Abstract:
A method of encapsulating a semiconductor device comprising the steps of providing a mold having top and bottom halves each with cavities for holding semiconductor devices, and further having gates and runners for feeding encapsulation material into said cavities; lining said cavities with protective plastic films; providing a plurality of semiconductor integrated circuit chips, each having an outline; providing an electrically insulating interposer; assembling said chip and said interposer, loading said assembly into said mold and introducing into said mold a low-viscosity, high adhesion encapsulation material; at least partially curing said encapsulation material, thereby forming a flat, high-luster surface; opening said mold and removing said interposer together with said encapsulated chips from said mold; attaching an array of solder balls to the exposed surface of said interposer; and singulating said encapsulated semiconductor devices, thereby forming devices having an outline substantially the same as said outline of said chips.

Description:
The present invention is related in general to the field of semiconductor devices and processes, and more specifically to encapsulation methods for integrated circuit chips resulting in thin and substantially flat packages having substantially the same outline as the circuit chip. 
     BACKGROUND OF THE INVENTION 
     During the last few years, a major trend in the semiconductor industry has been the efforts to shrink semiconductor packages so that the package outline consumes less area and less height when it is mounted onto customer circuit boards, and to reach these goals with minimum cost (both material and manufacturing cost). One of the most successful approaches has been the development of so-called “chip-scale packages”. These packages have an outline adding less than 20% to the chip area. A chip-scale package which has only the outline of the chip itself, is often referred to as “chip-size package”. While a number of designs have been successfully demonstrated to achieve substantially flat packages having substantially the same outline as the circuit chip, the reduction in device thickness has been quite limited and essentially elusive. 
     One of the earliest successful approaches to chip-scale packaging includes the use of sheet-like polymer interposers between and/or around elements of the semiconductor packages in order to reduce and/or redistribute the strain and stress on the connections between the semiconductor chip and the supporting circuitized substrate during operation of the chip. A description can be found for instance in U.S. Pat. No. 5,148,266, Sep. 15, 1992 (Khandros et al.) entitled “Semiconductor Chip Assemblies having Interposer and Flexible Lead”. It introduces a spacer layer (made of compliant or elastomeric material) between a top surface of a sheet-like substance and a contact bearing surface of a semiconductor chip, wherein the substrate has conductive leads thereon, the leads being electrically connected to terminals on a first end and connected to respective chip contacts to a second end. 
     In U.S. Pat. No. 5,776,796, Jul. 7, 1998 (Distefano et al.) entitled “Method of Encapsulating a Semiconductor Package”, the encapsulation method introduces curable liquids such as electronic grade silicone-based-or epoxy-based resins by using needle-like dispensers, moving around the periphery of the chip, until the desired level of encapsulant has been substantially uniformly dispensed. Dependent on the material, the encapsulant is then cured by radiant energy, thermal energy, moisture, or ultraviolet light. Typically, this encapsulation method is performed simultaneously on a plurality of chips which are sharing a common frame. Finally, a dicing saw, water jet, ultrasonic knife, rotary razor, or laser separates the encapsulated chip assembly structure from the frame so that the resultant chip package is no, or only slightly, wider than the periphery of the chip itself. With some effort, the encapsulate will not flow onto the back surface of the chip; it can thus subsequently be connected to a heat sink without an insulative material impeding the dissipation of heat from the chip. 
     Within the semiconductor memory product families, one of the most promising concepts for chip-scale packages is the so-called “board-on-chip” design. Recently, patent application entitled “Chip-size Integrated Circuit Package” has been filed by Texas Instruments in Singapore on Jul. 2, 1997 under Ser. No. 9702348-5, and in the USA on Dec. 19, 1997 under Ser. No. 08/994,627. An approach to reduce the package height and to reach a low device profile with the board-on-hip design has been described in the patent applications entitled “Thin Chip-size Integrated Circuit Package and Method of Fabrication” filed in Singapore on Jan. 2, 1998 under Ser. Nos. 9800005-2 and 9800006-0. These devices use wire ball bonding for assembly; because of the extreme sensitivity of the thin wires against mechanical disturbances, careful protection by reliable encapsulation is needed; liquid potting material is dispensed by needle-like syringes and later cured. If solder balls are used to assemble the devices on circuit boards, this encapsulation has to withstand the mechanical stress caused by the difference in thermal expansion coefficients of the material involved. 
