Abstract:
A package, a method of making and a method of assembly of packages for semiconductor chips are disclosed. The package includes a die attach pad on which a semiconductor die is mounted. A lead is electrically connected to the semiconductor die which is encapsulated in packaging material. The lead is exposed at opposed sides of the package. Exposing the lead on both sides of the package allows the package to be stacked or assembled so that the leads of adjacent pairs of packages are in electrical contact. Making the semiconductor packages includes forming a piece of electrically conductive material into a die attach pad and at least one lead associated with the die attach pad. A semiconductor die is mounted on the die attach pad and an electrical connection is made between the semiconductor die and the lead. The package is formed by encapsulation with the lead exposed on opposite sides of the package.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a package, and more particularly, to a stackable semiconductor device package, and related methods of manufacture and assembly. 
     BACKGROUND OF THE INVENTION 
     Current computer and signal processing systems are typically very complex and include a number of highly integrated semiconductor chips mounted on one or more printed circuit boards. Printed circuit boards typically include contacts that mate with corresponding contacts on the chip packages and electrical traces that provide electrical connections between contacts of different chips on the board or to external elements, such as power supplies, data input and output points, or even other circuit boards. 
     Market forces often require that consumer or other electronic products, such as cell phones, personal digital assistants, lap tops, etc, not only include more features, but be implemented in even smaller product sizes. This poses a number of serious problems for system designers. 
     Smaller products necessarily mean the circuit board or boards inside the product need to be smaller. With smaller boards, there is less real estate to “stuff” with integrated circuits. This problem is only exacerbated when new functionality requires the addition of more chips to the system. 
     SUMMARY OF THE INVENTION 
     A package, a method of making and a method of assembly of packages for semiconductor chips are disclosed. The package includes a die attach pad on which a semiconductor die is mounted. A first lead is electrically connected to the semiconductor die. Packaging material encapsulates the semiconductor die. The package has opposed sides, and the lead is exposed at the first and second sides. The exposed lead on the first and second sides allows the package to be stacked or assembled so that the leads of an adjacent pair of packages are in electrical contact. 
     The method of making the semiconductor packages includes forming a piece of electrically conductive material to form a die attach pad and at least one lead associated with the die attach pad. A semiconductor die is mounted on the die attach pad. An electrical connection is made between the semiconductor die and the lead. A package is formed by encapsulating the semiconductor die, connection and pad, with the lead exposed at opposite sides of the package. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with further advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a schematic side view of package according to the present invention; 
     FIG. 2 is a schematic plan view of the package shown in FIG. 1; 
     FIG. 3 shows a schematic side view of a package  200  mounted on a printed circuit board; 
     FIG. 4 is a schematic side view of an assembly of packages according to an aspect of the invention; 
     FIG. 5 is a schematic side view of an assembly of packages according to another embodiment of an aspect of the invention; 
     FIG. 6 is a schematic plan view of a lead frame strip used in a method aspect of the invention; 
     FIG. 7 is a magnified view of a part of the lead frame strip shown in FIG. 7; and 
     FIGS. 8A to  8 D are schematic cross sectional side views illustrating steps in a method of making packages according to the present invention; 
    
    
     In the Figures, like reference numerals refer to like components and elements. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to a semiconductor device package. FIGS. 1 and 2 respectively show schematic side and plan views of a package  100  according to an embodiment of the present invention. Package  100  includes a die attach pad  102  having a first side and a second side. Four leads are arranged along a first side of the package and towards the edge of the package and another four leads are arranged along a second opposed side of the package towards the edge of the package. Each lead  114  has a substantially T shape, with a lead tip  116 ,  118  at each end and a trunk  120  providing a bonding shelf for wires as will be described below. The die attach pad  102  and associated leads  110 ,  112  are fabricated from a single piece of copper as will also be described below. The die attach pad includes ties  104 , 106  which are artifacts from the lead frame used in manufacture of the packages. Ties  104 , 106  are not shown in FIG. 1 for the sake of clarity. 
     Although FIG. 2 shows leads along two sides of the package only, other lead arrangements can be used as appropriate. Leads can be provided around one, two, three or four sides of the package. Eight leads are shown in FIG. 2 for the sake of clarity. More or fewer leads can be used. In particular in some embodiments up to a very large number of leads can be used, depending on the number of leads required by a particular die. The leads are shown around the periphery of package  100 , but they can be located at any suitable location within the package. A rectangular package geometry is shown in FIG. 2, but the package can have other shapes such as other polygons, including squares, or curved shapes, such as circles. 
