FIELD OF THE INVENTION 
   This invention pertains to conductive core substrates, and particularly to apparatus and methods to reduce impedance mismatch issues with differential signaling in conductive core substrates, while maintaining adherence to the common mode noise assumption. 
   BACKGROUND OF INVENTION 
   Trends in microelectronic devices are toward increasing miniaturization, circuit density, operating speeds and switching rates. These trends have directly impacted the complexity associated with the design and manufacture of dies, microelectronic devices, which include the microelectronic die and a substrate, microelectronic packages, which include the microelectronic device, as well as computing devices in general. Examples of computing devices include, but are not limited to servers, personal computers and “special” purpose computing devices. Personal computers may have form factors, such as desktop, laptop, tablet, and the like. “Special” purpose computing devices may include set top boxes, personal digital assistants, wireless phones, and the like. 
   Accordingly, substrates, including but not limited to those used in microelectronic packages, have also evolved to enable the microelectronic devices to operate at higher speeds and efficiencies. Substrates include, but are not limited to, interposers, printed circuit boards, motherboards, and the like. One such advancement includes the use of conductive core substrates. One example of a conductive core substrate is a metal core substrate, which comprises a single or multiple metal layers encapsulated in a dielectric material. Metals used in metal core packages include, but are not limited to, copper, molybdenum, copper-Invar-copper and other conductive metals. Metal core substrates have become more prevalent due to their low coefficient of thermal expansion (CTE), low inductance, low resistance, high thermal conductivity and lower cost. The metal core also provides structural support to allow the substrate to carry large and heavy components, and to function in environments where shock, vibration, heat, and survivability are a factor. 
   Another advancement in substrate technology is the incorporation of differential signaling for the transmission of signals/data to and from a microelectronic die. Differential signaling provides a pair of conductive pathways, in one example, also known as traces, formed within and/or on the substrate to conduct the signal. A fist trace typically carries a positive signal, and a second trace carries a signal that is of equal magnitude, but opposite in phase, i.e. a negative signal. Differential signaling provides a number of benefits, including, but not limited to, lower voltage swings, faster switching rates, reduced power consumption, and reduced electromagnetic interference (EMI). Differential signals carried on the first and second traces are also less sensitive to electrical cross talk or interference, and have better overall noise immunity. For example, noise generated by spurious conditions within the microelectronic package or noise generated from an outside source adds to both signals equally. Thus, when the receiver subtracts the noise in the negative signal from the noise in the positive signal, the noise in each signal trace effectively cancels out. This is known in the art as “common mode noise assumption.” 
   With differential signaling, it is important to design the trace pairs in such a way that the characteristic impedance of the first and second trace is equal and constant. Substrate design configurations often include the first trace and the second trace to traverse from one substrate layer to another, which can require passing through the conductive core. 
     FIG. 5  is a top view of a portion of a current metal core substrate  10  comprising a first trace  12 , adapted to carry a positive signal, that runs generally parallel to a second trace  12 ′, adapted to carry a signal having an equal but opposite magnitude. First and second traces  12 ,  12 ′ traverse a layer of the substrate. The first trace  12  interconnects with a first via  14  that extends through the metal core  10 . Second trace  12 ′ interconnects with a second via  14 ′ also extending through the metal core  10 . First and second vias  14 ,  14 ′ allow the differential signals to pass through metal core  10  to a different layer. 
     FIG. 6  is a side view of the metal core  10  in accordance with  FIG. 4 , showing the path of the first trace  12  from a first side of metal core  10  to a second side of metal core  10 . Trace  12  traverses a layer  15  above the metal core  10 , passes through metal core  10  using the path formed by a via  14  extending through the metal core  10 , such as a plated through hole (PTH), and then again traverses a different layer  15 ′ on the second side of metal core  10 . Though not shown, second trace  12 ′ traverses a generally parallel but separate path as that of first trace  12 . 
     FIG. 5  also shows the electric field distribution between the first and second trace  12 ,  12 ′. While traversing the same layer, there is unimpeded signal coupling between the signal carried on trace  12  and the signal carried on trace  12 ′, as shown by arrows  16  representing electric field lines. This is an optimal distribution, as there is little or no impedance mismatch since there is no conductive obstruction between first and second traces  12 ,  12 ′ to block the signal coupling. This holds true regardless of which layer is being traversed. 
