Patent Publication Number: US-7902850-B2

Title: Versatile materials probe

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a Continuation application of U.S. Ser. No. 11/551,062, filed Oct. 19, 2006 now U.S. Pat. No. 7,508,226, the contents of which are incorporated by reference herein in their entirety. 
    
    
     TRADEMARKS 
     IBM® is a registered trademark of International Business Machines Corporation, Armonk, N.Y., U.S.A. Other names used herein may be registered trademarks, trademarks or product names of International Business Machines Corporation or other companies. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to measurement of material properties, and particularly to a versatile probe for measuring electrical impedance of materials. 
     2. Description of Background 
     The composition of materials used in electronic devices is determined for a variety of reasons. For example, a European Union directive, “Restriction of Hazardous Substances” (RoHS), bans the placing on the EU market of new electrical and electronic equipment containing more than agreed levels of lead, cadmium, mercury, hexavalent chromium, polybrominated biphenyl (PBB) and polybrominated diphenyl ether (PBDE) flame retardants, and material composition is determined to ensure compliance with RoHS. One method of determining the material composition is by measuring the electrical impedance of the material. Traditionally, such impedance measurements have been accomplished through the use of a single-point or surface probe resulting in an impedance value of the material measured in ohms/square. 
     A measurement is performed by placing the single-point or surface probe on the material. The measurement can be influenced by an amount of force applied to the probe when placing it on the surface. However, the measured impedance can be erroneously high or low if the applied force is not sufficiently controlled. Furthermore, when taking several measurements on a material sample, the applied force must be consistent to ensure the measured impedance values are comparable. In a traditional probe, a nickel plated gasket surrounds the probe and is intended to control the amount of force applied to the probe. The gasket, however, wears or changes characteristics over time requiring its replacement. Additionally, the gasket often deforms to a compressed state over time which can influence the amount of force applied to the probe, thereby changing the impedance measurements. 
     Finally, the standard probe is limited to taking one particular type of measurement (e.g. ohms/square) and only one measurement at a time. To measure more than one location on a sample, the probe must be moved. To take measurements requiring more than one interface to the material sample, more than one probe or some additional devices must be used in addition to the standard probe. 
     What is needed is a measurement probe that can accurately control the force applied to the probe during measurement. In addition, a probe is needed that is versatile, allowing for many measurements to be performed without moving the probe, and allowing for differing types of measurements to be performed without the use of additional probes or other ancillary equipment to facilitate the measurement. 
     SUMMARY OF THE INVENTION 
     The shortcomings of the prior art are overcome and additional advantages are provided through the provision of an electrical measurement probe comprising two probe blocks, each probe block having a connection face and a measurement face. Each probe block includes a plurality of spring loaded pogo pins. Each pogo pin having a first end that extends to the connection face and a second end that protrudes from the measurement face. The two probe blocks are attached to a top plate. The top plate is attached to a face of each probe block opposite to the measurement face of the probe block. 
     Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with advantages and features, refer to the description and to the drawings. 
     TECHNICAL EFFECTS 
     As a result of the summarized invention, technically we have achieved a solution which improves measurement versatility by providing a plurality of electrically conductive pogo pins, and improves measurement accuracy and repeatability by controlling the maximum travel of each of the spring loaded pogo pins. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  illustrates a side view of one example of a versatile materials probe. 
         FIG. 2  is a perspective view of a plurality of L-shaped pogo pins. 
         FIG. 3  is a section view of an L-shaped pogo pin. 
         FIG. 4  illustrates a perspective view of one example of a versatile materials probe including an adjustment knob. 
         FIG. 5  illustrates one example of load cells attached to the probe blocks. 
         FIG. 6  illustrates one example of a versatile probe configured for ohms/square measurement. 
         FIG. 7  illustrates one example of a versatile probe configured for statistical data collection. 
         FIG. 8  illustrates one example of a versatile probe configured for measurement using an Anderson loop method. 
         FIG. 9  illustrates an alternative embodiment of a versatile probe configured for measurement using an Anderson loop method. 
     
    
    
