Abstract:
A shear test device for testing the strength of attachment between a bond and an electronic substrate. The device incorporates a clamping mechanism and a shear test tool that are mounted on a baseplate. During a set-up procedure, the shear test tool is movable relative to the baseplate. During a test procedure, the shear test tool is clamped by the clamping mechanism in a fixed position relative to the baseplate. At least one abutment is provided that is fixed in position relative to the baseplate. During the test procedure, a drive mechanism provides relative movement between the shear test tool and the bond to cause the shear test tool to shear the bond off the substrate. The at least one abutment provides an additional clamping force on the test tool while the test tool is shearing the bond off of the substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of application Ser. No. 13/035,217, filed Feb. 25, 2011 (pending) which claims the priority of European Patent Application No. 10002332.4, filed Mar. 5, 2010 (pending), the disclosures of which are hereby incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to shear testing tools. In particular, the invention relates to a device and method that allows movement of a shear testing tool relative to a mounting plate during a set-up procedure but maintains the shear testing tool in a fixed position relative to the mounting plate during a shear test procedure. 
     BACKGROUND TO THE INVENTION 
     Semiconductor devices are very small, typically from 5 mm square to 50 mm square, and typically comprise numerous sites for the bonding of electrical conductors to the semiconductor substrate. Each bond consists of a solder or gold ball deposit adhered to the substrate. It is necessary to test the strength of the attachment between the bond and the substrate, known as the bond strength, in order to be confident that the bonding method is adequate and that the bond strength is sufficient. Because of the very small size of the bonds, tools used to test the bond strength must be both very accurately positioned and able to measure very small forces and deflections. 
     A known test device, as described in WO2007/093799, has a test tool for engagement with a bond. The test tool is used to shear a bond off a semiconductor substrate and the force required to shear the bond is recorded. A force transducer is incorporated into the test tool in order to measure the force. 
     In order to ensure repeatability, it is essential for the tip of the test tool to engage the side of the bond at a predetermined height above the surface of the substrate. This distance is small but critical, since the bond is usually dome-shaped. A predetermined spacing from the surface eliminates both sliding friction from the test tool on the substrate, and ensures that the shear load is applied at a precise location in relation to the bond interface. Accordingly, in practice, the test tool is first moved into contact with the substrate surface and then withdrawn by a predetermined distance, typically 0.05 mm or less before the shear test is performed. 
     Several difficulties arise. Friction and stiction in the mechanism of the device itself may cause difficulties in sensing contact with the substrate surface. Imprecise surface sensing will inevitably affect the distance by which the test tool is withdrawn, and thus the height at which the bond is sheared. The distances involved are very small and so care needs to be taken to sense the exact moment of surface contact, without compression of the substrate. Care must also be taken to prevent uncontrolled movement of the test tool at the test height prior to or during the shear test. Such movement may seriously affect the accuracy of the test results and significant movement of the test tool at the test height may damage an adjacent bond or wire. 
     The twin objectives of both a low contact force when sensing the surface of the substrate and accurate control of the test height are difficult to resolve. 
     U.S. Pat. No. 6,078,387 discloses a device for sensing contact of a test head of a test tool with the substrate which is adapted to immediately stop downward drive of the test head when contact is sensed. The test tool is supported on the free end of a pair of cantilever arms which are secured at their opposite ends to a baseplate and deflects to allow some vertical movement of the test head with respect to the baseplate. To prevent vertical movement of the test head during the shear tests, test tool is spring biased by the cantilever arms against the baseplate. The test head can be moved away from the baseplate by an air-bearing to allow the test head to move vertically in a substantially frictionless manner for initial contact sensing. Thus, when the test head first touches the substrate surface, it is pushed back by the substrate surface on the cantilever arms. Movement of the test head relative to the baseplate or movement of the cantilever arm can be detected by an optical detector, and the air-bearing is then switched off to ensure that the test head is fixed relative to the baseplate by the spring bias of the cantilever arms against the baseplate. Once the test head is fixed relative to the baseplate, the baseplate is raised by a predetermined amount so as to leave a clearance between the lower end of the test tool and the substrate of the desired “step off distance”. 
     An alternative system is to have the cantilever arms bias the test tool away from the baseplate to allow for substantially friction-free movement of the test head relative to the base plate during initial positioning, but then to press the test tool into contact with the baseplate using a piston driven by compressed air to create a clamping force on the test tool against the baseplate during a test procedure. 
