Abstract:
Packages and methods for mitigating plating stub effects. The semiconductor package includes an interposer substrate having a first side, a second side, a peripheral edge connecting the first side with the second side, a signal line on the first side, and an electrode pad on the first side. A semiconductor element is mounted on the first side of the interposer substrate. The semiconductor element is connected with the electrode pad by the signal line. A terminating resistor is mounted on the interposer substrate. A plating stub, which is located on the interposer substrate, has a first end portion that terminates near the peripheral edge of the interposer substrate and a second end portion that is electrically connected to the electrode. The first end portion is electrically connected through the terminating resistor to an electrical ground.

Description:
BACKGROUND 
       [0001]    The present invention relates to semiconductor fabrication and, in particular, to semiconductor packages having a semiconductor element mounted thereon, for transferring a high-speed signal, and particularly to the wiring configuration on an interposer substrate of the device package. 
         [0002]    Integrated circuit (“IC”) packaging is the final stage of semiconductor device fabrication per se, followed by IC testing. Contemporary electronic apparatuses include components that transmit very high-speed signals having a pulse width that corresponds, after being converted to frequency domain, to several hundreds of megahertz to one gigahertz. As technology advances, these signal speeds have been increased even more, and there is a demand for the transmission of a signal that corresponds to a frequency of several gigahertz or higher. 
         [0003]    Further, multifunctional ICs and IC modules have been developed that are like system large scale integration (“LSI”) chips, and these ICs are mounted in multi-terminal packages, such as ball grid arrays (“BGA”) or chip scale packages (“CSP”). That is, an IC having a high-speed signal transmission interface tends to be mounted in a multi-terminal semiconductor package, such as a BGA or a CSP. Ball grid array packages and their variants have existed since the 1970s. The BGA is descended from the pin grid array (“PGA”), which is a package with one face covered (or partly covered) with pins in a grid pattern. These pins are used to conduct electrical signals from the integrated circuit to the printed circuit board (“PCB”) on which it is placed. In a BGA, the pins are replaced by balls of solder stuck to the bottom of the package. The device is placed on a PCB that carries copper pads in a pattern that matches the pattern of solder balls. The assembly is then heated, either in a reflow oven or by an infrared heater, causing the solder balls to melt. Surface tension causes the molten solder to hold the package in alignment with the circuit board, at the correct separation distance, while the solder cools and solidifies. 
         [0004]    Generally, in a semiconductor package, a semiconductor element is connected by wire bonding to electrode pads on a resin substrate (an interposer) on which the semiconductor element is mounted. These electrode pads are connected to the interposer by signal lines that extend radially on the interposer. The electrode pads are also connected through vias to ball pads that are provided on the reverse face of the interposer, which connect to the solder balls of the BGA to attach the semiconductor package to a motherboard or other type of PCB. 
         [0005]    Gold plating is typically required for the electrode pads on the interposer. In order to perform the gold plating for the electrode pads, the electrode pads must be rendered conductive from the outer edge of the interposer. Therefore, in addition to wiring connected to the mounted semiconductor element, other wiring is extended from the outer edge of the interposer to the individual electrode pads. Wiring extended from an individual electrode pad to the outer edge of the interposer is called a “plating stub”. Plating stubs have an open end at the outer edge of the interposer, along the transmission line. The length of these stubs is generally about 1 to 4 mm for a BGA package, of a peripheral type, with 1 mm pitches and four rows. However, the stub length may be increased as package sizes grow with higher I/O count and ball array row count. 
         [0006]    When a period during which a signal reciprocates along a signal line in the open state is longer than the rise time for the signal, a reflected waveform occurs in the signal waveform and causes waveform distortion. For a signal for which the waveform is trapezoidal, the rise time for the signal is generally equal to about 5% of the cycle. Therefore, for a conventionally employed signal having a frequency of 1 GHz, the cycle is about 1.0 nsec and the rise time, which is 5% of the cycle, is 0.050 nsec. Through a calculation performed by employing a signal transfer rate of 6 nsec/m for a common glass epoxy substrate, the equivalent length obtained, for both directions is 8.30 mm, and the wiring length obtained that corresponds to one direction is 4.15 mm. That is, in the open state, a plating stub of about 1 to 4 mm in length does not greatly affect the quality of the waveform. 
