Abstract:
A test plate for holding miniature electronic circuit components as a part of batch processing for parametric testing purposes, including passive, two-terminal, ceramic capacitors, resistors, multilayer inductors, inductor beads, varistors, thermistors, fuses, sensors, actuators, and the like, or another type of device under test (DUT), includes a multilayer DUT-holding plate having a rotational axis and at least two layers centered on the rotational axis. A conductive layer of the two layers includes oversize holes in alignment with DUT-engaging holes in a nonconductive layer of the two layers that enable use of the first conductive layer as a guard layer held at a guard potential for electrical testing purposes in order to eliminate or at least significantly reduce the effects of stray impedances on test results. Additional conductive guard layers and nonconductive layers may be included. Conductive layers may take the form of conductive patterns etched into copper laminates on nonconductive layers composed of epoxy printed circuit board material.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    This invention relates generally to the batch processing of miniature electronic circuit components, including passive, two-terminal, ceramic capacitors, resistors, inductors, and the like. More particularly, it concerns a test plate for holding such components or other type of device under test (DUT) as part of the batch processing for purposes of parametric testing.  
           [0003]    2. Description of Related Art  
           [0004]    The tiny size of electronic circuit components of interest herein complicates processing. Typically fabricated in parallelepiped shapes having dimensions as small as 0.020″ by 0.010″ by 0.010,″ more or less, these difficult-to-handle components require appropriate equipment and precision handling techniques. What is sometimes referred to as a “carrier plate” holds many hundreds of the components upright in spaced-apart positions as the ends of each component are coated with a conductive material to produce electrical terminals. After adding terminals, a “test plate” holds the large batch of components for movement past a contactor assembly of a testing system for parametric testing purposes and eventual sorting. Thoughtful design of each of these components promotes efficient processing. Reference may be made to U.S. Pat. Nos. 6,204,464; 6,294,747; 6,194,679; 6,069,480; 4,395,184; and 4,669,416 for examples of some prior art component handling systems and testing techniques.  
           [0005]    The test plate is of particular interest. Mechanically, the test plate must hold the DUTs securely enough as they move past the contactor assembly so that they are presented to the contactor assembly in a repeatable, mechanically stable position. Electrically, the test plate must not degrade test results. But the mechanical and electrical functions are conflicting. Various forms of grease, grime, dirt, dust and other electrically conductive material on the test plate and/or on the DUTs provide unwanted conductive paths (i.e., stray impedances) to the DUT terminals. The stray impedances can render test results inaccurate. Thus, manufacturers engaged in batch processing of miniature electronic circuit components seek improvement in test plate design in that respect and so a need exists for a better test plate.  
         SUMMARY OF THE INVENTION  
         [0006]    This invention addresses the concerns outlined above by providing a multilayer test plate. It may be used in testing any of various passive components, including capacitors, resistors, multilayer inductors, inductor beads, varistors, thermistors, fuses, sensors, actuators, and the like. The multilayer test plate has at least two layers, one conductive and one nonconductive. The nonconductive layer holds the DUTs while the conductive layer functions as a guard layer that enables the measurement system to eliminate, or at least significantly reduce the effects of stray impedances. The test plate can be configured as a direct replacement for existing test plates, and one embodiment even includes an additional guard layer that includes a pattern of rings (guard tracks) between rings of DUT-engaging holes in the two nonconductive layers.  
           [0007]    To paraphrase some of the more precise language appearing in the claims, the invention provides a test plate in the form of a DUT-holding plate having a rotational axis and at least two layers centered on the rotational axis. A nonconductive layer of the two layers is composed of an electrically nonconductive material (e.g., epoxy printed circuit board material) that defines a plurality of DUT-engaging holes. A conductive layer of the two layers is composed of an electrically conductive material (e.g., copper) that defines a plurality of oversized holes such that each of the oversized holes is in alignment with a respective one of the DUT-engaging holes. The oversized holes have a size larger than the DUT-engaging holes in order to avoid having the conductive layer contact a DUT held by the test plate. That arrangement enables use of the conductive layer as a guard layer held at a guard potential for electrical testing purposes. It can be held at a desired guard potential and thereby eliminate, or at least significantly reduce, the effect of stray impedances on test results.  
