Patent Publication Number: US-6911835-B2

Title: High performance probe system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a continuation-in-part of pending U.S. patent application Ser. No. 10/142,548, filed May 8, 2002. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a system for providing paths suitable for high frequency signals passing between an integrated circuit (IC) test equipment and pads on the surfaces of ICs to be tested. 
   2. Description of Related Art 
   Integrated circuits (ICs) are often tested while still in the form of die on a semiconductor wafer. The following U.S. patents describe exemplary probe board assemblies for providing signal paths between an integrated circuit tester and input/output (I/O), power and ground pads on the surfaces of ICs formed on a semiconductor wafer: U.S. Pat. No. 5,974,662 issued Nov. 2, 1999 to Eldridge et al, U.S. Pat. No. 6,064,213 issued May 16, 2000 to Khandros, et al and U.S. Pat. No. 6,218,910 issued Apr. 17, 2001 to Miller. 
     FIG. 1  is a plan view and  FIG. 2  is a sectional elevation view of an exemplary prior art probe board assembly  10  for providing signal paths between an integrated circuit tester  12  and ICs  14  formed on a semiconductor wafer  16 . Tester  12  implements one or more tester channels, each providing a test signal as input to one of ICs  14  or receiving and processing an IC output signal to determine whether the IC output signal is behaving as expected. Probe card assembly  10  includes a set of pogo pin connectors  26  and a set of three interconnected substrate layers including an interface board  20 , an interposer  22  and a space transformer  24 . Pogo pins  28  provide signal paths between tester  12  and contact pads  30  on the upper surface of interface board  20 . Interface board  20  is typically a multiple layer printed circuit board including microstrip and stripline traces for conveying signals horizontally and vias for conveying signals vertically between pads  30  on its planar upper surface and a set of contact pads  32  on its planar lower surface. 
   Interposer  22  includes one set of spring contacts  34  mounted on its upper surface and a corresponding set of spring contacts  36  mounted on its lower surface. Each spring contact  34  contacts a separate one of the pads  32  on the lower surface of interface board  20 , and each spring contact  36  contacts one of a set of pads  38  on the upper surface of space transformer  24 . Vias passing through interposer  22  provide signal paths between corresponding pairs of spring contacts  34  and  36 . 
   Space transformer  24  provides signal paths linking spring contacts  36  to a set of probes  40  arranged to contact I/O, power and ground pads  44  on the surfaces of a set of ICs  14  to be tested. A chuck  42  positions wafer  16  with probes  40  in alignment with IC pads  44  of the ICs  14  to be tested. After one group of ICs  14  have been tested, chuck  42  repositions wafer  16  so that probes  40  access the IC pads  44  of a next group of ICs to be tested. 
   Various types of structures can be used to implement probes  40  including, for example, wire bond and lithographic spring contacts, needle probes, and cobra probes. In some probe systems, probes  40  are implemented as spring contacts formed on the lower surface of space transformer  24  with their tips extending downward to contact IC pads  44  on the surfaces of ICs  14 . Alternatively, spring contact type probes  40  are attached to the IC&#39;s pads  44  with their tips extending upward to contact pads on the lower surface of space transformer  24 . 
   A test signal generated by a tester channel implemented within one of circuit boards  18  travels through a pogo pin  28  to one of pads  30  on the surface of interface board  20 , and then travels through traces and vias within interface board  20  to one of pads  32  on its lower surface. The test signal then passes through one of spring contacts  34 , through a via within interposer  22 , and through one of spring contacts  36  to one of contacts  38  on the surface of space transformer  24 . Traces and vias within space transformer  24  then deliver the test signal to a probe  40  which then conveys the test signal to an IC pad  44  on the surface of one of ICs  14 . An IC output signal produced at one of IC pads  44  follows a similar path in an opposite direction to reach a channel within one of circuit boards  18 . As described in detail in the aforementioned U.S. Pat. No. 5,974,662, interposer  22 , with its flexible spring contacts  34  and  36 , provides compliant electrical connections between interface board  20  and space transformer  24 . Probes  40  may be made sufficiently resilient to compensate for any variation in elevation of the IC pads  44  on the upper surfaces of ICs  14 . 
     FIG. 2  has an expanded vertical scale to more clearly show the various components of probe board assembly  10 . The horizontal area over which pogo pins  28  are actually distributed is typically many times larger than the area over which probes  40  are distributed. Probe card assembly  10  is well adapted for connecting I/O ports of tester channels that are distributed over a relatively wide horizontal area to a set of probes  40  that are aligned to access IC pads  44  that are densely packed into a relatively small horizontal area. 
   One problem probe board assembly  10  shares to some degree with any interconnect system, is that the signal paths it provides tend to distort and attenuate signals, particularly signals having high frequency components. What is needed is a probe board assembly for providing signal paths between an IC tester and pads on one or more ICs, wherein at least some of the IC pads transmit and receive high frequency signals. 
   BRIEF SUMMARY OF THE INVENTION 
   A system for providing signal paths between an integrated circuit (IC) tester and input/output (I/O). Exemplary embodiments of the invention may include a probe board assembly, a flex cable and a set of probes arranged to contact the IC&#39;s pads. The probe board assembly includes one or more substrate layers (which may be rigid) and signal paths through the substrate layer(s) for linking the tester to one set of the probes. The flex cable includes a flexible substrate structurally linked to a layer of the probe board assembly and a set of signal paths through the flexible substrate for linking the tester to another set of the probes. A flex strip may alternatively be disposed behind a substrate to which the probes are attached. 
