Abstract:
A chip package having a lead frame, a chip, a plurality of bonding wires, and an insulation material is provided. The lead frame comprises a die pad, a plurality of leads, a plurality of signal pads and a plurality of non-signal pads. The signal pads and non-signal pads are underneath the signal leads and non-signal leads respectively. The non-signal pad is directly connected to a non-signal plane in the circuit board through its own vias. The signal pad has a structure which extends toward its adjacent non-signal pads. With the signal pad size enlarged, the capacitance between the non-signal plane in the circuit board and the signal pad is increased. The increased capacitance compensates the inductance induced from the bonding wires and improves the response of the signal propagation path for RF applications.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a mounting pad structure of wire-bonding type lead-frame packages. More particularly, the present invention relates to a mounting pad structure which can improve the performance of wire-bonding type lead-frame packages.  
         [0003]     2. Description of the Related Art  
         [0004]     Modern electronic products typically enclose a semiconductor chip and a carrier for electrically connecting with the semiconductor chip. At present, there are three major techniques for connecting a chip to a carrier, namely, a wire-bonding process, a flip-chip process and a tape-automated-bonding (TAB) process. If the carrier is a lead frame, the wire-bonding process is often used to connect the chip with the leads on the lead frame. Bonding wires of a package induces parasitic inductance which becomes more significant when the operating frequency is beyond several giga-hertz (GHz) for Radio Frequency (RF) products. The parasitic inductance has adverse effects on the circuit performance. Therefore, a method for reducing the parasitic inductance and improving the frequency response at high frequencies shall be introduced.  
         [0005]      FIG. 1A  is a schematic cross-sectional view of a conventional QFN (Quad Flat Non-Lead) package mounted on a circuit board for RF applications.  FIGS. 1B and 1C  are perspective view and top view respectively of the coplanar bonding wire structure of single-ended mode shown in  FIG. 1A . As shown in  FIGS. 1A, 1B  and  1 C, the chip package  100  comprises a chip  110 , a lead frame  120 , a plurality of bonding wires  130  and an insulation material  140 . The lead frame  120  has a die pad  121  and a plurality of ground leads  122  and signal leads  123 . The ground leads  122  and signal leads  123  are distributed evenly at the peripheral area of the die pad  121 . The chip  110  is attached to the die pad  121  through adhesive glue. The bonding wires  130  connecting the chip  110  to the respective leads  122 ,  123  are formed in a wire-bonding process. The die pad  121  and the ground lead  122  are electrically grounded to a ground plane  170  through a ground pad  181 , a die pad landing and ground vias  160 . The insulation material  140  encapsulates the chip  110 , the lead frame  120  and the bonding wires  130 . The die pad  121 , the ground leads  122 , and signal leads  123  of the chip package  100  are attached to the circuit board  190  through the application of adhesive materials.  
         [0006]      FIG. 1D  is a top view of the mounting pad structure of single-ended mode shown in  FIG. 1A . In the conventional pad design, the signal pad  182  is underneath the signal lead  123  and has a shape which corresponds to the signal pad  182 . Similarly for the ground pad  181 , it is underneath the ground lead  122  and has a shape which corresponds to the ground pad  181 . The ground pad  181  is connected to the die pad landing which is underneath the die pad  121  and is electrically grounded to a ground plane  170  through ground vias  160 .  
         [0007]      FIG. 2A  is a perspective view of a conventional coplanar bonding wire of differential mode.  FIG. 2B  is a top view of a conventional coplanar bonding wire of differential mode. The differential mode involves one transmission line  231  carries a positive signal and the other transmission line  231  carries a negative signal for signal pins. The signals are equal in amplitude and opposite in polarity. The two transmission lines  231  have a differential impedance of around 100 Ohms. The two signal leads  223  are adjacent to each other in a differential-mode wire-bonding type package. To the opposite side which is adjacent to the other signal lead  223 , there is an adjacent ground lead  222 . Underneath the signal lead  223 , there is a signal pad  282  and similarly, there is a ground pad  281  underneath the ground lead  222 . The size and shape of the ground pad  281  and the signal pad  282  correspond to the ground lead  222  and the signal lead  223  respectively. The ground pad  281  is connected to the die pad landing which is underneath the die pad and is electrically grounded to a ground plane  170  through ground vias  160 .  
       SUMMARY OF THE INVENTION  
       [0008]     Accordingly, the present invention is to provide mounting pad structures which can improve the performance of wire-bonding type packages.  
         [0009]     In accordance with the purpose of the invention, as embodied and broadly described herein, the invention extends the signal pad towards its adjacent non-signal pads to overlap with part of the non-signal leads which enlarges the size of the signal pad. The signal pad can extend to the direction of one adjacent non-signal pad or the directions of both two adjacent non-signal pads. The non-signal pads are connected to the non-signal plane through their own vias which leave some spaces for the signal pad to extend. The enlargement of the signal pad increases the capacitance between the signal pad and the non-signal plane in the circuit board and helps to compensate the parasitic inductance induced from the bonding wires. As a result, the frequency response is substantially improved.  
