Abstract:
An apparatus applies a pull test to a bond of a semi-conductor assembly, the bond including a ball or a bump of solder. The apparatus includes a probe including a straight, thermally conductive pin; a heater for heating a tip of the probe; a holder for supporting the probe and including a clamping mechanism that is configured to provide a clamping force on the probe; an actuation device for moving the holder and the probe up and down; and a pull force applier for applying a pull force on the holder. A force measuring system measures a force applied to the probe during the pull test to determine the strength of the bond.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a system and method for pull testing of bonds of semiconductor assemblies. In particular, the invention relates to a system and method for testing the pull strength of bonds formed from a ball or bump of meltable material, such as solder. 
       BACKGROUND TO THE INVENTION 
       [0002]    In order to test the quality of the manufacturing processes used in the production of printed circuit board (“PCB”) and/or semiconductor assemblies, destructive and non-destructive mechanical strength tests are performed. Typically the testing is performed on a bond which is providing electrical or thermal continuity between two materials. For bonds that are of a suitable shape and size the test force can be applied by gripping, hooking or shearing one of the materials making the bond. 
         [0003]    There are some bonds where this is not possible, typically where the bond is a ball or bump of solder on the surface of a PCB or a semiconductor substrate, but is of unusual shape or size or which is difficult to access using gripping jaws. As an alternative means of applying a test force, it is known to melt the bond, and then allow the bond to re-solidify around a test tool. The test tool can then be moved to apply a test force to the re-solidified bond. An example of this type of system used for pull testing is described in U.S. Pat. No. 5,641,913 to Watanabe. 
         [0004]    In practice, this operating principle has been implemented by adapting existing bond testing machines designed for hooking or gripping bonds to perform a pull test. A machine currently used in this way is the Dage 4000 multifunction Bondtester, available from Dage Holdings Limited, 25 Faraday Road, Rabans Lane Industrial Area, Aylesbury, Buckinghamshire, United Kingdom. A test pin is attached to the hook ordinarily used for pull testing. The test pin is held in place by the hook, which is directly attached to a beam fitted with strain gauges to measure the force applied during the test. One end of the pin has a 90 degree bend formed which engages the hook and transfers the force to the tip of the pin. The system uses a cartridge heater inserted into a relatively large titanium block which mechanically supports a heater and a thermocouple. The hook is accurately aligned above the titanium heater block so that the straight portion of the test pin passes through a close fitting hole running through it. 
         [0005]    The method of operation of this equipment is as follows. The sample to be tested is rigidly fitted into a work holder attached to a horizontally movable table. The operator uses joystick controls to move the specific test site on the sample directly under the test pin, typically using a microscope to achieve the required accuracy. The operator lowers the whole load cell and test pin assembly, which is mounted to a motorized vertical stage, using the joystick control until the tip of the test pin is resting on the top of the solder ball/bump to be tested. The test button is pushed which heats the probe, through the titanium heater block, to a pre-determined temperature. Once the solder ball/bump melts, the very tip of the test pin, under its own weight, drops into the molten pool of solder. When the desired temperature has been reached, the heater is turned off, which allows the test pin to start cooling and the solder to solidify. Once solidified, the pin, the solder and heater block are cooled more rapidly by means of a cold air jet directed at them. Once a predetermined temperature has been achieved the test pin is anchored into the body of the solder ball/bump and the destructive pull test can be started. The whole load cell assembly is automatically driven upwards which causes the pull hook to apply an axial load on the pin and therefore the bond. A beam in the load cell flexes and calibrated strain gauges measure the force. As the bond fails, the strain gauges see the force dropping off and the maximum force prior to failure is recorded. The recorded force is then stored in a database. 
         [0006]    This bond testing apparatus and method suffers from a number of disadvantages in measurement accuracy, speed of operation, and usability, and it is an object of the present invention to overcome some or all of these disadvantages, or at least to provide a useful alternative. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention is defined in the appended independent claims, to which reference should be made. Preferred aspects of the invention are set out in the dependent claims. 
         [0008]    In one aspect, the invention comprises an apparatus for applying a pull test to a bond of a semi-conductor assembly, the bond comprising a ball or a bump of solder, the apparatus comprising: 
         [0009]    a probe, said probe having a longitudinal axis; a heater for heating a tip of said probe to a temperature at or above a temperature at which the bond is melted; 
         [0010]    a holder for supporting said probe; an actuation device for moving up and down said holder and said probe supported in said holder; 
         [0011]    a means for applying a pull force on said holder which in turn applies a pull force to said probe along the longitudinal axis of the probe; and a force measuring system for measuring a force applied to said probe during the pull test, wherein after said probe tip has been heated to a temperature at or above a temperature at which the bond is melted, said probe tip is brought into contact with the bond, the bond is melted by heating of the probe and the bond is cooled and solidified to fix the probe tip in the bond, the probe is then retracted by the pull force applying means to apply a pull force to the bond, which pull force is measured by said force measuring system. 
         [0012]    By applying the pull force along the longitudinal axis of the probe, measurement accuracy is improved in comparison with the prior art. In the prior art system, employing a bent probe pin, the force applied via the bend in the test pin is not directly above the test site and generates a small bending moment. This can also result in increased friction between the test pin and the hole in the heater block, which can adversely affect the force measurement. By applying the force in line with the longitudinal axis of the probe, bending moments are eliminated and frictional problems reduced. 
         [0013]    Furthermore, in the prior art system, the maximum force applied by the pull hook engaging the test pin is limited by the strength of the bend in the test pin. When this maximum force is exceeded, the pin straightens out. By applying the pull force in line with the longitudinal axis of the probe, and not through a bend, this problem is eliminated. 
