhesive44is disposed proximal to such lateral edges. In such embodiments, an adhesive failure may occur at the bond between a stiffener51and CSP12, as depicted for example inFIG. 5.FIG. 6illustrates the deployment of supplemental adhesive46proximal to lower major surface20of CSP12but distal from the bond of primary adhesive44in accordance with a preferred embodiment of the present invention, and the resulting improvement in the geometry of distorted solder joints61and the coplanarity of flex circuitry30and CSP12.

In various circuit module configurations, one or more integrated circuit packages may have depopulated contact locations or deactivated contacts that can be removed to create depopulated contact locations. Flex circuitry on which such integrated circuit packages are mounted may have populated and depopulated contact locations corresponding to the contacts of such integrated circuit packages. In addition or alternatively, the flex circuitry may have vacant areas disposed proximal to an integrated circuit package or other rigid component. For example,FIG. 7Adiscloses an embodiment having flex circuitry30having two contact arrays, each 3 by 15 in dimension. Also shown on flex circuitry30are various contact array locations populated with contacts24, various contact array locations38having no contacts and thus depopulated of contacts, and vacant area39between the respective contact arrays.

FIG. 7Bdepicts supplemental adhesive46disposed on flex circuitry30in accordance with a preferred embodiment of the present invention. In the illustrated embodiment, supplemental adhesive46is disposed to form supplemental adhesive bonds on vacant area39of flex circuitry30between the respective contact arrays and locations38of flex circuitry30depopulated of contacts.FIG. 8depicts a side view of such embodiment.

Circuit modules frequently have components bonded together by an adhesive. Previous configurations of such circuit modules know in the art use bonds that comprised only a thermoset adhesive film or epoxy. Such configurations typically require components to clamped or otherwise held under load during thermal cure of the adhesive or epoxy, which could be a slow and complicated process. Alternate configurations using an adhesive that sets or cures rapidly typically exhibit adhesive bonds having undesirable flexibility, elasticity, or compliance at transient or operating temperatures.

In various embodiments of the invention, a circuit module has bonded components in which the bond comprises plural adhesive types. For example, various embodiments disclosed herein bond components with a rapid bond adhesive and a structural bond adhesive. Such embodiments may be employed to advantage with many of the wide range of CSP and leaded package configurations available in the art. Modules in accordance with various preferred embodiments of the present invention may comprise plural base elements exclusively, as in a memory circuit module having plural memory integrated circuit packages as base elements12, or may comprise one or more base elements deployed with support elements, as in a system circuit module having a microprocessor as base element12and memory and other support circuitry packaged in a variety of configurations as support elements depicted inFIGS. 9D,9F, and9G for example as support elements14and16. Those of skill in the art will readily appreciate, that the invention can employed to advantage with a variety of combinations of packages including leaded packages and CSPs and other configurations of packaged ICs.

FIG. 9Adepicts a preferred embodiment of the present invention having two base elements12disposed in a stacked configuration connected by flex circuitry30. As discussed above, in this embodiment supplemental adhesive46is disposed as indicated between flex circuitry30and each of base elements12. Base elements12are bonded in this embodiment by rapid bond adhesive47and structural bond adhesive48, as more fully discussed below with respect toFIGS. 10-15.

FIG. 9Bdepicts a preferred embodiment of the present invention having base element12and a CSP support element16disposed in a stacked configuration on upper major surface18of base element12. As discussed above, in this embodiment supplemental adhesive46is disposed to bond base element12and support element16, respectively, to flex circuitry30. Although supplemental adhesive46disposed to bond base element12to flex circuitry30is not illustrated inFIG. 9B, those of skill in the art will appreciate its configuration from the disclosure above. Base element12and support element16are bonded in this embodiment by rapid bond adhesive47and structural bond adhesive48, as more fully discussed below with respect toFIGS. 10-15.

FIG. 9Cdepicts a preferred embodiment of the present invention in which a base element12is disposed in a stacked configuration with a leaded support element16. As discussed above, in this embodiment supplemental adhesive46is disposed to bond base element12and support element16, respectively, to flex circuitry30. Although supplemental adhesive46disposed to bond base element12to flex circuitry30is not illustrated inFIG. 9C, those of skill in the art will appreciate its configuration from the disclosure above. Base element12and support element16are bonded in this embodiment by rapid bond adhesive47and structural bond adhesive48, as more fully discussed below with respect toFIGS. 10-15.

FIG. 9Ddepicts a circuit module10devised in accordance with a preferred embodiment of the invention comprising base element12disposed in a stacked configuration with support elements14and16. This embodiment aggregates base element12and support element14each deployed as CSPs with support element16deployed as a leaded package device having leads31. In accordance with the discussion above, in this embodiment supplemental adhesive46is disposed to bond base element12, support element14, and support element16, respectively, to flex circuitry30. Although supplemental adhesive46disposed to bond base element12to flex circuitry30is not illustrated inFIG. 9D, those of skill in the art will appreciate its configuration from the disclosure above. Base element12is bonded to support element14and support element16, respectively, by rapid bond adhesive47and structural bond adhesive48, as more fully discussed below with respect toFIGS. 10-15.

FIG. 9Edepicts an alternative preferred embodiment of the invention employed to aggregate leaded packages. Depicted base element12and support element16are each deployed as a leaded package device having leads31. In accordance with the discussion above, in this embodiment supplemental adhesive46is disposed to bond base element12and support element16, respectively, to flex circuitry30. Although supplemental adhesive46disposed to bond base element12to flex circuitry30is not illustrated inFIG. 9B, those of skill in the art will appreciate its configuration from the disclosure above. Base element12and support element16are bonded in this embodiment by rapid bond adhesive47and structural bond adhesive48, as more fully discussed below with respect toFIGS. 10-15.

