Abstract:
An improved knee probe for probing electrical devices and circuits is provided. The improved knee probe has a reduced thickness section to alter the mechanical behavior of the probe when contact is made. The reduced thickness section of the probe makes it easier to deflect the probe vertically when contact is made. This increased ease of vertical deflection tends to reduce the horizontal contact force component responsible for the scrub motion, thereby decreasing scrub length. Here “thickness” is the probe thickness in the deflection plane of the probe (i.e., the plane in which the probe knee lies). The reduced thickness probe section provides increased design flexibility for controlling scrub motion, especially in combination with other probe parameters affecting the scrub motion.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. patent application Ser. No. 11/450,977, filed on Jun. 9, 2006, entitled “Knee Probe having Increased Scrub Motion”. Application Ser. No. 11/450,977 claims the benefit of U.S. patent application Ser. No. 10/850,921, filed on May 21, 2004, entitled “Freely Deflecting Knee Probe with Controlled Scrub Motion”. 
     
     FIELD OF THE INVENTION 
       [0002]    This invention relates to probes for testing electronic circuits or devices. 
       BACKGROUND 
       [0003]    Testing of electrical devices and circuits has been an important component of electronic manufacturing processes for some time. Such testing typically entails probing a circuit with a fixture including multiple flexible probes, each probe making electrical contact to a contact pad on the circuit chip. Various practical issues that have arisen in this context have been addressed in the prior art, especially in connection with providing reliable, low-resistance electrical contact. 
         [0004]    It is well known that electrical contact between the probe and the contact pad can be hampered by the presence of non-conductive material on the pad and/or the probe (e.g., a thin oxide film). Accordingly, considerable attention has been devoted to details of how the tip of the probe moves relative to the contact pad in order to improve the resulting electrical connection. This relative motion is usually referred to as a scrub motion. For example, U.S. Pat. No. 5,923,178 considers a probe having a shape which provides a scrub motion which is primarily a rocking motion without appreciable sliding. U.S. Pat. No. 5,952,843 considers a probe having a canted tip portion to facilitate penetration of the passivation layer. U.S. Pat. No. 6,529,021 considers a probe arrangement where the probe tip can be made to move in a reciprocating manner to reduce contact resistance. 
         [0005]    As circuit manufacturing technology continues to evolve to smaller critical dimensions, new practical issues relating to probing tend to arise which are not fully addressed by prior art approaches. For example, the decreasing size of contact pads as critical dimensions are reduced leads to increasingly demanding requirements on the ability to precisely control the probe scrub motion. Excessive scrub motion can cause loss of electrical contact, if the probe moves off the contact pad. 
         [0006]    Accordingly, it would be an advance in the art to provide greater control of probe scrub motion. 
       SUMMARY 
       [0007]    To better appreciate the present invention, it is helpful to consider some aspects of prior work by the present inventor. In particular, U.S. patent application Ser. No. 11/450,977 by the present inventor considers a knee probe where the knee curves away from the probe axis and then curves back to connect to the probe tip, crossing the probe axis on the way to the tip. This configuration can be described as having a negative tip offset, in contrast to probes having no tip offset (i.e., the probe tip is aligned with the probe axis), or probes having a positive tip offset (i.e., the knee section does not cross the probe axis). Other parameters being comparable, probes having negative tip offset tend to provide longer scrub marks than probes having zero or positive tip offset. In some cases, it is desirable to decrease the scrub length provided by a probe having negative tip offset. 
         [0008]    Such reduction in scrub length can be provided according to the present invention by modifying the probe shape. More specifically, the probe knee section includes a reduced thickness section to alter the mechanical behavior of the probe when contact is made. Providing a reduced thickness section of the probe makes it easier to deflect the probe vertically when contact is made. This increased ease of vertical deflection tends to reduce the horizontal contact force component responsible for the scrub motion, thereby decreasing scrub length. Here “thickness” is the probe thickness in the deflection plane of the probe (i.e., the plane in which the probe knee lies). 
         [0009]    The reduced thickness section of the probe can be described in terms of a probe thickness function h(z), where z is distance along the probe, having a local minimum. A probe having uniform thickness would have a constant h(z), and a tapered probe would have a monotonically decreasing h(z). In either of these two conventional cases, h(z) would not have a local minimum. 