         [0014]    Preferably, the probe is a straight pin coupled to the holder by a probe retaining mechanism. Preferably, the probe is retained in the holder by a clamping mechanism. Preferably the clamping mechanism provides a clamping force on a longitudinal shaft of the probe. The use of a clamp of this type, together with a straight probe pin, allows the probe to be readily removed from the apparatus when a test is completed. In contrast, in the prior art system, once the test has been completed the test pin is mechanically trapped in the heater block, as it has the solder ball on one end and the bend on the other. This means either the pin has to be cut or the heat cycle has to be re-run and the solder ball removed manually whilst molten so that the probe can be removed. This has a significant impact not only on speed of operation but also on cost. The test pins are precisely formed and for a consumable item are relatively expensive, and so it is desirable to be able to clean and reuse them. 
         [0015]    Also, with the use of a straight, clamped pin, the operator requires less contact with the hot end of the holder during loading of the probe as the holder can be simply driven down onto a pin that is held vertically in a loading tray, and then clamped. A thermal shield can also be provided surrounding the heater, to reduce the risk of burn injuries and to reduce undesirable heat loss from the probe during the heating phase of operation. In contrast, with the prior art system manual loading of the probe was necessary, which was more time consuming and cumbersome. It also exposed the operator to a risk of burn injuries when contacting the holder, as parts of it can be hot, particularly if the solder ball has been re-melted following a test. 
         [0016]    With the probe rigidly clamped both a ‘Push’ and ‘Pull’ test can be performed. It also enables a ‘Fatigue’ test to be run where the load is cycled between a compressive and a tensile test up to predetermined forces or for a fixed number of cycles. With the prior art system, it was not possible to apply push force to the pin. Therefore, push tests and fatigue tests were not possible with the prior art system. 
         [0017]    Preferably, the clamping mechanism is pneumatically operated. However any suitable operating mechanism may be used, such as, for example, electrical or magnetic mechanisms. The clamping mechanism may comprise a collet surrounding the probe, the collet having a tapered external surface, and a piston, wherein, in use, actuation of the piston causes the piston to travel along the tapered surface, or causes the tapered surface to be driven against another surface, to clamp the collet around the probe. An additional or alternative, manually operable clamping mechanism may also be provided. Preferably, the collet is biased into an unclamped position. 
         [0018]    Preferably, the heater comprises a thermally conductive tube which surrounds at least a part of said probe and a heating element which surrounds at least a part of said tube. In this way, the heat source is much closer to the test pin than in the prior art, reducing the energy required to perform the test as well as the time to perform the complete cycle. The thermal mass that needs to be heated and cooled has also been significantly reduced by the present invention. 
         [0019]    Preferably, the heater wire is connected to an electrical power source that acts to heat the heater wire. Preferably, the thermally conductive tube is electrically insulating. Preferably, the thermally conductive tube is formed from a ceramic material. An example of a suitable ceramic material is an Aluminium Nitride/Boron Nitride ceramic composite, available from Dynamic Ceramic Ltd, of Crewe Hall, Weston Road, Crewe, CW1 6UA United Kingdom. 
         [0020]    Preferably, the apparatus further includes a thermocouple located adjacent to the probe. Preferably, the thermocouple is located on the thermally conductive tube, and more preferably at the end of the thermally conductive tube closest to the bond under test when in use. An electrically insulating tube allows the thermocouple to be placed directly on it. The thermocouple can be used to determine the temperature of the probe both during a heating phase of the bond test and during a cooling phase. The temperature of the probe can be recorded and controlled throughout the bond test. It is advantageous to control the melting and cooling of the bond under test to mimic as closely as possible the process carried out during manufacture. This ensures that the form of the bond and its material properties under test match those of manufactured bonds. 
         [0021]    Preferably, the apparatus includes a cooling system for cooling the probe. Preferably the cooling system comprises a source of compressed air, a nozzle or outlet connected to the source of compressed air, the nozzle or outlet arranged to provide a flow of compressed air in the vicinity the probe, and a valve configured to control the supply of compressed air from the source to the nozzle or outlet. 
         [0022]    Preferably the apparatus further includes a movable platform on which a semiconductor sample to be tested is mounted. Preferably, an array of probes is also mounted on the movable platform. By having an array of probes, preferably pre-loaded into a carrier tray, and with known positions for the test sites, an automatic test routine is possible. Motorized controls can be used to move the holder and platform relative to each other to automate picking up probes, performing a test and then dropping the used probes into a collection receptacle. 
         [0023]    In another aspect, the invention comprises an apparatus for applying a pull test to a bond of a semi-conductor assembly, the bond comprising a ball or a bump of solder, the apparatus comprising: 
         [0024]    a probe; 
         [0025]    a heater for heating a tip of said probe to a temperature at or above a temperature at which the bond is melted; 
         [0026]    a holder for supporting said probe; 
         [0027]    an actuation device for moving up and down said holder and said probe supported in said holder; 
         [0028]    a means for applying a pull force on said holder which in turn applies a pull force to said probe; and 
         [0029]    a force measuring system for measuring a force applied to said probe during the pull test, wherein after said probe tip has been heated to a temperature at or above a temperature at which the bond is melted, said probe tip is brought into contact with the bond, the bond is melted by heating of the probe and the bond is cooled and solidified to fix the probe tip in the bond, the probe is then retracted by the pull force applying means to apply a pull force to the bond, which pull force is measured by said force measuring system, and wherein said heater comprises a thermally conductive tube which surrounds at least a part of said probe and a heating element which surrounds at least a part of said tube. 
         [0030]    With this heater arrangement, the heat source is much closer to the test pin than in the prior art, reducing the energy required to perform the test as well as the time to perform the complete cycle. The thermal mass that needs to be heated and cooled is also significantly reduced. This speeds up the testing process. 
