Abstract:
A compliant electrical interconnect having a first component and a second component interlockingly engaged with the first component. Each component has two cantilever arms lockingly engaged and continuously biased against each other. Contact springs are captivated by the cantilever arms providing a contact force for the first and second components.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S)  
       [0001]     This application claims the benefit of U.S. Provisional Patent Application No. 60/689,755, filed Jun. 10, 2005, the disclosure of which is hereby incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to electrical contact probes for forming electrical interconnects and, more particularly, to a compliant electrical interconnect between a printed circuit board and the external leads of an integrated circuit package or other electrical circuit, such as an electronic module, during functional testing of the devices.  
       BACKGROUND OF THE INVENTION  
       [0003]     Conventional spring-loaded contact probes generally include a moveable plunger, a barrel having an open end for containing an enlarged diameter section or bearing of the plunger, and a spring for biasing the travel of the plunger in the barrel. The plunger bearing slideably engages the inner surface of the barrel. The enlarged bearing section is retained in the barrel by a crimp near the barrel&#39;s open end.  
         [0004]     The plunger is commonly biased outwardly a selected distance by the spring and may be biased or depressed inwardly into the barrel, a selected distance, under force directed against the spring. Axial and side biasing of the plunger against the barrel prevents false opens or intermittent points of no contact between the plunger and the barrel. The plunger generally is solid and includes a head, or tip, for contacting electrical devices under test. The barrel may also include a tip opposite the barrel&#39;s open end.  
         [0005]     The barrel, plunger and tip form an electrical interconnect between the electrical device under test and test equipment and, as such, are manufactured from an electrically conductive material. Typically, the probes are fitted in cavities formed through the thickness of a test plate or socket. Generally, a contact side of the electrical device to be tested, such as an integrated circuit, is brought in to pressure contact with the tips of the plungers protruding through one side of the test plate or test socket for maintaining spring pressure against the electrical device. A contact plate connected to the test equipment is brought to contact with the tips of the plungers protruding through the other side of the test plate or test socket. The test equipment transmits test signals to the contact plate from where they are transmitted through the test probe interconnects to the device being tested. After the electrical device has been tested, the pressure exerted by the spring probes is released and the device is removed from contact with the tip of each probe. In conventional systems, the pressure is released by moving the electrical device and probes away from one another, thereby allowing the plungers to be displaced outwardly away from the barrel under the force of the spring, until the enlarged diameter bearing the plunger engages the crimp of the barrel.  
         [0006]     The process of making a conventional spring probe involves separately producing the compression spring, the barrel and the plunger. The compression spring is wound and heat treated to produce a spring of a precise size and of a controlled spring force. The plunger is typically turned on a lathe and heat treated. The barrels are also sometimes heat treated. The barrels can be formed in a lathe or by a deep draw process. All components may be subjected to a plating process to enhance conductivity. The spring probe components are assembled either manually or by an automated process. The assembly process for these probes is a multiple step process. Considering that probes are produced by the thousands, a reduction in the equipment and the steps required to produce the probes will result in substantial savings.  
         [0007]     An important aspect of testing integrated circuit boards is that they are tested under high frequencies. As such, impedance matching is required between the test equipment and integrated circuit so as to avoid attenuation of the high frequency signals. Due to the numerous probes that are used in relatively small area in the socket, the spacing between probes is minimal making impedance matching infeasible. In such situations, in order to avoid attenuation of the high frequency signals, the length of the electrical interconnects formed by the probes must be kept to a minimum. With current probes, when the interconnect length is minimized, so is the spring length and thus spring volume.  
         [0008]     A spring&#39;s operating life, as well as the force applied by a spring are proportional to the spring volume. Consequently, the spring volume requirements for a given spring operating life and required spring force are in contrast with the short spring length requirements for avoiding the attenuation of the high frequency signals. Since the diameter of the spring is limited by the diameter of the barrel which is limited by the diameter of the cavities in the test sockets, the only way to increase the spring volume for increasing the spring operating life, as well as the spring force, is to increase the overall barrel length. Doing so, however, results in a probe having an electrical interconnect of increased length resulting in the undesirable attenuation of the high frequency signals.  