         [0010]     Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.  
         [0012]      FIG. 1A  is a schematic cross-sectional view of a conventional QFN (Quad Flat Non-Lead) package mounted on a circuit board for RF applications.  
         [0013]      FIG. 1B  is a perspective view of the coplanar bonding wire structure of single-ended mode shown in  FIG. 1A .  
         [0014]      FIG. 1C  is a top view of the coplanar bonding wire structure of single-ended mode shown in  FIG. 1A .  
         [0015]      FIG. 1D  is a top view of the mounting pad structure of single-ended mode shown in  FIG. 1A .  
         [0016]      FIG. 2A  is a perspective view of a conventional coplanar bonding wire of differential mode.  
         [0017]      FIG. 2B  is a top view of a conventional coplanar bonding wire of differential mode.  
         [0018]      FIG. 3A  is a top view of a protrudent T-shaped mounting pad according to a first preferred embodiment of this invention.  
         [0019]      FIG. 3B  is a perspective view of a protrudent T-shaped mounting pad according to a first preferred embodiment of this invention.  
         [0020]      FIG. 4A  is a graph showing the S 11  frequency response curves in various cases.  
         [0021]      FIG. 4B  is a Smith chart showing the frequency response curves in various cases.  
         [0022]      FIG. 5A  is a perspective view of protrudent L-shaped mounting pads according to a second preferred embodiment of this invention.  
         [0023]      FIG. 5B  is a top view of protrudent L-shaped mounting pads according to a second preferred embodiment of this invention.  
         [0024]      FIG. 6A  is a perspective view of protrudent L-shaped mounting pads with multi-stepped transmission lines according to a third preferred embodiment of this invention.  
         [0025]      FIG. 6B  is a top view of protrudent L-shaped mounting pads with multi-stepped transmission lines according to a third preferred embodiment of this invention.  
         [0026]      FIG. 7A  is a graph showing the S 11  frequency response curves in various cases of the differential mode.  
         [0027]      FIG. 7B  is a Smith chart showing the frequency response curves in various cases of the differential mode.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]     Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.  
       First Embodiment  
       [0029]      FIG. 3A  is a top view of a protrudent T-shaped mounting pad according to a first preferred embodiment of this invention.  FIG. 3B  is a perspective view of a protrudent T-shaped mounting pad according to a first preferred embodiment of this invention. Referring to  FIGS. 3A and 3B , instead of connecting ground pad  181  to the die pad landing underneath of the die pad  121  for connection to the ground plane  170  as seen in the prior art, the ground pads  381  are now connected to the ground plane  370  through their own vias  360 . The ground pads  381  are extended in a direction which is away from the chip  110  (not shown in the figure) and then they are grounded through vias  360 . With this new ground pad  381  design, some free spaces are available for the signal pad  382  to extend toward its adjacent ground pads  381 . The signal pad  382  is extended towards its adjacent ground pads  381  to overlap with part of the adjacent ground leads  322 , but without touching the adjacent ground pads  381  and the adjacent ground lead  322 . In addition, the signal pad  382  can overlap with only a small part of the ground leads  322  or it can be extended to exceed the ground leads  322 . In this embodiment, a protrudent T-shaped signal pad is formed with two adjacent ground leads  322  on each side of the signal lead  323 . Since the protrudent T-shaped signal pad  382  has a larger size than the signal pad in the prior art, the capacitance between the signal pad  382  and the ground plane  370  in the circuit board can be increased. The increased capacitance can compensate the parasitic inductance induced from the bonding wires  130 , which improves the impedance matching and enhances the performance of the package at high frequencies. One advantage of the present invention is that the size of the signal pad  382  can be freely adjusted to achieve an optimum circuit performance.  
         [0030]      FIG. 4A  provides a clear picture of comparing the performance of the first preferred embodiment with the conventional pad design as seen in  FIG. 1D .  FIG. 4A  is a graph showing the S 11  frequency response curves in these 2 cases of singled-ended mode. The curve labeled “single” represents the frequency response of the conventional pad design of singled-ended mode, and the “single_Tpad” curve shows the frequency response of the invented T-shaped pad of singled-ended mode. This graph shows S 11 , impedance matching versus frequency in the frequency range of 1 to 6 GHz. It can be observed that frequency response of the invented T-shaped pad structure is superior to the conventional pad design, especially in the lower frequency range.  
         [0031]      FIG. 4B  is a Smith chart showing the frequency response curves for the conventional pad and the invented T-shaped pad. Once again, “single” represents the conventional pad design and “single_Tpad” represents the invented T-shaped pad. Since the top half of the Smith chart exhibits inductive impedance and the bottom half exhibits capacitive impedance, the best impedance matching is achieved with a S 11  curve located in the middle of the chart (indicated as 1.00 in  FIG. 4B ). The invented T-shaped pad curve, “single_Tpad” is closer to the center (indicated as 1.00 in  FIG. 4B ) than the conventional pad design curve, “single” means that the inductance has been compensated in the invented T-pad. This explains why better impedance matching can be obtained with the invented T-shaped pad.  
