Abstract:
A circuit board according to the invention is made from two or more laminates each made of a fusible dielectric material, which laminates are bonded to each other along respective inner faces thereof. Each such laminate is preferably a pre-preg sheet containing both a heat-fusible resin and a reinforcing fiber filler to provide the desired stiffness and strength. A number of first electrical contacts are exposed on an outer face of the first laminate, and second electrical contacts are exposed on an outer face of the second laminate. The circuit board further includes a plurality of electrical conductors each running from a first contact to a second contact, the conductors including elongated conductive lines extending along one of the first or second laminates, and vias extending through the first and second laminates which have been filled with an electrically conductive via filler.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to circuit boards and integrated circuit packages and, more particularly, to method for making electrical circuits on circuit boards or integrated circuit packages.  
         BACKGROUND OF THE INVENTION  
         [0002]    Semiconductor chip package designs have had difficulty keeping pace with ever-increasing chip frequencies and industry demands for higher circuit densities at reduced cost. Castro et al U. S. Pat. Nos. 6,107,683 and 6,248,612 describe a substrate package wherein the circuit is built up upon a copper heat sink, the integrated circuit (IC) die is mounted in a face up in a central cavity, and wires are used to interconnect the package bond pads with the die. This type of chip package has proven reliable, but requires wire bonding and a copper plate as a support. Substrate packages built on a copper heat sink are generally more expensive to make than ones formed on commonly available heat-curable pre-preg materials, wherein plastic or glass fibers are disposed in a resin matrix that can be cured by heating. However, pre-preg materials do not have the rigidity of metal plates and are difficult to laminate to one another in a manner that maintains a highly accurate registration between interconnects called for by the circuit design. The present invention addresses this limitation.  
           [0003]    Flip chip substrate packages do away with the need for wire bonding by connecting the IC die directly to contacts formed on the substrate package. This is occasionally done in a “die down” configuration wherein the die pads and the bond pads for the solder balls are on the same side of the copper heat sink, and it is generally necessary to mill a cavity in the center of the heat sink to fit the die, or to build up circuit layers around a central die space.  
           [0004]    Die-up flip chip carriers are used in a majority of applications wherein the die is mounted on one side of the support and the solder balls for connection to a circuit board are on the opposite side. See, for example, U. S. Pat. No. 6,229,209 (Matsushita). Conductive interconnects must be provided through the thickness of the support, which in the &#39;209 patent is a single glass-ceramic circuit board. However, the described carrier, showing only a single support, is of limited utility as compared to multilayer chip packages known in the art.  
           [0005]    Ormet Corporation has introduced a family of transient liquid phase sintering conductive adhesives which can be used to form electrical interconnects. See U.S. Pat. Nos. 5,716,663, 5,376,403, 5,853,622 and 5,922,397. The paste mixture when sintered creates intermetallic compounds from metal powders that provide an interconnect with good thermal, mechanical and electrical properties. These materials have nonetheless found limited application as compared to solder, plating through holes and other conventional interconnect materials. The present invention provides a new use for such sinterable, electrically conductive adhesive materials.  
         SUMMARY OF THE INVENTION  
         [0006]    A circuit board according to the invention is made from two or more laminates each made of a fusible dielectric material, which laminates are bonded to each other along respective inner faces thereof Each such laminate is preferably a reinforced polymer system such as a thin pre-preg sheet containing both a heat-fusible resin and a reinforcing fiber filler to provide the desired stiffness and strength, or may be a layer of reinforced polymer that is formed in situ, such as by spraying. If there are three or more laminates, the first and second laminates are the ones disposed on the outside. A number of first electrical contacts are exposed on an outer face of the first laminate, and second electrical contacts are exposed on an outer face of the second laminate. The circuit board further includes a plurality of electrical conductors each running from a first contact to a second contact, the conductors including elongated conductive lines extending along one or both of the first or second laminates and vias extending through the first and second laminates which have been filled with an electrically conductive filler. For purposes of the invention, expressions such as “formed on” or “superposed on” mean do not require direct contact between the parts or layers referenced, unless so specified, and other components or layers may intervene.  
