Abstract:
A test apparatus applies high speed impact load to a sample to test the shear strength of attachment of a component part to the sample, by use of a rotary drive mechanism driving an impact tip. A support mechanism provides alignment between the impact tip and a portion of the sample to receive a test force, and prevents relative movement of at least one of the sample and the impact tip. The rotary drive mechanism establishes a impact force between the impact tip and the sample, and a force transducer receives the resultant force and providing a corresponding output. In one example the force transducer uses a piezoelectric film for sensing. The testing may be used, for example, to provide stable impact speed to a solder ball, and provide, as an output a force and displacement relationship curve. The stable speed can be acquired by clutch, and the data collection

Description:
RELATED APPLICATION 
       [0001]    The present Patent Application claims priority to Provisional Patent Application No. 60/996,718filed Dec. 3, 2007, which is filed by the inventors hereof and which incorporated by reference herein. 
     
    
     BACKGROUND 
       [0002]    1. Field 
         [0003]    This disclosure relates to measuring and testing, and more particularly to testing of solder balls and similar structures, such as found in a ball grid array. 
         [0004]    2. Background 
         [0005]    Because ball grid arrays (BGAs) have a wide application in electronic packaging, the packaging strength of BGAs to the attachment substrate is crucial for industry. Industry uses a test procedure called a “single ball shear test”, which provides a simple and efficient technique for evaluating the quality of solder balls. Experimental observations from low shear rate tests of solder joints are not accurate predictors of failure behaviour at high strain rates. That is because low shear rate tests of solder joints cannot accurately predict the mode of deformation and failure behavior at high strain rates. Due to strain rate effect, brittle failures often take place when the solder joints are subjected to dynamic loadings, and such brittle failures may not be seen under low strain rate shear tests. 
         [0006]    Thus, it is desired to develop an improved test procedure capable of evaluating the impact strength characterization of component parts. One such example is a procedure for evaluating the impact strength characterization of solder joints. Such impact strength characterization becomes critical during package design and manufacturing for high reliability. This is particularly true for lead-free solder, for example, lead-free solder used in handheld devices. 
         [0007]    Conventional techniques of testing solder balls use a linear accelerating system to shear the solder ball at different speeds.  FIGS. 1A and 1B  are schematic diagrams showing the movement of a tester tip against a solder ball. As  FIGS. 1A  and  1 B depict, the tip moves in the linear direction from a significant distance. At an expected speed, the tip will shear the solder ball, resulting in the removal of the solder ball ( FIG. 1B ).  FIG. 2  is a graph showing measured load vs. displacement. During this impact situation, the force versus displacement curve will be recorded, resulting in a graph as shown in  FIG. 2 . The load may be measured at the tip&#39;s holder or at the clamp for the substrate to which the solder ball is attached. From the curve depicted in  FIG. 2 , the resistance of shearing the solder ball can be observed. 
       SUMMARY 
       [0008]    A test is performed by an apparatus applying a high speed impact load to a sample to test the shear strength of attachment of a component part to the sample. A support mechanism provides an alignment between an impact tip and a portion of the sample to receive a test force, and prevents relative movement of at least one of the sample and the impact tip. A rotary drive mechanism applies relative motion between the tip and the sample to establishing an impact force between the impact tip and the sample, and a force transducer receives a force proportional to said impact force and provides a corresponding output. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0009]      FIGS. 1A and 1B  are schematic diagrams showing the movement of a tester tip against a solder ball. 
           [0010]      FIG. 2  is a graph showing measured load vs. displacement. 
           [0011]      FIGS. 3A and 3B  are schematic diagrams showing rotational movement of a tester tip against a solder ball, implementing a rotational accelerating system. 
           [0012]      FIG. 4  is a diagram showing the basic components of the rotational accelerating system. 
           [0013]      FIG. 5  is a photomicrograph showing the engagement of a tester tip with a solder ball on a substrate. 
           [0014]      FIG. 6  is a diagram showing a configuration of a tester. 
           [0015]      FIG. 7  is a diagram showing the impact system and drive mechanism. 
           [0016]      FIG. 8  is a schematic diagram depicting platform and sampling system. 
           [0017]      FIG. 9  is a graphical diagram depicting voltage vs. time of a piezoelectric sensor. 
           [0018]      FIG. 10  is a graphical diagram depicting voltage vs. time of a piezoelectric sensor depicting a test performed with a glass test sample. 