         [0031]    Preferably, the heater wire is connected to an electrical power source that acts to heat the heater wire. Preferably, the thermally conductive tube is not electrically conductive. Preferably, the thermally conductive tube is formed from a ceramic material. 
         [0032]    Preferably the apparatus further includes a thermocouple located adjacent to the probe. Preferably, the thermocouple is located on the thermally conductive tube, and more preferably at the end of the thermally conductive tube closest to the bond under test in use. The thermocouple can be used to determine the temperature of the probe both during a heating phase of the bond test and the cooling phase. The temperature of the probe can be recorded and controlled throughout the bond test. It is advantageous to control the melting and cooling of the bond under test to mimic as closely as possible the process carried out during manufacture. This ensures that the form of the bond and its material properties under test match those of manufactured bonds. 
         [0033]    The apparatus preferably further includes a thermal shield surrounding the heater to reduce the risk of burn injuries and to reduce undesirable heat loss from the probe during the heating phase of operation. 
         [0034]    Preferably, the apparatus includes a cooling system for cooling the probe. Preferably the cooling system comprises a source of compressed air, a nozzle or outlet connected to the source of compressed air, the nozzle or outlet arranged to direct a jet of compressed air at the probe, and a valve configured to control the supply of compressed air from the source to the nozzle or outlet. 
         [0035]    Preferably, the probe has a longitudinal axis and said holder applies a pull force to said probe along the longitudinal axis of the probe 
         [0036]    Preferably, the probe is a straight pin coupled to the holder by a probe retaining mechanism. Preferably, the probe is retained in the holder by a clamping mechanism. Preferably the clamping mechanism provides a clamping force on a longitudinal shaft of the probe and is symmetrically disposed about the shaft. 
         [0037]    Preferably, the clamping mechanism is pneumatically operated. However any suitable clamping mechanism may be used which may be operated, for example, electrically or magnetically. The clamping mechanism may comprise a collet surrounding the probe, the collet having a tapered external surface, and a piston, wherein, in use, actuation of the piston causes the piston to travel along the tapered surface, or causes the tapered surface to be driven against another surface, to clamp the collet around the probe. An additional or alternative, manually operable clamping mechanism may also be provided. Preferably, the collet is biased into an unclamped position. 
         [0038]    Preferably the apparatus further includes a movable platform on which a semiconductor sample to be tested is mounted. Preferably, an array of probes is also mounted on the movable platform. By having an array of probes, preferably pre-loaded into a carrier tray, and with known positions for the test sites, an automatic test routine is possible. Motorized controls can be used to move the holder and platform relative to each other to automate picking up probes, performing a test and then dropping the used probes into a collection receptacle. 
         [0039]    In a further aspect, the invention comprises a method of testing a bond on a semiconductor assembly, the bond comprising a ball or a bump of solder, the method comprising the steps of: 
         [0040]    applying a tip of a thermally conductive probe to the bond; heating the tip of said probe to a temperature at or above a temperature at which the bond is melted; 
         [0041]    cooling the tip of the probe, or allowing the tip of the probe to cool, to a temperature at which the bond is solidified, wherein the tip of the probe is embedded in the bond; 
         [0042]    applying a pull force on the probe, and recording the force applied to the probe during the step of applying a pull force, wherein the probe has a longitudinal axis and the step of applying a pull force comprises applying a pull force on the probe along the longitudinal axis of the probe. 
         [0043]    Preferably, the method includes the step of applying a force on the probe during the step of heating, to push the probe into the bond as it melts. Alternatively, the method includes the step of resting the probe on the bond during the step of heating, to allow the probe to sink into the bond as it melts under its own weight. 
         [0044]    In a still further aspect, the invention comprises a method of testing a bond on a semiconductor assembly, the bond comprising a ball or a bump of solder, the method comprising the steps of: 
         [0045]    applying a tip of a thermally conductive probe to the bond; heating the tip of said probe to a temperature at or above a temperature at which the bond is melted; 
         [0046]    cooling the tip of the probe, or allowing the tip of the probe to cool, to a temperature at which the bond is solidified, wherein the tip of the probe is embedded in the bond; 
         [0047]    applying a pull force on the probe to remove the bond from the semiconductor assembly, and recording the force applied to the probe during the step of applying a pull force, 
         [0048]    wherein the step of heating comprises providing a thermally conductive tube which surrounds at least a part of said probe and a heating element which surrounds at least a part of said tube, and passing a current through the heating element to heat the probe. 
         [0049]    In a further aspect, the invention comprises a method of testing a bond on a semiconductor assembly, the bond comprising a ball or a bump of solder, the method comprising the steps of: 
         [0050]    clamping a thermally conductive probe into a probe holder, wherein the probe comprises a straight pin, and the step of clamping the probe comprises moving the probe holder and the probe relative to one another so that the probe is positioned within the holder and clamping a longitudinal shaft of the probe; 
         [0051]    applying a tip of the probe to the bond; 
         [0052]    heating the tip of said probe to a temperature at or above a temperature at which the bond is melted; 
         [0053]    cooling the tip of the probe, or allowing the tip of the probe to cool, to a temperature at which the bond is solidified, wherein the tip of the probe is embedded in the bond; 
         [0054]    applying a pull force on the probe through the probe holder, and recording the force applied to the probe during the step of applying a pull force, and 
         [0055]    releasing the probe from the probe holder. 
         [0056]    Preferably, the step of moving the probe holder and the probe relative to one another so that the probe is positioned within the holder is performed under automated control. 
         [0057]    Preferably the step of moving the probe holder and the probe relative to one another so that the probe is positioned within the holder comprises moving the probe laterally so that it is aligned with the probe holder and subsequently moving the probe holder vertically over the probe. 
         [0058]    Preferably, the method further includes the step of moving a receptacle to a position underneath the probe holder subsequent to the step of applying pull force and the step of releasing the probe from the probe holder comprises releasing the probe into the receptacle. 
