Thin multichip flex-module Number:7,520,781 from the United States Patent and Trademark Office (PTO) owispatent

Title: Thin multichip flex-module

Abstract: A socket assembly for multichip in-line modules comprises: at least three parallel in-line sockets, one of which is an edge-card socket adapted to matably engage electrodes on the edge of a printed circuit board, and the others of which are module sockets adapted to accept multichip in-line modules; and, internal connections between respective pins in each of the parallel sockets, whereby signals from the printed circuit board may be simultaneously carried to each of the multichip in-line modules. Alternatively, a socket assembly for multichip in-line modules comprises: a substantially rigid housing structure; at least two parallel in-line sockets adapted to accept multichip in-line modules; a set of electrodes adapted for soldering to a printed circuit board; and, internal connections between respective pins in each of the parallel sockets and the set of electrodes, whereby signals from the printed circuit board may be simultaneously carried to each of the multichip in-line modules.

Patent Number: 7,520,781 Issued on 04/21/2009 to Clayton,   et al.

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Inventors:	Clayton; James E. (Raleigh, NC), Fathi; Zakaryae (Raleigh, NC)
Assignee:	Microelectronics Assembly Technologies (Research Triangle Park, NC)
Appl. No.:	11/715,206
Filed:	March 7, 2007

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Related U.S. Patent Documents

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	Application Number	Filing Date	Patent Number	Issue Date
	60780440	Mar., 2006

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Current U.S. Class:	439/631 ; 257/686; 439/196; 439/61
Current International Class:	H01R 24/00 (20060101)
Field of Search:	439/190,191,192,196,629-637,67,61,62,77,541.5 257/686

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References Cited [Referenced By]

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U.S. Patent Documents

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7122889	October 2006	Belgacem
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Foreign Patent Documents

	3424929		Feb., 1996		JP

Other References

Anon. Thermal Performance of ArctiCore(tm) FBDIMMs and Conventional FBDIMMs, Staktek Corp. Technology Whitepaper, Mar. 2006. cited by other.

Primary Examiner: Gushi; Ross N
Attorney, Agent or Firm: Lauf; Robert J.

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Parent Case Text

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CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Provisional Patent Application No. 60/780,440 by the present inventors, filed on Mar. 8, 2006, the entire disclosure of which is incorporated herein by reference.

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Claims

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We claim:

1. A socket assembly for multichip in-line modules comprising: at least three parallel in-line sockets, one of which is an edge-card socket adapted to matably engage electrodes on the edge of a printed circuit board, and the others of which are module sockets adapted to accept multichip in-line modules; internal connections between respective pins in each of said parallel sockets, whereby signals from said printed circuit board may be simultaneously carried to each of said multichip in-line modules; and, a rigid adaptor having; a first connector strip adapted to engage one of said module sockets; a second connector strip adapted to engage said edge-card socket, said first and second connector strips disposed substantially in parallel and spaced apart adequately to allow a second socket assembly to be engaged thereon; and, internal connections between respective pins in each of said connector strips.

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Description

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to means for packaging microelectronic devices. More particularly, the invention relates to improved, SIMM and DIMM type memory modules.

2. Description of Related Art

Multichip module (MCM) assembly is currently an enabling technology for improving system performance in high-end workstation and super computers. By interconnecting multiple bare dice on a single substrate, packaging density is increased and chip-to-chip communication distance is consequently shortened, enabling higher operating speeds. Small, lightweight consumer products such as notebook or handheld computers and telephony products are expected to benefit from the improved miniaturization that MCM technology accomplishes. Unfortunately, this technology is currently too expensive for most of these applications.

A principal reason for the expense associated with multichip modules is that the technology is constrained to low volume, custom applications which cannot attain sufficient market volumes to help drive manufacturing costs down. Part of this problem is exacerbated by a current shortage of reliable, high-volume sources for the large variety of "known good die" required by many MCM applications. "Known good die" are semiconductor IC chips that are fully tested and screened to the same level of reliability as individually packaged parts, and are a fundamental necessity for attaining high MCM assembly yields with minimum repair. There has been recent progress in solving some of the handling issues with respect to bare die testing for both single die and dice still in wafer form, so this problem appears to be resolvable. However, identifying and implementing a high-volume, industry standard MCM application is still proving elusive.