           [0007]    In a preferred form of the invention, the first electrical contacts are configured as flip-chip die pads and the second electrical contacts are configured as solder ball bond pads in ball grid array (BGA) configuration, so that the circuit board can be used as a flip-chip integrated circuit package substrate. For this purpose, a preferred conductive filler consists essentially of an adhesive containing conductive metal particles, especially a transient liquid phase sintering conductive adhesive wherein the metal particles have been sintered after filling of the adhesive into the via, and the conductive lines consist essentially of a plated metal disposed between an outer surface of at least one of the first and second laminates and an external soldermask layer.  
           [0008]    According to another aspect of the invention, some or all of the electrical conductors are embedded between a pair of fused laminates. In an IC package substrate, it is especially useful to make the substrate using three fused laminates, locating the power supply conductor and the ground conductor (which are larger than individual signal lines or conductors) between the first and third and second and third laminates respectively, and locate the signal lines on the outside of the first laminate, the outside of the third laminate, or both. This isolates the power and ground connections from the signal lines while permitting double density signal lines.  
           [0009]    The invention also provides a process for making the foregoing circuit board or IC package substrate. Such a process includes a step of forming a first subassembly, wherein the first subassembly includes a first rigid support plate, a first laminate made of a fusible dielectric material bonded to the rigid support, and a first circuit pattern including a number of vias through the first laminate filled with an electrically conductive filler. A second subassembly of similar construction is formed, wherein the second subassembly includes a second rigid support plate, a second laminate made of a fusible dielectric material bonded to the rigid support, and a second circuit pattern including a number of vias through the second laminate filled with an electrically conductive filler. Vias in on inner surface of the first laminate are brought into electrical contact with the circuit pattern of the second laminate, and vias on an inner surface of the second laminate are brought into electrical contact with the circuit pattern of the first laminate. In some cases, as where the via penetrates straight through both laminates, two (or more) filled vias will be in alignment. In others, the filled vias will be offset from one another, with a conductive line or plane running between them as discussed hereafter. The inner surfaces of the first and second laminates are bonded together to form electrical connections at the filled vias, and the rigid supports are then removed from outer faces of the first and second laminates. Such a process permits more precise alignment of electrical interconnections, and the heat used to bond the laminates together can be used to cure the conductive filler.  
           [0010]    In a preferred form of this process, the step of forming the first subassembly proceeds by forming a first release layer on a face of the first rigid support, forming a first electrically conductive metal layer on the release layer, placing a first laminate made of a dielectric material comprising fibers having a resin impregnated therein over the first release layer and first conductive layer, forming vias (preferably by laser drilling) through the first laminate at locations overlying the electrically conductive metal layer, and filling the vias in the first laminate with the electrically conductive filler material. Similarly, the second subassembly is made by forming a second release layer on a face of the second rigid support, forming a second electrically conductive metal layer on the second release layer, placing a second laminate made of a dielectric material comprising fibers having a resin impregnated therein over the second release layer and second conductive layer, forming vias through the second laminate at locations overlying the electrically conductive metal layer, and filling the vias in the second laminate with the electrically conductive filler material. The step of removing the rigid supports then proceeds by removing the first rigid support from the first release layer and removing the second rigid support from the second release layer, after which the release layers can be removed, leaving the surface circuit patterns exposed. Additional subassemblies can be interposed to provide additional circuit layers, for example, the embedded power and ground connections described above.  
           [0011]    According to another aspect of the invention, a heat sink may be combined with a flip-chip integrated circuit package substrate of the invention to provide additional mechanical stiffness and thermal management. The heat sink is bonded to the outer face of the first laminate and has a central opening therein wherein the die pads are accessible. In a resulting, flip-chip integrated circuit package, a heat conductive material may be added to the encapsulant so that heat is more readily transferred from the die to the heat sink, which is spaced from the die.  
