Patent Publication Number: US-2007115014-A1

Title: Incorporation of Isolation Resistor(s) into Probes using Probe Tip Spring Pins

Description:
BACKGROUND  
      Connector-less probing has emerged as an attractive form of probing for logic analyzers and other test equipment. In connector-less probing, a customer may design their printed circuit board (PCB) to incorporate a “landing pattern” of test points. The customer then attaches a connector-less probe to their test equipment, and mounts the connector-less probe to their PCB so that a plurality of spring-pins on the probe engage the plurality of test points in their PCB&#39;s landing pattern.  
      One embodiment of a connector-less probe is disclosed in the United States Patent Application of Brent A. Holcombe, et al. entitled “Connector-Less Probe” (Ser. No. 10/373,820, filed Feb. 25, 2003). An alignment/retention device for mounting a connector-less probe to a PCB is disclosed in the United States Patent Application of Brent A. Holcombe, et al. entitled “Alignment/Retention Device For Connector-Less Probe” (Ser. No. 10/644,365, filed Aug. 20, 2003).  
      Connector-less probes for probing a plurality of breakout vias on the backside of a printed circuit board to which a grid array package is attached are disclosed in the United States Patent Application of Brock J. LaMeres, et al. entitled “Backside Attach Probe, Components Thereof, and Methods for Making and Using Same” (Ser. No. 10/781,086, filed Jul. 28, 2004).  
      Agilent Technologies, Inc. (headquartered in Palo Alto, Calif.) markets a number of connector-less probing solutions under the name “Soft Touch”.  
     SUMMARY  
      One aspect of the invention is embodied in a probe tip spring pin comprising a conductive sleeve and a plunger. The plunger is spring loaded within, and electrically coupled to, the conductive sleeve. The plunger comprises an isolation resistor.  
      Another aspect of the invention is embodied in a method for constructing a probe tip spring pin. The method comprises doping a ceramic material to give the material a resistivity sufficient to serve as an isolation resistor. A plunger is formed from the ceramic material, and is spring-loaded in a conductive sleeve. The spring-loading of the plunger electrically couples the plunger to the conductive sleeve.  
      Yet another aspect of the invention is embodied in alternate method for constructing a probe tip spring pin. The alternate method comprises constructing a plunger from first and second electrically coupled materials, at least a first of which has a resistivity sufficient to serve as an isolation resistor. The plunger is spring-loaded in a conductive sleeve, with the spring-loading serving to electrically couple the plunger to the conductive sleeve.  
      An additional aspect of the invention is embodied in a probe apparatus comprising a printed circuit board and a probe tip spring pin. The printed circuit board is provided with 1) first and second traces, 2) an isolation resistor that is embedded in at least one of the traces, 3) a via that electrically couples the first and second traces, and 4) upper and lower blind plated holes that respectively intersect the first and second traces. The probe tip spring pin is retained within the upper blind plated hole (and a fixed pin may be retained in the lower blind plated hole).  
      Other embodiments of the invention are also disclosed.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      Illustrative and presently preferred embodiments of the invention are illustrated in the drawings, in which:  
       FIGS. 1 &amp; 2  illustrate the coupling of an exemplary backside attach probe to a PCB;  
       FIG. 3  illustrates a first exemplary construction of the backside attach probe shown in  FIG. 1 ;  
       FIG. 4  illustrates, in exploded form, various exemplary layers of the probe portion shown in  FIG. 3 ;  
       FIG. 5  illustrates a first exemplary elevation of one of the probe tip spring pins shown in  FIG. 1 ; and  
       FIG. 6  illustrates an exemplary schematic of a probe tip network.  
    
    
     DESCRIPTION OF THE INVENTION  
       FIGS. 1 &amp; 2  illustrate the coupling of an exemplary backside attach probe  100  to a printed circuit board (PCB)  102 .  
      Attached to the PCB  102  is a grid array package  104 . By way of example, and as shown in the figures, the package  104  may be a ball grid array (BGA) package. However, the package  104  could also take other forms (such as that of a land grid array (LGA) package).  
