Abstract:
An electrical component handler that tests electrical circuit components and includes a self-cleaning lower contact offers reduced yield loss and mean time between assists. A preferred embodiment of the electrical component handler includes multiple sets of upper and lower contacts, each set of which is spatially aligned to electrically contact a single device-under-test (DUT). Each DUT is seated in a test plate that transports the DUT to and from a test measurement position between the upper and lower contacts. The lower contact includes a contact tip that a biasing mechanism urges against the electrical component as it undergoes a test process and against a surface of the test plate as it transports the electrical component. The lower contact rubs against the test plate, thereby contributing to removal of contaminant material acquired by the contact tip during component handler operation.

Description:
TECHNICAL FIELD  
       [0001]     The invention relates to electrical component handlers that test electrical circuit components, and in particular, to a self-cleaning lower contact for use in an electrical component handler.  
       BACKGROUND OF THE INVENTION  
       [0002]     Electrical component handlers receive electrical circuit components, e.g., ceramic capacitors, present the electrical circuit components to an electronic tester for testing, and sort the electrical circuit components according to the results of the testing. An exemplary electrical component handler is described in U.S. Pat. No. 5,842,579 to Garcia et al. (the &#39;579 patent), which is assigned to Electro Scientific Industries, Inc., the assignee of the present patent application. Design and operational advantages of the electrical component handler of the &#39;579 patent include (1) the elimination of manual seating of components for test purposes and manual sorting; (2) the ability to handle a greater quantity of components per unit time than prior art electrical component handlers are able to handle; (3) the ability to take a randomly oriented heap of components and properly orient them; (4) the ability to present the components to a tester in multiples; and (5) the ability to sort the tested parts into a plurality of receiving or sorting bins.  
         [0003]      FIG. 1  is a pictorial drawing of an electrical component handler  2  as described in the &#39;579 patent. In electrical component handler  2 , one or more concentric rings  3  of component seats  4  formed in an annular test plate  5  are rotated in a clockwise direction around a turntable hub  6 . As test plate  5  turns, component seats  4  pass beneath a loading area  10 , a contact head  11  of five contact modules  12  (two shown in  FIG. 1 ), and an ejection manifold  13 . In loading area  10 , electrical circuit components or devices-under-test (DUTs)  14  ( FIG. 3 ) are poured into concentric rings  3 , causing unseated DUTs  14  to tumble randomly until they are seated in test plate seats  4 . DUTs  14  are then rotated beneath contact head  11 , and each DUT  14  is electrically contacted and parametrically tested. Once DUTs  14  have been tested, ejection manifold  13  ejects DUTs  14  from their seats by blasts of air from selectively actuated, spatially aligned pneumatic valves. Ejected DUTs  14  are preferably directed through ejection tubes  15   a  into sorting bins  15   b.    
         [0004]      FIGS. 2 and 3  show prior art contact head  11  of the &#39;579 patent in greater detail. Specifically,  FIG. 2  shows a pictorial drawing of contact head  11  with less than a full complement of contact modules  12  mounted thereon; and  FIG. 3  is a fragmentary sectional view taken along lines  3 - 3  of  FIG. 2  juxtaposed with a fragmentary cross-sectional view of a DUT  14  seated in test plate  5 . With reference to  FIGS. 2 and 3 , contact module  12  includes a plurality of upper contacts  16  and lower contacts  18  (one each shown in  FIG. 3 ) for coupling DUT  14  to test plate  5 . Upper contacts  16  are resilient flat metal cantilevered leaves with inclined elongated tips that project away at a shallow angle from test plate  5 . Upper contacts  16  flex slightly when they encounter seated DUTs  14  to provide a downward contact force that is largely dictated by the thicknesses and/or end widths of the leaves. The elongated tips prevent seated DUTs  14  from popping out of their seats (as a consequence of a “tiddlywink” effect) as the leaves pass over the back edges of DUTs  14  as test plate  5  advances forward. The tips of upper contacts  16  may be coated with a metal alloy to minimize contact resistance.  
