Patent Publication Number: US-2023137302-A1

Title: Apparatus and method for calibrating a shear test tool

Description:
FIELD OF THE INVENTION 
     The invention generally relates to shear tests conducted on interconnect bonds formed on electronic devices, such as a wire bond formed on the electronic device, or a die bond formed between a die and a substrate, and more specifically to calibrating a shear test tool for such bonded joints. 
     BACKGROUND 
     During semiconductor assembly and packaging, shear tests may be performed to determine the bond strength of interconnect bonds or the degree of adhesion between a die and a substrate. It is important to test the mechanical strength of these interconnect bonds in electronic devices so as to accurately evaluate the quality of these bonds so as to determine whether the bond strength is sufficient and/or whether bonding parameters need to be modified. 
     To accurately measure bond strength using a shear test tool, it is necessary to calibrate the shear test tool regularly so that compensation and/or correction actions may be carried out on the shear test tool when the test result shows any variation and deviation from a predetermined allowable tolerance. In prior art force calibration devices for shear test tools, bearing pivots are typically used to form a pivot between a fixed element and a pivotable element to conduct calibration. During such force calibration, bearing friction in mechanical bearings may cause wear and tear on the bearings, which in turn will reduce the lifespan of the bearings and force calibration reliability. Further, the calibration results determined using conventional force calibration devices may not be accurate since the bearing friction is not constant and may vary due to the use of different weights during the calibration process. In addition, the bearing components may become rusted and corroded over time if they contain iron. Although lubricants may be used to reduce friction and rust, this would incur greater maintenance effort and higher costs. 
     It would therefore be beneficial to design a new force calibration device for a shear test tool which may avoid at least some of the aforesaid shortcomings faced by conventional force calibration devices. 
     SUMMARY OF THE INVENTION 
     It is thus an object of the invention to seek to provide an improved force calibration apparatus that utilizes a resilient pivot mechanism so as to improve the lifespan and reliability of the force calibration apparatus. 
     According to a first aspect of the present invention, there is provided an apparatus for calibrating a shear test tool. The apparatus comprises a fixed element, a pivotable element configured to be rotatable relative to the fixed element, and a resilient pivot mechanism coupled between the fixed element and the pivotable element to form a pivot such that the pivotable element is rotatable about the pivot to lift a weight coupled to the pivotable element when the shear test tool applies a force on the pivotable element in order to rotate the pivotable element and lift the weight. 
     In the apparatus for calibrating a shear test tool, a resilient pivot mechanism is used to replace the bearing pivot used in prior art force calibration devices. Thus, no friction will be produced between the resilient pivot mechanism and other components of the force calibration device. The problems caused by bearing friction in prior art force calibration devices can therefore be avoided accordingly. The lifespan and the reliability of the force calibration device will be significantly improved, and the calibration results will be more accurate despite usage over time. Further, no lubricants are required to reduce friction and rust and therefore the maintenance effort and costs will be reduced. 
     In some embodiments, the pivotable element may define a first length along a horizontal direction and a second length along a vertical direction relative to the pivot, the weight is coupled to an end of the first length distal from the pivot, and the shear test tool is operative to apply a substantially horizontal force on an end of the second length distal from the pivot to rotate the pivotable element and lift the weight. 
     In one embodiment, the resilient pivot mechanism comprises a cross spring pivot mechanism. The cross spring pivot mechanism may be formed by a first spring sheet having an opening and a second spring sheet which is sized to pass through the opening of the first spring sheet, the first and second spring sheets being arranged at an angle to each other. Each of the first and the second spring sheets has a first edge fixed to the fixed element and a second edge fixed to the pivotable element such that each of the first and the second spring sheets is deflectably in contact with both the fixed element and the pivotable element. Preferably, the first spring sheet and the second spring sheet are mounted orthogonally with respect to each other. 
     In another embodiment, the resilient pivot mechanism comprises a first pair of spring sheets and a second pair of spring sheets. The fixed element has opposite first and second sides located along an interface between the fixed element and the pivotable element. The first pair of spring sheets is mounted on the first side of the fixed element and the second pair of spring sheets is mounted on the second side of the fixed element. Each of the spring sheets is deflectably in contact with both the fixed element and the pivotable element. Preferably, respective spring sheets of the first pair of spring sheets are mounted orthogonally with respect to each other, and respective spring sheets of the second pair of spring sheets are mounted orthogonally with respect to each other. 
