Abstract:
A grid array assembly is provided employing a thin copper or steel carrier frame having apertures extending longitudinally of the frame. A series of semi-flexible substrate printed circuit boards are mounted in seriatim to peripheral edges of the apertures, the circuit boards including bonding pads and metallization on a first surface and conductive vias in the circuit boards extending to a second opposite surface containing a contact pad array. The carrier strip with the mounted circuit boards are passed to a station where an IC die is mounted on the board first surface, wire bonds are placed from the die to the bonding pads and the assembly encapsulated using a portion of the carrier strip as a mold gate to form a package body. Subsequently each grid array assembly is singulated from the carrier strip.

Description:
RELATED APPLICATION 
     This application relates to commonly assigned U.S. patent application Ser. No. 08/214,339 filed Mar. 16, 1994 in the name of Bruce J. Freyman et al., now U.S. Pat. No. 5,635,671, issued on Jun. 3, 1997, and the commonly assigned and concurrently filed U.S. patent application Ser. No. 08/637,578 filed Apr. 24, 1996 in the name of Bruce J. Freyman and Robert F. Darveaux, now. U.S. Pat. No. 5,859,475, issued on Jan. 12, 1999, the disclosure of which concurrently filed application is incorporated herein by reference. It is also a divisional of 08/637,877, filed Apr. 24, 1996, now U.S. Pat. No. 5,852,870, issued on Dec. 29, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to grid array semiconductor packages, including integrated circuit chips or dies, produced from an assembly of packages formed together in an elongated, connected strip using standard packaging equipment, with a maximized yield, and at a low cost. 
     2. Related Art 
     As integrated circuits have become more complex, a need has arisen for a packaged integrated circuit having a large number of high density, reliable external package connections. It is also desirable to include in a packaged integrated circuit one or more conductive layers for signal routing and provision for ground and power metallization traces. To meet these needs, the ball grid array has been developed as illustrated in FIG. 1 of the related application Ser. No. 08/214,339, now U.S. Pat. No. 5,635,671. 
     A typical ball grid array includes a semiconductor die, sometimes called herein an integrated circuit (IC) or chip, attached to a die attach surface on a printed circuit board (PCB) with adhesive. Electrically conductive bond pads on the die are connected by electrically conductive bond wires to electrically conductive traces and/or electrically conductive regions formed on or adjacent to the die attach surface of the substrate. Electrically conductive vias are formed through the substrate from the traces and/or regions on the die attach surface to a mounting surface of the substrate opposite the die attach surface. The electrically conductive traces formed on the mounting surface extend to solder pads formed on the mounting surface. Solder bumps are formed on each of the solder pads. The solder bumps are reflowed to attach the substrate to a larger mother board. An encapsulant such as plastic is formed to enclose the semiconductor die, the bond wires and a portion of the die attach surface of the substrate including most of the traces and/or regions. The vias are shown outside the encapsulant but can be inside the encapsulant. The encapsulant is frequently formed by injection or transfer molding or by conventional molding equipment to form the encapsulant over the die in a ball grid array. Liquid (glob top) encapsulant also can be used. 
     Recent related art is seen in the illustration of a wire bond TBGA (Tape Ball Grid Array) 1-Metal Microflex Circuit published by the Electronic Product Division of 3M where an IC chip is adhered to a stiffener/heatsink with a polyimide tape first surface containing metallization, wire bonded to the chip and providing conductive vias, outside of a central encapsulant over the chip and bond wires, to solder balls on an opposite surface of the tape. 
     Currently, molded plastic BGAs are assembled using a printed circuit board (PCB) strip containing multiple BGA (ball grid array) circuits that is rectangular in shape. A single unit BGA image is successively replicated on the PCB strip in order to maximize productivity during assembly. In this way, several BGAs are processed at the same time in many of the assembly processes. The PCB strip format also helps to reduce material handling expense in other areas of the assembly process that operate on only one site at a time. 