         [0059]    Preferably the step of clamping comprises clamping a collet around a longitudinal shaft of the probe under automated control. 
         [0060]    In yet a further aspect, the invention comprises an apparatus for applying a pull test to a bond of a semi-conductor assembly, the bond comprising a ball or a bump of solder, the apparatus comprising: 
         [0061]    a probe, said probe comprising a straight, thermally conductive pin; 
         [0062]    a heater for heating a tip of said probe to a temperature at or above a temperature at which the bond is melted; 
         [0063]    a holder for supporting said probe, the holder comprising a clamping mechanism that is configured to provide a clamping force on the probe; 
         [0064]    an actuation device for moving up and down said holder and said probe supported in said holder; 
         [0065]    a means for applying a pull force on said holder which in turn applies a pull force to said probe; and 
         [0066]    a force measuring system for measuring a force applied to said probe during the pull test, wherein after said probe tip has been heated to a temperature at or above a temperature at which the bond is melted, said probe tip is brought into contact with the bond, the bond is melted by heating of the probe and the bond is cooled and solidified to fix the probe tip in the bond, the probe is then retracted by the pull force applying means to apply a pull force to the bond, which pull force is measured by said force measuring system. 
         [0067]    The use of straight probes allows the loading of the probes into the holder to be simplified and even automated. The holder simply needs to be driven down onto an aligned probe pin and then clamped about its shaft. In contrast, in the prior art system using a bent probe pin engaged with a hook, loading of the probe pin had to be done manually and required considerable dexterity. 
         [0068]    Preferably, the apparatus further comprises a plurality of probes arranged in predetermined positions, and an automatic probe loading mechanism configured to move either the holder or at least one of the probes, to bring a probe into alignment with the holder. Preferably, the apparatus further includes a collection receptacle that can be positioned underneath the probe holder to receive used probes following a test. 
         [0069]    Preferably the apparatus further includes a movable platform on which a semiconductor sample to be tested is mounted. Preferably, an array of probes is also mounted on the movable platform. By having an array of probes, preferably pre-loaded into a carrier tray, and with known positions for the test sites an automatic test routine is possible. An automatic probe loading mechanism can be used to move the holder and platform relative to each other to automate the picking up of probes. The test can then be performed and the used probe dropped into a collection receptacle. 
         [0070]    In yet another aspect, the invention comprises an apparatus for applying a pull test to a bond of a semi-conductor assembly, the bond comprising a ball or a bump of solder, the apparatus comprising: 
         [0071]    a plurality of thermally conductive probes positioned in predetermined locations; 
         [0072]    a holder for supporting a probe, the holder comprising a clamping mechanism that is configured to provide a clamping force on a probe; 
         [0073]    an automatic probe loading mechanism configured to move either the holder or at least one of the probes, to bring a probe into alignment with the holder; 
         [0074]    an actuation device for moving said holder and said probe supported in said holder towards and away from the bond; 
         [0075]    a heater for heating a tip of said probe to a temperature at or above a temperature at which the bond is melted; 
         [0076]    a means for applying a pull force on said holder which in turn applies a pull force to said probe; and a force measuring system for measuring a force applied to said probe during the pull test, wherein after said probe tip has been heated to a temperature at or above a temperature at which the bond is melted, said probe tip is brought into contact with the bond, the bond is melted by heating of the probe and the bond is cooled and solidified to fix the probe tip in the bond, the probe is then retracted by the pull force applying means to apply a pull force to the bond, which pull force is measured by said force measuring system. 
         [0077]    Preferably, the automatic probe loading mechanism comprises an electronic controller and a memory in which the position of each of the plurality of probes is stored. 
         [0078]    Preferably, the apparatus further includes a collection receptacle that can be positioned underneath the probe holder to receive used probes following a test. Preferably the automatic probe loading mechanism is configured to control the position of the collection receptacle relative to the probe holder. 
         [0079]    Preferably the apparatus further includes a movable platform on which a semiconductor sample to be tested is mounted. Preferably, an array of probes is also mounted on the movable platform. By having an array of probes, preferably pre-loaded into a carrier tray and with known positions for the test sites, an automatic test routine is possible. An automatic probe loading mechanism can be used to move the holder and platform relative to each other to automate the picking up of probes. The test can then be performed and the used probe dropped into a collection receptacle. 
         [0080]    Preferably, each of the plurality of probes comprises a straight thermally conductive pin. 
         [0081]    Each probe can be arranged to be upstanding so that, in use, the holder can be lowered over each probe and the probe then clamped prior to performing a test. Following a test, the used probe can be unclamped and can be slid out of the holder or may simply fall out of the holder under its own weight. 
         [0082]    In a further aspect, the invention comprises a method of testing a bond on a semiconductor assembly, the bond comprising a ball or a bump of solder, the method comprising the steps of: 
         [0083]    providing a plurality of thermally conductive probes at predetermined positions; 
         [0084]    moving either at least one of the probes or a probe holder under automatic control to bring a probe into alignment with the probe holder; clamping the probe to the probe holder, moving the probe and holder relative to the bond to apply a tip of the probe to the bond; 
         [0085]    heating the tip of said probe to a temperature at or above a temperature at which the bond is melted; 
         [0086]    cooling the tip of the probe, or allowing the tip of the probe to cool, to a temperature at which the bond is solidified, wherein the tip of the probe is embedded in the bond; 
         [0087]    applying a pull force on the probe through the probe holder, and recording the force applied to the probe during the step of applying a pull force, moving a receptacle relative to the probe holder; and releasing the probe from the probe holder into the receptacle. 
         [0088]    Preferably the step of moving either at least one of the probes or a probe holder under automatic control to bring a probe into alignment with the probe holder comprises moving the at least one of the probes laterally to a position underneath the probe holder. 
