Abstract:
A contact spring probe includes a plunger and a spring with a pair of opposed closed coils separated by an open coil. The plunger is secured at one end to one set of closed coils with a shoulder, flange or barb extending from the body of the plunger.

Description:
FIELD OF THE INVENTION 
   This invention relates to contact spring probes and, more particularly, to a contact spring probe with a dual tapered spring and plunger retained by the spring. 
   BACKGROUND OF THE INVENTION 
   Contact spring probes are known in the art. The typical contact spring probe includes a barrel, a plunger and a spring which urges the plunger outwardly from the barrel. The spring and plunger are encapsulated by the barrel which is press-fit into a socket of a test fixture. The electrical conduction path from the probe tip to the socket is typically from the probe to the spring, to the barrel to the socket, or from the probe to the barrel to the socket. Long signal paths may reduce or degrade the electrical performance of the probe and may contribute to mechanical degradation or failure. 
   SUMMARY OF THE INVENTION 
   The present invention provides a contact spring probe with a dual tapered spring with closed coils at each end separated by open active coils, and a plunger with an elongated tail and secured by the closed coils at one end. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an illustration of the probe of the present invention shown prior to assembly. 
       FIG. 2  is an illustration of the probe of  FIG. 1  shown assembled with a sectional view of the spring. 
       FIG. 3  is an illustration of several probes mounted in a fixture. 
       FIG. 4  is an illustration of the probe of  FIG. 1  shown prior to assembly and including an end cap. 
       FIG. 5  is an illustration of the probe of  FIG. 4  shown assembled with a sectional view of the spring and end cap. 
       FIG. 6  is an illustration of several probes with end caps mounted in a fixture. 
   

   DETAILED DESCRIPTION 
   Referring to  FIGS. 1 and 2 , a contact spring probe of the present invention is generally indicated by reference numeral  10 . Probe  10  includes a plunger  12  and a spring  14 . Plunger  12  includes a tip  16 , a shaft  18 , a flange or shoulder  20 , a body  22  and a tail  24 . Plunger  12  may be fabricated by various methods such as machining from a single piece of conductive material such as beryllium copper or steel or the like and may be plated with gold or other material. The tip  16  illustrated is a 120° convex tip which is used to test plated through holes, pads and lands. The smooth cone shape allows plated through holes to be tested with minimal wetness marks to the device or unit under test. The point of the tip  16  is used to test pads and lands. Tip  16  is one example of many probe tips that may be used such as a spear point tip, a flat tip, a 60° chisel tip or an 8-point crown tip, for example. The particular tip configuration may be chosen as appropriate for a particular use or application. It should be understood that the tip illustrated and the types of tips listed hereinabove are for illustrative purposes only and not as limitations or as an exhaustive list. 
   Shaft  18  extends from the tip  16  to the shoulder  20 . Shoulder  20  may be an inverted frustoconically shaped element which flares outwardly from shaft  18  to present a flange and then tapers to the body  22 . The diameter of the shaft  18  may be approximately equal to the diameter of the body  22 . The body  22  is generally elongated and extends to the tail  24  which has a diameter approximately equal to or smaller than the diameter of the plunger body  22 . 
   The spring  14  is generally helical. The first closed coils  26  are tightly wound with an inside diameter approximately equal to the diameter of the plunger shaft  18 . The first closed coils  26  transition into the open coils  28 . The open coils  28  are active with an inside diameter greater than the diameter of the plunger body  22 . The open coils  28  transition into the second set of closed coils  30 . The second closed coils  30  are tightly wound with an inside diameter equal to or slightly less than the diameter of the plunger Body  22 . Spring  14  may be symmetrical with the number of coils in and the inside diameter of the first set of closed coils  26  equal to the number of coils in and the inside diameter of the second set of closed coils  30 . A symmetrical configuration of spring  14  helps reduce the cost of manufacturing the spring  14  and assembly of the probe  10 . With a symmetrical spring  14  there is no orientation of the spring  14  for assembly. Thus, the plunger  12  may be inserted into either end of the spring  14  to reduce or eliminate assembly complexity, details and errors that may accompany assembly of asymmetrical components. 
