Abstract:
A high-frequency package comprises a ground lead occupying a side of the high-frequency package; and a signal lead comprising at least a protrusion protruding from a central portion of the signal lead; wherein the ground lead and the signal lead perform as a transmission line, and the at least a protrusion forms capacitance of the transmission line.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional application No. 62/204,972, filed on Aug. 13, 2015 and incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a high-frequency package, and more particularly, to a high-frequency package capable of reducing high-frequency loss. 
     2. Description of the Prior Art 
     Future mobile communication systems and satellite communication systems are usually required to operate at high frequencies. However, a traditional package is not customized for high frequency operation. Significant loss is caused at high frequencies, and degrades performance of the package. In detail, in the traditional package, a wire bonding is performed to connect a die with leads via bonding wires. After the wires are bonded, a molding process is performed. Since the molding compound is usually made of a lossy material, an inductive effect is unavoidable and a serious loss at high frequencies is caused. 
     For example, please refer to  FIGS. 1A-1C , which are schematic diagrams of a sectional side view, a top view and a bottom view of a package  10  in the prior art. In the package  10 , a die  100  is bonded on a die pad  102  and connected to leads  104  via bonding wires  106 . The bonding wires  106  and the leads  104  would be covered by a molding compound after the molding process, and thus, inductance is formed around the bonding wires  106  and the leads  104 . Notably, a top surface of the die  100  is not coplanar with a top surface of the adjacent leads  104 , such that the bonding wires  106  is required to be sufficiently long, which causes a significant inductive effect and brings serious loss at high frequencies. 
     Therefore, it is necessary to improve the prior art. 
     SUMMARY OF THE INVENTION 
     It is therefore a primary objective of the present invention to provide a high-frequency package capable of reducing high-frequency loss, to improve over disadvantages of the prior art. 
     The present invention discloses a high-frequency package. The high-frequency package comprises a ground lead connected to a die, occupying a side of the high-frequency package; and a signal lead comprising at least a protrusion protruding from a central portion of the signal lead; wherein the ground lead and the signal lead perform as a transmission line, and the at least a protrusion forms capacitance of the transmission line. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-1C  are schematic diagrams of a sectional side view, a top view, and a bottom view, respectively, of a package in the prior art. 
         FIG. 2  is a schematic diagram of a circuit model 
         FIGS. 3A and 3B  are schematic diagrams of a top view and a sectional side view, respectively, of a high-frequency package according to an embodiment of the present invention. 
         FIG. 4  is a frequency response diagram of transmission coefficient. 
         FIGS. 5A-5H  are schematic diagrams of signal leads and corresponding ground leads according to embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Inductance formed around the bonding wires and the leads is illustrated in  FIG. 2 , a schematic diagram of a circuit model of a package. In  FIG. 2 , an inductance L 1  represents an inductance around the bonding wires, and an inductance L 2  represents an inductance around the leads. Compensating (or reducing) the inductance effect brought by the inductance L 1  and the inductance L 2  would alleviate loss at high frequencies and enhance package performance at high frequencies. 
     To compensate the inductance effect brought by the inductance L 1  and the inductance L 2 , the leads are designed (or perform) as transmission lines to form capacitance effect.  FIGS. 3A and 3B  are schematic diagrams of a top view and a sectional side view, respectively, of a high-frequency package  30  according to an embodiment of the present invention, where  FIG. 3B  illustrates the sectional side view of the high-frequency package  30  along an A-A′ line (in  FIG. 3A ). The high-frequency package  30  comprises a signal lead  346  and a ground lead  344 . The signal lead  346  and the ground lead  344  are designed as transmission lines and disposed on a side S 1  of the high-frequency package  30 . The ground lead  344 , occupying the side S 1  of the high-frequency package  30 , comprises ground segments  340 ,  342 . A hollow area is formed within the ground lead  344 , and the ground segments  340 ,  342  are separated with each other by the hollow area. The signal lead  346  is disposed within the hollow area. The signal lead  346  comprises a central portion  3460  and protrusions  3462 ,  3464 . The protrusions  3462 ,  3464  protrude from the central portion  3460  of the signal lead  346 . A shape of the hollow area varies in corresponding to a shape of the signal lead  346 , such that the ground lead  344  is separated from the signal lead  346 . In such a situation, the protrusions  3462 ,  3464  form a capacitance effect of transmission line. The capacitance effect formed by the protrusions  3462 ,  3464  plays a role of compensating the inductance effect brought by the inductance L 1  and the inductance L 2 , and utilizing the signal lead  346  and the ground lead  344  to perform as transmission line would effectively reduce the inductance L 2  around the leads, such that the high-frequency performance of the high-frequency package  30  is enhanced. 