   The claims appended to this specification particularly point out and distinctly claim the subject matter of the invention. However those skilled in the art will best understand both the organization and method of operation of what the applicant(s) consider to be the best mode(s) of practicing the invention, together with further advantages and objects of the invention, by reading the remaining portions of the specification in view of the accompanying drawing(s) wherein like reference characters refer to like elements. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a plan view of a prior art probe board assembly for providing signal paths between an integrated circuit (IC) tester and input/output, power and ground pads on an array of ICs, 
       FIG. 2  is a sectional elevation view of the prior art probe board assembly of  FIG. 1 , 
       FIG. 3  is a plan view of a probe system in accordance with an exemplary embodiment of the invention for providing signal paths between an IC tester and pads on one or more ICs, 
       FIGS. 4 and 5  are sectional elevation views the probe system of  FIG. 3 , 
       FIG. 6  is a plan view of the flex cable termination block of  FIG. 5 , 
       FIG. 7  is a plan view of the lower surface of the space transformer of the probe system of  FIG. 3 , 
       FIG. 8  is a sectional elevation view of a probe system in accordance with a second exemplary embodiment of the invention for providing signal paths between an IC tester and pads on one or more ICs, 
       FIG. 9  is an expanded partial sectional elevation view of the probe system of  FIG. 8 , 
       FIG. 10  is a plan view of the lower surface of the space transformer of the probe system of  FIG. 8 , 
       FIG. 11  is a plan view of the lower surface of a space transformer of a third exemplary embodiment of the invention for providing signal paths between an IC tester and pads on one or more ICs, 
       FIG. 12  is an expanded partial sectional elevation view of the probe system of  FIG. 11 , 
       FIG. 13  is a sectional elevation view of a probe system in accordance with a fourth exemplary embodiment of the invention for providing signal paths between an IC tester and pads on one or more ICs, 
       FIG. 14  is a sectional elevation view of a probe system in accordance with a fifth exemplary embodiment of the invention for providing signal paths between an IC tester and IC pads on one or more ICs, 
       FIG. 15  is an expanded partial sectional elevation view of the probe system of  FIG. 14 , 
       FIG. 16  is a plan view of the lower surface of the space transformer and the flex cables of the probe system of  FIG. 14 , 
       FIG. 17  is an expanded plan view of an area of flex cable of  FIG. 16  containing a single substrate island, 
       FIG. 18  is a sectional elevation view of a probe system in accordance with a sixth exemplary embodiment of the invention for providing signal paths between an IC tester and spring contacts formed on pads on one or more ICs, 
       FIG. 19  is an expanded partial sectional elevation view of the probe system of  FIG. 17 , 
       FIG. 20  is a plan view of the lower surface of the space transformer and the flex cables of the probe system of  FIG. 17 , 
       FIG. 21  is an expanded plan view of an area of flex cable of  FIG. 20  containing a single substrate island, 
       FIG. 22A  is a side elevation view of a probe system in accordance with a seventh exemplary embodiment of the invention for providing signal paths between an IC tester, remote test equipment and pads on one or more ICs, 
       FIG. 22B  is a block diagram illustrating signal paths within the flex cable of  FIG. 22A , 
       FIG. 23A  is a plan view of a probe system in accordance with an eighth exemplary embodiment of the invention, 
       FIG. 23B  is a side elevation view of the probe system of  FIG. 23A , 
       FIGS. 24A-24G  illustrate steps in an exemplary embodiment of a process for forming probe tips on a flex cable in accordance with the invention, 
       FIGS. 25A and 25B  illustrate bottom and top views of an exemplary probe board assembly, 
       FIGS. 26A and 26B  illustrate full and partial side cross-sectional views of the probe board assembly of  FIGS. 25A and 25B , 
       FIGS. 27A-27E  illustrate partial bottom and cross-sectional views of en exemplary mutlilayer flex cable, 
       FIGS. 28A-28E  illustrate bottom and cross-sectional views of an exemplary tile attached to the flex cable of  FIGS. 27A-27E , 
       FIG. 29  illustrates a bottom view of the tile of  FIGS. 28A-28E  with added traces and probe pads, 
       FIGS. 30A and 30B  illustrate bottom and side views of the tile of  FIG. 29  added probes, and 
       FIGS. 31A and 31B  illustrate top and side-cross sectional views of another exemplary probe board assembly. 
   

   DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION 
   The present invention is directed to a probe board assembly for providing signal paths between an integrated circuit (IC) tester and input/output (I/O), power and ground pads of one or more ICs to be tested either while the ICs are still in the form of die on a semiconductor wafer or after they have been separated from one another. The specification describes exemplary embodiments and applications of the invention considered by the applicant(s) to be the best modes of practicing the invention. It is not intended, however, that the invention be limited to the exemplary embodiments described below or to the particular manner in which the embodiments operate. 
     FIG. 3  is a plan view and  FIGS. 4 and 5  are sectional elevation views of a probe system  50  in accordance with an exemplary embodiment of the invention for providing signal paths between an IC tester  52  to I/O, power and ground pads  54  on the surfaces of ICs  56 , for example, while still in the form of die on a semiconductor wafer  58 . Vertical dimensions in  FIGS. 4 and 5  are exaggerated so that the individual components forming probe system  50  may be more easily distinguished. 
   Probe system  50  includes a probe board assembly  51  having multiple interconnected substrate layers, including an interface board  60 , an interposer  62  and a space transformer  64 . Pogo pin connectors  66  within IC tester  52  include a set of pogo pins  68  providing signal paths between the tester  52  and contact pads  70  residing in on the upper surface of interface board  60 . Interface board  60  preferably, though not exclusively, comprises one or more layers of rigid insulating substrate material upon which are formed microstrip and/or stripline traces for conveying signals horizontally and through which are provided vias for conveying signals vertically between the pads  70  on its upper surface and a set of contact pads  72  on its lower surface. 
   Interposer  62  preferably, though not exclusively, includes a rigid insulating substrate having a set of flexible spring contacts  74  mounted on its upper surface and a corresponding set of flexible spring contacts  76  mounted on its lower surface. Each spring contact  74  contacts a separate one of the pads  72  on the lower surface of interface board  60 , and each spring contact  76  contacts one of a set of pads  78  on the upper surface of space transformer  64 . A set of conductive vias passing though interposer  62  provide signal paths between corresponding pairs of spring contacts  74  and  76 . 
   Space transformer  64  provides signal paths linking the pads  78  on its upper surface to a set of probes  80  arranged to contact IC pads  54  on the surfaces of a set of ICs  56  to be tested. Wafer  58  resides on a chuck  82  for positioning wafer  58  so that probes  80  contact the pads  54  of the ICs  56  to be tested. After one group of ICs  56  have been tested, chuck  82  repositions wafer  56  so that probes  80  access the pads  54  of a next group of ICs  56  to be tested. 