           [0019]      FIGS. 11-13  are diagrams illustrating the package geometry and the detailed information. 
           [0020]      FIGS. 14A-D  are SEM micrographs of typical solder ball fracture surfaces tested at different shear speeds. 
           [0021]      FIG. 15  is a graphical depiction of load-displacement curves. 
           [0022]      FIG. 16  is a graphical depiction comparing ductile, intermediate and brittle impact responses obtained from the packages from different suppliers. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    Overview 
         [0024]    Good solder joint strength at high strain rates is a critical reliability requirement for portable electronic devices. Experimental observation from low shear rate tests of solder ball joints cannot precisely reflect their deformation and failure behaviors at high strain rates. 
         [0025]    Testing of solder balls is given by way of non-limiting example. The testing apparatus is designed to test the strength of single solder ball in one chip. It can provide stable impact speed to a single solder ball, and at the same time to acquire its force and displacement relationship curve. The stable speed can be acquired by clutch, and the data collection can be obtained by piezoelectric film. The example solder ball testing demonstrates an ability to perform impact testing with short-range acceleration, controllable impact momentum and provides accurate measurement. 
         [0026]    The technique provides impact testing for a wide variety of general product reliability testing involving impact or force measurements. The technique is performed with short-distance acceleration, controllable impact speed and provides accurate measurement. 
         [0027]      FIGS. 3A and 3B  are schematic diagrams showing rotational movement of a tester tip  311  against a solder ball  315  fused to a substrate  317 , implementing a rotational accelerating system. The rotational accelerating system is used to replace the former linear acceleration system to shear the solder ball.  FIG. 4  is a diagram showing the basic components of the rotational accelerating system.  FIG. 4  depicts a sampling platform  411  consisting of resting surface  415  and clamp  417 . Tester tip  421  is controlled by motor  427  to rotate in an arc indicated at dashed circular line  431 . A sample substrate  441  is placed on sampling platform  411  and clamped by clamp  417 . Solder ball  445  on sample  441  is engaged by tip  421 , and if the force exerted by tip  421  exceeds the fusion strength of solder ball  445  to substrate  441 , solder ball is destructively removed.  FIG. 5  is a photomicrograph showing the engagement of a tester tip with a solder ball on a substrate. 
         [0028]    By using the rotational accelerating system, the speed can be more accurately controlled and the displacement for speed acceleration will be shorter than is the case with a conventional linear accelerating system. Less displacement for accelerating results in less solder balls being affected during the test, and more information concerning the solder balls will be obtained. 
         [0029]    In order to acquire impact data, a new sensor-piezoelectric film is used. This sensor can produce an electrical output by pressure force, and it has a high sampling rate, which is necessary for high speed impact on one single solder ball. Compared with the former sensor design, this design has low vibration effect from mechanical waves caused by dynamic loading. 
         [0030]    By using this testing method, more solder balls can be tested compared to linear accelerating system. This increase in number of solder balls tested is due, in part, to the accelerating distance being much shorter in the rotational accelerating system than in a linear accelerating system. 
         [0031]    Configuration 
         [0032]      FIG. 6  is a diagram showing a configuration of a tester  600  used to perform the functions diagramed in  FIG. 4 . The major components of the tester of  FIG. 6  include four parts:
       alignment system  610 , which includes adjusters  611 ,  612 ,  613 ;   impact system, which includes support  621 , impact tip  623 , motor  625  and clutch  627 ;   monitor system  630 ; and   platform and sampling system  64 O, which includes clamp  641 .         
         [0037]    Clamp  641  is used to support a sample (not shown in  FIG. 6 ; see sample substrate  441  with solder ball  445 ,  FIG. 4 ). Clamp  641  is used to apply a pre-tightening load to the substrate ( 441 ,  FIG. 4 ). The pre-tightening load allows measurement of force at clamp  641 , as will be explained infra. 
         [0038]    Adjusters  611 ,  612 ,  613  on alignment system  610  provides adjustment as an X-Y-Z table. The X-Y-Z adjustment preciously locates platform and sampling system  640 , and is used to put the solder ball in alignment with impact tip  623 . 
         [0039]    Impact System 
         [0040]    The impact follows five steps:
       Step 1: The sample is aligned in three directions by XYZ table  610 .   Step 2: The ram height is adjusted.   Step 3: After alignment, impact tip  623  is rotated in an arc away from the front of the single solder ball. This allows for acceleration of impact tip  623 .   Step 4: Impact tip  623  is accelerated to a stable speed prior to engaging the solder ball.   Step 5: The solder ball is impacted by tip  623 .       
 