     One of the problems with procuring PCBs in strip form that have successively replicated BGA images is that the PCB vendor needs to provide 100% good units on the PCB strip so that the assembly facility does not process “bad” units. If the vendor or in-house manufacturing operation fabricates BGAs in strip form and includes a bad BGA image, all of the remaining good BGA images on the strip have to be thrown away. If the BGA assembly facility accepts bad BGA images on each strip, then the throughput of certain assembly processes is reduced because “bad” units are being processed. Also, the cost of the PCBs increases if the vendor cannot ship PCB strips that contain bad units. 
     An obvious method of using only “good” units in the BGA assembly process is to have the PCB vendor supply all good PCBs in single unit format instead of in a strip format. The problem with processing singles in the BGA assembly process is that almost all of the off the shelf assembly equipment used by industry today is configured to run PCBs in the strip format. The other drawback of processing single unit PCBs is that the resultant throughput of many of the assembly processes is reduced as compared to throughput in PCB strip processing. 
     In the manufacture of related art devices by molding operations, it is well recognized that during encapsulation and subsequent removal of a molding die section, the package encapsulant not only encloses the die and die bonds but also extends along the surface of the die-mounting substrate where a mold runner is located leading to a molten encapsulant supply pot. The excess encapsulant sometimes called “flash” or “bleed”, i.e., encapsulant other than that necessary to enclose the die and die bonds, must then be removed. However, when the excess encapsulant is peeled away from the substrate surface, the encapsulant adheres to the substrate surface, twisting the substrate and tearing or rupturing the substrate surface, thereby causing damage to the packaged device. This damage can be cosmetic (e.g. marring of the substrate surface) and/or functional (e.g. fracturing of the substrate; destruction of the electrically conductive traces on the substrate surface; tearing away of the solder mask on the substrate surface to undesirably expose, for instance, copper, and/or weakening or breaking of the seal between the encapsulant and the substrate surface). 
     Further, in production, it is desirable to integrally form a plurality of substrates in a strip having alignment holes that are located so as to be captured by tooling pins of a fixture, allowing the packaging process (including encapsulation) to be automated. The excess encapsulant must be removed from the strip prior to further processing since, if left attached to the strip, the excess encapsulant extends past the edge of the strip prohibiting automated handling in subsequent processes. Adherence of the excess encapsulant to the substrate during removal of the excess encapsulant may cause torquing of the strip that distorts the strip and renders the strip unusable for further processing. In the first related application, a novel degating region with noble metal plating is formed at each runner or gate location such that the molding compound (encapsulant) bonds weakly with the plated runner area on the PCB substrate allowing removal of excess encapsulant without damaging the remainder of the package assembly. 
     A plurality of individual grid array semiconductor packages are singulated from an assembly of packages fabricated together in the form of an elongated, connected strip using standard packaging equipment, with a maximized yield, and at a low cost. The strip assembly comprises a series of pre-tested and pre-accepted printed circuit boards mounted seriatim in apertures extending longitudinally along an elongated carrier strip. The circuit boards may comprise a semi-flexible plastic, e.g., an epoxy, and the carrier strip may comprise a metal, e.g., copper or steel. 
     Each of the circuit boards has an outer peripheral edge connected with an adhesive tape to an inner peripheral edge of a portion of the carrier strip bounding the apertures therein. An integrated circuit die is attached to each of the circuit boards. Each die has conductive pads that are wire bonded to bonding pads on a first surface of a corresponding one of the circuit boards. An encapsulant is molded over each die, corresponding wire bonds, and onto a portion of the first surface of the corresponding circuit board to form a protective body thereon. 
     Each of the circuit boards in the strip assembly includes an array of contact pads on an opposite second surface thereof, and conductive vias that extend from the contact pads to a metallization on the respective first surface thereof. A solder ball may extend from each of the contact pads to function as an input-output terminal. 