FIG. 9Fdepicts a preferred embodiment of the present invention that employs a CSP base element12and CSP support elements14and16. Heat spreader34is disposed between base element12and support elements14and16. As depicted inFIG. 9F, heat spreader34is in contact with a portion of casing36of an environment in which circuit module10is deployed. As discussed above, in this embodiment supplemental adhesive46is disposed to bond base element12, support element14, and support element16, respectively, to flex circuitry30. Although supplemental adhesive46disposed to bond base element12to flex circuitry30is not illustrated inFIG. 9F, those of skill in the art will appreciate its configuration from the disclosure above. Each of base element12, support element14, and support element16are respectively bonded to heat spreader34by rapid bond adhesive47and structural bond adhesive48, as more fully discussed below with respect toFIGS. 10-15.

FIG. 9Gdepicts base element12and support elements14and16each deployed as CSPs, with support elements14and16extending beyond the physical boundaries of base element12. As discussed above, in this embodiment supplemental adhesive46is disposed to bond base element12, support element14, and support element16, respectively, to flex circuitry30. Although supplemental adhesive46disposed to bond base element12to flex circuitry30is not illustrated inFIG. 9G, those of skill in the art will appreciate its configuration from the disclosure above. Base element12is bonded to support element14and support element16, respectively, by rapid bond adhesive47and structural bond adhesive48, as more fully discussed below with respect toFIGS. 10-15.

FIGS. 10 and 11depict a preferred embodiment of a circuit module having bonded components in which the bond comprises plural adhesive types. In this embodiment, plural CSPs12are connected to flex circuitry30using supplemental adhesive46as discussed above. A rapid bond adhesive47and a structural bond adhesive48are applied to the upper major surface18of a CSP12as indicated, thus resulting in the disposition of respective adhesive bonds. Flex circuitry30is folded so that upper major surfaces18of each CSP12are adjacent and brought into contact as shown inFIG. 11under an initial application of force sufficient to bond CSPs12with rapid bond adhesive47. Accordingly, additional adhesive bonds are disposed on CSP12not receiving the initial application of rapid bond adhesive47and a structural bond adhesive48. While CSPs12are held in such configuration by rapid bond adhesive47, structural bond adhesive48is allowed to set or cure and create another bond between CSPs12.

Rapid bond adhesive47preferably is a pressure sensitive adhesive that quickly forms a bond and maintains the bond during successive solder reflow operations used to form high-temperature solder joints, sources of which are known in the art. Structural bond adhesive48preferably is thermoplastic bonding film with high shear and peel strength, sources of which also are known in the art. During assembly the application of heat and the termoplastic properties of such structural bond adhesive48allow structural bond adhesive48to conform to and fill bounded volumes defined by rapid bond adhesive47and/or other structures. In addition, components can be debonded with the application of heat and force to perform any required rework of the circuit module. Accordingly, embodiments using such types of rapid bond adhesive47and structural bond adhesive48offer quick and uncomplicated assembly along with a more stable and rigid bond at transient or operating temperatures.

FIG. 12depicts additional preferred embodiments of the invention. In each of these embodiments, rapid bond adhesive47is applied to the upper major surface18of a CSP12as indicated, resulting in the disposition of an adhesive bond. Again, rapid bond adhesive47preferably is a pressure sensitive adhesive that quickly forms a bond and maintains the bond during successive solder reflow operations used to form high-temperature solder joints. Structural bond adhesive48also is applied as indicated to dispose another adhesive bond, but in these embodiments structural bond adhesive48is a thermoset adhesive or epoxy or an RTV adhesive, sources of which are known in the art. As with previously described embodiments, flex circuitry30then is folded so that upper major surfaces18of each CSP12are adjacent and brought into contact (similar to the depiction ofFIG. 11) under an initial application of force sufficient to bond CSPs12with rapid bond adhesive47, thus disposing respective adhesive bonds on CSP12not receiving the initial application of rapid bond adhesive47and a structural bond adhesive48. While CSPs12are held in such configuration by rapid bond adhesive47, structural bond adhesive48is allowed to set or cure and create another bond between CSPs12. Depending on the structural bond adhesive48used, a rapid-cure system or ultraviolet light curing system alternatively may be employed to decrease the cure time. Such rapid-cure systems and ultraviolet light curing systems are known in the art.

FIGS. 13-15depicts an additional preferred embodiments of the invention. In this embodiment, rapid bond adhesive47is applied to the upper major surface18of a CSP12as indicated inFIGS. 13 and 14.FIG. 14Bdepicts a cross section along line14B-14B shown inFIG. 14A. Again, rapid bond adhesive47preferably is a pressure sensitive adhesive that quickly forms a bond and maintains the bond during successive solder reflow operations used to form high-temperature solder joints. As with previously described embodiments, flex circuitry30then is folded so that upper major surfaces18of each CSP12are adjacent and brought into contact (similar to the depiction ofFIG. 11) under an initial application of force sufficient to bond CSPs12with rapid bond adhesive47. After the bond of rapid bond adhesive47is established, structural bond adhesive48is injected into gaps between upper major surfaces18of CSP12, resulting in the disposition of additional adhesive bonds on each of the CSPs. In this embodiment, structural bond adhesive48comprises a thermoset, snap-cure, or ultraviolet light curable adhesive known in the art.

Although the present invention has been described in detail, it will be apparent to those skilled in the art that the invention may be embodied in a variety of specific forms and that various changes, substitutions, alterations, and additions can be made without departing from the spirit and scope of the invention. The described embodiments are only illustrative and not restrictive, and therefore do not restrict or limit the scope of the invention, which is defined by the following claims.