Abstract:
A bond strength testing apparatus comprising a main body, a test tool mount for holding a test tool, and an axial drive mechanism including a screw and nut assembly. The drive mechanism couples the test tool mount to the main body and allows for relative movement between the test tool mount and the main body in an axial direction. A backlash control element is coupled to the main body and the test tool mount, and, in operation, biases the test tool mount relative to the main body in an axial direction. The backlash control element is switchable between a first state in which the test tool mount is biased in a first axial direction by the backlash control element, and a second state in which either the test tool mount is biased in a second axial direction by backlash control element, or the backlash control element applies no biasing force to test tool mount. The apparatus can be automated to apply the appropriate backlash control for a particular, selected test type.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a device for testing the strength of electrical bonds on semiconductor devices. In particular, the invention relates to a device that is able to perform different types of tests such as a shear test, a pull test and a push test on electrical bonds. 
     BACKGROUND TO THE INVENTION 
     Semiconductor devices are very small, typically from 5 mm×5 mm square to 50 mm×50 mm square, and typically comprise numerous sites for the bonding of electrical conductors to a semiconductor substrate. Each bond consists of a solder or gold ball deposit adhered to the substrate. Very thin wires, usually about 0.025 mm in diameter, may be embedded in the ball deposits. 
     It is necessary to test the bond strength of the bonds, in order to be confident that a particular bonding method is adequate. Because of the very small size of the bonds, tools used to test the bond strength of these bonds must be able to measure very small forces and deflections accurately. 
     There are several different types of bond tests that are used to test bond strength. For example, shear testing tests the shear strength of a bond by applying a shear force to the side of the bond and shearing the bond off the substrate. Pull testing tests the pull strength of the bond by pulling a wire embedded in a ball deposit away from the substrate. In a push test, a force, or load, is applied in the vertical plane directly downward onto a bond. 
     Machines that perform these tests typically comprise a bond test tool, be it a shear test tool, push test tool or a pull test tool, that can be positioned relative to the bond under test and then either the bond or the tool are moved in order to perform the test by measuring the force needed to break the bond. 
     As mentioned above, in these tests it is necessary to be able to measure very small forces and deflections. Positioning of a test tool is typically achieved using some form of screw and nut rotational drive assembly. For example, a test tool may be mounted to an assembly of components that includes a nut which moves along a threaded screw when the threaded screw is driven by a servo motor. This mechanism may be used for positioning the tool correctly prior to a shear, push or pull test and may be used to drive the test tool during a pull test. 
     When using of a screw and nut arrangement for providing the movement of the test tool towards and away from the substrate, inevitable clearances must be provided between the mating components of the screw and nut to prevent jamming, allow for thermal expansion and manufacturing variances, etc. This clearance is referred to as “backlash”. This clearance limits the accuracy to which the test tool can be initially positioned and to which the desired position of a test tool can be accurately maintained during a bond strength test. 
     In prior art bond testing machines, wherein the test tool is driven up and down along the vertical axis by a screw and nut drive mechanism, a spring in tension has been positioned above the tool and used to bias the tool upwardly to close the clearance between the upper thread surfaces of the nut and the thread surfaces of the screw. This has reduced backlash when the tool is used for shear testing or push testing, because these tests cause an upward force to be applied by the tool during the test. For example, in shear testing, as the test tool shears a ball deposit off the substrate, a vertical force component results, causing the ball deposit to push up on the tool. Since the tool is already being biased in the upward position by the spring, the backlash clearance already been closed, and thus, the shear or push test itself does not cause a tool position problem associated with backlash. 
     However, the use of a biasing spring has not completely eliminated the backlash problem during shear and push tests because the force applied by the spring, which as mentioned is in tension, changes as the spring is stretched. The more the spring is stretched, the greater the biasing force it applies. Therefore, a varying force is applied by the spring over the range of travel of the nut along the screw. Consequently, the spring applies more force to reduce backlash clearance when the spring has been stretched to the lower end of travel of the nut along the screw than it does when the spring is stretched to a lesser extent at the upper end of travel of the nut along the screw. Thus, the use of the spring has reduced backlash problems in shear and push tests, but it has not eliminated backlash problems. 
