Abstract:
Wireless communications devices, such as mobile telephones and pagers, has have recently been allowed to operate at higher frequencies in the 27-32 Giga-Hertz range. These higher-frequency devices typically include a multi-tiered electronic assembly, which includes an integrated-circuit chip, a chip carrier, and a main circuit board, with the chip carrier sandwiched between the chip and the main circuit board. Testing these multi-tiered assemblies conventionally entails manually coupling test probes to specific contact regions of the circuit board, applying test signals to the board, and ultimately keeping or discarding the entire board based on the testing. This method is not only slow and wasteful, but sometimes requires the circuit board to include extra ground contacts that can disrupt normal circuit operation. Accordingly, the present inventors have devised unique test probes and related systems and methods for testing these and other high-frequency electronic assemblies. One unique probe structure includes at least one signal contact surface for contacting a signal-port trace of an electronic assembly and at least one substantially larger ground contact surface for contacting a ground pad of the electronic assembly. In another unique probe structure, a ground probe has a contact surface and a non-contact surface for overhanging a portion of a signal-port trace and thereby establishing a desired characteristic impedance. And yet another unique probe structure includes contacts for communicating electrical bias signals to the electronic assemblies, facilitating more rapid and cost-effective testing.

Description:
RELATED APPLICATION  
       [0001]    The present application is a continuation of U.S. Provisional Application No. 60/221550, which was filed on Jul. 28, 2000. This application is incorporated herein by reference. 
     
    
     
       TECHNICAL FIELD  
         [0002]    The present invention concerns testing equipment and methods for high-frequency devices, particularly test probes for wireless communications devices.  
         BACKGROUND OF INVENTION  
         [0003]    The increasing popularity of wireless communications devices, such as mobile telephones and pagers, has placed considerable demand on the limited range of broadcast frequencies that the federal government allots for these devices. In response, the federal government has extended this range to include higher frequencies. For example, the range for these devices now includes frequencies in the range of 27-32 Giga-Hertz. (A Giga-Hertz is one billion cycles or oscillations per second.)  
           [0004]    In turn, makers of communication devices now offer or intend to offer devices that function at these higher frequencies. At the heart of many of these devices is a multi-tiered electronic assembly, which includes an integrated-circuit chip, a chip carrier, and a main circuit board. The chip is soldered onto one side of the larger, and sturdier, chip carrier. The other side of the chip carrier is soldered to the main circuit board, sandwiching the chip carrier between the chip and the main circuit board. The main circuit board, known as a motherboard, includes circuitry that electrically communicates with the chip through conductors inside and on the surface of the chip carrier.  
           [0005]    One important aspect in making these multi-tiered electronic assemblies is testing their electrical capabilities. The conventional testing procedure tests each motherboard with the chip and chip carrier mounted to it. This testing, which is typically done manually, entails using test probes not only to apply test signals to inputs of the motherboard, but also to measure output signals at its outputs. A network analyzer, coupled to the test probe at the outputs, shows whether the output signals are acceptable or unacceptable. Unacceptable assemblies are generally discarded, because of the difficulty in salvaging the chip, chip carrier, or motherboard for reuse.  
           [0006]    One conventional type of test probe that is considered suitable for testing high-frequency electronic assemblies is the ground-signal-ground (GSG) single or dual signal-port probe. This probe type places each signal probe tip between two grounded probe tips, which electrically shield the signal probe tip during testing. The ends of the ground and signal tips—that is, the ends which contact the device under test—are substantially identical in structure, each having a sharp pointed end to facilitate its precise placement on conductive portions of the device under test. One example of this type probe is the PICOPROBE brand test probe from GGB Industries. (PICOBROBE appears to be a trademark of GGB Industries.) Another example is shown in U.S. Pat. No. 5,565,788.  
           [0007]    There are at least two problems that the present inventors have recognized with high-frequency applications of conventional test probes and test methods. The first problem is that proper probe operation requires the device under test, such as a motherboard assembly, include at least two ground pads, or contacts, next to each signal port being tested. The ground contacts engage the ground probe tips at the sides of the signal probe tip to shield the probe from electrical interference during testing. However, at high frequencies, these adjacent ground pads can generate parasitic resonances which frustrate normal operation of the devices.  
