Abstract:
The specification describes a leadframe that is aimed at high-performance digital IC devices with high-pin counts, and packaged using wire bond technology. According to the invention the configuration of the paddle is modified to add a new dimension to the leadframe design. In a preferred embodiment, one or more slots are formed in the paddle to allow the length of selected wire bonds to be reduced. This reduces the susceptibility of these selected leads to parasitic capacitances. The selected leads are typically those that carry very high-speed signals.

Description:
FIELD OF THE INVENTION 
     The field of the invention is integrated circuit (IC) packaging and more specifically to design of leadframes for overmolded wire-bonded IC packages. 
     BACKGROUND OF THE INVENTION 
     Wire-bonding technology for integrated circuit packages remains a staple in IC manufacturing. For high pin count devices with fine pitch it allows an element of precision that is difficult to match with flip-chip solder bump technology. Typical high-pin count packages, for example thin quad flat pack TQFP packages, have a square or rectangular paddle, on which the IC chip is bonded, with leads extending from the four sides. The leads are typically fanned out to transition the lead pitch from very fine at the IC chip, to a wider pitch at the wire bond sites, and even wider pitch at the interconnection sites, i.e. the lead ends. In state-of-the-art high-speed digital devices the length and configuration of the wire bonds and the leadframe fingers to which the wire bonds are attached adds a circuit element that needs to be controlled for optimum performance. A variety of leadframe designs have been developed to address these issues but improvements are continually sought. 
     STATEMENT OF THE INVENTION 
     We have designed a new leadframe, and IC package, that is aimed at high-performance digital IC devices with high-pin counts, and packaged using wire-bond technology. According to the invention the configuration of the paddle is modified to add a new dimension to the leadframe design. In a preferred embodiment, one or more slots are formed in the paddle to allow the length of selected wire bonds to be reduced. This minimizes the susceptibility of the wire bonds to these selected leads to parasitic resistances/inductances. The selected leads are typically those that carry very high-speed digital signals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a section view of a conventional TQFP IC package; 
         FIG. 2  is a schematic plan view of a typical leadframe design for the device of  FIG. 1 ; 
         FIG. 3  is a plan view showing the leadframe of  FIG. 2  after die bonding and wire bonding the IC chip; 
         FIG. 4  is a schematic plan view of a leadframe with a modified paddle geometry illustrating one embodiment of the invention; 
         FIG. 5  is a plan view showing the leadframe of  FIG. 4  after die bonding and wire bonding the IC chip; 
         FIG. 6  is a detailed view of the slot portion of  FIG. 5 ; 
         FIGS. 7–9  show different embodiments for the configuration of the selected leads that extend into the slot. 
     
    
    