         [0009]     An alternative type of conventional probe consists of two contact tips separated by a spring. Each contact tip is attached to a spring end. This type of probe relies on the walls of the test plate or socket cavity into which it is inserted for lateral support. The electrical path provided by this type of probe spirals down the spring wire between the two contact tips. Consequently, this probe has a relatively long electrical interconnect length which may result in attenuation of the high frequency signals when testing integrated circuits.  
         [0010]     Thus, it is desirable to reduce the electrical interconnect length of a probe without reducing the spring volume. In addition, it is desirable to increase the spring volume without decreasing the spring compliance or increasing the electrical interconnect length. Moreover, a probe is desirable that can be easily manufactured and assembled.  
       SUMMARY OF THE INVENTION  
       [0011]     The present invention is an improved electrical contact probe with compliant internal interconnect which has been designed to address the drawbacks of prior probe designs. The purpose of the invention is to provide a compliant electrical interconnect between a printed circuit board (PCB) and the external leads of an integrated circuit (IC) package or other electrical circuit, such as an electronic module, during functional testing of the devices. The probe of the present invention consists of two moving fabricated electrically conductive components with one or more electrically conductive compliant helical springs or compliant non-conductive structures in between the components. The compliance of the interconnect is maximized in order to accommodate mechanical tolerances in the interconnect application. In additional to maximizing the mechanical compliance, the helical spring(s) or compliant non-conductive structures of the contact provide adequate normal force to the part under test and PCB in order to provide electrical contact that maintains stable contact resistance. The overall length of the contact is minimized, in order to maximize high frequency response of the overall connection system thereby minimizing electrical inductance and optimizing the AC electrical transmission path.  
         [0012]     Many compliant interconnect designs that utilize springs as compliant members also use the same spring to provide a biasing force between the moving parts of the interconnect. Some designs use an angle bearing surface to provide a slight offset force between the parts during the compliant stroke of the assembly. This bias action between the components improves electrical connectivity between them, however often also influences the force that the assembly can provide to the IC and PCB in such a way that could degrade the interconnect. The present invention provides a bias between the upper and lower component completely independent of this spring(s) or compliant non-conductive structures that provide the contact force for the device under test. The probe of the present invention consists of four flexible cantilever arms that interconnect with each other. During the deflection of the components during the stroke of the probe, the interlocking cantilever arms are always in intimate contact with the mating arm, as the arms are designed specifically with a slight amount of interference with each other. This interference causes the arms to slightly deflect perpendicularly to the force provided by the springs. The design is such that even if the components are slightly rotated and angled with respect to each other, this perpendicular cantilever normal force always maintains contact at least on one point on each arm. By guaranteeing that intimate contact is always maintained, the overall electrical integrity of the probe is maximized by maintaining stable contact resistance from one end of the assembly to the other.  