           [0008]    The second problem is that conventional test methods only test complete motherboard assemblies—that is motherboards with mounted chips and chip carriers. Because of the difficulty in separating chip carriers from motherboards, defective motherboard assemblies are discarded as waste, increasing manufacturing cost.  
           [0009]    Accordingly, there is a need for better test probes and testing methods for high-frequency electronic assemblies.  
         SUMMARY  
         [0010]    To address this and other needs, the present inventors have devised unique test probes for testing high-frequency electronic assemblies, such as those for wireless communications devices. One unique probe structure includes at least one signal contact surface for contacting a signal-port trace of an electronic assembly and at least one ground contact surface for contacting a ground pad of the electronic assembly, with the ground contact surface substantially larger than the signal contact surface. Another unique probe structure includes at least one signal contact surface for contacting the signal-port trace and a ground probe having a contact surface for contacting the ground pad and a non-contact surface for overhanging a portion of the contacted signal-port trace and thereby establishing a characteristic impedance. Other unique probe structures include not only the larger ground contact surface or the ground with a non-contact surface, but also contacts for communicating electrical bias signals to devices under test.  
           [0011]    Other aspects of the invention include systems and methods that incorporate one or more of unique probe structures. One exemplary system mounts one or more of the unique probe structures to a programmable XYZ table to facilitate rapid testing of chip-carrier assemblies. And, one exemplary method entails testing one or more millimeter-wave chip-carrier assemblies using a unique probe structure prior to mounting the assembly to a main circuit board, such as a motherboard. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a bottom perspective view of a high-frequency test head  100  incorporating teachings of the present invention.  
         [0013]    [0013]FIG. 2 is top perspective view of exemplary test head  100  in FIG. 1.  
         [0014]    [0014]FIG. 3 is a top perspective view of probe-support fixture  110 , a component of test head  100 .  
         [0015]    [0015]FIG. 4 is a back perspective view of front plate  117 , another component of test head  100 .  
         [0016]    [0016]FIG. 5. 1  is a perspective view of a central ground probe  160 , one component of test head  100 .  
         [0017]    [0017]FIG. 5. 2  is a cross-sectional view of central ground probe  160  taken along line  2 - 2  in FIG. 5. 1 .  
         [0018]    [0018]FIG. 6 is a perspective view of test head  100  in overhead alignment with an exemplary chip-carrier assembly  600 .  
         [0019]    [0019]FIG. 7 is a simplified cross-sectional view of test head  100  in contact with signal port traces  611  and  612  and ground pad  614  of chip-carrier assembly  600 .  
         [0020]    [0020]FIG. 8 is a perspective view of an exemplary test system  800  which incorporates exemplary test head  100 .  
         [0021]    [0021]FIG. 9 is a perspective view of a z-axis translator  818 , one component of test system  800  in FIG. 8. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0022]    The following detailed description, which references and incorporates FIGS.  1 - 9 , describes and illustrates specific embodiments of the invention. These embodiments, offered not to limit but only to exemplify and teach the concepts of the invention, are shown and described in sufficient detail to enable those skilled in the art to make and use the invention. Thus, where appropriate to avoid obscuring the invention, the description may omit certain information known to those of skill in the art.  
         [0023]    [0023]FIG. 1, a bottom perspective view, shows an exemplary high-frequency test head  100  which incorporates teachings of the present invention. Test head  100  includes a probe-support fixture  110 , left and right signal probes  120  and  130 , front and back DC probes  140  and  150 , and a central ground probe  160 . Probe-support fixture  110  holds probes  120 - 160  in a fixed spatial relationship that corresponds to the input-output arrangement of a high-frequency chip-carrier assembly (not shown in this view.) Left and right signal probes  120  and  130  include respective coaxial couplings  122  and  132 , and signal probe tips  124  and  134 . Front DC probe  140  includes front DC probe tips  142 . 1 ,  142 . 2 , and  142 . 3  which are coupled to respective DC bias feeds  144 . 1 ,  144 . 2 ,  144 . 3 , and back DC probe  150  includes back DC probe tips  152 . 1 ,  152 . 2 , and  152 . 3  which are coupled to respective back DC bias feeds  154 . 1 ,  154 . 2 , and  154 . 3 . Central ground probe  160  includes a ground offset (or non-contact) surface  166 . 1  and a ground contact  166 . 2 .  