     Both of these systems are effective for accurately positioning a test head above a substrate whilst providing a relatively low touchdown force in initial positioning of the test tool above the substrate surface. However, they still suffer from some disadvantages. 
     A first disadvantage is the relatively high cost of these systems. The air-bearing is a relatively expensive component in the overall cost of a shear testing tool. 
     A second disadvantage is that during a test procedure much of the load has to be supported by the cantilever arms. This means that for tests at higher loads, larger, and consequently more massive, cantilever arms must be used. This in turn leads to a larger touchdown force when initially positioning the sensor, which can lead to damage to the substrate surface. This has also resulted in different cantilever arm assemblies being used for different load tests, further increasing cost. 
     A third disadvantage is that in both the air-bearing solution and the compressed air activated piston solution, the test head is moved laterally as the cantilever arms are urged or pressed against the baseplate, so that it can be positioned relative to the substrate but prior to the test being performed. The switching off of the air-bearing or the clamping of the test tool against the baseplate inevitably means some vertical movement will occur in addition to the lateral movement of the test head. This movement can reduce the accuracy of the resulting test, especially given the extremely small step off distance involved. 
     It is therefore an object of the invention to address the abovementioned problems or at least to provide a useful alternative. 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the invention, there is provided a shear test device for testing the strength of attachment between a bond and an electronic substrate, such as for testing the bond strength between a solder ball deposit and an electronic substrate, comprising: 
     an x-y table to which the substrate is attached; 
     a shear test tool; 
     a drive mechanism that provides relative movement in a test direction between the table and the test tool; 
     a baseplate, the shear test tool attached the baseplate by a resilient connector that allows for movement of the shear test tool relative to the baseplate in an axial direction which is perpendicular to the test direction; 
     at least one abutment rigidly fixed relative to the baseplate, the at least one abutment being in contact with the shear test tool; and 
     a clamping device coupled to the baseplate and moveable between a rest position wherein the device is not forced into contact with the shear test tool and an actuated position wherein the device is forced into contact with the shear test tool to maintain the shear test tool stationary relative to the baseplate in the axial direction, such that when the drive mechanism provides relative movement in the test direction between the x-y table and the shear test tool to cause the shear test tool to shear the bond, or ball deposit, off the substrate during a shear test, the at least one abutment provides an additional clamping force on the shear test tool. 
     Preferably, the clamping device comprises a single element that contacts the shear test tool in the actuated position. Alternatively, the clamping device may comprise two or more elements that contact the shear test tool in the actuated position. 
     Preferably, when the clamping device is in the actuated position, the shear test tool is held between the clamping device and the least one abutment. 
     Preferably, in the actuated position, the clamping device urges the shear test tool into contact with first and second abutments. 
     Preferably, the clamping device is moveable between the rest position and the actuated position via a pneumatic mechanism. 
     Preferably, the position of the at least one abutment, and the first and/or second abutments, is adjustable along an axis that is parallel to an axis aligned with the test direction. 
     Preferably, the resilient connector that allows for movement of the shear test tool relative to the baseplate comprises a pair of cantilever arms. 
     Preferably, the shear test device further comprises a sensor configured to generate a signal when movement of the shear test tool relative to the baseplate in the axial direction is detected, and a controller connected to the sensor and to the clamping device, wherein the controller is configured to move the clamping device into the actuated position in response to the signal from the sensor. 
     In a second aspect of the invention, a new method of testing the strength of attachment between a bond and an electronic substrate is described, using a shear test device in accordance with the first aspect of the invention, comprising the steps of: 
     positioning the test tool a predetermined distance from the substrate in the axial direction; 
     providing relative movement between the test tool and the bond in the test direction to cause the test tool to shear the bond off the substrate, wherein the force applied by the bond to the test tool causes the at least one abutment in the shear test device to provide a clamping force on the test tool; and 
     recording the force applied to the test tool by the bond. 
