Patent Document

FIELD OF THE INVENTION  
       [0001]     The present invention pertains in general to circuit design and in particular to creating and utilizing conductive trenches to improve power delivery, EMI suppression, and/or thermal dissipation within a circuit structure.  
       BACKGROUND OF THE INVENTION  
       [0002]     As printed circuit board designs have increased in complexity, the need for additional interconnect lines between the components coupled to the printed circuit boards have increased. To address this need, manufactures have provided multiple layer printed circuit boards where several layers of conductors are separated by layers of dielectric material. Printed circuit boards (PCBs) generally contain four or more conductive layers, where at least one conductive layer is a ground plane, one or more conductive layers are power planes and outer conductive layers that provide a high density interconnect for coupling various components or sockets, which have been mounted to the PCB. These multiple layer circuit boards are fabricated such the conductive layers are each separated by a dielectric layer so that the intermediate conductor layers providing power and the ground planes to the printed circuit board are not in contact except by vias.  FIG. 1  illustrates a multiple layer printed circuit board where layered beneath the interconnect layers (not shown) is a first metalized layer to provide power, and a second metalized layer to provide ground and where the two metalized layers are separated by a dielectric layer (removed for clarity). A clearance space in a metalized layer may be provided around a via to avoid connecting with that layer.  
         [0003]     The conductive layers of the multiple layer printed circuit boards can be connected to each other using vias, which are plated with conductive material to provide plated through holes. The vias are located across the printed circuit board and connected to mounting locations on the outer conductive planes using conductive traces. That is, mounting pads for integrated circuits and surface mount components may not be directly connected to plated through holes, but can be connected to the plated through hole locations using a patterned conductive trace. With the increased population density of integrated circuits, concerns about electromagnetic interference (EMI), power/heat dissipation, and power delivery increase.  
         [0004]     For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a printed circuit board which addresses the above concerns while maintaining current circuit board assembly quality, including solder joints.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0005]     The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like references indicate similar elements and in which:  
         [0006]      FIG. 1  illustrates a multiple layer printed circuit board where layered beneath the interconnect layers (not shown) is a first metalized layer to provide power, and a second metalized layer to provide ground.  
         [0007]      FIG. 2  is an illustration of one embodiment of segmented power and ground planes connected by conductive trenches  
         [0008]      FIG. 3A  is an illustration of a patterned first metal coating layer over a core dielectric material.  
         [0009]      FIG. 3B  is an illustration of the PCB after a dielectric layer is deposited over the patterned first metal coating layer, a second metal coating deposited/laminated over the second dielectric, and a via drilled through the PCB layers.  
         [0010]      FIG. 3C  is an illustration of the start of trench formation by first creating a groove through the second metal layer and the second dielectric layer to expose portions of the first metal layer.  
         [0011]      FIG. 3D  is an illustration of the trenches and via after a metalizing process.  
         [0012]      FIG. 4A-4D  are an illustrations of another embodiment for fabricating metalized trenches.  
         [0013]      FIG. 4A  is an illustration of the alternate embodiment for fabrication of metal circuitry.  
         [0014]      FIG. 4B  is an illustration of the alternate embodiment for adding more layers to the PCB.  
         [0015]      FIG. 4C  is an illustration of the alternate embodiment for using a die to imprint the PCB.  
         [0016]      FIG. 4D  is an illustration of the die imprint onto the PCB.  
         [0017]      FIG. 5  is an illustration of other embodiments of conductive trenches connecting segmented metalized planes on a PCB.  
         [0018]      FIG. 6  is an illustration of a 3D view of metalized trenches and segmented surfaces.  
         [0019]      FIG. 7A  illustrates cross-sectional area increase with the number of metalized trenches and the increased surfaces that define the trench.  
         [0020]      FIG. 7B  illustrates the metalized trenches from the side (perpendicular to the length of the trench).  
         [0021]      FIG. 8A  is an illustration of an alternate embodiment where conductive trenches can be used to fabricate a Faraday cage within the PCB substrate.  
         [0022]      FIG. 8B  is an illustration of the alternate embodiment with a top view of conductive trenches that form part of the Faraday cage.  
     
    
     DETAILED DESCRIPTION  
       [0023]     A method and apparatus is disclosed for creating and utilizing conductive trenches to improve power delivery, EMI suppression, and/or thermal dissipation within a PCB structure. This method and apparatus can segment one or more metalized layers (planes) separated by a dielectric such that the use of the trenches can be realized effectively in a package interconnect/via field. For purposes of discussing the invention, it is to be understood that various terms are used by those knowledgeable in the art to describe apparatus, techniques, and approaches.  