     In addition, in shear testing, it is very important that the lower end of the test tool maintain a very small, closely controlled standoff distance from the substrate. In that the upward bias of the spring varies depending on the length of the spring, the ability of the upward bias force to close off clearances also varies, making it difficult to accurately and reliably control the standoff distance for every position of the test tool above the substrate. 
     In a pull testing, moreover, a more significant backlash problem exists which is not solved by the use of the spring. In a pull test, the test tool is in the shape of the hook and it hooks under a wire that is bonded to a substrate. The test tool pulls up on the wire to pull the wire off a bond on the substrate and the force required to break that bond is measured. Typically the wire has been bonded to a solder ball on the substrate. As the wire is pulled, it exerts a downward force on the test tool. This downward force is pulling the tool down against the force of the spring. At some point, this force can overcome the spring force and cause the nut to move downward away from contact between its upper threaded surfaces and the screw until it makes contact with its lower threaded surfaces and the screw, closing the backlash clearance between the lower threaded surfaces of the nut and the threads of the screw. This movement to close the backlash clearance below the nut distorts the signals provided by the strain gauges, or other transducers, that are later described, and causes inaccurate force readings. 
     Furthermore, in the case of shear, push and pull tests, it is desirable that the nut and screw remain engaged in contact in a constant fashion to accurately control the axial movement of the test tool in a repeatable way. This has not always been achievable with the spring solution of the prior art. 
     SUMMARY OF THE INVENTION 
     The present invention is defined in the appended independent claims to which reference should be made. Preferred features are defined in the dependent claims. 
     To solve the aforementioned problems of the prior art spring solution, in the preferred embodiment of the present invention, a backlash control element is used to apply a constant biasing force to the nut that travels along the drive screw in a bond testing machine, which can be used for shear testing, push testing or pull testing. 
     The backlash control element can be in the form of a pneumatic piston and cylinder, with the cylinder fixed in position relative to the drive screw, and the piston fixed for movement with the nut that travels along the drive screw. This solution allows a desired bias force to be set, by setting the pressure in the cylinder, and then maintained regardless of the position of the nut along the drive screw. Thus, unlike the spring solution of the prior art, the biasing force applied to the nut can be specifically selected, and once selected, can be maintained constant as the nut moves along the length of the screw. 
     By providing a constant bias to the nut in both axial directions along the screw, the positional accuracy of the test tool relative to the substrate is improved relative to the prior art. This improvement in positional accuracy increases the reliability and repeatability of the force measurements made by the bond testing device. 
     A backlash control element in the form, for example, of a piston and cylinder, can also be used in such a way that bias is applied to the nut in the upward direction by the piston during shear and push tests, but no bias is applied during pull testing by venting the air pressure from the cylinder so that the piston can move freely. When the pressure is vented, instead of applying a bias by means of the piston and cylinder, the weight of the moving components themselves is relied upon to provide adequate bias to close the backlash clearance. This was not possible with the prior art spring solution in that the spring was always active to support at least part of the weight of the moving components of the system because the spring bias could not be removed during pull tests. 
     A backlash control element in the form, for example, of a piston and cylinder, can also be used to augment the drive force supplied by the screw and nut drive mechanism. For example, if the machine is used for a pull test, the screw drive will exert a given amount of upward force on the nut and test tool. If the piston is biased in the upward direction against the nut, the force of the piston will then be added to the drive force of the screw to produce a greater total upward force on the bond. Likewise, if the machine is used for push test, the screw drive will exert an amount of downward force on the nut and test tool. If the piston is biased in the downward direction against the nut, the force of the piston will be added to the drive force of the screw to produce a greater total downward force on the bond. 
     Consequently, in one aspect, the invention provides a bond testing apparatus comprising a main body, a test tool mount for holding a bond test tool, an axial drive mechanism, preferably comprising a screw and nut assembly, the drive mechanism coupling the test tool mount to the main body and allowing for relative movement between the test tool mount and the main body in an axial direction, and a backlash control element, coupled to the main body and the test tool mount, that, in operation, biases the test tool mount relative to the main body in an axial direction, wherein the backlash control element is switchable between a first state in which the test tool mount is biased in a first axial direction and a second state in which the test tool is biased in a second axial direction or in which the backlash control element provides no biasing force in the axial direction. 
     Preferably, the backlash control element comprises a pneumatically operated piston and cylinder. 
     Preferably, the force applied by the backlash control element can be set at a desired level, but once the force has been set, that force can be maintained at a constant level regardless of the axial position of the test tool, to improve the reliability and repeatability of bond tests. 
