Patent Publication Number: US-8523579-B2

Title: Spring contact assembly

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a continuation of U.S. patent application. Ser. No. 12/912,683, filed Oct. 26, 2010, now U.S. Pat. No. 8,231,416, which is a continuation of U.S. patent application Ser. No. 12/206,659, filed Sep. 8, 2008 and issued as U.S. Pat. No. 7,862,391 on Jan. 4, 2011, which claims priority to U.S. Provisional Patent Application Nos. 60/973,370, filed Sep. 18, 2007, and 61/080,607, filed Jul. 14, 2008, the entire contents of which are hereby expressly incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to electrical contact probes forming electrical interconnects and, more particularly, to a spring contact assembly having two movable and overlapping plungers having flat contact surfaces surrounded by an external spring. 
     BACKGROUND OF THE INVENTION 
     Conventional spring loaded contact probes generally include a movable plunger and a barrel having an open end for containing an enlarged diameter section of the plunger, and a spring for biasing the travel of the plunger in the barrel. The plunger bearing slidably engages the inner surface of the barrel. The enlarged bearing section is retained in the barrel by a crimp near the barrel open end. 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. 
     The barrel, plunger and tips 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 into 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 into pressure contact with the tips of the plungers protruding through one side of the test plate or test socket for manufacturing 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 from the other side of the test plate or test socket. The test equipment transmits 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. 
     The process of making conventional spring probes 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. 
     An important aspect of testing integrated circuits is that they are tested under high frequencies. As such impedance matching is required between the test equipment and the integrated circuit so as to avoid attenuation of the high frequency signals. Considering that spacing within a test socket is minimal, in order to avoid attenuation of the high frequency signals, the length of the electrical interconnect formed by the probes must be kept to a minimum. To address this problem external spring probes have been developed having a shorter length than conventional probes. External spring probes consist of two separate sections each having a tip and a flange. A contact component extends from each probe section opposite the tip. The two contact components contact each other and the spring is sandwiched between two flanges that surround the contact components. Typically the first contact component is a barrel while the second contact component is a bearing surface. The bearing surface is slidably engaged to the inner surface of the barrel. These probes are fitted into cavities formed in the test sockets used during testing. A problem associated with these type of external spring probes is the expense to manufacture due to costly machining operations. 
     In response thereto external spring probes were designed having flat components which can be produced less expensively by stamping. Typically these designs incorporate two components which are connected orthogonally and the electrical path between the two components is through a protruding end surface. A problem with this design is that the components wear out rather quickly and have a short life span requiring constant replacement. 
     Consequently a need exists for a new spring contact assembly design that is less expensive to manufacture. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a spring contact assembly having two movable and overlapping contact members or plungers surrounded by an external spring. Each plunger has a contact portion and a tail portion wherein the tail portion has a flat surface that passes over and makes contact with an opposing plunger tail portion inside the spring when assembled. The spring has end coils that press onto each of the opposing plungers to prevent the plungers from separating from the spring, thus fixing the plunger contact portion and the tail portions with respect to each end of the spring. Utilizing the natural torsional movement of the spring while it is compressed, the flat surfaces of the plunger tail portions maintain contact throughout the compression stroke of the contact assembly. The contact between the opposing flat sections prevents the twisting or torsional movement of the spring from translating to the tips on the contact portions. The opposition to the natural twisting enhances the electrical conductivity of the components, which in turn improves performance of the spring contact assembly. The spring can also have reduced diameter coil sections along the length of the spring to further constrain the plunger tails and enhance the interaction between the two plungers, or further biasing effect can be created by adding an offset coil section in the spring. 
     The flat surface on the tail portion of the plunger would normally be formed parallel at the midline of the cylindrical tail portion diameter for each plunger. In an alternative embodiment, the flat surface may be formed parallel above the midline of the cylindrical tail diameter to increase the resulting combined thickness of the assembly, creating additional interaction between the two plungers. In further embodiments the flat surface on the tail portion may be formed at either an angle to or in a helix about the midline of the cylindrical tail portion diameter. 