One potential mass market is represented by industry standard Single Inline Memory Modules (SIMM) and Dual Inline Memory Modules (DIMM). These products have annual volumes reaching millions of units. Memory modules typically consist of identical IC device types, eliminating the need for stocking a large variety of "known good die". Memory modules, however, are noted for being a highly competitive, low-margin product. Because memory modules are assembled on small area, printed circuit boards, using highly automated processes, they have low associated material and assembly costs. Hence, the standard memory module business is widely thought to be too cost-driven to be considered a good candidate for MCM technology. However, high-end computing platforms, such as blade servers, tower servers and graphic accelerator cards are anticipated to require higher performance memory modules operating above 800 MHz which would benefit from improved module designs.

Some patents relating to memory modules include the following: U.S. Pat. No. 4,656,605, "Single In-Line Memory Module;" U.S. Pat. No. 4,727,513, "Single In-Line Memory Module;" and U.S. Pat. No. 4,850,892, "Connecting Apparatus for Electrically Connecting Memory Modules to a Printed Circuit Board." Additional patents in this area include U.S. Pat. Nos. 5,661,339; 5,708,297; 5,731,633; 5,751,553; 6,049,975; 6,091,145; 6,232,659; 6,665,190; and Japanese Patent 3424929. None of the foregoing discloses means for cooling the interior of the modules.

Objects and Advantages

Objects of the present invention include providing an improved memory module design that uses bare die or chip-scale packaged (CSP) memory chips; providing a method by which memory modules can be produced in higher volumes than modules built on PCB panels using surface mount soldering; providing a memory module that is cheaper to manufacture than modules built on PCB panels using surface mount soldering; providing a memory module that can be actively cooled; providing a memory module having higher component and interconnect density; and, providing an improved method for manufacturing memory modules that are backward-compatible with industry standard components.

SUMMARY OF THE INVENTION

According to one aspect of the invention, a socket assembly for multichip in-line modules comprises:

at least three parallel in-line sockets, one of which is an edge-card socket adapted to matably engage electrodes on the edge of a printed circuit board, and the others of which are module sockets adapted to accept multichip in-line modules; and,

internal connections between respective pins in each of the parallel sockets, whereby signals from the printed circuit board may be simultaneously carried to each of the multichip in-line modules.

According to another aspect of the invention, a socket assembly for multichip in-line modules comprises:

a substantially rigid housing structure;

at least two parallel in-line sockets adapted to accept multichip in-line modules;

a set of electrodes adapted for soldering to a printed circuit board; and,

internal connections between respective pins in each of the parallel sockets and the set of electrodes, whereby signals from the printed circuit board may be simultaneously carried to each of the multichip in-line modules.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective drawing of a partially assembled first embodiment of the present invention. Two integrated circuit (IC) devices are shown mounted on a flex circuit substrate, which is in the process of being wrapped around the bottom edge of a frame that is molded into the shape of a SIMM circuit board.

FIG. 2 is a vertical cross-section taken from FIG. 1 after it has been assembled. It illustrates how integrated circuits are nested into cavities formed on both sides of the molded frame.

FIG. 3 is a vertical cross-section of a second embodiment of the present invention. It illustrates how integrated circuits can be mounted on interior and exterior surfaces of the flex circuit, to enable four interconnect levels of IC devices in a single module. A total of four ICs are shown, two on each side of the molded frame. An encapsulant is also illustrated surrounding the ICs.

FIG. 4 is similar to FIG. 2, but illustrates a thin, plate-like, battery integrated along the centerline of the molded frame to provide power to the IC devices when the module is removed from its socket.

FIG. 5 is also similar to FIG. 2, but illustrates 3-D assemblages of stacked IC's positioned into both cavities of the molded frame. Two thin metal plates enclose the module on either side.

FIG. 6 is similar to FIG. 3, but illustrates an embodiment with some of the IC's mounted to an interconnect circuit forming part of the frame to electrically bridge IC's on both sides of the center fold of the flex, to minimize time delays and enhance performance.

FIG. 7 illustrates an embodiment with a combination of features found in both FIG. 3 and FIG. 6. A total of six ICs are shown in cross-section; two mounted to the internal, electrically functional frame and four mounted on both surfaces of the flex circuit sections.

FIG. 8 is a perspective illustration of a strip of flex circuit film with four mounted ICs. The flex circuit film strip has sprocket holes along both edges to enable automated handling of the film in a reel-to-reel fashion.

FIG. 9 is similar to FIG. 8, and illustrates how the electrically functional frame can be molded or inserted in place on the flex circuit while the circuit is still in strip form.

FIG. 10 is an exploded view of an embodiment of the present invention in which a simple foldable frame, with one center of folding axis, is mated to a flex circuit.