     When processing of the strip assembly is complete, the individual grid array packages are singulated from it by cutting through at least each circuit board around a periphery of the corresponding body of encapsulant, and in some embodiments, through the carrier strip as well. In one such embodiment, the packages are singulated from the strip assembly such that a portion of the carrier strip remains on each grid array package to function as a heat sink and a stiffener. 
     A better understanding of the above and other features and advantages of the present invention may be had from a consideration of the detailed description below of some exemplary embodiments thereof, particularly if such consideration is made in conjunction with the appended drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block flow diagram of the method according to an embodiment of the invention for forming a grid array assembly. 
     FIG. 2 is a plan view of a carrier strip showing on the right side a molded grid array package therein. 
     FIG. 3 is a plan view of a carrier strip section showing an obverse side of a mounted printed circuit board prior to die mounting, wire bonding and encapsulation. 
     FIG. 4 is a plan view of a strip section showing a reverse side of two printed circuit boards prior to forming ball bonds on an array of contact pads. 
     FIG. 5 is a schematic side partial view of a molding die and cavity for encapsulating the grid array die and wire bonds. 
     FIG. 6 is a schematic side view partially in cross-section showing the start of automolding operation of encapsulating a mounted die and wire bonds. 
     FIG. 7 is a schematic side view partially in cross-section showing the encapsulated die and wire bonds. 
    
    
     DETAILED DESCRIPTION 
     The invention provides an assembly of substrate-based grid array semiconductor packages fabricated together in the form of an elongated, connected strip, from which a plurality of individual packages may be singulated. A series of conforming (the same) generally planar printed circuit boards (PCB) are manufactured with suitable conductive traces (metallization), an IC die attach area(s) and bonding pads on one surface and an array of contact pads on an opposite surface with conductive vias extending from the metallization on the one surface to the contact pads on the opposite surface. In the past as described above, PCB&#39;s have been manufactured in strip form so that four, six, eight or more identical PCB&#39;s are formed in situ in strip format. In the present invention, following manufacturing either singly or on the normal plastic strip the PCB units are tested to ensure that each PCB meets a prescribed detailed performance specification. Any PCB not meeting the specification is rejected. Those PCB&#39;s meeting the specification, herein termed “accepted” boards, are singulated, if not already in a single form, and then attached to a disposable or re-usable carrier strip generally in the form of a strong, semi-flexible web of copper or stainless or other steel or other material stock of from about 0.2 mm to about 1.0 mm in thickness. The carrier strip is compatible with all the subsequent ball grid array (BGA) manufacture and assembly steps. Each carrier strip has a series of spaced apertures over which a single PCB is attached. The invention specifically allows for standard automolding of each PCB in the carrier strip. 
     FIG. 1 is a diagram of a process  10  for forming a ball grid array semiconductor package according to one embodiment of the invention. 
     In step  11  the PCB is manufactured with prescribed metallization including patterned conductive traces of copper or the like, die attach areas, conductive bonding pads, conductive vias extending from metallization on one PCB surface to an opposite surface forming through board interconnections with an array of contact pads on that opposite surface. 
     In step  20  each PCB is tested to the prescribed performance specification and those PCB not meeting the specification become rejects  21 . Those PCB&#39;s meeting the specification are accepted boards  22 . 
     In step  30  a prescribed number of the accepted boards are attached to an incoming carrier strip  31  by automatic assembly apparatus (not shown) which attaches, by adhesive bonding, welding, riveting, soldering or the like, marginal outer peripheral edges of each PCB to edge margins of a carrier strip aperture, as described herein with respect to FIG. 2. A PCB with no marginal outer peripheral edges which is attachable to the carrier strip by adhesive bonding in the carrier strip aperture may also be used. In this embodiment, the area dimension of the accepted boards is the same as the area dimension, e.g. 27 mm×27 mm, as the singulated assembly described below. 
