Abstract:
A lead frame based semiconductor die package includes a lead frame having a die pad that supports a semiconductor die and lead fingers that surround the die and die pad. The die is electrically connected to the lead fingers with bond wires. The die and bond wires are covered with an encapsulant with ends of the lead fingers projecting out from the encapsulant. One set of the lead fingers are bent and project down and another set of the lead fingers are bent and project inwardly, and under a bottom surface of the encapsulant. The encapsulant includes a slot or groove for receiving the lead fingers of the second set.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a integrated circuit packaging and, more particularly, to packaging a lead frame based semiconductor die package. 
     A semiconductor die is a small integrated circuit formed on a semiconductor wafer, such as a silicon wafer. Such a die is typically cut from the wafer and packaged using a lead frame. The lead frame is a metal frame, usually of copper or nickel alloy, that supports the die and provides external electrical connections for the packaged die. The lead frame usually includes a flag (die pad), and associated lead fingers (leads). The semiconductor die is attached to the flag and bond or contact pads on the die are electrically connected to the lead fingers of the lead frame with bond wires. The die and bond wires are covered with an encapsulant to form a semiconductor die package. The lead fingers either project outwardly from the encapsulation or are at least flush with the encapsulation so they can be used as terminals, allowing the semiconductor die package to be electrically connected directly to other devices or to a printed circuit board (PCB). 
     Semiconductor die packages are being manufactured with an increased functionality to package pin count (external terminal or I/O count). This is partly because of improved silicon die fabrication techniques that allow die size reductions. However, the number of lead fingers is limited by the size of the package and the pitch of the lead fingers. In this regard, a reduced lead finger pitch generally increases the likelihood of short circuit faults particularly when the package is mounted to a circuit board. 
     One solution that may overcome or alleviate circuit board shorts due to reduced lead finger pitch is to space adjacent lead fingers in different planes. The mounting feet at the ends of adjacent lead fingers are spaced at different distances from the package housing and thus this increases the spacing of the circuit board pads to which the mounting feet are soldered. Although useful, the spacing of adjacent lead fingers in different planes can increase the manufacturing process complexity and requires accurate jig alignment and precision lead bending. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention, together with objects and advantages thereof, may best be understood by reference to the following description of preferred embodiments together with the accompanying drawings in which: 
         FIG. 1  is a plan view of an electrically conductive lead frame in accordance with a preferred embodiment of the present invention; 
         FIG. 2  is a plan view of partially assembled package, formed on the electrically conductive lead frame of  FIG. 1 , in accordance with a preferred embodiment of the present invention; 
         FIG. 3  is a plan view of a partially assembled electrically coupled package, formed from the partially assembled package of  FIG. 2 , in accordance with a preferred embodiment of the present invention; 
         FIG. 4  is a plan view of an encapsulated semiconductor die package in accordance with a preferred embodiment of the present invention; 
         FIG. 5  is a plan view of a singulated semiconductor die package resulting after detaching lead fingers from a frame member of the lead frame of  FIG. 1 , in accordance with a preferred embodiment of the present invention; 
         FIG. 6  is a plan view of a semiconductor die package after bending (forming) the lead fingers of the package of  FIG. 5 , in accordance with a preferred embodiment of the present invention; 
         FIG. 7  is a side view of part of the semiconductor die package of  FIG. 6 ; 
         FIG. 8  is a cross sectional view of the semiconductor die package of  FIG. 6 , through  6 - 6 ′; and 
         FIG. 9  is a flow chart illustrating a method of packaging a semiconductor die in accordance with a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The detailed description set forth below in connection with the appended drawings is intended as a description of presently preferred embodiments of the invention, and is not intended to represent the only forms in which the present invention may be practiced. It is to be understood that the same or equivalent functions may be accomplished by different embodiments that are intended to be encompassed within the spirit and scope of the invention. In the drawings, like numerals are used to indicate like elements throughout. 
     Furthermore, terms “comprises,” “comprising,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that module, circuit, device components, method steps and structures that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such module, circuit, steps or device components. An element or step proceeded by “comprises” does not, without more constraints, preclude the existence of additional identical elements or steps that comprises the element or step. 
     Certain features in the drawings have been enlarged for ease of illustration and the drawings and the elements thereof are not necessarily in proper proportion. Further, the invention is shown embodied in a quad flat pack (QFP) type package. However, those of ordinary skill in the art will readily understand the details of the invention and that the invention is applicable to all leaded package types and their variations. 
     In one embodiment, the present invention provides for a method of packaging a semiconductor die. The method includes providing an electrically conductive lead frame with a least one die pad, a frame member surrounding the die pad. There are a plurality of lead fingers attached to the frame member, and disposed between the frame member and die pad, such that each of the lead fingers has a proximal end near the die pad and a distal end located away from the die pad. The method also includes attaching a semiconductor die to the die pad and electrically coupling contact pads on the semiconductor die with respective proximal ends of the lead fingers. There is also performed a process of encapsulating at least the die, the die pad and the proximal ends of the lead fingers with an encapsulation material. The encapsulation material provides a housing with edges from which the lead fingers extend, and the housing has an underside that has a least one slot therein. The method further includes detaching the lead fingers from the frame member and bending the lead fingers into a first set and second set of lead fingers. The distal ends of the first set of lead fingers are located away from the housing, and the distal ends of the second set of lead fingers are located at least partially in the slot. 