           [0008]    One embodiment of the invention includes a second conductive layer on an opposite side of the nonconductive layer. The second conductive layer can be used as a second guard layer held at a second guard potential that is the same or different from the guard potential at which the first conductive layer is held. The second conductive layer may include oversized holes and/or one or more radially spaced-apart conductive rings (i.e., guard tracks) disposed intermediate radially spaced-apart rings of DUT-engaging holes in the nonconductive layers. In addition, the invention can be readily fabricated by etching conductive patterns on double-sided, copper-clad, printed circuit board material to result in the first nonconductive layer being sandwiched in between the two conductive layers. Additional conductive and nonconductive layers can be added thereafter.  
           [0009]    Thus, the invention provides a multilayer test plate that improves upon and better balances its mechanical and electrical functions. It includes at least two layers, at least one of which is a conductive guard layer. It may include one or more additional guard layers and/or nonconductive layers. It can be fabricated using printed circuit board techniques, and it can be configured as a direct replacement for existing test plates. The following illustrative drawings and detailed description make the foregoing and other objects, features, and advantages of the invention more apparent.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 of the drawings is a top plan view of a multilayer test plate constructed according to the invention;  
         [0011]    [0011]FIG. 2 is a reduced-in-size exploded view of the multilayer test plate showing the first and second nonconductive layers and the first and second conductive layers;  
         [0012]    [0012]FIG. 3 is a top plan view of just the first nonconductive layer of the test plate;  
         [0013]    [0013]FIG. 4 is a top plan view similar to FIG. 3 of just the second nonconductive layer of the test plate;  
         [0014]    [0014]FIG. 5 is a top plan view of just the first conductive layer of the test plate;  
         [0015]    [0015]FIG. 6 is a top plan view similar to FIG. 5 of just the second conductive layer of the test plate;  
         [0016]    [0016]FIG. 7 is an enlarged cross sectional view of a portion of the multilayer test plate as viewed in a vertical plane bisecting one of the DUT-receiving holes;  
         [0017]    [0017]FIG. 8 is an enlarged top plan view of a portion of a second embodiment of the invention, having a second conductive layer with four, circular guard tracks intermediate radially spaced apart rings of DUT-receiving holes;  
         [0018]    [0018]FIG. 9 is a diagrammatic representation of the multilayer test plate in its usual environment, holding a DUT for movement past a contactor assembly of a component testing system;  
         [0019]    [0019]FIG. 10 is an enlarged cross sectional view of a portion of a third embodiment of the invention, having just two layers;  
         [0020]    [0020]FIG. 11 is an enlarged cross sectional view of a portion of a fourth embodiment of the invention similar to the third embodiment in FIG. 10 except that it has a blind hole with an air inlet hole instead of a through hole;  
         [0021]    [0021]FIG. 12 is an enlarged cross sectional view similar to FIG. 7 of a portion of a fifth embodiment of the invention having a companion nonconductive layer that includes a contact;  
         [0022]    [0022]FIG. 13 is an enlarged cross sectional view similar to FIG. 12 of a portion of a sixth embodiment of the invention having a companion nonconductive layer with a contact formed by top and bottom etched patterns connected by a via; and  
         [0023]    [0023]FIG. 14 is an enlarged cross sectional view similar to FIG. 13 of a portion of a seventh embodiment of the invention having a companion nonconductive layer with small inlet holes through the contact that enable use of pressurized air for dislodging a DUT from the DUT-holding hole, only one such inlet hole being shown for illustrative convenience. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    FIGS.  1 - 7  of the drawings show various aspects of a multilayer test plate  10  constructed according to the invention. Generally, a test plate constructed according to the invention includes at least two layers such that at least one layer is conductive and at least one layer is nonconductive. That way, the conductive layer can be used as a guard layer held at a desired guard potential for testing purposes. A test plate constructed according to the invention may have more than two layers and more than one conductive guard layer. The illustrate test plate  10  includes four layers of which two are conductive and two are nonconductive.  
         [0025]    The layers of the test plate  10  combine to define an array of DUT-receiving holes  10 A, just one such DUT-receiving hole  10 A being identified in FIG. 1 for illustrative convenience. The illustrated test plate  10  includes four hundred, circularly shaped, DUT-receiving holes  10 A arranged in four radially spaced apart, concentric rings of one hundred, circumferentially spaced apart holes each. Of course, the number, shape, and pattern of the holes can vary without departing from the inventive concepts disclosed.  