         [0005]     Lower contacts  18  are typically stationary contacts in the form of cylinders. As shown in  FIG. 4 , an exemplary prior art lower contact  18  is an elongated cylinder having upper and lower planar surfaces, a central conductive core  22 , and an electrically insulating outer sleeve  24 . Lower contact  18  extends through holes  30  formed in a vacuum plate  32  and set between adjacent vacuum channels  34  such that lower contact  18  is in alignment with its corresponding upper contact  16  and its corresponding component seat ring  3 . A base member  36  positioned below vacuum plate  32  includes an upwardly projecting wall  38  formed of contiguous cylindrical scallop segments  40  that receive a row of cylinders  18 . A releasable clamping mechanism  42  pushes and thereby pins outer sleeves  24  of lower contacts  18  against their associated scallop segments  40  of wall  38  to maintain their orientation normal to test plate  5 . Thus, for each row of lower contacts  18 , there is a clamping mechanism and a pinning wall. A corresponding plurality of spring-biased pin contacts  44  (e.g., “pogo” pins) extends through a plurality of slots (not shown) in the bottom of base member  36  to make electrical contact with central cores  22  of lower contacts  18 . There is one base slot for each row of lower contacts. Pin contacts  44  are preferably mounted lengthwise by their spring-biased ends in holders  46 , four for each holder to match a row of lower contacts  18 . Each holder  46  is affixed in a different base slot. Pin contacts  44  are coupled to the tester electronics through wires  48 .  
         [0006]     Contact head  11  includes five contact modules  12 . This embodiment includes 20 upper contacts  16 , five for each ring  3  of component seats  4 . Each of 20 lower contacts  18  is positioned on the opposite side of test plate  5  and in alignment with a different one of the 20 upper contacts  16 , as indicated in  FIG. 3  for one pair of upper and lower contacts. Thus, contact head  11  includes a full complement of contact modules  12  in which the terminals of 20 DUTs  14  can be contacted simultaneously, thereby simultaneously coupling all 20 of them to test plate  5 .  
         [0007]     Upper and lower contacts  16  and  18  of contact modules  12  become contaminated during operation of electrical component handler  2 . Exemplary contamination sources include friction polymerization; external debris, such as material deposits from previously tested devices; and naturally occurring oxide formation on the contact surface. Additionally, some amount of debris, such as broken devices, plating media, or fragments of refractory carriers, is typically present in or on DUTs  14 . This debris is often introduced into the test system and subsequently placed in contact with lower contacts  18 . Contamination of upper and lower contacts  16  and  18  creates contact resistance variation that is additive to the actual resistance measurement for each DUT  14 . This contamination of upper and lower contacts  16  and  18  results in rejection of acceptable DUTs  14 , resulting in yield loss and a reduction in the mean time between assists (MTBA) associated with electrical component handler  2 . When stationary lower contacts  18  are used in electrical component handler  2 , about 5.89% of DUTs  14  are falsely rejected.  
         [0008]     Consequently, periodic cleaning of upper and lower contacts  16  and  18  is required to facilitate accurate DUT measurement. The most common prior art method of cleaning upper and lower contacts  16  and  18  entails stopping operation of electrical component handler  2  and mechanically cleaning upper and lower contacts  16  and  18 . However, stopping electrical component handler  2  results in lost productivity and reduces machine throughput by lowering the MTBA.  
         [0009]     Another prior art method of removing contamination and debris entails the use of jam sensors or jam-clearing mechanisms. Implementing these additional devices increases the manufacturing and repair costs, as well as the mechanical complexity, of electrical component handler  2 .  
         [0010]     Thus a need exists for a device that is capable of carrying out an effective and efficient method of cleaning the contacts of an electrical component handler.  
       SUMMARY OF THE INVENTION  
       [0011]     The present invention is a device that carries out an effective and expedient method of cleaning a lower contact of an electrical component handler during its operation and thereby reducing yield loss and increasing the MTBA associated with the electrical component handler.  
         [0012]     A preferred electrical component handler includes a contact head having multiple sets of associated upper and lower contacts. The upper contact and the lower contact in each set are spatially aligned to electrically contact the terminals of a single DUT. Each DUT is seated in a test plate that transports the DUT to and from at least one testing area in the component handler. The test plate has upper and lower surfaces that are positioned adjacent the upper and lower contacts, respectively. The upper contact exerts a downward force against the upper surface of the test plate as it transports the DUT and against one terminal of the DUT as it undergoes a test process. The lower contact includes a housing and an extensible contact element. The housing is preferably formed of an electrically insulating material, and the extensible contact element is movable within and outwardly of the housing and has an end portion that terminates in a contact tip. A biasing mechanism applies a force to the contact tip to urge the contact tip against a terminal of the DUT as it undergoes the test process and against the lower surface of the test plate as it transports the DUT. The urging of the contact tip against the lower surface of the test plate contributes to maintaining the DUT in a measurement position and to removal of contaminant material acquired by the contact tip during component handler operation. A roller type upper contact obviates a need for rubbing against the upper surface of the test plate to keep the upper contact free from contaminant material.  
