Stacked module systems and methods

Abstract

The present invention provides methods for constructing stacked circuit modules and precursor assemblies with flexible circuitry. Using the methods of the present invention, a single set of flexible circuitry whether articulated as one or two flex circuits may be employed with CSP devices of a variety of configurations either with or without form standards.

Description

TECHNICAL FIELD

The present invention relates to aggregating integrated circuits and, in particular, to stacking integrated circuits in chip-scale packages and methods for creating stacked modules of chip-scale packages.

BACKGROUND OF THE INVENTION

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

CSP refers generally to packages that provide connection to an 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 as in “leaded” packages, in a CSP, contacts are placed on a major surface and typically emerge from the planar bottom surface of the package. The absence of “leads” on package sides renders most stacking techniques devised for leaded packages inapplicable for CSP stacking.

A variety of previous techniques for stacking CSPs typically present complex structural arrangements and thermal or high frequency performance issues. For example, thermal performance is a characteristic of importance in CSP stacks. Further, many stacking techniques result in modules that exhibit profiles taller than may be preferred for particular applications.

Staktek Group L.P., the assignee of the present invention, has developed a variety of stacked module designs that employ a form standard or mandrel that can provide thermal and/or construction advantages while providing a standard form that may allow use of a flexible circuit design with a variety of CSP types and body sizes. The mandrel or form standard stack designs come in a variety of shapes and sizes and materials. Some form standards extend beyond the perimeter edge or the extent of the CSP body and thus provide a form about which the flex circuitry transits.

Stacked module design and assembly techniques and systems that provide a thermally efficient, reliable structure that perform well at higher frequencies but do not add excessive height to the stack that can be manufactured at reasonable cost with readily understood and managed materials and methods are provided.

SUMMARY OF THE INVENTION

The present invention provides methods for constructing stacked circuit modules and precursor assemblies with flexible circuitry. Using the methods of the present invention, a single set of flexible circuitry, whether articulated as one or two flex circuits, may be employed with CSP devices of a variety of configurations either with or without form standards.

SUMMARY OF THE DRAWINGS

FIG. 1 is an elevation view of an assembly devised in accordance with a preferred embodiment of the present invention.

FIG. 2 depicts a two CSP embodiment of a module that employs an assembly devised in accordance with a preferred embodiment of the present invention.

FIG. 3 is an enlarged depiction of the area marked “A” in FIG. 1.

FIG. 4 illustrates an exemplar optional form standard that may be employed in some preferred embodiments of the present invention.

FIG. 5 is a plan view of an exemplar module from below depicting the relationship between an optional form standard and flex circuitry employed in a module in accordance with a preferred embodiment of the present invention.

FIG. 6 depicts two flex circuit edges in an arrangement according to a preferred embodiment of the present invention.

FIG. 7 depicts two flex edges in accordance with an alternative preferred embodiment of the present invention.

FIG. 8 depicts a tooling apparatus devised in accordance with a preferred embodiment of the present invention.

FIG. 9 depicts an enlarged depiction of the area marked “B” in FIG. 8.

FIG. 10 illustrates a tooling apparatus in accordance with a preferred embodiment of the present invention.

FIG. 11 illustrates another step in devising an assembly in accordance with a preferred embodiment of the present invention.

FIG. 12 depicts another step in devising an assembly in accordance with a preferred embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is an elevation view of assembly 12 devised in accordance with a preferred embodiment of the present invention. In this embodiment, assembly 12 includes what, in an assembled stacked module, will become a lower CSP which is here identified with reference 18. Constituent CSPs of preferred module embodiments have an upper surface 20 and a lower surface 22 and opposite lateral edges 24 and 26 and include at least one integrated circuit typically surrounded by a plastic body 27 with a lateral extent “L”. The body need not be plastic, but a large majority of packages in CSP technologies are plastic. Those of skill will realize that the present invention may be devised to create modules and precursor assemblies with different size CSPs and that the constituent CSPs may be of different types within the same module. The disclosed methods allow a single set of flex circuitry, whether comprised of one or two flex circuits, to be employed with a variety of body sizes of CSPs. For example, one of the constituent CSPs of an example module may be a typical CSP having lateral edges 24 and 26 that have an appreciable height to present a “side” while other constituent CSPs of the same module may be devised in packages that have lateral edges 24 and 26 that are more in the character of an edge rather than a side having appreciable height.

