Abstract:
A method is disclosed for fabricating a PCB so that is can easily be determined if a via in the PCB has not been counterbored to a desired depth. A PCB fabricated according to the method also is disclosed.

Description:
FIELD OF THE INVENTION 
     The invention pertains to printed circuit board (PCB) fabrication. More particularly, the invention pertains to counter boring of vias on printed circuit boards. 
     BACKGROUND OF THE INVENTION 
     Printed circuit boards are used in many electronic devices for purposes of routing signals between electrical and electronic components that are (1) mounted on the PCB, (2) connected to the PCB via edge connectors, or (3) fabricated directly into the PCB. PCBs generally are flat square or rectangular boards primarily formed of a dielectric material with copper or other conductive traces formed therein to route signals between the various aforementioned electronic components. The term circuitry shall be used herein to refer broadly to any form of electrical or electronic components, including analog components, digital components, ground planes, and simple conductors such as copper traces. 
     A PCB has two opposing external major surfaces, one or both of which may bear circuitry. In addition, multilayer PCBs are well known in which one or more layers of circuitry are disposed in between the two opposing external major surfaces. Vias are commonly used to connect signals on any one of these layers to any other one of these layers. A via essentially is a hole that is drilled or otherwise formed in the PCB between any two layers and plated or filled with copper or another conductor. Due to ease of fabrication issues, vias typically are drilled completely through the PCB even if the via is used to connect circuitry on two internal layers or one of the two external layers (topmost layer or bottommost layer) and an internal layer. 
     “Backplane” is the common terminology used for specific type of PCB found in many electronic devices, such as computers, that is usually large in size (e.g., greater than about 9 inches per edge) and that contains connectors on the edge of the PCB into which additional electronic components may be plugged, such as computer peripheral cards (often called plug-in cards). 
     Commonly, the edge connectors on the PCB for connecting a plug-in card or other electronic component or device are installed on the surface or edge of the PCB and connected to signals on other layers with vias. Specifically, one common method is to install a connector comprising a conductive pin that extends into the via hole from one of the external major surfaces of the PCB that makes electrical contact with the metalized wall of the via. These metal pins are called press-fit pins and rely on mechanical forces to ensure electrical connection. 
     Vias generally comprise a change point in signal flow that tends to cause a great deal of signal degradation, particularly with respect to signals in the radio and microwave frequency range, and more specifically signals having frequencies over 100 MHz. Vias generally look like a capacitance to high frequency signals passing therethrough. A copper trace on a PCB typically has an impedance of 50 for a single ended trace or 100 ohms for differential signals, whereas a via typically has a much lower impedance, that impedance being primarily capacitive. Typically, the longer the via, the greater the capacitance and, therefore, the greater the signal degradation. This problem is particularly acute with respect to vias used for connectors because such vias must have a certain close spacing (usually standardized) in order to properly mate with the connector pins of a plug-in card. Also, the backplanes on which such connector vias are commonly found tend to be rather thick PCBs because they commonly must accommodate a large number of layers due to the need to route a large number of signals over the PCB. 
     Accordingly, it is common practice in PCB fabrication to drill vias completely through the PCB, plate the entire via with copper or another conductive material, and then counterbore the vias to remove the unnecessary copper therein. The unnecessary copper in any given via is the copper that runs between any layers of the PCB that are not being electrically interconnected by that via. For instance, if a particular via is provided to interconnect the topmost external layer with the second topmost, internal layer of the PCB, then the via would be counterbored from the bottommost external layer up to but just short of the second topmost layer. 