     In some embodiments, the apparatus may further comprise a coupling device configured to detachably couple the weight to the pivotable element. In one embodiment, the coupling device may comprise a hook from which the weight is configured to hang. 
     In some embodiments, the shear test tool may comprise a shear test bar and a force sensor coupled to the shear test bar. The force sensor is operative to measure the force applied to the pivotable element with the shear test bar in order to rotate the pivotable element and lift the weight. In one embodiment, the shear test bar may have a tip with a tapered shape. 
     In some embodiments, the apparatus may further comprise a processor which is operatively connected to the force sensor. The processor is configured to determine a relatively constant force measured with the force sensor after the weight has been lifted from a rest position by the force exerted with the shear test bar. 
     According to a second aspect of the present invention, there is provided a method for calibrating a shear test tool. The method comprises: providing a calibration apparatus which comprises a fixed element, a pivotable element configured to be rotatable relative to the fixed element, and a resilient pivot mechanism coupled between the fixed element and the pivotable element to form a pivot such that the pivotable element is rotatable about the pivot; and applying, with the shear test tool, a force on the pivotable element such that the pivotable element is rotated about the pivot to lift a weight coupled to the pivotable element. 
     In some embodiments, the pivotable element defines a first length along a horizontal direction and a second length along a vertical direction relative to the pivot. Accordingly, the method further comprises: coupling the weight to an end of the first length distal from the pivot; and wherein the step of applying the force on the pivotable element comprises: applying, with the shear test tool, a substantially horizontal force on an end of the second length distal from the pivot to rotate the pivotable element and lift the weight. 
     In one embodiment, the resilient pivot mechanism comprises a cross spring pivot mechanism. The cross spring pivot mechanism may be formed by a first spring sheet having an opening and a second spring sheet which is sized to pass through the opening of the first spring sheet, the first and second spring sheets being arranged at an angle to each other. Preferably, the first spring sheet and the second spring sheet are mounted orthogonally with respect to each other. 
     In another embodiment, the resilient pivot mechanism comprises a first pair of spring sheets and a second pair of spring sheets. The fixed element has opposite first and second sides located along an interface between the fixed element and the pivotable element. The first pair of spring sheets is mounted on the first side of the fixed element and the second pair of spring sheets is mounted on the second side of the fixed element. Each of the spring sheets is deflectably in contact with both the fixed element and the pivotable element. Preferably, the respective spring sheets of the first pair of spring sheets are mounted orthogonally with respect to each other, and respective spring sheets of the second pair of spring sheets are mounted orthogonally with respect to each other. 
     In some embodiments, the shear test tool may comprise a shear test bar and a force sensor coupled to the shear test bar. The method further comprises: moving the shear test bar to apply the force to the pivotable element and measuring, with the force sensor, the force applied to the pivotable element with the shear test bar in order to rotate the pivotable element and lift the weight. 
     In some embodiments, the method may further comprise: determining, with a processor or a computer system, a relatively constant force measured with the force sensor after the weight has been lifted from the rest position by the force exerted on the pivotable element by the shear test bar. 
     These and other features, aspects, and advantages will become better understood with regard to the description section, appended claims, and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG.  1 A  is a cross-sectional view of an apparatus for calibrating a shear test tool according to some embodiments of the invention. 
         FIG.  1 B  is a cross-sectional view of the apparatus as shown in  FIG.  1 A  wherein fixed and pivotable elements of the apparatus have been separated. 
         FIG.  2 A  and  FIG.  2 B  show front and isometric views respectively of a resilient pivot mechanism according to a first embodiment of the invention. 
         FIG.  2 C  is a perspective view of the apparatus for calibrating a shear test tool according to the first embodiment of the invention. 
         FIG.  3 A  shows a resilient pivot mechanism which comprises two pairs of spring sheets according to a second embodiment of the invention. 
         FIG.  3 B  and  FIG.  3 C  show respective perspective and front views of the apparatus for calibrating a shear test tool according to the second embodiment of the invention. 
         FIG.  4    is a cross-sectional view of the apparatus for calibrating a shear test tool when it is used to calibrate a shear test tool according to one embodiment of the invention. 
         FIG.  5    is a line graph illustrating the calibration results of a shear test tool using the apparatus for calibrating a shear test tool according to the first embodiment of the invention. 
         FIG.  6    is a flow chart illustrating a method for calibrating a shear test tool according to one embodiment of the invention. 
     
