Patent Publication Number: US-2006006532-A1

Title: Flip-chip without bumps and polymer for board assembly

Description:
FIELD OF THE INVENTION  
      The present invention is related in general to the field of electronic systems and semiconductor devices, and more specifically to structure and fabrication methods of wafer-scale devices without bumps and polymer layers and their assembly onto wiring boards.  
     DESCRIPTION OF THE RELATED ART  
      In their book “Chip Scale Package” (McGraw-Hill, 1999), John H. Lau and Shi-Wei Ricky Lee describe various semiconductor devices and packages of contemporary “chip-scale” families, as they are fabricated by a number of semiconductor companies worldwide. The newest designs and concepts in microelectronics assembly and packaging are aiming for a package with a planar area not substantially greater than the silicon chip itself, or at most 20% larger area. This concept, known as Chip-Scale Package (CSP), is finding particular favor with those electronics industries where the product size is continually shrinking such as cellular communications, pagers, hard disk drivers, laptop computers and medical instrumentation. Most CSP approaches are based on flip-chip assembly with solder bumps or solder balls on the exterior of the package, to interface with system or wiring boards.  
      A typical flip-chip process calls for solder-compatible contact pads on the circuit surface of the chip, and the deposition of solder bumps or balls thereon. The semiconductor wafers have to be separated into chips before flip-chip attachment to the board. Existing solder bump processes include solder through metal masks, electroplated solder or screen printing a mound of solder paste onto each metallic contact. Typically the solder bumps are reflowed in a chain type furnace. Alternatively, pre-fabricated solder balls may be placed on the chip pads and reflowed in a similar chain type furnace.  
      The chip-to-be-flipped may then be attached to a second interconnection surface such as an interposer, or alternatively, coupled directly to a printed circuit board (PCB). Attaching the flip-chip to the next interconnect is carried out by aligning the solder bumps or balls on the chip to contact pads on the second level interconnection and then performing a second solder reflow operation. During the reflow, the bumps or balls liquefy and make a bond to the next interconnect level which has pads or traces to receive the solder. Following the solder reflow step, flip-chips often use a polymeric underfill between the chip and the interposer or PCB to alleviate mechanical stress caused by the mismatch in the coefficients of thermal expansion (CTE) between the semiconductor chip, the interposer, if any, and the PCB. Many reliability problems occur due to the stress placed on the solder bumps or balls when the integrated circuit is cycled from hot to cool during operation. The interposers and underfills of the prior art are used to reduce or eliminate the mechanical stress generated by thermal cycling on the solder bumps or balls, but they help little to dissipate thermal energy.  
      When another set of solder balls on the opposite side of the interposer is employed to complete the bonding process to a PCB, this second set may also be aligned and reflowed for attachment by the end user. When the chip is attached to the board as described, the final consumption of board area is usually not much larger than the area of the chip (about 20% larger). Consequently, this family of products is classified as “chip-scale packages”. However, these devices contribute little to reduce the device height, which is critical in small appliances.  
      An example of a solder ball flip-chip structure designed for stress absorption after mounting is described in the Japanese Patent # 1-209746, issued on 23 Aug. 1989 (Moriyama Yoshifumi, “Semiconductor Device”). The perimeter of the under-bump metallization of the solder ball is supported by a polyimide layer; further, a second thick polyimide layer is shown under the bump metallization as low stress, heat tolerant resin.  
      Structures such as the one quoted above are based on the publication “A Silicon and Aluminum Dynamic Memory Technology” by Richard A. Larsen (IBM J. Res. Develop., vol. 24, no.  3 , May 1980, pp. 268-282). The article includes description of a flip-chip packaging technology using a solder bump on an under-bump metallization, which is resting its perimeter on a thick polyimide layer. The bump structure is supported by another polyimide layer.  
      Problems exist with conventional process equipment and flows for chip-scale packages using flip-chip technology. First, a typical solder bumping process is very equipment intensive, resulting in a large capital cost. Evaporation, plating and screening are environmentally unfriendly in that they make use of excess of solder, often containing lead. Both processing and clean-up costs are high in these operations.  
