Abstract:
Methods and apparatus are provided for securely and cost effectively attaching one or more shielded cables to a planar substrate. A cable assembly includes a printed circuit board (PCB) coupled to a distal end of the one or more shielded cables. Perpendicular alignment of the distal cable ends promotes a dense array that is achieved using angular mounting brackets for coupling cable shields to electrical contacts on an engagement surface of the PCB. Mounting brackets are attached between the cable shield and shield contacts using electrically conductive attachment techniques including soldering and laser welding. The PCB also includes one or more signal contacts for each cable. Distal ends of the internal conductors are each bent about 90 degrees from the vertical cable axis to align with the horizontal engagement surface. Internal conductors are surface mounted to their respective signal contact using one or more of soldering and laser welding.

Description:
RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 60/957,350, filed on Aug. 22, 2007, the entire teachings of which are herein incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The embodiments herein generally relate to shielded cable assemblies and more particularly to a shielded cable interface module for a high-performance semiconductor tester interface. 
     BACKGROUND 
     Automatic test equipment (ATE) employed in the functional test of semiconductor devices can include a number of channel cards generating a multitude of electrical test signals directed to a semiconductor device under test (DUT). The test signals are routed through a semiconductor interface, such as a probe card for contacting electrical interconnects on the DUT. Often times, the test signals operate at high frequencies ranging from hundreds of megahertz to gigahertz. To achieve efficient signal distribution while controlling signal interference, shielded cables are typically employed. 
     Routing of test signals from the channel cards to the probe card often requires that the shielded cables be brought into closer and closer proximity with each other. Such compact routing is due at least in part to the small scale of the DUTs. An interface is typically provided between a distal end of the shielded cables to terminate the cables and route the signals to the probe card or interposer. Others have proposed cable terminations for such applications. 
     SUMMARY 
     What is needed is a method of manufacturing an assembly for interconnecting a plurality of shielded cables to a planar substrate. The method may be adapted for interfacing to a semiconductor device under test through an interposer. The assembly should accommodate a dense array of shielded cables capable of accommodating high-frequency signals while maintaining a high fidelity. 
     Various embodiments can provide an apparatus and method for manufacture for securely attaching at least one shielded cable to a planar substrate, such that a distal end of the shielded cable is aligned substantially perpendicular to the planar substrate. Substantially perpendicular alignment allows multiple shielded cables to be attached to the planar substrate, such as a printed circuit board, forming a dense array of shielded cables that is well suited to space restrictions encountered in semiconductor test applications. 
     In one aspect, the embodiments relate to a process for attaching a distal end of a shielded cable to an engagement surface of a planar substrate without using a connector. The shielded cable has at least one internal conductor and an external shield conductor. A first portion of an electrically conductive mounting bracket is attached to the engagement surface. A distal end of the internal conductor is bent such that the distal end is substantially perpendicular to a central axis of the shielded cable. The distal end of the internal conductor is attached to the engagement surface, such that the central axis of the shielded cable is not parallel to the engagement surface. In some embodiments, the distal end of the shielded cable is substantially perpendicular to the engagement surface. A distal portion of the external shield conductor is attached to the first portion of the electrically conductive mounting bracket. A second portion of an electrically conductive mounting bracket is also attached to the distal portion of the external shield conductor and the engagement surface. The distal end of the shielded cable is securely attached to the engagement surface at least in part through attachments of the distal ends of the internal conductor and the external shield conductor. 
     In another aspect, the embodiments relate to a shielded cable assembly comprising a planar substrate including an engagement surface. The engagement surface has at least one electrically conductive signal contact and at least one separate electrically conductive shield contact. The assembly includes a shielded cable having at least one internal conductor and an external shield conductor. A distal end of the internal conductor extends beyond a distal end of the external shield conductor. The distal end of the internal conductor includes a bend that is attached to and in electrical communication with the electrically conducting signal contact. The assembly also includes an electrically conducting mounting bracket. The mounting bracket is attached between and in electrical communication with each of the distal end of the external shield conductor and the electrically conductive shield contact. A distal end of the shielded cable is securely attached to and perpendicularly aligned with the engagement surface without using a connector. 
