Abstract:
A manipulator for supporting heavy as well as lighter test heads in a small footprint includes a body and an interface for supporting a test head from behind. The interface includes a first portion fixedly attached to the body and a second portion fixedly attached to the rear of the test head. The first and second portions of the interface are rotatably coupled together to allow rotation of the test head about its approximate center of mass. Although the weight of the test head is entirely borne from the rear, the test head can still be moved with relatively little applied force, thereby satisfying the requirements for compliant docking.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS  
       [0001]     Not Applicable.  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT:  
       [0002]     Not Applicable.  
       REFERENCE TO MICROFICHE APPENDIX  
       [0003]     Not Applicable  
       BACKGROUND OF THE INVENTION  
       [0004]     1. Field of the Invention  
         [0005]     This invention relates generally to precisely positioning heavy objects, and more particularly to precisely positioning a test head of an electronic automatic test system for docking the test head with a prober, handler, or other peripheral for testing electronic devices.  
         [0006]     2. Description of Related Art  
         [0007]     Manufacturers of semiconductor chips and assemblies use automatic test equipment (“ATE”) to verify the performance of devices before the devices are shipped to customers. ATE systems typically include a “test head” and a “tester body.” The test head houses portions of the test system that are preferably located as close as possible to the device under test, and connects to the tester body via one or more cables. For testing electronic devices, the test head connects or “docks” with a peripheral. The peripheral feeds a series of devices to the ATE system for testing, and the ATE system tests the devices.  
         [0008]     Constraints affecting semiconductor test processes often make it impractical to move the peripheral to the test head. In most manufacturing facilities, therefore, the peripheral that feeds the chips remains stationary, and the test head is moved into position for docking with the peripheral.  
         [0009]     A device called a “manipulator” is used to move the test head to the peripheral. As is known, a common type of manipulator is the fork-arm manipulator, an example of which is shown in  FIG. 1 . Here, a manipulator  100  holds a test head  112  from its sides via fork arms  114 . The manipulator  100  raises and lowers the test head  112  on a linear stage  118 , and rotates the test head about a twist axis upon a twist gear  120 . A fork arm manipulator like the one shown in  FIG. 1  is disclosed in U.S. Pat. No. 5,949,002, entitled, “Manipulator for Automatic Test Equipment with Active Compliance,” which is assigned to Teradyne, Inc., of Boston, Mass., and is hereby incorporated by reference.  
         [0010]      FIG. 2  shows another type of manipulator used for positioning a test head. Rather than using fork arms, the manipulator  200  of  FIG. 2  supports a test head  210  internally. The test head  210  is mounted to a central stiffener  212 , which is, in turn, mechanically coupled to the manipulator via a central blade (not shown), which extends approximately through the center of the test head. The manipulator  200  can raise and lower the test head  210  on linear bearings  224 , and can rotate the test head in the twist direction via twist bearing  214 . It can also swing the test head  210  via a swing bearing  222 . A manipulator like the one shown in  FIG. 2  is disclosed in U.S. patent application Ser. No. 09/615292, entitled “AUTOMATIC TEST MANIPULATOR WITH SUPPORT INTERNAL TO TEST HEAD.” This application is also assigned to Teradyne, Inc. and is hereby incorporated by reference.  
         [0011]     Both types of manipulators generally include actuators such as motors (not shown) on their respective bearings and linear stages. The actuators move the test head to the peripheral, and orient the test head for docking. The test head is then docked with the peripheral by finely adjusting the position and orientation of the test head.  
         [0012]     Manipulators commonly provide a range of “compliance” that allows a test head to be rotated about one or more axes as the test head and peripheral are being docked. The range is “compliant” because the test head literally complies with forces applied to the test head, which during docking tend to cause the mating surface of the test head to become coplanar with the mating surface of the peripheral.  
         [0013]     In the fork arm manipulator of  FIG. 1 , the test head  112  can be made to nod compliantly in a “tumble” direction (NU and ND) by rotating the test head about pins  116 . One pin  116  is provided within each fork arm on each side of the test head  112 . The test head can also be made to turn compliantly in a “theta” direction (ΘL and ΘR), via the movement of mechanisms within each fork arm, which allow the pins  116  to move slightly back and forth along the length of each fork arm. Opposing movements of the pins  116  on opposite fork arms effects compliant theta rotation.  