   As described in more detail in the aforementioned U.S. Pat. No. 5,974,662, interposer  62 , with its flexible spring contacts  74 ,  76 , provides compliant electrical connections between interface board  60  and space transformer  64 . Probes  80  may be sufficiently resilient to compensate for any variation in elevation of the pads  54  on the upper surfaces of ICs  56 . 
   Various types of structures can be used to implement probes  80  including, for example, wire bond and lithographic spring contacts, needle probes, and cobra probes. Spring contacts may be formed on a pad or other base structure of a substrate in any of a number of ways. As one example, a spring contact may be formed by wire bonding a wire to the pad and overcoating the wire with a resilient material, such as disclosed in U.S. Pat. No. 6,336,269 issued Jan. 8, 2002 to Eldridge et al., incorporated herein by reference. As another example, a spring contact may be formed lithographically by depositing material in one or more molds formed over the pad and substrate. examples of such lithographic techniques can be found in U.S. Pat. No. 6,255,126 issued Jul. 3, 2001 to Mathieu et al., and U.S. patent application Ser. No. 09/710,539 filed Nov. 9, 2000, both of which are incorporated herein by reference. U.S. patent application Ser. No. 09/746,716 filed Dec. 22, 2000 (also incorporated herein by reference) discloses yet another exemplary spring contact. 
   When spring contacts are employed to implement probes  80 , they can be formed on the pads  54  of ICs  56  when space transformer  64  includes pads  81  on its lower planar surface arranged to contact the tips of the spring contacts. Alternatively, spring contact probes  80  may be formed on the pads  81  on the lower planar surface space transformer  64  and arranged so that their tips contact the pads  54  of ICs  56 . 
   U.S. Pat. No. 6,064,213, issued May 16, 2000 to Khandros et al. (incorporated herein by reference) disclose and example of a card assembly designed to contact spring contacts formed on an IC. The following patents, each incorporated herein by reference, describe examples in which spring contact formed on a probe board assembly function as probes: U.S. Pat. No. 5,974,662 issued Nov. 2, 1999 to Eldridge et al.; U.S. patent application Ser. No. 09/810,874 filed Mar. 16, 2001; and U.S. Pat. No. 6,218,910 issued Apr. 17, 2001 to Miller. 
   A test, power or ground signal provided at an I/O port of an IC tester  52  travels through one of pogo pins  68  to one of pads  70  on the surface of interface board  60 , and then travels through traces and vias within interface board  60  to one of pads  72  on its lower surface. The test signal then passes through one of spring contacts  74 , through a via within interposer  62 , and through one of spring contacts  76  to one of pads  78  on the surface of space transformer  64 . Traces and vias within space transformer  64  then deliver the test signal to one of probes  80  which forwards the test signal to one of IC pads  54 . An IC output signal generated at one of IC pads  54  follows a similar path in an opposite direction on its way back to an I/O port of a channel within tester  52 . 
   As best seen in  FIG. 5 , probe system  50  provides a signal path between IC tester  52  and IC pads  54  suitable for conveying high frequency signals, for example up to approximately 100 GHz in frequency. A pogo pin connector  84  mounted on a lower edge of a printed circuit board within IC tester  52  provides pogo pins  83  for conveying high frequency signals between a tester channel I/O port and pads  85  formed on an end of a flex cable  86  linked to conductors within the flex cable. Opposite ends of the conductors within flex cable  86  are terminated on the lower surface of space transformer  64 . Flex cable  86  includes a flexible substrate holding conductors for conveying signals. Various types of well-known flex cables may be used to implement flex cable  86 . For example flex cable  86  may include one or more substrate layers of flexible polyimide, teflon, or other dielectric material upon which microstrip and/or strip-line conductors of copper or other conductive material are formed, for example through lithographic techniques, to provide uniform transmission line environments over the entire length of the flex cable. A flex cable  86  may provide parallel pairs of traces providing paths for high noise immunity differential signals. 
   Flex cable  86  may alternatively consist of or include one or more coaxial cables and may include other types of transmission lines formed on or within the flexible substrate for providing signal paths through the flex cable. 
   While the exemplary embodiments of the invention illustrated in  FIGS. 4 and 5  employ pogo pin connectors  66  or  84  as signal paths between IC tester  52  and probe board  50 , the signal paths between IC tester  52  and probe board  50  may be implemented in many other ways, such as for example through coaxial or flex cables or various types of well-known connectors such as SMB, SMP or SMA connectors. 
   As illustrated in  FIG. 6 , an upper end of flex cable  86  may be encased in epoxy  90  or other suitable insulating material to form a cable termination block  92 . The top of termination block  92  may be ground to a flat surface to expose ends of the conductors. Conductive material deposited on the exposed conductor ends may provide pads  85  for receiving pogo pins  83 . Each termination block  92  is suitably held by adhesive within an opening in interface board  60  with the termination block positioned so that the pads  85  on its upper surface reside in same plane on the upper surface of the interface board as pads  70  (FIG.  4 ). 
     FIG. 7  is an upward-directed plan view of the lower surface of space transformer  64  upon which four flex cables  86  are terminated. For simplicity, space transformer  64  is depicted as having an array of 36 probe pads  81  on its under surface upon which probes  80  ( FIG. 5 ) may be formed, though in practice space transformer  64  may include a much larger array of pads  81 . Exposed lower ends  94  of the conductors provided by flex cables  86  are connected (as by solder, wire bonds, conductive adhesive, or other means) to pads  96  on the lower surface of space transformer  64 . Traces  98  formed on the lower surface of space transformer  64  link some of pads  96  to some of probe pads  81 . Upward extending vias (not shown) may link other conductors  94  to traces (not shown) formed on higher layers of space transformer  64 . The higher layer traces extend to other vias (not shown) passing downward to other probe pads  81 . 
   A signal path between tester  52  and spring contacts  80  provided by pogo pins  83  and flex cable  86  of  FIG. 5  can have a higher bandwidth than a signal path passing through probe board assembly  51  because most of the higher bandwidth path consists of a highly uniform transmission line environment having evenly distributed impedance. Also the higher bandwidth path includes substantially fewer junctions between dissimilar transmission lines that can cause signal attenuation and distortion. As described above, a signal path through pogo pins  66  (FIG.  4 ), interface board  60 , interposer  62  and space transformer  64  may include 10 or more such junctions. A signal path though pogo pins  83  (FIG.  5 ), flex cable  86 , and traces  96  ( FIG. 7 ) on the lower surface of space transformer  64  includes only three transmission line junctions. 