     Since all these approaches to chip-scale package the process of encapsulating the device comprises distributing semi-viscous material from the openings of syringes onto preselected regions of the device surfaces to be covered, and then distributing the material over the whole area and into openings to be filled with the help of capillary forces, this technology obviously suffers from-several shortcomings. Foremost, the existing technology is not economical. In order to keep the number of dispensers in practical limits, only a modest number of packages can be encapsulated in one fabrication step; the process does not lend itself to mass production. Secondly, the process is hard to control uniformly, and prone to statistical variations such as uneven fillings, pronounced meniscus formation, or flaws such as voids. It has been the experience over several years of production that a high percentage of the devices exhibit cosmetic flaws. Further, the choice of materials is limited to liquid materials which typically require prolonged “curing” times for polymerization and hardening, causing high mechanical stress in the product. In addition, the liquids tend to splatter onto exposed semiconductor surfaces, thus causing particulate contamination inhibiting the application of flat heat sinks. 
     Understandably, efforts have been expanded to apply the conventional transfer molding technology to produce thin semiconductor products. The transfer molding technology was introduced to semiconductor devices (U.S. Pat. Nos. 3,716,764 and 4,043,027) as an encapsulation process which is both gentle and reliable, and exceedingly well applicable to mass production. Over the years, transfer molding has been applied to almost all semiconductor device types, but it has proved extraordinarily difficult to produce devices thinner than about 0.8 mm total thickness. The main difficulty has been the adhesion of the molding material to the cavity walls of the steel molds, which proved to become dominant over the adhesion of the molding material to the device parts when the molded layers shrink below about 0.2 mm thickness (dependent on the chemistry of the material). The thin molded layers also tended to break after polymerization. 
     A partial solution arrived with U.S. Pat. No. 5,098,626 of Mar. 24, 1992 (Pas, “Method for Packing a Measured Quantity of Thermosetting Resin and Operating a Mold for Encapsulating a Component”) and U.S. Pat. No. 5,431,854 of Jul. 11, 1995 (Pas, “Method for Pressing a Plastic, which Cures by means of a Reaction, into a Mold Cavity, a Pressing Auxiliary in Pill Form to be Used in this Method, and a Holder Composed of such Material”). The emphasis is placed on cleanliness of the molding material by prepacking and sealing it in plastic forms which are only ruptured at time of usage, and on preventing the deleterious adhesion to the mold cavity walls of the molding material by covering thin continuous plastic films over the mold walls. For this purpose, the pulling forces from vacuum “dispensed” from numerous openings pressure the flexible films against the walls, thus keeping the molding material away from the walls. However, until now the so-called “3-P” technology is intended for encapsulating only standard-size semiconductor devices and does not offer the unique processes, mold designs or molding materials needed for encapsulating devices with at least one minimized geometry. 
     An urgent need has therefore arisen for a coherent, low-cost method of encapsulating thin semiconductor chip-scale packages. The method should be flexible enough to be applied for different semiconductor product families and a wide spectrum of design and process variations, should eliminate all cosmetic and substantial flaws in those products, and should allow the usage of various formulations of encapsulating materials. Preferably, these innovations should be accomplished while shortening production cycle time and increasing throughput. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of encapsulating a semiconductor device; it especially relates to high density integrated circuits in packages which have an outline similar to the integrated circuit chip itself, and a low profile. These circuits can be found in many device families such as processors, digital and analog devices, memory and logic devices, high frequency and high power devices, especially in large chip area categories. The invention helps to alleviate the space constraint of continually shrinking applications such as cellular communications, pagers, hard disk drives, laptop computers and medical instrumentation. 
     In accordance with the present invention, a molding method and apparatus are provided in conjunction with specially developed encapsulation materials of low viscosity and high adhesion The transfer molding process based on the invention results in ultra-thin devices free of any voids and having unusually flat surfaces and high luster. The combination of system and special materials for the transfer molding process further allows an order of magnitude higher production throughput compared to conventional potting encapsulation. The method comprises the option of retaining one chip surface free from any contamination, or to overmold it with a protective layer of predetermined thickness. 
     It is an object of the present invention to be applicable to a variety of different semiconductor chip-scale package (CSP) designs, for example: Active chip surface assembled on connection-carrying interposer, passive chip surface assembled on connection-carrying interposer, board-on-chip assembly, peripheral bonding, center-line bonding, area bonding, wire bonding and flip-chip. 
     For the miniaturized assembly feature sizes to be encapsulated free of voids, the invention replaces the conventional potting method by a transfer molding technique. Low viscosity of the encapsulation material is required. In order to achieve the desired low profile on the order of 0.5 mm, strong adhesion of the encapsulation material to all differentiated materials of the assembly is required. The invention solves these conflicting needs by developing new encapsulation material formulations and processing these materials in a modified “3-P” molding technique with its film-protected mold surfaces. 