     A first die  124  is mounted on the first side of the die attach pad  102  and secured in place using an epoxy  126 . A suitable conductive epoxy is ABLESTIK 84-1 and a suitable non-conductive epoxy is ABLESTIK 84-3, both as provided by Ablestik Laboratories, California. A second die  128  is mounted on the second side of the die attach pad  102  and is also secured in place using the same epoxy  130 . Dice  124 ,  128  are semiconductor, integrated circuit based devices which can provide various functionalities depending on the intended application of the package. An electrical connection is provided between the inputs and outputs of the dice and the leads  110 ,  112  by gold wire bonding  132  between the dice and the bonding shelf of each lead  114 . 
     The package is encapsulated in a packaging material  136 . In one embodiment of the package  100 , the packaging material can be a mold compound such as EME 6300 or CR 720, as provided by Sumitomo Chemical Company Limited of Japan. In other embodiments in which one or more of the dice is sensitive to radiation, the encapsulating material is selected to be substantially transparent to the appropriate radiation. For example, in an embodiment for use in optical imaging applications, the encapsulating material is chosen to be one that is transparent to radiation in the visible light part of the light spectrum. A suitable encapsulating material would be a clear mold compound. In another embodiment for applications using infrared radiation, the encapsulating material is selected to be one that is transparent to light in the infrared part of the light spectrum. In other embodiments, the encapsulating material can be selected to be one that is substantially transparent to light in the invisible light parts of the light spectrum. 
     The package has a first surface  138  and a second surface  140  opposed to the first surface. Each lead  114  extends between the first  138  and second  140  surfaces of the package  100  and each end surface  142 ,  144  of each lead  114  is exposed at the first  138  and second  140  package surfaces. 
     FIG. 3 shows a schematic side view of a package  200  mounted on a printed circuit board  210 . The printed circuit board (PCB)  210  has a substrate  212  of FR4 on which is a complex pattern of electrically conducting copper tracks  214 ,  216  for carrying electrical signals to and from the package  200 . In other embodiments, substrate  212  can be of tape, polyimide or ceramic materials. Connection terminals for the tracks are arranged on the PCB in a pattern corresponding to the pattern of the leads of the package. The package  200  is mounted on the PCB using solder  218 ,  220  or alternatively electrically conductive epoxy. Package  200  is configured for an imaging application and includes an imaging die  222  and an image data processing die  224 . Encapsulation material  226  is substantially transparent to radiation  228  in the visible part of the electromagnetic spectrum. 
     An example application for package  200  could be as a video camera for a wireless communications device, such as a cellular telephone. Package  200  has a small footprint on the PCB and provides two semiconductor devices within that footprint. Some of the leads of the package can provide a common signal to the dice, e.g. a power supply signal, and so a reduced number of circuit board tracks can be used with the package. Hence the package facilitates miniaturization. Some of the leads can be dedicated to one or the other of the dice and the wire bonding pattern is selected so as to ensure that the correct input or output of the dice is electrically connected to the correct circuit board track via the appropriate lead  114 . Some of the leads can be wire bonded to only a one of the dice, as appropriate. 
     In some embodiments a single die may be used in a package where the application is appropriate. 
     FIG. 4 shows a further embodiment illustrating aspects of the invention. FIG. 4 shows a stack  400  of four packages  402 ,  404 ,  406 ,  408  mounted on a printed circuit board  410 . Each of the die in each package is a semiconductor memory device. The stack of packages therefore provides a high capacity memory within a small footprint. The fourth package  408  is connected to tracks  412 ,  414  on the PCB  410  by solder  416 . The third package  406  is mounted on the bottom package  408  and secured in place by solder  418 . An interface  420  is provided between the third  406  and second  404  packages. The interface  420  has a flexible substrate  424  comprising a strip or sheet of dielectric material, such as mylar, which has a pattern of conducting copper tracks on its upper and lower surfaces (not shown in FIG. 4 for the sake of clarity). The interface is connected to the third  406  and second  404  packages by solder between leads of the respective packages and terminals of the interface tracks. An end of the interface is electrically connected to PCB tracks  415  by solder  426  between interface track terminals and the PCB tracks  415 . The leads of the first package  402  are soldered to the leads of the second package  404 . 