     Preferably, the backlash control element comprises a coupling between the moving portions of the apparatus and the non-moving portions of the apparatus, and most preferably, comprises an element that supports the nut in a screw and nut drive mechanism. 
     Preferably, the backlash control element comprises a pneumatically operated piston and cylinder which applies an upward biasing force during a shear test or push test, and a downward biasing force, or no biasing force, during a pull test. 
     Preferably, the apparatus further comprises a controller connected to a user interface, the controller and user interface being configured to allow a user to select a type of bond test, and wherein the controller is connected to the backlash control element and controls switching of the backlash control element between the first state and the second state in dependence on the type of bond test selected. 
     In another aspect, the invention provides a method of testing the bond strength of a bond on a substrate by a either a shear test or a pull test, using a bond testing device comprising a test tool that is coupled to a main body by a coupling, comprising the steps of: selecting either a shear test or a pull test; and if a shear test is selected, applying a biasing force on the test tool in a first direction to remove backlash from the coupling; positioning the test tool relative to the bond; providing relative movement between the test tool and the substrate to shear the bond off the substrate; and recording the force applied to the test tool by the bond. 
     Preferably, the method further comprises the step of applying a biasing force on the test tool in a second direction to remove backlash from the coupling, if a pull test is selected. 
     Preferably, the step of applying a biasing force comprises operating a pneumatic cylinder and piston coupled between the test tool and the main body. 
     Preferably, the first direction is an upward direction and the second direction is a downward direction. 
     Preferably, the method further comprises the step of initially positioning the test tool relative to the bond prior to applying a biasing force on the test tool, and more accurately positioning the test tool relative to the bond subsequent to applying a biasing force on the test tool. 
     The arrangement of the present invention allows a test tool to perform shear tests and pull tests, in which backlash is substantially eliminated. It allows pull and shear tests to be performed by the same apparatus, with good positional accuracy. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1   a  is a front view of a test apparatus in accordance with the present invention; 
         FIG. 1   b  is a cross-sectional view showing the test tool supported by the tool mounting bracket; 
         FIG. 1   c  is a perspective view showing the test tool clamped to the tool mounting bracket; 
         FIG. 2   a  is a perspective view of a test tool mount and drive, including an anti backlash cylinder in accordance with the present invention; 
         FIG. 2   b  is a side view of the assembly of  FIG. 2   a;    
         FIG. 2   c  is a rear view of the assembly of  FIG. 2   a;    
         FIG. 2   d  is a cross-sectional view of an alternative embodiment of the anti backlash cylinder in accordance with the present invention; 
         FIG. 3   a  is a schematic illustration of the pneumatic control system for the piston and cylinder shown in  FIG. 2   c;    
         FIG. 3   b  is a schematic illustration of the pneumatic control system for the piston and cylinder shown in  FIG. 2   d;    
         FIG. 4  is a perspective, cut away view of the test tool mount and drive illustrated in  FIG. 2   a - 2   c;    
         FIG. 5   a  is a schematic diagram illustrating the control elements of a system in accordance with the present invention; 
         FIG. 5   b  is cross-sectional view showing the strain gauges attached to the test tool; 
         FIG. 6   a  is a flow diagram illustrating steps performed by an apparatus in accordance with the present invention during a bond test procedure; and 
         FIG. 6   b  is a flow diagram illustrating steps performed by an apparatus in accordance with an alternative embodiment of the present invention during a bond test procedure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1   a  is an illustration of a bond testing device in accordance with the present invention. The device comprises a test tool  10  mounted to a cartridge  11 , which is itself mounted to the main body of the device  12 . Beneath the test tool is a motorized stage table  13 , on which samples, or substrates  100 , to be tested can be mounted. 
     The test tool  10 , mounted to cartridge  11 , can be a shear tool, push tool or a pull tool and can be switched in order to perform different tests. An example of a suitable shear tool is described in U.S. Pat. No. 6,078,387, the contents of which are incorporated herein by reference. An example of a suitable pull tool is described in U.S. Pat. No. 6,301,971, the contents of which are incorporated herein by reference. 