      Second, the manufacturing of flip-chip assembly can have a long cycle time. Typically, reflows which are carried out in infrared or forced convection ovens have cycle times of 5 minutes or longer. These furnaces are usually very long (&gt;3 m) and massive structures, occupying much space on the assembly floor. Moving parts in such furnaces are a significant source of particulate contamination.  
      Third, present day assembly of flip-chips is processed in chip form. The assembly process starts after the chip has been diced from the wafer. This type of production strategy causes a disconnect between the wafer fabrication and test plant (“wafer fab”) and the assembly and final test site because the dicing of the wafer must occur outside the clean room environment of a wafer fab. In addition, there are substantial costs in shipping valuable wafers worldwide. After packaging is completed in the assembly sites, the devices must undergo final testing before they can be shipped to the customer. A need thus exists for a packaging method that provides for wafer-scale assembly of integrated circuits, preferably in the clean room facility of the wafer fab itself.  
     SUMMARY OF THE INVENTION  
      The present invention describes a semiconductor chip having a planar active surface including an integrated circuit protected by an inorganic overcoat; the circuit has metallization patterns including a plurality of contact pads. Each of these contact pads has an added conductive layer on the circuit metallization. This added layer has a conformal surface adjacent the chip, including peripheral portions of the overcoat, and a planar outer surface; this outer surface is suitable to form metallurgical bonds without melting. The chip contact pads may have a distribution arrayed in the center of the chip in close proximity to the chip neutral line. The chip may further have a non-conductive adhesive layer over the overcoat, filling the spaces between the added conductive layers on each contact pad.  
      The inorganic overcoat is moisture-impenetrable and stiff. This overcoat may have an additional top layer of organic overcoat which is heat-resistant and compliant.  
      The chip contact pads may have a distribution such that an area portion of the active chip surface is available for attaching a thermally conductive plate; this plate has a thickness compatible with the thickness of the conductive pad layer.  
      The present invention further discloses several embodiments of semiconductor assemblies in which a chip as described above is bonded to a wiring board which has a plurality of planar, metallurgically bondable terminal pads in a distribution aligned with the distribution of the chip contact pads. The bonding is performed by a technique selected from 
      Direct welding by metallic interdiffusion;     Attaching by solder paste;     Attaching by conductive adhesive.    

      In the first embodiment, the chip, with or without the heat spreader, is directly bonded to the board.  
      In the second embodiment, the chip, with or without heat spreader, is first attached to a substrate. In the “ball-grid array” version of this embodiment, the substrate comprises solder balls for the attachment to the wiring board. In the “land-grid-array” version of this embodiment, the device comprises bondable pads for the attachment to the wiring board.  
      In the third embodiment, the chip, with or without heat spreader, is first attached to a relatively thicker metal patterned in the mirror image of the chip contact pads. This thicker metal, in turn, is the directly bonded to the wiring board. For the third embodiment, as well as for the second embodiments, an encapsulation, for example in protective molding compound, of chip and substrate is preferred. This feature, in turn, provides the condition for assembling extra-thin silicon chip material, a welcome contribution for fabricating low-height chip-scale devices.  
      The first and the third embodiment can be designed and manufactured as true chip-size devices.  
      It is a technical advantage of the present invention that a wide variety of materials and techniques can be employed for the proposed metallization and assembly steps.  
      Other technical advantages of the present invention include a reduction of manufacturing cost, a lead-free assembly solution, improved thermal performance of the package, and improved reliability of the device.  
      The technical advances represented by the invention, as well as the aspects thereof, will become apparent from the following description of the preferred embodiments of the invention, when considered in conjunction with the accompanying drawings and the novel features set forth in the appended claims.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic cross section of an IC contact pad having under-bump metallization and a solder ball for flip-chip attachment according to known technology.  