      The grid array package  104  is attached to the PCB  102  at a number of pads (e.g., pad  106 ) on one side of the PCB  102 . The pads (e.g.,  106 ) to which the package  104  is attached are coupled to a plurality of breakout vias  108 ,  110 ,  112 ,  114 ,  116 ,  118  that present on a side of the PCB  102  opposite the side of the PCB to which the package  104  is attached. For purposes of illustration, each of the breakout vias  108 - 118  is shown to be bounded above and below by a somewhat thick pad (e.g., pad  106 ). Typically, however, these pads will be very thin. Also,  FIG. 1  shows that each of the breakout vias  108 - 118  is a through-hole type via. Although through-hole vias reduce the lengths of electrical paths between the package  104  and the probe  100 , the vias  108 - 118  need not be through-hole vias, and could for example, traverse only some of the layers of the PCB  102 . In this case, the breakout vias would not extend to package  104 , and would instead be coupled to package  104  by means of internal traces and/or other vias of PCB  102 . Also, if the breakout vias  108 - 118  are not through-hole type vias, they may not be vertically aligned with the contacts (e.g., solder balls) of package  104 , as shown in  FIG. 1 .  
      As shown, the probe  100  may comprise a plurality of probe tip spring pins  120 - 130 . The probe  100  may also comprise one or more mechanisms  132 ,  134  that may be used to mechanically couple the probe  100  to the PCB  102 . As shown in  FIG. 1 , probe  100  comprises two such mechanisms  132 ,  134 , and PCB  102  comprises two corresponding mechanisms  136 ,  138 . However, the number of securing mechanisms  132 - 138  on the probe  100  and PCB  102  may vary. By way of example, the securing mechanisms  132 ,  134  of the probe  100  may be pop rivets, and the corresponding mechanisms  136 ,  138  of the PCB  102  may be through-holes.  
      To probe the grid array package  104 , the probe  100  is first aligned with the plurality of breakout vias  108 - 118  (see  FIG. 1 ). The probe  100  is then moved toward the PCB  102  until its probe tip spring pins  120 - 130  engage the PCB&#39;s breakout vias  108 - 118  (see  FIG. 2 ). As the spring pins  120 - 130  engage the breakout vias  108 - 118 , they apply pressure to the breakout vias  108 - 118 , thereby ensuring good electrical connections with the breakout vias  120 - 130 . At this point, the probe  100  may be mechanically coupled to the PCB  102  to keep the spring pins  120 - 130  engaged with the breakout vias  108 - 118 . In some embodiments, movement of the probe  100  toward the PCB  102  may cause the securing mechanisms  132 ,  134  of the probe  100  to automatically engage their corresponding mechanisms  136 ,  138  on the PCB  102 . In other embodiments, the securing mechanisms  132 ,  134  may require manual engagement.  
      Providing a probe  100  with probe tip spring pins makes the probe  100  more “user friendly” by giving some relief to its user. That is, the user can worry less that he/she is pressing too hard (and damaging the probe  100 ) or too soft (and not ensuring a good electrical connection between the probe  100  and breakout vias  108 - 118 ).  
       FIG. 3  illustrates a first exemplary construction of the probe  100 . As shown, the probe  100   a  may comprise a PCB  300  having first and second circuit traces  302 ,  304  that are electrically coupled by a via  306 . Preferably, and as shown, the traces  302 ,  304  are formed as inner traces, and the via  306  is formed as a buried via. Alternately, one or both of the traces  302 ,  304  could be formed as surface traces, and the via  306  could be formed as either a blind via (i.e., a via drilled from one side of the PCB  300 ) or a through-hole via. Depending on the types of traces and via employed, the traces and via may be formed prior to, or during, assembly of the various layers of the PCB  300 .  FIG. 4  illustrates, in exploded form, various exemplary layers  400 - 428  of the probe portion shown in  FIG. 3 . Note that the layers  400 - 428  comprise alternating signal and dielectric layers (with only some of these layers being specifically referenced in  FIG. 3 ).  
