Patent Document

CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of pending U.S. patent application Ser. No. 12/758,141, filed on Apr. 12, 2010, which is a Continuation of pending U.S. patent application Ser. No. 11/539,239, filed on Oct. 6, 2006, which claims the benefit of provisional Application No. 60/731,779, filed on Oct. 31, 2005, the entirety of which are incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to leadframes for semiconductor packages and in particular to leadframes for high frequency applications. 
     2. Description of the Related Art 
     Semiconductor dies are enclosed in plastic packages that provide protection from hostile environments and enable electrical interconnection between the semiconductor die and a printed circuit board via a metal leadframe. The conventional leadframe type semiconductor package has a central supported die pad for supporting semiconductor die, a plurality of leads peripherally located therein, a plurality of bonding wires for electrically connecting the semiconductor die to the leads, and a mold compound, such as plastic, for encapsulating these components in a package structure. 
     In most semiconductor package configurations, a portion of the leadframe is internal to the package, (i.e., completely surrounded by the mold compound). Portions of the leads of the leadframe typically extend externally from the package body for electrically connecting the package to the printed circuit board. 
     In the electronics industry, there is continued demand for developing semiconductor dies which have increasing processing speeds and higher degrees of integration. For a semiconductor package to accommodate these enhanced semiconductor dies, the number of leads included in the semiconductor package must be significantly increased. To avoid an undesirable increase in the size of the semiconductor package attributable to the increased number of leads, a common practice is to reduce or narrow the spacing between the leads. However, a decreased spacing between the leads increases the capacitance between the leads, and increases the level of self inductance and mutual inductance. This inductance adversely affects the quality of signals transmitted on the leads of the leadframe by increasing signal reflections; causing greater impedance mismatches. 
     Especially, in high frequency applications the semiconductor package has the greatest influence on total performance of the circuit, and one of the main causes of performance degradation is inductance of the interconnections between chip and printed circuit board. Therefore, as the operating frequency of these circuits increases, there is a need for even lower impedance mismatches packages. As shown in  FIG. 2A , conventionally, the lead route or lead distribution of the leadframe is substantially symmetrical for desired producibility or manufacturability and lower process cost, but do negatively affect the impedance match. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention provides leadframe for semiconductor packages and a method utilizing the same, providing flexible impedance match design, improving the electrical performance of the resulting electronic products. 
     The invention provides a leadframe for semiconductor packages comprising a die pad, a side rail, a tie bar, and a plurality of leads. The side rail is around the die pad. The tie bar connects the die pad and the side rail. The leads extend from the side rail to close proximity to the die pad. The lead includes a first lead and a second lead being at opposite locations of the leadframe relative to a center line through the die pad. The first and second leads are substantially asymmetrical with each other relative to the center line and have different impedance values. The plurality of leads are disconnected from each other. 
     The invention further provides a method of achieving a desired impedance value for a leadframe for semiconductor packages. The method includes providing a leadframe comprising a die pad, a side rail around the die pad, a tie bar connecting the die pad and the side rail, and a plurality of leads extending from the side rail in close proximity to the die pad; and designing a layout of the plurality of leads comprising a first lead and a second lead being at opposite locations of the leadframe relative to a center line through the die pad. The first and second leads are substantially asymmetrical with each other relative to the center line and have different impedance values. The plurality of leads are disconnected from each other. 
     Further scope of the applicability of the invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIGS. 1A through 1E  illustrate top views of a preferred embodiment of the invention; 
         FIGS. 2A through 2B  illustrate a conventional symmetrical leadframe; 
         FIGS. 3A and 3B  illustrate a first experimental example of the invention; and 
         FIGS. 4A and 4B  illustrate a second experimental example of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
       FIGS. 1A through 1E  are top views of a preferred embodiment of the invention. Referring to the  FIGS. 1A through 1E , the leadframe comprises a die pad  10 , a side rail  30 , tie bars  21  through  24 , and a plurality of leads. The side rail  30  is around the die pad  10 . The tie bars  21  through  24  connect the die pad  10  and the side rail  30 . The leads extend from the side rail  30  close proximity to the die pad  10 . In some cases, the side rail  30  is removed in a trimming or separation step of the subsequent semiconductor packaging process. 