         [0024]    More particularly, probe-support fixture  110 , which is electrically insulated from probe tips  124  and  134  and DC probes  140  and  150 , includes left and right portions  112  and  116 , a center portion  114 , and a front plate  117 . Left portion  112 , which supports left signal probe  120 , includes a bottom face  112 . 1  and a tuning conductor  112 . 2 , and right portion  116 , which supports right signal probe  130 , includes a bottom face  116 . 1  and a tuning conductor  116 . 2 . Bottom face  112 . 1  includes notches  112 . 11  and  112 . 12  and a hole  112 . 13 , which are linked via a transverse through-hole  112 . 14  for tuning conductor  112 . 2 . Likewise, bottom face  116 . 1  includes notches  116 . 11  and  116 . 12  and a hole  116 . 13 , which are linked via a transverse through-hole  116 . 14  for tuning conductor  116 . 2 .  
         [0025]    [0025]FIG. 2, a top perspective view of test head  100 , shows that center portion  114  includes a central bore  114 . 1  which directly contacts ground probe  160 . (Some embodiments may insulate probe  160  from portion  114 .) A screw  164 . 2  in probe  160  facilitates rotation of probe  160  within bore  114 . 1 , and a set screw  114 . 3  fixes the vertical and angular position of ground probe  160  within center portion  114  of fixture  110 . Additionally, set screw  114 . 3  allows one to replace ground probe  160  with another ground probe providing a different characteristic impedance or the same characteristic impedance for a different device under test. (Holes  114 . 4  and  114 . 5  are used for mounting the test head to an actuation assembly as shown in FIGS. 6 and 7.)  
         [0026]    [0026]FIG. 3, a top perspective view of probe-support structure  110 , shows that center portion  114  further includes front and back surfaces  114 . 6  and  114 . 7  which confront respective interior surfaces of front plate  117  and a back plate  118  (shown in FIG. 2.) Front surface  114 . 6  includes substantially parallel grooves  114 . 61 ,  114 . 62 , and  114 . 63 . Back surface  114 . 7  includes substantially parallel grooves  114 . 71 ,  114 . 72 , and  114 . 73 .  
         [0027]    [0027]FIG. 4 shows a perspective view of front plate  117 , which is structurally identical to back plate  118 . Front plate  117  includes respective narrow, broad, and intermediate sections  117 . 1 ,  117 . 2 , and  117 . 3  as well as parallel grooves  117 . 4   117 . 5 , and  117 . 6 . Narrow section  117 . 1  terminates in a 45-degree bevel; intermediate section  117 . 2  includes holes  117 . 21  and  117 . 22  and tapers at 45 degrees from narrow section  117 . 1  to broad section  117 . 3 . Grooves  117 . 4 ,  117 . 5 , and  117 . 6  correspond to those of front surface  114 . 6 . Front DC bias feeds  144 . 1 ,  144 . 2 , and  144 . 3  are sandwiched respectively between grooves  11 . 71 ,  114 . 72 , and  114 . 73  and grooves  117 . 4 ,  117 . 5 , and  117 . 6 . Similarly, back DC bias feeds  154 . 1 ,  154 . 2 , and  154 . 3  are sandwiched respectively between grooves  114 . 71 ,  114 . 72 , and  114 . 73  of the back surface  114 . 7  and corresponding grooves (not shown) in back plate  118  (in FIG. 2).  
