Abstract:
An integrated circuit encapsulation apparatus comprises a molding press, which has a mold defining a cavity adapted to receive an integrated circuit die and an attached leadframe for encapsulation thereof. The molding press is operable by an electro-pneumatic driving mechanism which utilises a geared servo motor for opening and closing the mold, and a pneumatic cylinder for providing clamping pressure. Both the servo motor and pneumatic cylinder act upon a threaded screw member for movement of the molding press. The press is provided with an integrated mold brushing unit which has transversely rotating brushes and travels over the mold faces to remove debris. In a further refinement, a spring-loaded bearing system is provided for easy removal of the mold.

Description:
This invention relates to apparatus for injection molding, for example, for encapsulating integrated circuits. 
    
    
     BACKGROUND OF THE INVENTION 
     In the electronics manufacturing industry, a high degree of cleanliness is required to prevent contamination of circuitry and devices, which may cause subsequent electronic failure. A common way of packaging electronic integrated circuits is by encapsulating the prefabricated integrated circuit and a portion of the attached leadframe in a plastics material. Typically this is performed utilising injection molding apparatus, often of the transfer mold variety. Known injection molding equipment employs hydraulic presses, and it is not uncommon for hydraulic fluids to leak from the hydraulic presses, which may contaminate the molds and molded products. Although cleansing of the molds is performed regularly, the cleaning techniques are not always effective in removing contaminating debris from the molds. Furthermore, removal of molds for cleaning is often difficult and time consuming. It is an object of the present invention to overcome these problems. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly the present invention provides an injection molding apparatus, comprising a molding press adapted to receive a mold having a cavity shaped for the formation of a molding product, the molding apparatus comprising: first and second platens which are movable relative to one another so as to allow respective mold parts mounted thereon to be moved between open and closed configurations; and 
     an electro-pneumatic drive mechanism comprising: a threaded screw member coupled to one of the first and second platens; an electric motor coupled to drive the threaded screw member by way of a gear mechanism for moving the first and second platens relative to one another; and a pneumatic cylinder mechanism for driving the screw thread of the threaded member separately from said electric motor. 
     The present invention is particularly well adapted for use in an integrated circuit encapsulation process. 
     Preferably, the mold is provided with a spring-loaded bearing system for easy removal of the mold from the press. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described in greater detail below by way of example only, with reference to the accompanying drawings in which: 
     FIGS. 1A,  1 B and  1 C are respective cross-sectional views through an exemplary molding press illustrating the operation for encapsulating an integrated circuit and leadframe; 
     FIGS. 2A and 2B show a plan and cross-sectional elevation view respectively of an electro pneumatic press; 
     FIGS. 3A and 3B show an elevation and side cross-sectional elevation respectively of a mold brushing system; and 
     FIGS. 4A and 4B are cross-sectional views showing a spring-loaded bearing system for removing the mold. 
     FIGS. 1A,  1 B and  1 C are cross-sectional views of an exemplary transfer molding press  1  adapted to receive two molds  2 . Each mold  2  is arranged within the molding press  1 , and comprises upper and lower mold parts  2   a ,  2   b  which fit together to define a mold cavity  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The molding press  1  is shown in FIG. 1A in a closed position, having been loaded with integrated circuit leadframes  6  within the respective mold cavities  4 , and a pellet of encapsulating material  3  in a gangpot  9 . Encapsulation of the integrated circuits  6  is achieved by heating the encapsulating material pellet  3  and pressing it within the gangpot using a transfer plunger  8 , which causes the pellet  3  to liquefy and flow into the mold cavities  4  through small passages between the gangpot and the mold cavities (see FIG.  1 B). After allowing the encapsulating material to solidify again, the molding press  1  is opened (FIG.  1 C), wherein the mold parts  2   a ,  2   b  are separated. The encapsulated integrated circuits  7  are lifted from the mold cavity by way of ejector pins  10 , so as to expose them for removal from the molding press. After removal of the encapsulated integrated circuits  7 , the open molding press is ready to receive new leadframe inserts  6  and encapsulating material pellet  3  to repeat the encapsulating process. 
     In order to move the mold parts  2   a ,  2   b  towards and away from each other between the open and closed configurations illustrated in FIGS. 1C and 1A, respectively, a driving mechanism is required. The upper and lower mold parts  2   a ,  2   b  are respectively mounted on upper and lower platens  16   a ,  16   b  of the molding press, and the upper and lower platens are movable relative to one another along linear guide rods  20  (see FIG.  2 B). Typically hydraulic driving mechanisms have been employed for movement of the molding press platens, and in the electronics encapsulation industry, at least, such driving mechanisms are subject to disadvantages as discussed hereinabove. However, hydraulic driving mechanisms have persisted even in the electronics encapsulation industry because other characteristics of the hydraulic systems make them well suited to the requirements for driving molding presses. In particular, hydraulic systems are able to easily provide a sufficient range of relative movement of the platens to permit access to the mold cavities when separated, and also enable generation of a sufficiently large clamping force between the mold parts when the press is closed. 
     The molding press driving mechanism of the present invention is able to provide the range of movement and force requirements of the molding press, but without the deleterious effects which can result from the prior art hydraulic driving systems. A particular preferred form of the molding press driving mechanism is illustrated in FIGS. 2A and 2B, in plan and cross-sectional elevation views, respectively. Representations of the upper and lower mold press platens are shown at  16   a  and  16   b  (FIG.  2 B), mounted for relative movement along guide rods  20 . In this driving mechanism, it is the upper platen  16   a  which is in fact movable along the guide rods  20 , to effect displacement relative to the lower platen  16   b  which is fixed to the guide rods. The driving mechanism comprises an electro-pneumatic system, since the two operative components are an electrically activated servo motor and a pneumatically motivated cylinder and piston arrangement. 