FIG. 11 is an exploded view of a module with a simple folding frame that folds in a direction opposite of that shown in FIG. 10.

FIG. 12 is a cross-sectional view of the folding module of FIG. 11.

FIG. 13 is a perspective view of an edge-card socket designed to mate with two modules similar to that illustrated in FIG. 2. It shows the socket and two inserted modules about to be placed as an assembly onto the edge of a motherboard circuit.

FIG. 14 is a cross-sectional view of the edge-card socket and inserted modules of FIG. 12. It shows one module partially inserted into the edge-card socket and the other slightly withdrawn from the connector. The edge-card socket illustrated accepts two modules, however more modules can be disposed above or below the plane represented by the motherboard. Another slot shown is reserved for engaging contacts on the edge of the motherboard.

FIG. 15 is a cross-sectional view of yet another embodiment, in which two modules are joined by a continuous strip of flex circuit film. The arrows indicate the direction the modules are being folded, accordion style, to form a double-joined module. Two differing frame choices are represented in this Figure. The frame shown on the left half is a cross section view of a foldable frame, similar to that of FIG. 10, which includes molded features for enabling the frame to be snapped together. The frame on the right half is similar to that shown in FIG. 2. A spacer element or battery is positioned at the center.

FIG. 16 is a cross sectional view of a double-joined module, similar to FIG. 15, being inserted into a double-rowed socket designed to mate with this module.

FIG. 17 is a perspective view of a preferred method for connecting IC chips to the flex circuit substrate.

FIG. 18 is a cross sectional view of the bottom portion of a module which employs an elastomeric material at the base of the frame to impart a spring-like compliance to the flex circuit contacts. The contacts on this embodiment are also repositioned along the bottom edge of the module.

FIG. 19 is a cross sectional view of the bottom portion of a module similar to FIG. 18 and includes a cam-like element that, when rotated, exerts an outward force or pressure behind the flex circuit contacts. This feature is intended to enable the module to be more easily inserted and removed from a mating socket.

FIG. 20 is a cross section view of the flex module from FIG. 5 with a modified frame member that is intended to provide a means for transporting a cooling gas or liquid through an interior cavity to remove internal heat from the enclosed semiconductor devices. A mating socket is also illustrated showing a means for connecting both inlet and outlet ports to a bottom portion of the module frame.

FIG. 21 is a cross section view of an alternative cavity for receiving coolant material within the interior portion of the flex module, formed by a hermetic seal of the enclosing flex circuit and coverplates.

FIG. 22 is a cross section view of another means for cooling the modules interior using a frame that incorporates an internal heat pipe to conduct the heat away from the contained semiconductor devices using a contained vapor phase change medium. This figure also includes a means for providing either a ground or voltage reference to the module coverplates through elevated pins contained in the mating socket.

FIG. 23 is a cross-sectional view of another means for cooling the module interior that incorporates a thin layer of carbon nano-tubes (CNT) that reduce the friction of fluid flow, thereby increasing the rate of fluid transport and efficiency of thermal conduction and removal.

FIG. 24 is similar to FIG. 14, but illustrates two edge-card sockets joined with a coupling section which acts as a bridging circuit to enable the two sockets to be mechanically and electrically joined together to function in unison and as a single socket. This increases the number of flex modules that can be mounted in a vertical stack.

FIG. 25 illustrates the flex module of FIG. 18, with bottom contact pads, inserted into a special socket designed to guide and latch the flex module in compression onto bumped contact pads of the motherboard surface.

FIG. 26 illustrates a technique for forming a multi-fold flex module. Two foldable frame members, joined with a continuous flexible circuit, are folded about three axes to form a multi-fold flex module with central-mated heat-sinks which leave no gap or space at the central axis.

FIG. 27 illustrates another technique for forming a multi-fold flex module. Two foldable frame members, joined with a continuous flexible circuit, are folded about three axes to form a multi-fold flex module with spaced-apart heat-sinks which leave a gap or hollow channel at the central axis.

FIG. 28 illustrates a multi-fold flex module similar to FIG. 27 which contains a foldable spacer element. Two foldable frame members, joined with a continuous flexible circuit and mounted on a foldable spacer, are folded about three axes to form a multi-fold flex module with spaced-apart heat-sinks which leave a wide gap or hollow channel at the central axis.

FIG. 29 is an enlargement of the cross-section (FIG. 28C) of FIG. 28. It illustrates two foldable frame members with pivot mechanism and snap latch mounted to a foldable spacer. An example of an end retainer, suitable for locking the multi-fold flex module together, is illustrated.