           [0012]    More generally, a flip-chip integrated circuit package of the invention includes a substrate having a plurality of exposed die pads on a die side of the substrate, a plurality of exposed solder ball bond pads on an ball side of the substrate, and a plurality of electrical conductors each running from a die pad to a solder ball bond pad, the conductors including elongated conductive lines extending along the substrate and interconnects extending through the substrate. The substrate may be a layer structure using the laminates of the invention as described above, or a substrate of another type known in the art. A heat sink having a central opening therein is bonded to the die side of the substrate, so that the die pads are accessible through the central opening in the heat sink. An integrated circuit die is positioned in the central opening of the heat sink in contact with the die pads, and a layer of an encapsulant such as an epoxy resin surrounds the die, wherein the layer of an encapsulant contains a heat conducting material that conducts heat from the die to the heat sink better than the encapsulant by itself For this purpose “encapsulant” means any flowable material that can be poured into a die cavity and then cured to a hard state, which encapsulant is electrically and thermally insulating. The heat conducting material may for example comprise metal particles distributed in the encapsulant between the die and the heat sink. These and other aspects of the invention are described further in the detailed description that follows. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    A more complete understanding of the invention may be had by reference to the following detailed description when taken in conjunction with the accompanying drawings, wherein:  
         [0014]    FIGS.  1  to  27  are a series of schematic sectional views, taken along the line of each circuit, illustrating the stages of making an integrated circuit package substrate according to the invention;  
         [0015]    [0015]FIG. 28 is a partial (quarter) top view illustrating an integrated circuit package substrate made by the method of the invention as illustrated in FIGS.  1 - 27 ;  
         [0016]    [0016]FIG. 29 is a schematic sectional view illustrating an embedded ground plane made by the method of the invention as illustrated in FIGS.  1 - 27 ;  
         [0017]    [0017]FIG. 30 is a partial schematic sectional view illustrating an alternative embodiment of an integrated circuit package substrate according to the invention;  
         [0018]    [0018]FIG. 31 is a schematic top view of the integrated circuit package of the embodiment of FIG. 30;  
         [0019]    [0019]FIG. 32 is an enlarged view of the dotted area shown in FIG. 31;  
         [0020]    [0020]FIG. 33 is a schematic bottom view of the integrated circuit package of the embodiment of FIG. 32;  
         [0021]    [0021]FIG. 34 is an enlarged view of the dotted area shown in FIG. 33; and  
         [0022]    [0022]FIG. 35 is a schematic diagram of a flip-chip integrated circuit package according to another embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0023]    Referring first to FIGS.  1  to  27 , a die-up, flip-chip integrated circuit package substrate according to the invention is made by a sequential build up that starts with a thin stainless steel plate  10  (0.062″ thick) that has been prepared by surface oxidation. Copper is flash plated to steel plate  10  as shown in FIG. 2 to form a thin (e.g., 10-20 micron, especially 15 micron) copper layer  11 . The adhesion of the copper layer  11  to plate  10  is relatively weak, such that plate  10  can be pulled off later. A thicker layer  12  of dry film dielectric photoresist, such as a polyclad aqueous photodevelopable resist, is coated onto layer  11  and a pattern is developed therein in a manner well known in the art. This pattern preferably corresponds to the locations of electrical contacts to be located on the outside of the finished IC package. An optional second conductive metal  13  such as tin is flashed into the resulting channels  14 , then flashed over with a thin layer (e.g., 3 microns) of copper, followed by plating to form a copper layer  16  in accordance with the location of the electrical contacts. The second metal layer  13  is applied for purposes of the manufacturing process as described hereafter but forms no part of the finished substrate. The resist layer  12  is then stripped by conventional methods, leaving the plate with copper layer  11 , and a first circuit pattern  15  of including tin and copper layers  13 ,  16 .  
         [0024]    A resin-impregnated fiber laminate  17 , known commercially as pre-preg, is then applied with a light tack to the back of the assembly as shown in FIG. 8. An aramid-fiber epoxy pre-preg laminate  17  is preferred. As shown, the resin of laminate  17  flows and embeds circuit pattern  15  on three sides. A laser such as an ESI 5200 YAG laser or Hitachi CO 2  laser is then used at a power level sufficient to burn through the beta-stage pre-preg laminate  17  but not sufficient to bum through the underlying copper. Holes or vias  21  are drilled at the location of each interconnect required by the circuit design. A conductive paste  22  of resin and conductive metal powder is then filled into holes  21 . As paste  22 , a sinterable material available commercially as Ormalink made by Ormet Corporation is preferred. Other preferred materials are described in U.S. Pat. Nos. 5,716,663, 5,376,403, 5,853,622 and 5,922,397, the contents of which are incorporated by reference herein. Ormalink contains copper powder in a resin base. The heat supplied during the fusing of one pre-preg laminate to another as described hereafter is effective to sinter the metal particles in the conductive adhesive to achieve electrical conductivity.  