         [0046]    In order to complete the impact, it is desired to establish a desired rotational impact speed of tip  623 . The acceleration system comprising motor  625  and clutch  627  is designed to provide stable impact speed. The use of a clutch allows the tip to acquire speed within a short time period and further allows the drive mechanism to provide a desired momentum without the motor being restricted to the arc of movement of the impact tip. In the example configuration, the impact speed is selected from a range of 0.3 m/s to 5 m/s. 
         [0047]    Impact Drive System 
         [0048]      FIG. 7  is a diagram showing the impact system. Depicted are support  621 , impact tip  623 , motor  625  and clutch  627 , as described above in connection with  FIG. 6 . Also depicted are shafts  731 ,  732 , coupling  741  and shaft encoder  751 . 
         [0049]    After the alignment, motor  625  is caused to rotate, which drives shaft  732  through its connection with coupling  741 . Clutch  627  includes three clutch components  761 ,  762 ,  763 . Clutch component  761  connects with shaft  731 , and clutch component  762  connects with shaft  732 . Clutch component  763  provides a magnetic field which causes clutch components  761  and  762  to engage, thereby causing the shaft  731  to quickly reach the speed of shaft  732 . Impact tip  623  rotates with shaft  731  and impacts the solder ball (not shown in  FIG. 7 ). Shaft encoder  751  is used to monitor the rotational speed by virtue of its connection with shaft  731 . 
         [0050]    This configuration is able to provide fast impact speed within a short time period. Clutch  627  may be a magnetic clutch as described or another type of clutch drive system or another type of drive system are given by way of non-limiting examples of techniques to provide a quick ramping of rotational speed and of providing stable impact force through impact tip  623 . The use of the clutch provides an ability to quickly achieve rotational speed while maintaining a predetermined momentum of the drive system, and transferring the predetermined momentum to impact tip  623 . The motor  625  working through clutch  627  provide sufficient energy to achieve a high linear speed. The high linear speed can be accomplished by motor  625  and clutch  627  using a short rotation arm for impact tip  623 . 
         [0051]    The impact system is thereby suited for quick acceleration for the purpose of testing of sheer strength. An example of such testing is given in the sheer testing of the solder ball  315  fused to a substrate  317  ( FIG. 3 ), but is also suitable for other types of impact testing. This impact system also can be applied to small scale joint strength measurement. 
         [0052]    Sampling System 
         [0053]      FIG. 8  is a schematic diagram depicting platform and sampling system  640  used in association with tester  600  ( FIG. 6 ). Platform and sampling system  640  includes clamp comprising two clamp halves  811 ,  812 , and piezoelectric sensor  821 . Piezoelectric sensor  821  functions as a load cell, which is a type of force transducer, and by way of non-limiting example, is configured as a piezoelectric film sensor. Sensor  821  is positioned between a sample  830  consisting of a substrate  831  and clamp half  811 . Substrate  831  has solder ball  835  fused to it. Clamp  811 ,  812  provides a pre-tightening load to sensor  821 . In the example configuration, sensor  821  includes a force distributing layer  851  formed of metal or ceramic which has high hardness. Force distributing layer  851  directly transfers the load to piezoelectric film  852  without absorbing a substantial amount of the force. Electric charge created by the force on piezoelectric film  852  is collected by a charge amplifier  861  for readout according to voltage produced by piezoelectric sensor  821 . Piezoelectric sensor  821  advantageously has a high sampling rate; however piezoelectric sensor  821  can be replaced by other force transducers which have the similar characteristics. 
