Abstract:
A lead-frame method and assembly for interconnecting circuits within a circuit module allows a circuit module to be fabricated without a circuit board substrate. Integrated circuit dies are attached to a metal lead-frame assembly and the die interconnects are wire-bonded to interconnect points on the lead-frame assembly. An extension of the lead-frame assembly out of the circuit interconnect plane provides external electrical contacts for connection of the circuit module to a socket.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation-in-part of U.S. application Ser. No. 09/956,190 entitled LEAD-FRAME METHOD AND ASSEMBLY FOR INTERCONNECTING CIRCUITS WITHIN A CIRCUIT MODULE filed Sep. 19, 2001. 
    
    
     STATEMENT RE: FEDERALLY SPONSORED RESEARCH/DEVELOPMENT 
     (Not Applicable) 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to circuit modules and, more specifically, to a method and assembly for interconnecting circuits within a circuit module. 
     2. Description of the Related Art 
     Circuit modules or cards are increasing in use to provide storage and other electronic functions for devices such as digital cameras, personal computing devices and personal digital assistants (PDAs). New uses for circuit modules include multimedia cards and secure digital cards. 
     Typically, circuit modules contain multiple integrated circuit devices or “dies”. The dies are interconnected using a circuit board substrate, which adds to the weight, thickness and complexity of the module. Circuit modules also have electrical contacts for providing an external interface to the insertion point or socket, and these electrical contacts are typically circuit areas on the backside of the circuit board substrate, and the connection to the dies are provided through vias through the circuit board substrate. Producing vias in the substrate adds several process steps to the fabrication of the circuit board substrate, with consequent additional costs. 
     Therefore, it would be desirable to provide a method and assembly for interconnecting circuits within modules that requires no circuit board substrate. It would also be desirable to provide an assembly for interconnecting circuits within a module that, in addition to not including a circuit board substrate, is specifically configured to be resistant to fracture failures. 
     BRIEF SUMMARY OF THE INVENTION 
     A circuit module assembly and method for interconnecting circuits within modules to provide a circuit module that may be fabricated without a circuit board substrate. A lead-frame assembly is connected to one or more dies and external contacts may be provided by an extension of the lead-frame assembly out of the plane of the die interconnect. 
     More particularly, in accordance with the present invention there is provided a circuit module which comprises a lead-frame having at least one die pad, a plurality of contacts, a plurality of conductive traces extending from respective ones of the contacts toward the die pad, and at least one reinforcement bar. Attached to the die pad is a semiconductor die which is electrically connected to at least one of the traces. A body at least partially encapsulates the lead-frame and the semiconductor die such that the contacts are exposed within a bottom surface defined by the body, and at least a portion of the body is reinforced by the reinforcement bar. 
     The present invention is best understood by reference to the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These, as well as other features of the present invention, will become more apparent upon reference to the drawings wherein: 
         FIG. 1A  is a pictorial diagram depicting a top view and  FIG. 1B  is a pictorial diagram depicting a cross section of a prior art circuit module; 
         FIG. 2A  is a pictorial diagram depicting a top view and  FIG. 2B  is a pictorial diagram depicting a cross section of a lead-frame in accordance with an embodiment of the invention; 
         FIG. 3A  is a pictorial diagram depicting a top view and  FIG. 3B  is a pictorial diagram depicting a cross section of a circuit module in accordance with an embodiment of the invention; 
         FIG. 4  is a top plan view of a lead-frame constructed in accordance with an alternative embodiment of the present invention to include an interleaf reinforcement for facilitating the prevention of fracture failure in the circuit module; 
         FIG. 4A  is a top plan view of a lead-frame including an interleaf reinforcement formed in an alternative configuration to that shown in  FIG. 4 ; and 
         FIG. 5  is a cross-sectional view of the circuit module of the present invention illustrating a portion of the interleaf reinforcement of the lead-frame as optionally bended in the z-direction. 
     
    
    
     Common reference numerals are used throughout the drawings and detailed description to indicate like elements. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the figures and in particular to  FIG. 1A , a top view of a prior art circuit module  10  is depicted. Circuit module  10  is depicted as a circuit module as used in various multimedia card memory applications. The present invention is also applicable to cards and modules having other outlines such as secure digital cards and to peripheral device cards (I/O cards), as well. 