   A problem arises, however, when the first and second traces  12 ,  12 ′ traverse through the metal core  10 . A portion  18  of the metal core  10  (designated by dashed lines) separates the first and second vias  14 ,  14 ′, which impedes the signal coupling of the first and second traces  12 ,  12 ′. This results in several undesirable effects. First, as shown by electric field lines  20 , there is impedance mismatch between the signals as they traverse first and second vias  14 ,  14 ′. Second, the common mode noise assumption no longer applies because the signals blocked by portion  18  of metal core  10  resulting in different degrees of noise couple to first and second traces  12 ,  12 ′ in differing amounts. These effects result in a significant degradation of the signal integrity, which impairs performance. 
   Accordingly, there is a need for apparatus and methods to employ differential signaling in conductive core substrates, which maintain matched impedance between the differential signal traces while traversing through the conductive core, while also maintaining the common mode noise assumption. 


   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIGS. 1 and 2  are top views of a conductive core substrates including differential signal traces in accordance with one embodiment of the present invention; 
       FIG. 3  is a side cross sectional view of the conductive core substrate shown in  FIGS. 1 and 2 . 
       FIG. 4  is a top view of a conductive core substrates including differential signal traces in accordance with another embodiment of the present invention; 
       FIG. 5  is a top view of a prior art metal core substrate; and 
       FIG. 6  is a side cross sectional view of the embodiment of the metal core substrate of  FIG. 5 ; and 
       FIG. 7  is a side view of a microelectronic package in accordance with one embodiment of the present invention. 
   


   DESCRIPTION 
   In the following detailed description, reference is made to the accompanying drawings which form a part hereof wherein like numerals designate like parts throughout, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. Therefore, the following detailed description is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims and their equivalents. 
     FIG. 7  illustrates a side view of a microelectronic package in accordance with one embodiment of the present invention. A microelectronic package  54  may be coupled to a substrate, such as a printed circuit board  52 , via socket connection  55 . Microelectronic package  54  may include a die  56  electrically coupled to substrate, such as an interposer  58 . Microelectronic package  54  may also be electrically coupled to power source  50 . 
     FIG. 1  is a top view of a portion of a substrate  31  having a conductive core  30  with conductive differential signal first and second traces  32 ,  32 ′ traversing substrate layers above and below conductive core  30 , as well as through conductive core  30 . Conductive core  30  can be any conductive material, but it has been found that conductive metal, including but not limited to copper, molybdenum, copper-invar-copper, are preferred. Conductive core  30  has an aperture  36  extending from the first side to the opposite second side. The aperture  36  contains a first conductive path  34  and a second conductive path  34 ′, which are electrically insulated from each other and electrically insulated from the conductive core  30 . First and second conductive paths  34 ,  34 ′ are spaced apart, such that there is no conductive core existing in the aperture between first and second conductive paths  34 ,  34 ′. 
   As shown in  FIG. 3 , first trace  32  traverse a first layer  42  on a first side of conductive core  30  and can electrically interconnect with first conductive path  34  in aperture  36 . The signal carried on first trace  32  then can traverse from the first side of conductive core  30  to the second side of conductive core  30  by way of the first conductive path  34 . Once having traversed conductive core  30 , first trace  32  then traverses a second layer  42 ′. Though not shown in the cross sectional view of  FIG. 3 , second trace  34 ′ also traverses first layer  42 , generally parallel to first trace  32 ′, passes through conductive core  30  through conductive path  34 ′ of aperture  36 , and then traverses second layer  42 ′. 
   By having the first and second conductive paths  34 , 34 ′ electrically insulated from each other and disposed in a spaced apart relationship within aperture  36 , the impedance remains substantially equal between the first and second conductive paths  34 ,  34 ′ as there is no conductive obstructions there between, which results in more optimal signal coupling. It can be appreciated by one skilled in the art that the first layer  42  and second layer  42 ′, for which conductive core  30  lies between, need not be directly adjacent to the conductive core. 