   The probe  10  is assembled by inserting the plunger  12  into the spring  14 . The plunger tail  24  and body  22  are first inserted into the closed coils  26 . As the plunger  12  is inserted, the first closed coils  26  are forced apart by frustoconical shoulder  20  until the first closed coils contact a base  32  of the plunger tip  16 , and are seated around the shaft  18 . The shoulder  20  grip the closed coils to secure the plunger  12  in place. One or more barbs or a flange may also be used to grip the closed coils and secure the plunger  12  in place. 
   Referring to  FIG. 3 , probe  10  is shown inserted in a fixture or interposer  50 . Fixture  50  includes upper  52  and lower  54  plates. The upper fixture plate  52  includes a tip bore  56  and an open coil bore  58 , which are axially aligned. Tip bore  56  has an inside diameter greater than the diameter of the plunger tip  16  and first closed coils  26 , and less than the diameter of the open coils  28  of spring  14 . The open coil bore  58  has an inside diameter greater than the outside diameter of the open coils  28 . 
   The lower fixture plate  54  includes an open coil bore  60  and a tail bore  62 , which may be axially aligned or the tail bore  62  may be offset to ensure good electrical contact with the closed coils  30 . Open coil bore  60  and tail bore  62  have inside diameters generally equal to the open coil bore  58  and tip bore  56  of the upper fixture plate  52 , respectively. The thickness of the lower fixture plate  54  may be generally equal to the thickness of the upper fixture plate  52 . 
   To assemble the probes  10  in the fixture  50 , the closed coils  30  and tails  24  of the probes  10  are inserted into the open coil bores  60  and tail bores  62 . The closed coils  30  may extend below the lower surface  64  of the lower fixture plate  54 . The upper fixture plate  52  is then placed over the probe tips  16  with the upper fixture plate bores  56  and  58  in axial alignment with the lower fixture plate bores  60  and  62 . When assembled, the probes  10  are captured by the fixture  50 , which permits the probe tip  16  and closed coils  30  to freely move in and out of the fixture bores  62  and  56  when contacting a device under test, but retains the open coils  28 . 
   When the probes  10 A and  10 B are in contact with a device under test  70 , each plunger tail  24  extends into the respective closed coils  30  to ensure good and consistent electrical contact from the device under test  70  to the probe tip  16 , through the probe body  22  and tail  24  to the closed coils  30  and to the test unit  72 . Additionally, the probe&#39;s resistance is relatively low because the electrical path is short and only travels laterally through the closed coils  30 . The probe body  22  and/or tail  24  may be slightly curved or bent (not shown) to promote contact between the tail  24  and the closed coils  30 . 
   Referring to  FIGS. 4–6 , probe  10  is shown with an end cap  31  which may be placed over the closed coils  30  to provide a double ended probe. End cap  31  may be hollow and may be soldered to the closed coils  31  or may include barbs or an internal ridge (not shown) to secured the cap to the coils. The tip of end cap  31  is illustrated is a 120° convex tip which is used to test plated through holes, pads and lands. This tip is one example of many probe tips that may be used. The particular tip configuration may be chosen as appropriate for a particular use or application. It should be understood that the tip illustrated and the types of tips listed hereinabove are for illustrative purposes only and not as limitations or as an exhaustive list. 
   When the probes  10 A and  10 B are in contact with a device under test  70 , each plunger tail  24  extends into the respective closed coils  30  to ensure good electrical contact from the device under test  70  to the probe tip  16 , through the probe body  22  and tail  24  to the closed coils  30  and end cap  31  to the test unit  72 . The probe body  22  and/or tail  24  may be slightly curved or bent (not shown) to promote contact between the tail  24  and the closed coils  30 . 
   It is to be understood that while certain forms of this invention have been illustrated and described, it is not limited thereto, except in so far as such limitations are included in the following claims and allowable equivalents thereof.