     In yet another embodiment, one plunger includes an essentially flat tail portion, centrally located along the axis of the component, and the opposite plunger has a mating slot which receives the flat tail portion of the opposite plunger. This design allows for two edges to be in slidable contact engagement, thus enhancing resistance performance. 
     Each of the plungers may be formed in a general cylindrical shape, suitable for lathe, screw machine or other similar manufacturing equipment. Alternatively the plunger may be formed in a generally flat shape, suitable for stamping, etching, photolithography or other similar manufacturing technique for creating substantially two dimensional geometries. 
     For generally flat shaped plungers, the plunger tail section may have a portion that extends beyond the opposite ends of the coils of the spring. This facilitates enhanced electrical contact and adds additional support to the opposite plunger tip or contact portion. The plunger tail portion that extends past the opposite end of the spring can have the edges on one side reduced to provide for maximum material utilization. A slot may also be provided in the spring interference area of the flat pattern version of the plunger to provide additional compliance, absorbing tolerance while providing a reliable press fit. Flat plunger designs also can benefit from having the plungers generally sliding together at an inclined configuration, thus transferring some of the axial forces supplied by the external helical spring to a perpendicular direction, which is normal to, mating plunger surfaces. This normal surface force enhances intimate electrical contact between the components. Methods for maintaining the probe together include enlarged tail portions extending past reduced diameter center coils and interlocking tabs on the contact portions. Kelvin configurations are also possible having two separate electrical paths isolated from one another within a single probe. 
     Further benefits provided by flat geometry allow multiple plungers to be easily fabricated as part of a lead frame assembly, facilitating plating and assembly in high volumes. Completed assemblies can also be supplied attached to a lead frame, making it easier for the end user to load the probe assembly into a finished test fixture or socket. Tip configurations can be single point, multiple point or three dimensional. 
     The present invention also contemplates a spring contact assembly having a cylindrical plunger in combination with a flat plunger. This hybrid combination can provide for cylindrical tip geometries to address sufficient testing for solder ball or other geometries while reducing the manufacturing costs for a fully cylindrical contact assembly by incorporating a flat plunger as the opposite plunger. These and other aspects of the present invention will be more fully understood with reference to the detailed description in combination with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the spring contact assembly of the present invention; 
         FIG. 2  is a side view of a first plunger of the spring contact assembly of  FIG. 1 ; 
         FIG. 3  is a side view of an alternative plunger design of the present invention; 
         FIG. 4  is a side view of a second plunger of the spring contact assembly of  FIG. 1 ; 
         FIG. 5  is a side view of a spring of the spring contact assembly of  FIG. 1 ; 
         FIG. 6  is a cross-sectional view of the spring contact assembly of  FIG. 1 ; 
         FIG. 7  is a side view of an alternative plunger design of the present invention; 
         FIG. 8  is a side view of an alternative plunger design of the present invention; 
         FIG. 9  is an axial cross-sectional view of the spring contact assembly of  FIG. 1  inserted into a test socket; 
         FIG. 10  is a perspective of an alternative embodiment spring contact assembly of the present invention; 
         FIG. 11  is a side view of one plunger of the spring contact assembly of  FIG. 10 ; 
         FIG. 12  is a schematic illustration of a stamping procedure for the plunger of  FIG. 11 ; 
         FIG. 13  is a side view of a second plunger of the spring contact assembly of  FIG. 10 ; 
         FIG. 14  is a cross-sectional view of the spring contact assembly of  FIG. 10 ; 
         FIG. 15  is an axial cross-sectional view of an alternative spring contact assembly of  FIG. 10 ; 
         FIG. 16  is a perspective view of an alternative spring contact assembly of the present invention; 
         FIG. 17  is a perspective view of another alternative embodiment spring contact assembly of the present invention; 
         FIG. 18  is a side view of another alternative embodiment spring contact assembly of the present invention; 
         FIG. 19  is a perspective view of another alternative embodiment spring contact assembly of the present invention; 
         FIG. 20  is a perspective view of another alternative embodiment spring contact assembly of the present invention; 
         FIG. 21  is a perspective view of another alternative embodiment Kelvin measurement spring contact assembly of the present invention; 
         FIG. 22  is a perspective view of three dimensional contact tip embodiment of the present invention; 
         FIG. 23  is a perspective view of a spring contact assembly with the tip configuration of  FIG. 22 . 