    
    
     In the drawings, like parts are denoted by like reference numerals. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
       FIG.  1 A  is a cross-sectional view of an apparatus  100  for calibrating a shear test tool according to some embodiments of the invention. As shown in  FIG.  1 A , the apparatus  100  comprises a fixed element  111 , a pivotable element  112  and a resilient pivot mechanism  113 . The pivotable element  112  is configured to be rotatable relative to the fixed element  111 .  FIG.  1 B  is a cross-sectional view of the apparatus  100  wherein fixed and pivotable elements  111 ,  112  of the apparatus have been separated. The resilient pivot mechanism  113  is coupled between the fixed element  111  and the pivotable element  112  to form a pivot such that the pivotable element  112  is rotatable about the pivot to lift a weight coupled to the pivotable element  112  when the shear test tool applies a horizontally-directed force on the pivotable element  112  in order to rotate the pivotable element  112  and lift the weight. 
       FIG.  2 A  and  FIG.  2 B  show front and isometric views respectively of the resilient pivot mechanism  113  according to a first embodiment of the invention.  FIG.  2 C  is a perspective view of the apparatus  100  according to the first embodiment of the invention. In this embodiment, the resilient pivot mechanism  113  comprises a cross spring pivot mechanism which is formed by a first spring sheet  113   a  having an opening and a second spring sheet  113   b  which is sized to pass through the opening of the first spring sheet  113   a . When the first and second spring sheets  113   a  and  113   b  are installed between the fixed element  111  and the pivotable element  112 , the first and second spring sheets  113   a  and  113   b  are mounted orthogonally with respect to each other, as shown in  FIG.  2 C . 
     In this embodiment, the first spring sheet  113   a  is coupled between the fixed element  111  and the pivotable element  112  by a first coupling means including four pairs of bolts and nuts, and the second spring sheet  113   b  is coupled between the fixed element  111  and the pivotable element  112  by a second coupling means including two pairs of bolts and nuts. Referring to  FIG.  2 A  to  FIG.  2 C , the first spring sheet  113   a  has a first edge  113   a - 1  fixed to the fixed element  111  and a second edge  113   a - 2  fixed to the pivotable element  112  such that the first spring sheet  113   a  is deflectably in contact with both the fixed element  111  and the pivotable element  112 . Similarly, the second spring sheet  113   b  has a first edge  113   b - 1  fixed to the fixed element  111  and a second edge  113   b - 2  fixed to the pivotable element  112  such that the second spring sheet  113   b  is deflectably in contact with both the fixed element  111  and the pivotable element  112 . It should be noted that the first coupling means and/or the second coupling means in this embodiment are provided for illustration only. In other embodiments, different coupling means may be used to install the first and second spring sheets  113   a ,  113   b  between the fixed element  111  and the pivotable element  112 . 
     It should be appreciated that the components and structure of the cross spring pivot mechanism in this embodiment are provided for illustration only. The cross spring pivot mechanism may have different structure and components, e.g., the cross spring pivot mechanism may be an integrally-formed component having two spring sheets which are unitary and arranged orthogonally with respect to each other. 
     In a second embodiment of the invention, the resilient pivot mechanism  113  comprises a first pair of spring sheets  113 A and a second pair of spring sheets  113 B.  FIG.  3 A  shows the two pairs of spring sheets  113 A and  113 B according to the second embodiment of the invention. Each pair of the spring sheets include two spring sheets installed between the fixed element  111  and the pivotable element  112  orthogonally with respect to each other. As shown in  FIG.  3 A , the first pair of spring sheets  113 A includes a first spring sheet  113 A- 1  and a second spring sheet  113 A- 2 , and the second pair of spring sheets  113 B include a first spring sheet  113 B- 1  and a second spring sheet  113 B- 2 .  FIG.  3 B  and  FIG.  3 C  show respective perspective and front views of the apparatus  100  according to the second embodiment of the invention. As shown in  FIG.  3 B  and  FIG.  3 C , the fixed element  111  has opposite first and second sides  111   a  and  111   b , located along an interface between the fixed element  111  and the pivotable element  112 . The first pair of spring sheets  113 A is mounted on the first side  111   a  of the fixed element  111  and the second pair of spring sheets  113 B is mounted on the second side  111   b  of the fixed element  111 . Each spring sheet is deflectably in contact with both the fixed element  111  and the pivotable element  112 . The two spring sheets  113 A- 1  and  113 A- 2  are mounted orthogonally with respect to each other, and the two spring sheets  113 B- 1  and  113 B- 2  are mounted orthogonally with respect to each other. 