         [0089]    Preferably, the step of providing a plurality of thermally conductive probes at predetermined positions comprises mounting a carrier tray holding the probes to a movable platform. 
         [0090]    Preferably, each of the plurality of probes comprises a straight thermally conductive pin. 
         [0091]    In a still further aspect, the invention provides an apparatus for applying a pull test to a bond of a semi-conductor assembly, the bond comprising a ball or a bump of solder, the apparatus comprising: 
         [0092]    a probe; 
         [0093]    a heater for heating a tip of said probe to a temperature at or above a temperature at which the bond is melted; 
         [0094]    a holder for supporting said probe, wherein the holder comprises a clamp for retaining the probe, a drive mechanism coupled to the clamp and having a active position and a resting position, wherein movement of the drive mechanism from the resting position to the active position causes the clamp to tighten around the probe, and a manual adjustment mechanism coupled to the drive mechanism and configured to adjust the resting position of the drive mechanism; 
         [0095]    an actuation device for moving said holder and said probe supported in said holder towards and away from the bond in use; 
         [0096]    a means for applying a pull force on the probe; and 
         [0097]    a force measuring system for measuring a force applied to said probe during the pull test, wherein after said probe tip has been heated to a temperature at or above a temperature at which the bond is melted, said probe tip is brought into contact with the bond, the bond is melted by heating of the probe and the bond is cooled and solidified to fix the probe tip in the bond, the probe is then retracted by the pull force applying means to apply a pull force to the bond, which pull force is measured by said force measuring system. 
         [0098]    Preferably, the clamp is a collet. Preferably, the drive mechanism is a piston and cylinder coupled to a pneumatic or hydraulic control system. Preferably, the piston abuts the collet and the manual adjustment mechanism is configured to adjust a resting position of the piston. 
         [0099]    Preferably the manual adjustment mechanism allows a probe to be held in the clamp in a friction fit prior to a pull test without activating the automated clamping mechanism. A greater clamping force can subsequently be applied during the pull test by activating the automated clamping mechanism. 
         [0100]    Preferably the manual adjustment mechanism can alternatively be used instead of the automatic clamping mechanism to cause the clamp to tighten around the probe sufficiently for applying the pull test to the bond. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0101]    An example of the invention is described in detailed with reference to the accompanying drawings, in which: 
           [0102]      FIG. 1  is a front view of a bond testing apparatus in accordance with the invention; 
           [0103]      FIG. 2  is a cross-sectional view of the apparatus taken along line X-X of  FIG. 1 ; 
           [0104]      FIG. 3  is a detailed view of portion A of  FIG. 2 ; 
           [0105]      FIG. 4  is a schematic illustration of the pneumatic components of the probe clamping system of  FIG. 3 ; 
           [0106]      FIG. 5  is a detailed view of portion B of  FIG. 3 ; 
           [0107]      FIG. 6  is a schematic illustration of the cooling system partially shown in  FIG. 3 ; 
           [0108]      FIG. 7  is a detailed view of the cartridge assembly shown in  FIGS. 1 and 2 , with the cover removed; 
           [0109]      FIG. 8  is a circuit diagram illustrating how the strain gauges in  FIG. 7  are used to provide a force measurement; 
           [0110]      FIG. 9  is a schematic diagram showing the control architecture of an apparatus in accordance with the invention; and 
           [0111]      FIG. 10  is a flow diagram of a method of bond testing in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0112]      FIG. 1  is a front view of a bond testing apparatus or machine  200  in accordance with the invention.  FIG. 2  is a lateral cross-section of the same machine  200  through the line X-X. The machine  200  comprises a stage table  1 , on which a semiconductor assembly  2 , having a bond or bonds to be tested, is mounted. 
         [0113]    The machine  200  shown in  FIGS. 1 and 2  is configured to perform pull tests and push tests on bonds, and in particular solder bonds. A probe  3  is held in a vertically movable cartridge assembly  4 . The probe is used to contact a bond, melt the bond, sink into the melting bond, cool the bond to re-solidify it and then apply a pulling force on the bond to test its strength. 
         [0114]    The machine  200  shown in  FIGS. 1 and 2  comprises a stationary chassis  5  to which the moving parts of the machine  200  are attached. The chassis  5  is designed to sit on a workbench or table. Attached to the chassis  5  is the movable stage table  1 , on which the semiconductor assembly or sample  2  under test is mounted. The stage table  1  is movable in the horizontal plane, herein referred to as the X-Y plane, relative to the chassis  5 . This movement of the stage table  1  is achieved achieve and controlled using stepper or servo motors (not shown). A sample holder  6  is fixed to the stage table  1  using a threaded bolt and nut assembly, although any suitable fixing means may be used. The sample holder  6  comprises a clamp in which semiconductor samples of different sizes can be held. The clamp is adjusted using a handle  7 . This type of stage table  1  and sample holder  6  arrangement is well known, and can be found in the Dage 4000 Multipurpose Bond Tester, available from Dage Holdings Limited, 25 Faraday Road, Rabans Lane Industrial Area, Aylesbury, Buckinghamshire, United Kingdom. However, the assembly under test can be secured in position on the stage table  1  by several different means. A vacuum chuck can be used to secure larger substrates or devices to the table. Also shown in  FIG. 1  is a tray of probes  80 , located on the stage table 1. 
         [0115]    The machine  200  also comprises the cartridge assembly  4  to which the probe  3  is coupled. The cartridge  4  and probe  3  are movable in a vertical direction, normal to the surface of the semiconductor assembly  2  (herein referred to as the Z direction) relative to the chassis  5  and stage table  1 . Movement of the cartridge  4  and probe  3  relative to the chassis  5  is again achieved using a stepper or servo motor (not shown) to drive a lead screw or ball screw  8  mounted to the chassis  5 . The cartridge  4  is mounted on a nut (not shown) on the ball screw  8  and so translates travels up or down in the Z direction when the screw  8  is rotated, but other suitable drive mechanisms may be used, such as a solenoid, as is well known in the art. The vertical drive arrangement illustrated in  FIGS. 1 and 2  can be found in the Dage 4000 Multipurpose Bond Tester, referenced above. 