       FIG. 2  is a schematic cross section of the first embodiment of the invention, showing an IC with an inorganic overcoat, the contact pad having added conductive layers, the outer layer having a planar surface suitable to form metallurgical bonds.  
       FIG. 3  is a schematic cross section of the second embodiment of the invention, showing a layer of non-conductive adhesive added to the device of  FIG. 2 .  
       FIG. 4  is a schematic top view of the active surface of a chip according to the third embodiment of the invention, showing the contact pads arrayed in the center of the chip.  
       FIG. 5  is a schematic cross section of the fourth preferred embodiment of the invention, showing an IC with an inorganic and an organic overcoat, the contact pad having added conductive layers, the outer layer having a planar surface suitable to form metallurgical bonds.  
       FIG. 6A  is a schematic top view of the fifth embodiment of the invention showing an assembled chip active chip surface having a plurality of metallized contact pads (in a peripheral array) and a heat spreader positioned in the remaining surface area.  
       FIG. 6B  is a schematic cross section of the fifth embodiment, having metallized contact pads and a heat spreader, attached to a printed circuit board.  
       FIG. 7A  is a schematic top view of a combination of the third and fifth embodiments showing the active chip surface having a plurality of metallized contact pads in a centralized array and a heat spreader positioned in the remaining surface area.  
       FIG. 7B  is a schematic cross section of the structure of  FIG. 7A , having metallized contact pads and a heat spreader, attached to a printed circuit board.  
       FIG. 7C  is a schematic cross section of the assembly of  FIG. 7B  including a protective device encapsulation.  
       FIG. 8  is a schematic cross section of the assembly in  FIG. 6B  attached to an insulator-based substrate, then encapsulated in a molded package, and finally attached to a board using solder balls.  
       FIG. 9  is a schematic cross section of the assembly in  FIG. 6B  attached to an insulator-based substrate, then encapsulated in a molded package, and finally attached to a board using bondable metal lands.  
       FIG. 10  is a schematic cross section of the assembly in  FIG. 6B  attached to a sheet-like metal, then encapsulated, and finally attached to a board using the bondable outer surface of the metal sheet.  
       FIG. 11  shows a schematic cross section of a chip with contact pad, redistribution line, and added conductive layers for connection to other parts according to the invention.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The present invention is related to U.S. patent applications Ser. No. 09/458,593, filed on Dec. 10, 1999 (Zuniga et al., “Bonding over Integrated Circuits”), and Ser. No. 10/006,576, filed on Dec. 6, 2001 (Koduri et al., “Back Side Coating of Semiconductor Wafers”). Further, the present invention is related to U.S. patent application Ser. No. 10/001,302, filed on Nov. 1, 2001 (Zuniga et al., “Bumpless Wafer-Scale Device and Board Assembly”), which is herewith incorporated by reference.  
      The impact of the present invention can be most easily appreciated by highlighting the shortcomings of the known technology. As a typical example of the known technology, the schematic cross section of  FIG. 1  illustrates the detail of the metallurgical requirements for the integrated circuit contact pad  100  in order to ready it for flip-chip assembly using solder balls. A semiconductor chip  101 , commonly silicon, is protected by a moisture-impermeable dielectric protective overcoat  102 , usually silicon nitride, an optional additional organic overcoat  108 , and a patterned metallization “under-bump”  103  over the aluminum or copper  104  of the circuit contact pads. Metallization  103  usually consists of a sequence of thin layers: When the circuit metallization  104  is aluminum, the conformal layer  105  adjacent to the circuit is typically a refractory metal  105 , such as chromium, titanium, tungsten, molybdenum, tantalum, or alloys thereof. When the circuit metallization  104  is copper, the conformal layer  105  is typically aluminum. The following buffer layer  106  is typically nickel. The outermost layer  107  has to be a solderable metal, such as gold, copper, nickel, or palladium.  