     In another aspect, the embodiments relate to an automatic test equipment (ATE) interface for coupling high-frequency tester channels to a device-under test. The interface includes a printed circuit board having a first planar surface and a second opposing planar surface. The first planar surface includes multiple signal contacts and shield contacts. The interface also includes multiple shielded cables, with each of the shielded cables having at least one internal conductor and an external shield conductor. A distal end of each of the shielded cables is perpendicularly aligned with the planar surface of the printed circuit board. A distal end of the at least one internal conductors is aligned with the planar surface. The interface also includes multiple electrically conducting mounting brackets. Each of the mounting brackets is coupled between a distal end of at least one of the shielded cables and at least one of the shield contacts. The electrically conducting mounting brackets support perpendicular alignment of the shielded cables coupled thereto without using connectors. The ATE interface couples high-frequency tester channels to a device-under test through the multiple shielded cables. 
     In yet another aspect, the embodiments relate to a shielded cable assembly including means for attaching a first portion of an electrically conductive mounting bracket to an engagement surface. A distal end of the internal conductor is bent substantially perpendicular to a central axis of the shielded cable. The assembly includes means for attaching the bent distal end of the internal conductor to the engagement surface, such that the central axis of the shielded cable is substantially perpendicular to the engagement surface. Means for attaching a distal portion of the external shield conductor to the first portion of the electrically conductive mounting bracket are also provided, as are means for attaching a second portion of an electrically conductive mounting bracket to the distal portion of the external shield conductor and the engagement surface. The vertically aligned end of the shielded cable is securely attached to the engagement surface. 
     An advantage of non-parallel orientations in the shielded cable assembly is that more shielded cable terminations can be accommodated per unit surface area which allows for a space savings that can accommodate a dense signal routing, useful in semiconductor test applications. Another advantage of perpendicular fashion of the mounting bracket in the shielded cable assembly is that the perpendicular alignment of the distal end of the shielded cable to the surface is promoted which increases the efficiency of the electrical conductive path therebetween. 
     An additional advantage of the shielded cable assembly is that the funneling of large arrays of conductors from a low-density array to a high-density array is efficiently manufactured. Another advantage of the shielded cable assembly is that impedance-matched transmission lines with low dielectric are utilized as signal conductors which maximizes signal fidelity over a wide bandwidth passing beyond several gigahertz. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
         FIG. 1  is a cross-sectional view of an exemplary embodiment of a shielded cable assembly in accordance with the principles of the present embodiments. 
         FIG. 2  is a perspective view of an exemplary embodiment of an electrically conductive mounting bracket in accordance with the principles of the present embodiments. 
         FIG. 3  is a planar view of a portion of an exemplary engagement surface of a planar substrate including electrically conductive shield contacts and electrically conductive signal contacts in accordance with the principles of the present embodiments. 
         FIG. 4  is a flow diagram of an exemplary process for attaching a distal end of a shielded cable to an engagement surface of a planar substrate in accordance with the principles of the present embodiments. 
         FIGS. 5A through 5H  are perspective diagrams of an exemplary shielded cable assembly during various stages of construction, the shielded cable assembly constructed in accordance with the principles of the present embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , a cross-sectional view of an exemplary embodiment of a shielded cable assembly  10  is shown. The assembly  10  includes at least one shielded cable  12  having a distal end attached to a planar substrate  14 . The shielded cable  12  includes at least one internal electrical conductor, or signal conductor  16  electrically insulated from an electrically conductive shield conductor  18 . One or more electrically insulating layers  20  are disposed between the signal conductor  16  and shield conductor  18 . In some embodiments, an external cable jacket  22  surrounds the signal and shield conductors  16 ,  18  electrically insulating the shielded cable  12 . 
     Prior to attachment, the distal end of each shielded cable  12  is prepared by removing a portion of the cable jacket  22  to expose a distal portion of the shield conductor  18 . A distal portion of the shield conductor  18  and the electrical insulation  20  is also removed to expose a distal portion of the signal conductor  16 . To promote surface attachment to the planar substrate  14 , a distal end portion  24  of the exposed signal conductor  16  is bent. Preferably, the bend is about 90 degrees measured from the cable axis, such that the bent end portion  24  is parallel to an engagement surface  26  of the planar substrate  14 , while the distal end of the shielded cable  12  is perpendicular to the engagement surface  26 . Surface attachment of the shielded cable  12  to the engagement surface  26  avoids the need for through bores for at least the one signal conductors  16  and shield conductor  18 . Avoiding through bores sized for cable conductors  16 ,  18  promotes control of physical surface contours on a second opposing surface  34 . Namely, surface protrusions and discontinuities that might otherwise result from protruding cable conductors  16 ,  18  are avoided allowing for the smoother second opposing surface  34 . A smooth surface  34  with controlled contacts  33 ,  37   a ,  37   b  is better adapted for abutting an interposer as used in semiconductor test applications. 