         [0026]    With a batch of DUTs in the DUT-receiving holes  10 A, the test plate  10  serves the function of holding the batch of DUTs for movement past a contactor assembly  10 B on a component testing system  10 C depicted diagrammatically in FIG. 9. The component testing system  10 C typically includes alignment pins (not shown) that seat in a pair of alignment holes  10 D and  10 E extending through the test plate  10  (FIG. 1) in order to help align the test plate  10  with supporting structure on the component testing system. Related details are described in the U.S. Pat. Nos. 6,204,464; 6,294,747; 6,194,679; 6,069,480; 4,395,184; and 4,669,416 mentioned previously. In addition, U.S. patent application Ser. No. 10/097,464 provides details of the contactor assembly  10 B and its operation in the testing system  10 C, and that patent application is incorporated herein by reference for the details provided.  
         [0027]    Generally, the illustrated multilayer test plate  10  includes four layers in the form of first and second nonconductive layers  11  and  12 , and first and second conductive layers  13  and  14 . Those four layers are disposed symmetrically about a rotational axis  15  of the test plate  10  that is identified in FIGS.  1 - 6 . The four layers  11 - 14  of the test plate  10  combine to function both mechanically and electrically. They physically hold a DUT for movement past a contactor assembly while reducing the effects of stray impedances that might otherwise degrade test results.  
         [0028]    Introducing the nomenclature used in this description and in the claims, the first nonconductive layer  11  is said to include opposite first and second sides  11 A and  11 B that extend parallel to each other and perpendicular to the rotational axis  15 . The first side  11 A of the first nonconductive layer  11  is identified in FIGS. 2 and 7. It faces the first conductive layer  13  as illustrated in FIG. 7. The second side  11 B of the first nonconductive layer  11  is identified in FIGS. 2, 3, and  7 . It faces the second conductive layer  14  as illustrated in FIG. 7.  
         [0029]    The four layers  11 - 14  are bonded together or otherwise suitably attached to form the test plate  10 . The resulting structure can be described as having the first nonconductive layer  11  sandwiched in between the first and second conductive layers  13  and  14 , the first conductive layer  13  sandwiched in between the first and second non conductive layers  11  and  12 , and the combination of the first nonconductive layer  11  and the first conductive layer  13  sandwiched in between the second nonconductive layer  12  and the second conductive layer  14 .  
         [0030]    Now consider the details of each of the illustrated layers  11 - 14  beginning with the first nonconductive layer  11  shown in FIGS. 2, 3, and  7 . It is a circularly shaped, measuring about 0.130 millimeters (mm) thick (between first and second sides  11 A and  11 B) and about 300 mm in diameter (perpendicular to the rotational axis  15 ). It is composed of an electrically nonconductive material (e.g., the G10 epoxy material of a piece of double-sided, copper-clad printed circuit board material), and it defines a first plurality of DUT-engaging holes  16 , just one individual DUT-engaging hole  16  being identified in FIG. 3 for illustrative convenience. There are four hundred such DUT-engaging holes  16  defined by the first nonconductive layer  11 , enough to hold four hundred DUTs.  
         [0031]    The DUT-engaging holes  16  are DUT-engaging in the sense that they hold DUTs with a close fit for movement past a contactor assembly (e.g., the contactor assembly  10 B in FIG. 9) for testing purposes. They are arranged in a pattern of four rings  17 ,  18 ,  19 , and  20  (FIG. 3), each ring consisting of one hundred holes spaced apart center-to-center at 3.6-degree intervals. Illustratively, the centers of ten of the DUT-engaging holes  16  in each of the four rings  17 - 20  are connected by lines in FIG. 3 in order to thereby identify the pattern of rings. The connecting lines are for illustration purposes and do not represent part of the design. The four rings  17 - 20  so illustrated are centered on the rotational axis  15  and spaced apart radially, having respective radii measuring about 120 mm, 126 mm, 133 mm, and 130 mm.  
