Patent Publication Number: US-6335221-B1

Title: Ball grid array (BGA) encapsulation mold

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a divisional of application Ser. No. 09/352,479, filed Jul. 13, 1999, now U.S. Pat. No. 6,164,946, issued Dec. 26, 2000, which is a continuation of application Ser. No. 08/898,812, filed Jul. 23, 1997, now U.S. Pat. No. 5,923,959 issued Jul. 13, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to semiconductor devices having molded housings. More particularly, the invention pertains to a mold for encapsulating integrated circuits (ICs) which include arrays of conductive balls, pins, or pads on substrates. 
     2. State of the Art 
     Modern packaged integrated circuits (ICs) comprise one or more encapsulated semiconductor dies or chips within a protective “package” of plastic or other moldable material. The integrated circuit chips are made from a semiconductor material such as silicon, germanium or gallium arsenide, and microscopic circuits are formed on a surface of each chip by photolithographic techniques. A plurality of external connections, typically designed for soldering, is connected to bond pads on one or more encapsulated chips, enabling the chips to be electrically interconnected to an external electrical apparatus. In one form of interconnection, a substrate such as a wiring board or circuit board has an array of conductors which is typically connected to the wire bond pads of the chips and which extends through the substrate to the opposite side for conductive, e.g. solder, connection to an electronic apparatus. In addition to one or more chips attached to the substrate in either packaged or unpackaged form, other devices such as resisters, capacitors, etc. may be mounted to the substrate and incorporated in the circuit. 
     Plastic encapsulation of semiconductor and other electronic devices by transfer molding is a well-known and much-used technique. Typically, a large number of components or devices is placed in a lower mold plate or half of an open multi-cavity mold, with one or more devices within each cavity. The mold is closed with a mating upper plate. The cavities of the mold are connected by “runners”, i.e. channels to a “pot” or reservoir from which pressurized liquified plastic is fed. Typically, constricted channels known as “gates” are located at the entrance to each mold cavity to limit the flow rate and injection velocity of liquified plastic into the cavity. 
     Where it is desired to encapsulate a circuit board or wiring board upon which one or more semiconductor devices have been mounted and wired, a peripheral portion of the board (or of a portion encompassing a mounted circuit) is compressed between the upper and lower plates to prevent leakage of liquified plastic from the mold cavity. Typically, the force required to compress the platen together is of the order of tons, even for molding machines having only a few mold cavities. 
     Typically, powdered or pelletized plastic, e.g. thermoset resin, is placed in the resin pot and compressed by a ram. The heated, pressurized plastic becomes liquified and flows through the runners and gates to surround each device and fill each mold cavity, where it subsequently hardens to encapsulate the board and the devices attached to it. Air is expelled from each cavity through one or more runners as the plastic melt fills the mold cavities. Following hardening by partial cure of the thermoset plastic, the mold plates are separated along the parting line and the encapsulated devices are removed and trimmed of excess plastic which has solidified in the runners and gates. Additional thermal treatment completes curing of the plastic package. 
     Following removal of each encapsulated unit from its mold cavity and curing, the peripheral portions of the board may be excised from the board and any flash is removed as known in the art, and the device is ready for use. 
     In many devices having a ball grid array (BGA) or similar array on a circuit board, the molding process is conducted so that the surface of the circuit board having the array connections forms an outer surface of the package, not being covered by the plastic material. Thus, any plastic which has solidified on the array connection surface is removed. 
     In most devices, the substrate material which is clamped between the platens may be variable in thickness or density from lot to lot, or may actually change in thickness during the molding process due to compression and/or release of gases therefrom. It is known, of course, that the high compression forces will typically compress a circuit board significantly. The variation in thickness at a given compression force may be ±5 mils or more. Unless there is compensation for this variability, a thinner-than-normal circuit board may result in leakage of encapsulant plastic past the mating portions of the upper and lower mold plates. 
     In U.S. Pat. No. 5,609,889 of Weber, a mold is described which has a biased plug that exerts pressure on a heat sink or circuit board to prevent molding compound from covering the heat sink or circuit board. A passage is provided in the substrate circuit board so that plastic flows latitudinally under the circuit board into a cavity. The plug is biased by a plate spring to accommodate variations in the thickness of the substrate and ensure that the exterior surface of the heat sink does not become significantly encased in plastic. The pressure exerted by the plate spring need only be nominal, enough to hold the circuit board or heat sink against the “floor” of the mold cavity. The circuit board is not clamped between the plate walls, so any variation in thickness of the board will not cause leakage of plastic past the mating plate surfaces. 