         [0007]    The frequency of a signal used for the semiconductor element has been repeatedly increased, and a signal having a frequency even greater than 2 GHz is now employed. For a signal having a frequency of 2 GHz, the cycle is 0.5 nsec and the rise time, which is 5% of the cycle, is 0.025 nsec. Through calculations performed using the signal transfer rate of 6 nsec/m, the equivalent length in both directions is 4.15 mm, and the wiring length corresponding to one direction is 2.08 mm. That is, in the open state, a plating stub of about 2 mm or longer would greatly affect the waveform of a signal to be transmitted. 
         [0008]    A differential transmission method may be adopted for high-speed signals. A differential pair of signal lines for which impedance matching is required must be provided on the interposer. In order to achieve impedance matching for the differential pair of signal lines, a predetermined clearance must be maintained between two signal lines of a differential pair of signal lines. However, it is challenging, while maintaining this clearance, for the differential pair of signal lines to be passed through a number of electrode pads and connected to the electrode pads nearest the outer edge of the interposer substrate. Inner row ball assignment is preferred from a perspective of low package loss rather than a perspective of impedance matching if the plating stub is not a concern, but from a board escape perspective, these differential signal assignments are preferred on outer ball rows. However, in practical link density applications, all rows are considered for high speed signal assignment, causing the lengths of the plating stubs to increase. Thus with the increased lengths of the plating stubs, the distortion of waveforms for signals to be transmitted cannot be avoided. Additionally, even relative short stubs of the outer row ball signals are now starting to make a sizeable impact on the signals as the speed increases toward 10 giga-bit per second and beyond. 
         [0009]    What is needed therefore is a method and a semiconductor package to mitigate the distortion of the waveforms for the transmitted signals. 
       BRIEF SUMMARY 
       [0010]    According to one embodiment of the invention, a semiconductor package includes an interposer substrate having a first side, a second side, a peripheral edge connecting the first side with the second side, a signal line on the first side, and an electrode pad on the first side. A semiconductor element is mounted on the first side of the interposer substrate, where the semiconductor element is connected with the electrode pad by the signal line. A plating stub is located on the interposer substrate. The plating stub has a first end portion electrically connected through a terminating resistor to a ground, where the terminating resistor is mounted on the interposer substrate. The first end portion of the plating stub terminates near the peripheral edge of the interposer substrate. The second end portion of the plating stub is electrically connected to the electrode pad. 
         [0011]    In some embodiments of the semiconductor package, the terminating resistor includes a resistor having a first end electrically connected to the first end portion of the plating stub and a second end electrically connected to ground. In these embodiments, the resistor may be a surface mounted resistor or a resistive film. The resistor may have a resistance in a range of 40 ohms to 300 ohms. In a specific embodiment, the plating stub is located on the first side of the interposer substrate and the terminating resistor is mounted on the first side of the interposer substrate. In an alternative embodiment, the plating stub is located on the second side of the interposer substrate and electrically connected to the signal line by a via extending through the interposer substrate. In this embodiment the terminating resistor is mounted on the second side of the interposer substrate. 
         [0012]    In other embodiments of the semiconductor package the electrode pad is a first electrode pad and the signal line is a first signal line. The second side of the semiconductor package contacts a ball grid array (BGA) containing a plurality of solder balls. In these embodiments, the semiconductor package further includes a second electrode pad connected to the second end of the resistor by a second signal line. The second electrode pad is further connected to a grounded solder ball of the plurality of solder balls in the ball grid array on the second side of the interposer substrate by a via extending through the interposer substrate from the first side to the second side. 
         [0013]    According to another embodiment of the invention, a system includes a motherboard and a semiconductor package. The semiconductor package is electrically connected to the motherboard. The semiconductor package includes an interposer substrate having a first side, a second side, a peripheral edge connecting the first side with the second side, a signal line on the first side, and an electrode pad on the first side. A semiconductor element is mounted on the first side of the interposer substrate, where the semiconductor element is connected with the electrode pad by the signal line. A plating stub is located on the interposer substrate. The plating stub has a first end portion electrically connected through a terminating resistor to a ground, where the terminating resistor is mounted on the interposer substrate. The first end portion of the plating stub terminates near the peripheral edge of the interposer substrate. The second end portion of the plating stub is electrically connected to the electrode pad. 