       FIG.  4    is a cross-sectional view of the apparatus  100  when it is used to calibrate a shear test tool  115  according to one embodiment of the invention. In use, a weight  114  with a predetermined weight value, e.g. a constant weight or a dead mass, is coupled to the pivotable element  112  by a coupling device. In this embodiment, the coupling device comprises a hook  116  from which the weight  114  is configured to hang. It should be noted that the weight  114  may be coupled to the pivotable element  112  by using any means in other embodiments of the invention, as long as a known vertically-directed force F1 is applied to the pivotable element  112  by the weight  114 . For example, the weight  114  may be directly attached to a bottom/top surface of the pivotable element  112  either with or without any additional coupling device. 
     The shear test tool  115  includes a shear test bar  115   a  and a force sensor  115   b  coupled to the shear test bar  115   a . The shear test tool  115  is moved such that the shear test bar  115   a  is in contact with the pivotable element  112  and exerts a force on the pivotable element  112  to rotate the pivotable element  112  in order to lift the weight  114 . The force sensor  115   b  is operative to measure a reaction force applied by the pivotable element  112  against the shear test bar  115   a  when the latter is rotating the pivotable element  112  to lift the weight  114 . The shear test bar  115   a  may have a tip with a tapered shape to minimize contact between the shear test bar  115   a  and a surface to which the interconnect bond is formed or located. 
     As shown in  FIG.  4   , in this embodiment, the pivotable element  112  defines a first length L1 along a horizontal direction relative to the pivot and a second length L2 along a vertical direction relative to the pivot. The weight  114  is coupled to an end of the first length L1 distal from the pivot and the shear test tool  115  is operative to apply a substantially horizontal force F2 on an end of the second length distal from the pivot to rotate the pivotable element  112  in order to lift the weight  114 . In this embodiment, the end of the first length L1 distal from the pivot is located at one end P of an arm of the pivotable element  112 . However, in other embodiments, the position of the end of the first length L1 distal from the pivot may be different, e.g., it may be positioned at a point which is not at the end P. 
     In some embodiments of the invention, the apparatus  100  may further include a processor or any computing system which is operatively connected to the force sensor  115   b  and configured to determine a relatively constant force measured by using the force sensor  115   b  after the weight  114  has been lifted from its resting position by the force exerted on the pivotable element  112  by the shear test bar  115   a.    
       FIG.  5    is a line graph illustrating the calibration results of the shear test tool  115  using the apparatus  100  according to the first embodiment of the invention. As shown in FIG.  5 , the reaction force measured by the force sensor  115   b  increases significantly with the distance moved by the shear test bar  115   a  along the horizontal direction towards the pivotable element  112  until the pivotable element  112  starts to rotate and the weight  114  is lifted. Once the processor determines that the force measured by the force sensor  115   b  remains relatively constant, i.e., there is no significant increase in the force measured by the force sensor  115   b  with further movement of the shear test bar  115   a  along the horizontal direction, a value of the relatively constant force C is taken as a calibration result for the calibration of the shear test tool  115 . It should be noted that the force measured by the force sensor  115   b  will not become a real constant value during the calibration although it may not obviously increase after the weight  114  is lifted. The reason is that to ensure the shear test bar  115   b  moves along the horizontal direction with a constant velocity, the force applied to the pivotable element  112  will still slowly increase with further movement of the shear test bar  115   b  as a pivot spring force will increase with further deformation of the spring sheets of the resilient pivot mechanism  113 . This increase in the measured force caused by the further deformation of the resilient pivot mechanism  113  is not obvious as the spring constant of the resilient pivot mechanism  113  is a very small value, e.g.  0 . 017   g /um. 
     In order to calibrate the shear test tool  115  accurately, a plurality of weights with different values may be used to conduct the calibration of the shear test tool  115 . 