     It has been discovered that gases are commonly released from substrates such as circuit boards during transfer molding, presumably because of substrate compression as well as degasification due to the elevated molding temperatures. Mold plates which clamp a substrate about a mounted IC device are completely sealed on the side of the substrate which is not to be encapsulated. The buildup of gases (and pressure) on one side of the substrate may cause temporary warping of the substrate. Upon removal from the mold plates, the residual forces in the substrate may cause cracking, spalling, etc. in the package. 
     SUMMARY OF THE INVENTION 
     The present invention provides a molding machine for providing one-side encapsulation of electronic devices having a substrate such as a circuit board upon which are mounted integrated circuits (ICs) optionally with other apparatus such as resistors and capacitors. 
     The present invention also provides a molding machine having means to compensate for batch-to-batch variations in substrate thickness. The present invention provides substrate thickness compensation which automatically adjusts for variations in substrate thickness about a nominal thickness, and which is easily adjustable for different nominal substrate thicknesses. The present invention provides a molding apparatus having substrate thickness compensation which is readily usable with existing commercial molding machines. 
     The present invention further provides apparatus for venting gases released from a substrate during one-side molding encapsulation of an electronic apparatus mounted on the substrate. 
     The present invention further provides apparatus which is easily constructed from existing components at low cost. 
     The invention comprises improvements in a molding machine for encapsulating electronic devices mounted on the first side of a substrate such as a circuit board or wiring board, as exemplified by a board having a ball-grid-array (BGA), pin-grid-array (PGA), land-grid-array (LGA) or similar set of multiple electrical terminals on its opposite side. Such a circuit board is configured to have the array terminals bonded to another apparatus following encapsulation of the electronic devices including chip(s), wiring and other devices on its first side. 
     One aspect of the invention comprises a biased floating plate apparatus which comprises a floating plate, which may be a mold plate or a platen, and a second member attached to the floating plate. The second member may be a mold chase, for example. A biasing means associated with the second member biases the combination toward a first thickness dimension t, at which no outward force is exerted by the floating plate apparatus. A compressive force P is required to diminish the thickness t of the apparatus, and the inverse relationship between the compressive force P and thickness t is readily adjustable over a desired working range. The biased floating plate apparatus is positioned within the compression train of the molding machine to provide the desired compensation for inherent or process-caused variations in substrate thickness. The floating plate is biased by a plurality of springs or other force-compressible means or suitable resilient means spacedly mounted in the second member, a mold chase, for example. 
     Another aspect of the invention comprises a relief area and vent within the cavity, on the lower mold plate, covering all or a portion of the “floor” of each mold cavity. The relief area and vent serve to collect and discharge gases emitted by the substrate. The emission of such gases is due to compression at the clamped portions of the substrate, and the increased temperature during encapsulation. Unless relieved, the pressure from such gases may warp the substrate during encapsulation. Return of the substrate to its original planar position after removal from the mold may lead to cracking and spalling of the encapsulant from the substrate. Use of the relief area and vent of the invention results in a stable electronic package. 
     The combination of the biased floating plate apparatus and plate relief/vent provides a molding machine with significant advantages in production methodology, including an increase in production up-time and cost-effectiveness. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The invention is illustrated in the following figures, wherein the elements are not necessarily shown to scale: 
     FIG. 1 is an exploded side view of a post of a molding machine for encapsulating semiconductor devices and or devices mounted on a substrate, and depicting a mold chase, floating bias, pate and lower mold plate of the biased floating plate apparatus of the invention; 
     FIG. 2 is a perspective view showing a mold chase and floating biased plate of the biased floating plate apparatus of the invention; 
     FIG. 3 is an enlarged cross-sectional side view of a mold chase and floating biased plate of the biased floating plate apparatus of the invention, as taken along line  3 — 3  of FIG. 2; 
     FIG. 4 is an exploded side view of a portion of a molding machine, showing another embodiment of a mold chase, floating biased plate and lower mold plate of the biased floating plate apparatus of the invention; 
     FIG. 5 is an enlarged top view of a portion of a lower mold plate including a gas collection/venting apparatus of the invention; 
     FIG. 6 is a cross-sectional side view of a portion of a lower mold plate including gas collection/venting apparatus of the invention, as taken along line  6 — 6  of FIG. 5; and 
     FIG. 7 is a graphical representation of an exemplary relationship between compressive movement and compressive force exerted by a biased floating plate apparatus of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An improved molding machine for semiconductors and/or other devices mounted on a substrate such as a circuit board is provided. The several aspects of the invention are particularly applicable to substrate-mounted arrays of ball grids, pin grids and land-grids with various encapsulatable devices mounted on the opposite side of the substrate. 