         [0014]    According to another embodiment of the invention, a method of mounting a semiconductor element on an interposer substrate having a first side, a second side, a peripheral edge connecting the first side with the second side, a signal line on the first side and an electrode pad on the first side, includes connecting the electrode pad with the semiconductor element by the signal line. In these embodiments, the method further includes electrically connecting a first end portion of a plating stub to the electrode pad and electrically connecting a second end portion of the plating stub, which terminates near an edge of the interposer substrate, through a terminating resistor to ground, where the terminating resistor is mounted on the interposer substrate. This method enables wirebond packaging with plating process to carry higher speed signals which otherwise cannot be accommodated in this type of package. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0015]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description given below, serve to explain the principles of the invention. 
           [0016]      FIG. 1  is a schematic perspective view of a semiconductor package mounted on a motherboard in accordance with an embodiment of the invention. 
           [0017]      FIG. 2  is a detailed perspective view of the semiconductor package of  FIG. 1 . 
           [0018]      FIG. 3  is a schematic top view of the semiconductor package of  FIG. 2 . 
           [0019]      FIG. 4  is a schematic top view of a semiconductor package with a single trace in accordance with an alternative embodiment of the invention. 
           [0020]      FIGS. 5A-5G  are diagrammatic views illustrating successive stages of a masking and etching process used to generate resistive films that can be utilized with the embodiments in  FIGS. 1-4 . 
           [0021]      FIG. 6  is a graphical view of signal loss over a frequency range. 
           [0022]      FIG. 7  is a perspective view of a semiconductor package in accordance with an alternative embodiment of the invention. 
       
    
    
       [0023]    It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features illustrative of the basic principles of the invention. The specific design features of the sequence of operations as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes of various illustrated components, will be determined in part by the particular intended application and use environment. Certain features of the illustrated embodiments have been enlarged or distorted relative to others to facilitate visualization and clear understanding. In particular, thin features may be thickened, for example, for clarity or illustration. 
       DETAILED DESCRIPTION 
       [0024]      FIGS. 1-3  illustrate an exemplary semiconductor package  10  configured with a ball grid array (“BGA”)  12 . A semiconductor element  14  is mounted on one side  30  of an interposer substrate  16 , which provides mechanical support to the semiconductor element  14 . The interposer substrate  16  is composed of an electrically insulating dielectric material, such as polyimide, and may be either a flexible sheet or a more rigid plate. Bond wires  20 , such as flexible lengths of gold wire, are used to electrically connect the semiconductor element  14  to electrode terminals  18  on the interposer substrate  16 . The interposer substrate  16  includes electrode pads  24   a ,  24   b ,  24   c ,  24   d  and signal lines  22   a ,  22   b ,  22   c ,  22   d  that electrically connect the electrode terminals  18  to the electrode pads  24   a - d . Generally, the length of the signal lines  22   a - d  is shorter for higher frequency signals, although the length is also subject to matching the pattern of the BGA  12 . 
         [0025]    The interposer substrate  16  further includes plating stubs  26   a ,  26   b ,  26   c ,  26   d  that extend from the electrode pads  24   a - d  to a peripheral edge  28  of the interposer substrate  16 . The peripheral edge  28  connects and extends between the opposite sides  30 ,  32  of the interposer substrate  16 . The signal lines  22   a - d , electrode pads  24   a - d , and plating stubs  26   a - d  are all located on the same side  30  of the interposer substrate  16 . 
         [0026]    As described above, the plating stubs  26   a - d  render the electrode pads  24   a - d  conductive to the outer peripheral edge  28  of the interposer  16 . Each of the plating stubs  26   a - d  has an open end along the transmission line at the peripheral edge  28  of the interposer substrate  16 . If unmitigated, the open end of each plating stub  26   a - d  may be a significant source of interference to the signals transmitted along the signal lines  22   a - d  because of signal reflections in the plating stubs  26   a - d . Accordingly, terminating resistors  34   a ,  34   b ,  34   c ,  34   d  are provided at the open ends of the plating stubs  26   a - d  in order to mitigate, alleviate, or otherwise reduce the interference from signal reflections. 