FIG. 30 illustrates the assembly sections and sequence of a folded flex module. The flex circuit is shown laminated to two coverplate heat-sinks with two rows of stacked chips on both sides of the module. This subassembly is then bonded to a foldable frame, in a flat configuration, which is later folded about a single axis into its final form factor.

FIG. 31 illustrates an enlargement of the cross-section (FIG. 30E) of FIG. 30. It illustrates two internal rows of chips meeting back-to-back at the centerline of the foldable frame member of the flex module and examples of two removable top retainer features for clamping and holding the flex module in a closed configuration. One top retainer includes a heat exchanging extension.

DETAILED DESCRIPTION OF THE INVENTION

The following embodiments introduce new construction concepts and assembly methods directed at providing lower-cost, higher-density, packaging solutions for next generation electronic module products. The module embodiments disclosed herein may be broadly classified as "Multichip Modules" in that the semiconductor devices illustrated are preferably in a "bare-die" or "chip" form. It will be understood, however, that in some instances packaged die may be used instead of bare die, depending on design objectives.

The electronics industry presently recognizes three main types or divisions of multichip modules based on the type of interconnecting substrate upon which the bare-die or chips are assembled: MCM-D (deposited thin film), MCM-C (ceramic), and MCM-L (laminate). MCM type-L refers to conventional epoxy/glass, printed circuit board laminate substrates and comes closest, of the three categories, for classifying the present invention. Epoxy/glass PCBs, however, are generally too thick, rigid and brittle for the purposes of this invention (although current research, directed at reducing PCB thickness, suggests that this may change in the future) and do not accurately characterize the thin flexible substrates preferred for these embodiments. Therefore a fourth classification, MCM-F, based on thin, flexible substrates is proposed as a more descriptive classification for modules of the present invention.

In addition to various module embodiments and assembly methods disclosed herein, new socket/connector designs are also described. These sockets are designed to complement the various module embodiments and extend their usefulness at the system level.

In a packaged semiconductor IC such as microprocessors die or memory die, heat escapes predominantly from the solder balls area than through the package itself. This holds true regardless of the package configuration, whether single, dual or quad die (these consist of stacked chips). In the innovative module hereby introduced the chips are mounted in the best thermal conductive path for optimal heat exhaust. The chips are mounted on a thin, flex circuit (for better thermal transfer), the flex circuit is laminated to a heat dissipating surface, the flex has provisions for optimal heat transport by using appropriately placed thermal vias) between the flex and the heat sink. The heat sink is of sufficient dimensions (thickness and surface area) to accommodate the required heat flux. The heat sink is in contact with a cool air (or other fluid) stream for best heat removal. All chips maintain the best thermal path for maximum heat exhaust. In some embodiments the chips may be cooled in two opposite paths.

A thermal via is defined as a thermal conduit extending through the flex circuit. These thermal vias may or may not have electrical functionality. The thermal vias can be built into the flex circuit by various fabrication methods including laser drilling and back filling with a high thermal conductivity material or through the well known combination of photo-definable processes along with metal electroplating. The density of thermal via in the Flex-circuit is another design consideration by which more or less heat can be exhausted. The higher the density of thermal vias the better, provided signal integrity in the flex circuit is not compromised.

In contrast, prior art modules using PCB cannot achieve optimal thermal exhaust because they are placed on a thermally insulating substrate.

The innovative memory module configuration disclosed in the current invention yields a combination of enhancements in each of the following areas: better thermal cooling (best heat removal), better thickness (very thin module compared to alternatives), lower height for Low Profile configurations and full rework capability. The reason rework capability is important is that defective chips resulting from faulty assembly steps during manufacturing can be corrected for with ease which enhances production yields.

In the best mode, the chips are mounted on Flex and the flex surface opposite the solder balls of the chip is attached using a thermally conductive material to a heat sink that has heat exchanging properties and is preferably placed in a stream of cooling fluid with minimal impediment.

The various embodiments of the FlexModule 10 of the present invention can be understood by referring initially to FIG. 1, which is a perspective view of the major components. The module of FIG. 1 comprises a module frame 12 (hereinafter referred to as either a "molded frame" or "module frame" or simply "frame"), thin flexible circuit 50 and integrated circuit (IC) devices 54. The molded frame 12 comprises a first and second major parallel plane or surface, noted as reference numerals 16 and 18, respectively, which are separated by a specified edge thickness 20. The frame shown in FIG. 1 is intended to represent a SIMM module, in that a corner notch 26 and two end holes 24 are fashioned into the frame, enabling it to mate with existing SIMM type sockets. Other embodiments may substitute a frame designed to match the shape of standard DIMM or other module form factors.