     As shown in  FIG. 1   b , the test tool  10  is typically attached to the cartridge  11  by a tool mount bracket  70  having cantilever arms  72 ,  74  fixed at one end to the cartridge  11  by screws  73 , with the free ends of the arms  72 ,  74  supporting a clamp  76 . As shown in  FIG. 1   c , the tool  10  is clamped in clamp  76  by means of clamp screw  78 .  FIG. 2   a  shows the retaining channel  71  on the cartridge mount plate  21  into which the cartridge  11  is slid and then secured using one or more screws  22 . The cartridge mount plate  21  includes a data port  23  that couples with an electrical connector on the cartridge  11  for transferring data from the transducers (later described) of cartridge  11  to a PC. A cartridge and test tool assembly of this type is well known in the prior art. See for example the Dage 4000 multipurpose bond tester available from Dage Holdings Limited, 25 Faraday Road, Rabans Lane Industrial Area, Aylesbury, Buckinghamshire, United Kingdom. However, any suitable means for attaching a test tool to the cartridge mount plate may be used in a system in accordance with the present invention. 
     The cartridge  11  is moveable in a direction normal to the surface of the substrate  100  on the stage table  13 . This allows the test tool  10  to be positioned relative to the substrate  100  under test so that it will contact a particular bond during a test. Relative movement between the test tool  10  and the table  13  in a direction parallel to the plane of the substrate  100  is typically achieved by moving the table  13 . Movement of the table  13  is achieved using suitable servo motors or stepper motors, coupled to the table  13  via a lead screw and nut, ball screw and nut, or suitable belt-drive mechanism (not shown), as is also well known in the prior art, such as the Dage 4000 Multipurpose Bond Tester referenced above 
     Also shown in  FIG. 1   a  are control devices, comprising two joystick controls  14 ,  15  to allow for controlling movement of the table  13 , and a keyboard  16 . A display  17 , a light  18  for illuminating the substrate  100  under test, and a microscope, aiding accurate positioning of the test tool  10 , are also shown. These features are also all well known in the prior art, such as the Dage 4000 Multipurpose Bond Tester referenced above. 
       FIG. 2   a  shows that the mounting plate  21  and its connection to the main body  25 . As has been described, the test tool (not shown in  FIG. 2   a ) must be moveable towards and away from a substrate under test. This is achieved by moving the cartridge mount plate  21 , to which the test tool  10  is attached, relative to the main body  25  of the device in a direction towards and away from the substrate, herein referred to as the z-axis direction or axial direction. The cartridge mount plate  21  is rigidly coupled to a moving block  24 , using screws  75 . The moving block  24  is coupled to the main body  25  via a ball screw (or lead screw)  33  and nut  34  and nut block  35  that is driven by a servo motor or stepper motor  26 . This is described in more detail below with reference to  FIG. 4 . 
     As described above, the use of a screw and nut arrangement for providing the movement of the test tool  10  towards and away from the substrate  100  leads to the problem of backlash in the mechanism. 
     In order to remove the problem of backlash, an anti-backlash mechanism is included. This mechanism is shown in  FIG. 2   a - 2   c  and  4 , and preferably comprises a pneumatic piston  27  and cylinder  28 . The piston  27  is coupled to the vertically movable components of the system which mainly comprise the coupling plate  29 , coupling block  30 , moving block  24 , mounting plate  21  (and cartridge  11  and test tool  10 ), nut block  35  and nut  34 . While only the shaft portion of the piston  27  is shown in  FIGS. 2   a - 2   c , piston  27  also includes a piston head portion at its lower end as shown in  FIG. 2   d . The cylinder  28  is connected to the components of the system which do not move vertically. These components comprise the cylinder mount plate  31  (later described), main body  25  and the components driving the screw  33 . As will be described in more detail below the piston  27  is coupled for movement with the moving block, or nut block,  35  which supports nut  34 , and the cylinder  28  is fixed in position relative to the screw  33  of the vertical axis drive mechanism. In this way, the pneumatically driven piston  27  is able to apply a controllable biasing force to the nut  34  so that backlash is removed from the system. The biasing force can also be switched off. 
       FIG. 2   b  is a side view of the arrangement of  FIG. 2   a , showing the connection of the piston  27  to the moving block  24 , via the coupling plate  29  and coupling block  30 , which is bolted by bolts  80  to the moving block  24   
       FIG. 2   c  is a rear view of the assembly of  FIG. 2   a , showing the connection of the cylinder  28  to the main body  25  via the cylinder mount plate  31 . Air inlet  32  can also be clearly seen. 
