Abstract:
A spring probe having a barrel, plunger and a back-drilled aperture is provided in which the centerline axis of the aperture is separate from the longitudinal axis of the plunger. A portion of the spring force directed along the longitudinal axis of the probe is transferred to a side force to bias the plunger against the barrel for electrical contact. The lighter, more uniform biasing and slight rotation/agitation of the plunger within that barrel increases the probe life and electrical performance.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to battery-type contacts and interconnect probes and, in particular, to spring-loaded contact probes and a method for biasing the probes which are used in electrical testing applications and battery contact applications. 
     Conventional spring-loaded electrical contact probes generally include an outer receptacle, a barrel containing a movable plunger and a spring, which exerts a force against the back of the plunger to bias the plunger outwardly against the barrel. The plunger may be depressed inwardly of the barrel a predetermined distance, under force directed against the spring. 
     Battery-type contacts and interconnect probe designs generally require compact, durable, highly reliable designs with circuit paths optimized for the best performance. These contacts are typically employed in battery charging applications, mobile telecommunication applications, docking applications, and other portable electronic devices in addition to applications for testing electronics, printed circuit boards and computer chips, for example. They may be used as either power conductors or as signal carriers and would be subject to a variety of environmental conditions. 
     As products continue to shrink in size or increase in performance while maintaining current size, the need for smaller contacts continues to grow. Compliancy of a probe contact though continues to be important to accommodate the tolerances of many parts in an assembly. Many times this compliancy requires a probe with a plunger travel much longer than a spring can supply in the spaced allotted. This is compensated by back drilling the plunger to supply additional space for the spring. The resultant probe performs well mechanically but the electrical performance in certain instances is compromised by the action of the spring and device under test. Specifically, if the device under test pushes directly down on top of the plunger and the spring generates a force pushing directly up the desired contact between plunger and barrel, which is required for optimal electrical performance, can be very light or nonexistent. The result is a poor, unreliable electrical performance for the probe. 
     As is known in the art, current travels in parallel down all available paths in a quantity dependent upon the path&#39;s resistance. A spring, by nature of its design, has a very large resistance and will cause poor performance if it is the main circuit path. Likewise, large resistances between the barrel inner diameter (“ID”) and plunger, referred to as the contact resistance, will also lead to poor performance or failure. Large contact resistances are generally due to low contact force between barrel ID and plunger, poor conductive material of barrel and plunger including plating material and contaminates such as dirt, lint, or even some lubricants. Good probe designs minimize the contact resistance by proper material selection, plating selection, attention to cleanliness/handling, and increasing the contact force between barrel ID and plunger through efforts called biasing, which is the action of forcing the plunger&#39;s bearing surface against the barrel ID. 
     In an effort to improve biasing in probes many designs have been generated. The most popular and successful has been applying a “bias cut” on the tail of the plunger. A large side force is created from the spring pushing against the bias cut creating firm, constant contact force between barrel and plunger. This contact force ensures that the current will flow from the plunger to the barrel and not through the spring and also provides the lowest contact resistance between barrel and plunger. The disadvantage to this type of design is the higher friction that is created between plunger and barrel resulting in failure of the probe due to mechanical wear. 
     With a back-drilled plunger, an angled surface cannot be generated to induce this biasing. Thus, other techniques must be employed to generate the biasing. Some techniques involve changing the plunger design on the front end to promote biasing while others require special barrels, tangs and such. 
     SUMMARY OF THE INVENTION 
     The present invention is a plunger with back-drilled hole or aperture with the centerline of the aperture separate from the plunger&#39;s longitudinal axis. The spring force against the plunger is no longer directly in line with the plunger longitudinal axis or centerline. When the plunger encounters the device under test or battery contact, for example, an immediate coupling or moment is created which transfers a portion of the longitudinal force exerted along the plunger axis into a side force. This moment creates the biasing needed by forcing the plunger&#39;s bearing surface against the barrel inner diameter. The pivot point is the contact point between plunger and device under test. The larger the spring force, the larger the moment and thus, the higher the contact force. 
