Abstract:
The invention provides a molding system for encapsulating a piece of substrate having a plurality of molding sections. The system comprises a molding device operative to separately encapsulate each molding section with a molding compound and indexing means operative to position said molding sections of the substrate relative to the molding device for encapsulation.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to a system for molding electronic components mounted on substrates with encapsulating material during a semiconductor assembly process.  
         BACKGROUND AND PRIOR ART  
         [0002]    Semiconductor manufacturing involves various processes, such as mounting a plurality of semiconductor dice to a substrate, forming electrical connections between the dice and the substrate, encapsulating the dice and substrate with a plastic encapsulating material, such as epoxy resin, and separating each molded electronic component from one another to form individual semiconductor packages.  
           [0003]    The electronic components are encapsulated using a molding process. During this process, the substrate is introduced into a mold cavity formed by two mold halves. The substrate is sandwiched between the mold halves with the semiconductor dice positioned in corresponding cavities of the mold. Encapsulating material is introduced into the cavities and is allowed to set and harden. The mold halves are then opened, the encapsulating material is allowed to cure, and the cull and other unwanted encapsulating material are then removed in a degating process.  
           [0004]    Conventionally, the mold halves are designed to accommodate and mold all the plurality of semiconductor dice on a substrate at the same time. Whilst this conventional method is a simple molding method, and has been used for many years, there are several disadvantages associated with its use. Since mold chases comprised in the mold halves have larger areas to accommodate an entire array of semiconductor dice on the substrate, they require larger presses and higher clamping tonnage to secure the molding areas and provide a better sealing effect to prevent encapsulating material from leaking during the molding process. Also, since there are more semiconductor dice to be molded in one molding cycle, it requires a more complex mold chase design comprising runners and mold cavities to properly channel the encapsulating material to each die position for effective molding. The use of the mold chase may be more expensive because a new mold chase design would be required to cater to each substrate length and width. Even if the basic patterns of the dice arrangements are the same, a new mold chase would be necessary for, say, a longer length of substrate. All these factors lead to increased costs in the implementations of conventional molding systems.  
           [0005]    Prior art molding systems also require pick-and-place transfer mechanisms for substrates, pre-heaters before molding and curing ovens after molding. This involves more complexity in operation and generally higher equipment cost to support the molding operation. Further, each molding shot to mold a whole substrate normally consumes seven to eight epoxy resin pellets. A fast pellet preparation turnaround time is therefore required.  
         SUMMARY OF THE INVENTION  
         [0006]    The invention seeks to provide a molding system that avoids some of the disadvantages that have been found with conventional molding systems, by sectional molding of a substrate.  
           [0007]    According to a first aspect of the invention, there is provided a molding system for encapsulating a piece of substrate having a plurality of molding sections comprising: a molding device operative to separately encapsulate each molding section with a molding compound; and indexing means operative to position said molding sections of the substrate relative to the molding device for encapsulation.  
           [0008]    According to a second aspect of the invention, there is provided a method for encapsulating a piece of substrate having a plurality of molding sections comprising the steps of: positioning said molding sections of the substrate relative to a molding device for encapsulation; and separately encapsulating each molding section with a molding compound.  
           [0009]    It will be convenient to hereinafter describe the invention in greater detail by reference to the accompanying drawings which illustrate one embodiment of the invention. The particularity of the drawings and the related description is not to be understood as superseding the generality of the broad identification of the invention as defined by the claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    An example of a preferred embodiment of a sectional molding system in accordance with the invention will now be described with reference to the accompanying drawings, in which:  
         [0011]    [0011]FIG. 1 is a schematic illustration of a sectional molding system according to the preferred embodiment of the invention;  
         [0012]    [0012]FIG. 2 is a perspective view of a piece of substrate located over a lower mold half of the molding system of FIG. 1;  
         [0013]    [0013]FIG. 3 illustrates exemplary molding designs that may be formed by the molding system on molding sections of a substrate; and  
         [0014]    [0014]FIG. 4 is a top view of the molding system according to the preferred embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]    [0015]FIG. 1 is a schematic illustration of a sectional molding system  10  according to the preferred embodiment of the invention. The molding system  10  generally comprises a molding device in the form of a top mold housing  12  and bottom mold housing  14  attached to a top mold platen  16  and bottom mold platen  18  respectively. The top mold housing  12  includes a top molding surface in the form of a top mold chase (not shown), which cooperates with a bottom molding surface in the form of a bottom mold chase  20  housed in the bottom mold housing  14  during molding to form molded semiconductor packages.  
         [0016]    The molding system preferably includes temperature control passages, such as first and second heat tunnels  22 ,  24  respectively at either side of the mold housings  12 ,  14 , a degating station  26  and a mold cleaner  28 . The temperature control passages are operative to bring pieces of substrates  30  to a predetermined temperature, such as by heating when molding with epoxy resin. Thus, substrates  30  are prepared for molding by preheating them in the first heat tunnel  22  during transportation through the tunnel  22  to prepare them for molding, before the substrates  30  are positioned over the bottom mold chase  20  for molding. After molding, the substrates  30  are passed through the second heat tunnel  24  during transportation through the tunnel  24  for post-mold curing. After curing, the substrates  30  enter the degating station  26  to remove unwanted cull and runner material formed during molding. Thereafter, a stack magazine  32  may be used to receive and store substrates  30  that have been molded. Other equipment may be positioned adjacent to the molding system  10  depending on what is to be done with the substrates  30  after molding. The mold cleaner  28 , which would typically include brushes and vacuum suction means, may move over the surfaces of the top and bottom mold chases to clean them after each substrate  30  has been molded.  