     A carrier  14 , to which integrated circuit dies  12  are attached and circuit contacts  13  are included on the bottom side, is covered by a cover  11  that is bonded to carrier  14 . The circuit module housing may be completely formed from an encapsulant, or the circuit may be encapsulated and a lid  15  applied over the encapsulant. Dies  12  are coupled to each other and to circuit contacts  13  by circuit traces  16 , which are typically etched from a metal layer on the top of carrier  14 . Circuit contacts  13  are coupled by means of plated-through holes  15  that pass through carrier  14 . The bottom side of carrier  14  is also typically etched from a metal layer on the bottom side forming electrical contacts  13  that are generally plated with a corrosion resistant material such as gold and circuit contacts  13  connect on the bottom side of carrier  14  to plated through holes  15  by circuit traces on the bottom side of carrier  14 . Circuit traces  16  include wire bonding areas  17  that may also be plated, permitting a wire bonding apparatus to electrically couple dies  12  by wires  18 . 
     Referring now to  FIG. 1B , a cross section end view of circuit module  10  is depicted. Dies  12  are covered by cover  11  and are bonded to carrier  14 . Circuit contacts  13  are disposed on the bottom side of carrier  14  to provide electrical connections to the external circuits via a socket in which circuit module  10  is inserted. 
     The present invention provides a circuit module that does not require a separate carrier, wherein the circuit paths between dies  12  and electrical contacts  13  are provided by a conductive lead-frame to which dies  12  are bonded and an encapsulant applied surrounding the lead-frame to provide support and electrical insulation. 
     Referring now to  FIG. 2A , a top view of a lead-frame  20  in accordance with an embodiment of the invention is depicted. Circuit traces  16 A are supported by a dam bar  22  that surrounds the periphery of the lead-frame  20 , providing rigidity during the fabrication and integration processes. Lead-frame  20  is generally stamped from a metal, such as copper, and integrated circuit dies are bonded to lead-frame  20  in die bonding areas  21 . Wire bonding pads  17 A are provided on circuit traces  16 A to permit attachment of wires from dies to the lead-frame  20 . The lead-frame  20  is then encapsulated and portions of dam bar  22  are cut, resulting in electrical isolation of circuit traces  16 A, after mechanical rigidity has been provided by the encapsulant. 
     In addition or in alternative to wire bonding pads  17 A, pads may be included for attachment of surface mounted passive components by soldering or conductive adhesive attachment, and pad grids may be included for attachment of pre-packaged integrated circuits. 
     Referring now to  FIG. 2B , a cross-section side view of lead-frame  20  is depicted. Dam bar  22  is shown at ends of the lead-frame  20  and is cut-away along the sides in the figure to illustrate that circuit traces  17 A are a half-thickness of metal with respect to dam bar  22 . This half-thickness may be produced by etching the bottom side of lead-frame  20  after applying an etchant resistant coating to dam bar and circuit contacts  13 A. Circuit contacts  13 A are also partially a half-thickness of metal, produced by etching the top side of lead-frame  20  after applying an etchant resistive coating to circuit traces  16 A and dam bar  22 . The etching of both sides of lead-frame  20  results in a circuit that has circuit contacts  13 A disposed as an extension out of the plane of circuit traces  16 A, while the full thickness portion of the electrical contacts/circuit trace combination produces a continuous conductive and mechanically rigid connection from circuit traces  16 A to circuit contacts  13 A. Thus, encapsulant may be applied beneath circuit traces  16 A and the circuit contact  13 A surfaces may protrude from the encapsulant, providing an interface connection external to a circuit module. 
     As an alternative, circuit contacts  13 A may be fabricated in the same plane as circuit traces  16 A and additional length supplied so that the circuit traces may be bent to provide an extension out of the plane of circuit traces  16 A so that circuit contacts  13 A may protrude from an encapsulant applied beneath lead-frame  20 . 
     The illustrative embodiments herein depict an etched lead-frame, but lead-frames may also be stamped in accordance with an embodiment of the present invention. The alternative embodiment depicted, wherein circuit traces are bent to provide circuit contacts especially lends itself to stamping, because the circuit traces may be formed and bent in a single stamping operation. 