     Some spring movement or “snaking” occurs due to the off-centered hole. The ends of the spring will tend to center themselves in the cavities made for them. Being that the two cavities are not aligned, the spring has no choice but to bend in the center of the coils. This bending action further amplifies the biasing of the plunger if the plunger cavity extends to the center of the spring. 
     An additional advantage of this design is that the force between plunger and barrel will not be as large as a normal biased plunger and will result in longer life through less wear. In a normal biased design the wear is localized between the plunger and barrel due to the severe biasing of the plunger. This new design with a less aggressive biasing, spreads the wear more evenly across the contact points of the barrel and plunger thereby reducing wear and increasing the life of the probe. Additionally, a slight random rotation of the plunger in the barrel due to the spring action further spreads and reduces the wear. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side elevation sectional view of a prior art spring probe with a bias cut plunger; 
     FIG. 2 is a cross-sectional side view of a prior art spring probe with a back-drilled plunger; 
     FIG. 3 is a cross-sectional side view of a spring probe of the present invention having a plunger with an eccentric back-drilled hole; 
     FIG. 4 is a diagrammatic left end view, along line  4 — 4 , of the plunger end of FIG. 3; 
     FIG. 5 is a partial diagrammatic side view of the plunger of FIG. 3 having an aperture with a centerline axis generally parallel to the longitudinal axis of the plunger; 
     FIG. 6 is a partial diagrammatic side view of another embodiment of the plunger of FIG. 5 having an aperture with a centerline axis which diverges from the longitudinal axis of the plunger; 
     FIG. 7 is a diagrammatic left end view of the plunger of FIG. 6; 
     FIG. 8 is a partial diagrammatic side view of another embodiment of the plunger of FIG. 5 having an aperture with a centerline axis which intersects the longitudinal axis of the plunger; 
     FIG. 9 is a diagrammatic left end view of the plunger of FIG. 8; 
     FIG. 10 is a partial diagrammatic side view of another embodiment of the plunger of FIG. 5 having an aperture with a centerline axis which is not coplanar with the longitudinal axis of the plunger; 
     FIG. 11 is a diagrammatic left end view of the plunger of FIG. 10; 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 illustrates a prior art electrical contact probe  10 . Prior art probe  10  includes a barrel  12  for receiving a plunger  14 . A crimp  16  in barrel  12  retains plunger  14  within barrel  12 . A bias cut  18  on the tail of plunger  14  is provided to create a transaxial force from a spring  20  pushing against the bias cut  18  and forcing plunger  14  against the inside diameter  22  of barrel  12 . The contact force between plunger  14  and barrel  12  provides an electrical path between the barrel and plunger. The angled surface  18  creates a bend in spring  20  causing the spring to rub against the inside of the barrel  12 , which reduces the life of the spring and increases the wear on the inside diameter  22  of the barrel  12 . 
     FIG. 2 illustrates a prior art conventional back-drilled electrical contact probe  30 . Probe  30  includes a barrel  32  for receiving a plunger  34 . Plunger  34  includes a back-drilled hole as indicated by reference numeral  36  to receive a spring  38 . Hole  36  is drilled coincident with the centerline axis  40  of plunger  34 . A crimp  42  retains plunger  34  within barrel  32 . Contact between the plunger  34  and barrel  32  is intermittent along the inner surface  44  of barrel  32 . 
     FIGS. 3-5 illustrate the electrical contact probe of the present invention generally indicated by the reference number  50 . Probe  50  includes a hollow barrel  52  for receiving a plunger  54 . Plunger  54  includes a top  56 , shoulder or flange  58  and end portion  60 . Plunger  54  is generally circular in cross-section having a diameter that diminishes from the end portion  60  to the tip  56  across flange  58 . Crimp  62  in barrel  52  retains plunger  54  within barrel  52 . 