The invention makes use of structural members with various characteristics and functions. The non-foldable frame 12 in this invention assumes various functions and roles. Some of these functions are mechanical and some of these functions are electrical. The non-foldable frame can contain depressions, recessed cavity(s) or window(s) 14 extending below the first major surface 16 of the non-foldable frame 12, stretching over a substantial portion of the length and width of the frame. The non-foldable frame 12 has provisions for contact pads, holes, windows, internal cavities, floor, and recesses and stepped ledges. The non-foldable frame can act as a spacer to prevent chips from crushing each other's. The foldable frame 13 can be used in combination with the non-foldable frame 12 or a spacer element 16 without any recesses and could be assembled to the surface(s) of frame 12 or spacer 16 for mechanical protection and heat sinking functions to the internally mounted chips.

Electrical functions are also intended for the non-foldable frame in some cases. The internally positioned non-foldable frame 12 used in combination with the foldable frame 13 has internal portions or windows inside the module that can electrically bridge chips on opposing folds of the module. This is done when the internal non-foldable frame 12 has internal portions or windows inside the module that can electrically bridge the flex circuitry upon which the chips are mounted across opposing folds inside the module. The internal non-foldable frame inside the module can be metallic with or without embossments and/or cutout windows. The internal frame inside the module can be a moldable thermoplastic. The internal non-foldable frame inside the module can be metallic or a moldable thermoplastic with flex circuit portions or PCB portions.

As with the non-foldable frame 12, the foldable frame 13 in this invention also plays several functions and roles. Some of these functions are mechanical and some of these functions are electrical. The foldable frame assumes a flat configuration when it is fully open. In this fully open and flat position the flex can be laminated onto the frame. Once the lamination process is done, the frame acts as a carrier for chip assembly onto the flex. As such the open frame goes through SMT solder reflow followed by underfill and cure, if deemed necessary (e.g., for flip chip). After assembly of the chips, the open and flat subassembly may be tested and repaired, if necessary. The frame is now ready for folding. The folding takes place in steps.

The outermost sections of the foldable frame 13 with mounted chips 54'' are folded first through a 180 degree arc towards the center in either an upward or downward direction followed by a center-folding step with both pre-folded sections rotating towards one another through a 90 degree arc to meet at the center in either an upward or downward direction. The folding of the frame is enabled by special provisions and these are obtained through several methods. One method is to thin down the metal areas along the folding area 108. Another method is to machine creases to enable easier folding along the folding area 108. The preferred method is to have several straight metallic sections that are articulated using pivot mechanisms 112 machined or formed in the sections. In this case, when the pivots 112 are engaged, the foldable frame can be closed in sections with minimal force. Furthermore, the foldable frame can be opened with minimal force.

Some surfaces in the foldable frame can be used as ground planes. The outer surfaces of the foldable frame can be used as an EMI shield.

The foldable frame has specially designed mechanisms or features that when the frame is folded the features are juxtaposed to close proximity to form useful shape and mechanisms that assume various functions. One mechanism formed through folding specially designed features in the foldable frame is the connector area 109 around which the flexible circuit is folded. This is how the module can have its connector that gets inserted into standard sockets. The module in the current invention has a foldable frame that is adapted for insertion into a SIMM or DIMM sockets. One mechanism formed through folding specially designed features in the foldable frame is a latch mechanism or snap latch 124 that once engaged through the fold operation the snap latch keeps the module in tight closed configuration. The foldable frame contains mechanism features that act as spacers when the frame is fully closed. The spacers are designed to prevent chips from crushing each other and would limit mechanical pressure applied to the chips 54''. The spacer elements may exist as part of the foldable frame or be independent structural members that get introduced separately. Another mechanism formed subsequent to the folding operation is internal fluid channels. In the case, features exist on the foldable frame and when the frame is folded these features form air channels or cavities 79 such as illustrated in FIG. 20. Another mechanism formed subsequent to folding the frame, is a fluid delivery conduit or orifice 77. These features exist on the foldable frame and these features form the inlet 77 and outlet 81 ports for fluid delivery when the module is folded. Another useful mechanism is the existence of a male perimeter protrusion that mates with a female perimeter recess in such a way that when the module is folded an internal chamber is sealed (not shown). Alternatively, a perimeter gasket can be adhered to the perimeter of said foldable frame in such a way to maintain a sealed internal volume inside the module in its fully folded position (not shown). Alternatively, a perimeter adhesive can be adhered to the perimeter of said frame in such a way to maintain a sealed internal volume inside the module in its fully folded position (not shown).