     As side benefits of using the “3-P” technology, the pre-assembled devices are protected against contamination and chip cracking, and the mold system avoids down time and cost due to customary routine cleaning requirements. 
     It is an object of the present invention to provide a low-cost method and system for packaging chip-scale devices in thin overall profile. 
     Another object of the present invention is to provide a significantly higher production throughput. 
     Another object of the invention is to improve product quality by eliminating cosmetic flaws and obtaining precise and reproducible geometries for the finished product, and to provide reliability assurance through in-process control at no extra cost. 
     Another object of the invention is to introduce assembly concepts for thin profiles and reliability which are flexible so that they can be applied to many families of semiconductor products, and are general so that they can be applied to several future generations of products. 
     Another object of the invention is to minimize the cost of capital investment and the movement of parts and product in the equipment. 
     These objects have been achieved by the teachings of the invention concerning systems and methods suitable for mass production. Various modifications have been employed for the encapsulation of assembled semiconductor chips as well as connective to interposers and substrates. 
     In one embodiment of the invention, the encapsulation material is pressured into the mold cavity when the gate is located above the plane defined by the interposer (so-called “top gate” arrangement). 
     In another embodiment of the invention, the encapsulation material is pressured into the mold cavity when the gate is located below the plane defined by the interposer (so-called “bottom gate” arrangement). 
     In another embodiment of the invention, the ranges and combinations of viscosity, glass transition temperature, and filler size of the encapsulation material are modified and varied in order to optimize the flow characteristics and adhesion of the material as a function of the molding system parameters. 
     In another embodiment of the invention, the apparatus for the transfer molding in the fabrication of a semiconductor device comprises pairs of runners with a plurality of dual gates supplying encapsulation material simultaneously and uniformly for forming ultra-thin chip-scale packages. 
     In another embodiment of the invention, the molding process parameters (temperature, time, pressure, transfer, curing, etc.) are modified in order to optimize them relative to the system and process parameters. 
     In yet another aspect of the invention, benefits are derived from the fast-speed singulation methods (such as sawing) of the encapsulated devices, and the lower costs of the molding compounds and methods (as compared to potting materials and methods). 
     The technical advances represented by the invention, as well as the objects 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 simplified cross section through the cavity of a mold, lined with plastic films on top and bottom mold halves, with integrated circuit chips pre-assembled on an interposer, for fabricating thin packages having an exposed chip surface according to the invention. 
     FIG. 2 is a simplified cross section through the cavity of a mold, lined with plastic films on top and bottom mold halves, for fabricating thin packages having the chip completely encapsulated. 
     FIG. 3 is a simplified top view of runners and the cavity of a mold loaded with an interposer and a plurality of pre-assembled integrated circuit chips. 
     FIGS. 4 and 5 are simplified schematic cross sections of semiconductor integrated circuit chips assembled on interposers and encapsulated according to one embodiment of the present invention. 
     FIGS. 6 and 7 are simplified schematic cross sections of semiconductor chips assembled on interposers and encapsulated according to yet another embodiment of the present invention. 
     FIG. 8 is a simplified schematic cross section of an integrated circuit chip assembled on an interposer with an opening and encapsulated according to another embodiment of the present invention. 
     FIG. 9 is a simplified top view of runners and the cavity of a mold loaded with an interposer and a plurality of pre-assembled integrated circuit chips, the runners being arranged in pairs comprising a plurality of dual gates according to the invention. 
     FIG. 10 is a simplified top view of runners and the cavity of a mold loaded with an interposer and a plurality of pre-assembled integrated circuit chips, the gates feeding through slots in the interposer to spaces forming a frame around each chip, to be filled with encapsulation material according to the invention. 
     FIG. 11 depicts another embodiment of the invention; a simplified cross section through the cavity of a mold, lined with plastic films on top and bottom mold halves, and the top half having castellations separating individual devices, as needed for fabricating specific thin semiconductor chip-scale packages with a molded frame around each chip. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention adopts essential features of the socalled “3-P” molding technology and modifies them for preferred embodiments as illustrated in FIGS. 1 and 2. The term “3-P” refers to the concept of using material as “pre-packaged product”. The encapsulation material is carefully cleaned from impurities and gaseous contents, and thereafter wrapped in thin plastic films. With this protection from the environment, the material is shipped to the customer, who can use it in clean condition. The wrapped product may be fabricated in various geometrical forms, for instance in elongated, narrow shapes reminiscent of pencils. Such “pencils” are preferred shapes of encapsulation due to their ease of operation in transfer molding. 