         [0054]    The sensing of the force must take into account the pre-tightening force applied by the clamp  811 ,  812 , since the clamping force is not part of the force applied to the solder ball  835 . The clamp will apply a pre-tightening force, but it doesn&#39;t affect the final results. Consequentially, there is no need to subtract the pre-tightening force from the measured total force results since the piezoelectric material is not sensitive to static loading applied as stable pre-tightening force. Piezoelectric sensor  821  will provide charge only by dynamic loading; however, it needs some calibrations before use. 
         [0055]    In the example configuration, piezoelectric sensor  821  is clamped by two force distributing layers, and is responsive to two-sided normal force resulting from pressure on two sides of the piezoelectric film transferred through the two force distributing layers. The electrical signal can thereby be produced by said pressure on the two sides of the piezoelectric film. The pressure applied to sensor  821  is the normal stress, which is vertical to said piezoelectric film sensor. Sensor  821  receives force transferred by copper pieces and metal wire to charge amplifier  861 , which is then be collected by an oscillograph or computer to perform analysis. The output from sensor  821  would be linearly proportion to the shear force applied by the impact tip  623 . As configured, piezoelectric sensor  821  has force distributing layers on two sides in order to improve the surface contact of sensor  821 . 
         [0056]      FIG. 9  is a graphical diagram depicting voltage vs. time of piezoelectric sensor  821  ( FIG. 8 ). When impact occurs, impact tip  623  provides a high speed load to the solder ball  835 , so the solder ball itself or solder joint fixing the solder ball  835  to sample substrate  831  will produce reaction force. Since the clamp  811 ,  812  is holding substrate  831 , substrate  831  will not move, but instead will transfer the force to sensor  821 . Therefore, sensor  821  &#39;s output signal can reflect the real resistance load of sample  830 . This sampling system can also be used to measure the high speed or high frequency force or pressure. 
         [0057]    While the example configuration shows the force measurement at sampling system  640 , it is also possible to measure force at the impact mechanism. Likewise, the rotary movement may be achieved by rotary movement of impact tip  623  or sample  830 . 
         [0058]    In addition to the force monitoring system, video and other systems for observing the movement of the unit under test are provided, as depicted by camera  630  ( FIG. 6 ). 
         [0059]    Experimental Work 
         [0060]    One wafer level BGA sample was chosen to do the test, which is depicted in  FIG. 9 , described above.  FIG. 10  depicts the test performed with a glass test sample. By using a testing machine similar to that described in connection with  FIGS. 6-8 , the load versus time curve could be displayed by oscillograph, as depicted in  FIG. 9 .  FIG. 9  shows that the load will vary with the displacement changes. The testing sample was then changed to glass, which is a brittle material, depicted in  FIG. 10 . From  FIG. 10 , it can be found that the load quickly falls down after it reaches to peak force, as indicated at  1011 . This is because the crack propagates faster than the impact speed, causing a sharp decrease in force. Those two experiments demonstrate the collection of sampling system as reflecting the resistance of an object (e.g., solder ball  835 ,  FIG. 8 ) to force. 
         [0061]    It is possible to obtain the load-time curve by use of sensors and oscillograph at first. It is presumed that the power of motor is much stronger than the solder joint strength; therefore, think the speed is constant during the impact. As a result, it is possible to obtain results by using displacement at a given speed and multiplying by time or by integrating displacement and speed over time. 
         [0062]    Six wafer level packages manufactured by four vendors were used in this study for solder joint strength test. Table  1  gives details regarding the packaging technology and the solder balls used per given leg. The alloys of the solder balls were Sn-1Ag-0.5Cu (SAC105) and Sn-1.2Ag-0.5Cu—Ni (LF35). 
         [0000]    
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Experimental matrix 
               