         [0116]    The machine  200  is typically controlled by a personal computer (PC)  90  and including suitable user interface devices, such as a keyboard and screen and one or more joysticks (not shown). The machine  200  also includes a microscope  9  directed at the sample  2  under test to aid accurate positioning of the probe  3 . These user control features can also be found in the Dage  4000  Multipurpose Bond Tester, referenced above. 
         [0117]      FIG. 3  shows the probe  3  of  FIG. 2 , and the manner in which the probe  3  is mounted to a pull shaft  10  of the cartridge  4  in more detail. The probe  3  comprises a thermally conductive pin  3  held in a probe holder  210  (hereinafter “holder”) using collet  12 , which is coupled to the pull shaft  10  within a probe housing  11 . The tip  3   a  of the probe  3  is coated or dipped in solder to ensure a good wetting action when in contact with bonds under test. 
         [0118]    The holder  210  may include a clamping mechanism  220 . The clamping mechanism  220  used to clamp the probe  3  to the pull shaft  10  is driven pneumatically. However, other mechanisms based on electrical or magnetic actuators may also be used. The clamping mechanism  220  comprises a steel collet  12  that surrounds a portion of the shaft of the probe  3 . The collet  12  is clamped tightly around the shaft of the probe  3  by the action of a piston  13  moving within a cylinder  14  that houses the collet  12 . The cylinder  14  is closed by a cylinder end cap  14   a  that is threaded onto the bottom of cylinder  14 . The piston  13  abuts the collet  12  and drives a tapered outer surface  12   a  of the collet  12  into a corresponding tapered portion of the cylinder end cap  14   a,  thereby clamping the collet  12  around the upper end of probe  3 . The piston  13  is driven pneumatically using compressed air which is supplied to piston chamber  13   a  that is provided between the piston  13  and the cylinder  14 . O-rings  15 ,  16  are used to provide a seal between the piston  13  and the cylinder  14  and the piston  13  and the pull shaft  10 . A probe backstop  17  is provided on the pull shaft  10  to limit the travel of the probe  3  within the holder  210 . The piston  13 , the cylinder  14  and the pull shaft  10  are all formed from steel in the embodiment shown. A spring  18  is provided between the piston  13  and the cylinder end cap  14   a  to bias the piston  13  away from the collet  12  in the absence of a pneumatic force on the piston  13 . The pneumatic components of the clamping mechanism  220  are also illustrated schematically in  FIG. 4 .  FIG. 4  shows the piston chamber  13   a  connected to a source of compressed air  40 , which may be mounted to the chassis  5 . The supply of compressed air  40  to drive the piston  13  is regulated by a supply valve  41 , which is electronically controlled by control  43  as described later. Exhausting of compressed air from the piston chamber  13   a  to release the clamp is controlled by an exhaust valve  42 , which is also electronically controlled by control  43 . 
         [0119]    Other clamping arrangements are possible. For example, a piston might be used to travel along a tapered surface of a collet to thereby squeeze it around the probe  3 . Alternatively, instead of a collet, directly actuated clamp members abutting the probe shaft may be employed. An advantage of all these clamping arrangements, in combination with the use of a straight probe  3 , is that the probe  3  can be easily loaded and unloaded from the holder  210  and the process can be completely automated. 
         [0120]    A manual clamp adjustment mechanism is also provided which may be used instead of or in addition to the pneumatic clamping mechanism  220  described above. When used in addition to the pneumatic clamping mechanism  220 , the manual clamp adjustment mechanism is used to set the resting position of the piston  13 , and so open up or close down the internal surfaces of the collet  12 . The manual clamp adjustment mechanism comprises an annular plate  19  that is fitted on a screw thread (not shown) on the back of the cylinder  14 . The plate  19  abuts one end of three downwardly extending dowel pins  20  (only one shown) that also abut the back surface of the piston  13  at their other ends. The plate can be moved down or up on the screw thread by rotating the plate  19 . When the plate  19  is moved down, it closes down the collet  12 . When the plate  19  is moved up, it the opens up the collet  12 . The manual clamp adjustment mechanism can be used by itself, without the need for the pneumatic clamping mechanism  220 , by simply rotating the annular plate  19  until it moves downwardly far enough to securely clamp the probe  3 . 
         [0121]    In order to heat the probe  3 , and in particular that portion of the probe  3  in contact with the bond  31  under test so that the bond  31  melts, a heater  230  is provided around a lower portion of the probe  3 . The heater  230  comprises a ceramic tube  21  that fits closely around the probe  3  and a heater wire  22  that is wound around or otherwise laid on the outer surface of the tube  21 . The tube  21  is formed from thermally conductive, but electrically insulating, ceramic with walls 0.95 mm thick. The material of the ceramic tube  21  is an Aluminium Nitride/Boron Nitride Ceramic composite. The heater wire  22  provides heat by resistive heating. The heater wire  22  is connected to an electric power source (not shown in  FIG. 3 ). An electric current is passed through the heater wire  22  to heat the ceramic tube  21  and the probe  3 . The heater wire  22  is formed from Nickel-Chrome (Nichrome). The ceramic tube  21  may be formed with grooves (not shown) on its outer surface that receive the heater wire  22  and ensure good thermal contact. Other forms of heating as an alternative or in addition to resistive heating may be used, such as, for example, induction heating or the use of a hot air jet or jets. An insulating barrier  23  is provided between the heater  230  and the clamp assembly  220 . A heat shield  24  is provided around the heater  230 . 