     The cross section of a mold is generally designated  100  in FIG. 1, and a similar cross section is designated  200  in FIG. 2 (and  300  in FIG.  11 ). The encapsulation material  11 , wrapped in plastic film  12 , is placed in a groove; as mentioned, a preferred shape of such groove is reminiscent of “pencils”. From the bottom of the groove, force F presses moveable piston  13  into the groove until the wrapping ruptures and the material is squeezed into runners  14 . Finally, the material is pressed through gates  15  into the cavities of the mold. The flow rate is controlled by the force F, the lengths and cross sections of the runners, the cross section of the gates, the temperature of the transfer operation, and the viscous and flow characteristics of the material. In “3-P” technology, groove  12 , runners  14  and gates  15  have thin plastic films on their inner walls, protecting the material against influences from the environment, and preventing any contact between the encapsulation material and the walls the apparatus. This feature of no-contact between material and walls allows the use of lower cost materials (such as ceramics) than the traditional special steel for fabricating the mold equipment. 
     In FIG. 1, the mold  100  comprises the top half  17  and the bottom half  18 ; together they form cavity  16 . The inner surfaces of cavity  16  are completely lined with thin plastic films  19 , preventing any contact between the walls of cavity  16  and the encapsulation material. Capitalizing on this pivotal feature, the present invention allows the application of extra high adhesion materials without running a risk that the material may stick to the mold walls. Consequently, the high adhesion capability can be fully utilized for insuring strong adhesion to the widely different materials found in semiconductor devices (parts made of semiconductor, metals, insulators, inorganic or organic materials, etc.), and thus guaranteeing high mechanical stress tolerance of the finished device—an essential prerequisite for very low profile components. 
     The encapsulation compounds especially suitable and developed for high adhesion comprise epoxy-based thermoset resins and silicone-based elastomerics. They are commercially available from Shin Etsu Chemical Corporation, Japan, or Kuala Lumpur, Malaysia, or Sumitomo Bakelite Corporation, Japan, or Singapore, Singapore. In addition, thermoplastics as well as liquid-crystal polymers are suitable. 
     In addition to high adhesion, these encapsulation materials exhibit very low viscosity at molding temperatures (while solid at room temperature). With this feature, the minute spaces found in thin semiconductor chip-scale packages (for instance around the electrical connections, see the device examples in FIGS.  4  through  8 ), can be reliably filled with encapsulation material. Voids or other cosmetic defects, which plague thin devices made by conventional technology, are completely eliminated. 
     Typical compound formulations comprise viscosities in the range from 30 to 200 poise, glass transition temperatures from 100 to 230° C., and filler sizes from 1 to 75 μm with a median from 15 to 25 μm (fillers are needed to lower the coefficient of thermal expansion and increase the strength and flexibility of the molding material). The molding process parameters are recommended in the following ranges: Molding temperature from 140 to 220° C. (preferred temperature 175° C.), cure times from 70 to 120 s, clamping pressure from 18 to 33 tons, transfer from 5 to 18 s, transfer pressure: initial pressure from 50 to 80 kg/cm 2 , final pressure from 15 to 30 kg/cm 2 . 
     With these materials and process parameters, molded profiles between 0.15 and 0.3 mm are achievable. 
     As defined herein, the term “profile” refers to the thickness or height of the part under discussion (here: the molded layer), and may be measured in, for example, millimeters (mm). 
     Another distinguishing feature of the present invention, which is based on the usage of the thin plastic films covering the mold cavity walls, is the high luster of all molded surfaces. This high luster appears on smooth as well as matte surfaces, and originates from the rounded contours of all surface features, which is caused by the rounding effect of the plastic film. Similar high luster could in the past only be achieved by careful polishing of all mold surfaces—a very expensive method. In addition, the high luster causes colors to appear more intensive; for instance, the conventional black molding compound color now appears significantly blacker. 
     In the cross section through the mold cavity depicted in FIG. 1, the plastic film  19  lining top mold half  17  is shown to rest tightly on the surfaces of the devices-to-be-encapsulated, forming interfaces  400 . With this arrangement, those surfaces are not receiving any encapsulation material; furthermore, they are protected against any contamination or any spill-over, flash, or spurious material which could accidentally be deposited on those surfaces. Consequently, the device surfaces emerge clean from the encapsulation process. 