         [0025]    After filling of vias  21 , the mylar release liner  18  of laminate  17  is peeled off, and a steel plate assembly  23  having a steel plate  10 A, copper coating  11 A, and tin and copper layers  13 A,  16 A forming a second circuit pattern  15 A thereon is inverted and positioned over filled vias  21  (FIG. 12). Assembly  23  may be made in the same manner as described leading up to FIG. 7, but with a different circuit design. Assembly  23  is pressed into face-to-face contact with the underlying layers so that connection points of copper layers  16 A contact and bond to the exposed tops of the filling material  22 , which forms a conductive pathway through to the underlying circuit pattern  15 . This is carried out with a circuit press known in the art that uses multiline tooling to precisely align the assembly.  
         [0026]    Steel plate  10 A is then removed by means of the weak adhesion between plate  10 A and copper layer  11 A, and layer  11 A is then removed by flash copper etching (FIG. 15), leaving layers  13 A,  16 A intact to form circuit pattern  15 A superposed on the contact pattern  15 . Tin layer  13 A (if applied) is then electrolytically stripped, leaving copper layer  16 A exposed. In the alternative, if the etching of a thin outer copper layer such as  11  or  11 A can be precisely controlled, the intervening tin layer  13 ,  13 A, etc. can be omitted, and the outer copper layer removed without stripping the underlying copper circuit layer. A second piece of pre-preg laminate  27  is placed over copper layer  16 A, resin side down, embedding copper layer  16 A as shown in FIG. 17. A second set of vias  28  are then drilled through laminate  27  at desired interconnect points, the laser again being stopped by the underlying copper. Additional conductive paste  29  such as Ormalink is filled into vias  28 , and the outer pre-preg liner  31  is removed. For the sake of uniformity, each of the pre-preg laminates used are preferably identical in type and dimensions.  
         [0027]    The resulting structure is now ready for final pairing. A second circuit assembly  33  is prepared having the same layer structure as shown in FIG. 16 except for differences in the specific circuit pattern. Assembly  33  is formed with second and third circuit patterns  15 B and  15 C on opposite sides of a third pre-preg laminate  35 . Assembly  33  is inverted and brought into precise alignment with the underlying structure so that contact portions of copper layer  16 B of assembly  33  are brought into registration with the filler material  29  in vias  28  (FIG. 22). The respective steel plates  10 ,  10 B and copper release layers  11 ,  11 B are then successively removed from opposite sides of the resulting paired assembly  34 , leaving the optional tin layers  13 ,  13 C exposed. Copper layers  11 ,  11 B can be removed using a chemical copper etchant which does not remove the underlying tin. These tin layers  13 ,  13 C are then electrolytically stripped in the same manner as layer  13 A, leaving copper layers  16 ,  16 C exposed. A soldermask layer  36  is then applied to both sides, then imaged and developed to expose desired contact points  37  on opposite sides of the assembly. The contacts  37  are then surface finished with a precious metal  38  such as silver, resulting in a circuit board substrate  41  having the structure shown in FIG. 27.  
         [0028]    In this example four circuit layers  15 ,  15 A,  15 B and  15 C are formed, with  15  and  15 C being disposed on the outside beneath soldermask layers  36 , and circuits  15 A,  15 B being disposed on the inside, embedded between the associated pieces of pre-preg which have been fused together. To minimize the number of holes laser drilled through the prepreg, it is preferred to run the signal lines along the opposite outer sides of the assembly as circuits  15 ,  15 C. Circuits  15 A and  15 B are preferably designed as power and ground planes, respectively. These planes are larger (wider, more planar) in comparison with the signal lines, and embedding them inside the pre-preg laminate isolates them from each other and the signal lines. die pad pads  42  on one side for flip-chip die attachment as well as an outer row or rows of solder ball pads  43  on the other side, each connected by a conductive line  44  of one of circuits  15  or  15 C (lines  44  on both sides of the substrate are shown in FIG. 28 for purposes of illustration.) FIG. 29 illustrates a ground plane  45  with current passing through the sintered conductive paste material in the vias when moving between a plane  45  and a pad  42  or  43 . Solder ball pads  43  may be given an OSP, silver or tin finish, and die pad pads  42  may have a copper/OSP or solder finish. Connections for lines  44  are similar, except that a single via including three stacked “cones” of conductive filler material penetrates the entire thickness of the assembly, either at the location of pad  42  or  43 .  