         [0028]    [0028]FIGS. 5. 1  and  5 . 2  show respective perspective and cross-sectional views of central ground probe  160 . Ground probe  160  includes a conductive cylindrical shaft  162  of substantially uniform diameter of 0.125 inches (3.17 millimeters) for example. Shaft  162  has an upper portion  164  and a lower portion  166 . Upper portion  164  includes a central axial bore  164 . 1  and a screw  164 . 2 . Screw  164 . 2  allows one to adjust the angular orientation of probe  160  relative to other portions of probe  100 . In the exemplary embodiment, axial bore  164 . 1  has an approximate diameter of 0.10 inches (2.50 millimeters) and an approximate depth of 0.20 inches (7.88 millimeters.) Lower portion  166  includes a ground offset surface  166 . 1  and a ground contact surface  166 . 2 . Ground contact surface  166 . 2  in the exemplary embodiment is a rectangular solid, with an exemplary depth of about 0.0045 inches (0.114 millimeters), an exemplary length of about 0.082 inches (2.08 millimeters), and an exemplary width of about 0.048 inches (1.22 millimeters.)  
         [0029]    When ground contact  166 . 2  contacts a ground contact of a device under test that has adjacent signal ports, a portion of ground offset surface  166 . 1  overhangs a portion of an one or more of the adjacent signal port trace of the device under test. Assuming an appropriate offset between surfaces  166 . 1  and  166 . 2  relative to the width of the adjacent signal trace, this arrangement establishes a desired characteristic impedance. For example, an offset of about 4.3 mils (0.144 millimeters) with a trace width of about 18 mils (0.457 millimeters) forms a nominal characteristic impedance of 50 Ohms.  
         [0030]    The exemplary embodiment machines probe-support structure  110  from aluminum 6061-T6 and finishes it with 0.00001-inch-thick, 24-carat-gold plating over 0.0002-inch-thick nickel. Front and back plates  117  and  118 , and ground probe  160  are fabricated similarly.  
         [0031]    [0031]FIG. 6 shows how exemplary test head  100  is intended to engage a exemplary high-frequency chip-carrier assembly (or surface-mount package)  600 . Chip-carrier assembly  600  includes a chip-carrier substrate  610  and an integrated circuit chip  620 . Though not shown, the exemplary embodiment provides chip  620  with a lid or cover for protection.  
         [0032]    More particularly, chip-carrier substrate  610  includes two high-frequency signal port traces or contacts  611  and  612 , a central ground pad  614 , and low-frequency or direct-current (DC) bias pads  617  and  618 . Signal-port traces  611  and  612 , which have a rectangular shape in this embodiment, are positioned directly opposite each other. Central ground pad  614 , which has an exemplary rectangular shape or peripheral outline, lies centered not only between signal-port traces  611  and  612 , but also between DC bias pads  617  and  618 . DC bias pads  617  includes a collinear arrangement of three pads  617 . 1 ,  617 . 2 , and  617 . 3  on one side of carrier  610 , and DC bias pads  618  includes a collinear arrangement of three pads  618 . 1 ,  618 . 2 , and  618 . 3 .  
         [0033]    [0033]FIG. 6 further shows that various portions of test head  100  are aligned with portions of chip-carrier assembly  600 . Specifically, left and right signal probe tips  124  and  134  are aligned to contact respective signal port traces  611  and  612 , front (and back) DC bias probe tips  142 . 1 ,  142 . 2 ,  142 . 3  are aligned to contact DC bias pads  617 . 1 ,  617 . 2 , and  617 . 3 , and central ground probe  160  is aligned to contact central ground pad  614 . (The figure does not clearly show alignment of back DC bias probe tips  152 . 1 ,  152 . 2 ,  152 . 3  with DC bias pads  618 . 1 ,  618 . 2 , and  618 . 3 , although this is what is intended in the exemplary embodiment. Also, it is intended in the exemplary embodiment that ground contact  166 . 2  register precisely with pad  614 .)  