         [0017]    [0017]FIG. 2 is a perspective view of a piece of substrate  30  located over a lower mold half of the molding system  10  of FIG. 1. The piece of substrate  30  may be in any form suitable for mounting semiconductor dice, such as a leadframes or printed circuit boards or film. The lower mold half includes the bottom mold housing  14  incorporating the bottom mold chase  20 . The bottom mold housing  14  is attached to the bottom mold platen  18 . The substrate  30  is divisible into a plurality of molding sections that may be separately encapsulated by the molding device. In the illustration of FIG. 2, the substrate  30  is divided into four sections or portions for sectional molding. In the illustration, the substrate  30  is movable generally from left to right of the drawing, such that a molded portion  30   a  has been formed on the right side of the substrate  30  that has been molded. There is a molding portion  30   b  positioned over the bottom mold chase  20  that is to be molded. The bottom mold chase  20  is designed to substantially enclose the bottom surface of the molding portion  30   b . Similarly, the top mold chase is designed to substantially enclose the top surface of the molding portion. Subsequent sections of the substrate  30  will be moved over the bottom mold chase  20  to be molded in turn.  
         [0018]    [0018]FIG. 3 illustrates exemplary molding designs that may be formed by the molding system  10  on molding sections of the substrate  30 . The substrate  30  has been divided into four sections or portions, including the molded portion  30   a , and the molding portion  30   b  to the left of the molded portion  30   a . The molding system  10  is designed such that in this embodiment, it molds each of the four molding portions of the substrate  30  separately. In FIG. 3( a ), the portion of substrate  30  has been molded in block form. The molded substance  38  formed from the molding surfaces or mold chases comprises a block of molding material  42  covering a majority of the molding portion  30   b  of the substrate  30 , runner material  44  and cull material  46 . In FIG. 3( b ), the substrate has been molded in matrix array form. The molded substance  40  formed from the molding surfaces or mold chases comprises an array of molding material covering individual semiconductor dice in the form of individual molded packages  48 , runner material  44  and cull material  46 .  
         [0019]    [0019]FIG. 4 is a top view of the molding system  10  according to the preferred embodiment of the invention. Substrates  30  are introduced into an input buffer  50  from one side of a molding machine incorporating the molding system  10 . Substrates  30  are moved into the first heat tunnel  22  for preheating, and thereafter into a position over the bottom mold chase  20  for molding. Where the encapsulating material is epoxy resin, the pellet carrier  52  carries resin pellets to a pellet compartment  54  for storage. When required, pellets are moved through a pellet feed  56  to a location where they may be heated and compressed to produce fluid epoxy resin material to enter mold cavities of the mold chases during molding of each portion of the substrates  30 .  
         [0020]    As mentioned above, the substrates  30  are molded portion by portion, and after molding, are moved into the second heat tunnel  24  for post-mold curing. After curing, they are moved into the degating station  26  to remove runner material  44  and cull  46 . After degating, the substrates  30  may then be stored in a stack magazine to undergo further processes, for example, for singulation by sawing.  
         [0021]    The molding system preferably uses indexing means to move the substrates  30  along a traveling path through the various parts of the molding system  10 , and to position the molding sections of the substrates  30  relative to the molding device for encapsulation. The indexing means preferably extend adjacent to the molding device and is substantially parallel with the traveling path of the substrates  30 , as shown in FIG. 4. A suitable indexing means is a clamp or pin indexer  58 . A clamp indexer comprises a clamping device to grip a side of a substrate  30  and moves it along the traveling path by coordinated clamping and unclamping action. On the other hand, a pin indexer comprises a pin to locate into a corresponding pinhole formed in the substrate  30  to push it along the traveling path. Either of these would be suitable for use with the invention.  
         [0022]    It should be appreciated that the molding system  10  as described significantly reduces press clamping tonnage required to mold a plurality of electronic components on a substrate  30 , thereby minimizing the cost and size of a mold press. With a reduced number of electronic components molded in one molding cycle, tooling investment can also be reduced by simplifying the design of a mold chase used. Furthermore, the molding system may flexibly be programmable for various substrate widths. Overall, a shorter machine time may be achieved due to various factors, such as a shorter cure time required by using a post-cure heat tunnel  24 , and by cleaning the mold chase after each completed strip of substrate  30  has been molded.  
         [0023]    In addition, complicated pick-and-place transfer mechanisms for substrates, pre-heaters before molding and curing ovens after molding are not necessary due to the use of indexing means and heat tunnels  22 ,  24 . Only two epoxy resin pellets may be consumed for each mold shot, as compared to seven or eight consumed for conventional molding systems, thereby reducing demands on pellet preparation turnaround time.  
         [0024]    The invention described herein is susceptible to variations, modifications and/or additions other than those specifically described and it is to be understood that the invention includes all such variations, modifications and/or additions which fall within the spirit and scope of the above description.