     On all sides extending from the interfaces  400 , the encapsulation material fills spaces  401 , forming completely flat interfaces  402  between the encapsulation material and the plastic film, which, after curing of the encapsulation material, extend the device surfaces as solid new surfaces of the encapsulant. These new surfaces have no meniscus or bulging, or any other surface irregularity since they have been formed along the flat, smooth boundary of the plastic film. They exhibit the high luster and intensive color described above. 
     In the cross section through the mold cavity depicted in FIG. 2, the thin plastic film  19  is also shown to surround mold cavity  20 , which is formed by top mold half  21  and bottom mold half  22 . In contrast to FIG. 1, however, the film lining top mold half  21  does not rest tightly on the devices-to-be-encapsulated, but rather provides space  23  on top of the devices-to-be-encapsulated. After the molding process, these spaces  23  are filled with encapsulation material, together with spaces  403  between the devices. With this arrangement, the devices are covered by an extensive layer of encapsulant, forming interface  404  to the plastic film, which, after curing, shows a flat, high-luster surface. Together with the surface of the adjacent filled spaces  23 , a continuous flat, high-luster surface has been acquired. 
     In the top view of the mold cavity shown in FIG. 3, a cavity  30  is depicted together with groove  31 , runners  32  and gates  33 . The encapsulation material in its pencil-shaped wrapping is positioned in groove  31 . The number, position and cross sectional shape of the runners and the gates depend on the design of the mold and the results of mold flow modeling which aim at creating a common and uniform front of the encapsulation material progressing into the cavity  30  during the molding process (and which try to avoid formation ofjets of encapsulation material emerging from the gates). The embodiment shown in FIG. 3 is only one of numerous solutions; another one are so-called “fan gates”, each of which cover about 80 to 100% of the widths of the packages to be produced, and which are enabled by the “3-P” molding technology. 
     In FIG. 3, an electrically insulating interposer  34  is loaded into cavity  30 , together with a plurality of integrated circuit chips  35  (examples of such semiconductor devices are detailed in FIGS.  4  through  8 ). The arrangement depicted allows the processing of numerous ICs in one process step, thus achieving a high production throughput. The throughput according to the present invention is between 8 to 15 times higher than the throughput by the conventional potting method (using needles and syringes). Further, the spaces between the devices are completely filled with encapsulation material. After this so-called array molding is completed and the encapsulation maternal cured, the separation of the devices is preferably performed by sawing because of the high-speed singulation. In this fashion, the devices obtain precise outlines and dimensions. 
     The mold shown in FIG. 3 indicates a margin  36  of the interposer which is clamped between the films of bottom and top halves of the mold such that no encapsulation material can be pressed out. The avoidance of any “flash” is another advantage of the “3-P” technology compared to standard mold design. Another embodiment of the present invention is described in FIG.  9 . 
     As defined herein, the term “outline” relates to the overall width and length of the integrated circuit package of the present invention. The outline of the package is also referred to as the footprint of the package, because it defines the surface area on a wiring or assembly board that the package will occupy. Chip-scale packages have an outline adding less than 20% to the chip area. A chip-scale package which has only the outline of the chip itself, is often referred to as “chip-size package”. The terms “substantially similar” and “substantially the same” refer to the relative outlines of certain parts of the package (such as the spacer layer, or the interposer) and the IC chip when they are within 10% of one another. In several devices, the difference in the outlines is about 2%, or may vanish altogether. 
     FIGS. 4 through 8 show schematic cross sections of different device types which can be fabricated as thin chip-scale packages using the present invention. In the device of FIG. 4, IC chip  40  has an outline given by the sawing of the semiconductor material, an active surface  41  and a passive surface  42 . The active surface  41  comprises the integrated circuit and contact pads  43  around the periphery of the IC. For silicon ICs, the thickness (distance between active and passive surface) is between 200 and 375 μm, typically 275 μm. Other semiconductor materials comprise silicon germanium and gallium arsenide. The contact pads are aluminum, often doped with 2% copper and 1% silicon, or copper, or gold. A compliant spacer layer  44 , typically a sheet-like elastomer, separates the major portion of the active circuit surface  41  from the interposer  45   a . Spacer layer  44  has a substantially similar outline as IC chip  40 , thus charactcrizing the device as a chip-scale device. This interposer is an electrically insulating tape, for instance a flexible polyimide layer. On its surface facing the IC, it comprises a plurality of metallic leads (made, for example, of gold) which are glued to the spacer layer  44  and on their free ends contact the circuit pads  43 . On the opposite surface, the interposer  45   a  has an array of solder balls  47  which are attached after the device has been singulated. Electrically conductive paths extending through interposer  45   a  connect leads  46  to the solder balls  47  on an individual basis. 