         [0010]    Although reduction of scrub length for negative tip offset probes is one application of the invention, the invention is also applicable to probes having no tip offset and to positive tip offset probes. In general, embodiments of the invention can provide improved control of scrub motion (e.g., by varying details of the reduced thickness section such as location, amount of thickness reduction, etc.), especially in combination with other probe parameters affecting scrub motion. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  shows a probe according to a first embodiment of the invention. 
           [0012]      FIGS. 2-3  show alternate embodiments of the invention having different tip offsets. 
           [0013]      FIGS. 4-5  show alternate embodiments of the invention having different upper knee section thickness profiles. 
           [0014]      FIG. 6  shows an embodiment of the invention having a tapered lower knee section. 
           [0015]      FIG. 7   a  shows an embodiment of the invention in an initial contact configuration. 
           [0016]      FIG. 7   b  shows an embodiment of the invention in an operating contact configuration. 
           [0017]      FIG. 8  shows a probe according to another embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]      FIG. 1  shows a probe  100  according to a first embodiment of the invention. A shank  102  defines a probe axis  116 . A curved knee section  104  is connected to shank  102  and includes an upper knee section  106  and a lower knee section  108 . A probe tip  112  is connected to an end of knee section  104  opposite from the shank. Upper knee section  106  extends outward from shank  102  and reaches a knee point  110  of maximum separation from probe axis  116 , thereby defining a lateral knee offset  118  from the probe axis. Lower knee section  108  extends from knee point  110  toward probe axis  116  and to a tip location  114 , thereby defining a lateral tip offset  120  from the probe axis. 
         [0019]    A probe plane includes and is thereby defined by probe axis  116  and knee point  110 . In this example, the plane of  FIG. 1  is the probe plane. A thickness of upper knee section  106  in the probe plane varies along the length of the upper knee section according to a thickness function h(z), where z is position along the probe. The upper knee section includes a reduced thickness section, as described above. More specifically, h(z) has a local minimum located between shank  102  and knee point  110 , as shown in the example of  FIG. 1 . 
         [0020]    Suitable materials for shank  102 , knee section  104  and probe tip  112  are well known in the art, and any such materials can be employed in practicing the invention. Suitable tip materials are electrically conductive and wear-resistant, and include Rh and Cr. Known probe fabrication methods are applicable for fabricating embodiments of the invention. These methods include, but are not limited to, standard multi-layer metal deposition techniques such as plating, sputtering, photolithographic techniques and microelectromechanical systems (MEMS) techniques. No unusual fabrication difficulties due to the reduced thickness section arise in fabricating probes according to the invention. 
         [0021]    Preferably, h(z) varies smoothly (i.e., h(z) is preferably continuous with a continuous first derivative) to avoid stress concentration at discontinuities and/or sharp corners of h(z). It is also preferred for the minimum probe thickness (i.e., the local minimum of h(z)) to have a value between about 0.5 h nom  and about 0.95 h nom , where h nom  is a nominal in-plane probe thickness. In some cases (e.g., as on  FIG. 1 ), lower probe section  108  has a roughly constant thickness h 1 , and in such cases, h nom  can equal h 1 . In other cases, the nominal probe thickness h nom  can be taken to be the maximum value of h(z) (i.e., the maximum thickness of the upper knee section). In either of these two cases, the nominal in-plane probe thickness h nom  is typically between about 25 μm and about 55 μm, although the invention can also be practiced outside of this thickness range. 
         [0022]    Preferably, h(z) varies smoothly along the entire length of upper knee section  106 , in order to minimize stress concentration for a given minimum thickness. It is also preferred for the probe thickness perpendicular to the probe plane to be somewhat higher than the nominal in-plane probe thickness, so that deformation of the probe is easiest in the probe plane. More specifically, the out of plane thickness is preferably between about 1.1 h nom  and about 1.5 h nom . 
         [0023]    For a configuration with a small knee offset and large tip offset one can expect a longer scrub length. For a configuration with large knee offset and small tip offset, a shorter scrub length is expected. Preferred probe design approaches depend on the friction between probe and contact pad. For large contact friction, probe designs that generate larger horizontal reaction force typically produced with larger scrub length are preferred. For smoother, less frictional contact pad surfaces, probe designs producing a shorter scrub length are preferred. 