         [0013]     At the end of each of the four cantilever arms is a small interlocking tab. During the assembly of the probe, the tab rubs against the tabs of the mating component, thus deflecting the cantilever arms and allowing the components to snap together, captivating the helical coil springs or compliant non-conductive structures between the components. Once captivated after the probe assembly process, the springs maintain a slight pressure exerting a force against the tabs on the cantilever arms in an axial direction. This force maintains a pre-load within the assembly that maintains a consistent assembled overall length. The interlocked arms of the probe assembly are permanently held within the inner diameter of the helical springs or compliant non-conductive structures, and the geometry is such that lateral motion of the assembled parts will not dislodge the locking tabs, thus the probe is self-contained and requires no external housing to hold it together. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is an exploded perspective view of an electrical contact probe of the present invention;  
         [0015]      FIG. 2  is a cross-sectional view of the probe of  FIG. 1  in an extended position;  
         [0016]      FIG. 3  is a cross-sectional view of the probe of  FIG. 1  in a compressed position;  
         [0017]      FIG. 4  is a perspective view of the electrical contact probe of the present invention as mounted into a semi-conductor test contactor;  
         [0018]      FIG. 5  is an exploded perspective view of a first alternative embodiment electrical contact probe;  
         [0019]      FIG. 6  is an exploded perspective view of second alternative embodiment electrical contact probe;  
         [0020]      FIG. 7  is a perspective view of the probe of  FIG. 6  in an extended position;  
         [0021]      FIG. 8  is a perspective view of the probe of  FIG. 6  in a compressed position;  
         [0022]      FIG. 9  is a perspective view of a third alternative embodiment electrical contact probe in an extended position;  
         [0023]      FIG. 10  is a perspective view of the probe of  FIG. 9  in a retracted position;  
         [0024]      FIG. 11  is an exploded perspective view of a fourth alternative embodiment electrical contact probe;  
         [0025]      FIG. 12  is a front view of a fifth alternative embodiment electrical contact probe;  
         [0026]      FIG. 13  is a perspective exploded view of a sixth alternative embodiment electrical contact probe;  
         [0027]      FIG. 14  is a perspective view of the probe of  FIG. 13  in a Kelvin configuration. 
     
    
     DETAILED DESCRIPTION  
       [0028]      FIGS. 1-3  illustrate an electrical contact probe  10  with compliant internal interconnect of the present invention. The probe  10  consists of two moving fabricated electrically conductive components, namely upper component or plunger  12  and lower component or barrel  14  with one or more electrically conductive compliant helical springs  16  and  18  in between upper component  12  and lower component  14 . The compliance of the interconnect is maximized in order to accommodate mechanical tolerance of the interconnect application. In addition to maximizing the mechanical compliance, the helical springs or other compliant non-conductive structures of the spring probe provide adequate normal force to the unit under test and the printed circuit board between which the spring probe is placed in order to provide electrical contact that maintains stable contact resistance. The overall length of the contact probe is minimized, in order to maximize high frequency response of the overall connection system, by minimizing electrical inductance and optimizing the AC electrical transmission path.  
         [0029]     The contact probe  10  provides a bias between the upper component  12  and the lower component  14  completely independently of the springs or other compliant non-conductive structures that provides the contact force for the unit under test. The contact probe accomplishes this objective by including four flexible cantilever arms, namely arms  20  and  22  on upper component  12  and arms  24  and  26  on lower component  14 . Cantilever arms  20  and  22  interlock with cantilever arms  24  and  26 . During deflection of the upper component with respect to the lower component during the stroke of the contact probe, the interlocking cantilever arms are always in intimate contact with its mating cantilever arm, as the arms are designed specifically with a slight amount of interference with each other. This interference causes the arms to slightly deflect perpendicularly to the force provided by the springs. This design maintains contact even if the upper component and the lower component are slightly rotated and angled with respect to each other because the perpendicular cantilever normal force always maintains contact on at least one point on each arm. By guaranteeing this intimate contact being maintained, the overall electrical integrity of the probe is maximized by maintaining stable contact resistance from one end of the assembly to the other.  
         [0030]     Positioned on the end of each cantilever arm is a small interlocking tab  28 . During the assembly of the probe, the tabs rub against the tabs of mating component, thus deflecting the cantilever arms and allowing the components to snap together captivating the helical coil springs or other compliant non-conductive structures between the components. Once captivated after the contact probe assembly process, the springs maintain a slight pressure exerting a force against the tabs on the cantilever arms in an axial direction. This force maintains a preload within the assembly that maintains a consistent assembled overall length. The interlocked cantilevered arms of the assembly are permanently held within the inner diameter of the helical springs or compliant non-conductive structures, and the geometry is such that lateral motion of the assembled parts will not dislodge the locking tabs, thus the contact probe is self-contained and requires no external housing to hold it together.  