         [0032]    The illustrated DUT-engaging holes  16  are circular with diameters of about 1.0 mm. They are sized and shaped to present a close fit for the DUTs that are to be held by the test plate  10  so that the DUT-engaging holes  16  engage and hold the DUTs. Thus, the size and shape of the DUT-engaging holes  16  may vary significantly from what is described and illustrated herein according to the DUTs to be held. The pattern in which the DUT-engaging holes  16  are arranged may vary too. Moreover, the other sizes, shapes, and particular materials described in this specification may also vary without departing from the in the broader inventive concepts disclosed, namely, a multilayer test plate having at least two layers enabling use of use of one conductive layer as a guard layer for testing purposes.  
         [0033]    With further regard to hole size, it depends on a number of things. If it is a round hole, the diameter of the hole must be slightly larger than the diagonal (from corner to corner) of the DUT. Generally, a DUT will be encouraged, perhaps by a combination of vibration and gravity, to fall into the hole so that the two terminated ends of the DUT are protruding slightly from the upper and lower surface of the test plate. If the hole is square, the dimensions of the hole must be slightly larger than the cross sectional dimensions of the DUT. Square holes are sometimes effectively used to capture DUTs that have a square cross section. Rectangular holes must have a diagonal that is slightly larger than the diagonal of the DUT. Rectangular holes are sometimes effectively used in a situation where the DUT length and width are close to the same dimension.  
         [0034]    The second nonconductive layer  12  shown in FIGS. 2, 4, and  7  is similar to the first nonconductive layer  11 . It is a circularly shaped, also measuring about 0.018 mm thick and about 300 millimeters in diameter. Of course, those dimensions apply to the illustrated test plate  10 . The thickness of any other test plate constructed according to the invention and the individual layers of the test plate are set according to the length of the chips to be tested. The thickness of the test plate should be slightly less than the length of the chip so that the two terminated ends extend out of the DUT-receiving holes. Similar to the nonconductive layer  11 , the nonconductive layer  12  is composed of an electrically nonconductive material (e.g., bare G10 epoxy printed circuit board material) and it defines a second plurality of DUT-engaging holes  21 , just one individual DUT-engaging hole  21  being identified in FIG. 4 for illustrative convenience. There are four hundred such DUT-engaging holes  21  defined by the second nonconductive layer  12 , and each one is aligned with a respective one of the DUT-engaging holes  16  in the first nonconductive layer  11 . In other words, each one of the second plurality of DUT-engaging holes  21  is centered on an axis common to it and a respective one of the DUT-engaging holes  16  as indicated by an axis  10 F in FIG. 7.  
         [0035]    Similar to the DUT-engaging holes  16 , the DUT-engaging holes  21  are arranged in a pattern of four rings  22 ,  23 ,  24 , and  25  (FIG. 4), each ring consisting of one hundred DUT-engaging holes  21  spaced apart center-to-center at 3.6-degree intervals. As with FIG. 3, the centers of ten of the DUT-engaging holes  21  in each of the four rings  22 - 25  are connected by lines in FIG. 4 in order to thereby identify the pattern of rings. The four rings  22 - 25  are centered on the rotational axis  15  and spaced apart radially, having respective radii measuring about 120 mm, 126 mm, 133 mm, and 130 mm.  
         [0036]    Similar to the DUT-engaging holes  16 , the illustrated DUT-engaging holes  21  are circular with diameters of about 1.0 mm. They are sized and shaped to present a close fit for the DUTs to be held by the test plate  10  so that the DUT-engaging holes  21  engage and hold the DUTs. Thus, the DUT is held by both the DUT-engaging hole  16  and the DUT-engaging hole  21  at spaced apart locations along its length to result in greater stability of the DUT as it is moved past the contactor assembly  10 B in FIG. 9 mentioned previously. As explained above for the first nonconductive layer  11 , the size and shape of the DUT-engaging holes  21  in the second nonconductive layer  12  may vary significantly from what is described and illustrated herein according to the DUTs to be held.  
         [0037]    Turning now to the first conductive layer  13  shown in FIGS. 2, 5, and  7 , it is similar in shape to the first and second nonconductive layers  11  and  12 . It is circularly shaped and about 300 mm in diameter. Unlike the first and second nonconductive layers  11  and  12 , however, the first conductive layer is composed of an electrically conductive material and it defines a first plurality of oversized holes  30 , just one individual oversized hole  30  being identified in FIG. 5 for illustrative convenience. The first conductive layer may, for example, take the form of a 0.018 mm thick copper laminate on the first side  11 A of the first nonconductive layer  11 . In that case, the second nonconductive layer  12  (e.g., bare G10 epoxy printed circuit board material) is affixed to the first conductive layer  13  by bonding or other suitable means after the laminate on the first conductive layer  13  is etched into the desired pattern of oversized holes  30 .  