         [0014]     The test head  210  of  FIG. 2  can compliantly rotate in theta, tumble, and twist via a spherical bearing (not shown) positioned approximately at the test head&#39;s center of mass. The spherical bearing has an inner race coupled to the central stiffener  212  and an outer race coupled, via the central blade, to the twist bearing  214 .  
         [0015]     Semiconductor manufacturers and semiconductor testing facilities place a high premium on minimizing the floor space that their ATE systems occupy. As semiconductor devices become more complex, however, the ATE systems used to test them tend to become larger, requiring more floor space. We have recognized that relatively small test heads (e.g., less than 200 kg) can be held effectively with fork arms that do not occupy much additional floor space. For larger test heads, however, fork arms are required to grow substantially in size, to the point where they occupy a significant percentage of the ATE&#39;s overall footprint.  
         [0016]     The internally supporting manipulator, like the one shown in  FIG. 2 , tends to hold heavier test heads with less floor space than an equivalent fork arm manipulator would require. ATE systems now include test heads weighing over 1300 kg. For these larger test heads, internally supporting manipulators are generally the more space efficient alternative.  
         [0017]     We have recognized that the centrally supporting manipulator places many constraints on the design of the test head that it supports. For instance, the test head generally must be provided in two portions  210   a  and  210   b,  which independently attach to the central stiffener  212 . Electrical cables connecting the two portions must be passed either through the central stiffener or around it. Because the central stiffener and associated hardware occupy the center of the test head  210 , this area is not available for other purposes, such as cooling and additional electronics.  
         [0018]     What would be desirable is a manipulator that is more space efficient for heavy test heads than the fork-arm manipulator but does not impose the design constraints associated with the centrally supporting manipulator.  
       BRIEF SUMMARY OF THE INVENTION  
       [0019]     In accordance with the present invention, a manipulator for supporting a test head includes a body and an interface for supporting the test head from one of its faces, for example, from the rear. The interface includes a first portion fixedly attached to the body and a second portion fixedly attached to the test head at one of its faces. The first and second portions of the interface are rotatably coupled together to allow rotation of the test head about its approximate center of mass. Because the test head is rotatable about its center of mass, it can be made to rotate in response to relatively small applied forces, to satisfy the requirements of compliant docking.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]     Additional objects, advantages, and novel features of the invention will become apparent from a consideration of the ensuing description and drawings, in which  
         [0021]      FIG. 1  is a perspective view of a fork arm manipulator according to the prior art;  
         [0022]      FIG. 2  is a perspective view of a manipulator for internally supporting a test head according to the prior art;  
         [0023]      FIGS. 3A-3C  are a series of front, elevation views of a manipulator according to a first embodiment of the invention, which shows the test head rotated to different orientations;  
         [0024]      FIG. 4  is an exploded, perspective view of an interface assembly used for rotatably coupling the manipulator to the test head in the arrangement of  FIGS. 3A-3C ;  
         [0025]      FIG. 5  is an elevated, perspective view of the interface assembly of  FIG. 4 ;  
         [0026]      FIG. 6  is an elevated, front view of the interface assembly of  FIGS. 4 and 5 ;  
         [0027]      FIG. 7  is an elevated, perspective view of a portion of an interface assembly and test head according to a second embodiment of the invention; and  
         [0028]      FIGS. 8A and 8B  are elevated, perspective views of the embodiment of  FIG. 7 , which show the test head rotated to different orientations.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0029]      FIGS. 3A-3C  show different views of a manipulator  310  for positioning a test head  312  according to a first embodiment of the invention. A device interface board (DIB)  316  is attached to the test head  312  for interfacing the test head with a peripheral (not shown), such as a prober or a handler. The manipulator  310  includes a body  310   a  mounted to a base  310   c.  An arm  310   b  extends from the body  310  for supporting the test head  312 . The manipulator  310  can move the arm  310   b  up and down, in a Z-direction, for positioning the test head vertically in space.  