     It should be mentioned that the term solder “balls” used herein does not imply that the solder contacts are necessarily spherical; they may have various forms, such as semispherical, half-dome, truncated cone, or generally bump. The exact shape is a function of the deposition technique (such as evaporation, plating, or prefabricated units) and reflow technique (such as infrared or radiant heat), and the material composition. Several methods are available to achieve consistency of geometrical shape by controlling amount of material and uniformity of reflow temperature. 
     When devices of the type shown in FIG. 4 are molded using the mold of FIG.  1  and the method of the present invention, the encapsulation material  48  is filling spaces around the periphery of chip  40  and surrounding bond pads  43  and leads  46 . Very noticeably, the encapsulation material  48  is forming surfaces  49   a  coplanar with passive chip surface  42  as a flat extension of it, and also surfaces  49   b  coplanar with interposer  45   a  as a flat extension of its surface opposite the IC. These surfaces, newly formed by the molding process of the invention, exhibit high luster and intensive color, and are free of cosmetic defects. 
     Electrically insulating interposer  45   a  comprises a first and a second surface and a plurality of electrically conductive paths extending through the interposer from the first to the second surface, forming electrical entry and exit ports on the interposer. The entry ports are attached to the leads  46 . After completing the molding process, an array of solder balls  47  is attached to the exit ports. 
     Due to the uniformity of the molded portions created by the molding process of the invention, sawing is the preferred method of device singulation. Using this technique, peripheral surfaces  49   c  are generated. In FIG. 4, the peripheral surface  49   c  incorporates a frame  45   b  of interposer material in order to define the edge of the package; this design option as well as others are within the scope of the present invention. The singulated device of FIG. 4 will thus comprise precise package outlines and dimensions. 
     The device shown in FIG. 4 is a thin chip-scale package. After attachment of the solder balls, the overall profile is between 0.4 to 1.7 mm. Using the advantages of the encapsulation method of the present invention, a typical device profile is 0.7 mm. 
     In another embodiment, the present invention delivers similar advantages for chip-scale packages of the type schematically-utlined in FIG.  5 . Here, chip  50  comprises an active surface area  51 , including a plurality of bond pads  53 ; an array of solder balls  54  replaces the compliant spacer  44  layer in FIG.  4  and also performs the function of the leads  46  in FIG.  4 . The array of solder balls  54  may be distributed uniformly over the whole active surface area  51 . Interposer  55   a  has a plurality of solder balls  56  attached for connecting the device to an outside board. After the molding method of the invention, molded material  57  is added filling spaces around the periphery of chip  50  and spaces between solder balls  54 . The encapsulation material  57  is forming surfaces  58   a  coplanar with the passive chip surface  52  as a flat extension of it, and also surfaces  58   b  coplanar to interposer  55   a  as a flat extension of its surface opposite to the IC. These surfaces, newly formed by the molding process of the invention, exhibit high luster, intensive color, and are free of cosmetic defects. 
     Using sawing as the preferred singulation method, peripheral surfaces  58   c  are generated. As in the example of FIG. 4, the device of FIG. 5 incorporates a frame  55   b  of interposer material in order to define the edge of the package. The singulated device of FIG. 5 will thus comprise precise package outlines and dimensions. 
     The device shown in FIG. 5 is a thin chip-scale package. After attachment of the solder balls, the overall profile is between 0.4 and 1.7 mm. Using the advantages of the encapsulation method of the present invention, a typical device profile is 0.7 mm. 
     Using the embodiment of the invention depicted in FIG. 2, devices as shown in FIGS. 6 and 7 can be manufactured. IC chip  60  has an outline given by the sawing of the semiconductor material, an active surface  61  and a passive surface  62 . The active surface  61  comprises the integrated circuit, including contact pads  63   a  around the periphery of the IC. The passive surface  62  is glued by tacky film  64  to one surface of an electrically insulating interposer  65 . In FIG. 6, this interposer is a sheet-like dielectric material, such as flexible polyimide (thickness about 0.1 to 0.2 mm). In FIG. 7, the interposer is made of a board-like dielectric material  70 , such as FR-4 (thickness about 0.2 to 0.4 mm). The surface of the interposer onto which the chip is glued, also comprises around its perimeter a plurality of electrical entry ports. Electrically conductive paths extend through the interposer from the entry ports on one surface to the opposite surface, where they form exit ports. Whisker wires  63   b  connect the IC contact pads  63   a  to the interposer entry ports; an array of solder balls  66  is attached to the interposer exit ports. 