The foldable frame has a thermal function consisting of spreading the thermal energy emanating from the chips and then transferring the thermal energy through the foldable frame surfaces into a stream of air or cooling fluid. A series of heat exchanging surfaces 105 and 107 may be formed once the module is folded. These heat-exchanging surfaces can form a hollow volume or hollow channel 120. The hollow channel enables higher performance heat exchange and would be used in applications with high heat dissipation. The center of the hollow channel 120 is aligned with the BB' center fold line defined in this invention. The dimensions and the morphology of the hollow channel inside the module depend on the foldable frame.

In this inventive multi-fold module, the foldable frame is folded in such a way that four heat-exchanging surfaces are formed. Two heat-exchanging surfaces are on the outside of the fully folded module are referred to as right and left sections-E. The walls of the hollow channel 120 present two heat-exchanging surfaces and are referred to right and left sections I. The four heat exchanging surfaces are preferably located opposite the semiconductor chip's contact pads 74 or solder balls 106 (shortest thermal path configuration), such that a majority of the chip's heat is conducted through the flex circuit and through the thermal vias and into the heat-exchanging surfaces. The heat-exchanging surfaces are in the path of maximum airflow and the thermal energy is carried away from the module. The flex circuit and the foldable frame are designed to allow for folds and bends in such a manner that the assembled chips assume specific placement when the frame is fully folded, all the chips are in the shortest thermal path configuration. The foldable module is designed to assume an overall thickness and height with specific dimensions. The configuration of the folded module has provisions for maximum heat transfer. The module is folded in such a manner as to maintain a maximum heat transfer path (that is to say a short path between the solder balls of the chips and the heat sink with minimal thermal impedances).

When the module is fully closed, a hollow channel 120 is formed in the interior of the module and presents two heat-spreading surfaces 105 from which the heat can be exhausted and through which an air stream can pass. For air to circulate efficiently through the parallel walls 105 of the hollow channel 120 there should be no obstructions in the air path and the gap between the parallel walls forming the hollow section inside the module has to be wide enough to enable air circulation. If appropriate dimensions are not designed in, air could simply circumvent the hollow channel.

When 105 and 107 are closed they define an internal volume in either the left or the right section of the module. This internal volume can be cooled through the use of a cooling fluid that can be circulated through an access port 77 congruent with one of the embodiments of the present invention. These internal volumes can be cooled using the methods described in this invention that consist of fluid inlets and outlets described in FIGS. 20 and 21.

The current inventive module (or Multi-fold Flex Module) has maximum heat exhaust capability in that the chips can be cooled using air-cooling on the external surfaces of each module section as well as circulating a fluid coolant in the internal volume defined within each section. Each chip in the multi-fold module can exhaust heat from two sides. Every chip is made to maintain a heat-exchanging surface with a minimal path between heat generation area and the heat exchanger. These attributes provides for maximum cooling efficiency for semiconductor chips assembled in the inventive multi-fold module.

The foldable frame has three operational positions open, partially open and fully closed. If the module is in the socket, the open and partially open positions the foldable module allows for probing into the electrical functionality of the module by having electrical probes contacting pads or electrical circuitry and for evaluating signal integrity. The fully open position allows provisions for rework by known methods in the state of the art. The rework is not done while the module is in the socket.

In order to insure that various embodiments of this invention are compatible with existing SIMM and DIMM connectors, the general thickness of the modules described herein, as measured across the contacts arrayed along the bottom edge(s) 20 of the module and end sections, is preferably 0.040 or 0.050 inch, standard thickness presently established for SIMM and DIMM circuit boards. With exception of multi-fold modules depicted herein and those that include molded protective overcoats 70 (as shown in FIG. 3 for example), the nominal thickness of the various embodiments of the present invention (measured within the section of the module containing devices 54), is also preferably 0.050 inch. Other embodiments of the present invention, described below, which include either molded protective overcoat 70 and/or coverplate(s) 48, are preferably 0.100 inch thick or less. As a general rule, the various module embodiments described herein are considerably thinner and lighter in weight than existing SIMM & DIMM type modules, yet are backward compatible with connectors developed for the earlier modules.