     The invention provides the capability to vary the impedance of an electrical device. As examples, impedance can be controllably varied by changing: the length of the leads; the pitch of the leads; the spacing between the leads; and/or the width of the leads. In consequence, in order to respectively adjust the impedance of the leads, the invention provides an asymmetrical leadframe structure. 
     In  FIGS. 1A through 1E , each lead has a corresponding lead relative to a predetermined center line at opposite location of the leadframe. In this embodiment, an exemplary center line  50  is shown in  FIGS. 1A through 1E . For example, the lead  241  corresponds to respective leads  141   a  through  141   e  in respective  FIGS. 1A through 1E , and the lead  245  corresponds to the lead  145 . In this embodiment, the leadframe comprises a pair of corresponding leads including the lead and the corresponding lead, substantially asymmetrical to each other. Specifically, this asymmetrical design serves for impedance matching. For example, the lead  241  is substantially asymmetrical with the respective leads  141   a  through  141   e  in  FIGS. 1A through 1E  relative to the predetermined center line  50 .  FIGS. 1A through 1E  show various examples of asymmetrical leads for the asymmetrical leadframe structure. 
     In some cases, the corresponding lead of a specific lead depends on the selected center line, such as the center line  50  of this embodiment. In  FIG. 1A , for example, the lead  141   a  corresponds to the lead  241  relative to line center  50 . Also, the lead  141   a  corresponds to the lead  146  relative to a center line (not shown) passing through the space between the leads  143  and  144 . Further, the lead  141   a  corresponds to the lead  341  relative a center line (not shown) passing through and aligned with the tie bars  21 . In this embodiment, the center line  50  is utilized as the exemplary center line in subsequent discussion. 
     Referring to  FIGS. 1A through 1E , lead  141   a  through  141   e  are asymmetrical to lead  241  relative to center line  50 , either the geometry of the lead or the route of the lead. Accordingly, the pair of corresponding asymmetrical leads related to the center line means that they are not identical in shape, dimension, or the relationship of itself to other corresponding parts of the leadframe. 
     In  FIG. 1A , the leads  141   a  and  241  have different lengths, and thus, are considered to be asymmetrical. In consequence, comparing the pair of corresponding lead  141   a  and lead  241 , the varied lead length results in varied resistance of the lead  141   a.  Thus, a desired impedance value can be achieved by adjusting the lead length. 
     In  FIG. 1B , the leads  141   b  and  241  have substantially the same widths. However, space S 1  between lead  141   b  and the adjacent lead, for example lead  142 , is larger than space S 2  between lead  241  and the corresponding adjacent lead, for example lead  242 . Further, the pitch P 1  is also larger than the pitch P 2 . Thus, the leads  141   b  and  241  are considered to be asymmetrical. In consequence, comparing the pair of corresponding lead  141   b  and lead  241 , varied space between the leads results in varied inductance between the leads. Thus, a desired impedance value can be achieved by adjusting space between the leads. 
     In  FIG. 1C , the leads  141   c  and  142   c  respectively have different widths from the corresponding leads  241  and  242 . Further, space S 1  between lead  141   c  and the adjacent lead, for example lead  142   c,  is less than space S 2  between lead  241  and the corresponding adjacent lead, for example lead  242 . And thus, the leads  141   c  and  241  are considered to be asymmetrical, and the leads  142   c  and  242  are considered to be asymmetrical. In consequence, comparing the pair of corresponding lead  141   c  and lead  241 , or lead  142   c  and lead  242 , varied lead width results in varied resistance of the lead. Thus, a desired impedance value can be achieved by adjusting the lead width. 
     In  FIG. 1D , the pitch P 1  between the lead  141   d  and the adjacent lead, such as lead  142   d,  is larger than the pitch P 2  between the lead  241  and the corresponding adjacent lead, such as lead  242 . Thus, the leads  141   d  and  241  are considered to be asymmetrical. In consequence, comparing the pair of corresponding lead  141   d  and lead  241 , varied lead pitch results in varied inductance between the leads. Thus, a desired impedance value can be achieved by adjusting the lead pitch. 