     FIGS. 8-10  illustrate another example probe system  100  having much in common with probe system  50  of  FIGS. 3-5  and, accordingly, similar reference characters refer to similar structures. However probe system  100  differs from probe board assembly  51  not only because it employs two flex cables  86  instead of four, but also because the lower ends of the conductors within flex cables  86  are coupled to IC pads  54  in a way that bypasses spring contacts  80 . 
   As illustrated in  FIGS. 8-10 , flex cable  86  includes serpentine substrate fingers  102  containing conductors forming signal paths extending into the area under space transformer  64  occupied by probes  80 . Bypassing various probes  80 A carrying signals between space transformer  64  and various IC pads  54 A, each finger  102  extends over one or more IC pads  54 B that are to transmit or receive high frequency signals via the transmission line(s) included in the finger. Pointed conductive tips  106  formed on the underside of fingers  102  act as probes to provide signal paths between the transmission lines residing within the fingers and the high frequency IC pads  54 B. 
   Ends of spring contacts  80 B that are somewhat shorter than the spring contacts  80 A that carry lower frequency signals to and from IC pads  54 A are bonded to the upper surfaces of flex cable fingers  102  to structurally link each finger  102  to the under surface of space transformer  64 . Spring contacts  80 B do not carry signals but instead act as flexible structural member for holding fingers  102  in place under space transformer  64 , and restricting their range of motion relative to the space transformer so that they are properly aligned with IC pads  54 B. Thus the uniform transmission line environments provided by conductors within flex cables  86  extend from pogo pins  83  all the way down to the tips  106  acting as probes to contact IC pads  54 B. Note that the flex cable termination arrangement of probe system  100  eliminates probe  80  and signal paths within space transformer  64  needed by the cable termination arrangement of probe system  50  of  FIGs. 5-7  and therefore reduces the number of transmission line junctions in the signal path. 
   ICs  56  may warm up and expand while they are being tested and thereby may cause IC pads  54  to move vertically and to move apart horizontally. Fingers  102  are flexible so that tips  106  can move vertically as necessary to allow them to remain in contact with IC pads  54 B. Fingers  102  preferably extend in a serpentine manner under space transformer  64  as illustrated in  FIG. 10  to provide them with longitudinal flexibility to permit tips  54 B to move horizontally relative to one another as necessary to remain in contact with IC pads  54 B. Space transformer  64  is preferably formed of a ceramic or other substrate material having a coefficient of thermal expansion similar to that of the semiconductor material forming wafer  58 . The temperature of space transformer  64  tends to track that of wafer  58  since it is positioned very close to the wafer. When space transformer  64  has the same coefficient of thermal expansion as wafer  58 , probes  80  tend to move apart at the same rate as IC pads  54 A so that probes  80  remain in contact with IC pads  54 A. Since the serpentine flex cable fingers  102  have the flexibility to move in the horizontal plane parallel to the plane of the wafer, and since spring contacts  80 B attached to finger  102  above finger tips  54  structurally link fingers  102  to space transformer  64 , finger tips  54  also move in a vertical direction perpendicular to the plane of the wafer surface as necessary to remain in contact with pads  54 B as pads  54 B move apart with increasing wafer temperature. 
     FIGS. 11 and 12  illustrate another exemplary embodiment of the invention employing an alternative approach for terminating conductors of the flex cables of probe system  100  under space transformer  64 .  FIG. 11  an upward-directed plan view of the undersides of flex cables  86  having fingers  120  extending under space transformer  64 .  FIG. 12  is a partial sectional elevation view of one finger  120  extending between space transformer  64  and an IC  56 . Fingers  120  extend over the IC pads  54 B that are to be accessed by conductors within fingers  120 . Tips  106  on the underside of fingers  120  provide signal paths between I/O pads  54 B and the conductors within fingers  120 . Probes  80 B connected between fingers  120  and pads  81  on the under surface of space transformer  64  do not carry signals, but instead act only as flexible structural members supporting fingers  120  and restricting their range of horizontal motion. 
   As they extend over pads  54 B, fingers  120  may pass over some contacts  54 C that are to be accessed via spring contacts  80 C attached to and extending downward from pads  81  on the underside of space transformer  64 . Lower ends of spring contacts  80 C are attached to upper surfaces of vias  122  extending vertically though flex cable fingers  120  to tips  124  mounted on the under surface of flex cable  86  for contacting IC pads  54 C. Lower frequency signals may therefore pass between IC pads  54 C and pads  81  on the lower surface of space transformer  64  through probes  80 C, vias  122  and probe tips  124  while higher frequency signals entering or departing IC pads  54 B pass through probe tips  106  and conductors implemented within flex cable fingers  120 . Lower frequency signals may also pass between pads  81  and IC pads  54 A directly through probes  80 A. 
     FIG. 13  is a sectional elevation view of a probe system  110  in accordance with another exemplary embodiment of the invention that is a variation on probe system  50  of  FIG. 5 , wherein similar reference characters refer to similar structures. Probe board assembly  110  differs from probe board assembly  50  in that upper ends of conductors within flex cables  86  are terminated on pads  112  formed on the lower surface of interface board  60 . Traces and vias (not shown) formed on and within interface board  60  link pads  112  to the pads  85  on the upper surface of the interface board contacted by pogo pins  83 . 
     FIG. 14  is a sectional elevation view of a probe system  120  in accordance with another exemplary embodiment of the invention that is a variation on probe system  100  of  FIG. 8 , wherein similar reference characters refer to similar structures.  FIG. 15  is an expanded sectional elevation view of the portion of the probe system  120  of  FIG. 14  residing between space transformer  64  and ICs  56 , and  FIG. 16  is a plan view looking upward from of wafer  58  toward the under sides of flex cables  86  and space transformer  64 . 