         [0013]     Additional aspects and advantages of this invention will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a pictorial view of an exemplary prior art electrical component handler.  
         [0015]      FIG. 2  is a pictorial view of a prior art contact head assembly to which are mounted less than a full complement of contact modules.  
         [0016]      FIG. 3  is a fragmentary sectional view taken along lines  3 - 3  of  FIG. 2  juxtaposed with a fragmentary cross-sectional view of a DUT seated in a test plate.  
         [0017]      FIG. 4  is a pictorial view of the test plate of the prior art electrical component handler of  FIG. 1 .  
         [0018]      FIG. 5A  is an isometric fragmentary view of test and vacuum plates with portions broken away to show a DUT in position between upper and lower contacts to undergo a test measurement.  FIG. 5B  is an enlarged, fragmentary side elevation view showing partly in cross-section the lower contact positioned against the DUT undergoing the test measurement depicted in  FIG. 5A .  FIG. 5C  is an enlarged, fragmentary isometric view of the DUT in position between the upper and lower contacts to undergo the test measurement depicted in  FIG. 5A .  
         [0019]      FIG. 6  is a fragmentary side elevation view showing partly in cross-section the arrangement of the test and vacuum plates and components of the lower contact as it and a roller type upper contact maintain a DUT in position in a test plate seat.  
         [0020]      FIG. 7A  is an isometric view of a double-sided, elastic lower contact configured in accordance with a preferred embodiment of the invention, and  FIG. 7B  is a sectional view taken along lines  7 B- 7 B of  FIG. 7A .  
         [0021]      FIG. 8  is an isometric view of a double-sided, inelastic lower contact configured in accordance with a preferred embodiment of the invention.  
         [0022]      FIGS. 9A and 9B  are, respectively, solid model and phantom line interior detail-revealing isometric views of a slotted extensible contact element inserted in a lower contact housing of single-piece construction.  
         [0023]      FIGS. 10A, 10B , and  10 C are, respectively, exploded, assembled, and enlarged top portion isometric views of a lower contact housing of multiple-piece construction.  
         [0024]      FIGS. 11A, 11B , and  11 C are, respectively, solid model, enlarged top portion, and partly cut away isometric views of an embodiment of a lower contact for use in four-terminal Kelvin-connection measurements.  
         [0025]      FIG. 12  is an isometric view of a common housing for multiple contact elements.  
         [0026]      FIGS. 13A and 13B  are fragmentary isometric views with portions cut away showing, respectively, a vacuum plate and a base member with an upwardly projecting wall modified to receive the common housing of  FIG. 12 .  
         [0027]      FIGS. 14A and 14B  are fragmentary isometric views of a lower contact similar to that of  FIG. 8  positioned between components of an optical contact wear sensor with a contact element of, respectively, nominal operational length and shortened length resulting from excessive wear. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0028]     A self-cleaning lower contact  100  constructed in accordance with a preferred embodiment of the present invention may be used to perform electrical measurements of miniature electrical circuit components, or devices-under-test (DUTs)  14 , in a high-speed measurement system such as a Model 3340 Multi-Function Tester, manufactured by Electro Scientific Industries, Inc. of Portland, Oreg.  FIGS. 5A, 5B ,  5 C, and  6  show the general construction of lower contact  100  and its operational relationship with an upper contact  102 . Lower contact  100  is spatially aligned with a corresponding upper contact  102 . DUTs  14  are seated in test plate  5 , which is positioned between lower contact  100  and upper contact  102 . Lower contact  100  is formed of an electrically insulating cylindrical housing  104  having a substantially planar upper end surface  106  and a substantially planar lower end surface  108 . An extensible contact element  110  is movable in an interior opening  112  of housing  104  and outwardly of upper surface  106 . Interior opening  112  is preferably a centrally located slot of generally rectangular cross section extending along the entire length of housing  104 . Contact element  110  has an end portion  114  that terminates in a contact tip  116 , which contacts a terminal  118  of DUT  14  as it undergoes a test process. The interior walls defining lengthwise slot  112  within housing  104  provide bearing surfaces for end portion  114  against which it moves to support straight line motion of contact element  110  along a longitudinal axis  120  of housing  104 .  