         [0034]    [0034]FIG. 7 shows a simplified cross-sectional view of left and right signal probes  120  and  130  and ground probe  160  of test head  100  in contact respectively with signal port traces  611  and  612  and ground pad  614 . Notably, when ground contact  166 . 2  contacts ground pad  614 , left and right portions of ground offset surface  166 . 1  overhang respective portions of signal port traces  611  and  612 . Assuming an appropriate depth (or height) of ground contact  166 . 2  (which establishes the distance between offset surface  166 . 1  signal port traces  611  and  612 , this arrangement sets a desired characteristic impedance between ground surface and the signal port trace. For example, in this embodiment, a depth of 0.0043 inches (0.114 millimeters) sets a characteristic impedance of 50 ohms. Replacement of the ground probe with another allows one to reconfigure the test head for other characteristic impedances, and/or electronic assemblies with other contact distributions, shapes, and/or dimensions.  
         [0035]    Other embodiments provide alternative ground probe dimensions and structures to effect impedance matching. For examples, some embodiments provide ground contact  166 . 2  as a set of two or more ground contact points. Variants of these embodiments form the ground contact points in hemispherical or conic forms. Still other embodiments provide the ground contact points as a set of angled fingers, similar in form to probe tips  124  and  134  to cushion impact of test head  100  with chip-carrier assembly  600 . Other embodiments may combine rigid or resilient contacts with one or more other resilient conductive or nonconductive features, such a spring member, to facilitate a soft landing of the ground probe.  
         [0036]    Additionally, some embodiments provide the ground probe with a variable offset-surface-to-contact-surface distance. For example, in some embodiments, ground contact surface is part of an axial insert within a cylindrical or rectangular ground sleeve. The ground sleeve has an end face which functions as an offset surface, and the axial insert slides within the sleeve, allowing one to adjust and set the distance between the offset surface and the contact surface and thus to set the characteristic impedance of the probe. Other more complex fine tuning mechanisms are also feasible with this variable mechanism. Indeed, with an automated adjustment mechanism and suitable feedback electronics, it is conceivable to dynamically match the characteristic impedance of the probe to each device under test in a mass-production environment using an automated test system.  
         [0037]    [0037]FIG. 8 shows an exemplary test system  800  that incorporates exemplary test head  100 . In addition to test head  100 , system  800  includes a programmable XYZ table  810 , and a network analyzer  820 . XYZ table  810  includes an x-axis translator  812 , an y-axis translator  814 , a substrate holder  816 , and a z-axis translator  818 . X-axis translator  812  moves z-axis translator  818  along an x-axis dimension  840 , and y-axis translator  814  moves substrate holder  816 , which holds one or more exemplary chip-carrier assemblies  600 , along a y-axis dimension  842  perpendicular to the x-axis dimension. Z-axis translator  818 , which includes bias circuitry  818 . 1  coupled to the dc bias feeds of test head  100 , moves the test head along a z-axis dimension  844 , perpendicular to the x- and y-axes, to engage its probe tips with each of chip-carrier assemblies  600  on substrate holder  816 . Network analyzer  820  includes network-analyzer ports  822  and  824 .  
         [0038]    In exemplary operation, a programmed computer controller (not shown) controls XYZ table  810 , using x-axis and y-axis translators  812  and  814  to align z-axis translator  818 , more precisely test head  100 , over one of the chip-carrier assemblies on substrate holder  816 . After achieving this two-dimensional alignment, the controller operates z-axis translator  818  to bring test head  100 , specifically signal probe tips  124  and  134  into contact with respective signal port traces  611  and  612 ; front and back DC probe tips  142  and  152  into contact with respective DC bias pads  617  and  618 ; and central ground probe  160  into contact with central ground pad  614 , as indicated in FIGS. 6 and 7.  
         [0039]    Some embodiments control movement of the test head in the z-dimension by established a predetermined stopping point for the test head. Other embodiments use the sensed flow of electrical current through the bias circuitry as a stop signal for downward movement of the test head. And still other embodiments may force gas through a nozzle mounted adjacent the test head on to the substrate or substrate holder, sense back pressure as the test head moves downward, and cease movement when the back pressure exceeds a certain threshold. Yet other embodiments may use optical control methods.  