     Preferably, the step of positioning the test tool comprises: moving the test tool in the axial direction towards the substrate; detecting contact between the test tool and the substrate; and stopping the moving of the test tool in the axial direction when contact is detected. Preferably, the method further comprises the step of clamping the test tool to fix the position of the test tool relative to the baseplate following the step of stopping. Preferably, the method further comprises the step of moving the test tool and baseplate away from the substrate a predetermined distance following the step of clamping. Preferably, the method further comprises the step of unclamping the test tool relative to the baseplate following the step of recording. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic illustration of a shear testing apparatus in accordance with the present invention; 
         FIG. 2  is a perspective view of a shear testing device, in accordance with a the present invention; 
         FIG. 3  is a cross-section view of a shear testing device in accordance with the present invention; 
         FIG. 4  is another perspective view of the shear testing device of the present invention, with the clamp support block  23  removed; 
         FIG. 5  is a cross sectional view showing the mounting of the shear tool  10  to the tool holder  200 , and the strain gauges  160   a - 160   d , in more detail; 
         FIG. 6  is a partial cross-sectional view of the elements of the optic sensor  202  and the touchdown adjusting screw  120 ; 
         FIG. 7  is a partial cross-sectional view the pre-clamp piston  33  and other elements of the device; 
         FIG. 8  is an enlarged perspective view of encircled area  8  showing a part of the shear testing device, with the clamp support block  23  removed to show the touchdown block  122  in more detail; 
         FIG. 9  is a schematic diagram showing the control elements of an apparatus in accordance with the present invention; 
         FIG. 10  is a flow diagram illustrating the method steps carried out in performing a shear test using an apparatus in accordance with the present invention; and 
         FIG. 11  is a schematic diagram of a circuit for detecting signals from the strain gauges  160   a - 160   d.    
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is an illustration of a shear testing device  12  in accordance with the present invention. The device  12  comprises a shear test tool  100  mounted to a cartridge  11 , which is itself mounted to the main body of the device  12 . Beneath the shear test tool  100  is a motorised stage table  13 , on which samples to be tested can be mounted. As shown in  FIG. 3 , the samples are typically substrates  300  upon which solder ball deposits  302  are formed that are connected to electronic circuitry (not shown) within the substrate  300 . 
     As best shown in  FIG. 3 , the shear test tool  100  includes a shear test head  10  that is secured in a tool holder  200 , which is in turn secured in a shear seam body  22 . As will later be described in more detail, the test head  10  applies a shear force to the ball deposits  302  on a substrate  300  under test. The shear test tool  100  includes strain gauges (later described) for measuring the shear force experienced by the test head  10  as the ball deposit  302  is sheared off the substrate  300  under test. 
     The shear tool  100  is held in a cartridge  11  (in a manner later described) and is moveable in a direction normal to the substrate  300 , herein after referred to as the Z-direction or axial direction. Movement of the shear tool  100  in the Z-direction is achieved by movement of the cartridge  11  relative to the main body  12 . The cartridge  11  is mounted to the main body  12  by a ball screw, or leadscrew and nut (not shown), and can be driven by an axial drive mechanism such as a servo motor or stepper motor (not shown), or any other suitable, controllable drive arrangement as 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. 
     The stage table  13  is movable parallel to the plane of a substrate  300  under test, herein after referred to as the X-Y plane. This allows ball deposits  302  to be moved along a test direction axis in a test direction towards and past the shear test tool  100  during a test procedure. Movement of the stage table  13  is again achieved using suitable servo motors or stepper motors coupled to the table via a leadscrew and nut, ballscrew and nut or suitable belt drive mechanism, as is also well known in prior art such as the Dage 4000 Multipurpose Bond Tester referenced above. 
     Also shown in  FIG. 1  are control devices, comprising two joystick controls  14 ,  15  for controlling the movement of the stage table  13 , and a keyboard  16 . A display  17 , a light  18  for illuminating the substrate under test, and a microscope  19  aiding accurate positioning of the test tool, are also shown. These features are also all well known in prior art such as the Dage 4000 Multipurpose Bond Tester referenced above. 
     With reference to  FIG. 2 , the cartridge  11  comprises a backplate  20 , to which one end a pair of cantilever support arms  21  is rigidly attached using screws  105 . The cantilever support arms  21  support shear test tool  100  at their opposite ends. Shear test tool  100 , in one preferred embodiment, comprises a shear beam body  22  to which is attached a tool holder  200 , to which is attached a test head  10 . As best shown in  FIG. 4 , shear beam body  22  has a split clamp design. Tool holder  200  is inserted into the body  22  and then screws  110  are tightened to clamp holder  200  securely into the body  22 . The tool holder  200  is fitted with a dowel pin protruding from the clamped portion of its shaft. The dowel pin is received in the split of the split clamp  22 , such that the rotational position of the tool relative to the shear beam body  22  is assured. The shear test head  10  is in turn attached to the tool holder  200  by a thumb screw  106  as best shown in  FIG. 5 . The test head  10  has a flat on a front surface, onto which the thumb screw  106  operates. Fastening the thumb screw against the flat ensures that the test head  10  is in the correct rotational orientation relative to the tool holder  200 . The cantilever arms  21  allow the shear test tool  100 , comprised of the shear beam body  22 , tool holder  200  and test head  10 , to move in the Z-direction with very little friction. 