         [0027]    Ball pads  36  are arranged in a grid or array on the reverse side  32  of the interposer substrate  16 . Electrically-conductive vias  38 , including vias  38   a - c , extend through the entire thickness of the interposer substrate  16  between the opposites sides  30 ,  32 . The electrode pads  24   a - d  are connected to the ball pads  36  by the vias  38  formed in the interposer substrate  16 . The ball pads  36  are attached to a motherboard  40 , or other type of printed circuit board (“PCB”), via solder balls  42 , including the representative solder balls  42   a - c , of the BGA  12 . The solder balls  42  may then connect to metallic contact pads  44  on the motherboard  40 . The metallic pads  44  are present in a grid or array pattern that matches the grid or array pattern of the solder balls on the backside of the package  10 , including solder balls  42   a - c  that represent ground connections. Vias  38   a - c  connect solder balls  42   a - c  with respective electrode pads  92   a ,  92   b ,  92   c . The signal lines  22   a - d  are also respectively connected to electrode pads  24   a - d , which are connected through vias  94   a ,  94   b ,  94   c ,  94   d  to other solder balls  42  in the BGA  12 . 
         [0028]    The assembly of package  10  and motherboard  40  is heated, either in a reflow oven or by an infrared heater, causing the solder balls  42   a - c  to melt. Surface tension causes the molten solder of the solder balls  42   a - c  to hold the package  10  in alignment with the motherboard  40 , at the correct separation distance, while the solder cools and solidifies to form physical connections between the vias  38  and the metallic pads  44 . The resulting physical connections electrically interconnect the package  10  with the motherboard  40 . 
         [0029]      FIG. 4  is a diagrammatic view of a semiconductor package  60  illustrating a single conductor  62  consisting of a high frequency signal line  64  and a plating stub  66  intersected by an electrode pad  68 . The signal line  64  is electrically connected to the semiconductor element  70  via a bondwire  72 . Electrode pad  68  is electrically connected to a pad (not shown) on the opposite side of the interposer substrate  74  through a via (not shown). The pad on the opposite side of the interposer substrate  74  may then be connected to a ball grid (not shown) which in turn connects to a motherboard (not shown) similar to the package substrate illustrated in  FIG. 1 . The plating stub  66  extends from the electrode pad  68  to the peripheral edge  76 . To reduce interference due to reflections in the plating stub  66 , the stub  66  is electrically connected to an electrode pad  78 , which is connected to ground, through a terminating resistor  80 . 
         [0030]    In a representative embodiment, each terminating resistor  34   a - d  ( FIGS. 1-3 ) or the terminating resistor  80  ( FIG. 4 ) may be a surface mount resistor that is added during the assembly process and after the design and manufacturing are complete. Such surface mount resistors are directly attached onto side  30  of the interposer substrate  16 . 
         [0031]    In an alternative embodiment, each terminating resistor  34   a - d  ( FIGS. 1-3 ) or the terminating resistor  80  ( FIG. 4 ) may be a resistive film, which then may be included as part of the overall design and manufacturing process for the packaging. These resistive films may be formed using a masking and etching process as is well known to a person having ordinary skill in the art. For example, in  FIG. 5A , a resistive film  82  is applied to a dielectric substrate  84 , such as the interposer substrate  16  in  FIGS. 1-3  or the interposer substrate  74  in  FIG. 4 . The resistive film  82  is then covered by an electrodeposited copper foil  86 . In  FIG. 5B , a photoresist layer  88  is applied over the area that is destined to become the resistor, such as terminating resistors  34   a - d  ( FIGS. 1-3 ) or terminating resistor  80  ( FIG. 4 ). The dielectric substrate  84  is then subjected to an etching process where excess material, which is not masked by the photoresist layer  88 , is removed as seen in  FIG. 5C . The photoresist layer  88  is stripped off in  FIG. 5D  with, for example, a chemical solvent or plasma ashing. 
         [0032]    A second photoresist layer  90  is applied over the copper foil in  FIG. 5E  to define an area of the conductive pads for the terminating film resistor. A second etch step is shown in  FIG. 5F  in which the copper foil  86  is removed to expose the resistive film  82  in areas not masked by the photoresist layer  90 . Finally in  FIG. 5G , the residual photoresist layer  90  is stripped off leaving the terminating resistor that includes the resistive film  82  and conductive end pads of the remaining copper foil  86 . 
         [0033]    With renewed reference to  FIGS. 1-4 , while the terminating resistor  80  may be used to terminate the plating stub  66  extending from the signal line  64  or each of the terminating resistors  34   a - d  may be used to terminate the plating stubs  26   a - d  extending from one of the signal lines  22   a - d , a person having ordinary skill in the art will appreciate that lower frequency signals will have longer wavelengths. Because of the longer wavelengths, it may not be necessary to use terminating resistors  34   a - d ,  80  on these signal lines  22   a - d ,  64 . 