     For instance,  FIGS. 1A and 1B  show a conventional via before and after counterboring. In these Figures, the actual PCB dielectric material is not shown for sake of clarity. However, it will be understood that the spaces between the conductive layers are occupied by the PCB dielectric material. In this example, the via  101  is a copper plated hollow tube running the entire depth of the PCB. This PCB has five conductive layers, namely, top external layer  103 , bottom external layer  105 , a first ground plane layer  107 , a second ground plane layer  109  and a signal layer  111  sandwiched between the two ground plane layers  107 ,  109 . The ground plane layers  107  and  109  generally are each essentially complete sheets of copper  108 ,  110 , respectively, that substantially occupy the entire layer except for areas  121 ,  123  immediately surrounding the vias since, generally, each via forms part of a signal path between two electronic components and, therefore should not be shorted to ground. These areas around the vias that do not comprise copper are commonly called anti-pads  121 ,  123 . On the top and bottom layers  103  and  105  and any signal layer, such as signal layer  111 , to which the via  101  is to make a connection, a copper pad is formed around and in contact with the copper plating of the via. See, for instance, top pad  113 , bottom pad  115  and signal layer pad  117 . A signal trace  119  connects to the signal layer pad  117  for carrying a signal and/or from the via  101  between the two (or more) electronic components that are to be connected using the via as part of the signal path therebetween, 
     In theory, signal lines like trace  119 , can run right up to the via  101  and the pads  113 ,  115 ,  117  can be eliminated. However, the use of pads such as pads  113 ,  115 , and  117  allows for lower manufacturing tolerances in terms of at least, position of the via and the signal traces. In this example, the via is to a connector that is to connect a signal placed on the top pad  113  by connecting a press fit pin of the connector to the signal path  119  in the signal layer  111 . 
     After the via is fully formed as shown in  FIG. 1A , the via is counterbored by drilling with a drill having a larger diameter than the via  101  from the bottom surface  105  up to, but just short of, the signal layer  111 . The shaded cylinder  125  in  FIG. 1B  represents the portion of the via and PCB that is removed by the counterboring drill. As can be seen, this counterboring eliminates almost all of the copper in the via between the bottom layer  105  and the signal layer  111 , all of which is unnecessary for purposes of connecting the top pad  113  to the signal trace  119  in the signal layer  111 . However, there is some copper left below signal layer  111  simply because the drilling can be performed only to certain practical tolerances and, therefore, room for such tolerance errors must be designed into the drilling operation to assure that the drill does not inadvertently drill into the signal layer  111 , which would break the desired signal path from the top pad  113  of the via to the signal path  119  in signal layer  111 . On the other hand, if the drill does not drill far enough, then more copper will remain in the via than is necessary, leading to increased signal degradation. 
     It can readily be detected if the drill drills too far by simple resistive testing. Specifically, an ohmmeter can be placed across the top pad  113  and the destination of signal trace  119 . If an open circuit is detected, the pad  117  has been breached and the PCB is defective. However, there is no easy way to determine if the drill did not drill as deeply as desired (so that the signal degradation caused by the via is greater than it needs to be). While there are ways to determine if the counterbore has not been drilled deep enough, they are not practical for standard PCB testing. For instance, the depth of the counterbore can be determined visually by observation with a microscope. However, this is not a solution that can be reasonably implemented on a production scale because of the labor and cost involved. It also is possible to test the via by placing a high frequency signal across the via and testing the output for signal degradation. However, this also is a time consuming and expensive proposition that is not reasonable to implement on a production scale. 
     SUMMARY OF THE INVENTION 
     A method is disclosed for fabricating a PCB so that is can easily be determined if a via in the PCB has not been counterbored to a desired depth. The method involves adding a conductive structure to the PCB prior to counterboring of the via that initially electrically connects the via to a layer of the PCB to which the via is not suppose to be connected when fabrication is completed and, after counterboring the via, testing the electrical continuity of the via to that layer. A PCB fabricated according to the method also is disclosed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are elevation views of a conventional via in a PCB before and after counterboring, respectively. 
         FIGS. 2A and 2B  are elevation views of a via in a PCB fabricated in accordance with the principles of a first embodiment of the invention before and after counterboring, respectively. 
         FIG. 3  is a cross-sectional plan view of the PCB of  FIGS. 2A and 2B  taken through section line  3 - 3  in  FIG. 2A . 
         FIG. 4A  is an elevation view of a PCB in accordance with the principles of an alternate embodiment of the invention. 
         FIG. 4B  is a see-through, plan view of multiple layers of the PCB of  FIG. 4A . 
         FIG. 5A  is an elevation view of a PCB in accordance with the principles of another alternate embodiment of the invention. 
         FIG. 5B  is a cross sectional plan view of the PCB of  FIG. 5A  taken through line  5 B- 5 B in  FIG. 5A . 
         FIG. 6  is an elevation view of a PCB in accordance with the principles of yet another alternate embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 2A and 2B  are elevation views before and after counterboring, respectively, of a via similar to the one shown in  FIGS. 1A and 1B , but modified in accordance with the principles of a first embodiment of the present invention. Most of the features shown in  FIGS. 2A and 2B  are the same as features shown in  FIGS. 1A and 1B . Hence, like components have been labeled with the same reference numerals in  FIGS. 2A and 2B  as in  FIGS. 1A and 1B . 