The invention is used with CSP packages of a variety of types and configurations such as, for example, those that are die-sized, as well those that are near chip-scale as well as the variety of ball grid array packages known in the art. It may also be used with those CSP-like packages that exhibit bare die connectives on one major surface. Thus, the term CSP should be broadly considered in the context of this application. Collectively, these will be known herein as chip scale packaged integrated circuits (CSPs) and preferred embodiments will be described in terms of CSPs, but the particular configurations used in the explanatory figures are not, however, to be construed as limiting. For example, the elevation view of FIG. 1 depicts a CSP of a particular profile known to those in the art, but it should be understood that the figures are exemplary only. The invention may be employed to advantage in the wide range of CSP configurations available in the art where an array of connective elements is available from at least one major surface. The invention is advantageously employed with CSPs that contain memory circuits, but may be employed to advantage with logic and computing circuits where added capacity without commensurate PWB or other board surface area consumption is desired.

Typical CSPs, such as, for example, ball-grid-array (“BGA”), micro-ball-grid array, and fine-pitch ball grid array (“FBGA”) packages have an array of connective contacts embodied, for example, as leads, bumps, solder balls, or balls that extend from lower surface 22 of a plastic casing in any of several patterns and pitches. An external portion of the connective contacts is often finished with a ball of solder. Shown in FIG. 1 are contacts 28 along lower surface 22 of illustrated CSP 18. Contacts 28 provide connection to the integrated circuit or circuits within the CSP package.

Flex circuits 30 and 32 are shown in conjunction with CSP 18. The entire flex circuit may be flexible or, as those of skill in the art will recognize, a PCB structure made flexible in certain areas to allow conformability around CSPs and rigid in other areas for planarity along CSP surfaces may be employed as an alternative flex circuit in the present invention. For example, structures known as rigid-flex may be employed. More than one flex circuit may be employed to implement the connections between constituent CSPs in a module.

As shown in FIG. 1, a substantially planar and optional form standard 34 is disposed along lower planar surface 22 of body 27 of CSP 18 in assembly 12. The depicted embodiment of assembly 12 is comprised of CSP 18 and optional form standard 34 and flex circuitry, which in this example is comprised of flex circuits 30 and 32. Flex circuits 30 and 32 have, respectively, upper portions 30U and 32U which terminate in edges 70A and 70B which are separated by gap “G” above the upper surface 20 of CSP 18. In some embodiments, gap G is preselected and imposed when assembly 12 is created as will be further shown.

Where employed, form standard 34 is disposed along a surface of a CSP even if literally separated from that surface by adhesive, for example. In this embodiment, form standard 34 is attached to flex circuits 30 and 32 with adhesive 35 and as shown, adhesive 35 has a portion 35B that extends beyond the extent of the form standard and about one major side of the flex circuitry.

Form standard 34 may take many configurations, but in preferred embodiments herein, it is substantially planar. A preferred embodiment is shown using a form standard 34 having a lateral extent smaller than the lateral extent L of CSP 18. Other embodiments may have a form standard 34 with a lateral extent larger than CSP 18. Other embodiments that employ form standards have a downward opening form standard shown in pending U.S. patent application Ser. No. 10/453,398, filed Jun. 3, 2003, now U.S. Pat. No. 6,914,324, commonly owned by Staktek Group L.P., the assignee of the present invention. In some cases, embodiments that employ downward opening form standards that are disposed across the upper surface of and arc underneath the lower surface of the CSP with which the form standard is associated may exhibit higher profiles. Module contacts 38 are shown through which a module may connect to an application environments in a preferred embodiment. Those of skill will recognize that module contacts 38 which are balls are not required to connect a module to an application environment and other connective strategies may be employed such as, for example, direct pad to pad connection schemes.

As shown in FIG. 1, adhesive 35 has portion 35A adjacent to form standard 34, and portion 35B extending beyond the lateral extent of form standard 34. Portion 35B may provide a number of benefits to the structure and assembly of a module. For example, the extension of adhesive portion 35B onto flex circuits 30 and 32 may help control the bend radius of curves 30A and 32A linking those portions of flex circuits 30 and 32 below CSP 18 to those portions 30U and 32U above CSP 18.

FIG. 2 depicts an exemplar module 10 in accordance with a preferred embodiment of the present invention that employs an optional form standard 34. Flex circuits 30 and 32 in FIG. 2 have adhesive portions or extended adhesive portions 35B of adhesive 35.