         [0024]     In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be evident, however, to one skilled in the art that the present invention may be practiced without these specific details. In some instances, well-known structures and devices are shown in gross form rather than in detail in order to avoid obscuring the present invention. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that logical, mechanical, electrical, and other changes may be made without departing from the scope of the present invention.  
         [0025]     This invention allows improved current carrying capability, decreased voltage droop, and/or improved thermal transfer by adding channels thereby increasing the surface areas in the direction of the current flow. The channels can be made from an electrically conductive material and the result can be an increase in cross-sectional area in the direction of current flow. Such electrically conductive material can be, for example, metals, metalized paste, or electrically conductive polymers. The channels can be made from a material that is thermally conductive and can result in an increase in thermal conduction along this increased cross-sectional area. Such thermally conductive materials can be materials such as, for example, metals, pastes, and filled-polymer composites. Further, the channels can be made from a material that is both thermally conductive and electrically conductive such as, for example, metals, metalized polymers, conductive composite pastes, etc.  
         [0026]     The channel (trench) is formed by selectively removing portions of the outer metalized layers and portions of base material layers placed between the outer metalized layers. The PCB can have as base material, i.e. as one or more non-conducting layers, a laminate such as, for example, an epoxy resin reinforced with glass fiber, where one such fiberglass version is FR4. In addition, the trench can be deep enough to also remove portions of one or more of the inner metalized layers placed between the outer metalized layers. The trench can be formed such that the direction (length) of the trench is along a path where added cross-sectional area is desired (in the case of power delivery, the trench can be in the direction of current flow). The trench can be formed by such techniques as laser ablation, photo-developed patterning, plasma, chemical, or mechanical. The bare trench can then be provided a coating such as with one of the electrically and/or thermally conductive materials mentioned above and hereafter referred to as a conductive coating. The cross-section of this conductive coating within the trench can provide a conductive path having the improved cross-sectional area. By increasing the cross-sectional area, the per unit length resistance drops accordingly and the reduced resistance decreases the IR (current multiplied by resistance) drop of a power delivery circuit. This increase in cross-sectional area that improves the ability of the path to carry electrical current can also conduct heat and the surfaces of the conductive coating can dissipate heat. The trench, now having a conductive coating (i.e. a conductive trench), can radiate and convect heat off the conductive surface to remove heat from the interior of the PCB. This can occur where the trench makes a conductive connection between an inner layered circuit/plane and an outermost (exposed) circuit/plane such that the thermal dissipation of the interior circuit/plane is improved.  
         [0027]      FIG. 2  is an illustration of one embodiment of segmented power and ground planes connected by trenches and vias. As shown in  FIG. 2 , an upper metalized plane  202  (plane) and a lower metalized plane  204  can be separated by a dielectric (removed for clarity) on a printed circuit board (PCB)  200 . The upper and lower metalized planes  202  and  204  are shown as generalized planes, i.e. no specific boundaries, with phantom lines. The upper and lower metalized planes  202  and  204  can be segmented, where the separated upper segments  206 ,  208 , and  210  can be connected to the separated lower segments  212 ,  214 , and  216  by conductive trenches  218 ,  220 , and  222 . The upper  206 ,  208 ,  210  and lower 212, 214, 216 segments can be stacked so that equal potential segments (i.e. having equal or nearly equal areas) are aligned in the vertical axis  224  for connection by the respective conductive trenches  218 ,  220 , and  222 .  
         [0028]     A trench is formed by pattern etching the metalized planes  202  and  204  and then by removing the dielectric material separating the planes  202  and  204 . Each trench  218 ,  220 , and  222  is then coated with a conductive material to provide an electrical and/or thermal connection between the upper segments  206 ,  208 , and  210  and lower segments  212 ,  214 , and  216 . The coating thickness of the conductive trench can be made to provide cross-sections greater than cross section areas attained without a trench for either of the metalized planes  202  and  204 . A segment  210  and  208  can be patterned to surround and electrically connect one or more vias  226  and  229  respectively and to a trench  222  and  220  resp., or alternatively, a segment  210  and  208  can be patterned to surround yet remain spaced apart (i.e. not connect) from a via  227  and  228  resp.  