         [0029]     A biasing mechanism  130 , including, for example, spring-loaded pin (“pogo pin”)  44 , applies a force to the end of contact element  110  positioned farther from terminal  118  of DUT  14 . Biasing mechanism  130  urges contact tip  116  against terminal  118  of DUT  14  as it undergoes the test process and against a lower surface  134  of test plate  5  as it transports DUT  14  to and from different corresponding pairs of upper and lower contacts  116  and  118 . Upper contact  102  is composed of a resilient flat metal cantilevered arm  136  with one end secured to a support module (not shown) and the opposite end terminating in a spring-biased roller contact  140  that contacts a terminal  138  of DUT  14 . Upper contact  116  is of a roller wheel type described in U.S. Pat. No. 6,040,705, which is assigned to the assignee of this patent application.  
         [0030]     When test plate  5  advances to place DUT  14  in physical contact with contact tip  116  of lower contact  100 , lower surface  134  of test plate  5  slides over at least a portion of contact tip  116 , depressing it so that it is flush with upper surface  106  of housing  104 . The upward force holds contact tip  116  in physical contact with lower surface  134  of test plate  5  and terminal  118  of DUT  14  during testing. When testing is complete, the position of test plate  5  is advanced (to present the next DUT  14  to lower and upper contacts  100  and  102  for testing) and the upward force causes contact tip  116  to rub against lower surface  134  of test plate  5 . Thus, contaminants or oxides are rubbed off of contact tip  116 , so that it is self-cleaned during operation of electrical component handler  2 , thereby eliminating the need for costly and time-consuming shutdown of the electrical component handler to clean lower contact  100 .  
         [0031]     Contact tip  116  undergoes wear removal of contact tip material that results in progressive shortening of contact element  110  as test plate  5  transports DUT. End portion  114  of contact element  110  is preferably in the shape of a flat blade, and contact tip  116  may be formed by automated stamping of a photochemical or highly conductive material, such as solid coin silver (90 wgt % Ag, 10 wgt % Cu) or plain, soft copper. Contact element  110  and contact tip  116  are preferably made of the same material, but contact tip  116  could be formed by depositing material having suitable wear characteristics on end portion  114  of different material. Contact tip  116  has a usable depth of about 3.8 mm (0.15 in) and has a sloped leading edge  144  to ensure terminal  118  of DUT  14  does not catch on contact tip  116  as test plate  5  advances DUT  14  to its measurement position between lower and upper contacts  100  and  102 . A conductive coating, such as gold, silver, or rhodium, may be applied to contact tip  116  to increase its resistance to oxidation during storage.  
         [0032]     To prevent the upward force from ejecting DUT  14  from test plate  5 , a downward force that is greater than the upward force may be simultaneously exerted on or by test plate  5  against lower contact  100 . The downward force may be exerted by, for example, vacuum plate  32 , a plate retainer roller  146 , or both of them. Further, contact tip  116  is preferably wider than seats  4  in test plate  5  such that the upward force applied against lower contact  100  does not eject DUT  14  upward and out of its seat  4 .  
         [0033]      FIG. 6  shows housing  104  of lower contact  100  extending through a hole in vacuum plate  32  such that housing  104  is positioned adjacent lower surface  134  of test plate  5 . A preferred vacuum plate  32  is in the form of a disk-like annulus and includes vacuum channels that are concentrically adjacent to seat rings  3 . Vacuum channels  34  are connected to a partial vacuum pressure source (not shown) and communicate partial vacuum pressure to seats  4  of each ring  3  by way of linking channels through the thickness dimension of test plate  5  at seats  4 . The partial vacuum pressure created by vacuum plate  32  holds it adjacent to lower surface  134  of test plate  5  and DUT  14 . This positioning results in contact tip  116  simultaneously being in contact with DUT  14  and lower surface  134  of test plate  5  during a test cycle. The partial vacuum pressure draws through vacuum channels  34  contact tip material dust produced by wear of contact tip  116 . Residual amounts of the dust accumulate on the bottom surface of test plate  5 , which is periodically cleaned to remove the dust.  
         [0034]     Recent technological advances in component miniaturization have resulted in the formation of DUTs  14  having length and width dimensions of about 0.016×0.008×0.008 (0402 “metric” components) and 0.024×0.012×0.12 (0201 “metric” components). Because of their small size, these DUTs are unable to tolerate large gaps, ledges, or interruptions in the surface geometry or channels along which they are transported during the testing process. If they encounter gaps, ledges, or interruptions, DUTs  14  can be easily damaged, since their terminal (electrode) ends are often coated with soft tin plating. Thus, in a preferred embodiment of electrical component handler  2 , DUT  14  passes over only a very small clearance gap. For example, in one preferred embodiment, contact tip  116  is wider than the device-receiving seats  4  in test plate  5 . This geometry also prevents the edge of contact tip  116  from interfering with a side wall of one of the device-receiving seats  4  in test plate  5 .  