FIG. 3 is an enlarged depiction of the area marked “A” in FIG. 1. In a two-CSP module 10, contacts 28 of an upper CSP 16 contact the flex circuitry that transits about the body of lower CSP 18. Form standard 34 may be fixed to the lower (or upper) surface of the respective CSP with an adhesive 35 which preferably is thermally conductive while a stabilizing fill may be optionally employed between flex circuits at different levels in the module.

In a preferred embodiment, portions of flex circuits 30 and 32 may be attached to form standard 34 by adhesive 35, which, in a preferred embodiment, is a laminate tape adhesive. Other methods for attaching form standard 34 to flex circuitry may be employed in the present invention including, for example, liquid adhesive. Preferably, the adhesive will be thermally conductive. The depicted adhesive 35 is preferably disposed, after assembly, over a large portion of the curve 30A connecting the depicted upper portion 30U and lower portion 30L of flex circuit 30.

FIG. 4 illustrates an exemplar optional form standard 34 that may be employed in some preferred embodiments of the present invention where an optional form standard is employed. Form standard 34 as depicted in the preferred embodiment of FIG. 4 is comprised of nickel-plated copper and exhibits two windows identified by references A and B to allow the array of contacts 28 that rise above lower surface 22 of the respective CSP to readily pass through form standard 34. Form standard 34 may take other configurations and may, for example, be devised in more than one piece or have only one piece with only one window.

FIG. 5 is a plan view of an exemplar module 10 from below depicting an exemplar module 10 in which flex circuit 32 has been deleted to allow a view of the relationship between form standard 34 passing along lower planar surface 22 of CSP 18 and the flex circuitry employed in the module. On the right-hand side of the view of FIG. 5, and visible through window B of form standard 34, contacts 28 are shown rising from lower surface 22 of CSP 18 and projecting into window B. On the left-hand side of the view of FIG. 5, flex circuit 30 is represented as being disposed over part of form standard 34 and substantially all of window A of form standard 34. Module contacts 38 are shown along flex circuit 30.

The depicted edge of form standard 34 in this embodiment is outside the lateral extent of CSP 18. Other embodiments may have extend further outside. Other embodiments, such as depicted earlier, may have a form standard 34 with a lateral extent smaller than that of CSP 18. Still other embodiments do not employ a form standard.

FIG. 6 depicts two flex circuit edges 70A and 70B in a proximal arrangement according to a preferred embodiment of the present invention. Referring to FIG. 6, upper side 33 of flex circuits 30 and 32 are depicted. As those of skill will recognize, upper contact arrays 72A and 72B have been abstracted to illustrate only a few exemplar flex contacts or pads 44 when in practice, upper arrays 72A and 72B will typically include a greater number of individual flex contacts than the few shown for illustrative purposes.

The depiction of FIG. 6 shows flex edges 70A and 70B separated by gap G. The depicted flex circuitry may be comprised of one or two flex circuits and thus, the depicted flex edges may be edges of a single flex circuit or, a single edge 70A from flex circuit 30 and an edge 70B from flex circuit 32. Flex edges 70A and 70B terminate respective upper portions 30U and 32U of flex circuits 30 and 32. Whether one or two distinct flex circuits are employed with, for example, CSP 18 in module 10, in one embodiment, gap “G” between edges 70A and 70B is controlled by a physical form during creation of assembly 12 and upper arrays 72A and 72B will, therefore, be localized or fixed in relative position. This allows use of one flex circuitry design for a module (whether implemented with one or two distinct flex circuits) with a large variety of CSPs of differing types and body sizes.

Other means may be employed to position or set edges 70A and 70B and, by extension, arrays 72A and 72B. For example, flex edges 70A and 70B may be devised to be jointly fittable with each other as shown in FIG. 7 to position upper arrays 72A and 72B. Protrusion 74 fits with receptive check 75 to both align laterally and transversely edges 72A and 72B. Other similar devices may be employed to laterally and/or transversely align edges 72A and 72B. Thus, array 72A and 72B are disposed in predetermined relation to each other by the jointly fittable configuration of edges 70A and 70B to mesh with each other. Consequently, in this depicted alternative embodiment, edges 70A and 70B are disposed in predetermined relation to each other by their jointly fittable configurations.