     In some embodiments, the planar substrate is a printed circuit board  14  including a respective electrically conductive signal contact  28  for each of the one or more signal conductors  16  and at least one electrically conductive shield contact  30   a ,  30   b  (generally “ 30 ”). Each signal contact  28  respectively includes an electrically conductive signal path  32  extending from the engagement surface  26  to a second opposing surface  34  of the printed circuit board  14 . Each signal path  32  is in electrical communication with the respective signal contact  28 . Similarly, each of the at least one of the shield contacts  30  respectively includes at least one electrically conductive shield path  36   a ,  36   b  (generally “ 36 ”) extending from the engagement surface  26  to the second opposing surface  34  of the printed circuit board  14 . Each of the at least one shield paths  36  is also in electrical communication with the respective shield contact  30   a ,  30   b.    
     At least one electrically conductive mounting bracket  38   a ,  38   b  is coupled between the exposed distal portion of the shield conductor  18  and a respective one of the shield contacts or pads  30   a ,  30   b . For example, a first mounting bracket portion  38   a  includes a base  40   a  aligned with the horizontal engagement surface  26  and coupled to one of the shield contacts  30   a . The first mounting bracket portion  38   a  also includes a vertical member  42   a  integrally formed with the base  40   a  and extending perpendicularly away from the engagement surface  26 . The vertical member  42   a  is coupled to a portion of the distal end of the exposed shield conductor  18 . The first mounting bracket portion  38   a  is in electrical communication with both the shield conductor  18  and the first shield contact  30   a , forming a low impedance electrically conductive path therebetween. 
     Generally, references to alignment of shielded cables to an engagement or mounting surface (e.g., printed circuit board) refers to alignment of distal end portions of the shielded cables in the immediate vicinity of the mounting surface. There are no intentions to impose restrictions on the routing of more proximal portions of the cable. Additionally, although “vertical” and “perpendicular” are used in exemplary embodiments, it is understood that other cable orientations are applicable. For example, a shielded cable may be aligned at any angle between 0 and 90 degrees with respect to a mounting surface. At least one advantage to non-parallel orientations is an accommodation of more shielded cable terminations per unit surface area. Such space savings accommodate a dense signal routing, useful in semiconductor test applications. 
     In some embodiments, a second mounting bracket portion  38   b  also having a base  40   b  and vertical member  42   b  is similarly coupled to a portion of the distal end of the exposed shield conductor  18 . The second mounting bracket portion  38   b  is also in electrical communication with both the shield conductor  18  and the second shield contact  30   b , forming a low impedance electrically conductive path therebetween. Preferably, or high-frequency applications, combination of the vertical members  42   a ,  42   b  of the first and second mounting bracket portions  38   a ,  38   b  are coupled to substantially the entire perimeter the exposed distal shield conductor  18 , to prevent the leakage of electrical current therethrough. 
     In some embodiments, an electrically conductive ferrule  44  is coupled between an outer periphery of the exposed distal end of the shield conductor  18  and the vertical members  42   a ,  42   b  of the first and second mounting bracket portions  38   a ,  38   b . An adhesive compound  43  can be applied to a portion of the engagement surface  26  in the immediate vicinity of the distal end of the shielded cable  12  to promote a secure attachment of the shielded cable  12  to the printed circuit board  14 . The adhesive compound  43  is in communication with one or more of the exposed distal end of the signal conductor  24 , an exposed end of the cable insulation  20 , the exposed distal end of the shield conductor  18 , one or more of the first and second mounting bracket portions  38   a ,  38   b , and one or more of the signal contact  28  and shield contacts  30 . Preferably, the adhesive compound  43  is non-conducting, such as an epoxy-based compound. More preferably, the adhesive compound  43  is matched to a structural impedance of the cable-printed circuit board assembly  10 . 
     Referring to  FIG. 2 , a perspective view of an exemplary embodiment of an electrically conductive mounting bracket  50  is shown. The bracket  50  includes a vertical member  52  defining an interior, perpendicular alignment surface  54 . The alignment surface  54  is adapted to abut at least one of the exposed distal end of the shield conductor  18  ( FIG. 1 ) and the electrically conductive ferrule  44  ( FIG. 1 ), when included. The mounting bracket  50  also includes a base or footing  56  disposed along a bottom edge  58  of the vertical member  52 . The footing  56  is perpendicularly aligned with the vertical member  52  and extends away from the vertical member  52  in an outward direction. Together, the vertical member  52  and the footing  56  form an L bracket. In some embodiments, the footing  56  extends along the entire bottom edge  58  as shown. In other embodiments, the footing  56  extends along one or more sub-regions  59  of the bottom edge  58 , leaving gaps therebetween. 