         [0122]    As shown in  FIG. 5 , a thermocouple  30  is positioned on the bottom end of the ceramic tube  21 . The thermocouple  30  provides an indication of the temperature of the probe  3  and so an indication of the temperature of the bond  31  under test. The thermocouple  30  is a Type K thermocouple. A suitable thermocouple  30  of this type is available from Omega Engineering Limited, Manchester, United Kingdom M44 5BD. The thermocouple  30  is used to determine when to perform the various stages of the pull test and to allow the temperature profile of the melting and solidifying stages to be controlled so as to closely mimic those in the manufacturing process of the semiconductor assembly  2 . The rate of heating and cooling can affect the material properties of the bond  31 . 
         [0123]    A cooling assembly  240 , illustrated schematically in  FIG. 6 , is also provided in order to cool the probe  3  rapidly once the bond  31  under test has melted. The cooling assembly  240  operates by providing a jet of compressed air in the vicinity the probe  3 . A source of compressed air  60  is connected to a nozzle  25  (shown in  FIG. 3 ) located close to the probe  3 , within the heater shield  24 . An electronically controllable valve  61  is provided between the source of compressed air  60  (which may be the same or different to the source of compressed air  40  used for the clamp mechanism  220 ) and the nozzle  25 . The connection between the air source  60 , the valve  61  and the nozzle  25  may be made by suitable flexible or rigid hoses. The air source may be a cylinder of compressed air that can be mounted to the chassis  5 . The control  62  used to control valve  61  is also shown in  FIG. 6 . Other forms of cooling may be provided as an alternative. For example, a fan for generating a flow of non-compressed air over the probe  3  and bond  31  may be used, or alternatively the probe  3  and bond  31  may simply be allowed to cool to the ambient temperature unaided. 
         [0124]    The ceramic tube  21  and the probe  3  have relatively low thermal mass compared with the heater block of the prior art, and so the heating and cooling cycles are much faster. 
         [0125]      FIG. 7  illustrates the cartridge assembly  4  and the manner in which forces on the probe  3  are detected. The cartridge comprises a rigid backplate  70 , which is rigidly connected to the nut on the leadscrew  8 . A pair of aluminium cantilever arms  71  a,  71  b are fixed at one end to the backplate  70  using bolts  75  and are free to flex in the Z direction at their opposite ends. Cantilever arms  71   a ,  71   b  of this type are well known and found in the Dage 4000 Multipurpose BondTester referenced above. The pull shaft  10  passes through apertures provided in the free ends of the cantilever arms  71   a ,  71   b  and is rigidly fixed to the cantilever arms  71   a ,  71   b  using a nut  72  fitted to a threaded portion of the pull shaft  10 . Clearly, other means of attaching the pull shaft  10  to the cantilever arms  71   a ,  71   b  are possible. 
         [0126]    Any force exerted on the pull shaft  10  in the Z direction will cause the cantilever arms  71   a ,  71   b  to flex. In order to measure the force exerted on the pull shaft  10 , strain gauges  73   a,    73   b  are located on the top surface of one of the cantilever arms  71   a  and strain gauges  74   a,    74   b  (not shown in  FIG. 7 ) are located on the opposite surface of that cantilever arm  71   a . The flexing of the arm  71   a  distorts the strain gauges  73   a,    73   b,    74   a,    74   b  and allows a measure of the force on the pull shaft  10  to be recorded. This type of force measurement system is known, for example from U.S. Pat. No. 6,301,971 and the Dage 4000 Multipurpose BondTester referenced above. 
         [0127]    A temperature compensation element  76  is provided with the cantilever arms  71   a ,  71   b .  FIG. 8  illustrates a circuit arrangement using the four strain gauges  73   a,    73   b,    74   a,    74   b  and the temperature compensation element  76 . The circuit uses a Wheatstone Bridge configuration for the strain gauges, which is excited using excitation voltage V excite . The output voltage V out  is an indication of the force on the pull shaft  10 . 
         [0128]    The strain gauges  73   a,    73   b,    74   a,    74   b  are used not only to detect the force on the pull shaft  10  during the pull or push test, but also to determine when the probe  3  contacts the bond  31  under test during initial positioning of the probe  3 , prior to commencing the test. The small deflection of the cantilever arms  71   a ,  71   b  as the probe  3  contacts the semiconductor assembly  2  is detected and the Z direction drive is then stopped to prevent damage to the bond  31  or substrate  2 . 
         [0129]      FIG. 9  is a schematic illustration of the control of the different parts of the machine  200 . The machine  200  is controlled by application software  91  running on a personal computer (PC)  90 . Dedicated electronics are provided for various parts of the machine  200 , which are controlled by the application. Motion control electronics  92  are provided for the Z direction drive  93  of the cartridge assembly  4 , and for the X-Y plane drive  94  of the stage table  1 . Clamp electronics  95  are also provided to control actuation of the clamping mechanism  220 , and specifically to control valves  41  and  42 . 
         [0130]    Temperature sensing and control electronics  96  are provided to control the heater  230  and the cooling system  240 . Signals from the thermocouple  30  are used by the temperature control electronics  96  to start and finish the heating and cooling cycles. 
         [0131]    Force measurement and touchdown sensing electronics  99  are provided to operate the strain gauges  73   a,    73   b,    74   a,    74   b  and to determine a measure of the force on the pull shaft  10  from their output. As mentioned above, signals from the strain gauges  73   a,    73   b,    74   a,    74   b  are used as a touchdown sensor to stop the Z direction drive  93  when the probe  3  is first positioned. 
         [0132]      FIG. 10  is flow diagram illustrating the steps performed in a bond test in accordance with the invention. 