   Conductive paths  34 ,  34 ′ can be vias, plated through holes, or any other path that will allow a signal to traverse from either first trace  32  or second trace  32 ′ through conductive core  30  through aperture  36 . In one embodiment in accordance with the present invention, and as shown in  FIGS. 1 and 2 , aperture  36  can be encapsulated with a dielectric material  35 . Though dielectric material may partially or completely fill aperture  36 , thereby encapsulating first and second conductive pathways  34  and  34 ′, this will not impede the signal coupling between the first and second conductive paths  34 ,  34 ′. Again, the conductive core  30  can be any conductive material, including but not limited to a conductive metal such as copper that is conductive and thus impedes the signal coupling of differential signals as they traverse the conductive core. Though a polyamide resin is preferred for the dielectric material  35  and the material making up first and second layers  42 ,  42 ′, the dielectric material  35  and layers  42 ,  42 ′ can be any non conductive material, including but not limited to epoxy resins, fiber reinforced resins, polymers and the like. 
   The impedance match and signal coupling between the first and second traces  32 ,  32 ′ and the first and second conductive paths  34 ,  34 ′ is illustrated by the electric field distribution lines  40  and  42 , as shown in FIG.  2 . When traversing a dielectric layer of substrate  31  on a first side of conductive core  30 , the impedance between the differential signal first and second traces  32 ,  32 ′ is matched and the signal coupling is more optimized. This is shown by electric field distribution lines  40  indicating coupling between the signal carried on first trace  32  and the signal carried on the second trace  32 ′. The same is true when the first and second traces  32 ,  32 ′ traverse a second layer on the second side of conductive core  30 . When traversing conductive core  30  by way of first and second conductive paths  34 ,  34 ′ spaced apart in aperture  36  the impedance between the differential signals carried by conductive paths  34 ,  34 ′ generally stays matched. Electric field distribution lines  42  illustrate a more desired signal coupling from the signal carried on the first conductive path  34  and the signal carried on the second conductive path  34 ′. 
   By keeping the impedance matched from a first layer on the first side of conductive core  30 , through conductive core  30  to a second layer on a second side of conductive core  30 , not only maintains a signal integrity, but also maintains the common mode noise assumption. As previously discussed, where differential signal traces are routed close together and have matched impedance, the external noise will be coupled into each signal substantially equally. This allows for the noise to be substantially cancelled out, leaving only the resulting signal. Because the differential signals are in the same environment when traversing the first layer and the second layer, as well as when traversing conductive core  30  by way of the first and second conductive paths  34 ,  34 ′, noise should be coupled to the signals in substantially equal amounts, and thus the common noise assumption is valid with aperture  36  in conductive core  30 . 
   As illustrated in FIG.  1  and  FIG. 2 , creation of aperture  36  in conductive core  30  substantially addresses the problem of impedance mismatch and allows common mode noise assumption to hold true. Creation of aperture  36  can be easily and cost effectively accomplished during the substrate manufacturing process. For example, two independent apertures can be created and plated as necessary to enable signal conductivity. A third aperture could then be created between the two independent apertures, effectively removing the portion of conductive core between the two independent apertures. The aperture can be created in a variety of ways, including, but not limited to laser etching, drilling an additional hole, or other known methods. Aperture  36  can also be created in conductive core  30  by creating a single aperture, coating the metal core and aperture with a dielectric material, then creating two conductive paths within the aperture such that the conductive paths are electrically insulated from each other, and such that there is no conductive core between the conductive paths to impede signal coupling. Another example of a single aperture embodiment in accordance with the present invention is shown in  FIG. 4 , where aperture  36  could be laser etched to a size adapted to sufficiently reduce the impedance causing metal core, where the first and second conductive paths  34 ,  34 ′ are created at opposite ends of aperture  36 . 
   Although specific embodiments have been illustrated and described herein for purposes of description of the preferred embodiment, it will be appreciated by those of ordinary skill in the art that a wide variety of alternate and/or equivalent implementations calculated to achieve the same purposes may be substituted for the specific embodiment shown and described without departing from the scope of the present invention. Those with skill in the art will readily appreciate that the present invention may be implemented in a very wide variety of embodiments. This application is intended to cover any adaptations or variations of the embodiments discussed herein. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.
This invention relates to an apparatus and methods for reducing the impedance mismatch problem encountered by differential signaling in conductive core substrates, while maintaining adherence to the common mode noise assumption. Specifically, the conductive paths that traverse through the conductive core are separated by a nonconductive material which minimize impedance and interruption of the signal coupling.