         [0024]    Preferably, the reduced thickness section is in the upper knee section as shown and described above, although the invention can also be practiced by having the reduced thickness section anywhere along the length of knee section  104 . Placing the reduced thickness section in the upper knee section tends to decrease scrub motion without appreciably decreasing the contact force, while placing the reduced thickness section in the lower knee section (e.g., as shown on  FIG. 8 ) tends to decrease both scrub motion and contact force. More specifically, a negative tip offset probe having an upper knee section reduced thickness section tends to rotate toward the knee during deflection, thereby decreasing scrub motion. A probe having a lower knee section reduced thickness section tends to have increased flexibility (which reduces contact force). This reduced horizontal scrubbing force decreases the scrub motion. Probes having multiple reduced thickness sections can also be employed (e.g., one being in the upper knee section and the other being in the lower knee section) in practicing the invention. 
         [0025]    As described above, the invention is applicable to probes having a positive tip offset, a negative tip offset, or no tip offset. The example of  FIG. 1  shows a probe having a negative tip offset. Here probe axis  116  is between knee point  110  and tip location  114 .  FIG. 2  shows an embodiment of the invention having no tip offset. Here tip location  114  is substantially on probe axis  116 .  FIG. 3  shows an embodiment of the invention having a positive tip offset. Here tip location  114  is between probe axis  116  and knee point  110 . For the probe of  FIG. 3 , the knee section does not cross the probe axis. 
         [0026]    Reduced thickness sections of probes according to the invention can be regarded as resulting from removing material from the left and/or right sides of a smooth, constant-thickness probe profile. For example,  FIG. 4  shows an embodiment of the invention where the reduced thickness section is formed by variation of a right probe boundary f 2 (z).  FIG. 5  shows an embodiment of the invention where the reduced thickness section is formed by variation of a left probe boundary f 1 (z).  FIG. 1  shows an embodiment of the invention where the reduced thickness section is formed by variation of both a left probe boundary f 1 (z) and a right probe boundary f 2 (z). 
         [0027]    In the preceding examples, lower knee section  108  has a roughly constant in-plane thickness. The detailed shape of lower knee section  108  is not critical in practicing the invention, and any other lower knee section shape can also be employed in practicing the invention. For example,  FIG. 6  shows an embodiment of the invention having a tapered lower knee section  108 . More specifically, the in-plane thickness of lower knee section  108  decreases monotonically along the length of the lower knee section from knee point  110  to the tip location  114 . 
         [0028]    Operation of the invention can be appreciated in connection with  FIGS. 7   a - b , which show an embodiment of the invention in an initial contact configuration and an operating contact configuration respectively. On  FIG. 7   a  a probe according to the invention makes initial contact with a contact pad  704 . It is convenient to describe the initial contact point between the probe and contact pad  704  in terms of an initial contact offset  708  defined with respect to probe axis  116 . In operation, the arrangement of  FIG. 7   a  is vertically compressed (e.g., by moving contact pad  704  up by a vertical deflection  706 ). Under this compression, the probe elastically deforms as schematically shown on  FIG. 7   b . As a result of this deformation, the probe tip moves relative to contact pad  704 . Typically this relative motion includes a translation (i.e., operating contact offset  708 ′ being different from initial contact offset  708 ) and a rocking motion of the probe tip relative to the contact pad surface. The rocking motion can be appreciated by noting the different orientations of a tip axis  702  on  FIGS. 7   a  and  7   b . The scrub mark length is the difference between offset  708  and offset  708 ′ 
         [0029]    In one example, a reference probe (probe A) having a nominal in-plane probe thickness of 52 μm and a negative tip offset (as shown on  FIG. 1 ), provided a 25 μm scrub length on an A 1  surface for 75 μm vertical deflection. A probe according to the invention (probe B) had the same shape as the reference probe, except that the upper knee section of probe B smoothly varied to provide a local minimum thickness of 33 μm in the upper knee section. This local minimum was located about halfway between the knee point and the shank. The thickness variation of the upper knee section was distributed over the entire length of the upper knee section. Probe B provided a 10 μm scrub length on the same Al surface used for testing probe A. For both probes A and B, the contact force was about the same (2 grams per 25 μm vertical deflection).