         [0029]      FIGS. 3A-3D  illustrate one embodiment of a method for fabrication of segmented metalized planes connected by conductive trenches and vias on a PCB.  FIG. 3A  is an illustration of a patterned first metalized layer over a core base material. The first metalized coating  302  can be copper that is blanket deposited by several methods such as, for example, CVD or lamination. Patterning, after depositing the metal layer  302 , can include developing an image in a photoresist coating placed over the metal layer. An etch process can then segment the copper layer  302 , exposing the underlying base material  304 .  
         [0030]      FIG. 3B  is an illustration of a second metalized layer and a dielectric layer deposited over the first metalized layer. A second dielectric layer  306  is deposited over the patterned first metalized coating  302 . Next, a second metalized coating  308  is deposited or laminated over the second dielectric  306 , and a via  310  can be drilled through the PCB layers. The second metalized coating  308  can be copper and the second dielectric  306  can be an epoxy resin.  
         [0031]      FIG. 3C  is an illustration of the start of trench formation. Grooves  312  and  314  can be etched or ablated through the second metal layer  308  and the second dielectric  306  layer to a depth that exposes circuit traces in the first metal layer  302 . Etching/ablating the grooves  312  and  314  in the dielectric  306  can be accomplished by any number of processes such as, for example, mechanical imprinting, chemical etching, mechanical routing, or laser ablation. For mechanical imprinting, one method can use a metal die (not shown) that has a male pattern of the trenches to be placed into the substrate. Mechanically pressing the die onto the substrate can displace material and form the trenches. The process to remove metal from an area of the trench  312  and  314  in metal layer  308  can be different from a process to remove the dielectric material  306  in the same trench area.  
         [0032]      FIG. 3D  is an illustration of conductive trenches  316  and  318  and coated via  320  after a coating process such as, for example, CVD, sputtering, electroless plating, electrolytic plating, or a combination of such processes. The coating can cover the trench  312  and  314  ( FIG. 3C  above) surfaces to create an increased conductive cross-section area where such area can be dimensionally tuned by controlling the volume of conductive material, such as, for example, metal deposits, polymers, or pastes, that is deposited.  
         [0033]      FIGS. 4A-4D  illustrate an alternate embodiment of a method for fabrication of segmented metalized planes connected by conductive trenches and vias on a PCB.  FIG. 4A  is an illustration of a patterned first metalized layer over a core base material that is a dielectric. The first metalized coating  402 , such as copper, can be blanket deposited by several methods such as, for example, CVD or lamination. Patterning, after depositing the metalized layer  402 , can include developing an image in a photoresist coating placed over the metal layer. Etching can then segment the metalized layer  402 , exposing the underlying base material  404 .  
         [0034]      FIG. 4B  is an illustration of a second metalized layer and a dielectric layer deposited over the first metalized layer. The second dielectric layer  406  can be deposited over the patterned first metalized coating  402 . Next, a second metalized coating  408  can be deposited or laminated over the second dielectric  406 , and then a via  410  can be drilled through the PCB layers. The second metalized coating  408  can be copper and the second dielectric  406  can be an epoxy resin.  
         [0035]      FIG. 4C  is an illustration of creating an imprint that results in conductive trench formation. Mechanical imprinting, such as by using a metal die  412  that has a male pattern  414  (i.e. mirror image of the trenches to be manufactured), can be pressed  416  onto the second or top metalized layer  412  of the substrate  400 .  
         [0036]      FIG. 4D  is an illustration of the metal die pressed into the substrate creating the conductive trenches. Mechanically pressing the metal die  412  onto the substrate  400  can simultaneously displace both the metalized layer  408  and the dielectric material  406  to form the conductive trenches  418  and  419 .  
         [0037]     The process to remove or displace dielectric material to create the trenches can be different from a process used to displace or remove the metalized layer.  
         [0038]      FIG. 5  is an illustration of other embodiments of conductive trenches connecting segmented metalized planes on a PCB. Using the same processes as described above ( FIGS. 3A-3D ), metalized trenches  502  and  510  can be made deeper and can connect more than two metalized layers  512 ,  514 , and  508 .  FIG. 5  illustrates a first metalized trench  502  that is formed such as by etch through two layers of dielectric  504  and  506  to expose a third metal layer  508  on the bottom of the PCB. A second metalized trench  510  is also illustrated that is placed through two dielectric layers  504  and  506  to connect tracings on three metal layers  508 ,  512 , and  514 . A third trench  516  can connect the second metalized layer  514  with the third metalized layer  408 .  