       FIG. 2   d  shows how the mounting plate  31  attaches the cylinder  28  to the main body  25  in more detail. An end cap  84  is secured to the top of the cylinder  28  with the shaft of piston  27  extending up through the end cap  84 . The plate  31  is inserted over the end cap  84  and secured to it by a nut  87  which is threaded onto the end cap  84 . One or more screws  86  attach the mounting plate  31  to the main body  25 , and thereby, rigidly attach cylinder  28  to main body  25 . Note that a second (optional) air inlet port  90  is formed in end cap  84 . End cap  84  may or may not include this optional inlet port  90 . As is explained in more detail below,  FIG. 3   a  shows an embodiment of the invention wherein cylinder  28  has only one air input port  32 , and  FIG. 3   d  shows an embodiment where cylinder  28  has the additional air inlet port  90 . 
     The piston and cylinder are coupled to a compressed air supply at air inlet connection  32  (and optionally air inlet port  90 ). The air supply can be controlled using suitable valves connected to control electronics. Suitable piston and cylinder assemblies and suitable electronically controlled valves are available from Bosch Rexroth, of 15 Cromwell Road, St. Neots, PE19 2ES, United Kingdom. Alternative suppliers are Festo Ltd of Caswell Road, Brackmills, Northampton, NN4 7PY, United Kingdom and Pneumax Ltd., Unit 7/8, Venture Industrial Park, Gosport, PO13 0BA, United Kingdom. 
       FIG. 3   a  is a schematic illustration of the control system for the pneumatic piston and cylinder. The piston  27  moves axially within cylinder  28 . Air is both supplied to the cylinder and escapes from the cylinder through port  32 . Port  32  is coupled to a pneumatic control system comprising a compressed air supply  40 , regulator  97 , a supply valve  41  and exhaust valve  42 . Supply valve  41  selectively connects the compressed air supply to the port  32 , to thereby drive the piston  27  upwardly within the cylinder  28  to provide the desired biasing force to the nut  34  by setting the desired air pressure at a regulator  97 . Exhaust valve  42  selectively vents air in the cylinder  28  to the environment, thereby removing any biasing force supplied by the piston  27 . When the interior of the cylinder is in open fluid communication with the environment via the exhaust valve  42 , the piston  27  is free to move within the cylinder  28 . The supply valve  41 , exhaust valve  42  and regulator  97  are connected to control electronics  56 , described with reference to  FIG. 5 . 
     In  FIG. 4 , the arrangement of screw  33  and nut  34  can be clearly seen. The nut  34  is rigidly coupled to moving block  24  via nut block  35 , while the screw  33  is supported by bearings  36  on the main body  25  of the device, and allowed to rotate about its axis. Nut block  35  is rigidly fixed to moving block  24  using screws  82  that can travel along a slot  83  in the main body  25 . The top end of slot  83  is shown in  FIG. 4 . A servo motor  26  is fixed to the main body  25  and is coupled to the screw  33  via a coupling  37 , to rotate the screw, thereby moving the nut  34  up and down the length of the screw  33 . The motor assembly fixed to the main body  25  comprises the motor  26 , an encoder  38  and a gearbox  39 , available from Trident Engineering Ltd., Trident House, King Street Lane, Winnersh, Wokingham, Berkshire, RG41 5AS, United Kingdom. Alternatively, a direct drive motor and encoder assembly may be used, without a gearbox. An assembly of this type is available from Maxon Motors Ltd., Maxon House, Hogwood Lane, Finchampstead, Berkshire, RG40 4QW, United Kingdom. Accurate control of the motor  26  using suitable control electronics provides accurate control of the vertical position of the cartridge mount plate  21  and hence the test tool  10 . 
     During a shear test, such as is described in U.S. Pat. No. 6,078,387, where the tool  10  is normally shearing a solder ball deposit off of a substrate, the bond under test exerts an upward force on the test tool. In order to ensure that the tip of the test tool remains accurately positioned during a test and that the test is repeatable, an upward biasing force is applied by the piston  27  on the moving body  24  which can be set at the desired level and maintained consistently. As previously mentioned, the same upward biasing force is used during a push test. This upward biasing force eliminates backlash between the screw  33  and nut  34  described above. 