      Finally, solder bump  109  is formed by reflowing the deposited (evaporated or plated) solder alloy (typically a mixture of tin and lead, indium, or other metals). These solder bumps assume various shapes after attaching the chip to the substrate, influenced by the forces of surface tension during the reflow process. The overall process depicted in  FIG. 1  is expensive, since typically ten or more process steps are involved: Sputter chromium and copper (or nickel or any of a wide selection of metals described in the literature); spin resist; bake; expose; develop; etch metal; remove resist; seed solder; evaporate or plate solder; reflow solder; flip-chip attach.  
      In some process flows of the known technology, a layer  108  of polymeric material (benzocyclobutene, BCB) is deposited over the silicon nitride layer  102  so that it can act as a stress-relieving buffer between the under-bump metal  103  and the solder material  109 . It has been shown to be useful in preventing solder joint failures when the solder bump has to withstand thermomechanical stresses in temperature variations (see the Japanese Patent # 1-209746 of 1989 and the publication by R. A. Larsen, IBM, of 1980, both cited and discussed above).  
      The bumped chip is then flipped so that its surface having the integrated circuit faces the assembly board, and solder-attached to that board. This assembly lends itself to the fabrication of so-called “chip-size” devices which have the same outline as the chip. Unfortunately, these devices suffer from the drawback that, in operation and temperature excursions, they are sensitive to thermomechanical stress due to the mismatch between the coefficients of thermal expansion of the semiconductor material and the substrate material.  
      These reliability risks, as well as the requirements for special pad metallizations, have been described in a series of detailed publications by the International Business Machines Corporation in 1969 (IBM J. Res. Develop., Vol. 13, pp. 226-296): P. A. Totta et al., SLT Device Metallurgy and its Monolithic Extension, L. F. Miller, Controlled Collapse Reflow Chip Joining, L. S. Goldmann, Geometric Optimization of Controlled Collapse Interconnections, K. C. Norris et al., Reliability of Controlled Collapse Interconnections, S. Oktay, Parametric Study of Temperature Profiles in Chips Joined by Controlled Collapse Techniques, B. S. Berry et al., Studies of the SLT Chip Terminal Metallurgy.  
      These investigations of the optimum height and volume of the solder joint and the expected onset of thermal fatigue and cracking showed that it is desirable to have 
          a highly ductile solder;     a high ultimate shear strength of the chip/joint and substrate/joint interfaces;     a large value for the ratio (radius of bump-to-chip contact)/(distance to neutral point of chip).        

      One method aims at absorbing part of the thermomechanical stress on the solder joints by plastic material surrounding the joints and filling the gap between chip and substrate. See for instance, U.S. Pat. No. 6,213,347 and 6,228,680 (Thomas, “Low Stress Method and Apparatus of Underfilling Flip-Chip Electronic Devices”). However, this method is expensive, because it represents an additional process step, and it may not be welcome since the customer may have to perform the process after device attachment to the motherboard.  
      Another method aims at elongating the solder joint after assembly into a column with concave surface contours. However, this method is constrained by solder volume and manufacturability.  
      The schematic cross section of  FIG. 2  illustrates the first preferred embodiment of the present invention.  201  is a portion of the semiconductor chip, with planar active surface  201   a , including an integrated circuit (IC). The semiconductor material is commonly silicon; other materials include silicon germanium, gallium arsenide, or any other semiconductor material used in IC fabrication. The IC has a metallization pattern  202  including a plurality of contact pads  202   a . The chip metallization  202  may consist of aluminum, copper, or alloys thereof. The IC surface is protected by a dielectric protective overcoat  203 , usually silicon nitride or silicon oxynitride of about 1.0 μm thickness. It is impermeable to moisture and mechanically stiff. As  FIG. 2  illustrates, the photoresist window defining contact pad  202   a  has been opened through the protective overcoat  203 .  
      When layer  202  is aluminum or aluminum alloyed with 0.5 to 2% copper and/or 0.5 to 1% silicon (layer 0.4 to 1.5 μm thick), the contact pad exhibits under the aluminum frequently a thin layer (4 to 20 nm thick) of titanium, titanium nitride, titanium tungsten, tantalum, tantalum nitride, tantalum silicon nitride, tungsten nitride, or tungsten silicon nitride (not shown in  FIG. 2 ).  