     With reference to  FIG. 2A , the top layer  103 , the bottom layer  105 , the upper ground plane layer  107 , and the signal layer  111  are essentially unchanged from  FIGS. 1A and 1B . However, the lower ground plane layer  109  differs in that, in the middle of the anti-pad  123 , a conductive pad  210  has been added surrounding the via (similar in nature to pad  117  in the signal layer  111 . This pad makes contact with the copper in the via  101 . Furthermore, a signal path  212  has been added that runs the short distance through the anti-pad area  123  between the pad  210  and the ground plane copper  110 . This connection of the via  101  to the ground plane  109  through pad  210  and trace  212  essentially short circuits to ground the signal that is supposed to be carried on trace  119  in the signal layer  111 . This, of course, is undesirable in operation. However, this short circuit will be eliminated if and when the via  101  is properly counterbored as shown in  FIG. 2B . That is, as is conventional, the via will be counterbored from the bottom surface  105  up to but just short of the signal plane  111  in order to eliminate as much of the excess, unnecessary conductor in the via  101  as possible. As discussed above in connection with  FIGS. 1A and 1B , that excess conductor is essentially all of the conductor below the signal plane  111 . Ideally, the counterbore reaches as close as possible to the signal layer  111  without breaching the pad  117  in the signal layer. 
     As previously mentioned in connection with  FIGS. 1A and 1B , after the via has been counterbored, it can be easily determined if the counterbore was drilled to deep and has breached the signal layer by means of a simple impedance test between the top pad  113  and the destination/source node of trace  119  in the signal layer. Particularly, if that impedance test measures an open circuit, then the counterbore is too deep and has breached the signal layer rendering the PCB defective. If, on the other hand, it measures a short circuit, then the counterbore has not reached deep enough. 
     However, there has been no cost- or time-effective way in the prior art to determine if the counterbore has been drilled too shallow, such that too much conductor remains below the signal layer  111 . In accordance with this embodiment, the placement of the pad  210  and trace  212  in the layer  109  directly beneath the signal layer  111  provides a simple way to determine if the counterbore is too shallow and, more specifically, to determine if the counterbore reached at least the layer  109  directly beneath the signal layer  111 . Specifically, it can be determined whether the counterbore extends at least to the layer  109  by another simple impedance test, this time between the top pad  113  and ground plane  109 . If the counterbore did not at least reach ground plane layer  109 , then the pad  210  and trace  212  will remain in layer  109  shorting the via  101  to ground. On the other hand, if the counterbore at least reached into layer  109 , then at least a portion of pad  210  and/or trace  212  will have been destroyed by the counterbore, thereby disrupting the electrical connection between the via  101  and ground. Thus, if the impedance test shows a short circuit between top pad  113  and ground, it means that the counterbore has not been drilled deep enough because it does not extend past the layer  109  underlying the signal layer. If, on the other hand, the impedance test reveals an open circuit between top pad  113  and ground, then the counterbore does, in fact, at least reach layer  109 . Thus, if the first impedance test shows a short circuit between the top pad  112  and the signal destination/source of trace  119  in the signal layer and the second impedance test shows an open circuit between top pad  113  and ground, the counterbore has been drilled to a reasonable depth. 
     In the example illustrated in  FIGS. 2A and 2B , several exemplary assumptions have been made. First, it has been assumed that the depth between ground plane layer  109  and signal layer  111  is large enough that the counterbore should extend past layer  109 . It also has been assumed that the depth between layer  109  and layer  111  is small enough that it is satisfactory if the counterbore extends at least through that layer  109 . However, these are merely exemplary assumptions. Neither is a requirement. For instance, if the former assumption is not reasonable, i.e., the distance between layer  109  and layer  111  is too small to expect every good counterbore to breach layer  109  and not breach layer  111 , then a different layer may be chosen that is farther away from layer  111  in which to add the extra pad  210  and trace  212 . In  FIGS. 2A and 2B , there is only one layer between the signal layer  111  and bottom external layer  105 . However, commonly, there will be more layers and the additional structure may be added in any of those layers. There is no requirement that the layer in which the additional structure is added be a next adjacent layer. 
     On the other hand, layer  109  were too far away from layer  111 , the design of the PCB could be modified to reduce the depth between the layers or to add another layer to the design the sole purpose of which is to add testing structure such as pad  210  and trace  212 . This type of design is discussed in more depth in connection with  FIG. 6 . 
