Patent Publication Number: US-2003234444-A1

Title: Lead frame

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to a conductive substrate, normally termed a lead frame, and a semiconductor package using such a conductive substrate.  
       [0003] 2. Description of the Prior Art  
       [0004] In order to reduce component cost and increase component circuit density, it is normal to fit more than one semiconductor chip into a semiconductor package. Many semiconductor packages use a conductive substrate, termed a lead frame, as a mounting surface for the semiconductor chips.  
       BRIEF SUMMARY OF THE INVENTION  
       [0005] The present invention seeks to provide an improved lead frame and improved semiconductor package.  
       [0006] Accordingly, the present invention provides a lead frame for use in a semiconductor package, the lead frame having at least two pedestals for supporting a chip during assembly of a semiconductor package.  
       [0007] The present invention also provides a conductive clip for a semiconductor package, the clip having at least two pedestals for electrically contacting a chip and supporting the chip during manufacture of a semiconductor package.  
       [0008] The present invention also provides a semiconductor chip for a semiconductor package, the chip having at least two solderable pads for electrically contacting a lead frame for supporting the chip during manufacture of a semiconductor package.  
       [0009] The present invention also provides a semiconductor package comprising a semiconductor chip and a clip and lead frame for electrically supporting the chip within the semiconductor package, wherein the chip has at least two solder pads on one surface thereof each of which is soldered to a respective pedestal on one of said lead frame and clip.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0010] The present invention is further described hereinafter, by way of example, with reference to the accompanying drawings, in which:  
     [0011]FIG. 1 is a partial section through a conventional semiconductor package;  
     [0012]FIG. 2 is a plan view of an upper surface of a chip of the package of FIG. 1; and  
     [0013]FIG. 3 is a inverse plan view of the chip of FIG. 2;  
     [0014]FIG. 4 is a partial section through a semiconductor package embodying a preferred form of the invention;  
     [0015]FIG. 5 is a plan view of an upper surface of a chip of the package of FIG. 4; and  
     [0016]FIG. 6 is a inverse plan view of the chip of FIG. 5. 
    
    
     DETAILED DESCRIPTION  
     [0017]FIG. 1 is a partial section through a semiconductor package  10  having several semiconductor chips  12  (only of one of which is shown) mounted on a conventional lead frame  14 . Electrical connection to each chip within the package is made by way of the lead frame  14  on one side (the underside) of each chip and a conductive formed clip  16  on the other (upper) side of each chip. The chip, lead frame and clip are all embedded in a moulded polymer insulation  19  to form the semiconductor package.  
     [0018]FIGS. 2 and 3 are respectively plan views of the upper and lower surfaces of one chip  12 . Each surface has a metallisation which is normally chosen as a combination of metal layers. These are typically four layers which may be patterned by etching through suitable masks. The top one or two layers are chosen to be solderable to tin based solders and form a solderable pad or metal area  18 ,  20  which is surrounded by an area of metal  22 ,  24  which is resistant to soldering. The metal areas  22 ,  24  are formed by the middle layers of the metallisation. The dimensions of the solderable metal areas  18 ,  20  are designed to match the areas of the pedestals on the lead frame and clip.  
     [0019] The chip shown in FIGS. 2 and 3 is designed for high voltage operation and has a periphery  26  which is designed to sustain the applied voltage. This voltage sustaining peripheral region  26  must be electrically isolated from the lead frame and the clip  16  and this is effected by the formation of a raised area or pedestal  28 ,  30  on each of the lead frame  14  and clip  16 . The dimensions of the pedestals are such that they lie within the area  22  (ideally matching the areas  18 ,  20 ) and do not contact the voltage sustaining periphery  26 .  
     [0020] The solders used to solder the chips to the lead frame and clip generally contain a proportion of tin and the materials of the lead frame and chip metalisation are chosen to offer good solderability to such solders.  
     [0021] During the soldering operation, the solder forms a liquid layer between the chip and the lead frame and clip. The only forces supporting the chip at this time and retaining it in position are the wetting forces and surface tension arising from metallurgical interaction between the solder and the metals of the chip surface, the lead frame and clip. When several chips are soldered into a single semiconductor package these forces may not be sufficient to prevent rotation or lateral movement of each chip, particularly as the pedestals and the solder pads  18 ,  20  of the chip, lead frame and clip lie on the same axis of rotation. As a result, there is a possibility of adjacent chips contacting one another. It is also possible for a chip to be displaced or rotated sufficiently to bring the chip or a part of a chip too close to the external surface of the moulded polymer  19  which may in turn effect the functionality of the chip over its useful life.  
     [0022] The semiconductor package shown in FIG. 4 is similar to that of FIG. 1 having a lead frame  114  and a clip  116  mounting a semiconductor chip  112 , the whole being embedded within moulded polymer  19 .  
     [0023] However, as can be seen from FIG. 6 each chip  112  in the package has two metalised, solderable pads  120 ,  150  surrounded by the area  24  and the peripheral region  26 . The pads  120 ,  150  are formed in the same manner as the area  20  of FIG. 3. The upper surface of the chip carries a single solderable metalised pad  18  and as can be seen, in this example, the arrangement of the metalised layers  18 ,  120 ,  150  and the voltage sustaining region  26  is the same as shown in FIG. 2.  
     [0024] Referring again to FIG. 4, the lead frame  114  is shown as having two pedestals  128 ,  158  each of which is intended to contact a respective one of the metalised areas  120 ,  150  of the chip  112 .  
     [0025] During manufacture, the pedestals  128 ,  158  of the lead frame  114  would be soldered to the respective pads  120 ,  150  of the chip. The pedestal  30  of the clip  116  would be soldered to be pad  18 .  
     [0026] Each chip  12  is therefore soldered in three areas, two on the chip undersurface and one on the upper surface. This arrangement reduces the possibility of rotation or movement of the chip during soldering, particularly with the solderable areas on the undersurface of the chip no longer being in the same axis of rotation as the solderable area on the top of the chip.  
     [0027] It will be appreciated that the chip may have two or more solderable pads on its upper or lower surface or on both surfaces for soldering to corresponding pedestals on the clip and lead frame. The solderable pads may also be part of a single continuous region with narrower sections connecting the areas which form the pads, for example, two overlapping circular pads in an approximate ‘ 8 ’ shape. It is also possible for the clip to have two or more pedestals and the lead frame only one, or both the lead frame and clip to have two or more pedestals  
     [0028] In one preferred embodiment, not shown in the drawings, only one pad is formed on the upper and lower chip surfaces with one pedestal on each of the lead frame and clip. However, the axes of rotation of the pedestals and pads are such that at least two are misaligned.