      For aluminum as well as copper chip metallization, the insulator under the metallization is typically silicon dioxide. However, with the recent trend towards dielectric materials of lower dielectric constants, films made of silicon-containing hydrogen silsesquioxane (HSQ), aerogels, organic polyimides and parylenes are also used in spite of the fact that they are less dense and mechanically weaker than previous standard insulators such as the plasma-enhanced chemical vapor deposited dielectrics. Also, stacks of dielectric layers may be used such as alternating layers of plasma-generated tetraethylorthosilicate (TEOS) oxide and HSQ, or ozone TEOS oxide and HSQ.  
      In order to strengthen the chip contact pad mechanically, reinforcing structures are sometimes placed under the bonding pad (not shown in  FIG. 2 ). Descriptions may be found in U.S. Pat. No. 6,1443,396, issued Nov. 7, 2000 (Saran et al., System and Method for Reinforcing a Bond Pad), and U.S. Pat. No. 6,232,662, issued May 15, 2001 (Saran, System and Method for Bonding over Active Integrated Circuits), and U.S. patent application Ser. No. 09/312,385, filed May 14, 1999 (Saran et al., “Fine Pitch System and Method for Reinforcing Bond Pads in Semiconductor Devices”).  
      In the embodiment of  FIG. 2 , the contact pad has an added conductive layer  205  on the pad metallization  202 . This layer  205  is conformal to the surface of the chip and provides a reliable, low resistance contact to the pad metallization  202 , as well as a strong and reliable adhesion to the inorganic overcoat  203 . As  FIG. 2  shows, layer  205  covers not only the area of the pad  202   a  proper, but also the window side walls of protective layers  203  and a portion  205   a  of the surface surrounding the pad window. The preferred material for layer  205  is aluminum in the thickness range of about 500 to 1000 nm. There are various methods of depositing layer  205 . The preferred method is sputtering, since this method allows an effective cleaning of the pad surface just before the actual metal deposition (especially useful for aluminum pad metallization  202 ). Other deposition techniques include evaporating and plating.  
      The second added layer  206  in  FIG. 2  has the important function to enable the transition from a conformal outline following the chip surface contours, to a “flat” outline defining a planar outer surface, which is substantially parallel to the chip surface. The second conductive layer  206  may also have to operate as a diffusion barrier if copper is used as the chip metallization  202 . The second conductive layer is selected from a group consisting of nickel, vanadium, titanium, tungsten, tantalum, osmium, chromium, and aluminum, and alloys thereof.  
      Dependent on the choice of the metal, there are several methods of depositing layer  206 : 
          Electroless plating. The complete process includes the following steps:     Coating the backside of the silicon IC wafer with resist using a spin-on technique. This coat will prevent accidental metal deposition on the wafer backside.     Baking the resist, typically at 110° C. for a time period of about 30 to 60 minutes.     Cleaning of the exposed bond pad metal surface using a plasma ashing process for about 2 minutes.     Cleaning by immersing the wafer, having the exposed metal of the bond pads, in a solution of sulfuric acid, nitric acids, or any other acid, for about 50 to 60 seconds.     Rinsing in overflow rinser for about 100 to 180 seconds.     Immersing the wafer in a catalytic metal chloride solution, such as palladium chloride, for about 40 to 80 seconds “activates” the metal surface, i.e., a layer of seed metal (such as palladium) is deposited onto the clean non-oxidized metal surface.     Rinsing in dump rinser for about 100 to 180 seconds.     Electroless plating of the barrier metal. If nickel is selected, plating between 150 to 180 seconds will deposit about 0.4 to 0.6 μm thick nickel. 
 
 More detail on this and closely related processes can be found in U.S. patent application Ser. No. 09/775,322, filed Feb. 1, 2001 (Stierman et al., “Structure and Method for Band Pads of Copper-Metallized Integrated Circuits”), which is herewith incorporated by reference. 