     In step  40  incoming semiconductor IC dies  41  are attached to each accepted PCB in a PCB die attach area using conventional materials such as epoxy or acrylic or polyimide adhesive, and by conventional processes. 
     In step  50  the mounted die  41  is wire bonded or otherwise connected to bonding pads on the PCB to form an assembly  51 . 
     In step  60 , the assembly  51  particularly the die and wire bonds are encapsulated to enclose the die and any other electronic device and at least a peripheral portion of the PCB encircling the die attach area, as explained in detail below. Briefly a mold die, having a cavity shaped generally to conform to the to-bemolded shape of the final BGA package, is positioned against peripheral edges of the carrier strip aperture and encapsulant transferred into the mold cavity and through a mold runner area on the carrier strip until the cavity is filled. 
     The encapsulant flows around die  41  and the wire bonds and adheres to interior portions of the PCB including metallization surrounding the die attach area. When the encapsulant hardens the mold die is removed or opened. Excess encapsulant (bleed) is formed on the surface of the carrier strip and poorly adheres thereto. It is easily removed in a later step by exerting a torqueing or tensile force on the mold runner attached to the strip carrier. This allows excess encapsulant to peel cleanly from the carrier and the package encapsulant without pulling any of the package encapsulant away from the PCB nor twisting the PCB so as to damage the resultant packaged electronic device, either cosmetically, mechanically or electrically. 
     In step  70  interconnection bumps, generally in a solder ball configuration, are formed. 
     In step  80  each so-formed ball grid array is singulated from the carrier strip. 
     FIG. 2 is a plan view of a portion of a carrier strip  31  showing on the left side an octagonal carrier strip aperture  32  under which a PCB is to be mounted. The preferably copper carrier strip includes tooling holes  33  and  34  which, respectively, are used to align a PCB  22  against the strip in aperture overlying position and to position the carrier strip on and with respect to the various apparati of standard manufacturing equipment e.g. the mold or wire bonder, used in the production of a packaged electronic device such as a BGA, in accord with this invention. As seen on the right side of FIG. 2 an accepted PCB  22  having in one embodiment a 27 mm by 29 mm rectangular size denoted by dashed outline lines  35  is positioned so as to underlie aperture  32  with the long ends  35   a  and  35   b  of the PCB  22  extending beyond holes  33 . The short ends overlap the side marginal edges  35   c  and  35   d  of the carrier strip aperture  32  by only about 0.5 mm to about 0.8 mm. The PCB is attached to the carrier strip at those four overlaps, or at long end overlaps  35   a  and  35   b  only, by suitable adhesive  56  (FIG.  6 ). The adhesive may be a double-sided adhesive tape suitable for the processing environment to which it will be subjected, such as Lead Lock tape from Brady Co. of Milwaukee, Wis. or comparable tape from Tomoegawa of Japan or Pyralux™ adhesive tape from DuPont Co. After encapsulation to form a molded body  61 , as in FIG. 7, a bevelled encapsulant edge  62  is formed encircling the body  61  inside a mold shut off area  63 . Typical bleed or mold flash  61   b  (FIG. 3) is shown extending on the top surface of the carrier strip  31 . Flash  61   a  (FIG. 3) may extend anywhere on this surface. Upon singulation of the BGA from the carrier strip in step  80  the finished/completed BGA package will have a 27 mm. by 27 mm. size with or without an attached portion of the carrier strip, as seen by the multi-dashed lines  64 . 