         [0030]     An interface assembly  314  connects the test head  312  to the manipulator  310 . The interface assembly includes a first portion attached to the body  310   a  of the manipulator (via the arm  310   b ) and a second portion attached to the test head  312 . The first and second portions are coupled together in a manner that supports the weight of the test head, yet allows the test head to be rotated about any of the conventional rotational components: theta, tumble, and twist.  
         [0031]      FIG. 3A  shows the test head  312  rotated to the left in theta, but with tumble and twist nominally at zero degrees. In  FIG. 3B , theta and twist are at zero degrees but the test head is rotated downwardly in tumble. In  FIG. 3C , theta is at zero degrees, but the test head is rotated upwardly in tumble and downwardly in twist. It should be appreciated that the dimensions and proportions of the equipment shown in  FIGS. 3A-3C  are presented for illustrative purposes and are not necessarily to scale.  
         [0032]     From these views, it is apparent that the first portion of the interface remains in a fixed position with respect to the manipulator and the second portion of the interface remains in a fixed position with respect to the test head. The first and second portions of the interface move with respect to each other, however, to accomplish rotation of the test head.  
         [0033]     Although it may not be immediately apparent, the relative movement of the first and second portions of the interface is not mere rotation, such as that accomplished by a hinge. Rather, the second portion of the interface moves in arc-shaped paths with respect to the first portion of the interface. By geometric projection, the arc-shaped paths can be seen to have a common center located within the body of the test head, approximately (and ideally) at its center of mass.  
         [0034]     Because the center of the test head&#39;s rotation is its center of mass (approximately), the test head rotates about its center of mass, as if it were being held from this point, even though the test head is being supported entirely from the rear. This means that the test head is easily moveable in compliance with normal docking forces. Docking forces need not be sufficient to lift the weight of the test head; they need only be sufficient to overcome friction and small imbalances that arise if the geometric center of rotation does not precisely coincide with the test head&#39;s center of mass.  
         [0035]     An illustrative embodiment of the interface  314  can be more clearly seen in the exploded view of  FIG. 4 , and in the perspective side view and front view of  FIGS. 5 and 6 . The first portion, i.e., the part that normally attaches to the manipulator, includes a support plate  410  and a pair of substantially U-shaped brackets  412 . The U-shaped brackets are fixedly attached to an upper region of the support plate  410  by any appropriate means, such as screws, bolts, welding, brazing, and so forth. Low friction contacts  414 , such as ball casters (also known as ball transfers), are held by the U-shaped brackets in a position that enables their contact regions, such as balls  414   a,  face to the rear, in the direction of the support plate. In particular, the ball casters  414  include a cylindrical back portion that is inserted into a hole  412   a  in each U-shaped bracket, from the inside of the respective U-shaped bracket. The ball casters are then fastened to the insides of the U-shaped brackets, for example, using bolts, although other types of fastening can be used.  
         [0036]     A pair of standoff blocks  418  is fastened to a lower region of the support plate  410 . Each standoff block  418  preferably includes a hole  418   a  for receiving an additional low friction contact, such as a ball caster  420 . The ball casters  420  are preferably of the same type as the ball casters  414  used at the upper region of the support plate  410 . The standoff blocks  418  and the U-shaped brackets  412  hold their respective low friction contacts at respective angles, which roughly conform to the plane of the surfaces with which they make contact, i.e., those of the dish-shaped section  440 .  
         [0037]     The dish-shaped section  440  forms part of the second portion of the interface, i.e., the portion that is attached to the test head. The dish-shaped section preferably has a central aperture  440   a,  an concave inner surface facing the test head, and a convex outer surface facing the first portion of the interface. The inner and outer surfaces are each preferably spherical in shape (i.e., they are each sections of a sphere), and the spherical surfaces are preferably concentric. The dish-shaped section fits into the U-shaped brackets  412  such that its inner surface makes contact with the balls  414   a  of the ball casters  414  and its outer surface makes contact with the balls  420   a  of the ball casters  420  (best seen in  FIG. 5 ). The dish-shaped section  440  is preferably attached to the test head via a cylindrical section  450 . The dish-shaped section  440  is preferably bolted to the cylindrical section  450 , which is preferably bolted in turn to the test head. As is usually the case here, the exact fastening technology is not critical, provided it is sufficient to support the expected loading with adequate safety margin.  