         [0035]     Electrically insulating housing  104  is preferably constructed of either ceramic- or plastic-based wear-resistant material. Plastic material includes polycarbonate or polyphenylene sulfide (PPS), and ceramic material includes transition-toughened zirconia ceramic or alumina. Housing  104  is preferably designed to be easily removed during periodic machine maintenance and to facilitate changing vacuum plate  32 . Electrically insulating housing  104  is preferably aligned relative to concentric rings  3  of component seats  4  such that upper surface  106  tracks the path of advancement of DUT  14 . This alignment can be achieved by incorporating aligning features on electrically insulating housing  104 , by using an alignment tool, or by a combination thereof.  
         [0036]     Various embodiments of lower contact  100  are in the form of multiple-component assemblies and are described below and shown in the drawing figures associated with their descriptions.  
         [0037]      FIG. 7A  shows lower contact  100  having a housing  104  of single-piece construction of molded or cast electrically insulating material.  FIGS. 7B and 8  show different embodiments of a double-sided contact element  110  set in slot  112  of housing  104  of single-piece construction. Double-sided contact element  110  is advantageous because the presence of two contact tips  116  prolongs its useful life. Double-sided contact element  116  has two end portions  114 , each terminating in a contact tip  116 . In the embodiment of  FIG. 7B , end portions  114  are connected on opposite sides by a sinuous resilient contact body portion  150  that does not contact the interior housing walls defining slot  112 . Resilient contact body portion  150  is made of phosphor bronze or beryllium copper for resilience. End portions  114  and contact tips  116  may also be made of these materials to provide a one-piece resilient contact element  110 . End portion surfaces  152  move along bearing surfaces  154  of the interior housing walls defining slot  112  in response to force applied by biasing mechanism  130 . In the embodiment of  FIG. 7B , resilient body portion  150  provides the elasticity in the biasing force and, therefore, constitutes a part of biasing mechanism  130 . In the embodiment of  FIG. 8 , end portions  114  constitute opposite distal ends of a unitary contact body portion  156  of generally rectangular cross-section, the end surfaces  152  of which extend along the length of contact element  110  and thereby continuously contact bearing surfaces  154 . Contact element  110  has a length defined as the distance measured between contact tips  116  along longitudinal axis  120 , and housing  104  has a length defined as the distance between upper surface  106  and lower surface  108 . The length of contact element  110  (which becomes progressively shorter over time with wear) is greater than the length of housing  104  to enable contact tip  116  in proximal position to a DUT  14  to contact its electrode and to enable biasing mechanism  130  to engage contact tip  116  in proximal position to its end  132 .  
         [0038]      FIGS. 9A and 9B  show housing  104  of single-piece construction in which a contact element  110  of generally rectangular cross-section such as that shown in  FIG. 8  but with a slot  170  extending along the length of contact element  110 . Housing  104  has a hole  172  through which a screw  174  or other suitable fastener passes to secure contact element  110  in place and that is suitably positioned to limit to the slot length the travel of contact element  110 .  
         [0039]      FIGS. 10A, 10B , and  10 C show housing  104  of multiple-piece construction of electrically insulating material composed of two body components in the form of generally cylindrical segments  176  and  178 . Cylindrical segments  176  and  178  have complementary mating surfaces that fit together to form slot  112 . In one embodiment, cylindrical segments  176  and  178  have axially aligned apertures through which screw  174  passes to assemble housing  104  and hold in place slotted contact element  110  in the same manner as that shown in  FIGS. 9A and 9B  for housing  104  of single-piece construction. In another embodiment, cylindrical segments  176  and  178  are glued together. In both of these embodiments, the opposing inner walls of cylindrical segments  176  and  178  defining slot  112  function as bearing surfaces  154  along which contact element  110  slides.  
         [0040]     Skilled persons will appreciate from the foregoing descriptions that housing  104  of either single-piece or multiple-piece construction may include a slotted or a nonslotted contact element  110 , a contact element  110  with a single-sided or double-sided contact tip  116 , or interior bearing surfaces provided by a lining material for slot  112  or the slot-defining interior surfaces of housing  104  itself.  