      Referring again to  FIG. 3 , the PCB  300  of the probe  100   a  comprises upper and lower blind plated holes  308 ,  310  that respectively intersect the first and second traces  302 ,  304 . In one embodiment, the blind plated holes  308 ,  310  are formed by drilling first and second holes into the PCB  300 , and then spin-coating the surfaces of the holes with a conductive material.  
      After formation of the blind plated holes  308 ,  310 , a probe tip spring pin  120  is inserted into the upper blind plated hole  308 . The spring pin  120  may be retained within the hole  308  in a number of ways. For example, the hole  308  and spring pin  120  could be sized to enable press fitting of the spring pin  120 . Alternately, the body of the spring pin  120  could be soldered (or otherwise conductively adhered) to the hole&#39;s plating, or to a conductive pad formed at the mouth of the hole  308 .  
      To facilitate the attach of test instrument leads or cabling to the probe  100   a , a fixed pin  312  may be inserted into the lower blind plated hole  310 , and retained therein using any of the ways used to retain spring pin  120  in hole  308 . Although  FIG. 3  shows that the spring pin  120  and fixed pin  312  are aligned, this need not be the case. For example, the lower blind plated holes  310  of the probe  100   a  could be formed at a different pitch or in a different pattern than the probe&#39;s upper blind plated holes  308 , thereby enabling the probe  100   a  to be coupled to a particular connector of a test instrument cable, or providing more spacing between the lower blind plated holes  310  so that a user can more easily probe individual ones of a grid array package&#39;s signals. In alternate embodiments of the probe  100   a , something other than a fixed pin may be inserted in the probe&#39;s lower holes  310 .  
       FIG. 5  illustrates a first exemplary elevation of one of the probe tip spring pins  120  shown in  FIGS. 1-3 . By way of example, the probe tip spring pin  120   a  comprises a conductive sleeve  500 , a plunger  502 , and a spring  504 . The spring  504  and plunger  502  are inserted into and mechanically retained in the sleeve  500  (e.g., by detents  506 ,  508  created after the plunger  502  is inserted into the sleeve  500 ), with the spring biasing the plunger  502  with respect to the sleeve  500 . Optionally, the plunger  502  may be provided with a crown tip  510 . In one embodiment, the crown tip  510  is an integral extension of the plunger&#39;s body. In another embodiment, the crown tip  510  is soldered or otherwise bonded to the plunger&#39;s body.  
      The components of the probe tip spring pin  120   a  may be formed from various metallic or composite materials. However, all of the components  500 - 504  are conductive so that a conductive path is formed between the tip  510  of the plunger  502  and the sleeve  500 . Although the crown tip  510  shown in  FIG. 5  is outwardly flared, it need not be.  
      Although the probe  100  (possibly constructed as probe  100   a ) may be provided to a user pre-assembled, it may also be provided to a user in kit form. That is, a user may be provided with a PCB  300  (constructed as shown), a plurality of spring pins (e.g., spring pins  120   a ), and the mechanism  132 ,  134  that is used to mechanically couple the probe  100  to the PCB  102 . Preferably, the securing mechanism  132 ,  134  is pre-assembled to the PCB  300 .  
      So long as the breakout vias  108 - 118  of a PCB  100  are provided at the same pitch as the upper holes  308  of the probe  100 , the user may configure the probe  100  of a kit by inserting probe tip spring pins  120  into holes  308  that are selected to match the layout of the breakout vias  108 - 118 .  
      As shown in  FIG. 6 , the probes or leads of modern test instruments typically comprise a tip network  600 . The tip network  600  usually comprises a tip capacitor  602  and tip resistor  604  that form a compensated resistive-divider circuit with the termination impedance of a test instrument. The electrical loading on signals being probed can be reduced by placing the tip network  600  as close as possible to a target signal (i.e., a signal being probed). Placing the tip network  600  closer to a target signal also increases the quality of signals that are sensed by a test instrument (e.g., by reducing signal reflections and “ringing”). However, spatial and capacitive loading problems often make it difficult to place the tip capacitor  602  and tip resistor  604  (which is often on the order of 20 k Ω) as close to the target signal as desired. As a result, the tip network  600  will sometimes also comprise an isolation resistor  606 . The value of this isolation resistor  606  may be on the order of  125  Ω. Being of smaller size than the tip resistor  604 , and being one component instead of two, the isolation resistor  606  can often be placed much closer to a target signal than the tip capacitor  602  and tip resistor  604 .  
