Abstract:
A compact tension testing apparatus for determining the peel strength of a circuit line or film bonded to a substrate. A force gauge is mounted on an apparatus using low friction air bearings to allow the force gauge to self-align with the peeling location of the circuit line or film, thereby providing essentially orthogonal positioning relative to the substrate.

Description:
FIELD OF THE INVENTION 
     The present invention is in the field of tensile testing machines. More particularly, the present invention provides an improved apparatus and method for measuring the peel strength of a circuit line or film bonded to a substrate such as a circuit board. Also, the present invention relates to universal testing machines providing tensile, compressive, shear, bending, and torsion tests on a material sample. 
     BACKGROUND OF THE INVENTION 
     Tensile testing machines are commonplace. Typically, a material test sample is clamped to a horizontal platform, and a clamping device attached to a force gauge is lowered in a vertical direction and attached to the test sample. Means are provided to move the clamping device and force gauge in an upward direction, thereby causing a tensile force to be applied to the test sample. 
     Commonly, samples are destructively tested in a large tensile testing machine, wherein a sample to be tested must be sacrificially extracted from a larger specimen for the sake of the test. Once the test has been completed, the sample is usually discarded, which can be prove to be very costly. 
     Typical tensile testing machines used for laminate bond testing only provide peeling movement in one direction, so that the electrical circuit line or other sample on a substrate being tested must be lined up along the one direction of movement. Unfortunately, this requires the substrate to be repositioned and clamped whenever a circuit line has a directional orientation different from the previous test direction. Further, when peeling a circuit line from a substrate, the force gauge must be constantly moved in order to keep the force gauge directly over the peeling location. This is necessary in order to ensure that a true force reading of the force perpendicular to the substrate is being measured. 
     SUMMARY OF THE INVENTION 
     The present invention avoids the disadvantages of the prior art by providing a compact mini-tension tester. The mini-tension tester includes a base plate, a x-axis slide apparatus, a y-axis slide apparatus, a z-axis slide apparatus, a servo actuator assembly, a force gauge, a cable, and a gripper clamp. 
     A substrate with a film or circuit line bonded to its surface can be attached to the base plate using clamps, vacuum means, or other attaching systems. The z-axis slide apparatus is slidably attached to the y-axis slide apparatus and the y-axis slide apparatus is slidable attached to the x-axis slide apparatus. Air bearings, or other frictionless type mechanisms, are used to provide essentially friction free motion. Therefore, the z-axis slide apparatus can move essentially friction free to any location within the x and y plane. 
     A servo actuator assembly is attached to the z-axis slide apparatus, and a force gauge is attached to the servo actuator assembly. The force gauge preferably comprises a strain gauge load cell, although other types of force measurement devices may be used. A cable connects the force gauge to a gripper clamp that is attached to a test sample located on a substrate. The test sample may include, for example, a circuit line or film formed on a substrate such as a printed circuit board. 
     In order to measure the pull force required to pull a circuit line or film from a substrate, the substrate is firmly attached to the platform using a vacuum system. In order to obtain test data without destroying the substrate or affecting the operation of the circuitry on the substrate, sample test circuit lines are applied to the substrate during the production process. Preferably, the sample test circuit lines are only used to monitor the production process, and are not involved with any part of the electronic functioning of circuitry on the substrate. Therefore, these circuit lines may be peeled off the substrate for testing, without sacrificing the operational circuitry on the substrate. 
     During testing, the end of a circuit line is peeled from the substrate and grasped by the gripper clamp. Next, a servo actuator assembly in the z-axis slide apparatus displaces the force gauge, cable, and gripper clamp upward in the z-direction at a constant velocity, thereby providing an upward force that peels the circuit line away from the substrate. The servo actuator assembly is force limited to provide a maximum of about 20 pounds of force. The desired force reading is the force applied in a direction perpendicular to the substrate. In the present invention, the friction free air bearings in the x-axis slide apparatus and y-axis slide apparatus allow the z-axis slide apparatus to “walk” with the circuit line release or peel point. This ensures that the force applied to the release point of the circuit line is always perpendicular to the substrate. Therefore, the force gauge is always measuring the desired force, that is, the force perpendicular to the substrate. 