   Probe system  120  of  FIG. 14  differs from probe system  100  in that flex cables  86  extend completely under space transformer  64  as best seen in  FIGS. 15 and 16 . Probe tips  130  mounted on the lower sides of flex cables  86  contact the IC pads  54 . Flexible spring contacts  80  attached between pads  81  on the lower surface of space transformer  64  and to pads  132  on the upper surfaces of flex cables  86  above probe tips  130  provide support for cables  86 . 
   Vias  133  through flex cables  86  may link one set of probe tips  130  to the pads  132  above the tips. IC tester  52  is therefore able to communicate with some IC pads  56  by way of paths extending through probe board  60 , interposer  62 , space transformer  64 , spring contacts  80 , vias  133  and probe tips  130 . 
   A second set of probe tips  130  formed on the lower surface of flex cables  86  are connected to the signal paths (not shown) provided by flex cable  86  so that IC tester  52  may also communicate with some of IC pads  54  through high frequency signals passing through flex cables  86  and probe tips  130 . The spring contacts  80  above the second set of probe tips  130  do not convey signals, but they do provide flexible support for the probe tips. 
     FIG. 17  is an enlarged plan view of area of flex cable  86  holding one of probe tips  130 . Parts of the substrate material of flex cable  86  are removed to create spaces  134  nearly surrounding an island  138  of flex cable substrate holding probe tip  130 . Two (or more) small, flexible serpentine bridges  140  of flex cable substrate remain to link each substrate island  138  to the main expanse of flex cable  86 . For the set of probe tips  138  that communicate with IC tester  52  through signal paths provided by flex cables  86 , those signal paths extend to that set of probe tips  130  through bridges  140 . 
   Bridges  140  and the spring contacts  80  connected to flex cable  86  above islands  138  also hold tips  130  in position above the IC pads  54  ( FIG. 13 ) they contact. As discussed above, IC pads  54  are not perfectly co-planar with one another, and they can move both vertically and horizontally as the ICs under test warm up and expand. Bridges  140  and the spring contact  80  above each probe tip  130  have sufficient flexibility to allow the probe tip  130  to move vertically as necessary to remain in contact an IC pad  54  even though the elevation of the pad may change as the IC wafer begins to warm up. 
   Although the substrate material of flex cable  86  may not have the same coefficient of thermal expansion as the semiconductor material forming wafer  58  (FIG.  1 ), the serpentine nature of bridges  140  provides them with sufficient flexibility to allow probe tips  130  to also move horizontally relative to one another and relative to the main body of flex cable  86  as necessary to remain in contact with the IC pads  54  when the pads move horizontally during thermal expansion of the ICs under test. 
     FIG. 18  is a sectional elevation view of a probe system  150  in accordance with another exemplary embodiment of the invention that is a variation on probe system  110  of  FIG. 14 , wherein similar reference characters refer to similar structures.  FIG. 19  is an expanded sectional elevation view of the portion of the probe system  150  of  FIG. 18  residing between space transformer  64  and ICs  56 ,  FIG. 20  is a plan view looking upward from wafer  58  toward the under sides of flex cables  86  and space transformer  64 , and  FIG. 20  is an enlarged view of a portion of the flex cable  86  of  FIG. 21  illustrating a single substrate island  138  linked to flex cable  86  thorough substrate bridges  140 . 
   Probe system  150  provides signal paths between IC tester of FIG.  18  and spring contacts  152  that are attached to the pads  54  of ICs  56 . Probe system  150  differs from probe system  110  of  FIG. 14  in that in probe system  150  pads  154  on the upper surface of flex cable substrate islands  138  are directly connected by a solder ball array  156  to pads  81  on the lower surface of space transformer  64 . Also a pad  158 , rather than a probe tip, is formed on the lower surface of each flex cable substrate island  138 . Pads  158  are positioned so that they may be contacted by tips of the spring contacts  152  extending upward from the IC pads  54 . 
   Vias  133  extending through some of islands  138  link one set of probe pad  158  to the pads  154  on the upper surface of the islands. IC tester  52  is therefore able to communicate with some IC pads  56  by way of paths extending through probe board  60 , interposer  62 , space transformer  64 , solder balls  156  vias  133 , pads  158  and spring contacts  152 . 
   A second set of pads  158  formed on the lower surfaces of substrate islands  138  are connected to the signal paths (not shown) provided by flex cable  86  so that IC tester  52  may also communicate with some of IC pads  54  through high frequency signals passing through flex cables  86 , substrate bridges  140 , pads  158  and spring contacts  152 . 
     FIG. 22A  illustrates another exemplary embodiment of the invention, a multiple-layer probe card assembly  160  for providing signal paths between an integrated circuit tester  162  and pads  163  on surfaces of IC dice  164  on a wafer  166  under test. Probe assembly  160  can also provide remote test equipment (not shown) with signal access to IC pads  163 . 
   Probe card assembly  160  includes a probe board  170  having a set of pads  172  on its upper surface for receiving tips of a set of pogo pin connectors  174  providing signal paths between tester  162  and pads  172 . Signal paths extending through one or more flex cables  175  interconnect a set of spring contacts  176  and  178  formed on the upper and lower surfaces of flex cable  175  provide signal paths between a set of pads  180  on the lower surface of probe board  170  and a set of pads  182  on an upper surface of a space transformer board  184 . A set of probes  186  provide signal paths between pads  188  on the lower surface of space transformer  184  and IC pads  163 . Probe board  170  and space transformer  184  may include single or multiple insulating substrate layers, traces formed on the substrate layers, and vias extending through the substrate layers for conducting signals horizontally and vertically between pads and/or contacts on their upper and lower surfaces. 
   Some of spring contacts  178  may contact signal paths within flex cables  175  that may extend to probe board  170 . Probe board  170  links some of its upper surface contacts  172  to the conductors within the flex cable  175 , thereby permitting high frequency or other signals traveling via flex cable  175  to spring contacts  178  and to by-pass transmission line junctions within probe board  170  and between pads  180  and contacts  176 . One or more conductors of flex cables  175  may extend to remote equipment (not shown) connected anywhere by any means to a rigid substrate.) 