     With reference to the drawings of FIGS. 1-5, and particularly to FIG. 1, portions of a molding machine  10  are depicted, including an upper mold plate  12  and a lower mold plate  14 . For the sake of easy depiction, it is assumed that the portions illustrated to the right of center line  16  are mirror images of the portions to the left of the center line. 
     The portions illustrated and described are those which pertain directly to the invention. Thus, a mold heater is generally included in the compression train of a molding machine  10  and may be located immediately below the lower mold plate  14 , for example. It is not shown nor described herein, not being directly related to the invention. 
     The terms “upper” and “lower” are used consistently herein for the sake of convenience only, inasmuch as the upper and lower mold plates may be inverted in position. 
     An upper mold plate  12  is shown as a conventional, generally rectangular plate member with multiple cavities  18  along its lower flat surface  20 . Each cavity  18  has a feed runner  22  for injecting the cavity with melted plastic, typically a thermoset resin, and a vent runner  24  for venting gas from the cavity during encapsulation. The upper mold plate  12  generally has a flat upper surface  26  upon which force  28  is applied to clamp the upper mold plate  12  to a mating lower mold plate  14  for encapsulation without leakage. 
     A lower mold plate  14  is shown as generally having a flat lower surface  30  upon which a clamping force  32  may be exerted. The upper mold plate  12  is shown with raised clamping ridges  34 A extending about each portion of the substrate  40  to be encapsulated. Lower mold plate  14  is shown with raised clamping ridges  34 B which “mate” to ridges  34 A when the mold plates  12  and  14  are clamped over a substrate  40 . 
     In one aspect of the invention, a raised relief portion  52  and through-vent  54  are formed in the lower mold plate  14  for collecting and venting gases emitted by a substrate  40  during the encapsulation process. As illustrated in FIGS. 1,  5  and  6 , the raised relief portion  52  comprises a pattern of ridges-and-valleys leading to the through-vent  54  which is shown passing through the lower mold plate  14  to the atmosphere. The total relief between the tops of ridges  56  and the bottoms of the striated valleys  58  need not be large, and may be, e.g., about twenty five (25) microns to about one hundred (100) microns. FIG. 6 shows elevations exaggerated for the sake of clarity, and includes a plurality of lowermost valleys  58 A, one of which intersects the through-vent  54 . Preferably, the relief portion  52  covers at least about one-fourth of the exposed area of the substrate underside  50 , i.e. within clamping ridges  34 B, and has an elevation which avoids possible damage to the solder balls, pins or lands (pads) on the substrate underside  50 . However, the pattern of ridges  56  and valleys  58  may be of any arrangement which will collect and vent gases from the non-encapsulated side of the substrate. The venting of gases from the substrate avoids the problems which such gas buildup and accompanying pressure rise can create. 
     An example of an electronic apparatus  38  which may be encapsulated in the molding machine  10  is shown as including substrate  40  such as a circuit board having a first side  42  upon which are mounted one or more semiconductor chips  44  and/or resisters and/or capacitors and/or other electronic devices, not shown, together with interconnected conduits including, for example, fine wires  46 . In this configuration, conduits on the first side  42  of the substrate  40  are connected through the substrate to a grid array of solder balls, pins or lands  48  on the underside (second side)  50  of the substrate. Such are well-known in the art. This particular type of electronic apparatus  38  is designed to be encapsulated only on the first side  42 , leaving the ball-grid-array uncovered for electrical attachment to another apparatus, not shown. In FIG. 1, each apparatus to be encapsulated is shown as having two chips. 
     A biased floating plate apparatus  60  of the invention is shown in FIGS. 1-3 as including a first member, i.e. floating plate  62 , and a second member  64  depicted as an H-shaped mold chase. The mold chase  64  is shown as comprising a horizontal plate  66  with a vertical compression wall  68  on each side. The first member  62  is positioned between the vertical compression walls  68  for vertical movement relative to the second member  64 , i.e. between an abutment position therewith and a maximum spaced-apart position. The “maximum spaced-apart position” is defined herein as the adjustable spaced position at which the first and second members  62 ,  64  are held by a fastening apparatus  90 , but from which the members may be compressed toward each other. It is noted that the first member  62  may be mounted above or below the horizontal plate  66  of the second member  64 , depending upon the configuration of the molding machine. The use of the terms “upper” and “lower” in referring to elements or positions in the floating plate apparatus is pertinent to the depictions of the figures, but it is understood that the elements and positions may be inverted. 