         FIG. 24  is a perspective view of an alternative contact tip design of the present invention; 
         FIG. 25  is a perspective view of  FIG. 24  illustrating a three dimensional contact tip; 
         FIG. 26  is a perspective view of an alternative contact tip design; and 
         FIG. 27  is a perspective view of  FIG. 26  illustrating a three dimensional contact tip. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1-11  illustrate a first embodiment spring contact assembly  10  of the present invention. The spring contact assembly  10  includes a first contact member or plunger  12 , a second contact member or plunger  14 , and a spring  16 . As shown in  FIG. 2 , plunger  12  includes a contact portion  18  and a tail portion  20 . A contact tip  22  is positioned at an end of the contact portion  18  and can have multiple contact geometries as also shown in  FIG. 3  and  FIG. 4 . A flange  24  is positioned between contact portion or section  18  and tail portion or section  20 . Flange  24  has a flat face  26  used for aligning the probe during assembly. The plunger tail portion  20  has a cylindrical surface  28  and a flat surface  30  extending along its length. 
     Plunger  14  as shown in  FIG. 4  also includes a contact portion or section  32  and a tail portion or section  34 . A flange  36  is positioned between contact section  32  and tail section  34  and also includes a flat surface  38  for positioning plunger  14  during assembly. Tail section  34  has a cylindrical surface  40  and a flat surface  42  extending along its length. Flat surfaces  30  and  42  pass over one another and make contact inside of spring  16  when assembled, as also seen in  FIG. 9 . Flat surfaces  30  and  42  increasingly engage one another during compression of the assembly. 
     As shown in  FIG. 5  the spring  16  has end coils  44  and  46  at opposite ends of the spring that press onto plunger tail sections  20  and  34  at cylindrical sections  48  and  50  adjacent flanges  24  and  36 . The end coils  44  and  46  have a slightly smaller diameter and therefore firmly grip cylindrical sections  48  and  50  to prevent the plungers from separating from the spring, thus fixing the plunger tips  22  and  52  and flat surfaces  30  and  42  with respect to each spring end. Utilizing the natural torsional movement of the spring  16  while it is compressed, the flat portions  30  and  42  of the plungers maintain contact throughout the entire stroke of the probe as also shown in  FIG. 6 . The contact between the opposing flat sections prevents twisting or torsional movement from translating it to the spring contact tips  22  and  52 . The opposition to the natural twisting enhances the electrical conductivity of the components, which in turn improves the performance of the contact. 
     The flat section  30  as shown in  FIGS. 2 and 3  can be formed parallel at the midline of the cylindrical tail portion diameter for each plunger or the flat section  42  may be formed parallel above the midline  54  of the cylindrical tail diameter for each plunger as shown in  FIG. 4  to increase the resulting combined thickness of the assembly, creating additional interaction between two plungers in an assembly. Although  FIGS. 1-4  show three different plunger tip designs, it is to be understood that any plunger tip design could be utilized depending upon its particular application. To further constrain the tail sections  20  and  34  and enhance the interaction between the two plungers, the spring  16  can employ reduced coil sections  56  as shown in  FIG. 5 . Further biasing effect can also be created by offsetting coil sections  56 . 
     Alternative designs for the plunger tail section can include the flat surface  58  formed in a helix about the midline of the cylindrical tail diameter as shown in  FIG. 7  or the flat section  60  may be formed at an angle to the midline of the cylindrical tail diameter as shown in  FIG. 8 . As with all of the various plunger designs, the tail section may have a reduced end section  62  that allows the spring to be threaded onto the tail portion before being press fit on the reduced diameter section adjacent the flange. The reduced section  62  allows the plunger to pilot into the spring, easing the assembly process. As previously indicated, the cylindrical sections  48  and  50  of the plunger tails creates an interference fit with the end coils of the spring and the gripping force created between the cylindrical sections and the end coils is sufficient to keep the assembly together during normal handling and use and in combination with the flat surfaces resist normal torsional forces applied by the spring. The generally cylindrical plunger designs of  FIGS. 1-9  are manufactured by machinings such as a lathe, screw machine or other similar manufacturing equipment. 