         [0029]    If desired, the intermediate steps used to form the intermediate circuit  15 A can be omitted (from FIGS.  12  to  20 ). In such a case the ground and power planes could be formed at offset locations in the same embedded circuit layer. Similarly, the procedure shown in FIGS.  12 - 20  may be repeated if needed to build in more than two embedded circuit layers. In addition, vias may be provided and filled for conducting heat away from the die, if needed.  
         [0030]    FIGS.  30 - 34  illustrate an alternative embodiment of a IC package substrate  51  of the invention which may have substantially the same layer structure as described for substrate  41 . However, a soldermask layer  46  on the die side is reduced in size (length and width), so that a square or rectangular heat sink  52 , which also acts as a stiffener for the assembly, can been adhered to one side of the layer structure by an adhesive or direct bonding, preferably by means of a layer  48  of a low cure adhesive that minimizes mechanical stress, such as of the Tovay YEF series. Heat sink  52  includes a relatively thick copper plate clad  53  with an outer finish layer  54  such as nickel. A square central opening  56  forms a die cavity. Solder mask  46  may optionally be slightly set back from the inner edge of heat sink  52 , following boundary  57  as shown in FIG. 31. Exposed surfaces of signal lines in this area are covered later during encapsulation of the die.  
         [0031]    A central area  61  of the die side, which area  61  will underlie the die when installed and is of smaller length and width than the die, is configured with an array of power and ground pads  62  which are connected back to enlarged, depthwise interconnects  63  by conductive lines  64 . Lines  64  often connect together several pads  62  and interconnects  63 . Pads  62  are generally arranged at regular intervals, and are positioned according to the die manufacturer&#39;s requirements. Central area  61  also includes a number of signal pads  66  which have associated interconnects  67 . A square peripheral area  71  surrounding central area  61  contains exposed signal pads  72  which connect directly to rows of pins along the periphery of the die. A further square area  73  surrounding area  71  contains signal lines  74  which run from pads  72  towards the periphery of the device, disappearing beneath the inner edge of heat sink  52 , and then penetrating through the thickness of the device to ball pads  89  on the opposite side.  
         [0032]    As shown in FIGS.  33 - 34 , the solder ball side  81  of the substrate  51  includes a central group of ball pads  82  (8 by 8 in this example) for power and ground connections which are connected back to interconnects  63  and power and ground pads  62  on the die side. Some of these may be joined by conductive lines  83  if it is necessary to reposition a ball pad location. Surrounding pads  82  is a square central area  86  free of pads through which conductive lines  93  on the ball side run, and around area  86  is a square outer area  88  in which a large number of signal ball pads  89  are positioned. These are generally arranged in rows and columns, in this example, nine pads deep on each side, with an innermost row  91  being 18 by 18 pads and the outermost row  92  being 34 by 34 pads, less pads omitted on the four corners. As shown in FIG. 34, some of the outer ball pads  89 A are connected by means of conductive lines  93  disposed just beneath the associated soldermask layer to depthwise interconnects  67  positioned near and among the central ball pads  82 , whereby pads  89 A are electrically connected to signal pads  66  on the die side. Other pads  89 B have adjacent interconnects  96  that emerge underneath heat sink  52  and connect to each of lines  74  on the opposite side, leading to the outer signal pads  72 .  
         [0033]    It is most convenient to locate the power and ground pads on the side of the substrate directly opposite the die connections, but of course such connections could be routed to the side in the same manner as the signal connections. Similarly, having conductive lines  74 ,  93  on both sides of the device permits up to double the number of signal connections at a given spacing as compared to a single-sided construction. The foregoing embodiments also conserve space by vertical stacking of vias to form the interconnects. Conventional manufacturing methods with less precise registration between adjacent layers generally require use of offset vias, greatly increasing the amount of space required. A substrate according to the invention can save up to 50% in required space as compared to a conventional package substrate with the same number of connections.  