     In another embodiment, the present invention provides for a semiconductor die package including a die pad and a first set of lead fingers that are spaced from and project outwardly from the die pad. The lead fingers have proximal ends close to the die pad and distal ends spaced from the die pad. There is a second set of lead fingers that are spaced from and project outwardly from the die pad. The second set of lead fingers also have proximal ends close to the die pad and distal ends spaced from the die pad. A semiconductor die is attached to the die pad and bonding pads on the semiconductor die are selectively electrically coupled to the proximal ends of the first and second sets of lead fingers with bond wires. There is an encapsulation material covering the bond wires, the semiconductor die and the proximal ends of the first and second set of lead fingers. The encapsulating material provides a housing with edges from which the first set and second set of lead fingers extend. The housing has an underside that has a least one slot therein and wherein distal ends of the first set of lead fingers are located away from the housing and the distal ends of the second set of lead fingers are located at least partially in the slot. 
     Referring now to  FIG. 1 , there is illustrated a plan view of an electrically conductive lead frame  100  in accordance with a preferred embodiment of the present invention. The lead frame  100  is part of a lead frame sheet and the lead frame  100  has a die pad  102 , a frame member  104  surrounding the die pad  102  and a plurality of lead fingers  106  attached to the frame member  104 . In this particular embodiment, the lead frame  100  has two sets of distinguishable lead fingers  106  which are a first set of lead fingers  108  and second set of lead fingers  110 . 
     The lead fingers  106  are disposed between the frame member  104  and the die pad  102  such that each of the lead fingers  106  has a proximal end  112 ,  114  near the die pad  102  and a distal end  116 ,  118  located away from the die pad  102 . As illustrated in this particular embodiment, each member of the second set of lead fingers  110  is shorter than each member of the first set of lead fingers  108 , and the first set of lead fingers  108  has distal ends  116  at the frame member  104 . In contrast, the second set of lead fingers  110  has distal ends  118  spaced the frame member  104 . Also, in this embodiment the proximal ends  112  of the first set of lead fingers  108  and proximal ends  114  of the second set of lead fingers  110  are spaced approximately at a constant distance from the die pad. 
     Struts  120  extend from the frame member  104  to attach and support the die pad  102  to the frame member  104 . The struts  120  in combination with dam bars  122  support and attach the first and second set of lead fingers  108 ,  110  to the frame member  104 . Furthermore, each of the struts  120  has an angled section  124  that downsets the die pad  102  relative to the lead fingers  106  as will be apparent to a person skilled in the art. 
     Referring to  FIG. 2 , a plan view of a partially assembled package  200 , formed on the electrically conductive lead frame  100  in accordance with a preferred embodiment of the present invention, is illustrated. The partially assembled package  200  includes a semiconductor die  202  that is typically attached to the die pad  102  by a bonding agent (not shown). Also, as various size semiconductor dice are known, it is understood that the size and shape of the die pad  102  will depend on the particular semiconductor die  202 . The semiconductor die  202  has contact pads  204  (that can be circuit electrodes) that are input, output or power supply nodes. These contact pads  204  are disposed on an upper or active surface  206  of the semiconductor die  202  as will be apparent to a person skilled in the art. 
       FIG. 3  is a plan view of partially assembled electrically coupled package  300 , formed from the partially assembled package  200  in accordance with a preferred embodiment of the present invention. As shown, the partially assembled electrically coupled package  300  has the contact pads  204  selectively electrically coupled (connected), by bond wires  302 , to the proximal ends  112  of the first set of lead fingers  108  and the proximal ends  114  of the second set of lead fingers  110 . 
       FIG. 4  is a plan view of an encapsulated semiconductor die package  400  in accordance with a preferred embodiment of the present invention. The encapsulated semiconductor die package  400  includes the package  300  after wire bonding has been completed for all the required contact pads  204 . The encapsulated semiconductor die package  400  includes an encapsulating material that provides a housing  402  molded to the conductive lead frame  100  so that the housing  402  encapsulates the semiconductor die  202 , die pad  102 , the bond wires  302  and the proximal ends  112 ,  114  of the lead fingers  106 . 
     The housing  402  has edges  404  from which the first set of lead fingers  108  and second set of lead fingers  110  extend. Also, an underside of the housing has slots  406  (shown in hidden detail, wherein the each of the slots  406  is adjacent a respective one of the edges  404 . Furthermore, there are recesses  408  in respective edges  404  of the housing  402  and each of the recesses  408  is aligned with a respective slot  406  to thereby provide channels  410  for the second set of lead fingers  110 . As illustrated in the magnified area, the recesses are tapered which allows easy access to the second set of lead fingers  110  during forming (bending) described later herein. 