     An alternative spring contact assembly  70  is illustrated in  FIGS. 10-15 . Spring contact assembly  70  includes two movable and overlapping plungers  72  and  74  surrounded by an external spring  76 . Plungers  72  and  74  are formed in a generally flat shape, suitable for stamping, etching, photolithography or other similar manufacturing technique for creating substantially two-dimensional geometries as generally referenced as  78  in  FIG. 12 . An additional benefit of flat plunger geometry allows multiple plungers to be easily fabricated as a part of a lead frame assembly, facilitating plating and assembly in high volume. Completed assemblies can also be supplied attached to a lead frame, making it easier for the end user to load the probe assembly into a finished test fixture or socket  110  (shown in  FIG. 9 ). 
     Plunger  72  includes a contact portion or section  80  and a tail portion or section  82 . Contact section  80  includes a contact tip  84  which can be any of a number of geometrical configurations. Considering the entire plunger has a flat configuration, plunger tail section  82  includes a flat surface  86 . A flange  88  is positioned between contact section  80  and tail section  82 . Tail section  82  includes an enlarged portion  90  for creating an interference fit with end coils of the spring  76  to retain the spring contact in its assembled configuration. Mating plunger  74  also includes a contact portion or section  92 , a tail portion or section  94 , and a flange  96  positioned between the contact section and tail section. Tail section  94  includes an enlarged portion  98  for creating an interference fit with the end coils of spring  76 . 
     In the flat configuration spring contact assembly  70 , the plunger tail sections  82  and  94  may have an end portion  100  that extends past the end coils of the spring as shown in  FIG. 10 . This design enhances the electrical contact between the plungers and adds support to the opposite plunger tip. One or both of the plunger tails can extend beyond the end coils for a particular application. As shown in  FIG. 11  and  FIG. 14  the end portion  82  which may extend beyond the end coils of the spring may have the corner edges  102  removed allowing for maximum material utilization. 
     As shown in  FIG. 13  plunger  74  may include a slot  106  in the spring interference area of the plunger to provide additional compliance, absorbing tolerance while providing a reliable press fit for the end coil. As shown in  FIG. 15  the flat plunger design benefits from the new result of having the plungers  72  and  74  generally sliding together in an inclined configuration with respect to a midline axis  108 . The plungers sliding together at an incline configuration transfers some of the axial forces supplied by the external helical spring  76  to a perpendicular direction, or normal to, the mating flat plunger surfaces. The normal force enhances intimate electrical contact between the plungers as shown in  FIG. 15 . 
     Yet another alternative embodiment spring contact assembly  112  is shown in  FIG. 16  comprising mating plungers  114  and  116  and an external helical spring  118 . In this configuration, plunger  114  has a flat tail portion  120  and plunger  116  has a tail portion  122  having a feature with internally opposed flat surfaces  124  for receipt of flat tail  120 . Tail portion  120  is centrally located along the axis of plunger  114  and the feature  124  is centrally located along the axis of plunger  116 . Spring contact assembly  112  allows for two flat edges  126  and  128  to be received in slidable contact with flat edges within feature  124 . This design provides for enhances resistance performance of the spring contact assembly. 
       FIG. 17  illustrates yet another alternative embodiment spring contact assembly  130  which is a hybrid combination of the spring contact assemblies of  FIGS. 1 and 10 . Spring contact assembly  130  has a cylindrical plunger  132  and a flat plunger  134  in slidable contact within an external helical spring  136 . Tail portion  138  of plunger  132  has a flat surface  140 , and tail portion  142  of plunger  134  also has a mating flat surface  144 . Flat surfaces  140  and  144  are in slidable engagement. 