     The upper platen  16   a  has an elongate ball-screw  15  mounted thereon, centrally arranged on top of the platen and rotatable about a central axis. The ball-screw  15  extends parallel to the guide rods  20  away from the lower platen  16   b , and has an external screw thread formed thereon. A fixed frame plate  40  is mounted at the top of the guide rods  20  having a central aperture aligned with the ball-screw  15 . Within the central aperture is mounted a flange member  42  which is fixed to prevent movement thereof relative to the frame plate  40  in the axial direction of the ballscrew, but to allow rotational movement about the ball-screw axis. The flange member  42  has a circular internally threaded opening, and the ball-screw  15  extends through the opening with the internal and external screw threads of the ball-screw and flange member interfitting. Accordingly, relative rotational movement between the ball-screw  15  and flange member  42  is translated into axial movement of the ball-screw relative to the frame plate, the direction of rotation determining the direction of axial movement. In view of the construction, therefore, relative rotational movement between the ballscrew  15  and the flange member results in relative linear movement of the upper platen  16   a , with respect to the lower platen  16   b , along the guide rods  20 . 
     As mentioned, the electro-pneumatic driving mechanism provides two active components, namely an electric servo motor  14 , and a pneumatic piston and cylinder arrangement  17 . Both the electric and pneumatic active components operate upon the ball screw arrangement above described, but provide different functions. In particular, the servo motor  14  is arranged to provide the required range of relative movement of the platens to permit sufficient access to the mould cavities when separated, whilst the pneumatic piston and cylinder is arranged to provide a sufficiently large clamping force between the mold parts when the press is closed. 
     Adjacent the top of the upper platen  16   a , a gear cog  12  is mounted for axial movement with the ball-screw  15 . The gear cog  12  intermeshes with a driving cog  11  which is driven by the servo motor  14  mounted on the upper platen  16   a . Accordingly, driving the servo motor  14  rotates the driving cog  11  which thereby causes rotational movement of the gear cog  12  and ball screw  15 . As discussed above, this rotational movement results in linear axial movement of the ball-screw  15  and upper platen  16   a , relative to the lower platen  16   b . Thus, by controllably driving the servo motor  14 , the upper platen  16   a  can be moved relative to the lower platen  16   b  to, in use, open and close the molds of the molding press. Whilst the ball-screw  15  is driven by the servo motor  14  the flange member  42  of course remains stationary. 
     When the molds of the molding press are positioned in the closed configuration by action of the servo motor  14 , it is then desired to ensure that sufficient clamping force is applied between the upper and lower mold parts. This function is provided by the pneumatic cylinder and piston arrangement  17 . A lever arm  18  is fitted to the flange member  42  by way of connecting bolts  19 , and a movable end of the piston of the cylinder and piston arrangement  17  is coupled to the end of the lever arm  18 . This arrangement enables movement of the piston by action of increased pneumatic pressure in the cylinder  17  to be translated to rotational movement of the lever arm  18 , and thus to the flange member  42 . An interlocking device  13  is also provided adjacent the gear cog  12 , which is operable to interlock with the gear cog  12  to prevent rotation thereof as well as the ball screw  15 . Thus, motion of the piston  17  which results in rotational movement of the lever arm  18  and flange member is translated to axial movement of the ball screw  15 . However, because of the nature of the mechanism the pneumatic cylinder is only able to produce a small rotational movement of the flange member  42 , which results in only a small axial movement of the upper platen, although significant clamping force can nevertheless be generated. 
     Utilising the construction of the driving mechanism illustrated in FIGS. 2A and 2B, the servo motor  14  is able to be controlled so as to move the upper and lower latens  16   a ,  16   b  apart and together between open and closed configurations of the molds. When the servo motor is controlled so as to configure the molds into a closed configuration, then the pneumatic cylinder  17  can be activated by increasing the gas pressure therein so as to rotate the flange member relative to the ball screw  15 . This action, combined with activation of the interlocking device  13  to prevent rotation of the ball screw  15  enables a clamping pressure to be applied by the pneumatic cylinder  17 . 
     In FIG. 3A, heat resistant brushes  21  are mounted along a series of parallel endless V-belts  23  which run on pulleys  22 . A motor  28  drives the pulleys  22  through a drive mechanism  26 . When the mold is opened, the brushing unit rotate in one direction is movable axially of the pulleys  22  (ie. transverse to the movement of brushes  21 ) across the mold-face brushing any debris away from the mold and then rotate in the another direction during the returning stroke of the cleaning process. The rotation of the brushes about a horizontal axis as the translation occurs provides a more effective cleaning action than that of known cleaning methods which have brushes rotating about vertical axes. 
     In FIGS. 4A and 4B, a roller-bearing system  34  is mounted in the press-table  36  and is loaded with a spring  33  set in the base plate  32 . When the mold  31  is tightened, it depresses the spring  33  and sits in contact with the press table  36 . When the mold is released, the spring  33  uncoils and raises the mold above the press table  36  allowing easy removal. The temperature of the mold which can be as high as 180° C. makes mold changing a difficult operation and this system increases the speed of changing considerably. As shown in FIG. 4A, the strength of springs  33  is such as to elevate the mold  31  by pressure of the bearings  34  to about 1 mm above the surface of the press table  36 . It will be immediately apparent that the bearings  34  may be of a form comprising rollers, or incorporating a ball bearing to allow two dimensional relative movement between the press table and mold, as will be appreciated by those skilled in the art. It is of course obvious that the roller-bearing system would be incorporated in the lower platen  16   b , and that the base plate  32  and press table  36  may comprise components thereof. 
     The above detailed description is by way of example only, and is not intended to limit the scope of the invention which is defined in the following claims.