     The use of the mini-tension tester is not restricted to only providing tensile testing, but can also provide compressive, shear and bending material testing, and strength testing. For instance, compressive testing can be conducted by providing a rigid member between the force gauge and the test object. Then the force cell is moved along the z-axis direction toward the test object, thereby creating a compressive force on the test object. For applying shear force, a test object can be clamped onto the base plate in a direction such that the desired shear force is in line with the z-axis of the mini-tension tester. In another embodiment, a shear force can be applied to the test object by attaching the servo actuator assembly in a direction perpendicular to the z-axis. For this case, a shear force can be applied to a test object in a direction parallel to the base plate. If a test object is attached to the base plate in a cantilevered manner, the servo actuator assembly can apply a force in the z-axis direction to the free end of the cantilever causing a bending moment in the test object. 
     The present invention additionally provides a mini-tension tester that is compact enough to fit inside an oven to provide elevated temperature testing. The mini-tension tester is portable and versatile since a variety of substrate sizes can be attached to the base plate. Also, the mini-tension tester is much less costly then the large tensile testing machines that it replaces. 
     Generally, the present invention provides an apparatus for measuring the peel strength of a material bonded to a substrate, comprising: 
     a gripper clamp for grasping a material bonded to a surface of a substrate; 
     a force gauge attached to a z-axis displacement system and coupled to the gripper clamp, wherein a displacement of the z-axis displacement system causes the material to peel away from the substrate; and 
     x and y-axis displacement systems attached to the z-axis displacement system for providing self-aligning orthogonal positioning of the force gauge relative to a release point of the material as the material is peeled away from the substrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention will best be understood from a detailed description of the invention and a preferred embodiment thereof selected for the purposes of illustration and shown in the accompanying drawings in which: 
     FIG. 1 illustrates a mini-tension tester according to a preferred embodiment of the present invention; 
     FIG. 2 illustrates a top perspective view of the mini-tension tester of FIG. 1; and 
     FIG. 3 illustrates a graph of force versus peel distance along a circuit line. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The features and advantages of the present invention are illustrated in detail in the accompanying drawings, wherein like reference numerals refer to like elements throughout the drawings. 
     A mini-tension tester  10  for measuring the peel strength of a film  12  bonded to a substrate  14  in accordance with a preferred embodiment of the present invention is illustrated in detail in FIGS. 1 and 2. The mini-tension tester  10  generally includes a base plate  16 , a vacuum surface  18 , an x-axis slide apparatus  20 , a y-axis slide apparatus  22 , a z-axis slide apparatus  24 , a servo actuator assembly  26 , a servo actuator controller  28 , a force gauge  30 , a cable  32 , and a gripper clamp  34 . 
     The x-axis slide apparatus  20  includes air bearing slides  32  and  31 , air bearings  36  and  38 , and bridge  40 . Air bearings  36  and  38  are attached to the bridge  40  and slide essentially “friction free” on top of the air bearing slides  32  and  31 . Therefore, the x-axis slide apparatus  20  allows essentially “friction free” motion of the bridge  40 , in the “x” direction  42  as shown in FIG.  2 . 
     The y-axis slide apparatus  22  includes an air bearing slide  44  and an air bearing  46 . The air bearing  46  slides essentially “friction free” on top of the air bearing slide  44  in the “y” direction  48  as shown in FIG.  2 . 
     The z-axis slide apparatus  24  includes a vertical post  50 , a slide  52 , and the air bearing  46 . Air bearing  46  is rigidly attached to the vertical post  50 . Slide  52  is slidingly attached to the vertical post  50 . The force gauge  30  is attached to the slide  52 . Servo actuator assembly  26  provides controlled relative motion between the slide  52  and the vertical post  50 . Cable  32  connects the force gauge  30  to the gripper clamp  34  (FIG.  1 ). 
