Abstract:
A single-piece contact probe includes a tip, coil and base formed from a single piece of electrically conductive material. A helical groove is machined around the center portion of the probe then a bore is drilled from the base toward the tip along the longitudinal axis of the probe to form the coils.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims the benefit of a prior filed, co-pending provisional application Ser. No. 60/545,882, filed Feb. 19, 2004, entitled SPRING PLUNGER PROBE. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates generally to battery contacts and interconnect probes and, in particular, to a single-piece contact probe which is used in electrical testing and battery contact applications.  
       BACKGROUND OF THE INVENTION  
       [0003]     In most probe designs, there are three distinct parts. First there is the plunger that makes contact with the target, DUT, battery contact, etc. The tip of the plunger that makes contact with the target can be of many different configurations, i.e. spear point, spherical point, crown point, etc., depending upon the target&#39;s geometry, cleanliness, contact material, for example. The other end of the plunger is housed in the second feature of the probe that is the barrel. The barrel has a three-fold purpose: 1) to capture or retain the plunger so it remains in place when the target is removed yet still allow the plunger to have compliancy (movement up and down or back and forth) 2) to transfer current or signal from the plunger to a source or receiver and 3) to supply a means of retaining the assembly in a housing, circuit board, etc. The third component of a probe is the spring that resides in the barrel between the end of the barrel and the plunger. The spring&#39;s sole purpose is to provide force to the plunger as it is moved into the barrel. It provides great compliancy for a probe by allowing the plunger to have large travel within a very small footprint. The compliancy of a probe provides the user great freedom in designing other components of a test fixture, machine, or device, and makes it useful for multiple applications.  
         [0004]     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.  
         [0005]     There are inherent problems with the conventional probe that have led to a myriad of designs. The greatest problem is that which makes the probe very useful. The great compliancy of a probe also works against it in electrical performance. As compliancy simply means that the probe has moving components, this movement also creates poor electrical contact between components.  
         [0006]     Plungers and barrels are dimensioned to provide spacing between them to allow for this movement to take place. Although this spacing is maintained as tight as possible (and still allow good manufacturing processes), any gap between two electrical contacts creates an open or failure in performance. Great efforts have been made in probe designs to ensure that contact between plunger and barrel always exist. But more than just contact is required; good, solid contact is necessary as intermittent contact or a light contact force results in intermittent opens or high resistances.  
         [0007]     The probe industry has sought to ensure reliable contact between probe and plunger by permanently connecting the spring to both the plunger and barrel via a soldering or welding process. Although this provided a somewhat reliable contact, it resulted in a probe with appearances of intermittent electrical opens. This was due to the nature of the spring, a long, thin wire that is coiled into a much shorter component resulting in very large resistances. Resistances in the ohm range could be obtained if the spring becomes the only current path; so, when a designer is expecting resistances in the realm of a few milliohms, this appears as an open. If a significant amount of current passes through the spring, the high resistance results in a sudden heating of the probe possibly leading to the spring annealing, destroying the probe.  
         [0008]     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 current 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. Generally, 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.  
       SUMMARY OF THE INVENTION  
       [0009]     The present invention provides a probe in which the plunger, spring, and barrel are combined into one single component eliminating a sliding or moving contact. The one-piece probe includes of a conventional probe tip. A spring is machined into the central body of the probe. Unlike a conventional wire spring, the machined spring consists of considerably more volume of material that carries the current more effectively.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a side elevation view of a probe of the present invention illustrating a spring and plunger within a cross-sectional side view of a barrel;  
         [0011]      FIG. 2  is a right end view of the probe of  FIG. 1 ;  
         [0012]      FIG. 3  is a side elevation view a spring and plunger of  FIG. 1 ;  
         [0013]      FIG. 4  is a right end view of the spring and plunger of  FIG. 3 ;  
         [0014]      FIG. 5  is a partial sectional view of a probe mounted in a fixture;  
         [0015]      FIG. 6  is an enlarged view of one of the probes of  FIG. 5 ;  
         [0016]      FIG. 7  is a side elevation view of a double-ended probe;  
         [0017]      FIG. 8  is a right end view of the probe of  FIG. 7 .  
     