       FIG. 3  is a cross sectional plan view taken along line  3 - 3  in  FIG. 2A . It is, in essence, a plan view of the ground plane layer  109 . As can be seen the pad  210  and trace  212  are in the anti-pad area  123  in ground plane  109 . Collectively, the pad  210  and trace  212  electrically connect the via  101  to the ground plane  109 . The circle  125  represents the diameter of the counterboring drill. It can be seen that, if the counterbore reaches ground plane layer  109 , it will completely eliminate the pad  210  as well as a portion of the trace  212 . It is only necessary that the counterbore diameter be sufficient to break the conductive continuity between the via  101  and the ground metal  110 . Thus, the drill diameter could actually be smaller that the diameter of the pad  210  or larger than the diameter of the anti-pad  123 . 
     In an alternative embodiment, it is not necessary to add a specific pad  210  and/or trace  212  in ground plane layer  109 . Rather, the anti-pad  123  could simply be eliminated so that the ground plane metal  110  reached right up to the via  101 . The counterbore would still break the conductive path between the via and the ground plane metal  110 . 
     Furthermore, while the invention had been illustrated so far with the additional structure in a ground plane, that feature also is merely exemplary. There is no reason that the pad  210  and trace  212  (or other structure) could not be added to any other type of layer, such as a signal layer. The only requirement is that the additional structure conductively connect the via to an electrical node of the PCB that is accessible for purposes of impedance testing. 
       FIGS. 4A and 4B , for instance, are elevation and plan views, respectively, of an alternative embodiment in which the additional counterbore-depth-testing structure is added to a signal layer. In this embodiment, the layers in the PCB include top, external layer  401 , first signal layer  403 , second signal layer  405 , ground plane layer  407 , and bottom, external layer  409 .  FIGS. 4A and 4B  illustrate two vias  41  land  413  that will be counterbored. The plan view of  FIG. 4B  is not a pure cross sectional view, but rather a view showing structure in each of layers  403 ,  405 , and  407 . Top and bottom layers  401  and  409  are not represented in  FIG. 4B  in order not to obfuscate the features and points being illustrated. In  FIG. 4B , each element is labeled with the reference numeral corresponding to the element as well as the reference numeral of the layer in which it is formed. The vias, of course, pass through all of the layers and, therefore, do not have a layer reference numeral. 
     Vias  411  and  413  are within a shared oval anti-pad  419  in the ground metal  418  of ground plane layer  407 , as is common. The two vias  411  and  413 , for example, serve the purpose of carrying the two ends of a differential signal to a destination node (not shown in the Figures) in layer  403  through pads  451  and  452  and signal traces  421  and  426 , respectively, in first signal layer  403 . Accordingly, it is desired to counterbore vias  411  and  413  from the bottom external surface  409  up to a depth between the second signal layer  405  and the first signal layer  403 . Accordingly, it would be desirable to position the additional structure for counterbore depth testing in layer  405 . Thus, in accordance with one particular embodiment, pads  415  and  417  are added surrounding vias  411  and  413  in second signal layer  405 . 
     Furthermore, a signal trace  421  is added running between pad  415  and a pad  422  in layer  405  surrounding and connected to a ground pin via  423  and another trace  425  is added between pad  417  and another pad  424  in layer  405  surrounding and connected to another ground pin via  427 . As noted, vias  423  and  427  are ground pin vias and therefore make contact with the ground plane of layer  407 . Such pins are already commonly formed in PCBs for purposes unrelated to any inventive features introduced in this specification. 
     In accordance with this embodiment, the depth of the counter bore can be assured to extend from the bottom layer  409  to a depth between first signal layer  403  and second signal layer  405  by testing continuity between each top pad  431 ,  433  of vias  411  and  413  on the top, external layer  401  of the PCB and any ground connection. 
     Even further, while the embodiments discussed herein above have all involved short circuiting vias to ground, that too is merely exemplary. As previously mentioned, the continuity that must be broken need not necessarily be to ground. Ground is merely frequently a convenient node, but the added structure can provide connectivity to any node on the PCB that can be accessed directly or indirectly for purposes of impedance testing. 