     In addition, the signal lead  346  is connected to a die  300  via bonding wires  306  and configured to deliver a high frequency signal of the die  300 . The ground lead  344  is connected to a ground of the die  300  via the bonding wires  306 . Notably, the ground lead  344  connected to the ground of the die  300  is held at a fixed ground voltage while the signal lead  346  delivers the high frequency signal of the die  300 . Under a condition that the ground lead  344  surrounds the signal lead  346  (as shown  FIG. 3A ), the ground lead  344  and the signal lead  346  form as aground-signal-ground (GSG) structure. Preferably, a top surface of the ground lead  344  and a top surface of the signal lead  346  are at a same horizontal level. That is, the top surface of the ground lead  344  is coplanar (or aligned) with the top surface of the signal lead  346 . Therefore, performance of the high-frequency package  30  at high frequencies is enhanced. Furthermore, the bonding wire  306  connecting the die  300  and the ground lead  344  is disposed between the bonding wires  306  connecting the die  300  and the signal lead  346 , which means that the bonding wires  306  also form as a GSG structure so as to enhance the high-frequency performance of the high-frequency package  30 . 
     As can been see, utilizing the signal lead  346  and the ground lead  344  to perform as transmission line would effectively reduce the inductance L 2  around the leads. Further, forming the capacitance effect of the transmission line using the protrusions  3462 ,  3464  of the signal lead  346  would compensate the inductance effect brought by the inductance L 1  and the inductance L 2 . Thus, performance of the high-frequency package  30  at high frequencies is enhanced. 
     In addition, performance of the high-frequency package  30  at high frequencies may be further enhanced by reducing the inductance L 1  around the bonding wires. To reduce the inductance L 1  around the bonding wires  306 , lengths of the bonding wires  306  may be shortened by lowering a horizontal level of a top surface of the die  300 . For example, as  FIGS. 3A and 3B  show, a notched area  308  may be formed on a die pad  302  of the high-frequency package  30 , such that after the die  300  is bonded on the die pad  302 , a top surface  320  of the die  300  would substantially align with top surfaces  324  of adjacent leads, where the adjacent leads may be the signal lead  346  or the ground lead  344 . In general, a difference in horizontal level between the top surface  320  of the die  300  and the top surfaces  324  of the adjacent leads is suggested to be smaller than 60% of a thickness of the die  300 . Preferably, the top surface  320  is coplanar (or aligns) with the top surfaces  324 . Therefore, the lengths of the bonding wires  306  are shortened, and the inductance L 1  is reduced. 
     The high-frequency performance of the high-frequency package  30  maybe evaluated by a frequency response diagram of insertion loss (i.e., the scattering parameter S 21 ) of the package  10  and the high-frequency package  30 , as shown in  FIG. 4 . In  FIG. 4 , a dashed line represents the frequency response of insertion loss of the package  10 , and a solid line represents that of the high-frequency package  30 . As can be seen from  FIG. 4 , a characteristic frequency (at which the insertion loss is 1 dB) of the package  10  is only 26 GHz. By forming the capacitance of transmission line using the protrusions (to compensate the unavoidable inductance) and shortening the bonding wires, a characteristic frequency of the high-frequency package  30  is significantly improved to approach 38 GHz. In addition, ripples in frequency response under the characteristic frequency of the high-frequency package  30  are minor than the package  10 , which benefits an operation of the high-frequency package  30 . 
     Notably, the embodiments stated in the above are utilized for illustrating the concept of the present invention. Those skilled in the art may make modifications and alternations accordingly, and not limited herein. For example, methods of forming the notched area are not limited. The notched area is formed by either topside etching or backside etching, and not limited herein. In addition, the signal lead and the ground lead may be realized using microstrip line or coplanar waveguide, and not limited herein. 
     In addition, the die pad  302  is not limited to having the notched area  308  formed therein. The die pad may be downset. As long as the surface  328  of the die pad is lower than the top surfaces  324  of the leads  304  such that the top surface  320  of the die  300  is substantially aligned (coplanar) with the top surfaces  324  of the leads  304 , the requirements of the present invention is satisfied. 
     In addition, a shape of the signal lead is not limited. The shape of the signal lead may be modified according to the practical situation. For example, please refer to  FIGS. 5A-5H , which are schematic diagrams of signal leads and corresponding ground leads according to embodiments of the present invention. As shown in  FIGS. 5A-5H , the shape of the protrusion of the signal lead may be semicircle, rectangular, triangular, trapezoidal, etc. The protrusion may protrude from the central portion of the signal lead unilaterally or bilaterally. In addition, the protrusion may protrude from the central portion of the signal lead symmetrically or asymmetrically. As long as the corresponding ground lead surrounds the signal lead and separates from the signal lead, the signal lead and the ground lead are within the scope of the present invention. 
     In summary, the signal lead of the present invention includes the protrusions to form capacitance effect for compensating the inductance effect brought by the inductance L 1  and the inductance L 2 . In addition, the notched area is formed on the die pad, so to align the top surface of the die with the top surface of the leads, shorten the lengths of the bonding wires, and reduce the inductance L 1 . Thus, performance of the high-frequency package is improved. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.