      Referring back to  FIG. 3 , one aspect of the invention comprises embedding an isolation resistor  606  in at least one of the traces  302 ,  304  of the PCB  300 . Although the isolation resistor  606  may be embedded in either of the traces  302 ,  304 , or may be distributed across the traces  302 ,  304  (e.g., as a combination of two series resistors), it is preferable that the isolation resistor  606  be embedded in the trace  302  that is closest to the probe tip spring pin  120 . As stated above, the closer an isolation resistor  606  can be placed to the point of probing, the greater the benefit to both 1) the device being probed, and 2) the test instrument receiving probed signals.  
      By way of example, the value of an isolation resistor  606  embedded in trace  302  may be controlled by choosing an appropriate metal and/or dimensions for trace  302 . For flying lead logic analyzers manufactured by Agilent Technologies, Inc., useful values for the isolation resistor  606  are believed to be values greater than 100 Ω, with values from 100-200 Ω being preferable, and a value of 125 Ω being ideal. Although these values may also be used in conjunction with other test instruments, the value of the isolation resistor  606  may, in some circumstances, assume other values.  
      Another aspect of the invention comprises incorporating an isolation resistor  606  into the plunger of a probe tip spring pin. One way to do this is to construct a plunger from first and second electrically coupled materials, with at least one of the materials having a resistivity that is sufficient to serve as the isolation resistor  606 . The remaining portion of the plunger may contribute to the value of the isolation resistor  606 , or may be a low resistivity conductor. By way of example, the two materials of the plunger may be electrically coupled using solder or a conductive adhesive.  
      Referring to the probe tip spring pin  120   a  shown in  FIG. 5 , the two electrically coupled materials of the plunger  502  may be the crowned probe tip  510  and body  512  of the plunger. Fabricating the crowned probe tip  510  to serve as the isolation resistor  606  would be ideal, but the irregular shape of the crown tip  510  makes it more difficult to precisely control its resistivity. Alternately, the body  512  of the plunger  502  could be fabricated to serve as the isolation resistor  606 . Given that the plunger body  512  shown in  FIG. 5  is cylindrical, it may be easy to control its impedance.  
      The plunger  502  of a probe tip spring pin  120   a  could also be formed of two electrically coupled materials by dipping one in the other. Thus, for example, the plunger  502  ( FIG. 5 ) could be manufactured as a whole (or assembled into a whole) and then dipped in a resistive fluid that coats the probe tip  510  of the plunger  502 . By way of example, the material into which the plunger  502  is dipped could be a thick film ink.  
      Another way to incorporate an isolation resistor  606  into the plunger  502  of a probe tip spring pin  120   a  is to dope a ceramic material to give the material a resistivity that is sufficient to serve as an isolation resistor. The plunger  502  may then be cut, stamped, shaped, or otherwise formed from the doped ceramic material. Alternately, only the portion of the plunger  502  that serves as the isolation resistor  606  (e.g., the body  512  of the plunger  502 ) could be formed from the doped ceramic, and the isolation resistor could then be electrically coupled to another portion of the plunger  502  (e.g., the crown tip  510 ).  
      It should be noted that spring pin plungers incorporating isolation resistors could be used in conjunction with the probes  100 ,  100   a  shown in  FIGS. 1-3 , or in any other probe using spring pins (such as those disclosed in United States Patent Application of Brent A. Holcombe, et al. entitled “Connector-Less Probe” (Ser. No. 10/373,820, filed Feb. 25, 2003), or those disclosed in United States Patent Application of Brock J. LaMeres, et al. entitled “Probes with Perpendicularly Disposed Spring Pins, and Methods of Making and Using Same” (Attorney Docket Number 10031039-1, filed on Feb. 17, 2004).  
      While illustrative and presently preferred embodiments of the invention have been described in detail herein, it is to be understood that the inventive concepts may be otherwise variously embodied and employed, and that the appended claims are intended to be construed to include such variations, except as limited by the prior art.