    
     DETAILED DESCRIPTION  
       [0018]     Referring to  FIGS. 1-4 , a spring probe of the present invention is generally indicated by reference numeral  10 . Probe  10  includes a plunger  12  and a barrel  14 . Plunger  12  is machined from a solid piece of stock material and includes a tip  16 , a machined spring or coil  18 , and a base  20 . Plunger  12  is press-fit into barrel  14  to ensure good electrical contact between the probe  12  and the barrel  14  presenting, in essence, a one-piece probe  10 . The plunger  12 , spring  18 , and barrel  14  are combined into one single component eliminating a sliding or moving contact between the plunger  12  and barrel  14 .  
         [0019]     The plunger  12  may be machined from a solid piece of stock material. The center portion of the stock material is machined to a diameter less than the base  20 . The tip  16  is machined opposite the base  20 . The plunger tip  16  may be configured in as many different ways as a conventional probe tip. A spiral cut is machined in the center portion of the stock material to a depth of the diameter of the hole  24 . The hole  24  is then drilled in the end  20  along the longitudinal axis of the stock material. Unlike a conventional wire spring, the machined spring  18  consists of considerably higher volume of material that carries the current more effectively.  
         [0020]     In one embodiment, the thickness of the spirals  30  of spring  18  may be 0.01 inch and the spacing  32  between spirals  30  may be 0.02 inch. The diameter of the coil may be approximately 0.110 inch with a coil thickness of approximately 0.06 inch.  
         [0021]     As shown in  FIG. 1 , the base  20  of plunger  12  includes a beveled shoulder  34  to help guide the plunger  12  into the barrel  14 . The plunger  12  is press-fit into the barrel  14 . The plunger tip  16  extends through an aperture  36  in the barrel  14 . The aperture  36  may be much larger than the diameter of the probe tip  16  to allow the plunger  12  to freely move in and out of the barrel  14  with little or no contact between the probe tip  16  and the aperture  36 .  
         [0022]     Resistance of a part is determined by the simple equation:
 
Resistance= rho ×length/area.
 
         [0023]     As seen from this equation, the greater the cross-section and the shorter the part, the lower the resistance (rho being the resistivity of the material). Thus short contacts are better conductors. The length of the wire of a spring in a standard probe may be very long (many inches) and the cross-section may be very small resulting in a very high resistance. The machined spring  18  of plunger  12  results in much larger cross-section and a shorter spring length. The larger cross-section of the machined spring  18  also results in a stronger spring coil as the spring rate is proportional to the thickness and width of the rectangular cross-section. A stronger coil means simply fewer coils resulting in a shorter spring length. As a result of the machined spring  18 , a more reasonable resistance can be expected as the current passes through the coils. Additionally, the machined spring  18  leads right into the base  20  of the plunger  12  from which connection may be made directly to an external power source or receiver (not shown).  
         [0024]     Only one point of contact exists between the plunger  12  and target (not shown). Unlike a conventional probe, a second, sliding contact does not exist which might detrimentally affect the electrical performance. Reliability is improved because current does not have to transfer from one component to another but moves directly through the plunger  12  to the external connection.  
         [0025]     Referring to  FIGS. 5 and 6 , the spring plunger  12  may be pressed directly into a plastic housing  38  with the plunger  16  contacting a target (not shown) and the base  20  soldered to a board (not shown). As shown in  FIG. 1 , the spring plunger  12  may be pressed into a conventional probe barrel  14 , which may then be soldered to a circuit board (not shown), for example. This latter use may enhance the electrical performance of the probe  10  as any contact between the plunger  12  and barrel  14  may reduce the resistance of the entire probe  10 .  
         [0026]     Referring to  FIGS. 7 and 8 , a double-ended probe is generally indicated by reference numeral  50 . Many of the same components the double-ended probe  50  are the same as described hereinabove for probe  10  and thus the same reference numerals are used. Probe  50  includes a plunger  12  and a barrel  52 . Plunger  12  is machined from a solid piece of stock material and includes a tip  16 , a machined spring or coil  18 , and a base  54 . Base  54  includes a conical nose  56 . A second tip  56  is pressed into aperture  24  in the base  54  to ensure good electrical continuity. Plunger  12  is slip-fit into barrel  54  with the tip  16  extending through aperture  57  at one end of barrel  52 . The open end  58  of barrel  52  is crimped to retain the entire assembly. The plunger  12  is allowed to free-float in the barrel  52 .  
         [0027]     Other benefits of probe  10  may include a longer life as the friction component in a sliding contact is removed. Reliability of a conventional probe degrades over its life due to the wearing away of plating on the plunger and the inner diameter of the barrel and resulting high, inconsistent resistances. Also, the particulate from such wear has a tendency to interfere with the motion of the probe even to the point of binding the plunger up so no movement is possible. This particulate is readily observed in high friction probes by a black residue that falls out of the bottom of the barrel or works its way up the shaft of the plunger. Because the present design does not rely upon physical contact for electrical performance, frictional wear is not an issue. Also as a result of removing this friction issue, the smoothness of operation of the probe  10  may be greatly increased. Probe  50  has an improved life and reliability due to the larger cross-section and higher strength of the machined spring  18  over a conventional spring.  
         [0028]     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.