     FIG. 22B  is a block diagram illustrating an exemplary signal routing scheme within flex cable  175 . A flex cable includes a flexible substrate that can be used like a circuit board to hold install small surface mounted devices on a flex cable, including passive devices such as resistors and capacitors and active devices including, for example, integrated circuit switches, multiplexers and the like which can act a signal routing devices.  FIG. 22B  shows a set of integrated circuit routing switches  191  powered and controlled by signals from one of tester channels for selectively linking the pads  192  on the lower surface of flex cable  175  accessed by spring contacts  178  of  FIG. 22A  to various other conductors including spring contacts  176  and flex cable conductors leading to tester  162  or to the remote equipment. 
   The switching arrangement of  FIG. 22B  is useful, for example, when IC tester  162  and other remote test equipment carry out different types of tests at the IC terminals. For example IC tester  162  may be adapted to carry out logic tests on ICs  164  while the remote equipment may be adapted to carry out parametric tests on the ICs. The remote equipment may also supply the power for the ICs being tested. Some paths to the remote equipment (such as for example those connected to power supplies) may connect directly to spring contacts  178  so that remote equipment and tester  162  can concurrently access various IC pins during a test. To increase the number of ICs that can be concurrently tested, more then one IC tester of the same type can concurrently access the ICs. This is particular feasible in low frequency testing applications where it is not necessary to minimize signal path distances between the test equipment and the ICs being tested. In such case routing switches  191  are not needed since each flex cable conductor and each spring contact  176  accesses a separate spring contact  179 . The flex cables  175  can be easily replaced with flex cables have different signal routing arrangements to accommodate changes in routing patterns resulting in changes to the ICs being tested or to accommodate changes in the test equipment. 
     FIG. 23A  is a plan view, of a pair of flex cables  86  passing under a rigid substrate  193  looking upward from a wafer  194  ( FIG. 23B ) being accessed via a set of probes  195  attached either to space transform  193  or to pads on the surface of wafer  194 . Probes  195  pass through a set of windows  196  in flex cables  86 .  FIG. 23B  is a sectional elevation view along cut line B—B of FIG.  23 A.  FIGS. 23A and 23B  illustrate an alternative approach to linking some of probes  156  to conductors  197  in flex cable  86 . A set of spring contacts  198  extending between pads on the upper surface of flex cable  86  linked to conductors  197  through vias  199  passing vertically through flex cable  86  and pads  200  on the lower surface of substrate  193 . Conductors (not shown) formed within or on the surface of space transformer  193  link pads  200  on the lower surface of substrate  193  contacted by or attached to probes  200 . This type of interconnect arrangement can be employed in lieu of or in addition to the interconnect arrangements illustrated in  FIGS. 7 ,  10 ,  11 ,  15  and  19 . 
     FIGS. 24A-24G  illustrate an exemplary process for forming contact tip structures and attaching them to pads of a flex cable so that the cables may be employed in various exemplary embodiments of the invention described herein above. As illustrated in  FIG. 24A , a set of pits  210  are suitably formed in a substrate  212  of any suitable material such as, for example, a silicon semiconductor wafer using photolithographic etching or any other suitable technique. A layer  214  of readily etchable releasing/shorting material, such as for example aluminum, is then formed over the upper surface of substrate  210  as illustrated in FIG.  24 B. As illustrated in  FIG. 24C  masking material  216  such as photoresist is then deposited on the releasing/shorting material  214  to form a set of molds  218  defining shapes of the contact tip structures. Referring to  FIG. 24D , conductive material  220  that is to form the contact tip structures is then deposited in the molds. The tip structure material  220  may be deposited by electroplating or any other known process of depositing material within a pattern masking material. As illustrated in  FIG. 24E , the masking material  216  is then removed to reveal a set of tips  222 . As shown in  FIG. 24F , the tip structures  222  are then attached to pads  224  on the flex cable  226  using joining material  228  such as, for example, conductive adhesive, solder, brazing material and the like. The release/shorting layer  218  is then removed, for example by etching, to release the tip structures  230  from substrate  210  shown in FIG.  24 G. 
   Releasing/shorting layer  214  thus not only facilitates the formation of tips  222  though electroplating, it also provides a base for tips  222  which can be easily etched to release tips  22  from substrate  210 . Releasing/shorting layer  214  may include one or more layers, with a releasing material layer being formed first and a shorting material layer being formed on the releasing material layer. 
   The particular size, shape or contour of tip structure  230  shown in  FIG. 24G  is not critical to the invention and other suitable tip structures of various sizes and shapes can be formed in a similar manner. Additional exemplary tip structures are disclosed in U.S. patent application Ser. No. 08/819,464, filed Mar. 17, 1997, now abandoned, and U.S. patent application Ser. No. 09/189,761, filed Nov. 10, 1998, both incorporated herein by reference. 
     FIGS. 25A-30B  show yet another exemplary embodiment of the invention.  FIGS. 25A-26B  show an exemplary probe board assembly  2500 .  FIG. 25A  is a bottom view and  FIG. 25B  is a top view of the probe board assembly  2500 .  FIG. 26A  is a side-cross sectional view of the probe board assembly  2500 , and  FIG. 26B  is an enhance, partial view of FIG.  26 A. 
   As shown in  FIGS. 25A and 25B , the exemplary probe board assembly includes an interface board  2502  with contact pads  2516   a - 2516   f  on one side for making contact with a tester. The interface board  2502  may be, for example, a printed circuit board, and the contact pads  2516   a - 2516   f  may be, for example, pads for making contact with pogo-style pins. The contact pads  2516   a - 2516   f  are electrically connected to electrical traces (not shown in  FIGS. 25A and 25B ) in a flex strip  2510 . The electrical traces in the flex strip  2510  make electrical connections with probes  2508  through a tile  2506  to which the probes are mounted. 
     FIGS. 25A-26B  shown one exemplary manner of make electrical connections from the contact pads  2516   a - 2516   f  to traces in the flex strip  2510  and then to the probes  2508 . As shown in  FIG. 25B , traces  2518   a - 2518   f  run from the contact pads  2516   a - 2516   f  to via pads  2520   a - 2520   f  on one side of the interface board  2502 . As shown in  FIGS. 26A and 26B , a via  2626  electrically connects each via pad  2520   a - 2520   f  to an electrical trace  2628  on the other side of the interface board  2502 . Traces  2628  are electrically connected to one or more connectors  2512  into which the flex strip  2510  may be plugged or other wise connected. In this way, traces  2610   a  of the flex strip  2510  are electrically connected to the traces  2628 . 