In the embodiment shown in FIG. 1, a depression, recessed cavity or window 14 extends below the first major surface 16 of the internal frame 12, and stretches over a substantial portion of the length and width of the frame. The internal frame 12 was defined and called in previous inventions "frame", "module frame", or "molded frame". This internal frame, as the name indicates, has provisions for contact pads, holes, windows, internal cavities, floor, recesses and stepped ledges. Metallic coverplates 48, as previously defined, could be assembled to the surface(s) of frame 12 for mechanical protection and heatsinking functions to the internally mounted chips (refer to the previous patents for full definition and explanation). In the preferred embodiment of the previous inventions, the semiconductor chips were mounted on a flexible circuit, which in turn was wrapped around internal frame 12, followed by the attachment of monolithic metallic pieces (coverplate 48) mounted on the outside of the module. Furthermore, provisions for nesting coverplates 48 inside frame 12 were described. In this case the coverplates were internal to the module for heat dissipation purposes. The cavity provides an area into which IC devices 54 are enclosed and protected when flex circuit 50 is assembled onto the frame. IC devices 54 are, therefore, preferably located on the interior surface(s) of flex circuit 50 such that devices 54 lie underneath the flex circuit after it is wrapped around frame 12. This assembly design enables devices 54 of module 10 to be positioned quite close together and as near as possible to the midpoint of the module's thickness. It differs substantially from existing module design practices, which place the devices on the external surfaces of conventional printed circuit boards (PCB) that are manufactured in a standard thickness. Since the PCB is imposed between the devices in this later example, the total module thickness is the combined thickness of the circuit board and individual components mounted on either side of the PCB.

Since many modules require more devices than can be fitted on a single side, a second cavity 14' may also be formed into the second major surface 18, as shown in FIG. 2 and subsequent figures. This enables chip devices 54 to be positioned on both sides of frame 12. Cavity 14 (and/or 14') generally extends below surface 16 (and/or 18) to a floor 28 at a depth equal to or greater than the thickness of IC devices 54. Alternatively, the cavity may extend entirely through edge thickness 20, thereby eliminating floor 28. In other embodiments (not shown), there may be no cavity(s) at all. In this case, devices 54 would not be enclosed within any recessed cavity formed within the frame 12, but would be positioned on the external surface(s) of flex circuit 50. A protective covering would be used in this example to protect devices 54. In yet another embodiment (not shown) separate cavities for individual devices 54 may be formed in frame 12, creating separate walled cavities for the devices and improving the frame's rigidity.

Frame 12 may typically be fashioned from an injection-molded thermoplastic or thermoset material, which may be transparent, translucent or opaque. Other materials that can be molded, pressed, or machined to the proper shape, are also acceptable. Examples include, but are not limited to, polycarbonate plastic, liquid crystal polymer plastics, Ryton.TM., ceramic, metal, glass, etc. In some embodiments, where IC devices 54 are electrically and mechanically attached directly to frame 12, as illustrated in FIGS. 6 and 7, frame 12 may be fashioned from conventional printed circuit board (PCB) materials. In other embodiments, frame 12 may include heat conductive filler materials or an internal heat pipe to improve the internal thermal dissipation characteristics of the module.

Flex circuit 50 is preferably fashioned from a thin, flexible, insulative film such as polyester, polyethylene terephthalate (PET), polyimide (Kapton.TM.), Goretex.TM., or other films well known within the flex circuit industry. Conceivably, even thin epoxy-glass printed circuit boards (PCB) may be used for the practice of this invention, assuming the plies are of sufficient elasticity to enable the circuit to be reliably folded into the preferred shape. The flex or "flexible" circuit 50 may be fabricated as either a single-sided circuit, double-sided circuit, or a lamination of two or more layers of films (i.e. multi-layered circuit). Examples of "flexible" circuits 50 are manufactured by Sheldahl (Northfield, Minn.); Poly-Flex Circuits, Inc. (Cranston, R.I.); CTS Corporation (Elkhart, Ind.); Nelco International Corporation (Tempe, Ariz.); Gould Electronics, Inc.; and others.

Flex circuit 50 of FIG. 1 is shown only partially assembled onto frame 12. When folded and attached in place, the flex circuit extends from the top edge and sides of first surface 16, wraps around the bottom edge of frame 12, and continues up the back side or second surface 18 of frame 12. In other words, flex circuit 50 is preferably of sufficient length, to extend from the top edge of the first plane 16 of frame 12, to the top edge of the second plane 18, thereby almost completely enclosing both planes 16 and 18 of frame 12. This is the preferred configuration for flex circuit 50 and is the configuration shown in most modules illustrated. However, other embodiments (not shown) of the present invention may only require the flex circuit to cover one surface of frame 12.