     The detailed description explains the preferred embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to the drawings in greater detail, it will be seen that in  FIG. 1  there is an improved materials probe  10 . The probe  10  comprises a pair of probe blocks  12  connected to a top plate  14 . Each probe block  12  is cube-shaped and comprises a measurement face  16  opposite the face of the probe block  12  which is connected to the top plate  14 , and a connection face  18  which is adjacent to the measurement face  16  and substantially parallel to a longitudinal face  20  of the top plate  14 . 
     Each probe block  12  contains an array of commercially available pogo pins  22 . Each pogo pin  22  is formed from a conductive material and, for example, is nickel plated. Referring to  FIG. 2 , the pogo pins  22  are L-shaped and comprise a fixed end  24  and a moveable end  26  enclosed in a shell  28 . As shown in  FIG. 3 , the moveable end  26  is spring-loaded, and electrical connection between the moveable end  26  and the fixed end  24  is provided by a spring  30 . Returning now to  FIG. 1 , the fixed end  24  of each pogo pin  22  is disposed on the connection face  18 , and the moveable end  26  of each pogo pin  22  is disposed through the measurement face  16 . Spring-loading the moveable end  26  allows the pogo pins  22  to absorb force applied to the probe  10  when measuring a material. 
     The top plate  14  includes a through hole  32 . As shown in  FIG. 4 , an adjustment knob  34  is disposed on top of the top plate  14 . A threaded portion  36  extends from a bottom face of the adjustment knob  34  and through the hole  32  in the top plate  14  and further extends between the two probe blocks  12 . An adjustment block  38  is disposed between the two probe blocks  12  and includes a threaded hole on its top face. The threaded portion  36  is threaded into the adjustment block  38 , so that when the adjustment knob  34  is turned in a clockwise direction, the adjustment block  38  is raised toward the top plate  14  and when the adjustment knob  34  is turned in a counter clockwise direction, the adjustment block  38  is lowered away from the top plate  14 . 
     The height of the adjustment block  38  relative to the measurement face  16  controls the amount of force applied when a measurement is taken. The greater an offset  40  between a stop face  42  and the measurement face  16 , the smaller the distance that the moveable ends  26  of the pogo pins  22  will travel before the stop face  42  rests on the material, and the lower the amount of force. Conversely, the smaller the offset  40 , the greater the distance moveable ends  26  of the pogo pins  22  will travel, and the greater the amount of force. Incorporation of the adjustment block  38  ensures that substantially equal amounts of force are applied when taking measurements, resulting in more accurate measurements since a known source of error (a variation in force applied to the probe) has been removed. 
     To set the applied force to a known value, as shown in  FIG. 5 , a load cell  44  can be fitted to each probe block  12 . When pressure is applied to the pogo pins  22 , the pressure is measured as strain in the load cell  44 . The strain measurement is outputted and translated into a force value. The position of the adjustment block  38  can be adjusted until the desired force value is measured. 
     The number of pogo pins  22  in the probe blocks  12  allows the probe  10  to be configured to perform a variety of measurements. For example, as shown in  FIG. 6 , by configuring each probe block  12  with one connection plate  46  on the connection face  18 , the fixed ends  24  of the pogo pins  22  in each probe block  12  are connected to a single lead wire  48 . With this configuration, the probe  10  is able to provide a measurement similar to a conventional ohms/square probe. Alternatively, and as shown in  FIG. 7 , if individual connections  50  are provided to the fixed ends  24  of each pogo pin  22 , multiple measurements can be obtained and statistically combined to obtain, for example, mean and standard deviation of the measurements without moving the probe. This configuration also allows for connection of the probe  10  to a voltage network analyzer, and a number of measurements may be performed without moving the probe  10 . 
     In one embodiment, the probe  10  is configured to test the impedance of one material compared to the impedance of a reference material. To have this capability, as shown in  FIG. 8 , an electrically conductive contact plate  52  is disposed at the stop face  42 . When comparing the impedance of two materials, a first probe block  12   a  is applied to a first material, a second probe block  12   b  is applied to a second material, and the contact plate  52  is disposed such that a first rib  54  contacts the first material and a second rib  56  contacts the second material. The provision of the contact plate  52  enables the probe  10  to compare the impedance of two materials via an Anderson loop method. In this case, a first row  58  of pogo pins  22  of each probe block  12  provides current injection or removal, a second row  60  and a third row  62  of pogo pins  22  of each probe block  12  are used to measure the voltage drop in each material, and the contact plate  52  provides current continuation between the first material and the second material. The resulting measured voltage drop across each material can be compared to determine the materials&#39; relative impedance. 
       FIG. 9  illustrates another embodiment of the probe  10  that may be configured to test the impedance of one material compared to a reference material. In this embodiment, each probe block  12  requires at least twelve pogo pins  22  disposed in four rows of three pogo pins  22  each. In this embodiment, there is no adjustment block  38  between the probe blocks  12 , so the probe blocks  12  can move independently. When comparing the impedance of two materials, a first probe block  12   a  is applied to a first material, and a second probe block  12   b  is applied to a second material. Because the two probe blocks  12   a  and  12   b  can move independently, the first material and the second material can be of different thicknesses. In order to control the force applied to the probe in this instance, each probe block  12   a  and  12   b  includes a sleeve  64  which extends from the measurement face  16 . The sleeve  64  is disposed with an offset  40  to the measurement face  16  to control the amount of force applied as described above. To measure impedance, this embodiment may utilize an Anderson loop method. In this case, a first row  58  of pogo pins  22  of each probe block  12  provides current injection or removal, a second row  60  and a third row  62  of pogo pins  22  of each probe block  12  are used to measure the voltage drop in each material, and a fourth row  66  provides current continuation between the first material and the second material. The resulting measured voltage drop across each material can be compared to determine the materials&#39; relative impedance. 
     While the preferred embodiment to the invention has been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.