     Referring now to  FIG. 3A , a top view of a circuit module  30 , in accordance with an embodiment of the invention, is depicted. The depiction shows the internal features after dies  12  have been bonded to lead-frame  20 , an encapsulant cover  11 A applied and the dam bar  22  is singulated from circuit module  30 . The resulting circuit module  30  has circuit traces  16 A that are isolated (but supported by the encapsulant) and wires  18  have been bonded from dies  12  to bonding pads  17 A. Circuit contacts  13 A are located at the bottom surface of encapsulant cover  11 A and protrude from or are conformal to the bottom surface to provide an external electrical connection. 
     Referring now to  FIG. 3B , a cross-section end view of a circuit module  30 , in accordance with an embodiment of the invention, is depicted. The plane of circuit traces  16 A adjacent to the plane of circuit contacts  13 A may be seen from the figure. Die  12  is shown as mounted above the plane of circuit traces  16 A, but a mounting within the plane of circuit traces  16 A is also possible. Additionally, circuit contacts  13 A may be attached using plating techniques to attach to circuit traces  16 A rather than including the circuit contacts  13 A within the lead-frame  20 . 
     As shown in  FIG. 3A , in the circuit module  30 , a relatively large minimally reinforced area of the encapsulant cover  11 A is defined between that die bonding area  21  disposed furthest from the circuit contacts  13 A, and the lateral side or edge of the encapsulant cover  11 A which is disposed furthest from the circuit contacts  13 A. The encapsulant material used to form the encapsulant cover  11 A is relatively brittle upon hardening, and is susceptible to fracture failure at consistent stress concentration points resulting from applied bending loads which sometimes are exerted during normal handling conditions. The encapsulant material in the above-described area is particularly susceptible to such fracture failure as a result of the absence of a high concentration of reinforcement members extending therein, i.e., only portions of the circuit traces  16 A extend within the encapsulant material in such area. This particular problem is exaggerated in those instances when the lead-frame  20  is formed in an alternative configuration, such as one wherein the larger die bonding area  21  disposed furthest from the circuit contacts  13 A is not included in the lead-frame  20 . 
     Referring now to  FIG. 4 , there is shown a lead-frame  32  constructed in accordance with an alternative embodiment of the present invention specifically adapted to address the fracture failure susceptibility in the encapsulant material described above. The lead-frame  32  comprises an outer frame or dam bar  34 . The dam bar  34  has a generally rectangular configuration defining an opposed pair of longitudinal sides or segments and an opposed pair of lateral sides or segments. The dam bar  34  further defines a fifth sloped side which extends angularly between one of the lateral sides and one of the longitudinal sides thereof. 
     In addition to the dam bar  34 , the lead-frame  32  includes at least one die attach area such as a die pad  36  which is disposed within the interior of the dam bar  34 . The die pad  36  defines opposed, generally planar top and bottom surfaces. Integrally connected to and extending inwardly from the lateral side of the dam bar  34  extending to the sloped side thereof is a plurality of contacts  38  of the lead-frame  32 . Each of the contacts  38  also defines opposed, generally planar top and bottom surfaces. Integrally connected to and extending from each of the contacts  38  is an elongate conductive trace  40 . Certain ones of the traces  40  are integrally connected to the dam bar  34 . Each of the traces  40  terminates in close proximity to respective ones of the peripheral sides of the die pad  36 . 
     The lead-frame  32  further comprises a plurality of interleaf reinforcement bars  42 . Each of the reinforcement bars  42  comprises an elongate base section  44  having a plurality of fingers  46  integrally connected to and extending perpendicularly from either a common longitudinal side thereof or each of the opposed longitudinal sides thereof. In the lead-frame  32 , the base section  44  of one of the reinforcement bars  42  including only one set of fingers  46  is attached to the lateral side of the dam bar  34  disposed furthest from the contacts  38 . Additional reinforcement bars  42  which also each include only one set of fingers  46  protruding from the base sections  44  thereof are disposed side-by-side adjacent the die pad  36  and are attached to respective ones of the longitudinal sides of the dam bar  34 . One of these particular reinforcement bars  42  is also integrally connected to the die pad  36 . Disposed between these two reinforcement bars  42  and the reinforcement bar  42  attached to the lateral side of the dam bar  34  are two additional reinforcement bars  42 , each of which includes two sets of fingers  46  extending from the base section  44  thereof. Importantly, the reinforcement bars  42  are sized, configured, and oriented relative to each other within the dam bar  34  such that the fingers  46  thereof are interleaved, thus facilitating the formation of serpentine gaps or openings between the reinforcement bars  42 . 