     DETAILED DESCRIPTION 
     The invention will be illustrated and described using an exposed paddle thin quad flat pack IC package (eTQFP) as a prototype. However, it should be understood that the invention is not so limited. It may apply to a variety of wire-bonded IC devices. Typically these will be overmolded plastic packages, as in the example illustrated here, or may be plastic cavity packages, or any other type of high pin count packaging. Also to be considered within the scope of the invention are IC or electrical component packages in which the paddle configuration is modified to influence other aspects of the electrical performance of the device. The package may contain hybrid ICs or integrated passive device (IPD) chips. It may contain optical sub-assemblies such as MEMS devices packaged with digital chips. 
     With reference to  FIG. 1 , a semiconductor chip  11  is shown bonded to a leadframe. The leadframe comprises paddle  12  with solder, or conductive adhesive,  13  as the medium for bonding the chip to the paddle. In this package design the paddle is exposed on the bottom of the package to allow a ground I/O connection to be made directly to the exposed paddle. The paddle also comprises a lead finger  14  extending from the side of the package. This form of semiconductor device package is characterized by wire bonds  15  bonded between bond pads  16  on the IC chip and leads  14 .  FIG. 1  also shows the usual plastic encapsulant  17 . 
       FIG. 2  is a plan view of the leadframe of  FIG. 1  that schematically shows the organization of the lead fingers  14  that extend away from paddle  12 . The leads in this design are fanned for the reason described above. The fanned array also provides approximately equal wire bonds lengths. The paddle  12  typically has square shape, with four edges as shown. In the general case the paddle has a quadrilateral shape, with length L, width W, and four edges. In  FIG. 2 , the plurality of lead fingers extends from the paddle along the four edges. Other designs are possible, but the lead fingers will usually extend from at least two edges. 
       FIG. 3  shows the leadframe of  FIG. 2  with the IC chip die bonded to paddle  12 . The IC chip in this design has a square shape but, again, in the general case the IC chip has a quadrilateral shape with length L′, width W′, wherein L′ is less than L (of the paddle) and W′ is less than W, and wherein the IC chip substantially covers the paddle except for the exposed regions along the edge of the paddle. (The exposed regions are a consequence of L′ and W′ being less than L and W respectively.)  FIG. 3  also shows the wire bonds  15  between the IC chip and the leads  14 . Due to the fanning of the lead fingers as described above, and the curved configuration of the array of lead fingers, the length of all of the wire bonds in the array is approximately equal. 
       FIG. 4  illustrates one embodiment of the invention. The paddle  12  is provided with one or more slots  21 .  FIG. 4  shows four slots  21  symmetrically placed one on each side of paddle  12 . However, there may be any number of slots, and they may be positioned at any place on the edge of the paddle. The center location is preferred because the slot geometry essentially matches the shape of the array of extended lead fingers. The slots will typically have a width S L , as measured along the length L (or width) of the paddle, that is substantially less than L, normally less than 0.25L. The width of the slot S W  preferably occupies most of the space allowed by the difference W–W′, for example, at least 50% and preferably 75% of W–W′. 
     The slots  21  accommodate a selected group of two or more lead fingers  22  that extend into the slots as shown in  FIG. 4 . This allows the wire bonds from the IC chip to the selected group of lead fingers to have substantially reduced length. 
       FIGS. 5 and 6  illustrate this in more detail.  FIG. 5  shows the leadframe of  FIG. 4  with the IC chip  11  attached and the wire bonds  15  bonded between the IC chip and lead fingers  14 . The selected group of lead fingers  22  extends into slot  21  as shown. Wire bonds  15 ′ have a length substantially shorter than wire bonds  15  to the conventional lead fingers, i.e. those that do not extend into a slot. In the preferred embodiment, the length of wire bonds  15 ′ is less than one-half of the length of the wire bonds  15 . In some cases it may be convenient to not use rectangular slots, but rather to slope the sidewalls of one or more slots to match the shape of the array of extended lead fingers in the region where the lead fingers extend within a slot. 
     From the description above and from  FIGS. 4–6  it is evident that the selected group of lead fingers  22  is substantially less than the overall number of lead fingers in the package. As shown in  FIG. 4 , there is a total of 160 lead fingers in the package, of which 28, or 17.5%, are extended according to the teachings of the invention. Obviously that percentage may be larger or smaller. Typically it will fall in a range of about 3–30%. 
     On examination of  FIGS. 5 and 6 , it will occur to those skilled in the art that if the pitch of all the fingers at the outer ends of the fingers, i.e. the outer edge of the fan array, is approximately the same, then the portion of the lead fingers that extends into the slot (the inner end of those fingers) will have a pitch less than the pitch at the inner end of the lead fingers that are not extended. This follows since all the lead fingers converge, and the lead fingers that extend into the slot are longer. Accordingly, it may be advantageous to adjust the spacing of the extended lead fingers to a pitch different from the pitch of the shorter lead fingers. An embodiment of this is shown in  FIG. 7 , where the extended lead fingers  72 , i.e. the lead fingers that extend into the slot, have a pitch at their outer end that is larger than the pitch at the outer end of the shorter lead fingers  14 . This results in the pitch of the extended lead fingers as they extend into the slot being approximately equal to the pitch at the inner end of the shorter fingers  14 . This expedient may be needed where the pitch at the inner end of all the lead fingers meets a minimum design rule. 
     Another alternative design is shown in  FIG. 8 . Here some of the extended lead fingers  82  are bent in a way that preserves the pitch of the inner ends of those lead fingers at a pitch that matches the pitch of the inner ends of the other lead fingers  14  in the array. 
     It may occur to those skilled in the art that by extending the length of selected lead fingers, the impedance of those fingers will change. To address this, according to the invention, the shape of the lead fingers is modified. An example of this is shown in  FIG. 9 , where the extended lead fingers  92  are tapered to adjust the impedance of the extended lead fingers. 
     It should be pointed out that any of the expedients described above may be used in alone, or in any suitable combination. 
     Various additional modifications of this invention will occur to those skilled in the art. All deviations from the specific teachings of this specification that basically rely on the principles and their equivalents through which the art has been advanced are properly considered within the scope of the invention as described and claimed.