     Whisker wires  63   b  are commonly gold, gold-beryllium alloy, or copper, between 15 to 32 μm thick (typically 20 μm). They form balls (typically 30 to 50 μm diameter) attached to the circuit contact pads  63   a , and stitch bonds to the interposer entry ports. Known technology allows a pitch of gold balls between 70 and 100 μm. Moving the bonder capillary containing the gold wire on a predetermined and computer-controlled manner through the air will create a wire looping of exactly defined shape; for instance, recent technical advances allow the formation of rounded, trapezoidal, linear or looped paths of different looping heights. With this capability, wire loops at a safe distance from the edge of the chip yet close to its surface can be manufactured, thus creating devices with thin profile. 
     The low viscosity materials employed by the present invention allow an encapsulation of the device without disturbing the whisker wires (so-called “wire sweep”) looped close to the chip surface. Furthermore, the molded material  67  can be kept thin, yet with a perfectly flat, high-luster surface  68   a . After curing of the encapsulation material and removing the encapsulated devices including interposer  65 , or  70  respectively, from the mold, an array of solder balls  66  is attached to the exit ports on the exposed surface of the interposer. Finally, the encapsulated devices are singulated, commonly by sawing. The sawing operation creates straight, sharp-commered edges  68   b . The device outline thus formed is substantially the same as the outline of the chips, producing chip-scale packages with thin profile. 
     FIG. 8 is a simplified schematic cross section of an integrated circuit chip  80  assembled on an interposer  81  with an opening  82  in its center, which is needed for so-called center-line bonding. ICs of this structure are especially common in dynamic random-access memories (DRAMs), synchronous DRAMS (SDRAMs), static random-access memories (SRAMs), erasable programmable read-only memories (EPROMs), but also in logic circuits (LOGIC), microprocessor, digital signal processor, and many other IC components. In center-line bonding, the balls  83  of the bonding wires are aligned approximately along the center-line of the chip, while the stitches  84  are affixed to pads on the interposer. 
     An electrically insulating interposer, or board,  81  with a first surface  81   a  and a second surface  81   b  has its second surface  81   b  attached to the active surface  80   a  of chip  80  by means of an adhesive layer  85 . Chip  80  has typically a thickness between 0.2 to 0.5 mm, preferably 0.28 mm. The passive surface  80   b  of chip  80  is facing the environment and is thus available for dissipating thermal energy and cooling of the heat generated by the chip in operation. The interposer  81  has a plurality of electrically conductive routing strips  86   a  and  86   b  and a plurality of contact pads  87  deposited on its first surface  81   a . Routing strips  86   a  and  86   b  and contact pads  87  comprise copper layers, or copper with gold-flash surface, or refractory metal layers followed by copper layers with gold flash. Alternatively, they may comprise copper layers followed by nickel layers. 
     Interposer  81  is made of electrically insulating material such as FR-4, which is an epoxy resin, or a cyanate ester resin, reinforced with a woven glass cloth. Alternatively, a polyimide-based interposer may be used. Interposer  81  has a thickness between 0.1 and 0.7 mm; a preferred thickness is 0.2 mm. It is designed to have substantially the same outline as IC chip  80 . The adhesive layer  85  may be made of, for example, a double-sided adhesive polyimide tape, adhesive glue, epoxy, or thermoplastic adhesive. 
     As shown in FIG. 8, one group of contact pads  86   a  is positioned immediately close to (and approximately parallel to) the edges of opening  82 . These contact pads  86   a  receive the stitch ends  84  of whisker wires  88 ; the surface of pads  86   a  are compatible with easy stitch bonding and comprise a gold flash surface. Lateral dimensions of stitch imprints are about three times the wire diameter; the exact shape of the stitches depend on the shape of the capillary used, such as capillary wall thickness and capillary footprint. Wires  88  connect to the chip bonding pads (and form gold balls  83 ). Attached to contact pads  87  are solder balls  89  which are needed for the final package attachment to the customer cation boards. The surface of contact  87  comprises gold, palladium, or platinum for easy attachment of solder balls. 