       Second Embodiment  
       [0032]      FIG. 5A  is a perspective view of protrudent L-shaped mounting pads according to a second preferred embodiment of this invention.  FIG. 5B  is a top view of protrudent L-shaped mounting pads according to a second preferred embodiment of this invention. The L-shaped mounting pads are applicable to differential-mode wire-bonding type packages. Referring to  FIGS. 5A and 5B , the ground pads  581  are connected to the ground plane  570  through their own vias  560 . The two signal leads  523  are adjacent to each other in a differential-mode wire-bonding type packages and contact the respective signal pads  582  which connect to 100-Ohm differential transmission lines for signal passing. To the opposite side which is adjacent to the other signal lead  523 , there is an adjacent ground lead  522 . Underneath the signal lead  523 , there is a signal pad  582  and similarly, there is a ground pad  581  underneath the ground lead  522 . Unlike the conventional pad design as seen in  FIGS. 2A and 2B , the signal pad  582  is extended towards its adjacent ground pad  581  to overlap with a part of the ground leads  522 . However, it shall be noted that the signal pad  582  shall not touch the ground pad  581  and the ground lead  522 . Two protrudent L-shaped signal pads  582  are formed in this differential mode circuit. As discovered earlier, when the signal pad  582  size is enlarged, the capacitance between the signal pad  582  and the ground plane  570  in the circuit board can be significantly increased. This increase in capacitance compensates the parasitic inductance induced from the bonding wires  130 . Hence, performance of the package at high frequencies can be substantially improved by enlarging the size of the signal pad  582 . In addition, by adjusting the size of the signal pad  582 , an optimum circuit performance can be achieved.  
       Third Embodiment  
       [0033]     To improve the frequency response even further, a multi-stepped transmission line  632  is instead of a conventional transmission line to connect the signal pad  682  in  FIGS. 6A and 6B . The multi-stepped transmission line  632  can contribute inductance which compensates the over-tuned capacitance from the large protrudent L-shaped pad  682 . To observe how the frequency response is affected by this multi-stepped transmission line  632 , reference is made to  FIGS. 7A and 7B .  FIG. 7A  is a graph showing the S 11  frequency response curves in various cases of the differential mode. There are 3 curves, labeled “diff”, “diff_Lpad” and “diff_stepL” in  FIG. 7A . “diff” represents the conventional pad design of differential mode as seen in  FIGS. 2A and 2B . “diff_Lpad” represents the protrudent L-shaped pad design of differential mode as seen in  FIGS. 5A and 5B . “diff_stepL” represents the protrudent L-shaped pad design with the multi-stepped transmission line of differential mode as seen in  FIGS. 6A and 6B . From the graph, it is observed that the frequency response of “diff_Lpad”, the invented L-shaped pad is enhanced comparing with “diff”, the conventional pad design. However, as frequency increases, the improvement becomes less significant and hence the multi-stepped transmission line is introduced to further improve the response. The “diff_stepL” curve, which is the protrudent L-shaped pad design with the multi-stepped transmission line, shows an excellent frequency response and it is no longer frequency dependent. There is an optimum point at around 5.5 GHz. The size of the L-shaped pad and the multi-stepped transmission line is adjustable to achieve an optimum frequency response at a particular frequency as desired.  
         [0034]      FIG. 7B  is a Smith chart showing the frequency response curves in various cases of the differential mode. Since the top half of the Smith chart exhibits inductive impedance and the bottom half exhibits capacitive impedance, the best impedance matching is achieved with a S 11  curve located in the middle of the chart. “diff_Lpad”, the protrudent L-shaped pad design has compensated the induced parasitic inductance and so this curve is closer to the center than “diff”, the conventional pad design. With the introduction of the multi-stepped transmission line, the “diff_stepL” curve occupies only the center of the chart, and therefore, a superior frequency response is achieved.  
         [0035]     The present invention is ideal for RF applications due to its excellent performance at higher frequencies. It is not only easy to implement, just by enlarging the signal pad size, but also there will be no extra cost involved in implementing this invention.  
         [0036]     Although ground pad/lead/plane are used as examples in the preferred embodiments, power pad/lead/plane may also be used to replace the ground pad/lead/plane. A term “non-signal” may be used to imply it is either a ground or a power.  
         [0037]     In summary, ground pads are directly connected to the ground plane in the circuit board though their own vias. Thus, some spare spaces are vacant for the signal pad to be extended toward its adjacent ground pads which has the effect of providing more capacitance to compensate the inductance induced from the bonding wires. The impedance is better matched and the frequency response between 1 GHz to 6 GHz is significantly improved. Moreover, the impedance matching can be further improved by inserting a multi-stepped transmission line between the signal pad and a 100-Ohm differential transmission line. In conclusion, the invented pad design exhibits excellent frequency response and makes it suitable for RF applications.  
         [0038]     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.