     In use, downward compressive forces  28  are applied by force exerting means, not shown, to the upper ends  69  of the vertical compression walls  68 , and the force is transmitted through the vertical compression walls  68  to the horizontal plate  66 , and through compressible biasing apparatus  70  to the floating plate  62 . The compressive forces  28  are then directed downwardly through the lower surface  75  of the floating plate  62  to an underlying mold plate or other plate by which forces are directed to a pair of mold plates. The compressive forces sealably clamp the substrate  40  between the mold plates  12 ,  14 . 
     Turning now to FIGS. 2 and 3, the biasing apparatus  70  is depicted as a series of stacked concentric conical spring members  72  compressively held against a first surface  74  of the floating plate  62  by a screw  76  in threaded hole  78  through horizonal plate  66 . Screw  76  is shown with a head  80  having threads  82  and a shaft  84  which maintains the spring members  72  in alignment. A portion  86  of threaded hole  78  is enlarged to permit radial expansion of the compressed spring members  72 . The number and/or compression characteristics of spring members  72  may be varied to achieve the desired amount of force and desired stress/strain ratio suitable for use with the mold material. 
     A plurality of biasing apparatus  70 , each having a center line  88 , is spaced to provide evenly distributed biasing over the entire floating plate  62 , and thus to the mold plates  12 ,  14 . Compressive forces are directed parallel to center line  88 . 
     In accordance with the invention, the first member  62 , e.g. floating plate, is attached to the second member  64 , e.g. mold chase, by a fastening apparatus  90 . The fastening apparatus  90  includes retainer bolt or screw  91  with a threaded stem  93  aligned along center line  102  having a head thereon having washer  96  thereunder passing through a hole  92  (thru-hole  92 ) in the horizontal plate  66  having spacing collar  94  located therein. The spacing collar  94  is shown within hole  92 , and the washer  96  underlies the retainer bolt or screw head  98 . The screw  91  is mounted in a threaded hole  100  in the floating plate  62 , and may be rotated to pre-compress, i.e. pre-load, the first member  62  to the second member  64 . The spacing collar  94  is slidably retained within hole  92  and may be used to adjust the spacing between plates  62  and  66  with respect to the floating plate  62 . Any desired number of fastening apparatus  90  which includes retainer bolt  91 , each located in a spacing collar  94  slidably retained within a hole  92 , may be used. 
     The biased floating plate apparatus  60  is configured for use in a pre-loaded state wherein it has a particular thickness dimension t between the upper surface  106  of the horizontal plate  66  and the lower surface  75  of the floating plate  62  (see FIG.  2 ). As pre-loaded, the retainer bolt or screw  91  is in a state of tension and the biasing apparatus  70  is in a state of compression. 
     As shown in FIG. 7, the relationship between the compressive force P exerted by the biased floating plate apparatus  60  and the thickness dimension t is inverse in nature. As applied to the apparatus of FIG. 1, the particular line is dependent upon the number n of spring members  72  in each biasing apparatus  70 . As n is increased, the compressive force P at a given position t increases, so that the apparatus may be easily adjusted for variations in substrate thickness. 
     The retainer bolt or screw  91  as well as the screw  76  may be configured for rotatable adjustment by a screwdriver, Allen wrench or other suitable tool or apparatus. 
     As shown in FIG. 4, the biased floating plate  62  may comprise the upper or lower mold plate  12  or  14 , respectively. Thus, the need for a separate floating plate is avoided. The upper mold plate  12  is shown as a floating plate attached to the mold chase  64  and spring-loaded thereby. The mold chase  64  and lower mold plate  14  are as previously described. As depicted, the feed runners  22  have inlets on the upper surface  106  of the upper mold plate  12  away from the vertical compression walls  68 . The vent runners  24  are also shown as avoiding interference by the vertical compression walls  68 . 
     It is apparent to those skilled in the art that various changes and modifications may be made to the biased floating plate apparatus and relief/venting apparatus of the invention as disclosed herein without departing from the spirit and scope of the invention as defined in the following claims.