         [0031]     The upper component  12  has a probe tip  30  with a v-shaped indentation for contact with the integrated circuit or unit under test. The probe tip  30  extends upwardly from shoulder  32  and cantilever arms  20  and  22  extend downwardly from shoulder  32 . Shoulder  32  includes a spring centering indentation  34  for receipt of the end coil of springs  16  and  18 . As stated previously, cantilever arms  20  and  22  have a tapered surface  36  and  38 , respectively, which engage interlocking tabs  28  of lower component  14 . Lower component  14  includes tapered surfaces  40  and  42  which engage interlocking tabs  28  of upper component  12 . Similarly, lower component  14  has a probe tip  44  for engaging a printed circuit board. Probe tip  44  extends down from shoulder  46  and cantilever arms  24  and  26  extend upwardly from shoulder  46 . Shoulder  46  also includes a spring centering indentation  48  for receipt of the bottom coil of springs  16  and  18 . Shoulder  46  also has stop surfaces  50  for the ends of cantilever arms  20  and  22 . Similarly, shoulder  32  of upper component  12  has stop surfaces  52  for the ends of cantilever arms  24  and  26 , as best seen in  FIG. 3 .  
         [0032]     As shown in  FIG. 4 , a plurality of electrical contact probes  10  are positioned in a semi-conductor test contactor  54  to provide a compliant electrical interconnect between a printed circuit board  56  and the external leads  58  of an integrated circuit package  60  or other electrical circuit during functional testing of the integrated circuit package. The interlocking upper component  12  and lower component  14  are constructed from a two-dimensional profile with a given thickness depending upon the particular application and unit under test being tested. The interlocking cantilever arms slightly flex during the complete actuation stroke of the probe thus providing intimate contact between the upper and lower components and guaranteeing that consistent electrical resistance is maintained. The slight taper on the cantilever arms ensures that the intimate contact is increased as the probe is compressed during normal operation. The contact probe can accommodate one or more springs or other compliant non-conductive structures in parallel so that the desired contact pressure can be achieved for optimum electrical stability while minimizing overall interconnect length.  
         [0033]      FIG. 5  illustrates a first alternative embodiment contact probe  62  of the present invention. Contact probe  62  includes an upper component  64  and a lower component  66  and a single compliant spring  68 . Upper component  64  has two cantilever arms  70  and  72  each having an interlocking tab  74  positioned at the end of the arm. Similarly, lower component  66  has two cantilever arms  76  and  78  each having an interlocking tab  80  positioned at the end of cantilever arm. In this embodiment, the upper component  64  and lower component  66  are arranged perpendicularly to one another such that cantilever arms  70  and  72  pinch lower component  66  and cantilever  76  and  78  pinch upper component  64 . Upper component  64  includes a stop  82  and lower component  66  includes a stop  84  for retaining spring  68 .  
         [0034]      FIGS. 6-8  illustrate another alternative embodiment contact probe  86 . Contact probe  86  includes an upper component  88  and lower component  90  and an extension helical coil spring  92  rather than a compression spring. When contact probe  86  is deflected the spring  92  is stretched. The compression spring is secured to the upper and lower components by aligning the formed ends  94  of the extension spring into notches  96  formed in the cantilever arms  98 ,  100 ,  102  and  104  of upper component  88  and lower component  90 , respectively.  
         [0035]      FIGS. 9 and 10  illustrate yet another alternative embodiment contact probe  106  of the present invention. Contact probe  106  has an upper component  108  and a lower component  110  and dual compression springs  112  and  114 . Both upper component  108  and lower component  110  have flanges  116  which extend into the inner diameter of the compression springs  112  and  114  to center the springs on the contact probe. The upper component  108  and lower component  110  are connected by an alternative latching mechanism that is not contained within the inner diameter of the springs, but is in a central region of the probe. Upper component  108  has a single cantilever arm  118  having an enlarged head portion  120  which is captivated between two cantilever arms  122  and  124  of lower component  110  by interlocking tabs  126  located on the end of each cantilever arm. Cantilever arms  122  and  124  have an angled inner surface  128  and  130 , respectively, to provide a slight offset force between the parts during the compliance stroke of the probe.  