       FIG. 5  is a plan view of a singulated semiconductor die package  500  resulting after detaching the lead fingers  106  from the frame member  104 , in accordance with a preferred embodiment of the present invention. As illustrated, the lead fingers  106  are separated from each other by cuts  502  in the dam bars  122  at locations between adjacent lead fingers  106 . In this particular embodiment parts of the dam bars  122  remain as lateral protrusions  504  in the lead fingers  106 , however the lateral protrusions  504  can be totally removed by suitable trimming (punching) during the detaching the lead fingers  106  from the frame member  104 . 
       FIG. 6  is a plan view of a semiconductor die package  600  after bending (forming) the lead fingers  106 , in accordance with a preferred embodiment of the present invention. As shown, the first set of lead fingers  108  are interleaved with the second set of lead fingers  110 . More specifically, members of the first set of lead fingers  108  are in an alternating arrangement with members of the second set of lead fingers  110 . In this particular embodiment the lateral protrusions  504  have been removed by suitable trimming, however if the protrusions remain there would not be an issue with shorting of adjacent lateral protrusions  504  due to the bent configurations of the adjacent lead fingers  106 . 
       FIG. 7  is a side view of part of the semiconductor die package  600 . As shown, after bending (forming) of the lead fingers  106  distal ends  116  of the first set of lead fingers  106  are located away from the housing  402  and the distal ends  118  of the second set of lead fingers  108  are located at least partially in a respective slot  406 . The first set of lead fingers  108  are bent so that they have mounting feet  702  at their distal ends  116  and the second set of lead fingers  110  are also bent so that they also have mounting feet  704  at their distal ends  116 . The mounting feet  702 ,  704  are aligned in a seating plane P1 and since the feet  704  are located in the slots  406 , in the underside  706  of the housing  402 , the height of the package may be reduced slightly. 
     Referring to  FIG. 8 , a cross-sectional side view through  6 - 6 ′ of part of the semiconductor die package  600  is shown. In this embodiment, each one of the first set of lead fingers  108  are spaced from and project outwardly from the die pad  102  and the proximal ends  112  are close to the die pad  102  and distal ends  116  (feet  702 ) are spaced from the die pad  102 . Each one of the second set of lead fingers  110  is spaced from and project outwardly from the die pad  102  and the proximal ends  114  are close to the die pad  102  and distal ends (feet  704 ) are spaced from the die pad  102 . More specifically, each one of the first set of lead fingers  108  is bent to have a first region  802  extending out of the housing  402  into a respective recess  408 , an upright mid-region  804  in the respective recess  408  and the distal end  118  (mounting foot  704 ) that is located in a respective slot  406 . 
     Referring to  FIG. 9 , a flow chart illustrating a method  900  of packaging a semiconductor die in accordance with a preferred embodiment of the present invention is shown. The method  900  will be described, where necessary, with reference to  FIGS. 1 to 8  however, the method is not limited to the specific embodiments of  FIGS. 1 to 8  as will be apparent to a person skilled in the art. The method  900  includes, at a block  910 , providing the electrically conductive lead frame  100 . At a block  920  there is performed attaching the semiconductor die  202  to the die pad  102  thereby resulting in the partially assembled package  200 . At a block  930  there is performed a process of electrically coupling the contact pads  204  of the semiconductor die  202  with respective proximal ends  112 ,  114  of the lead fingers  106 . This electrically coupling is typically performed by a conventional wire bonding process and results in the formation of the partially assembled electrically coupled package  300 . 
     At a block  940 , the method  900  performs encapsulating the die  202 , the die pad  102  and the proximal ends  112 ,  114  of the lead fingers  106  with the encapsulation material to provide the housing  402 . The housing  402  includes the edges  404  from which the lead fingers  106  extend and there may be individual slots (slots  406 ) in the underside  706  for each individual lead finger of the second set of lead fingers. Alternatively, there may be a continuous single slot on the underside  706  or a single slot on each edge of the underside. 
     At a block  950  a process of detaching the lead fingers from the frame member  104  is performed to provide the singulated semiconductor die package  500 . The method  900 , at a block  960 , then performs bending the lead fingers into the first set  108  and second set of lead fingers  110  to provide the semiconductor die package  600  which is typically a Quad Flat type package. 
     Advantageously, the present invention potentially reduces or alleviates the possibility of short circuit faults between adjacent lead fingers  106  because the slots  406  and recesses  408  that form the channels  410  separate and provide an insulating barrier between adjacent lead fingers  106 . The slots  406  have the further advantage of increasing the spacing of the circuit board pads to which the mounting feet are soldered, and the slots may also potentially reduce the height or footprint the semiconductor die package  600 . 
     The description of the preferred embodiments of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or to limit the invention to the forms disclosed. It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiment disclosed, but covers modifications within the spirit and scope of the present invention as defined by the appended claims.