         [0038]     The role of the cylindrical section  450  is to provide spacing between the dish-shaped section  440  and the test head to which it attaches. This spacing is needed to prevent the U-shaped brackets  412  and standoff blocks  418  from making contact with the rear of the test head as the first and second portions of the interface are moved over their full allowable range. Rather than providing the dish-shaped section  440  and the cylindrical section  450  as separate pieces, they could instead be provided as a single, continuous piece. Alternatively, the cylindrical section  450  could be replaced with separate standoffs that attach around the outer circumference of the dish-shaped section  440 , for maintaining the requisite spacing. If this variation is adopted, care should be taken, however, to prevent users from pinching body parts between the dish-shaped section and low friction contacts.  
         [0039]     To limit the range of upward tumble rotation, the U-shaped brackets  412  preferably include bumper pins  416 . The bumper pins  416  preferably each have a central spring-loaded pin that engages holes  412   b  in each U-shaped bracket, for holding them near the inside bottom of the respective U-shaped stiffener. They also each have an outer rubber tube surrounding the spring-loaded pin. The rubber tubes prevent damage to the dish-shaped section  440  by providing cushioned contact between the dish-shaped section  440  and the U-shaped brackets  412  when the test head is rotated to its maximum tumble angle. Other cushioned stops (not shown) are preferably also used, to provide cushioned contact at minimum tumble angle, as well as at maximum and minimum theta angles. These other cushioned stops can be implemented in a variety of ways, or instance, by attaching cushioned right angle brackets at appropriate positions and orientations on the support plate  410 .  
         [0040]     As indicated in the figures, the dish-shaped section  440  is held in place at four contact points (i.e., two instances at  414   a  and two instances at  418   a ). To prevent the dish-shaped section  440  from riding on only three of these contact points, the ball casters  414  and  420  are preferably spring-loaded. All four ball casters thus share the load induced by the test head approximately equally, in spite of minor tolerance errors.  
         [0041]     Because the inner and outer surfaces of the dish-shaped section  440  are preferably spherical and concentric, and because the centers of these spherical surfaces approximately intersect the test head&#39;s center of mass, the forces between the dish-shaped section and each of the low friction contacts are almost entirely normal. This means that, with the test head in place, there is substantially no rotational moment between the first and second portions of the interface. The interface therefore remains stable and does not tend to fall or tilt under the weight of the test head. The forces needed to rotate the test head are only those needed to overcome friction and loading imbalances.  
         [0042]     It is apparent from the foregoing that the interface  314  allows the test head to be compliantly rotated about its center of mass in tumble (up and down). Since the dish-shaped section  440  is spherical, it should also be apparent that the test head can be compliantly rotated about its center of mass in theta (left and right). In addition, since the dish-shaped section  440  forms a complete ring, it should further be apparent that the test head can be rotated about its center of mass in twist (clockwise and counterclockwise).  
         [0043]     This design has other advantages. Because it is open in the center, electrical cables emanating from the test head can be passed through the center of the interface back to the manipulator, through a hole  410   a  in the support plate  410 . This keeps the cables out of sight and causes their weight to induce a minimum load imbalance. The central region is large enough to allow other components, such as air ducts, blowers, or other cooling equipment, to be positioned therein for cooling the electronics within the test head in a highly space-efficient manner.  
         [0044]     The interface  314  and all of its parts are preferably steel, although other metals or materials can be used that meet the loading requirements with adequate safety margin. The dish-shaped section  440  is preferably hardened steel. Hardened steel is preferred because it minimizes distortion in the shape of the dish-shaped section  440 , particularly under the load of heavy test heads. In the preferred embodiment, the dish-shaped section is approximately 0.6 m in diameter and approximately 1.5 cm thick. In general, the smaller the diameter of the dish-shaped section  440 , the greater the thrust forces between it and the low-friction contacts. Therefore, decreasing the diameter of the dish-shaped section  440  generally requires increasing its thickness and increasing the thrust ratings of the low-friction contacts.  