         [0034]    Once formation is complete, a circuit board substrate  41  or  51  of the invention is singulated, tested, inspected and packed for shipment. Final processing is carried out by the end user, who applies the die to the die pads and solder balls to the solder ball pads, then encapsulates the die in the conventional manner to form the finished integrated circuit package. Such a circuit board substrate according to the invention provides numerous advantages over other known IC package designs. The flip-chip die connection eliminates the expense of connecting bonded wires and avoids the need to form a cavity for the die. The invention further provides a highly effective technique for forming conductive vias and achieving registration of fine features such as electrical contact points during the production process even when a large number of substrates are formed at the same time as a panel and later singulated by cutting. The use of bonded pre-preg laminates makes circuit board substrate  41  highly cost effective as compared to substrate packages requiring milled copper supports/heat sinks.  
         [0035]    The invention permits fine resolution of small features. For example, a substrate of dimensions 35 by 35 mm with a minimum line width and line spacing of 35 microns can accommodate an I/O count of 816, bump pad diameter 125 microns, via pad diameter 90 microns, signal capture pad diameter 125 microns, signal via diameter 90 microns, staggered bump pad pitch 160 microns, solder ball pitch of 1.00 mm, with 2 signal layers and 2 power/ground layers. The filled vias, due to the laser used to burn the vias, tend to be conical, for example, 90 microns diameter on the outside, 75 microns diameter on the inside, with a depth of about 80 microns (equals the thickness of the surrounding dielectric layer.) The sinterable conductive paste material, once sintering occurs, forms a conductive network of metal particles that is highly effective for conducting current. In subsequent sintering cycles, as needed when the second or subsequent pre-preg laminates are added, the previously sintered material in the vias does not re-melt and retains its superior electrical conductivity.  
         [0036]    The method of the invention keeps each half of a laminate pair bonded to a rigid backing or support (the steel plate) during assembly. This provides an enormous advantage in obtaining accurate registration of interconnections that cannot be matched by attempting to laminate a piece of pre-preg or adhesive tape onto a circuitized piece of pre-preg. Both halves of the assembly should be rigidly secured to the support during pairing to ensure a superior product.  
         [0037]    According to a further aspect of the invention, the end user encapsulates the die in a novel manner in order to maximize heat dissipation through the heat sink. Unlike in other formation processes where the die is mounted adjacent to the core or heat sink, in the present invention the die  100  as shown in FIG. 35 is spaced from the heat sink  52 . The encapsulant used to cover the die is typically an epoxy that is non-conductive of both heat and electricity. According to a further aspect of the present invention, the die is placed in contact with a heat-conductive bridge that conducts heat from the die to the heat sink to a degree substantially better than a conventional encapsulant. This could be done, as shown in FIG. 35, by use of a layer  101  of an encapsulant having heat-conductive filler particles  102  distributed therein (e.g., aluminum or the like) surrounding die  100  which would improve the heat conductivity to the heat sink  52  preferably without creating any electrical conductivity. A further layer  103  of conventional encapsulant lacking the conductive filler  102  may be filled in over layer  101  if needed. Such an arrangement may become essential as integrated circuits are designed to operate at ever-increasing speeds.  
         [0038]    The heat conductive bridge is not limited to the filler particles  102  shown. For example, after the space around the die is filled with encapsulant to the level of the die, a backing plate made of copper, similar to the heat sink itself, can be inserted like a lid behind the die  100 . The backing plate has the same length and/or width as recess  56  and thereby acts as a heat bridge. A further layer of encapsulant is filled in behind the heat conductive plate, which could be perforated or cut-away (e.g., X-shaped) so that the encapsulant could be filled in both above and below in one step after placement of the plate.  
         [0039]    The invention described herein is not limited to the applications described above and can be used for making circuit boards other than those for mounting an IC die. While the invention has been described with reference to the illustrated embodiment, it is not intended to limit the invention but, on the contrary, it is intended to cover such alternatives, modifications and equivalents as may be included in the spirit and scope of the invention.