   Flex strip  2510  may comprise a flexible, relatively thin insulative material  2610   b  on or in which are formed a plurality of electrical traces  2610   a , a simplified example of which is shown in FIG.  26 B. The flex strip  2510  runs behind atile  2506  to which probes  2508  are attached. As best shown in  FIG. 26B , pads  2630  on the flex strip  2510  are secured to corresponding pads  2632  on the tile  2506 . Vias  2636  through tile  2506  electrically connect pads  2632  to the probes  2508 . 
   Electrically conductive paths are thus formed between the contact pads  2516   a - 2516   f  and the probes  2508 . In the example shown in  FIG. 26B , such a conductive path comprises a trace  2518  on a top side of interface board  2502 , a via pad  2520  and via  2626  through interface board  2502 , another trace  2628  on a bottom side of interface board  2502 , a connector  2512 , a trace  2610   a  on the flex strip  2510 , a pad  2630  on the flex strip, a corresponding pad  2532  on the tile  2506 , and a via  2636  through tile  2506 . It may be advantageous to match the impedances of the various parts of such a conductive path. This may be particularly advantageous if high frequency electrical signals are to traverse the conductive path. 
   As also shown in  FIGS. 25A-26B , one or more electronic components  2522  may be connected to the conductive paths described above. Examples of such electronic components include decoupling capacitors and isolation resistors. In the example shown in  FIGS. 25A-26B , such an electronic component  2522  is connected to a trace  2610   a  of flex strip  2510  near where the traces  2610   a  is connected to pad  2632  on tile  2506 . Such electronic components may alternatively be connected at other locations along the path. For example, such electronic components could be secured to either side of the flex strip  2510  between connector  2512  and tile  2506 . 
   As also shown in  FIGS. 25A-26B , the flex strip  2510  may be secured to a spacing substrate  2504 , which itself is secured to interface board  2502 . Interface board  2502  and spacing substrate  2504  may include an opening  2524 , which may provide, for example, direct access to electronic components  2522  mounted to the flex strip  2510 , as shown in  FIGS. 25B ,  26 A, and  26 B. The spacing substrate  2504  may be secured to the interface board  2502  in any manner. For example, the spacing substrate  2504  may be adhered, bolted, clamped, fastened, etc. to interface board  2502 . As another example, spacing substrate  2504  may be secured to interface board  2502  using a planarizing assembly similar to that shown in  FIG. 5  of U.S. Pat. No. 5,974,662, which is incorporated herein in its entirety by reference. Flex strip  2510  may be adhered or otherwise secured to spacing substrate  2504  and/or tile  2506 . 
   Flex strip  2510  may include one or more layers of traces.  FIGS. 27A-30  show a partial view of an exemplary flex strip  2710  with multiple layers of traces. In addition,  FIGS. 27A-30  also illustrate exemplary interconnections between traces of the flex strip  2710  and probes  3008  on a tile  2806 , which may be used with any flex strip, whether single layer or multiple layer. As will also be seen, exemplary flex strip  2510  includes optional ground planes and shielding vias. 
     FIG. 27A  shows a partial view of the bottom surface of flex strip  2710  with multiple layers of traces (two layers of traces in this example). As shown, the bottom surface of flex strip  2710  includes pads  2730   a - 2730   p . As shown in  FIGS. 27B-27E , which show various cross-sectional side views of flex strip  2710 , this exemplary flex strip  2710  includes a layer of unshielded signal traces  2738  (four are shown) and a layer of shielded signal traces  2742  (two are shown), which are shielded by ground planes  2744  and  2740 . As is known, shielded traces are particularly useful for carrying high frequency signals. The signal traces  2738 ,  2742  and ground planes  2740 ,  2744  are encased in an insulative material  2746 . Preferably, flex strip  2710  is thin and flexible. It should be noted that  FIGS. 27A-30B  are not necessarily proportional, but may in some cases be depicted out of proportion for illustration purposes. It should be noted that planes  2740  and  2744  and plane  2840  (to be discussed below) need not be grounded but may be connected to a voltage source set to a desired voltage. As is known, such an arrangement may be used to control the impedances of traces and vias. 
   Vias in the flex strip  2710  connect signal traces  2738 ,  2742  and one or both ground planes  2740 ,  2744  to pads  2730   a - 2730   p  on the bottom side of the flex strip  2710 . In the example shown in  FIGS. 27A-27E , vias  2750  connect unshielded signal traces  2738  to pads  2730   m - 2730   p  (see FIG.  27 B), and vias  2752  connect ground plane  2740  to pads  2730   i - 2730   l  (see FIG.  27 C). If, as shown in  FIG. 27C , vias  2752  must pass through signal traces  2738 , insulated passages  2756  through signal traces  2738  are provided so that vias  2752  do not make electrical contact with signal traces  2738 . As shown in part in  FIGS. 27D and 27E , vias  2766  connect ground plane  2744  to pads  2730   e  and  2730   h , and vias  2764  connect shielded signal traces  2742  to pads  2730   f  and  2730   g . Insulated passages  2762  are provided through ground plane  2740 , and insulated passages  2756  are provide through signal traces  2738  as needed (examples are shown in FIG.  27 D). As shown in  FIG. 27D , pads  2748  for an electronic component (such as electronic component  2522  shown in  FIGS. 25B ,  26 A, and  26 B) may be provided with vias  2754 ,  2755  and insulated passage  2768  as needed for making electrical connection to a signal trace. 
   As mentioned above, ground planes  2740  and  2744  shield signal traces  2742 . Vias  2766  connected to ground plane  2744  and vias  2752  shield signal vias  2764 . Thus, not only do the ground planes  2740  and  2744  provide shielding, but grounded vias  2752  and  2766  also provide shielding for the vias  2764  that are electrically connected to the shielded traces  2742 . Of course, fewer or additional shielding vias may be provided. 