    Screen printing. A plurality of openings are prepared in a mask stencil so that their position mirror-images the position of the contact pads on the IC surface. The mask stencil is then aligned with the semiconductor wafer so that each mask opening aligns with its respective IC contact pad. The metal or metal alloy is liquefied and deposited through the mask openings so that it equalizes the overcoat contours of the pad window remaining from the etch step through the overcoat layers.     Support by islands of protective overcoat. Applying a modification of the U.S. patent application Ser. No. 09/458,593 cited above, the contact pads are opened so that a plurality of silicon nitride “islands” remain unetched in the window area, creating numerous silicon nitride “mesas” with narrow vias between them. In the following metal deposition step (for instance, aluminum as the first layer, nickel as the second layer), the vias are quickly filled, while metal is also deposited on top of the mesas. The overall window metal surface approaches progressively a uniformly flat configuration.        

      In  FIG. 2 , the outermost metal layer  207  has a surface suitable to form metallurgical bonds without melting. As an example, layer  207  is made of a metal selected from a group consisting of gold, palladium, platinum, silver, and alloys thereof. The bondability function of layer  207  is emphasized. The overall thickness of the bondable metal layer  207 , usually a noble metal, is kept to a minimum, thus saving cost. Acceptable thicknesses range from about 0.02 to 0.1 μm; for some metals, a flash is sufficient.  
      Without necessarily resorting to melting solder, these surfaces are suitable for the following attaching methods to outside board surfaces, which have been prepared with appropriately bondable metal surfaces (typically gold flash on copper-metallized terminal pads): 
          Direct welding by metallic interdiffusion. For this bonding, the clean surfaces of the chip contact and board terminal pads are subjected to slight compression or thermo-compression at elevated temperatures and moderate pressures for a period of time on the order of 1 s.     Attachment by conductive adhesives. This preferred embodiment permits small pad sizes and fine pad pitch, since conductive adhesives do not tend to bridge.     Attachment by solder paste. Both the chip contact pads and the board terminal pads are solderable. Consequently, metallurgically strong solder connections are easily established.        

      It is an important feature of the present invention that the number of added conductive layers are not restricted to three. There may be only one layer, or two layers, or more than three layers added to the chip contact pads metallization. It is only required that the added conductive layer or layers have a conformal surface adjacent the chip and a planar outer surface, and that the outer surface be suitable to form metallurgical bonds, without necessarily resorting to melting of solder.  
      It is another important feature of the present invention that the described added conductive layers can be formed onto any number of chip contact pads. The described technique is thus applicable chips with a small number of I/O&#39;s (for example, 12 I/O&#39;s as illustrated in  FIG. 6A ) as well as to chips with a high number of I/O&#39;s (for example, 200 or 600 I/O&#39;s). The contact pads may be shaped as a rectangle, a square or a circle with a side length of about 40 to 150 μm, preferably 90 to 100 μm. The pitch between neighboring contact pads is typically in the range from 0.4 to 0.6 mm with a separation between pads from 50 to 200 μm, for chips with high numbers of input/output terminals preferably between 50 and 75 μm. It is a technical advantage of the present invention that the pitch between neighboring contacts can be reduced to 100 μm center-to-center, which is a major improvement over solder ball contacts.  
      It is another important feature of the present invention that the described added conductive layers can be formed whatever the arrangement and distribution of the contact pads may be. As an example, the contact pads may be arrayed along the periphery of the active chip surface, as illustrated in  FIG. 6A . In another example, the contact pads may be arrayed in the center of the active chip surface, as illustrated in the schematic top view of the active chip surface in  FIG. 4 . In this third embodiment of the invention, the distribution of contact pads  402  on the active chip surface  401  is arrayed in the center of the chip in close proximity to the neutral line of the chip so that any stress on the contact joints after assembly is minimized. In yet another example, the contact pads may be arrayed in an arbitrary configuration across the active chip surface.  