     Is it advantageous for achieving minimum device profile to position the bus bars  86   b  (supplying power and ground potential) parallel to and in immediate proximity to the edges of chip  80 . With the bus bars positioned far away from any wire crossing, any electrical insulation against wire shorting is no issue, and bonding wires  88  may be affixed with minimum looping by moving the bonding capillary the shortest feasible distance over the surface of interposer  81 . As a consequence, the looping of wires  88  can cross the interposer  81  at a height of approximately 10 μm and approach the pad surface to at a low angle (at the stitch, there is no minimum height for a ball needed, nor is there a feat-affected zone like near the ball which would be mechanically weak for bending or other deformation stresses). 
     After wire bonding, the opening  82  is filled according to the teachings of the present invention with encapsulation material  90  up to a height of about 20 to 50 μm on top of surface 81 a , forming surface  90   a . The encapsulation material will be contained by polymer stopping walls  91  which are 20 to 50 μm high and have been pre-attached to interposer  81 . The thickness, or profile, of the filled opening is between 0.15 and 0.3 mm; the preferred profile of the filled opening is 0.2 mm. The surface 90 a  of the encapsulation material is coplanar with stopping walls  91 , completely flat, and exhibits a high luster. 
     The diameter, or thickness of the solder balls  89  must exceed the thickness of the molding material  90  extending over surface  81   a . Since the minimum wire looping permits thin molding material thickness, the solder balls need only the relatively small diameter of typically 0.1 to 0.3 mm; preferred solder ball diameter is 0.2 mm. Consequently, the thickness, or profile, of the overall chip-scale package is 0.3 to 1.2 mm; the preferred profile is about 0.5 mm. 
     In variation to FIG. 3, FIG. 9 shows another embodiment of the invention for fabricating ultra-thin packages, namely the preferred runner design for so-called “dual gating” in a mold used for encapsulating device types which exhibit openings like the example shown in FIG.  8 . For optimizing the dimensional design of the runners, computerized process flow modeling is employed based on the encapsulation material selected. Depicted in FIG. 9 is one mold half  92  (may be bottom or top) with its cavity for holding the electrically insulating interposer  93  with a plurality of pre-assembled IC chips  94 . According to the 3-P technology, thin plastic films cover the inner walls of the mold cavity. A plurality of runners  95  for feeding encapsulation material to the cavities is laid out so that the runners form pairs  96  of runners parallel to each other. The runners comprise gates  97  opposite to each other, forming dual gates. These gates open to the devices to be filled, for instance to both ends of the center opening of devices described in FIG.  8 . Each pair of runners is configured so that the encapsulation material pressed through the runners will exit from adjacent gates concurrently. Further, each pair of dual gates is structured such that they fill the cavity between them uniformly with encapsulation material, thus enabling to fill the narrow openings of thin devices without disturbing sensitive whisker wires. 
     Another embodiment of the present invention is illustrated in FIGS. 10 and 11, which are a variation of the high throughput mold depicted in FIG. 9, for an alternative way to encapsulate devices as shown in FIG.  1 . In the schematic top view, the cavity  101  is depicted together with groove  102 , runners  103  and gates  104 . The shape and dimensions of the runners and gates are determined by computerized mold flow modeling for synchronized and equalized filling of the cavities. The molding compound in its pre-packaged, pencil-shaped wrapping is positioned in groove  102 . The molding compound is pressured through runners  103  to the plurality of gates  104 , which open into slots of the interposer  105  so that a molded frame  106  around each device can be fabricated. In FIG. 1, the location of the interposer slots is indicated by designation  45   b  (marking the newly molded surfaces), and the molded fame by designation  48 . 
     The mold half depicted in FIG. 10 is preferably the bottom half of the mold, because it is easy load it with interposer  105  and the pre-assembled chips. Then the top half has to be designed with castellations as indicated in FIG.  11 . The schematic cross section of the mold is generally designated as  300 . The wrapped molding material  111  is located in groove  112 . The top mold half  113  (made of steel, ceramic or other high temperature tolerant material) exhibits castellations  114 , separating the individual devices to be encapsulated. The inner surfaces of the top mold-half  113  are completely lined with thin plastic film  115  (similar to the bottom mold half) according to the 3-P technology. 
     In FIG. 11, the devices are indicated by silicon chips  116  attached to spacer layers  117 , glued onto interposer  118 . This interposer  118  exhibits slots  119  through which the molding compound is pressured from the bottom up. The molding compound will then form a molded frame around the device including the flat surface coplanar with the passive surface of the chip, as described in conjunction with FIG. 4 (designating numbers  48 ,  49   a ,  49   b , and  49   c ). 
     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. It is therefore intended that the appended claims encompass any such modifications or embodiments.