         [0036]      FIG. 11  illustrates yet another alternative contact probe design  132  having a latching component for upper component  134  and lower component  136  which is the same as the latching mechanism for contact probe  106 . However probe  132  is a single spring  138  design and the latching component is contained within the inner diameter of spring  138 .  
         [0037]      FIG. 12  illustrates another alternative embodiment contact probe  140  of the present invention. Contact probe  140  is identical in construction to contact probe  10  with the exception of the probe tip  142  for upper component  144 . In this embodiment, probe tip  142  is offset and has an angled surface leading to a contact point and provides a manufacturable solution to be able to contact a very small pad  146  of an integrated circuit device  148  with two independent probes for a Kelvin connection. This is accomplished by placing two contact probes  140  adjacent each other in mirrored fashion so that the contact points of the two probes are adjacent one another and in contact with the pad  146  on the integrated circuit device.  
         [0038]      FIGS. 13 and 14  illustrate another alternative embodiment contact probe  150  for a practical Kelvin configuration. Probe  150  includes an upper component  152  and a lower component  154  and a single compression spring  156 . Upper component has two cantilever arms  158  and  160  and lower component  154  has two cantilever arms  162  and  164 . Each cantilever arm has an electrical contact bump  166  located at an end of the arm and an interlocking tab  168  located midway down the cantilever arm. Bump  166  is cleanly radiused and is designed to smoothly slide down the flat surface of the opposite component and is the main electrical contact point between the upper and lower components. The upper component  152  and lower component  154  are joined perpendicular to one another such that the cantilever arms and interlocking tabs  168  slideably interlock the upper and lower components together inside of compression spring  156 . The upper and lower components captivate the helical spring or non-conductive elastomeric member that provides a spring force that is axial to the overall length of the probe. The upper component  152  has a probe tip  170  similar to probe tip  142  of contact probe  140 . As seen best in  FIG. 14 , the Kelvin configuration places two contact probes adjacent one another in mirrored fashion so that the probe tips  170  can contact the unit under test.  
         [0039]     The components and features of the various embodiments are interchangeable for a particular application, and as can be seen from the previous examples a contact probe can be constructed with one or more springs that compress between the upper and lower of two conductive components. The contact probe can also be designed such that the compliance of the assembly is made by the use of one or more helical extension springs. The contact probe of the present invention can also be designed such that the compliance of the assembly is made by the use of one or more compressible conductive elastomers.  
         [0040]     The contact probe of the present invention can be constructed with a variety of different tip styles that are optimized to achieve the most stable resistance to an integrated circuit under test. Tip styles include, but are limited to, curved radius, single point sharp tip and dual point sharp tip, and can be positioned either centrally, on the outside portion of the upper or lower component or necked inside.  
         [0041]     The contact probe of the present invention can be designed with different latching mechanisms for the upper and lower component. Additional wiping arms can be added to the components to reduce overall path resistance. In addition, one of the components of the contact probe can have a tail such that it can be soldered or press fit into a printed circuit board to reduce overall path resistance. Spring probe tip geometries can be offset, such that adjacent probes could be positioned in such a manner that allow for Kelvin testing of the unit under test. Kelvin testing is often required to test very sensitive parts, or allow circuitry to designed that carry a forcing current through one probe and a sensing voltage drop can be easily measured through an adjacent probe. In order to achieve this in a practical socket design, the probe tips must be located very close to one another and mate with a single unit under test signal pad. In addition, contact geometries and configurations can be designed for optimized matched impedances or other optimized RF signal perimeters.  
         [0042]     Although the present invention has been described and illustrated with respect to multiple embodiments thereof, it is to be understood that changes and modifications can be made therein which are within the full intended scope of the invention as hereinafter claimed.