       FIG. 18  illustrates another alternative embodiment spring contact assembly  146  of the present invention. The spring contact assembly  146  is a flat configuration having a first contact member  148  and a second contact member  150 . Each of the tail portions  152  and  154  of contact members  148  and  150  include an enlarged tail section  156  and  158  which pass through reduced diameter center coil sections  160  of helical spring  162 . The enlarged tail portions are opposite contact tips  164  and  166  of each contact member. The enlarged tail section passes through the reduced diameter coils and the force of the spring is sufficiently low such that the contact members do not disengage from the spring. 
       FIG. 19  illustrates another alternative embodiment spring contact assembly  168  which illustrates another alternative method for retaining contact members  170  and  172  to helical spring  174 . In this embodiment, the center sections  176  and  178  of tail portions  180  and  182  respectively, are pierced through forming a tab  184 ,  186 , which when contact members  170  and  172  are assembled within the spring  174 , the tabs interlock. 
       FIG. 20  illustrates another alternative spring contact assembly  188  which illustrates another alternative for connecting the contact members. Assembly  188  includes a first contact member  190  and a second contact member  192  having tail portions  194  and  196  positioned within a helical spring  198 . The center section  200  and  202  of the tail portions have a non-centered tab  204  and  206  which when assembled interlock. Tabs  204 ,  206  are formed by a forming or folding operation. 
       FIG. 21  illustrates yet another alternative embodiment spring contact assembly  208  which is a Kelvin measurement configuration. Assembly  208  has two separate electrical paths within a single spring  210 . This is accomplished by having two separate contact members  212  and  214  adjacent one another. Members  212  and  214  are electrically isolated from each other and from spring  210  by having a non-conductive coating positioned upon adjacent surfaces. Contact member  212  has a first contact segment  216  and a second contact segment  218 . Similarly, contact member  214  has a first contact segment  220  and a second contact segment  222 . Each of segments  216  through  222  have a tail portion  224 ,  226 ,  228  and  230 , respectively. Tail portions  224 ,  226 ,  228  and  230  have conductive surfaces  232  such that only sections  224  and  226  electrically communicate with one another and only sections  228  and  230  electrically communicate with one another. Tail sections  224  through  230  are positioned within spring  210 . Assembly  208  has two separate electrical paths within a single spring wherein each contact path consists of two slidably engaging tail portions and each contact path is electrically isolated from its neighboring contact path. 
       FIGS. 22 and 23  disclose another alternative embodiment spring contact assembly  234  having a first contact member  236  and a second contact member  238 . Contact member  236  includes sections  240  and  242  each having a slot  244  so that sections  240  and  242  can engage one another to create a three dimensional contact tip  246 . Contact tip  246  comprises four contact points  248 . Sections  240  and  242  are illustrated as being connected perpendicularly, however, it is to be understood that slots  244  can be machined such that the two sections can be assembled at an angle other than perpendicular. Contact section  236  and contact section  238  have tail portions  250  and  252  which are positioned within spring  254 . 
       FIGS. 24 and 25  illustrate an alternative embodiment three dimensional contact tip configuration. Contact member  256  includes a tail portion  258  and a contact tip portion  260 . Contact member  256  is a flat configuration as shown in  FIG. 24  and contact tip  260  can have a three dimensional configuration as shown in  FIG. 25  by bending or otherwise deforming the flat contact tip into a three dimensional shape. It is to be understood that the contact tip configuration although illustrated with three contact points  262 ,  264  and  266  can have any number of contact points depending upon the particular application. The tip geometry can be V-shaped, U-Shaped or other shapes to transform an otherwise two dimensional contact tip into a three dimensional contact tip. This can be done by bending or other forming type operations. Similarly,  FIGS. 26 and 27  illustrate yet another alternative three dimensional contact tip design for contact member  268 . Contact member  268  initially is formed in a flat two dimensional configuration as shown in  FIG. 26  wherein contact member  268  includes a tail portion  270  and a contact tip portion  272 . As shown in  FIG. 27 , contact tip portion  272  is folded upon itself to form the three dimensional configuration. In this embodiment, four contact points  274 ,  276 ,  278  and  280  are illustrated. 
     Although the present invention has been described and illustrated with respect to several embodiments thereof it is to be understood that changes and modifications can be made therein which are within the full scope of the invention as hereinafter claimed.