     Although the example illustrated in  FIG. 2  uses a pair of cantilever arms  21  in order to allow the shear test tool  100  to move in the Z-direction, other resilient mounting arrangements may be used, for example a compression spring. 
     A clamp support block  23  is rigidly fixed to the backplate  20 . The clamp support block  23  has a hood portion  112  that extends around the shear beam body  22 , and a backplate portion  114  which is attached to backplate  20  such as by screws (not shown). A portion  116  of the clamp support block  23  is positioned on an opposite side of the shear beam body  22  to the backplate  20 . When the clamping mechanism (later described) is not in its actuated position, the shear beam body  22 , and therefore the shear test tool  100 , is free to move up and down within the clamp support block  23 . 
     The cartridge  11  includes an optical sensor  202  for detecting when the test head  10  of shear tool  100  contacts the substrate. As best shown in  FIG. 6 , the sensor  202  is supported in the clamp support block  23 . The sensor  202  includes an emitter  204  and a receiver  206 . The emitter  204  projects a light beam through aperture  208  formed in the clamp support block  23 . 
     A touchdown adjusting screw  120  is threaded into a touchdown block  122 . Touchdown block  122  is attached to shear beam body  22  by screws  124  best shown in  FIG. 8 . With reference to  FIG. 6 , touchdown adjusting screw  120  projects through a slot  130  (shown in  FIGS. 6 and 7 ) and can be threaded down into contact with the bottom  132  of slot  130  to block the light beam emitted from emitter  204  and prevent it from being detected by receiver  206 . 
     As this screw  120  is threaded down against the surface  132 , it exerts an upward force on the test tool  100 , lifting the test tool  100  against the bias of the cantilever arms  21 . The greater the distance that the test tool  100  is lifted by screw  120 , the greater the force of the cantilever arms that biases the screw  120  against the surface  132  of the clamp support block  23 . 
     When the tool  100  is moved downwardly into contact with the substrate  300 , the bottom end of the screw  120  is lifted off of the surface  132  to a position where it no longer blocks the light beam from emitter  204 . This causes the receiver  206  to detect the beam and thereby indicate to the control system (later described) that the tool  100  has contacted the substrate  300 . 
     If the screw  120  is only lightly contacting the surface  132 , only a light touchdown force is required to lift the end of screw  120  off the surface  132 . If, on the other hand, the screw  120  is threaded down to contact the surface  132  with greater force, a greater touchdown force is required to move the end of screw  120  up from contact with surface  132 . Therefore, the degree to which screw  120  is threaded down through the touchdown block  122  into contact with surface  132  of the clamp support block  23  determines the amount of touchdown force required to allow the optic sensor  202  to detect contact of the shear test tool  100  with the substrate  300 . 
     The sensor  202  is coupled to touchdown electronics  47  (later described with reference to  FIG. 9 ) that also controls the axial drive mechanism for the cartridge  11  and the shear tool  100 , and the movement of the x-y table  13 . When movement of the shear tool  100  upward relative to the backplate  20  is detected by detection of the optic beam by receiver  206  as described above, further downward movement of the cartridge  11  relative to the main body  12  by the axial drive mechanism is stopped. At this point, it is necessary to prevent further movement of the test tool  100 , so that the test tool  100  can be accurately positioned relative to the substrate  300 . The system of the present invention uses a pre-clamp mechanism (later described) to hold the shear test tool  100  in place relative to the cartridge backplate  20 , once touchdown of the tool  100  on the substrate has been detected. 
     As shown in  FIG. 3 , the shear beam body  22  is positioned between a rear abutment which, in this embodiment, is a clamp screw  30  and a front abutment which, in this embodiment, is a clamp screw  31 . Rear clamp screw  30  is accessed through a hole (not shown) in backplate  20  and is threaded through the back plate portion  114  of clamp support block  23 . Front clamp screw  31  is threaded through the front of the hood portion  112  of clamp support block  23 . The front and rear clamp screws  30 ,  31  are adjustable during a setup procedure when the shear beam body  22  is in a neutral position with no lateral forces being applied to it. In this neutral position, the front and rear clamp screws  30 ,  31  are threaded inwardly towards the test tool  100  until they are in only very light contact with the shear beam body  22  and tool holder  200 , respectively, causing little or no friction when the shear test  100  is moved in the Z-direction on the cantilever arms  21  relative to the clamp support block  23 . 