     To form a circuit module through the use of the lead-frame  32 , a semiconductor die  48  is preferably attached to the bottom surface of the die pad  36  through the use of an epoxy or adhesive. Subsequent to such attachment, the terminals  50  of the semiconductor die  48  are electrically connected to respective ones of the traces  40  of the lead-frame  32  through the use of conductive wires  52 . The conductive wires  52  effectively place the terminals  50  of the semiconductor die  48  into electrical communication with the lead-frame  32  and, more particularly, respective ones of the contacts  38  thereof. 
     Subsequent to the electrical connection of the semiconductor die  48  to the lead-frame  32  in the above-described manner, the lead-frame  32  is preferably subjected to a bending operation wherein the traces  40  are bent so as to facilitate the creation of an angled or sloped portion therein which is located between the contacts  38  and the die pad  36  as shown in  FIG. 5 . Bending of the traces  40  removes the contacts  38  from their original co-planar relationship to the die pad  36 . Thus, the contacts  38  and the die pad  36  extend along spaced, generally parallel planes. The bending of the lead-frame  32  in the above-described manner may occur either prior to the attachment of the semiconductor die  48  to the die pad  36 , or subsequent to the extension of the conductive wires  52  between the terminals  50  and traces  40 . It should be noted that in  FIG. 5 , the lead-frame  32  is depicted as including a lesser number of reinforcement bars  42  as compared to the showing in  FIG. 4 . In this regard, those of ordinary skill in the art will recognize that any number of reinforcement bars  42  may be included in the lead-frame  32 , with the number and placement of such reinforcement bars  42  being dependent on various parameters and, most notably, the size of the space or gap defined between the die pad disposed furthest from the contacts  38  and the longitudinal side of the dam bar  34  disposed furthest from the contacts  38 . 
     Subsequent to the bending of the lead-frame  32 , an encapsulant material is applied to the lead-frame  32 , semiconductor die  48 , and conductive wires  52 . The encapsulant material is preferably a plastic (e.g., thermoset, thermoplastic) which, upon hardening, forms a body  54 . The completely formed body  54  defines a generally planar top surface  56 , an opposed, generally planar bottom surface  58 , and angled or sloped side surfaces  60 . The package body  54  is preferably formed such that the bottom surfaces of the contacts  38  are exposed within and generally flush with the bottom surface  58  of the body  54 . As seen in  FIG. 5 , the body  54  may be formed such that the top surface of the die pad  36  (i.e., the surface opposite that including the semiconductor die  48  attached thereto) is covered by the body  54 . Alternatively, the body  54  may be formed such that the top surface of the die pad  36  is exposed in and substantially flush with the top surface  56  of the body  54 . Subsequent to the formation of the body  54 , the lead-frame  32  is cut or singulated in a manner facilitating the removal of the dam bar  34  as is needed to electrically isolate the traces  40  and hence the contacts  38  from each other. In this regard, the body  54  is preferably formed on the lead-frame  32  such that the dam bar  34  remains exposed (i.e., is not covered by the body  54 ). The exposure of the dam bar  34  allows for the removal of the same from the completely formed body  54 . 
     The formation of the body  54  completes the fabrication of a circuit module  62  which includes the lead-frame  32 , semiconductor die  48 , conductive wires  52 , and body  54 . In the circuit module  62 , the semiconductor die  48  is in a “die down” configuration. More particularly, the semiconductor die  48  is directed downwardly within the circuit module  62  since it is located between the bottom surface of the die pad  36  and the bottom surface  58  of the body  54 . As indicated above, the bottom surfaces of the contacts  38  are exposed within the bottom surface  58  of the body  54 , and define the connector of a memory card in which the circuit module  62  may ultimately be employed. 