         [0039]     With further alternate embodiments, a variety of combinations are possible, such as, for example, it may be that trenches such as, for example, deeper trenches connect to only one metal layer to increase the surface area of a first metal layer only to improve heat conduction and/or to improve electrical conductivity.  
         [0040]      FIG. 6  is a 3D illustration of one embodiment for patterns that define segments of a single plane pair  602  and  604 , a first metal layer  602  and a second metal layer  604  where the segments of the two metal layers  602  and  604  to be connected by trenches  605  can be positioned opposing each other. In this embodiment, the upper mask image  602  can later be connected to a (+) potential and the lower mask image  604  to a (−) potential. The larger surface area  606  in the upper mask image  602  can connect to the series of smaller surface area segments or fingers  609  and  609 ′ of the lower mask image  604 . Conversely, the larger surface area  608  in the lower mask image  604  can connect to the series of smaller surface area segments or fingers  607  and  607 ′ in the upper mask image  602 .  FIG. 6  is one embodiment that allows multiple electrical or thermal potentials to be realized in two or more surfaces that are conductively connected via the trenches to allow for segmenting two or more surfaces for connections into typical alternating power-ground fields used in electronic packages.  
         [0041]      FIGS. 7A and 7B  are illustrations of the cross-sectional of a metalized trench. Illustrated in  FIG. 7A , cross-sectional area increases with the number of metalized trenches  702  and  702 ′ and the increased surfaces that define the trench. In the case of a rectangular trench, the increase in surface area, for each metalized trench  702  and  702 ′ is related to the length of trench sidewall a and width of sidewall b. As shown in  FIG. 7B , when viewing the metalized trenches  702  and  702 ′ from the side (perpendicular to the length of the trench), the cross-sectional area is unchanged and the conductive path  704  is longer by (2a) for each trench  702  and  702 ′ crossed. In the case of power delivery, through the interstitial grid of a package (not shown), the number of metalized trenches that can be placed between package pins is a function of the metalized trench width (a) and aspect ratio capability of the metalization method The surface area of the trench and cross-sectional area after conductive coating can be controlled or increased by shaping edges and contours of the trench surface.  
         [0042]      FIGS. 8A &amp; 8B  illustrate an alternate embodiment of conductive trenches that can be used to fabricate a Faraday cage within the PCB substrate. The present invention can be a novel device structure for shielding individual circuitry from conductive and/or radiated energy. Such as, for example, electromagnetic interference (EMI) from radiation originating outside the printed circuit board or from adjacent devices on the PCB. In an alternate embodiment, a Faraday cage is constructed on and/or within a PCB substrate, to enclose PCB circuitry within a structure of metal.  
         [0043]     Usually a complete conductive shell, a Faraday cage collects stray charges and, because like charges repel, stores them on the outside surface (where they can be further apart than on the inside). The electric fields generated by these charges then cancel each other out on the inside of the cage. A Faraday cage is often used to protect sensitive radio equipment.  
         [0044]     As shown  FIG. 8A  is a cross-section of a PCB having individual circuitry surrounding by a Faraday cage.  FIG. 8B  is a top view of a conductive trench forming part of the Faraday cage. A number of conductive trenches  804  can be fabricated by the methods described above. In one embodiment, circuitry  801  and  801 ′ can be formed on layers within the PCB  800 . Trenches can be formed in both sides  802  and  804  of the PCB. The top surface  802  and the bottom surface  806  can have a metalized layer deposited. The trenches  804  and  804 ′ can be formed in a closed or nearly closed loop, i.e., for example, a square as shown here. As a result, selected circuitry can be enclosed within the trench-bottom/top surfaces thus shielding the selected circuitry with a Faraday cage. Other circuitry  801 ′ and vias  808  can be placed outside the Faraday cage. Alternatively, vias (not shown) can be placed through the structure of the Faraday cage allowing for some degradation of the effectiveness of the Faraday cage.  
         [0045]     Thus a method and apparatus for joining two or more metalized planes with a series of metalized trenches having cross-section areas that can increase the thermal and/or electrical conductive path from one or more metalized planes has been described. In addition, a series of metalized planes and conductive trenches can be positioned such as to create a Faraday cage protecting electrical circuitry and/or electrical devices within. Although the present invention has been described with reference to specific exemplary embodiments, it will be evident that various modifications and changes may be made to these embodiments without departing from the broader spirit and scope of the invention as set forth in the claims. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.

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