         [0045]     The interface  314 , and a manipulator constructed therewith, is suitable for supporting both exceedingly heavy and relatively light test heads. For supporting heavy test heads, it avoids the need for large fork arms, which require considerable space. It also avoids the need for internally supporting the test head, which places many constraints on the test head&#39;s design.  
         [0000]     Alternatives  
         [0046]     Having described one embodiment, numerous alternative embodiments or variations can be made. For example, in the above-described embodiment, the dish-shaped section  440  has been shown and described as part of the second portion of the interface  314  (i.e., attached to the test head), and the low friction contacts have been shown and described as part of the first portion of the interface  314  (i.e., attached to the manipulator). This could be reversed, however, by attaching the dish-shaped section  440  to the manipulator and the low friction contacts to the test head.  
         [0047]     As shown and described, the U-shaped brackets  412  reach to the inner surface of the disk-shaped section  440  through its central aperture  440   a.  Alternatively, the U-shaped brackets could be made to reach around the outside of the disk-shaped section  440 . The cylindrical section  450  could then be reduced in size and made to attach near the inner diameter of the dish-shaped section. This variation would allow the aperture  440   a  to be “removed” from the disk-shaped section  440 , leaving it more in the shape of a “cap” than a “ring.” 
         [0048]     As shown and described herein, the disk-shaped section  440  forms a complete ring, which allows the test head to be rotated in twist over an arbitrarily large angle. Alternatively, the disk-shaped section  440  could be constructed as a partial ring, such as a horseshoe, for providing a smaller range of twist rotation. As yet another variation, the disk-shaped section  440  could be provided as a pair of of opposing sections, for instance, left and right or top and bottom sections, with the other portions of the ring removed. The size of the sections could be made large enough to allow small twist rotations, such as those needed for compliant docking, but would not necessarily be large enough to allow gross adjustments of the test head&#39;s twist angle.  
         [0049]     Depending on the desired performance, the inner and outer surfaces of the disk-shaped section  440  need not be perfectly spherical. For example, slightly opening the curvature of the outer surface, closing the curvature of the inner surface, or both, induces a rotational moment that tends to restore the test head to a centered position, once it has been moved from its centered position.  
         [0050]     The low friction contacts  414  and  420  have been particularly described herein as thrust ball caster assemblies; however, other types of contacts can be used, such as other types of rolling contacts or frictional contacts. An example of a “frictional” low-friction contact is Teflon pads. Teflon or other low friction plastics or materials may be used instead of roller bearing assemblies, particularly in lightweight, low cost test systems. “Frictional” low friction contacts may be better used against some surfaces than others. Therefore, if this variation is adopted, it may be advantageous to cover the dish-shaped section  440  with a surfacing material having a particularly low coefficient of friction when used in connection with the “frictional” low friction contacts. For some test heads, it may be advantageous to use air bearings in place of conventional roller bearings to reduce friction to negligible levels. Suitable air bearings are available commercially from Space Electronics, Inc., of Berlin, Conn.  
         [0051]     As describe above, four bearing assemblies support the test head via the dish-shaped section  440 . The number of bearing assemblies could clearly be varied within the scope of the invention. In addition, the contacts need not be identical. For instance, a pair of thrust bearings could be used at the top and bottom of the support plate  410  for supporting the weight of the test head, and a pair of “frictional” contacts could be used at the left and right of the support plate for maintaining its position and guiding its theta rotation.  
         [0052]     The interface  314  need not be provided as a separate unit, to be later integrated with a manipulator and a test head. Instead, one portion could be built into the manipulator and another portion could be built into the test head. The parts of the interface could then be mated together when the test head is integrated with the manipulator.  
         [0053]     The interface  314  can be varied in a more basic manner within the scope of the invention. For example, as shown in  FIGS. 7-9 , a second embodiment of an interface for supporting a test head provides compliant rotation by breaking down the test head&#39;s rotations into separate components of theta and tumble. This interface preferably provides four channels  718  for allowing rotation in theta and four channels  722  for allowing rotation in tumble. Cam followers preferably ride within the arc-shaped channels to allow relative rotation of different parts of the interface. For each component of rotation, the respective channels share a common axis of rotation (i.e., they are aligned concentrically), which approximately (and ideally) intersects the test head&#39;s center of mass. Because the axis of rotation for each component intersects the center of mass, the test head has no rotational moment, and thus tends not to fall or tilt, even though it is held entirely from the rear.  