         [0038]    There are four hundred such oversized holes  30  defined by the first conductive layer  13 , and each one is aligned with a respective one of the first plurality of DUT-engaging holes  16  in the first nonconductive layer  11  and a respective one of the second plurality of DUT-engaging holes  21  in the second nonconductive layer  12 . Each one of the first plurality of oversized holes  30  is centered on an axis common to it and the respective ones of the DUT-engaging holes  16  and  21  as indicated by the axis  10 F in FIG. 7. The oversized holes  30  are oversized in the sense that they are larger than the DUT-engaging holes  16  and  21  so that the first conductive layer  13  does not contact a DUT held in the DUT-engaging holes  16  and  21 . With DUT-engaging holes  16  and  21  of diameters 1.0 mm, for example, each of the first plurality of oversized holes  30  has a diameter measuring about 3.0 mm.  
         [0039]    In a general sense, the oversized holes of a test plate constructed according to the invention should be large enough so that the voltages applied for insulation resistance test (e.g., 1000 volts in some cases) will not arc from the test contact or DUT terminal to the guard layer. This concern is more critical when the guard layer is on the outside of the test plate (e.g., the conductive layer  14 ) as compared to being embedded within the inner layers (e.g., the conductive layer  13 ). On the other hand, the oversized holes should not be any larger than necessary in order for the guard layer to provide the maximum guarding effect possible. Based upon the foregoing and subsequent descriptions herein, one of ordinary skill in the art can readily implement a test plate with guard layers having suitably oversized holes.  
         [0040]    The extra size of the oversized holes  30  results in the first conductive layer  13  circumscribing a DUT held in the DUT-engaging holes  16  and  21  without contacting the DUT. The first conductive layer  13  extends fully around the DUT without contacting the DUT as a first guard layer that can be held at ground potential or other desired guard potential in a known way during component testing. An operational amplifier may couple the guard potential of a measuring bridge to the first guard layer, for example, using vias fabricated in the test plate  10  for that purpose. So connected, the first guard layer serves the important function of helping to reduce the effect of stray impedances on test results, stray impedances referring to the various impedance paths that may otherwise be coupled electrically to DUTs held by the test plate  10  with undesired effects on test results.  
         [0041]    Similar to the DUT-engaging holes  16  and  21 , the oversized holes  30  are arranged in a pattern of four rings  31 ,  32 ,  33 , and  34  (FIG. 5), each ring consisting of one hundred oversized holes  30  spaced apart center-to-center at  3 . 6 -degree intervals. As with FIGS. 3 and 4, the centers of ten of the oversized holes  30  in each of the four rings  31 - 34  are connected by lines in FIG. 5 in order to thereby identify the pattern of rings. The connecting lines are not part of the design. The four rings  31 - 34  so identified are centered on the rotational axis  15  and spaced apart radially, having respective radii measuring about 120 mm, 126 mm, 133 mm, and 130 mm.  
         [0042]    The second conductive layer  14  shown in FIGS. 2, 6, and  7  is similar in many respects to the first conductive layer  13 . It is circularly shaped, about 300 mm in diameter, composed of an electrically conductive material (e.g., a 0.018 mm thick copper laminate on the second side  11 B of the first nonconductive layer  11 ), and it defines a second plurality of oversized holes  35 , just one individual oversized hole  35  being identified in FIG. 6 for illustrative convenience.  
         [0043]    There are four hundred such oversized holes  35  also. Each one is aligned with a respective one of the first plurality of DUT-engaging holes  16  in the first nonconductive layer  11 , a respective one of the second plurality of DUT-engaging holes  21  in the second nonconductive layer  12 , and a respective one of the first plurality of oversized holes  30  in the first conductive layer  13 . Each one of the second plurality of oversized holes  35  is centered on an axis common to it, the respective ones of the DUT-engaging holes  16  and  21 , and the respective one of the first plurality of oversized holes  30 , as indicated by the axis  10 F in FIG. 7. The extra size of the oversized holes  35  results in the second conductive layer  14  circumscribing a DUT held in the DUT-engaging holes  16  and  21  without contacting the DUT.  