     The end surfaces of the leads of the packages are exposed and the leads of adjacent pairs of packages are electrically connected by the solder. Other ways of connecting the stacked packages can be used. For example, electrically conductive epoxy could be used. Also, the packages could be fastened together with an adhesive between the bodies of the packages and with the leads directly contacting each other. 
     Any electrical signals common to all the dice, e.g. a power supply signal, can be provided to all the dice using a single lead so that the signal propagates up and down the stack using the same respective lead in each package. Those signals which are specific to a particular die or package can propagate up and down the stack using a particular vertical path of leads and appropriate wire bonding connections. 
     Interface  420  can provide a number of features. The pattern of tracks on the interface can be used to supply electrical signals across a package to different leads. For example, a track on the under side of the substrate  424  can pass through the substrate  424  and emerge on the upper side of the substrate thereby connecting different leads of the third  406  and second  404  packages. Other track patterns can be used at the interface so as to connect leads of the same package or different packages as required. 
     The interface  420  can also be used to allow electrical signals to be connected between packages of the stack and PCB tracks other than those to which the fourth package is connected. This allows signals to be supplied to a selected package or packages of the stack or to connect packages of the stack to other devices on the PCB. For example, PCB track  415  can be attached by solder  426  to a track on the substrate  424  of interface  420  which track is connected to a lead of the second package so that an electrical signal can be supplied to the second and/or first packages only. In one example embodiment, track  412  could be used to supply signals common to all the memory dice in the stack and track  414  could be used to supply signals from a first processing unit to the fourth and third packages and track  415  to supply signals from a second processing unit to the second and third packages. Such a configuration provides a stack of the memory with a reduced footprint and also helps to reduce the number of PCB tracks required by the stack of memory. 
     FIG. 5 shows a schematic side view of a further example embodiment of the invention. FIG. 5 shows a stacked assembly  500  of three packages  502 ,  504 ,  506  mounted on a PCB  510 . In this embodiment, the packages in the stack can communicate both wirelessly and via electrical connection of the leads. An infrared transmitter and receiver device  514  is mounted in a recessed part of the PCB and aligned with the stack. Each of the dice  516  includes an infrared transmitter and receiver and the encapsulating material  517  is substantially infrared transparent at the wavelengths of the infrared radiation  518  used by the transmitters and receivers. A laser diode can be used as the transmitter and an image/IR sensor as the receiver. The transmitter and receiver devices can be provided as parts of the dice or separate to the dice and the dice can have other functionalities which process, store or otherwise utilize the data transmitted to them. 
     Electrical signals required by the dice can be supplied via the leads and data, and other information, can be transmitted at high rates between the dice of the stack by being encoded in the infrared signals. Hence, data required by the top package  502  can be transmitted to it from the PCB infrared transmitter/receiver device  514 , by being relayed between the dice on the bottom  506  and middle  504  packages. High rate transmission of data between the packages using wireless communication obviates the problems associated with wire based connections at high rates, such as the wire impedance. 
     A method of manufacturing a package will now be described with reference to FIGS. 6,  7  and  8 A to  8 D. FIG. 6 shows a lead frame strip  600  having a plurality of six by six arrays  602 ,  604 ,  606  of lead frames for the individual packages. FIG. 7 is a magnified view of the lower left hand corner of lead frame strip  600 . The lead frame for each package includes the leads  114 , and the die attach pad  124  connected to the lead frame by ties  104 ,  106 . The lead frame strip is made by appropriately patterning a strip of approximately 40 mm thick copper and then etching the strip from both sides to a thickness  804  of approximately 3 mm. FIG. 8A shows a cross sectional view through four items of the array  604  along line AA′. After the lead frame has been etched, dice  124 ,  128  are attached using epoxy  126  to each side of each of the die attach pads  124 . The dice are then wire bonded  132  to the leads  114  using thermal sonic bonding. The packages are then formed by encapsulation in a packaging material  136  using conventional molding techniques or methods. The ends of the leads  114  are exposed at each surface of the packages. The mold clamps the lead frame during the molding stage and prevents mold compound from covering the ends of the leads. In the event that there is any bleed of mold compound onto the surface it can be chemically removed. The individual packages  804 ,  806 ,  808 ,  810  are then separated by using a saw  802  in a singulating step. 
     Although the foregoing invention has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. Some of the features of the various embodiments may be combined with features of other of the embodiments. Therefore, the described embodiments should be taken as illustrative and not restrictive, and the invention should not be limited to the details given herein but should be defined by the following claims and there full scope of equivalents.