     FIG. 28A  shows the bottom surface of an exemplary tile  2806  whose top surface is connected to the bottom surface of flex strip  2710 . On the top surface of tile  2806  (which is not visible in FIG.  28 A), pads are disposed in a pattern matching the pattern of pads  2730   a - 2730   p  disposed on the bottom of the flex strip  2710  (see FIG.  27 A). In  FIG. 28A , the locations of pads  2832   a - 2832   d ,  2832   e ,  2832   h , and  2832   i - 2832   l  on the top surface of tile  2806  are shown in dashed lines. The locations of pads  2832   f ,  2832   g , and  2832   m - 2832   p  on the top surface of tile  2806  correspond to the locations of pads  2870   f ,  2870   g , and  2870   m - 2870   p  on the bottom surface of tile  2806 . Pads  2730   a - 2730   p  on the bottom surface of flex strip  2710  may be interconnected to pads  2832   a - 2832   p  on the top surface of tile  2806  by any means, including without limitation solder, brazing, welding, etc. Other alternatives are possible. Clamping, adhesion, bolting, or other means may be used to hold the tile  2806  and flex strip  2710  in place against one another, and the interconnections between the pads  2730   a - 2730   p  on the bottom surface of the flex strip  2710  and the pads  2832   a - 2832   p  on the top surface of the tile  2806  may be mere pressure connections. 
   Vias  2850  (see  FIGS. 28C and 28D ) connect pads  2832   a - 2832   e  and  2832   h - 2832   l  on the top surface of the tile  2806  with a ground plane  2840  embedded in tile  2806 . Other vias  2852  (see  FIG. 28B ) and  2864  (see  FIG. 28D ) connect pads  2832   f ,  2832   g , and  2832   m - 2832   p  on the top surface of the tile  2806  with corresponding pads  2870   f ,  2870   g , and  2870   m - 2870   p  on the bottom surface of the tile. Insulated passages  2856  are provided through the ground plane  2840  as needed. Pads  2832   m - 2332   p  are thus connected to unshielded traces  2738  in flex strip  2710 , and pads  2870   g  and  2870   f  are thus connected to shielded traces  2742  in flex strip  2710 . 
   Probes  3008  may be attached to or formed on pads  2870   f ,  2870   g , and  2870   m - 2870   p  on the bottom surface of tile  2806 . Pads  2870   f ,  2870   g , and  2870   m - 2870   p  may not, however, be disposed in a pattern that corresponds to a desired pattern for the probes  3008 . In such a case, probe pads  2972  may be disposed on tile  2806  to correspond to a desired pattern of probes  3008 . Traces  2974  electrically connect pads  2870   f ,  2870   g , and  2870   m - 2870   p  to probe pads  2972 . 
   As shown in  FIGS. 30A and 30B , probes  3008  are attached to or formed on probe pads  2972 . Probes  3008  may be any type of probe. Nonlimiting examples of probes suitable for probing semiconductor devices include needle probes, buckling beam probes, bump probes, and spring probes. Nonlimiting examples of spring probes are described in U.S. Pat. No. 5,917,707, U.S. Pat. No. 6,482,013, U.S. Pat. No. 6,268,015, and U.S. Patent Application Publication 2001/0044225-A1 and U.S. Patent Application Publication 2001/0012739-A1, all of which are incorporated herein in their entirety by reference. 
     FIGS. 31A and 31B  illustrate an exemplary probe board assembly  3100  in which flex strip  3110  passes through a passage  3176  in interface board  2502  and connects with a connector  3112  located on the same surface of the interface board  2502  as contacts  2516   a - 2516   f . In the example shown in  FIGS. 31A and 31B , contacts  2516   c - 2516   e  are electrically connected through traces  2518   c - 2518   e  to connector  3112 , which makes electrical connections with flex strip  3110 . In this way, the electrically conductive paths from contacts  2516   c - 2516   e  to the flex strip  2510  are shortened. This may be particularly advantageous for transmission of high frequency signals. Indeed, such an arrangement is particularly well suited for signal frequencies in the range of about 3 to 6 gigahertz, although signals may be transmitted at less than 3 gigahertz or greater frequency than 6 gigahertz. In the example shown in  FIGS. 31A and 31B , probe board assembly  3100  is otherwise similar to probe board assembly  2500  shown in  FIGS. 25A-26A . 
   The forgoing specification and the drawings depict exemplary embodiments of the best modes of practicing the invention, and elements of the depicted best modes exemplify elements of the invention as recited in the appended claims. It is not intended, however, that the invention be limited to the exemplary embodiments described herein above or to the particular manner in which the embodiments operate. For example, while  FIGS. 7 ,  10 ,  11  and  15  illustrate exemplary embodiments of the invention employing two or four flex cables  86 , it should be understood that the number of flex cables and the number of conductors included in each flex cable can be chosen to suit the requirements of each particular test interconnect application. While pogo pin connectors  66  or  84  ( FIG. 4 ,  5 ,  13  or  14 ) may be used to link flex cables  86  or interface board  60  to tester  52 , other types of connectors known to those of skill in the art may be employed. The probe board assemblies illustrated in  FIGS. 4 ,  8 ,  13  and  14  are exemplary and may be implemented using more or fewer interconnected substrate layers. For example, the interposer  62  shown in  FIGS. 5 ,  8 ,  13 ,  14  and  18  may be eliminated and the space transfer  64  connected directly to interface board  60 . As another example, interposer  62  and space transformer  64  of those figures may be eliminated when probes  80  are formed directly on interface board  60 . Also the suggested signal frequency ranges for the various types of signal paths through the probe board assembly and flex cable are exemplary and not intended to be limiting. While the exemplary embodiments of the invention described above are adapted for linking an IC tester to ICs while still in the form of die on a semiconductor wafer, it should be understood that other embodiments of the invention may be used for linking an IC tester to ICs after they have been separated from one another, for example when held in an array on a tray. Of course, with reference to  FIGS. 25A-31B , fewer or more than six contacts and probes may be implemented in a probe board assembly, and a flex strip with any number of layers of signals may be used. 
   The appended claims are therefore intended to apply to any mode of practicing the invention comprising the combination of elements or steps as described in any one of the claims, including elements that are functional equivalents of the example elements of the exemplary embodiments of the invention depicted in the specification and drawings.