         [0044]    Thus, the second conductive layer  14  circumscribes the DUT held in the DUT-engaging holes  16  and  21  as a second guard layer that can be held at a common or different guard potential than the first conductive layer  13  during component testing. The second guard layer can be coupled to the first guard layer, for example, with vias fabricated in the test plate  10  or by means of a conductive lining in one or both of the alignment holes  10 D and  10 E (not shown). As with the first guard layer, the second guard layer serves the function of helping to reduce the effect of stray impedances on test results.  
         [0045]    The oversized holes  35  defined by the second conductive layer  14  are also arranged in a pattern of four rings  36 ,  37 ,  38 , and  39  (FIG. 6), each ring consisting of one hundred oversize holes  35  spaced apart center-to-center at 3.6-degree intervals. As with FIGS. 3, 4 and  5 , the centers of ten of the oversized holes  35  in each of the four rings  36 - 39  are connected by lines in FIG. 6 in order to thereby identify the pattern of rings. The four rings  36 - 39  so identified are also centered on the rotational axis  15  and spaced apart radially, having respective radii measuring about 120 mm, 126 mm, 133 mm, and 130 mm.  
         [0046]    [0046]FIG. 8 shows a second embodiment of the invention in the form of a multilayer test plate  100 . The test plate  100  is similar in many respects to the test plate  10  and so only differences are described in further detail. For convenience, reference numerals designating parts of  20  the test plate  100  are increased by one hundred over the reference numerals designating corresponding or related parts of the test plate  10 .  
         [0047]    The multilayer test plate  100  includes first and second nonconductive layers  111  and  112  (e.g., G10 epoxy printed circuit board substrates). The first and second nonconductive layers  111  and  112  define DUT-engaging holes  116  and  121  that are arranged in rings similar to the rings of DUT-engaging holes  16  and  21  described above for the test plate  10 . Only one pair of holes  116  and  121  is identified in FIG. 8 for illustrative convenience. The test plate  100  also includes first and second conductive layers  113  and  114  in the form of electrically conductive patterns formed by etching them into copper laminates on opposite sides of the first nonconductive layer  111  during the fabrication process. The first conductive layer  113  defines oversized holes  130 , only one of which is identified in FIG. 8, and that configures the first conductive layer  113  for use as a first guard layer. The second conductive layer  114  does not define oversize holes, but includes four conductive rings or guard tracks  141 ,  142 ,  143 , and  144  instead. The guard tracks  141 - 144  extend circumferentially as illustrated to establish a guard potential for DUTs held in the DUT-engaging holes  116  and  121 . Any of various other patterns may be employed according to the guard attributes desired.  
         [0048]    [0048]FIG. 10 shows a portion of a third embodiment of the invention in the form of a multilayer test plate  200 . The test plate  200  is similar in many respects to the test plate  10  (see FIG. 7) and so only differences are described in further detail. The major difference is that the test plate  100  includes only one nonconductive layer  241  and one conductive layer  242 . The nonconductive layer  241  defines DUT-engaging holes  243  and the conductive layer defines oversized holes  244  that are aligned with the DUT-engaging holes  243  so that the conductive layer  242  can function as a guard layer.  
         [0049]    [0049]FIG. 11 shows a portion of a fourth embodiment of the invention in the form of a multilayer test plate  300 . The test plate  300  is similar in many respects to the test plate  200  and so only differences are described in further detail. Similar to the test plate  200 , the test plate  300  includes only one nonconductive layer  341  and one conductive layer  342  that define DUT-engaging holes  343  and oversized holes  344 , only one of each being illustrated and identified. The major difference is that the DUT-engaging holes  343  are blind holes instead of being through holes like the DUT-engaging holes  243 . They are blind holes in the sense that they narrow sufficiently in cross section so that a DUT cannot pass fully through the hole. The illustrated DUT-engaging hole  343  narrows abruptly at a region identified in FIG. 11 by reference numeral  343 A. Preferably, an inlet hole  345  is provided in fluid communication with the DUT-engaging hole  343  so that pressurized air can be forced through the inlet hole  345  to the DUT-engaging holes  343  for purposes of dislodging a DUT held within the DUT-engaging hole  343  (i.e., blow the DUT out of the hole).  