FIG. 8 depicts a tooling apparatus 80 devised in accordance with a preferred embodiment of the present invention illustrating the use of a physical form to set gap G between edges 70A and 70B of flex circuitry employed in creating assembly 12. Tooling apparatus 80 includes an flex aligner 82 as shown in FIG. 8. When forming tool 84 disposes flex circuit 30 adjacent to upper surface 20 of example CSP 18 in forming assembly 12, edge 70B of flex circuit 32 is limited in lateral placement along upper surface 20 of CSP 18 by flex aligner 82. Gap “G” is, therefore, preselected by the dimensions of flex aligner 82 when disposed between edges 70A and 70B and gap G is determined and thus, edges 70A and 70B and therefore, upper arrays 72A and 72B are positioned during assembly.

FIG. 9 depicts an enlarged depiction of the area marked “B” in FIG. 8. As shown in the construction of the example assembly 12, flex circuit 30 is appended to an optional form standard 34. When assembly 12 comprising IC 18, optional form standard 34 and flex circuit 30 is disposed in cavity 88 of fixed form tool 86, flex 30 is deflected in an upward direction as shown in FIG. 9.

FIG. 10 illustrates a step in a method of devising an assembly 12 in accordance with a preferred embodiment of the present invention. As indicated, forming tools 84 are moveable as indicated by the arrow 84_M to indicate with the “+”, sign, movement of forming tool 84 to dispose flex circuits 30 and 32 over CSP 18. The ends 70A and 70B are set apart at distance “G” apart by flex aligner 82.

FIG. 11 illustrates another step in a method for devising an assembly 12 in accordance with a preferred embodiment of the present invention. Press tool 89 is imposed on assembly 12 after flex circuits 30 and 32 have been disposed over the upper surface 20 of the subject CSP. Press tool 89 is preferably heated.

FIG. 12 depicts another step in a method for devising an assembly 12 in accordance with a preferred embodiment of the present invention. Press tool 89 has moved up off of assembly 12 as indicated by motion arrow 89_M. Flex aligner 82 may now be withdrawn and assembly 12 is ready for combination with either another assembly 12 or a CSP 16 to form a module 10.

Claims

1. A method for constructing an assembly devised for employment in a stacked circuit module, the method comprising the steps of: providing a first CSP having first and second lateral sides and upper and lower major surfaces and a body having a CSP lateral extent;providing flex circuitry to connect the first CSP to other CSPs, the flex circuitry having first and second upper portions terminated by first and second edges, respectively, and disposing said first and second upper portions of the flex circuitry above the upper major surface of the first CSP while disposing the first and second edges a preselected distance apart and employing a physical form to impose the preselected distance while constructing the assembly.

2. The method of claim 1 further comprising the steps of providing a second CSP and disposing the second CSP above the assembly and connecting the second CSP to the flex circuitry.

3. The method of claim 1 or 2 in which the flex circuitry comprises two flex circuits.

4. The method of claim 1 or 2 further comprising the step of providing a form standard and disposing said form standard along the lower major surface of the first CSP.

5. The method of claim 1 or 2 further comprising employing a flex aligner as the physical form to impose the preselected distance.

6. The method of claim 5 in which a portion of the flex aligner is placed between the first and second edges.

7. A method of devising an assembly for employment in a stacked module, the method comprising the steps of: providing a first CSP having first and second lateral sides and upper and lower major surfaces;providing flex circuitry to connect the first CSP to other CSPs, the flex circuitry having first and second upper portions that exhibit first and second contact sets, the first and second upper portions being terminated by jointly fittable first and second edges, respectively, which jointly fittable first and second edges are configured to mesh with each other; anddisposing the first and second upper portions of the flex circuitry above the upper major surface of the first CSP and meshing together said jointly configurable first and second edges.

8. The method of claim 7 further comprising providing a second CSP and connecting the second CSP to the first and second contact sets of the flex circuitry.

9. The method of claim 7 or 8 in which the flex circuitry comprises first and second flex circuits.

10. The method of claim 7 or 8, further comprising attaching a form standard to the flex circuitry and the flex circuitry not exceeding the lateral extent of the first CSP.

11. The method of claim 7 further comprising the step of providing a second CSP having first and second lateral sides and upper and lower major surfaces with CSP contacts along the lower major surface;providing a form standard and disposing said form standard along the lower major surface of the first CSP, the form standard having first and second edges defining a lateral extent of the form standard;connecting the form standard to the flex circuitry; andfolding the first and second end portions of the flex circuitry over the upper major surface of the first CSP to form an assembly; anddisposing the second CSP above the assembly.