     Each of the vertical member  52  and the footing  56  are each formed from an electrically conductive material, such as a metal. In some embodiments, the vertical member  52  and the footing  56  are integrally formed together. For example, the mounting bracket  50  can be formed by first cutting a shape from sheet metal stock. The sheet metal cut out can then be bent along a first axis to form the two regions of the L bracket. The sheet metal cutout can be further bent along a radius about perpendicular axis adding contour shaped to engage the shield conductor  18  or ferrule  44 . Beneficially, the L-bracket shape configures the cable alignment surface in a perpendicular fashion to the engagement surface  26 , thereby promoting perpendicular alignment of a distal end of the shielded cable  12  to the printed circuit board  14 . 
     Referring to  FIG. 3 , a planar view of a portion of an exemplary engagement surface of a printed circuit board  14  is shown including electrically conductive shield contacts  30   a ,  30   b ,  30   c  and electrically conductive signal contacts  28   a ,  28   b  formed on the engagement surface. A pair of shield contacts  30   a ,  30   b  surround a corresponding pair of signal contacts  28   a ,  28   b . Such a configuration of shield and signal contacts  30   a ,  30   b ,  28   a ,  28   b  is adapted for a twin axial shielded cable including two internal signal conductors surrounded by a cylindrical cable shield. To promote transfer of signals from the cable signal conductors to the second opposing side of the printed circuit board  14 , each of the signal contacts  28   a ,  28   b  respectively includes at least one electrically conductive path, or via  32   a ,  32   b  (generally  32 ) viewed end on (see  FIG. 1 ). Similarly, to promote efficient transfer of electrical currents from the cable shield to the second opposing side of the printed circuit board  14 , each of the shield contacts  30   a ,  30   b , respectively includes one or more vias  36   a ,  36   b ,  36   c ,  36   d ,  36   e ,  36   f  (generally  36 ), also viewed end on. 
     At least one consideration in the positioning of the shield and signal vias  36 ,  32  on the printed circuit board  14  involves maintaining a characteristic impedance environment vertically through the board  14 . Any mismatch or discontinuity in the characteristic impedance of the cable will result in a loss of signal fidelity due at least in part to signal reflections. This can be straightforward in a coaxial cable, where the shield continuously surrounds the center conductor, but is not so simple to solve when transitioning from a shielded cable to a printed circuit board  14 . Because the engagement surface  26  is formed on a printed circuit board  14  substrate, forming surface structures on the board  14  through plating and etching techniques are straightforward and relatively inexpensive. 
     At least one positioning of the shield vias  36  around the signal vias  32  in a precise alignment can be used to accomplish a preferred characteristic impedance, such as 50 Ohms. For a configuration of six shield vias  36  to a two signal via  32  grouping, the ground vias circumscribe an oval  39  (shown in phantom) having a first diameter along a minor axis of about 0.190 inch and a second diameter along a major axis of about 0.250 inch. 
     Referring to  FIG. 4 , a flow diagram of an exemplary process for attaching a distal end of a shielded cable  12  to an engagement surface  26  of a planar substrate  14  ( FIG. 1 ) is shown. The process includes attaching a first portion of an electrically conductive mounting bracket  38   a  ( FIG. 1 ) to the engagement surface  26  at Step  60 . This can be accomplished by any conventional means of attaching that preserves electrical continuity. For example, a base portion  40   a  ( FIG. 1 ) of the mounting bracket  38   a  can be soldered to the first shield contact  30   a . Alternatively or in addition, the base portion  40   a  can be welded to the first shield contact  30   a . Laser welding is well suited to welding in such small space as energy can be focused onto the base portion  40   a  using a high energy beam of light. Such a method of attaching two surfaces is well known to those skilled in the art. 
     In preparation for attachment to the engagement surface  26 , a distal end of the internal signal conductor  16  ( FIG. 1 ) is bent to be substantially perpendicular to a central axis of the shielded cable  12  at Step  62 . The bent distal end  24  ( FIG. 1 ) of the internal signal conductor  16  is attached to the engagement surface  26  at Step  64 . When attached, the central axis of the shielded cable  12  is substantially perpendicular to the engagement surface  26 . The distal end  24  of the signal conductor  16  can also be attached to the corresponding signal contact  28  by soldering. When soldering is also used to attach the first mounting bracket  38   a , a lower melting point solder is used, such that the application of heat to the bent distal end  24  does not disrupt the attachment already made to the first mounting bracket  38   a . Alternatively or in addition, laser welding can also be used to establish attachment of the bent distal end  24  to the signal contact  28 . 