             
          
           
               
                 Vendor 
                 Leg 
                 WLP Technology 
                 Solder Balls Composition 
               
               
                   
               
               
                 Vendor 1 
                 Leg 1 
                 Double Polyimide Layer 
                 SAC 105 
               
               
                 Vendor 2 
                 Leg 1 
                 Double Polyimide Layer 
                 SAC 105 
               
               
                 Vendor 3 
                 Leg 1 
                 Copper Post 
                 SAC 105 
               
               
                 Vendor 3 
                 Leg 2 
                 Copper Post 
                 LF 35 
               
               
                 Vendor 4 
                 Leg 1 
                 Double Polyimide Layer 
                 SAC 105 
               
               
                 Vendor 4 
                 Leg 2 
                 Double Polyimide Layer 
                 LF 35 
               
               
                   
               
             
          
         
       
     
         [0063]      FIGS. 11-13  are diagrams illustrating the package geometry and the detailed information. The parameters are given in Table 2. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Chip parameters 
               
             
          
           
               
                   
                 Parameter 
                 Distance (μm) 
               
               
                   
                   
               
             
          
           
               
                   
                 A 
                 250 
               
               
                   
                 B 
                 4400 
               
               
                   
                 C 
                 4400 
               
               
                   
                   
               
             
          
         
       
     
         [0064]    The diameter and height of the balls were 250 μm and 200 μm, respectively. Both static and dynamic shear tests were conducted for those samples. The test conditions are listed in Table 3. Scanning electron microscopy (SEM, JEOL 6300) and EDX (INCA) were applied to investigate the fracture surface after the shear tests. 
         [0000]    
       
         
               
             
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Test parameters 
               
             
          
           
               
                 Shear Test Method 
                 Static Shear Test 
                 High Speed Impact Test 
               
               
                   
               
               
                 Equipment 
                 DAGE 4000S 
                 Lab-made single ball 
               
               
                   
                   
                 impact tester 
               
               
                 Shear Rate 
                 500 μm/s 
                 0.5; 1.0; 1.5; 2.0; 2.5; 
               
               
                   
                   
                 3.0; 3.5; 4.0 m/s 
               
               
                 Ram Height 
                 30 μm 
                 30 μm 
               
               
                 Solder Ball Components 
                 SAC105, LF35 
                 SAC105, LF35 
               
               
                   
               
             
          
         
       
     
         [0065]    The sample marked as Vendor 1 Leg 1 was chosen as the typical sample.  FIGS. 14A-D  are SEM micrographs of typical solder ball fracture surfaces tested at shear speeds ranging from 500 μm/s to 3.0 m/s. It appears that the strain rate had a significant effect on the fracture behavior of the solder joints. With the impact speed increasing, the fracture mode changed from complete ductile (at 500 μm/s and 0.5 m/s) to a semi-ductile (1.5 m/s) and eventually, to brittle fracture (3.0 m/s). The differences in peak stress and elongation at break points are able at different speeds are able to be determined. The corresponding load-displacement curves were recorded and are presented in  FIG. 15 . It is noted that at a low shear rate of 500 μm/s, the test was stopped after about 130 μm of shearing, because it was tested by another commercial device. 
         [0066]      FIG. 16  is a graphical depiction comparing ductile, intermediate and brittle impact responses obtained from the packages from different suppliers. The depiction compares the fracture mode of the ball joints, i.e. ductile, intermediate (semi-ductile or semi-brittle) or brittle. 
         [0067]    By examining the fracture surfaces obtained at various shearing impact speeds, one can establish a way the cracks initiated and propagated. At a low shear rate, when the failure mode was ductile, the fracture cracks started and propagated along the tip movement direction. At a high speed, however, the fracture was interfacial and the cracks followed the intermetallic compound (IMC). 
         [0068]    Conclusion 
         [0069]    It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated to explain the nature of the subject matter, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.