Flex circuit 50 is typically attached to frame 12 by means of a pressure sensitive adhesive (PSA) film (not shown) applied to either the frame or flex circuit prior to assembly. Alternatively, frame 12 may be molded directly onto flex circuit 50 or adhere thereto by means of a variety of bonding agents placed on either the frame or flex circuit.

Attached to flex circuit 50 are IC devices 54 and other surface mount components (not shown) such as chip capacitors, resistors, and inductors. These ICs may be of any variety of semiconductor digital, analog, or mixed-signal devices, but are typically memory chips such as DRAM, SRAM, Flash RAM and others intended for specific applications. Only two such devices are represented in FIG. 1, though in most applications more devices may be required on front or back sides of flex circuit 50.

In FIG. 1 and all following figures, the semiconductor devices illustrated comprise "bare" silicon dice (i.e. the chips are not pre-encapsulated into separate plastic packages). This is the preferred form factor for IC devices used in this invention. The "bare" IC chips are also preferably "Known Good Die" (KGD), meaning that the devices have been verified to be functional and reliable prior to assembly on the flex circuit. This is typically accomplished by screening the dice through a pre-stressing process which subjects the parts to rigorous electrical testing and functional operation while the devices are being temperature cycled.

Devices 54 are attached to the flex circuit using a variety of "Direct Chip Attach" (DCA), "Flip-chip" or "Chip On Board" (COB) techniques. The preferred DCA process makes use of an Anisotropic Conductive Adhesive placed between matching pad grid arrays (PGA) on the semiconductor dice and flex circuit. An example of this is shown as a perspective view in FIG. 17, where the anisotropic adhesive film 58 is illustrated as a thin layer of material situated between device 54 and a PGA site on flex circuit 50. Anisotropic conductive adhesive is formulated to function as an electrical conductor across the thin bond line formed when the material properly joins the IC device to the interconnect (flex) circuit. But, it exhibits an electrically isolative property in the plane perpendicular to the bond line thickness. Anisotropic conductive adhesives are familiar in the art and are manufactured by companies such as 3M (Minneapolis, Minn.), UNIAX Corp. (Bloomfield Hills, Mich.) and ZYMET Inc. (E. Hanover, N.J.). Other acceptable DCA or flip chip assembly technologies include conductive polymer inks, manufactured by companies such as Epoxy Technology, Inc. (Billerica, Mass.) and Alpha Metals (Jersey City, N.J.), as well as solder bumped reflow and conductive PSA tapes.

In addition to using "bare" IC chips, the semiconductor devices can be pre-encapsulated or packaged in "Chip Scale" form. This is a minimum area packaging technique, which approaches the same size as the sectioned dice itself, yet provides a degree of physical robustness to the dice for improved handling and reliability. "Chip Scale" packaged devices may be attached to the flex circuit by conventional surface mount soldering processes. Chip scale packaged devices are in the process of being developed and manufactured by numerous industry suppliers including: Tessera (San Jose, Calif.); Micro SMT; IBM; Motorola; Mitsubishi; Matsushita; Toshiba; and others.

Arrayed along the folded centerline of flex circuit 50, shown in FIG. 1, are numerous contact pads 22. These provided a means of electrical connection and communication between the module's IC devices 54 and a variety of existing SIMM and DIMM connector sockets that are typically mounted on the motherboard. Contact pads 22 of module 10 are preferably positioned on the external surface of flex circuit 50 along the bottom edge of the first 16 and second 18 major surfaces of frame 12. As a point of reference in discussing examples where the flex circuit is attached to both sides of frame 12, it should be noted that corresponding to first and second surfaces of frame 12 are first and second, interior and exterior, surfaces of flex circuit 50. In FIG. 2, for example, it can be seen that contact pads 22 are disposed on first and second exterior surfaces of flex circuit 50 adjacent to the bottom edge of frame 12, and that devices 54 are located on both first and second interior surfaces of flex circuit 50.

Electrical communication between devices 54 and pads 22 of flex circuit 50 is provided through electrically conductive interconnect traces 61 (refer to FIG. 8) that are fabricated on top and bottom surfaces and internal laminate layers of the flex circuit. Interconnect traces common to flex circuit technology included vacuum deposited thin copper films, photo-etched or plated copper films, screen printed silver-filled conductive inks and other materials well known within the PCB industry.

An advantage for the present invention is found in the incorporation of contact pads 22 as an integral part of thin flex circuit 50. In prior work, contact pads 22 were fashioned as an element placed on frame 12 which were subsequently electrically coupled to flex circuit 50. This required two separate interconnect levels within the module. First, an electrical coupling between IC d