      It is a technical advantage of the present invention that it permits protection of the passive surface of the chip, indicated by the layer  210  in  FIG. 2 . Preferably, layer  210  is an electrically non-conductive material of black color to protect the chip against disturbing influences of light or other electro-magnetic radiation, of environmental degradation such as moisture and dust, and of mechanical disturbances such as scratches. Methods of fabricating layer  210  include: 
      Screen-printing of polymeric material, undoped or doped;     Spinning of insulating material which will harden;     Attaching tape or film;     Spraying low-viscosity material which will harden; and     Transfer molding a protective layer.    

       FIG. 3  illustrates the second preferred embodiment of the invention in a schematic cross section analogous to  FIG. 2 . A non-conductive adhesive layer  301  has been deposited over the inorganic overcoat  203 , adhering to overcoat  203  as well as to an outside part after chip assembly. This adhesive layer has the same height  301   a  as the added metallization  200  over the contact pad. In this manner, the adhesive layer  301  serves to fill the spaces between the added metallization on each of the contact pads.  
       FIG. 5  depicts the fourth embodiment of the invention. A polymeric layer  501  in the thickness range between 1 and 3 μm is deposited over the inorganic overcoat  203 . This organic overcoat  501  is heat-resistant and mechanically compliant. It thus provides the stress-relieving buffer between the under-bump metallization and the solder material, as described in the above-cited publication by R. A. Larsen (1980). Preferred materials for layer  501  include polyimide and benzocyclobutene (BCB).  
       FIGS. 6A and 6B  illustrate the fifth preferred embodiment of the invention in a schematic top view ( FIG. 6A ) of the chip  601  after assembly, and a schematic cross section ( FIG. 6B ) of the chip attached to the wiring board  604 . In the fifth embodiment as illustrated in  FIG. 6A , the chip contact pads  602 , covered with the added conductive layers as described above, are distributed such that an area portion of the active chip surface is available for attaching a thermally conductive plate  603 . In the example of  FIG. 6A , this area portion is the central area of the active chip surface  601 . Since the plate is operable as a heat spreader, it is preferably made of a good thermal conductor (yet low cost material) such as copper and has a thickness compatible with the thickness of the added conductive pads layers (0.2 to 0.8 μm).  
      The plate  603  has an outer surface suitable for metallurgical bonds. Especially, the outer surface may be solderable. Preferably, the outer plate surface is a noble metal layer similar to layer  207  in  FIG. 2 , for instance a gold flash. The plate can then be attached to a wiring board metal pad, pre-fabricated in aligned position, concurrently with the attachment of the contact metal structures  602  to the board. For this attachment to the board, the same options are available, which are discussed above: 
          Direct welding by metallic interdiffusion;     Attachment by conductive adhesives;     Attachment by solder paste. 
 
 The schematic cross section of  FIG. 6B  depicts the assembled system. IC chip  610  has a plurality of metallized contact pads  611  arrayed along the chip periphery. Further, it has a heat-spreading plate  612  in the chip surface area not utilized by contact pads. The wiring board  620  has mirror-image counterparts: terminal pads  621  and thermal sink  622 . The assembly establishes electrical contact between the IC chip and the board, and forms gaps  630  in the spaces between the metallized areas. The width of the gaps is approximately equal to the thickness of the added conductive layers  611  on the chip contact pads, plus any height of the aligned metallization  621  on the board. Gaps  630  are filled with non-conductive adhesive when chips featuring the second embodiment of the invention, as described in  FIG. 3 , are used. 
       

      The assembly board  620  is typically selected from a group consisting of organic materials, including FR-4, FR-5, and BT resin, with or without strengthening or thermally modulating fibers; metals; and ceramics. It is a technical advantage to fill board  620  with particles of oxidized silicon (size about 10 to 100 μm) in order to approximate the board coefficient of thermal expansion (CTE) to the silicon CTE and thus minimize the stress exerted on the contact joints due to CTE mismatch.  