       FIGS. 5A and 5B  illustrate yet another embodiment of the invention in which the added structure does not connect the via(s) that are to be counterbored and tested to ground.  FIG. 5A  is an elevation view of the PCB and  FIG. 5B  is a cross sectional view taken along line  5 B- 5 B in  FIG. 5A .  FIG. 5B , therefore, is essentially a plan view of layer  505 . In this embodiment, the added counterbore-testing structure comprises pads and a trace that electrically connected these two vias to each other in the layer adjacent to the layer on which those vias are to carry signals to and/or from during operation. Particularly, this exemplary embodiment comprises five layers, including a top, external layer  501 , a first signal layer  503 , a second signal layer  505 , a ground plane layer  507 , and a bottom, external layer  509 . In this embodiment, vias  511  and  513  carry the two ends, respectively, of a differential signal between top pads  518  and  519 , respectively, and signal destinations (not shown) on first signal layer  503  through pads  514  and  516  and signal traces  515  and  517 , respectively. In accordance with this embodiment, pads  521  and  523  surrounding vias  511  and  513 , respectively, and a trace  525  running between pads  521  and  523  are all added in second signal layer  505  so that the two vias  511  and  513  are shorted to each other in second signal layer  505 . Thus, if the counterbore is deep enough (i.e., between first and second signal layers  503  and  505 , then, it will disconnect the short circuit between vias  511  and  513  in layer  505 . Thus, a simple impedance test between top pad  518  of via  511  and top pad  519  of via  513  will reveal if the via has been counter bored past layer  505 . Of course, it also can be determined if the counter bore was drilled too deeply so as to breach layer  503  by further testing continuity between each of vias  511  and  513  and the respective signal destinations of traces  515  and  517 , respectively. 
       FIG. 6  is an elevation view of another embodiment illustrating the concept of adding a conductive layer to the PCB strictly for the purpose of counterbore depth testing. This embodiment has six layers, including top layer  601 , first ground plane layer  603 , signal layer  605 , second ground plane layer  607 , testing layer  609 , and bottom, external layer  611 . Also, the diameter and desired depth of the counter bore is shown at  613 . 
     Layer  609  is an additional layer that has been added solely for the purpose of providing a layer that can be used in testing counter boring depths. Particularly, as previously mentioned in connection with the discussion of  FIGS. 2A and 2B , the traditional design of a particular PCB may not provide any layer that is appropriately spaced from signal layer  605  to provide depth testing that is deemed adequate. For instance, ground plane layer  607  may be too close to signal layer  605  to expect the counter boring drill to drill past that layer but not into the signal layer  605 . If there were another layer beneath second ground plane layer  607  in the normal design of the PCB, that layer might be too far away from signal layer  605 . Accordingly, a layer such as layer  609  may be added for the purpose of allowing testing of counter boring depth with respect to vias, such as via  602 , that carry signals to and/or from the signal layer  605  through pad  610 , and trace  612  and top pad  617 . As shown, the metal  621  on layer  609  may be wholly continuous so that it contacts the via  602  without the need to fabricate anti-pads and pads within those anti-pads with small traces connecting the pad to the main metal on that layer. 
     Furthermore, the additional layer  609  need not necessarily strictly be a conductor such as copper. It could be a resistive or capacitive material (all of which exist in PCB fabrication). For instance, layer  609  could be formed of a material that provides 50 ohms of impedance and is coupled to ground. Then, standard test equipment may determine if there is a 50 ohm impedance between top pad  617  and ground before counter boring and then determined if there is an open circuit after counter boring. This would indicate first that the additional layer  609  has been properly formed and that, after counter boring, the counterbore drill has reached past that layer, thereby breaking the connection to ground. 
     While the invention has been described above primarily in connection with the drilling of counterbores, this is merely exemplary. The invention may be applied in PCBs in which the counter bores are formed by any other present or future techniques. Furthermore, the invention has primarily been described above in connection with structure that conductively connect a via to an electrical node on the PCB. However, the additional structure need not necessarily even provide a conductive connection to another node on the PCB. Embodiments are envisioned in which the additional structure for helping determine counter boring depth provides an inductance or a capacitance between the via and a node of the PCB that can be accessed for capacitance or inductance testing. Essentially any form of electromagnetic coupling that can be disrupted or altered in a measurable way by removing the additional structure is possible. 
     Having thus described a few particular embodiments of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements as are made obvious by this disclosure are intended to be part of this description though not expressly stated herein, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only, and not limiting. The invention is limited only as defined in the following claims and equivalents thereto.