     Plunger  54  includes a back-drilled hole or aperture  64  to receive a spring  66 . In the preferred embodiment, the centerline axis  68  of aperture  64  is generally parallel to the longitudinal axis or centerline axis  70  of plunger  54  and cavity  53 . Because aperture  64  does not share a common axis with plunger  54  and cavity  53 , spring  66  bends slightly as its ends center themselves in cavity  53  and aperture  64 . The spring force is not directly in line with the plunger centerline  70 , thus when the plunger  54  encounters a device under test or battery contact, for example, an immediate coupling or moment of force is created. The moment transfers a portion of the axial force exerted on the plunger  54  by the spring  66 , into a side or transaxial force generally perpendicular to the longitudinal axis  70  of plunger  54 . This moment or torque creates the biasing by forcing the plunger&#39;s bearing surface (the outside surface of end  60 ) against the inner diameter  55  of barrel cavity  53 . The pivot point for the moment is the tip  56  of the plunger  54  or the contact point between the plunger  54  and the device under test or electrical device. The larger the spring force the larger the moment created which in turn creates a higher contact force between the plunger  54  and barrel  52 . 
     The biasing or contact between the barrel  52  and plunger  54  is necessary for good electrical conduction between the barrel  52  and plunger  54 . However, contact between the surfaces also causes probe  50  to wear and may eventually fail. In addition to the moment created by the slight bend in spring  66 , the off-centered aperture  64  induces the spring  66  to snake or rotate in the cavity  53  and aperture  64  causing plunger  54  to rotate slightly within cavity  53 . This slight rotation allows the contact area between plunger  54  and barrel  52  to change as the probe  50  is used. Thus the wear is spread over a larger area resulting in an increased probe life. It should be appreciated that the slight rotation of plunger  54  within barrel  52  may be somewhat random depending on the amount the spring  66  is compressed and the spring force. 
     Referring to FIGS. 6 and 7, another embodiment of a plunger  54  is shown. Plunger  54  is physically the same as the plunger shown in FIGS. 3-5, having a tip  56 , shoulder  58  and end portion  60 , with the exception that centerline  72  of hole  74  is not generally parallel to the axis of rotation  70  of plunger  54 . Hole  74  extends from the center of end portion  60  at a slight angle to centerline  70  toward the tip  56  of plunger  54 . 
     Because hole  74  does not share a centerline axis with plunger  54  and cavity  53 , spring  66  bends slightly as its ends center themselves in cavity  53  and hole  74  (see FIG. 3 for barrel cavity  53  and spring  66  configuration). The spring force is not in line with plunger centerline  70 , thus when the plunger  54  encounters a device under test, an immediate coupling or moment is created biasing the plunger against the inner diameter of barrel cavity  53 . Compression of spring  66  causes slight rotation or agitation of the plunger  54  within cavity  53  of barrel  52 . 
     Referring to FIGS. 8-13, three additional embodiments are shown for plunger  54 . In each of these embodiments the centerlines  76 ,  78  and  80  of apertures  82 ,  84  and  86 , respectively, are generally not parallel to the longitudinal axis  70  of plunger  54 . As shown in FIGS. 8 and 9, centerline  76  of aperture  82  begins above longitudinal axis  70  on the left side of FIG. 8, intersects axis  70  at approximately the mid-point of plunger  54  and ends at below axis  70  on the right side of FIG.  8 . In the embodiment shown in FIGS. 8 and 9, the centerline axis  76  of aperture  82  is coplanar with plunger longitudinal axis  70 . 
     Referring to FIGS. 10 and 11, centerline axis  78  of aperture  84  is not coplanar with longitudinal axis  70  of plunger  54 . In this embodiment, aperture  84  is drilled, for example, off center and at a slight angle with respect to longitudinal axis  70 . 
     It is to be understood that while certain forms of this invention have been illustrated and described, is it not limited thereto except insofar as such limitations are included in the following claims.