     In a pull test, such as is described in U.S. Pat. No. 6,301,971, a wire is pulled off a bond that attaches the wire to the substrate. The wire being pulled exerts a downward force on the test tool  10  during the test. In this case, the tip of the test tool would be a hook which hooks under the wire. To ensure that any backlash problems are eliminated, the piston  27  and cylinder  28  can be operated to apply an downward biasing force on the nut  34 . However, instead of applying a downward biasing force using the piston  27 , the weight of the vertically moving components of the system associated with test tool  10 , as described above, may be enough to eliminate any backlash. Therefore, in pull tests, one option with the present invention is to turn off the air pressure to the cylinder  28  at valve  41 , open the exhaust valve  42  to remove the air pressure below the piston head of piston  27  and allow the piston  27  to move freely in the cylinder  28 . As mentioned above, this option of allowing the weight of the moving components to provide the necessary backlash control during pull tests was not possible with the prior art machines which utilized a spring that constantly provided an upward biasing force on the nut  34 . 
       FIG. 3   b  shows the alternate system wherein the weight of the vertically moving elements is not sufficient to control backlash. In this case, an additional air inlet port  90  is provided at the top of the cylinder  28  so that air pressure, set to the desired level at regulator  97 , can be applied to the top side of the piston head of piston  27 . When a downward bias is to be applied to the nut  34  using the piston  27 , exhaust valve  42  is opened, exhaust valve  92  is closed, regulator  97  is set at the desired air pressure level and control valve  94  directs pressurized air through inlet  90  into chamber  28  to apply the desired downward biasing force to piston  27  and nut  34 . When an upward bias is to be applied to nut  34  using the piston  27 , exhaust valve  42  is closed, exhaust valve  92  is opened, the air pressure is set to the desired level at regulator  97  and control valve  94  directs pressurized air through inlet  32  into chamber  28  to apply the desired upward biasing force to piston  27  and nut block  35 . 
       FIG. 5   a  illustrates the control elements of a system in accordance with the present invention. The system is controlled by application software  51  running on a personal computer  50 , which includes a user interface. The test tool  10  typically includes force sensing components  52 , such as a piezoelectric crystal or a strain gauges.  FIG. 5   b  shows one example of a test tool  10  which has four strain gauges  110 ,  112 ,  114 , and  116  mounted thereon. Since this test tool is used for shear tests, the strain gauges  110 - 116  will become distorted as the test tool flexes while shearing a ball deposit off a substrate. Using well-known technology, the physical distortion of the strain gauges will produce electric signals that are processed by known circuitry to produce an indication of the force required to shear the ball deposit off the substrate. As mentioned previously, by accurately controlling the standoff distance of the test tool above the substrate during a shear test using the anti-backlash solution of the present invention, electric signals produced by the strain gauges more accurately, and more repeatedly, measure the force required to shear the ball deposit off the substrate. U.S. Pat. No. 6,301,971 describes another example wherein, in a pull test device, the strain gauges are mounted on the cantilever arms that support the test tool. Here again, accurate positioning of the test tool using the anti-backlash solution of the present invention improves the accuracy and repeatability of the force measurements produced. WO 2006/016136 A2 describes yet another example wherein a piezoelectric crystal is mounted on the test tool to measure a shear forces. Again, accurate control of the position of the test tool, using the present invention, improves test accuracy and repeatability. 
     Regardless of whether strain gauges or a piezoelectric crystal is used as the force sensing component  52 , the electrical output from this component  52  is processed by the data capture electronics  53  in  FIG. 5   a  to detect the force applied to the test tool during a test procedure. The data capture electronics  53  communicates with the application software  51 . The application software  51  also controls the movement and positioning of the test tool  10 . In  FIG. 5   a , only the Z axis motor  54  for driving the screw  33  shown in  FIG. 4  is shown. However, motion control electronics  55  would also control the motor moving the stage table  13 . The valves  41 ,  42 ,  92  and  94 , and regulator  97 , of  FIGS. 3   a  and  3   b  comprise the pneumatic control circuit  56  in  FIG. 5   a . These valves and regulator are operated to apply the desired upward or downward biasing force to the anti-backlash piston  27  within cylinder  28 , or no biasing force, as described above. The piston  27  and cylinder  28  are referred to as the antibacklash component  57  in  FIG. 5   a . 