     FIG. 3 illustrates the mounting of a PCB  22  in an overlying aperture  32  of a series of apertures in the carrier strip  31 . A die attach area  36  is provided for mounting the IC die or electronic device. Metallization traces  37  including gold-plated or other electrically conductive bonding pads  38  extend over the top surface of the PCB  22  to vias through one or more board levels. The standard PCB substrate may be, for example, epoxy-glass or polyimide-glass or BT/epoxy (bismaleimide-triazine and epoxy). If multilayer PCB&#39;s are used electrically conductive traces or regions (e.g. ground planes or power planes) can be formed by conventional photoresist and masking processes, within the multilayer substrate and connected to the exterior opposite surface with conductively plated vias or through holes formed by well-known techniques including mechanical or laser drilling and followed by electroless or electro plating. Annular conductive rings may be provided at the end of the vias and connected to the adjacent areas containing the contact or solder pads on which the interconnection bumps will be formed. The strip  31  further functions as a mold gate at area  61   b  and due to the ease of removal of any flash  61   a  leaves a clean package outline when the mold halves  68  and  69  (FIG. 5) are separated. 
     FIG. 4 shows the reverse or underside of the attached accepted board  22  where the edges 27 mm. by 29 mm of the PCB are seen by lines  35 . Vias  42  interconnect metallization  37  (FIG. 3) and contact pads  39 . Interconnect balls or bumps  71  (FIG. 7) are later formed on contact pads  39 . Parallel conductive traces  43  extend from certain bump pads to facilitate electroplating. 
     FIG. 5 illustrates the automolding of a package body  61  by the action of a molding die  68  having a cavity  64  which clamps against and around the periphery of the boundary surface edges  65  of the carrier strip apertures. The inner peripheral edges  66  (FIG. 6) of the strip aperture act as a stop or dam for the encapsulant with the strip aperture edges  65  acting as a mechanical shut-off for the liquid molding compound e.g. Plaskon molding compound, being dispensed into the molding die cavity. Arrows  67  indicate clamping of the mold die against the carrier strip  31  which is backed by a fixed mold die-supporting surface  69  during the clamping and encapsulating operations. When encapsulant is supplied into the cavity (with the top side of the PCB facing downwardly as shown in FIG. 4) a mold body  61  with or without a bevelled side edge  62  is formed upon hardening of the mold compound. A mold runner  61   a  (FIG. 3) extends from a mold supply inlet (not shown) to the mold cavity. For illustration purposes a non-existent gap is seen between the die  68  and the molded body  61 . By molding against the carrier strip aperture edges poor adhesion of the molding compound results and the compound does not tenaciously stick to PCB surfaces, other than at portions within the mold body surrounding the die attach area. 
     FIG. 6 shows the portion of the mold die  68  immediately before completion of clamping against the carrier strip  31 . When clamping is completed against the carrier strip  31 , encapsulant is injected into cavity  64   a  to encapsulate die  41 , wire bonds  55 , the outer edges of the attach adhesive  53  and an annular portion  58  of the PCB  22 . Flash is prevented or minimized by the clamping force of the molding die against the carrier strip. 
     FIG. 7 illustrates the final packaged assembly resultant from step  70  namely, the forming of interconnecting bumps, e.g. solder balls  71 , on the contact pads  39  in electrical connection to vias  42  and to the metallization  37  on and the die  41  mounted to the PCB. V-cut edge  25  which aids in the singulation step  80  may be included around the edge periphery of PCB  22 . The substrate is not adhesively bonded to the carrier strip in the area  56   a  inside of the V-cut and is bonded only in area  56   b  at an interface with the carrier strip outside of the V-cut, to facilitate the singulation of each BGA from the carrier strip. 
     In the singulation step  80  the finished BGA is broken out by pushing on the package to break it out of the carrier strip along the V-cut or by punching or sawing off 1.0 mm of the long sides of the original 27×29 mm PCB to result in a 27×27 mm packaged device. 
     An annular ring of copper from around the carrier strip aperture may be removed or left on to function as a heat sink and package stiffener to reduce package warpage. The carrier strip may be reused if no permanent damage results from excising the assembly from the carrier strip. 
     The above description of embodiments of this invention is intended to be illustrative and not limiting. Other embodiments of this invention will be obvious to those skilled in the art in view of the above disclosure.