         [0054]     A manipulator supports the interface by attaching to a first frame section  712 . Cam followers  716  preferably project upwardly and downwardly from corners of the first frame section  712 , for engaging channels  718 , which are preferably formed within a second frame section  714 . Movement of the first frame section  712  with respect to the second frame section  714  thus effects theta rotation of the test head (See  FIG. 8A ).  
         [0055]     To effect tumble rotation, tabs  714   b  are fixedly attached to and extend from the rear of second frame section  714  (from the viewpoint of  FIG. 7 ). Cam followers  720  extend from these tabs  714   b  for engaging channels  722 , which are preferably formed within tabs  710   a  of the test head frame  710 . Movement of the second frame section  714  with respect to the test head thus accomplishes the desired tumble rotation of the test head (See  FIG. 8B ).  
         [0056]     This second embodiment of the interface is analogous to the first embodiment. The first and second frame sections  712  and  714  and their associated hardware constitute a first portion of the interface (i.e., one to be attached to the manipulator), and the tabs  710   a  extending from the test head constitute the second portion of the interface. The first and second portions move relative to each other to allow compliant rotation of the test head approximately about its center of mass. Unlike the first embodiment, however, this embodiment does not support compliant rotation in twist. If needed, compliant twist rotation can be accomplished though the use of separate equipment.  
         [0057]     Where twist rotation is externally provided, it is preferable to prevent the tabs that extend from the second frame section  714   b  from rubbing against the tab sections  710   a.  This can be accomplished by placing low-frictional pads, ball casters, of similar components, between the tabs  714   b  and  710   a.    
         [0058]     To limit the speed at which the test head can be rotated, the interface of this embodiment preferably includes rate limiters, such as gas springs  724 . A first pair of gas springs is preferably coupled between the first and second frame sections  712  and  714 , and a second pair of gas springs is preferably coupled between the second frame section  714  and the test head.  
         [0059]     The second embodiment of the interface can be varied in numerous ways, many of which are similar to those described in connection with the first embodiment, and will not be repeated here. Other variations include replacing the channels  718  and  722  with tracks, for example, curved linear bearings. In addition, the relative positions of the channels and cam followers can be exchanged. For instance, channels can be formed within the first frame section  712  (or a tab extending therefrom) for engaging cams extending from the second frame section  714 .  
         [0060]     The channels can be made arbitrarily long or short, depending upon how much rotation is desired, whether gross or merely fine, compliant adjustment is required, and other design considerations.  
         [0061]     The shape of the channels (or tracks) can depart from perfectly circular arcs. By opening the curvature of the channels, the test head will be subjected to a restoring force that tends to drive it toward a centered position.  
         [0062]     Although it provides compliant rotation in both theta and tumble, the interface of  FIGS. 7 and 8  could be simplified to provide rotation in one direction only, i.e., either theta or tumble. For example, by removing the first frame section  712  and attaching the second frame section  714  directly to the manipulator, the interface can be made to provide tumble rotation only. Similarly, by attaching the second frame section  714  directly to the test head, the interface can be made to provide theta rotation only.  
         [0063]     As used herein, the “rear” of the test head conventionally refers to the face of the test head that attaches to the manipulator. This designation is clearly arbitrary, as the test head could attach to the manipulator at a number of its different faces. Also, the interface  314  and the corresponding interface of the second embodiment each have a second portion attached to the rear of the test head. However, the particular means of attaching to the rear of the test head need not be accomplished via connections at the rear surface of the test head. For instance, the second portion of the interface could be connected to the rear of the test head via brackets or other hardware extending from its sides, top, bottom, or inside. Moreover, the second portion of the interface can be “attached” to the test head by being formed coextensively with the test head, rather than being a separate part. This variation is used in the second embodiment, wherein the tabs  710   a  are actually formed within the frame  710  of the test head.  
         [0064]     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.