         [0050]    [0050]FIG. 12 shows a portion of a fifth embodiment of the invention in the form of a test plate  400 . The test plate  400  is similar in many respects to the test plate  10  (see FIG. 7) and so only differences are described in further detail. For convenience, reference numerals designating parts of the test plate  400  are increased by four hundred over the reference numerals designating corresponding or associated parts of the test plate  10 .  
         [0051]    Similar to the test plate  10 , the test plate  400  includes first and second nonconductive layers  411  and  412  that define DUT-engaging holes  416  and  421 , and it includes conductive layers  413  and  414  that define oversize holes  430  and  435 . The test plate  400  also includes an additional, separate, companion nonconductive layer  450  upon which the second nonconductive layer  412  (the lower nonconductive layer) rests when in use testing DUTs. An electrically conductive contact  452  is provided within a hole  457  in the companion nonconductive layer  412  for contacting a terminal on the lower end of a DUT held within the DUT-engaging holes  421  and  416 . That way, the test system can make contact with the terminal on the lower end of the DUT without requiring that the terminal on the lower end of the DUT slide across a contact. The contact  452  is an intermediate contact, and this companion-layer technique can be used with any test plate constructed according to the invention with through holes.  
         [0052]    [0052]FIG. 13 shows a portion of a sixth embodiment of the invention in the form of a test plate  500 . The test plate  500  is similar in many respects to the test plate  400  and so only differences are described in further detail. For convenience, reference numerals designating parts of the test plate  500  are increased by one hundred over the reference numerals designating corresponding or associated parts of the test plate  400 .  
         [0053]    Similar to the test plate  400 , the test plate  500  includes a nonconductive layer  512  that defines a DUT-engaging hole  521 . It also includes a companion layer  550  having an electrically conductive contact extending through a hole  551  in the companion nonconductive layer  550 . The major differences is that the contact is formed by joining an upper conductive pad  560  (a portion of an upper etched pattern on an upper surface of the nonconductive layer  560 ) to a lower conductive pad  561  (a portion of a lower etched pattern) with a via  562  using known printed circuit board techniques.  
         [0054]    [0054]FIG. 14 shows a portion of a seventh embodiment of the invention in the form of a test plate  600 . The test plate  600  is similar in many respects to the test plate  500  and so only differences are described in further detail. For convenience, reference numerals designating parts of the test plate  600  are increased by one hundred over the reference numerals designating corresponding or associated parts of the test plate  500 .  
         [0055]    Similar to the test plate  500 , the test plate  600  includes a nonconductive layer  612  that defines a DUT-engaging hole  621 . It also includes a companion layer  650  having an electrically conductive contact extending through a hole  651  in the companion nonconductive layer  650 . The contact includes an upper conductive pad  660  and a lower conductive pad  661  that are connected with a via  662 . The major differences is that the via defines an inlet hole  663  in fluid communication with the DUT-engaging hole  621  that can be used to couple a source of pressurized air or other case to the DUT-engaging hole  621  for purposes of disloding a DUT from the hole. Although only one inlet hole is illustrated for the test plate  600 , it is within the scope of the invention to use several small inlet holes. Vias can be fabricated with diameters measuring only a few mils, and the use of several small inlet holes rather than one larger one can be advantageous because using several small inlet holes distributes the blow-off air more evenly in order to provide a more positive blow-off action.  
         [0056]    Thus, the invention provides a test plate having at least two layers, a nonconductive layer and a conductive layer. The conductive layer can be held at a desired guard potential for testing purposes in order to eliminate or at least significantly reduce the effect of stray impedances. The test plate can be fabricated using known printed circuit board techniques and it can be configured as a direct replacement for existing test plates. It can have any shape, including circular, square, rectangular, polygonal. It can be any size desired. It can be configured with through holes or blind holes for chips having terminals on only one end. The holes can be any shape desired to hold and guard the particular DUTs to be tested, and it can be configured with a thickness suitable for a particular use. In other words, a test plate constructed according to the invention is a guarded test plate that can be used for testing any type of passive component. It has a minimum of two layers and no maximum. There is no restriction on shape, size, and thickness. There is no restriction on shape, size, quantity, or position of the DUT-receiving holes. Although exemplary embodiments have been shown and described, one of ordinary skill in the art may make many changes, modifications, and substitutions without necessarily departing from the spirit and scope of the invention.