     Though the lead-frame  32  shown in  FIG. 4  includes a total of seven contacts  38 , those of ordinary skill in the art will recognize that the lead-frame  32  may be formed to include any number of contacts  38  depending on the desired application. Along these lines, the lead-frame  32  shown in  FIG. 4  may further be alternatively configured to define two or more die pads for accommodating additional semiconductor dies or other devices. Further, more than one semiconductor die or other device could be attached to a single die pad. Thus, the configuration of the lead-frame  32  shown in  FIGS. 4 and 5  is exemplary only, in that the number of die pads, contacts, and conductive traces may be varied as needed to satisfy the requirements of a particular application. As indicated above, also variable is the number of reinforcement bars  42  included in the lead-frame  32 . 
     As will be recognized, in the lead-frame  32 , the traces  40  must be of sufficient length to facilitate the bending thereof in the above-described manner. Additionally, those of ordinary skill in the art will recognize that the circuit module  62  may be formed in a manner wherein the lead-frame  32  is not subjected to any bending operation, but rather is subjected to a half-etch process to create regions of reduced thickness therein as shown in relation to the lead-frame  20  in  FIG. 2B . In such half-etched version of the lead-frame  32 , the semiconductor die  48  would be attached to the top surface of the die pad  36 . The half-etching would preferably be facilitated in a manner wherein the bottom surfaces of the contacts  38  and the bottom surface of the die pad  36  extend along respective ones of a spaced, generally parallel pair of planes. 
     Referring now to  FIG. 4A , an exemplary lead-frame  32 A which may be employed in the circuit module  62  as an alternative to the lead-frame  32  is shown. In its preliminary, unbent state, the lead-frame  32 A also comprises an outer frame or dam bar  34 A. The dam bar  34 A has a generally rectangular configuration defining an opposed pair of longitudinal sides or segments and an opposed pair of lateral sides or segments. The dam bar  34 A further defines a fifth sloped side which extends angularly between one of the lateral sides and one of the longitudinal sides thereof. 
     In addition to the dam bar  34 A, the lead-frame  32 A includes a die attach area or die pad  36 A which is disposed within the interior of the dam bar  34 A. Integrally connected to and extending from one lateral side of the dam bar  34 A is a plurality of contacts  38 A of the lead-frame  32 A. Integrally connected to and extending from each of the contacts  38 A is a conductive trace  40 A. The traces  40 A terminate in close proximity to the die pad  36 A. Disposed within the dam bar  34 A between the die pad  36 A and the lateral side of the dam bar  34 A disposed furthest from the contacts  38 A is a reinforcement structure  64 A. The reinforcement structure  64 A is integrally connected to the die pad  36 A, and both longitudinal sides and the outermost lateral side of the dam bar  34 A. Formed within the reinforcement structure  64 A is a plurality of serpentine gaps  66 A and generally square or rectangular gaps  68 A. In this alternative embodiment of the lead-frame  32 A, the conductive traces  40 A may also be bent in the same manner previously described in relation to the lead-frame  32 . As an alternative, the lead-frame  32 A may be subjected to a half-etching technique to impart the thickness variations also described above in relation to the lead-frame  32 . 
     In the circuit module  62  including either the lead-frame  32  or the lead-frame  32 A, the reinforcement bar(s)  42  of the lead-frame  32  or the reinforcement section  64 A of the lead-frame  32 A provides internal reinforcement to the circuit module  62  which assists in preventing fracture failure or breaking. Certain ones of the reinforcement bars  42  or the reinforcement structure  64 A which are each integrally connected to the die pad  36 ,  36 A also provide a path for thermal spreading/dissipation. Thus, the configuration of the lead-frames  32 ,  32 A improves mechanical performance and durability for the circuit module  62 . The improved mechanical performance relates to the increased resistance to bending and twisting (torsion) attributable to the inclusion of either the reinforcement bars  42  or reinforcement structure  64 A. Typically, the lead-frames  32 ,  32 A will be configured in a manner wherein the reinforcements are disposed closest to the planes of maximum tension and compression. Though not shown, such location could be between the die pad  36 ,  36 A and the contacts  38 ,  38 A. 