         [0041]      FIGS. 9A and 9B  and  FIGS. 10A, 10B , and  10 C show a chamfer  180  formed in upper surface  106  of housing  104  of single-piece construction and multiple-piece construction, respectively. Chamfer  180  provides for DUT  14  a slide ramp of positive slope as test plate  5  moves DUT  14  off of lower contact  100  upon completion of a test measurement. This ensures that DUT  14  will not catch on an edge of slot  112  as DUT  14  is moved away from lower contact  100 .  
         [0042]      FIGS. 11A, 11B , and  11 C show an embodiment of lower contact  100  for use in four-terminal Kelvin-connection measurements. The Kelvin contact embodiment of lower terminal  100  includes two lower terminal contact elements  110   1  and  110   2  that are separated within housing  104  by an electrical insulator  190 . Terminal contact elements  110   1  and  110   2  are angularly inclined within housing  104  relative to longitudinal axis  120 , such that their respective contact tips  116   1  and  116   2  converge at upper surface  106  of housing  104 . For housing  104  of single-piece construction, insulator  190  is an integral part of the interior of housing  104 , in which terminal contact elements  110   1  and  110   2  move along angularly inclined slots formed in housing  104 . For housing  104  of multiple-piece construction, insulator  190  could be either a separate internal spacer component positioned between terminal contact elements  110   1  and  110   2  when housing  104  is assembled or an integral part of one of the housing body components. Biasing mechanism  130  comprises separate spring-loaded pin (pogo pin) contacts  44   1  and  44   2  that apply force against the respective ends of terminal contact elements  110   1  and  110   2  proximal to lower surface  108  of housing  104  to urge contact tips  116   1  and  116   2  against different regions of terminal  118  of DUT  14 .  
         [0043]      FIG. 12  shows a single-piece common housing  104   m  in the form of a rectangle with rounded edges and four slots  112  spaced apart to receive multiple separate contact elements  110 . Contact elements  110  may be of either single-sided or double-sided type. The interior construction of slots  112  is similar to that of housing  104  shown in  FIGS. 7A, 7B , and  8 .  
         [0044]      FIG. 13A  shows a vacuum plate  32   m , which is vacuum plate  32  modified to accommodate common housing  104   m . Vacuum plate  32   m  is formed by removing the material separating holes  30  to form an elongated slot  30   m  sized to receive housing  104   m .  FIG. 13B  shows a base member  36   m , which is base member  36  modified to clamp common housing  104   m  between upwardly projecting wall  38  and releasable clamping mechanism  42 . Base member  36   m  is formed by removing the scallop segments  40  of base member  36  to form a smooth surface recess  40   m  into which common housing  104   m  can fit securely when clamped.  
         [0045]      FIGS. 14A and 14B  show an optical contact wear sensor  200  that detects when contact tip  116  is worn down and therefore contact element  110  needs either replacement or, if double-sided with an unused side available, reversed in direction of movement of housing  104 . Contact wear sensor  200  includes a light emitter  202  and a light detector  204  positioned on either side of contact element  110  protruding from lower surface  108  of housing  104  and arranged to provide a direct line of sight light propagation path. As shown in  FIG. 14A , whenever contact tip  116  has sufficient electrode material remaining, contact element  110  interrupts the light propagation path between light emitter  202  and light detector  204 . As shown in  FIG. 14B , whenever contact tip  116  has undergone an amount of electrode material removal that shortens the length of contact element  110  by a predetermined amount, light propagating from light emitter  202  reaches light detector  204  to produce a signal indicating excessive wear of contact tip  116 .  
         [0046]     A contact head designed in accordance with the present invention was tested in a Model 3300B Multi-Function tester. The test conditions were as follows: the DUT was a 100 pF capacitor, the test frequency was set to 1 MHz, the upper limit for the decontamination factor (DF) was set at 0.0007, and the DUT actual DF was set to between about 0.0002 and about 0.0003. The results of the test were as follows: of the 113,257 DUTs that were tested, only 0.51% of them were falsely rejected. In comparison to the about 5.89% false rejection rate of prior art contact heads, this is a significant decrease in false rejections. This decrease in false rejection results in a consequent increase in productivity and machine throughput.  
         [0047]     It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments without departing from the underlying principles of the invention. For example, preferred embodiments of the contact head of the present invention include a plurality of lower contacts for simultaneously testing more than one DUT, but can also include a single lower contact. The scope of the present invention should, therefore, be determined only by the following claims.