Stacked module systems and method

Description

RELATED APPLICATIONS

U.S. patent application Ser. No. 10/453,398, filed Jun. 3, 2003, is hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to aggregating integrated circuits and, in particular, to methods for creating high density modules from chip-scale type devices.

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.

The predominant package configuration employed during the past decade has encapsulated an integrated circuit (IC) 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 packaging or “CSP” have recently gained market share.

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, 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 may present complex assembly problems. What is needed, therefore, is a technique and system for stacking CSPs that provides a thermally-efficient, reliable structure that performs well at higher frequencies but does not add excessive height to the stack yet allows efficient production at reasonable cost with readily understood and managed materials and methods.

SUMMARY OF THE INVENTION

The present invention stacks chip scale-packaged integrated circuits (CSPs) into modules that conserve PWB or other board surface area. Although the present invention is applied most frequently to chip scale packages that contain one die, it may be employed with chip scale packages that include more than one integrated circuit die. Multiple numbers of CSPs may be stacked in accordance with the present invention. The CSPs employed in stacked modules devised in accordance with the present invention are connected with flex circuitry. That flex circuitry may exhibit one or two or more conductive layers.

A combination composed from a form standard and a CSP is attached to flex circuitry. Solder paste is applied to first selected locations on the flex circuitry and adhesive is applied to second selected locations on the flex circuitry. 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 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 embodiments, the methods of the invention may be used to attach a CSP without a form standard to flex circuitry.

SUMMARY OF THE DRAWINGS

FIG. 1 illustrates a step in a prior art method for constructing a high-density circuit module.

FIG. 2 depicts a step in a prior art method for constructing a high-density circuit module.

FIG. 3 depicts a step in a prior art method for constructing a high-density circuit module.

FIG. 4 depicts a step in a method for constructing a high-density circuit module in accordance with a preferred embodiment of the present invention.

FIG. 5 depicts a step in a method for construction of a high-density circuit module in accordance with a preferred embodiment of the present invention.

FIG. 6 depicts a step in a method for construction of a high-density circuit module in accordance with a preferred embodiment of the present invention.

FIG. 7 depicts a high-density circuit module in accordance with a preferred embodiment of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1–3 depict steps in a prior art method for constructing a high-density circuit module. FIG. 1 depicts a CSP 18 attached with adhesive 36 to form standard 34. In the configurations depicted, form standard 34 is devised to be employed with a CSP to provide a standard form for flex circuitry connector(s). Contacts 28 of CSP 18 have been compressed in a solid or semi-solid state and solder paste 41 and adhesive 43 have been applied to flex circuitry 30 and 32. FIG. 2 depicts a step in a prior art method for construction of a high-density module. Contacts 28 and solder paste sites 41 have come into contact as have form standard 34 and adhesive sites 43. FIG. 3 illustrates how solder paste sites 41 and compressed contacts 28 have merged to form solder joints and flex circuitry 30 and 32 has been wrapped about CSP 18.

FIG. 4 depicts a method in accordance with a preferred embodiment of the present invention. CSP 18 and form standard 34 are attached with adhesive 36 to form a primary combination 50. The depicted configuration of form standard 34 is one of many that can provide a standard form about which flex circuitry may be disposed. This allows a connective design implemented in flex circuitry to be used with CSPs of a variety of designs and configurations. Form standard 34 may also provide thermal advantages particularly when devised from metallic materials such as copper and copper alloys for example. Other configurations of form standard 34 may be employed with the present invention including but not limited to those that extend across the bottom surface 19 of CSP 18. Further, some form standards may not extend beyond the perimeter of CSP 18. Still other embodiments may not employ a form standard and may use the methods of the present invention to affix flex circuitry to CSP bodies.

Flex circuitry in this embodiment is comprised of flex circuits 30 and 32. Other embodiments may use one contiguous flex circuit or several and the flex circuitry may be flexible throughout or flexible in some areas and rigid in other areas. Flex circuitry has solder paste applied at selected sites as represented by reference 41 and an adhesive at selected sites identified by reference 44. The adhesive is, preferably, a thermoset adhesive or epoxy that will not soften during subsequent reflow operations such as exposure to 200–250 degrees Centigrade, for example.

In FIG. 5, the primary combination and the flex circuitry have been disposed in proximity to each other. Typically, there will be contact between contacts 28 and solder paste sites 41 but a large gap “G” between flex circuitry and form standard 34 will be exhibited because primary combination 50 is suspended above flex circuits 30 and 32 by the adhesive 44 and the uncompressed height of contacts 28 and solder paste 41. Weight 52 is disposed above CSP 18 on primary combination 50 while flex circuits 30 and 32 are supported from beneath by work support 54. Work support 54 is preferably a carrier that is in motion through an assembly process or may be stationary. Primary combination 50 and 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 combination 50 and flex circuits 30 and 32 under force F which tends to move them closer together, primary combination 50 collapses toward the flex circuitry as contacts 28 melt in the solder reflow operation and merge with the solder paste on flex circuits 30 and 32 to form solder joints 56 as adhesive 44 is compressed as well. In preferred modes, adhesive 44 cures after the solder has melted. Unit 58 is formed by such a process and comprises CSP 18, form standard 34 and flex circuitry 30 and 32. A unit 58 devised in accordance with the preferred methods described is shown in FIG. 7. 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 contacts 28. An alternate system using a fixture 38 to apply force F is shown in FIG. 6. These processes are amenable to implementation in a standard pick and place operation. Once devised, unit 58 may then be employed as a unit in a stacked module such as that shown in FIG. 8.

FIG. 8 depicts a high-density circuit module 60 comprised of unit 58 in combination with upper CSP 16 which has its own attached form standard 34. To form module 60, one or more upper CSPs 16 may be combined with unit 58. When aggregating more than two CSPs in a module 60, further iterations of flex circuitry will typically be employed as those of skill will understand. Each of the upper CSPs may optionally include an upper form standard 34 such as the one illustrated in FIG. 8 associated with upper CSP 16. Module 60 is shown with module contacts 38.

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 and alterations can be made without departing from the spirit and scope of the invention. The described embodiments are only illustrative and not restrictive and the scope of the invention is, therefore, indicated by the following claims.