       FIG.  6    is a flow chart illustrating a method  600  for calibrating a shear test tool using the apparatus  100  according to one embodiment of the invention. 
     At Step  601 , a calibration apparatus  100  is provided which comprises a fixed element  111 , a pivotable element  112  configured to be rotatable relative to the fixed element  111 , and a resilient pivot mechanism  113  coupled between the fixed element  111  and the pivotable element  112 . 
     At Step  602 , a weight  114  having a predetermined value is coupled to the pivotable element  112 . 
     At Step  603 , a shear test tool  115  is moved relative to the pivotable element  112  so that a shear test bar  115   a  of the shear test tool  115  contacts the pivotable element  112 , and applies a force to rotate the pivotable element  112  and to lift the weight  114 . 
     In this embodiment, the shear test tool  115  may be moved downwards first until the shear test bar  115   a  is positioned at a predetermined height, and the shear test tool  115  is then moved along the horizontal direction at a constant speed till the shear test bar  115   a  is in contact with the pivotable element  112  at a predetermined position, e.g., the end of the second length L2 distal from the pivot defined by the pivotable element  112 . Once the shear test bar  115   a  contacts the pivotable element  112 , a substantially horizontal force is exerted on the pivotable element  112  to rotate the pivotable element  112  and to lift the weight  114 . 
     At Step  604 , a processor or a computing system which is operatively connected to the shear test tool  115  determines a relatively constant force measured by a force sensor  115   b  of the shear test tool  115  after the weight  114  has been lifted by the force exerted by the shear test bar  115   a  on the pivotable element  112 . 
     Specifically, during movement of the shear test bar  115   a  along the horizontal direction, the processor or the computing system may record the forces measured by the force sensor  115   b  as the distance moved by the shear test bar  115   a  increases along the horizontal direction. A value of a relatively constant force is determined based on the recorded forces. 
     To accurately calibrate the shear test tool  115 , a plurality of weights with different weight values may be used to conduct the steps at Step  602  to Step  604 . 
     As will be appreciated from the above description, the apparatus and method for calibrating a shear test tool provided in the described embodiments of the invention utilize a resilient pivot mechanism to form a pivot between the fixed element and the pivotable element such that when a force is exerted on the pivotable element by a shear test tool, the pivotable element is rotated about the pivot to lift a weight. Compared to prior art force calibration devices formed with bearing pivots, the problems caused by the bearing friction in various mechanical bearings will be avoided since friction is not produced by the resilient pivot mechanism installed between the fixed element and the pivotable element. Specifically, the lifespan and reliability of the force calibration apparatus will be significantly improved, since no lubricants are required to reduce friction and rust, the maintenance effort and costs of the force calibration apparatus will be reduced. Further, more accurate calibration results will be obtained, especially over long-term use, since the inaccuracy caused by the bearing frictions is avoided. 
     Table 1 below illustrates the calibration results obtained using the apparatus  100  according to the first embodiment of the invention. Weights having different values are used to conduct the calibration process. It can be seen from the results shown in Table 1 that the force ratios obtained from the apparatus  100  based on the measurement results are relatively constant. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Weight (Gram) 
                 Force (N/g) 
                 Ratio 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 5.000 
                 8.600 
                 1.7201 
               
               
                 10.000 
                 17.210 
                 1.7210 
               
               
                 25.000 
                 43.084 
                 1.7234 
               
               
                 50.000 
                 86.259 
                 1.7252 
               
               
                 100.000 
                 172.523 
                 1.7253 
               
               
                   
               
            
           
         
       
     
     Although the present invention has been described in considerable detail with reference to certain embodiments, other embodiments are possible. Therefore, the spirit and scope of the appended claims should not be limited to the description of the embodiments contained herein.