     The personal computer  50  is connected to the keyboard, joysticks and display shown in  FIG. 1  to allow for user configuration and control. 
       FIGS. 6   a  and  6   b  show the method steps that the system described in above goes through in performing a shear, push or pull test. 
     With reference to  FIG. 6   a , which describes the system as operated using the pneumatic control system of  FIG. 3   a , in the first stage  600 , the test type is selected using a user interface, such as the keyboard shown in  FIG. 1 . 
     If the test is a pull test, the backlash control cylinder is not required in this embodiment and is not activated. The backlash is eliminated by the weight of the vertically moving components of the system biasing the nut into a downward position as described above. This is shown as step  610 . In this situation, the exhaust valve  42  is open. 
     However, if a push or shear test is to be performed, at step  620 , the test tool  10  is driven into the desired test position, adjacent a bond to be tested. Once the test tool has been correctly positioned, an automated test procedure can be initiated under software control, via a suitable user interface such as the keyboard, shown as step  630 . The first step in the automated test procedure is that pressurized air, at the pressure set at regulator  97 , is supplied to the anti-backlash cylinder  28  under the piston head of piston  27  so that the piston  27  operates to bias the nut  34  into an upward position with the desired biasing force. This is shown as step  640 . Backlash is thereby removed from the system. After step  640  has been performed, the test tool is raised from the surface of the substrate to be tested by a predetermined step off distance in step  650 , and the shear test is then performed in step  660 , with a constant biasing force maintained throughout the test. Once the test is complete, air supply valve  41  is closed and exhaust valves  42  is opened to remove air pressure from under the piston head of piston  27  in step  670 . The system is reset in step  680 . A new test can then be performed as indicated in block  690 . 
     With reference to  FIG. 6   b , which describes the system when operated with the pneumatic control system of  FIG. 3   b , many of the method steps are the same as described above with reference to  FIG. 6   a , however, some steps are different. 
     In the first step  700 , the test type is selected using a user interface, such as the keyboard shown in  FIG. 1 . 
     If the test is a shear or push test, at step  720 , the test tool  10  is driven into the desired test position, adjacent a bond to be tested. Once the test tool has been correctly positioned, an automated test procedure can be initiated under software control, via a suitable user interface such as the keyboard, shown as step  730 . The first step in the automated test procedure is that pressurized air, at the pressure set at regulator  97 , is supplied to the anti-backlash cylinder  28  under the piston head of piston  27  through inlet  32  so that the piston  27  operates to bias the nut  34  into an upward position with the desired biasing force. During this step, vent valve  92  is open. These actions are indicated at step  740 . Backlash is thereby removed from the system. After step  740  has been performed, the test tool is raised from the surface of the substrate to be tested by a predetermined step off distance in step  750 , and the shear test is then performed in step  760 , with a constant biasing force maintained throughout the test. Once the test is complete, air supply valve  41  is closed and exhaust valves  42  is opened to remove air pressure from under the piston head of piston  27  in step  770 . The system is reset in step  780 . A new test can then be performed as indicated in block  790 . 
     If a pull test is selected at step  700 , at step  800  the test tool  10  is driven into the desired test position, adjacent a bond to be tested. Once the test tool has been correctly positioned, an automated test procedure can be initiated under software control, via a suitable user interface such as the keyboard, shown as step  810 . The first step in the automated test procedure is that pressurized air, at the pressure set at regulator  97 , is supplied to the anti-backlash cylinder  28  above the piston head of piston  27  through inlet  92  so that the piston  27  operates to bias the nut  34  into an downward position with the desired biasing force. Vent valve  42  is open. These actions are indicated at step  820 . Backlash is thereby removed from the system. After step  820  has been performed, the test tool is raised from the surface of the substrate to be tested by a predetermined step off distance in step  830 , and the pull test is then performed in step  840 , with a constant biasing force on nut  34  maintained throughout the test. Once the test is complete, air supply valve  41  is closed and exhaust valves  92  is opened to remove air pressure from above the piston head of piston  27  in step  850 . The system is reset in step  780 . A new test can then be performed as indicated in block  790 . 
     In the foregoing description, the means for removing backlash is a pneumatically operated piston and cylinder. However other means of biasing the nut relative to the screw are possible, such as a compression spring that can be mechanically or manually moved into and out of engagement with the test tool mount or the main body so that the biasing can be switched on and off.