Abstract:
A plating tail connected to a signal trace for use during an electroplating operation is fabricated such that it has a substantially different impedance from the signal trace at a characteristic frequency in use, so that adverse reflections during operation are reduced below a threshold of significance.

Description:
TECHNICAL FIELD  
       [0001]     The field of the invention is that of packaging integrated circuits (“IC”s), in particular substrates for supporting ICs.  
       BACKGROUND OF THE INVENTION  
       [0002]     In the field of packaging ICs, including supporting ICs on printed circuit boards and other substrates, the standard method of forming interconnecting members, referred to as signal traces, includes a step of electroplating a conductor, such as gold. The electroplating process requires a conductive path between the trace receiving the gold and a power supply.  
         [0003]     The standard physical construction is the fabrication of a conductive path attached to the signal trace at some convenient location and extending through or along the surface of the substrate to a suitable connection point. The conductive path is referred to as a plating tail. In high-frequency (high-speed) circuits, there can be a significant problem resulting from plating tails, in that the signal can reflect off the end of the tail or a step in the tail and then interfere with an IC, e.g. by canceling the desired signal.  
         [0004]     A common solution to this problem is to remove all or most of the tail. e.g. by etching selected areas on the substrate. Such a selective etching process requires that the areas be defined by a mask that protects the areas that are not to be etched, and imposes an additional cost on the product, even if there are no other problems.  
         [0005]     It would be advantageous to eliminate the need to define selected areas and then etch them.  
       SUMMARY OF THE INVENTION  
       [0006]     The invention relates to a substrate for supporting ICs that leaves plating tails in place, but structures them so they do not interfere with signals being processed.  
         [0007]     A feature of the invention is the connection of a conductive via to a signal trace, the via being connected to a plating tail that has a characteristic impedance such that the tail does not interfere with the signals.  
         [0008]     A feature of the invention is the modification of a ground plane to alter the impedance of a plating tail.  
         [0009]     Another feature of the invention is the alteration of a transverse dimension of a plating tail to alter its characteristic impedance.  
         [0010]     Yet another feature of the invention is the limitation of the length of the vias to an amount that reduces the reflected amplitude.  
         [0011]     Yet another feature of the invention is the grouping of plating tails in a location where they can be removed by a non-critical process. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  shows, in partially pictorial, partially schematic form, an exploded view of an embodiment of the invention.  
         [0013]      FIGS. 2A and 2B  show an alternative embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0014]     In  FIG. 1 , an IC  10  at the top of the Figure, is connected by wire bond  12  to wirebond pad  14  that, in turn, is connected to conductive line  16  on the surface  120 - 4  of a substrate. Contact pad  18 , at the end of the signal trace collectively denoted with numeral  15  and comprising elements  12 ,  14 ,  16  and  18  attaches to an external contact that carries the signal in question off the substrate.  
         [0015]     A plating connection sets up a conductive path during the plating process between signal trace  15  and a power supply. The plating connection comprises a stub  20 , connecting pad  18  to conductive via  30  that extends vertically down and away from surface  120 - 4  through other surfaces. An example of other surfaces that are part of the substrate include a ground plane  120 - 3 , a second ground plane  120 - 2  and a bottom surface  120 - 1 .  
         [0016]     At the bottom of the figure, rectangle  120 - 1  represents the bottom surface of the substrate, to which is attached the end of via  30  and plating tail  36 . Those skilled in the art will be aware that plating tail  36  will have a tail characteristic impedance at the operating frequency of the signals carried by trace  15  that is determined by the cross section of tail  36 , and the thickness  122 - 1  of the dielectric between tail  36  and the adjacent ground plane  120 - 2  (plus the dielectric constant, horizontal distance to other conductors, etc.). The standard impedance used in the industry is 50 ohms, though other values could be used.  
         [0017]     According to the invention, the impedance of tail  36  is changed from the impedance used elsewhere in the apparatus, (for example 50 ohms) to a value that is sufficiently different that the signal will be reflected as though it were an open circuit (i.e. the portion of the signal that travels down via  30  will be reflected back with the same polarity and a reduced amplitude). The impedance of the plating tail does not have to be infinite, of course. An impedance of 100 ohms (compared to a nominal trace impedance of 50 ohms) will be sufficient for practical purposes. The magnitude of impedance difference that will be satisfactory depends on the magnitude of the reflected signal and also on the sensitivity of the next IC in the system. Since the electrical path length of the plating tail is reduced to the length of via  30  and since that length is only a millimeter or so (characteristic of substrate thickness in contemporary practice), the amplitude is reduced considerably. A rule of thumb is that a distance of about {fraction (1/10)} wavelength will reflect signals with an amplitude that can be ignored; i.e. the amplitude of the reflected signal is below a “reflection threshold” that would adversely affect the circuit and can be ignored. The thickness of via  30  will be in that range for most ICs of current interest. The effect of reflecting at an open circuit instead of at a short is that the reflection is positive, so that the reflection does not tend to cancel the signal.  
         [0018]     Referring back to  FIG. 1 , the mechanism used to accomplish the desired impedance change is that ground plane  120 - 2  has a “voided area”  34 , meaning that the conductive sheet in that area is removed. The conductive sheet in the remainder of the ground plane is available to set the impedance for other interconnections positioned above plane  120 - 2  or below it (i.e. elsewhere on surface  120 - 1 ). Ground plane  120 - 2  is spaced from surface  120 - 1  by distance  122 - 1 , which is set such that other traces on surface  120 - 1  have the nominal impedance that is designed. Alternatively, the transverse dimensions of tail  36  could be set to differ from the trace characteristic impedance by increasing the width substantially from the standard trace width.  
         [0019]     For purposes of illustration, a second conductive plane  120 - 3 , which may be a ground plane or may be a plane carrying signal traces has a clearance space  32  separating via  30  from any conductor in that plane, so that undesired reflections (or short circuits) do not come from the conductor and affect signals on trace  15 . The view in  FIG. 1  is simplified for purposes of illustration. There may be other ICs in view and there may be other layers, conductive or dielectric in the substrate.  
         [0020]     The operating frequency referred to above will, as is apparent to those skilled in the art, be the fundamental frequency of the Fourier decomposition of the signal. In addition, the harmonics of the fundamental frequency of the Fourier decomposition are to be included in the analysis of circuit performance.  
         [0021]     Referring now to  FIG. 2 , there is shown an alternative embodiment, in which plating tail  36  is removed. In this embodiment, the substrate has only a single layer, seen in cross section in  FIG. 2B . Signal trace  16  extends along the bottom of layer  120 - 4  to via  30 . Plating tail  36  would have been at the interface between layers  120 - 4  and  120 - 3 , but was removed before the two layers were bonded. The location of plating tail  36  is shown as a dotted line in top view  2 A, showing the same chip  10 , wire bond  12 , bond pad  14  and trace  16  and contact pad  18 .  
         [0022]     In his case, the end of via  30  is a true open circuit. Preferably, the substrate is laid out so that plating tails are grouped together in areas that do not contain signal-carrying conductors. This feature is not essential, but is convenient, because the mask defining the etching are is then non-critical. Alignment of the mask can be very loose if no signal traces to be protected are in the vicinity. The surface carrying the plating tails can have groups of them located at various positions, so long as there is a clear separation between the blocks that can be etched (referred to as a “block etch”) with a mask region that covers a number of plating tails, rather than having a mask opening for each plating tail to be removed. A mechanical process, such as grinding or scraping could also be used if the manufacturer prefers.  
         [0023]     While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced in various versions within the spirit and scope of the following claims.