     A clamp, or pre-clamp, piston  33  is positioned directly opposite the front clamp screw  31  as best shown in  FIG. 7 . Clamp piston  33  is located within a chamber, or hole,  142  in backplate  114  of clamp support block  23 . Compressed air is supplied into chamber  142  through the compressed air supply line  144  shown in  FIGS. 2-4  and  7 - 8 . Once in the chamber  142 , the compressed air enters the cross drilled holes  146 , which have been drilled through piston  33 . The air then enters bore  148  and exits the back of the piston  33  to pressurize the back chamber  150  which is formed in backplate  20 . Back chamber  150  is sealed by O-ring  152 , and consequently, the pressurized air trapped in chambers  142  and  150  forces piston  33  to the left in  FIG. 7  to force it against shear beam body  22 . When the pressurized air is removed, there is no appreciable force exerted by piston  33  against shear beam body  22 , because piston  33  is freely movable within the hole  140  in backplate  14 . 
     Thus, piston  33  is operable to move between a rest position where it does not exert an appreciable force against the shear beam body  22 , and an actuated position, wherein it does exert an appreciable force against shear beam body  22  to clamp shear beam body  22 , and thereby the test tool  100 , between it and the front clamp screw  31 . Clamp piston  33  when in its actuated position, holds the shear beam body  22 , and shear tool  100 , in a fixed position relative to the clamp support block  23  and back plate  20 . This prevents any movement in the Z-direction, so that no further movement on the cantilever arms  21 , or shear tool  100 , is possible. There are other possible actuating mechanisms for the clamp piston. For example, it could be actuated by a solenoid or by a servo motor. 
     The clamp piston  33  is moved into the actuated position, fixing the position of the shear beam body  22 , and tool  100 , only when it has been determined by the detection of the optic beam by receiver  206  (as previously described) that the test head  10  of shear tool  100  has touched down on the substrate  300  under test. The clamp piston  33  can be referred to as a pre clamp mechanism because it provides an initial clamping mechanism to the shear tool  100  to prevent the shear tool  100  from moving as the ball deposit  302  is moved into engagement with the shear tool  100 . However, as explained further below, the shear forces that act upon the shear tool  100  during a shear test enhance the clamping force on the shear tool  100 . 
     Note that the shear tool  100  includes clamp pads  35  made from tungsten carbide or similar hard, tough material that are positioned opposite to the front and rear clamp screws  30 ,  31  and to the clamp piston  33 . This ensures a good contact between the shear test tool  100  and the clamp screws  30 ,  31  and clamp piston  33 , and ensures that there is minimal wear so that the system has good repeatability and an extended lifetime. Similarly, the front and rear clamp screws  30 ,  31  have contact surfaces made from hardened steel. The clamp piston  33  contact surface is made from brass or similar material to ensure smooth operation and is not hardened. 
     Once the clamp piston  33  has immobilised the shear test tool  100  relative to the backplate  20 , the cartridge  11  is moved in the Z-direction away from the substrate a predetermined step off distance prior to performing a shear test. This procedure is standard in shear testing tools of this type, and is described in detail in WO 2005/114722. 
     During a shear test, an object under test exerts a shear force on the test tool head  10  of the shear tool  100  in a test direction, which is the direction of relative movement between the object and the test tool head  10  of the shear test tool  100 . The test direction is indicated by arrow  36  in  FIG. 3 . The shear force exerted on the test tool head  10  of the shear tool  100  urges the test tool  100  against the front and rear clamp screws  30 ,  31 . The load of the shear force is thereby transferred to the backplate  20  through the clamp screws  30 ,  31 , rather than through the cantilever arms  21 . The greater the shear force, the greater the load applied to the clamp screws  30 ,  31 . This arrangement reduces the load experienced by the cantilever arms during a shear test. Thus, while the pre-clamp piston  33  initially clamps shear tool  100  in position, during a shear test, the shear forces themselves enhance the clamping force on the test tool  100 . 
     In prior cartridge assemblies, the cantilever arms  21  had to withstand a large proportion of the shear forces experienced during a test. This leads to a requirement for more robust and hence more massive cantilever arms when carrying out shear tests at greater test loads. The more massive the cantilever arms, the greater the touchdown force that the shear test tool  100  exerted on a substrate  300  during the touchdown procedure prior to a shear test. However, with the clamp screw arrangement of the present invention, the cantilever arms  21  take minimal load, and consequently, they can be made relatively light weight. 