      As described in the third embodiment of the invention in  FIG. 4 , the contact pads may be arrayed in the center of the active chip surface. This case is depicted in  FIGS. 7A and 7B , with the contact pads  702  and  711 , respectively, in the center of the surface  701  of chip  710 . The area portion of the active chip surface available for attaching a thermally conductive plate is the chip periphery. The heat spreader is now formed as a broad frame  703  and  712 , respectively, around the contact pads. This distribution is particularly favorable for minimizing thermomechanical stress at the assembled contact pads, when the system is subjected to temperature variations and the mismatch between the CTE&#39;s of IC chip and board become important. The mirror-image metallized areas on board  720  are designated  721  for the contact pads and  722  for the heat-conducting frame.  
      It is yet another important feature of the present invention that an encapsulation can be formed protecting at least the chip surface opposite to the active surface, and that the encapsulated device can be assembled on a board. The preferred encapsulation material is a molding compound, and the preferred encapsulation method the transfer molding technique. The device variation exhibiting a molded encapsulation is especially recommended when the I/C chip is thinner than the customary thickness of 275 μm.  FIG. 7C  shows the assembly of  FIG. 7B  with a device having an encapsulation  730 .  
      In  FIGS. 8 and 9 , a molded device analogous to the embodiment in  FIG. 6B  is shown in an assembly on relatively thin substrates, preferably insulator-based substrates such as a polyimide film. In  FIG. 8 , this assembled device is then completed as a ball-grid array device having solder balls. In  FIG. 9 , the assembled device is completed as a land-grid array (lead-less, or no-lead) device having bondable or solderable attachment pads.  
      In the schematic cross section of  FIG. 8 , the I/C chip  810  has added conductive contact metallization  811  for the plurality of pads and a heat spreader  812  similar to  FIG. 6B . Molding compound  813  encapsulates the device. The film-like substrate  820  has mirror-image terminals  821  for the contact pads  811  and heat sink  822  for heat spreader  811 . Molding compound  813  encapsulates the passive surface of the chip, attaches strongly to substrate  820 , and fills the gaps  830 . Using solder balls  840 , the device is attached to printed circuit board  850 .  
      In the schematic cross section of  FIG. 9 , bondable or solderable lands  940  replace the solder balls  840  of  FIG. 8 . In other respects, the structure of the device and the assembly onto the board are analogous to  FIG. 8 .  
      In the embodiment of  FIG. 10 , the contact metallization  1011  of chip  1010  is attached to the counterparts  1021  on a “leadframe”-like, relatively thick sheet of metal  1020 . This sub-assembly is encapsulated in molding compound  1013 , which also fill the gaps  1030 . The outer surface of the sheet-like metal  1021  is bondable or solderable. It is attached to printed circuit board  1050 . It should be mentioned that the embodiment of  FIG. 10  lends itself to fabricating a chip-size package. Instead of encapsulating individual chips, the whole wafer is encapsulated by molding compound. Subsequently, the devices are singulated, preferably by sawing, along lines  1014 , which are determined by the outline of chip  1010 .  
       FIG. 11  shows schematically an example for applying the contact metallization of the invention to the case where a redistribution of connecting lines from chip contact pads to remote contact pads at predetermined locations is desired. Such redistribution may often be required for connection to printed wiring boards with fixed terminals. Chip  1101  has a contact pad  1102 , made of copper or aluminum, and a first protective overcoat  1103 , preferably made of silicon nitride or oxynitride (or a stack of inorganic and organic layers). A metal line  1104 , preferably made of copper, connects chip contact pad  1102  to the redistributed pad  1105 . Pad  1105  exhibits the layer sequence and fabrication methods described above. The redistributing metal lines  1104  may be protected by another layer  1106  (or stack or organic an inorganic layers).  
      While this invention has been described in reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. As an example, the mutual arrangement of the contact pads and the heat spreader may be designed in a number of different configurations. As another example, the selection of the added metal layers to the contact pads can be varied to be best suitable for the assembly technique selected for board attachment.  
      It is therefore intended that the appended claims encompass any such modifications or embodiments.