         [0034]    The resistance of each of the terminating resistors  34   a - d ,  80  may generally be in the range of 30 ohms to 350 ohms, preferably 40 ohms to 300 ohms. The exact resistance value will depend, among other factors, on the frequency range of the signals on the signal lines such as signal lines  22   a - d  or signal line  64 . As apparent from the curves on the graph in  FIG. 6 , the addition of each terminating resistor  34   a - d ,  80  generally reduces the losses over the entire frequency range. Although the open-ended plating stubs  26   a - d ,  66  experience better losses at lower and higher frequencies, use of the terminating resistor  34   a - d ,  80  removes the stop frequency band of the signal existing in the open plating stubs  34   a - d ,  80  and provides about a 16 dB improvement for this particular example in the range of about 7 GHz to about 14 GHz, where a 50 ohm resistor was used to terminate each signal line to match the signal line impedance and source termination. 
         [0035]    Selection of the terminating resistance value for each terminating resistor  34   a - d ,  80  may be based on the signal operating data rates and the quarter wavelength resonance associated with the plating stub lengths. As the resistance of the terminating resistor  34   a - d ,  80  increases, there may be less improvement near resonance frequency but instead less loss outside of the resonance frequency zone as a tradeoff. Therefore, the selection of a resistance value may mainly depend on whether the operating signal frequency range is within or outside the stop band centered at the resonance frequency of the plating stub  26   a - d ,  66 . For example, if the frequency of the operating signal is near the plating stub resonance frequency, a resistance of 50 ohms or less may be more effective. If the operating signal frequency range is away from the stop band of the plating stubs  26   a - d ,  66 , higher resistance values may be more effective. 
         [0036]    Because of differences in length of each of the plating stubs  26   a - d , a corresponding terminating resistor  34   a ,  34   b ,  34   c , and  34   d , each with a resistance corresponding to the length of the plating stub  26   a - d  may be used to terminate the plating stubs  26   a - d . In some embodiments, the trace lengths of the plating stubs  26   a - d  may be characterized as having approximately the same length with the resistance values for each of the terminating resistors  34   a - d  being the same. Each of the terminating resistors  34   a - d  are connected to the closest one of the ground electrode pads  92   a - c  that is itself connected to ground. In some configurations, multiple plating stubs  26   b ,  26   c  may be connected to the same electrode pad  92   b.    
         [0037]    In an alternative embodiment and as visible in the semiconductor package  120  shown in  FIG. 7 , a plating stub  122  similar to plating stubs  26   a - d  may be located on a reverse side  125  of the interposer substrate  124  from the semiconductor element  126 . In this embodiment, the semiconductor element  126  is mounted on the interposer substrate  124  and is connected to an electrode terminal (not shown) on the interposer substrate  124  by a bond wire (not shown) similar to the embodiments in  FIGS. 1-4 . The electrode terminal is connected by a signal line  128  to an electrode pad  130 . The plating stub  122  is formed on the reverse side  125  and extends from electrode pad  132  to a peripheral edge  134  of the interposer substrate  124 . Electrode pad  130  is connected to electrode pad  132  by a via  136 , which electrically connects the signal line  128  to the plating stub  122 . 
         [0038]    The plating stub  122  may also be connected to a ground connection  138  through a terminating resistor  140 . Similar to the embodiments discussed above, the terminating resistor  140  may be a surface mount resistor or, alternatively, may be a resistive film. The resistance of the terminating resistor  140  may generally be in the range of about 40 ohms to about 300 ohms, depending on the frequency range of the signals on the signal line  128  as similar to the embodiments described above. The terminating resistor  140  is generally mounted on the same side  125  of the interposer substrate as the plating stub  122 . In this embodiment, the terminating resistor  140  is mounted on the reverse side of the interposer substrate  124  from the semiconductor element  126 . The ground connection  138  for the resistor  140  may also be established through a ball grid array or by other known connection methods. 
         [0039]    In an alternative embodiment, multiple plating stubs like plating stub  122  and multiple terminating resistors like terminating resistor  140  may be provided on the reverse side  125  of the interposer substrate  124 . 
         [0040]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
         [0041]    The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.