         [0133]    The first step in the process is to load a probe  3  onto the pull shaft  10 . This is shown as step  100 . This can be achieved by manually loading a probe  3  into the holder  210  and then manually or automatically clamping the probe  3  in position. Alternatively the process may be completely automated. The stage table  1  may be moved so that a probe  3  in the tray of probes  80  is located directly beneath the pull shaft  10 . By recording the position of the probes  3  in the tray  80  on the stage table  1 , the stage table  1  may be controlled to automatically move the next probe  3  into position. The cartridge assembly  4  is then lowered using the Z direction drive  93  until the probe  3  is in position. 
         [0134]    The probe  3  can then be clamped by actuating piston  13  in step  105 , or it may be that the collet  12  is sufficiently tight without actuating the piston  13  to support the probe  3  in place against its weight so that clamping is not necessary at this stage. The probe  3  and cartridge assembly  4  is then withdrawn a predetermined distance above the stage table  1 . 
         [0135]    The stage table  1  is then moved so that the bond  31  to be tested is directly underneath the probe  3  in step  110 . This can be done manually, with the aid of the microscope  9  and joystick controls (not shown). Alternatively it may be done automatically if the position of the bond  31  on the stage table is known and electronically recorded. 
         [0136]    In step  115 , the probe  3  is then moved down towards the bond  31  to be tested using the Z direction drive  93 . In step  120 , contact with the bond  31  is detected using output from the strain gauges  73   a,    73   b,    74   a,    74   b  and the Z direction drive  93  is then stopped in step  125 . 
         [0137]    In step  130  the probe  3  is unclamped or loosened so that the probe  3  rests on the top of the bond  31 . The heater  230  is then activated in step  135  to heat the probe  3  and so heat the bond  31 . When the thermocouple  30  detects that the probe  3  has reached a predetermined temperature at or above the melting temperature of the bond  31 , the heater  230  is stopped. The probe  3  sinks into the melting bond  31  under its own weight. Alternatively, the probe  3  may remain clamped during the heating step, but held in a position so that the solid bond  31  exerts an upward force on the probe  3 , causing the cantilever arms  71   a ,  71   b  to flex. As the bond  31  melts the probe  3  is then forced downwards into the molten solder by the action of the cantilever arms  71   a,    71   b.    
         [0138]    In step  140  the cooling cycle begins. The valve  61  is opened and a jet of compressed air is directed onto the probe  3  to rapidly cool it and the bond  31 . When the thermocouple  30  detects that a desired temperature, at which the bond  31  is solid, has been reached, the valve  61  is closed and cooling is stopped. At this point the bond  31  has solidified around the tip  3   a  of the probe  3 . 
         [0139]    The thermocouple  30  can be used to provide a record of the temperature profile of the probe  3  during the heating and cooling cycles. This can be used to control the rate of heating and cooling and so ensure that the re-solidified bond  31  has close to the same material properties as the original bond  31  following manufacture. This is done by mimicking the thermal profile used in the typical manufacturing process as closely as possible. 
         [0140]    After cooling is completed, if the probe  3  is unclamped, in step  145  the probe  3  is tightly clamped using the pneumatic clamping mechanism  220 . Alternatively the step  145  of clamping the probe may be carried out prior to the step  140  of cooling. Alternatively, the probe  3  may remain clamped throughout the process. 
         [0141]    In step  150 , the pull test is performed and the force on the pull shaft  10  recorded throughout the test using the output from the strain gauges  73   a,    73   b,    74   a,    74   b.  The pull test is performed by driving the pull shaft  10  in the Z direction away from the stage table  1  using the Z axis drive  93 . When the bond  31  is removed from the semiconductor substrate  2  during the pull test there will be a sudden reduction in the force exerted on the pull shaft  10 . The maximum force achieved is recorded. 
         [0142]    It can be seen from  FIGS. 2 ,  3 , and  7  that in the illustrated embodiment the longitudinal axis of the probe  3  is aligned with the longitudinal axis of the pull shaft  10 , and that the pull force exerted on the probe  3  by the pull shaft  10  during a pull test is directly in line with the longitudinal axis of the probe  3 . This ensures that there are no bending moments on the probe  3  that might lead to inaccurate and unrepeatable test results. It also increase the maximum pull force that can be applied and significantly reduces the likelihood of damage to the probe  3  during a pull or push test, and so increases the useful lifetime of the probes  3 . 
         [0143]    As well as being suitable for performing pull tests, the machine  200  of the present invention is also suitable for performing push tests on bonds  31 , by driving the probe  3  into the bond  31  in the Z direction (without melting) and recording the force on the pull shaft  10  as is travels over a predetermined linear distance. It is also possible to perform a fatigue test, consisting of a series of alternating pull and push tests up to a predetermined number of cycles or a predetermined force. 
         [0144]    Following completion of the pull test (or push or fatigue test) the semiconductor assembly  2  is moved away from the probe  3  by moving the stage table  1  in step  155 . To facilitate cleaning of the probe  3 , any bond material that is attached to the probe  3  may be re-melted while it is still attached to the holder  210  and housed within the ceramic heating tube  21 . A new semiconductor assembly  2  may be mounted on the stage table  1  if desired. 
         [0145]    In step  160 , the probe  3  is unloaded from the holder  210 . This can be done manually by rotating the plate  19  to move it upwardly to open collet  12 , or automatically by releasing the pneumatic clamping force. A receptacle  250  (schematically shown in  FIG. 2 ) for receiving used probes  3  may be fixed to the stage table  1  and moved underneath the pull shaft  10  at this stage. The probe  3  can then fall into the receptacle  250  on being unclamped and can be subsequently cleaned and reused. 
         [0146]    The process is then complete and a new bond  31  can be tested. To start the process again the stage table  1  is moved into a position to receive a new probe  3  in step  165 , and the process begins again at step  100 .