         [0040]    Contact of one or more of the probes, such as ground probe  160  with ground pad  614 , completes an electrical circuit for the DC bias circuitry  818   a  to apply appropriate DC bias voltages, through DC probes  140  and  150  to bias pads  617  and  618 . Control software senses the flow of current through the bias feeds, and waits a predetermined period of time, for example 10 seconds, to allow for establishing a steady-state condition. Once the steady-state condition is established, the control software directs network analyzer to output a test signal, for example in the 27-32 Giga-Hertz range, from port  822 , through left signal probe  120 , and into signal-port trace  611  of chip-carrier assembly  600 .  
         [0041]    Assembly  600  outputs a signal through signal-port trace  612  and right signal probe  130  to network-analyzer port  824 . Network analyzer  820  measures one or more electrical properties (such as S-parameters, power, delay, and so forth), compares the one or more measured properties to acceptance criteria, and records the results of the test along with a part identifier for the chip-carrier assembly, indicating whether the assembly has passed or failed. The controller then operates the z-axis translator to disengage the test head from the chip-carrier assembly; operates the x-axis and y-axis translators to align the test head with the next chip-carrier assembly for testing. Those assemblies that pass the test will be mounted to a motherboard or other circuitry using conventional mounting procedures, whereas those that fail will be discarded or salvaged.  
         [0042]    [0042]FIG. 9 shows a perspective view of exemplary z-axis translator  818  without bias circuitry  818 . 1 . Translator  818  includes a spring-biased vertical actuation assembly  900 , which is shown in its extended or actuated position. Assembly  900  includes table-mount bracket  910 , an actuator bracket  920 , an actuator  930 , a test-head bracket  940 , and a bias spring  950 .  
         [0043]    Table-mount bracket  910 , which is used to fasten assembly  900  to y-axis translator  814 , is fastened or secured to left and right stem portions  922 . 1  and  922 . 2  of actuator bracket  920 . Actuator bracket  920 , which forms an inverted “L,” includes a lower stem portion  922  and an upper portion  924 . Stem portion  922  includes a central slot  922 . 3  between left and right portions  922 . 1  and  922 . 2 . Fastened to upper portion  924  is one end of actuator  930 .  
         [0044]    Actuator  930 , which in various embodiments is hydraulic, pneumatic, or electric, includes a rod  932  mounted to test-head mount bracket  940 . Test-head bracket  940  forms a “T”, and includes an upper portion  942  and a lower portion  944 . Upper portion  942  slidably engages central slot  922 . 3 . Lower portion  944  includes a slot  944 . 1 , which defines left and right end portions  944 . 2  and  944 . 3 . Slot  944 . 1  receives front and back DC bias feeds  144  and  154  of test head  100 , and left and right end portions  944 . 2  and  944 . 3  are fastened to test head  100  using its holes  114 . 4  and  114 . 5  (shown in FIG. 2).  
         [0045]    Bias spring  950 , which is connected between lower portion  944  and upper portion  924  of actuator bracket  920 , bias the actuator toward a disengaged position, that is, away from substrate holder  816  (in FIG. 8.)  
       CONCLUSION  
       [0046]    In furtherance of the art, the inventors have presented unique test probes and related systems and methods for testing high-frequency electronic assemblies, such as those for wireless communications devices. One unique probe structure includes at least one signal contact surface for contacting a signal-port trace of an electronic assembly and at least one ground contact surface for contacting a ground pad of the electronic assembly, with the ground contact surface substantially larger than the signal contact surface. Another unique probe structure includes a non-contact ground surface for overhanging a portion of the contacted signal-port trace and thereby establishing a characteristic impedance. And yet another includes conductors for communicating electrical bias signals to devices under test.  
         [0047]    The embodiments described above are intended only to illustrate and teach one or more ways of making and using the present invention, not to restrict its breadth or scope. The actual scope of the invention, which embraces all ways of practicing or implementing the teachings of the invention, is defined only by the following claims and their equivalents.