     In  FIG. 1E , pitch P 1  between the lead  141   e  and the adjacent lead, such as lead  142 , is less than the pitch P 2  between the lead  241  and the corresponding adjacent lead, such as lead  242 . Thus, the leads  141   e  and  241  are considered to be asymmetrical. In consequence, comparing the pair of corresponding lead  141   e  and lead  241 , the varied lead pitch results in varied inductance between the leads. Thus, a desired impedance value can be achieved by adjusting the lead pitch. 
     Next, a conventional symmetrical leadframe is shown in  FIGS. 2A through 2B , and two experimental examples of the invention are respectively shown in  FIGS. 3A ,  3 B and  FIGS. 4A ,  4 B verifying the improved performance of the embodiment. 
     In  FIG. 2A , a top view of a conventional semiconductor package is shown. The package comprises a leadframe, a semiconductor chip  2100  attached to a die pad  2010  of the leadframe, a plurality of bonding wires  2200  electrically connecting the semiconductor chip  2100  and the leads of the leadframe, and an encapsulant (not shown) encapsulating the semiconductor chip  2100 , the leadframe, and the bonding wires  2200 . The leadframe comprises a die pad  2010 , four tie bars  2021  through  2024  for supporting die pad  2010 , and a plurality of leads. The side rail was trimmed during the packaging process. The conventional leadframe which the routes of the leads are substantially symmetrical. 
     In  FIG. 2B , a magnified drawing of the exemplary leads  1145 ,  1146 ,  1148 , and  1149  in  FIG. 2A  is shown. For an electronic signal with a frequency of approximately 750 MHz, the differential impedance values of a differential pair of the leads  1145  and  1146  is near 68 ohm. Similarly, the differential impedance value of a differential pair of the leads  1148  and  1149  is near 68 ohm. And the single-ended impedance values of those leads  1145 ,  1146 ,  1148 , and  1149  are near 50 ohm. In some case, however, the desired differential impedance values for some leads are required between 80 and 120 ohm, and preferably approximately 100 ohm. Or the desired single-ended impedance values for some leads are required between 40 and 60 ohm, and preferably approximately 50 ohm. Thus, the utilization of the conventional symmetrical leadframe cannot achieve the desired impedance value. 
     In  FIG. 3A , a top view of a semiconductor package of a first experimental example of the invention is shown. Compared to that shown in  FIG. 2A , the lengths of the leads  1145 ,  1146 ,  1148 , and  1149  are reduced by D, which is approximately 60 mils in this embodiment. Thus, the leadframe utilized in the package shown in  FIG. 3A  can act as another embodiment of the invention. 
     A magnified drawing of the shortened leads  1145 ,  1146 ,  1148 , and  1149  is shown in  FIG. 3B . For an electronic signal with a frequency of approximately 750 MHz, the differential impedance values of a differential pair of the leads  1145  and  1146  is near 84 ohm, which achieve the desired values. Similarly, the differential impedance value of a differential pair of the leads  1148  and  1149  is near 84 ohm, which achieves the desired values, too. And the single-ended impedance values thereof are near 58 ohm. It is appreciated that the package of the first experimental example utilizes the leadframe structure of the invention to cause the impedance values of the predetermined leads fulfilling the desired values for impedance match. 
     In  FIG. 4A , a top view of a semiconductor package of a second experimental example of the invention is shown. Compared to that shown in  FIG. 3A , spaces between the leads  1145  and  1146 , and the space between the leads  1148  and  1149  are broader. Thus, the leadframe utilized in the package shown in  FIG. 4A  can act as another embodiment of the invention. 
     A magnified drawing of the leads  1145 ,  1146 ,  1148 , and  1149  of  FIG. 4A  is shown in  FIG. 4B . For an electronic signal with a frequency of approximately 750 MHz, the differential impedance values of a differential pair of the leads  1145  and  1146  is near 108 ohm, which achieve the desired values. Similarly, the differential impedance value of a differential pair of the leads  1148  and  1149  is near 108 ohm, which achieves the desired values, too. And the single-ended impedance values thereof are near 62 ohm. It is appreciated that the package of the second experimental example utilizes the leadframe structure of the invention to cause the impedance values of the predetermined leads fulfilling the desired values for impedance match. 
     The efficacy of the inventive leadframes at developing asymmetrical lead route or lead distribution provides effective impedance match for the resulting products. 
     While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.

Technology Category: 5