     Referring again to  FIG. 5 , circuit module  62  including either the lead-frame  32  or lead-frame  32 A is typically incorporated into a memory card  70 . In addition to the circuit module  62 , the memory card  70  comprises a lid or skin  72 . The skin  72  is attached to the body  54  of the circuit module  62  in the manner shown in  FIG. 5 . The skin  72  is formed to include angled surfaces, the slopes of which are complementary to the side surfaces  60  of the body  54 , thus achieving a mating engagement therebetween. The attachment of the skin  72  to the circuit module  62  is preferably accomplished through the use of an adhesive. The attachment of the skin  72  to the circuit module  60  imparts to the completed memory card  70  a desired or prescribed form factor. When the skin  72  is attached to the circuit module  62 , the top surface  56  of the body  54  is completely covered or shielded by the skin  72 . If the semiconductor die  48  is oriented in the above-described die down configuration, the optional exposure of the top surface of the die pad  36 ,  36 A within the top surface  56  of the body  54  would be of no consequence since such exposed top surface of the die pad  36 ,  36 A would be covered by the skin  72 . A label  74  may optionally be applied to the exposed top surface of the skin  72 . In the completed memory card  70 , any flash on the exposed bottom surfaces of the contacts  38 ,  38 A is preferably removed through the implementation of a de-flash technique. 
     The memory card  70  has the form factor of a multi-media card. Those of ordinary skill in the art will recognize that the circuit module  62  may be employed in a memory card format other than a multi-media card format (e.g., a secure digital card format). Additionally, as an alternative to including the separate skin  72 , insert molding using plastic molding processing techniques may be employed to facilitate the complete formation of the memory card  70 . A more thorough discussion of various device configurations which may employ the use of the circuit module  62  including either the lead-frame  32  or lead-frame  32 A is described in Applicant&#39;s co-pending U.S. application Ser. No. 10/266,329 entitled DIE DOWN MULTI-MEDIA CARD AND METHOD OF MAKING SAME filed Oct. 8, 2002, the disclosure of which is incorporated herein by reference. 
     Referring again to  FIG. 5 , it is contemplated that one or more of the fingers  46  of one or more of the reinforcement bars  42  may be bent upwardly or downwardly to provide additional strength in the z-direction. As shown in  FIG. 5 , the fingers  46  of one of the reinforcement bars  42  of the lead-frame  32  are depicted as being bent upwardly. As will be recognized, such bending preferably occurs at the same time the traces  40  are bent, and thus prior to the encapsulation of the lead-frame  32  with the body  54 . 
     It is further contemplated that one or more of the fingers  46  of one or more of the reinforcement bars  42  may be provided with multiple bends so as to allow portions of each of the bent fingers  46  to act as heat fins. For example, the bent finger  46  shown in  FIG. 5  includes a sloped portion which extends from the die pad  36  downwardly toward the bottom surface  58  of the body  54 , a horizontal portion which extends in generally parallel relation to the bottom surface  58 , and a distal sloped portion which extends upwardly toward the top surface  56  of the body  54 . The bending of the finger  46  could be facilitated in a manner such that one surface of the horizontal portion of the bent finger  46  is exposed in and extends in substantially flush relation to either the top or bottom surface  56 ,  58  of the body  54 . Provided that such bent finger  46  is integrally connected to a reinforcement bar  42  which is itself integrally connected to the die pad  36 , the exposed surface of such finger  46  will act as a heat sink. The same holds true in relation to the lead-frame  32 A shown in  FIG. 4A , wherein portions of the reinforcement structure  64 A can be selectively bent as needed to facilitate the heat dissipation functionality described above in relation to the lead-frame  32 . In either case, a result of bending portions of either the reinforcement bars  42  or reinforcement structure  64 A away from the midplane of the body  54  facilitates thermal conduction efficiency away from those memory modules that may generate excessive heat. 
     To provide even further structural integrity to any circuit module  62  including either the lead-frame  32  or lead-frame  32 A, thermosets or thermoplastics used to form the body  54  may optionally be provided with fiber reinforcement or, alternatively, be modified by rubber for purposes of improving damage tolerance. 
     This disclosure provides exemplary embodiments of the present invention. The scope of the present invention is not limited by these exemplary embodiments. Numerous variations, whether explicitly provided for by the specification or implied by the specification, such as variations in structure, dimension, type of material and manufacturing process, may be implemented by one of skill in the art in view of this disclosure.