     A distal portion of the external shield conductor  18  ( FIG. 1 ) is attached to the first portion of the electrically conductive mounting bracket  38   a  at Step  66 . Finally, a second portion of an electrically conductive mounting bracket  38   b  ( FIG. 1 ) is attached to the distal portion of the external shield conductor  18  and the engagement surface  26  at Step  68 . When completed, the distal end of the shielded cable  12  is vertically aligned with and securely attached to the engagement surface  26 . Once again, soldering, laser welding, or a combination of soldering and laser welding can be used to make each of the attachments. When solder is used for more than one of the attachments, solders having lower melting points are used for subsequent attachments so as not to disturb earlier attachments. 
     Referring to  FIGS. 5A through 5H , perspective diagrams of an exemplary shielded cable assembly is shown during various stages of construction. In  FIG. 5A , an exemplary printed circuit board  70  is shown. The printed circuit board  70  includes an engagement surface  72  with a number of different contacts formed thereon. In the exemplary embodiment, the engagement surface includes five pairs of shield contacts  74   a ,  74   b ,  74   c ,  74   d ,  74   e  (generally “ 74 ”), with each pair  74  including opposing shield contacts  76   a ,  76   b . Also disposed between each of the pairs of shield contacts  74  are five pairs of signal contacts  78   a ,  78   b ,  78   c ,  78   d ,  78   e  (generally “ 78 ”). Thus, the exemplary engagement surface  72  is adapted to attach to a rectangular 5×5 array of 25 shielded twinaxial cables (not shown). Other array shapes are possible, such as hexagonal arrays and spiral arrays. A first electrically conductive mounting bracket  80   a  is positioned in alignment with a first opposing shield contact  76   a  of the first pair of shield contacts  74   a.    
     Referring to  FIG. 5B , the first mounting bracket  80   a , also referred to as a grounding fence  80   a , includes a base portion  82   a . The base portion  82   a  is attached to and in electrical communication with the first opposing shield contact  76   a . Referring to  FIG. 5C , a first shielded twinaxial cable  84   a  is suitably prepared by exposing distal ends of the two internal signal conductors  86   a ,  86   b  and a distal portion of the shield conductor  88 . As shown in  FIG. 5D , the end portions of the two exposed distal ends of the internal signal conductors  86   a ,  86   b  are each bent to an angle α of about 90 degrees measured from the cable axis  90 . The pair of bent signal conductor ends  86   a ,  86   b  are aligned above a respective pair or signal contacts  78   a.    
     As shown in  FIG. 5E , the signal conductor ends  86   a ,  86   b  are coupled to and in electrical communication with the respective pair of signal contacts  78   a  using any of the attachment techniques described herein. The process is repeated for each of the remaining four cables  84   b ,  84   c ,  84   d ,  84   e  (generally “ 84 ”) forming a first row of attached cables  84 . In some embodiments, referring to  FIG. 5F , an adhesive compound, such as a non-conductive epoxy  92  is applied to the signal conductor ends  86   a ,  86   b  and surrounding engagement surface for each of the cables  84 . Care can be taken to ensure that the applied epoxy  92  does not interfere with later attachment of a second portion of the cable shield or ground fence. 
     As shown in  FIG. 5G , a second ground fence  80   b  is attached to a second one of the shield contacts  76   b  and to an opposite side of each distal end of the cables  84 . The second ground fence  80   b  also includes a base portion, or footing  82   b  that is attached to and in electrical communication with the second shield contact  76   b . The second grounding fence can be attached to one or more of the exposed shield conductors  88  (not shown) and the shield contact  76   b  using any of the electrically conductive attachment means described herein. Referring to  FIG. 5H , the process illustrated in  FIGS. 5A through 5G  can be repeated for the remaining four rows of cables in the exemplary 5-row assembly resulting in a cable assembly  94  including a dense array of 25 shielded cables, each perpendicularly coupled to one side of a printed circuit board  70 . 
     Those skilled in the art will appreciate the many benefits and advantages afforded by embodiments of the present invention. In particular, funneling large arrays of conductors from a low-density array to a high-density array may be accomplished by an efficient and highly manufacturable method. Additionally, by enabling the use of impedance-matched transmission lines with low dielectric as signal conductors, signal fidelity can be maximized over a wide bandwidth passing beyond several gigahertz. 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.