     The servo actuator controller  28  provides control signals to the servo actuator assembly  26  through control cable  54  to control the displacement of the slide  52  and attached force gauge  30  relative to the substrate  14 . Preferably, a constant velocity motion is generated between the substrate  14  and the force gauge  30 . However, a variable velocity motion may be used, depending on the type of testing being performed by the tester  10 . As shown in FIG. 1, for example, with the gripper clamp  34  grasping the end  33  of the film  12  on the substrate  14 , a constant velocity motion provided by the servo actuator assembly  26  results in the film  12  being peeled from the substrate  14  at a constant velocity. As the film  12  is being peeled from the substrate  14 , the output from the force gauge  30  provides a continuous measurement of the force being applied to the film  12 . Advantageously, the x-axis slide apparatus  20  and y-axis slide apparatus  22  are configured to continuously position, i.e., self-align, the force gauge  30  directly above the release point of the film  12  on the substrate such that the force gauge  30  is always measuring a force perpendicular to the substrate. 
     Referring again to FIG. 1, a computer or other type of processing system  76  can be used to gather force measurement data through cable  70  and positional data through cable  72 . Cable  70  connects the force gauge  30  to the computer  76 , and cable  72  connects the servo actuator controller  28  to the computer  76 . The servo actuator controller  28  provides information regarding the position and movement of the servo actuator assembly  26 . Therefore, the computer  76  can be used to gather the force measurement, along with time and displacement measurements, as a test is being conducted. 
     FIG. 2 illustrates the peel strength testing of a circuit line  60  on a substrate  14 . The circuit line  60  may be a sample test line or may comprise a portion of the operational circuitry on the substrate  14 . Initially, a first end  62  of the circuit line  60  is peeled off of the substrate  14  and gripped by the gripper clamp  34 . Next, the servo actuator assembly  26  in the z-axis slide apparatus  24  displaces the slide  52 , force gauge  30 , cable  32  and gripper clamp  34  at a constant velocity in an upward “z” direction  64  (see FIG.  1 ). This upward motion provides an upward force that peels the circuit line  60  away from the substrate  14  (FIG.  2 ). Since the force gauge  30  is positioned above the release point of the circuit line  60 , the force measured by the force gauge  30  is the force applied to the circuit line  60  in the “z” direction  64  that is perpendicular to the substrate  14 . 
     As the slide  52 , force gauge  30 , cable  32  and gripper clamp  34  continue to be displaced at a constant velocity in an upward “z” direction, the z-axis slide apparatus  24  “walks” with the circuit line  60  release point, even if the circuit line  60  changes direction (FIG.  2 ). That is, the force required to peel the circuit line  60  away from the substrate  14  additionally causes the z-axis slide apparatus  24  to be pulled along with, and continuously positioned above, the release point of the circuit line  60 . Such self-aligning displacement of the z-axis slide apparatus  24  is provided through the use of the air bearing structure of the x-axis slide apparatus  20  and the y-axis slide apparatus  22 . Therefore, essentially “friction free” motion of the z-axis slide apparatus  24  is provided in the “x-y” plane. Thus, in the preferred embodiment of the present invention, the force gauge  30  is always measuring the force that is perpendicular to the substrate  14 . 
     Referring to FIG. 3, a graph of the force measured by the force gauge  30  versus the peel distance along the substrate  14  can be used for evaluation of the bonding strength along the circuit line  60 . If the bonding strength is uniform along the circuit line  60 , the graph of force versus peel distance will form an essentially horizontal line as illustrated in region (A) on FIG.  3 . If the bonding strength is higher in one region along the circuit line  60 , then the graph of force versus peel distance will form an upward spike, as illustrated in region (B) on FIG.  3 . If the bonding strength is lower in one region along the circuit line  60 , then the graph of force versus peel distance will form a downward spike, as illustrated in region (C) on FIG.  3 . Therefore, the graph of force versus peel distance provides information on the quality of the bonding strength along the circuit line  60 . 
     The cable  32  preferably has a predetermined minimum length to limit the effect of a temporary deflection of the cable  32  on the force value measured by the force gauge  30 . Such a temporary deflection may occur, for example, if a large section of the circuit line  60  suddenly releases from the substrate  14  during testing. The minimum length of the cable  32  is chosen to minimize the deviation of the force application angle on the force gauge  30 . In the preferred embodiment of the present invention, a minimum cable length of about 18 inches has proven to be adequate. 
     The foregoing description of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and many modifications and variations are possible in light of the above teaching. Such modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of this invention as defined by the accompanying claims.