       FIG. 9  is a schematic illustration of the control elements of a device in accordance with the present invention. The moving parts in the invention are: the axial drive mechanism, shown as a Z-axis motor and encoder  41 ; the motor  42  that drives the movement of the stage table  13  in the X-Y plane; and the clamp piston  43 , which in the illustrated embodiment is a pneumatic piston. Each of these devices is controlled by dedicated motion control electronics  44 , which is connected to a personal computer (PC)  45 . The PC  45  also receives a signal from the optical sensor  202  to indicate the touchdown of the test tool  100  on a substrate  300 . The optical sensor  202  is illustrated as box  46 , and also has its own dedicated controller, referred to herein as touchdown electronics  47 . There is a direct link between motion control electronics  44  and touchdown electronics  47 . This allows swift signal response to ensure that the axial drive mechanism stops in the quickest possible time, to prevent over-travel and associated “heavy” touchdown. 
       FIG. 10  illustrates the steps taken in setting up and performing a shear test using an apparatus in accordance with the present invention. In a first step  505 , the cartridge  11  and test tool  100  are moved down until test head  10  contacts the substrate  300 . At step  510 , the touchdown of the test head  10  on the surface of the substrate  300  is detected by detection of the optic beam by receiver  206 , as previously described. When touchdown is detected, operation of the Z-axis motor that moves the shear cartridge  11  towards the substrate  300  is stopped at step  515 . At step  520 , compressed air is supplied to chambers  142  and  150  to move the pre-clamp piston  33  to its actuated position were in it forcibly contacts the shear beam body  22  to immobilise shear tool  100  relative to the back plate  20 . At step  525 , the cartridge is then moved away from the substrate to a predetermined step off distance in preparation for a shear test. At step  530 , the shear test is commenced by moving the motorized table  13 , and thereby the substrate  300  attached to it, so that a ball deposit  302  on the substrate  300  contacts the test tool head  10  to shear the ball deposit  302  off the substrate  300 . 
     During the shear test, in step  535 , a load, or force, is applied to the shear test head  10  as the test head  10  shears the ball deposit  302  off the substrate  300 . This shear force is preferably picked up by strain gauges. As best shown in  FIG. 5 , in one preferred embodiment, four strain gauges  160   a ,  160   b ,  160   c  and  160   d  are bonded to tool holder  200 . A cover  165  protects the strain gauges  160   a - 160   d . Gauges  160   a  and  160   b  are on the front side of tool holder  200 , facing the ball deposit  302  that will be sheared off substrate  300  during the shear test. Gauges  160   c  and  160   d  are on the opposite, or rear side, of tool holder  200 . 
     The strain gauges  160   a - 160   d  are connected by wires (not shown) to an electric circuit, such as, for example, the full bridge circuit  160  shown schematically in  FIG. 11 . As is well known, such circuits convert electric signals caused by distortion of the strain gauges  160   a - 160   d  to force measurements, which indicate the force that was required to shear the ball deposit  302  off the substrate  300 . 
     Alternatively, the shear force could be detected by a piezoelectric crystal mounted on the shear tool as described in the previously mentioned WO 2007/093799 A1. Using either force detection technology, as indicated at step  540 , as the ball deposit  302  is driven against the shear tool  100 , the front and rear clamp screws or abutments  30 ,  31  further enhance the clamping action on the shear tool  100 , as described above. 
     At step  545  the shear test is completed and the shear test tool  100  is moved away from the substrate  300  and returned to a start position in step  550 . At step  555  the clamp piston  33  is deactivated by removal of air pressure from chambers  142  and  150  so that the piston  33  can assume its rest position. This allows the shear test tool  100  to be moved in the Z-axis on the cantilever arms  21  by the drive mechanism. At step  560  the shear test can be repeated by performing the same method steps again, as shown in step  565 . 
     Thus, referring to  FIG. 2 , a system in accordance with the present invention allows for a shear tool  100  to touchdown on a substrate under test with a relatively low, and adjustable, touchdown force. This low touchdown force is due to the relatively low mass of the cantilever arms  21  that is required for reasons described above. The system also allows the test tool  100  to be fixed relative to a mounting cartridge back plate  20  so that the vertical position of the test tool  100  can be accurately controlled. This is important for accurate and repeatable shear tests. As the shear load experienced by the shear test tool  100  is transferred to the cartridge backplate  20  via the clamping arrangement described, rather than via the cantilever arms  21 , it is not necessary to have different cartridge assemblies for different load tests.