Abstract:
Signal modules and methods for electrically interfacing with an electronic device are provided. The signal module includes a dielectric and a conductor extending through a surface of the dielectric. The surface of the dielectric is located away from perpendicular relative to an axis of the conductor and is located based on an electromagnetic field produced as a result of a signal flowing through the conductor.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to electronic interfacing and more particularly, an apparatus and method for substantially reducing electromagnetic reflections in a signal interface module. 
       BACKGROUND OF THE INVENTION 
       [0002]    In the manufacture of integrated circuits (ICs) and other electronic devices, testing with automatic test equipment (ATE) is performed at one or more stages of the overall process. IC testing systems typically include a test head and a probe card. Packaged part testing systems typically include a test head and a device under test (DUT) board. The probe card or DUT board includes a pattern of contacts for electrically probing or connecting to portions of an integrated circuit. The test head is configured to drive various contacts of the probe card or DUT board to carry out particular test procedures within the IC. In the course of a test procedure, the test head receives output signals from the IC via the contacts of the probe card or DUT board. The output signals are indicative of electrical characteristics of the IC under test. The probe card or DUT board and the test head are uniquely configured for a particular IC and, in some cases, a particular test procedure. Accordingly, the probe card or DUT board and/or the test head must be changed for different ICs and test procedures. 
         [0003]    The test head is electrically coupled to the probe card or DUT board with an interface apparatus. The interface apparatus may be, for example, a zero insertion force socket or a “pogo” unit. A pogo unit engages the test head, or some intermediate coupling structure associated with the test head, and the probe card or DUT board. The pogo unit includes an array of spring-loaded contact pins referred to as Pogo Pins®. The spring pins act as signal and ground conductors, and are arranged to electrically couple contacts on the probe card or DUT board to corresponding contacts on the test head. The spring force of the spring pins helps to maintain uniformity of electrical contact between the various contacts of the probe card or DUT board and the test head. When the test head and probe card or DUT board are engaged with the pogo unit exerting pressure against the spring pins, the spring pins respond with a spring force that enhances coupling pressure. The resilience of the pins generally ensures adequate coupling pressure despite planar deformation of the test head or the probe card or DUT board during a test procedure. 
         [0004]    In many applications, the conductors are required to carry signals having very high frequency components, from 100&#39;s of MHz to 10 GHz in the near future and to 10&#39;s of GHz in the more distant future. Accordingly, the transmission line characteristic impedance of the signal path between the probe card or DUT board and the test head is of prime interest. For optimal signal transfer between the test electronics and the device being tested, the characteristic impedance of all elements in the signal path should be closely matched. Usually, it is desired that all signal paths have the same impedance, for example 28, 50, or 75 Ohms, though it may be required that several different values of characteristic impedance be provided in the same interface. 
         [0005]    ATE interface signal modules typically employ dielectric materials to structurally support electrical transmission lines. These dielectric materials provide an electrically insulating boundary between adjacent transmission line elements, but also cause discontinuities in the characteristic impedance along the path of the transmission line. Discontinuities in characteristic impedance along a transmission line can cause undesirable effects that include increased reflection coefficients levels, consequent decreased transmission coefficient levels, both of which are frequency dependent, that causes unleveled channel performance detrimental to the signal integrity of the signal module and its ability to perform ATE signal characterization. In many applications it is thus desirable to remove the reflections of electromagnetic fields as they propagate through the impedance discontinuities caused by the differing dielectric constants. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention is embodied in a signal module for electrically interfacing with an electronic device. The signal module includes a dielectric and a conductor extending through a surface of the dielectric. The surface of the dielectric is located away from perpendicular relative to an axis of the conductor and is located based on an electromagnetic field produced as a result of a signal flowing through the conductor. 
         [0007]    The present invention is also embodied in a method of transmitting an electromagnetic signal. The method includes the step of providing a signal module. The signal module includes a dielectric and a conductor extending through a surface of the dielectric. The surface of the dielectric is away from perpendicular relative to an axis of the conductor and the surface is based on an electromagnetic field produced as a result of a signal flowing through the conductor. The method further includes the steps of providing the electromagnetic signal to one end of the conductor in the signal module and transmitting the signal through the conductor. 
         [0008]    The present invention is further embodied in a signal connector. The signal connector includes a dielectric having a countersink provided on each of a first surface and a second surface opposite the first surface. The signal connector also includes a conductor that extends through the countersink of the first surface of the dielectric and the countersink of the second surface of the dielectric. The dielectric provides mechanical support to the conductor. The countersinks are provided at a bevel angle relative to an axis of the conductor and located based on an electromagnetic field produced as a result of a signal flowing through the conductor. 
         [0009]    The present invention is further embodied in a signal module. The signal module includes a plurality of spring pins and a first and second retainer cap. Each retainer cap is of dielectric material. Each retainer cap includes a top portion having a plurality of countersinks provided in each of a first surface and a second surface opposite the first surface and a plurality of bores extending between the plurality of countersinks provided in the first surface and the second surface. Each retainer cap also includes a side portion that is coupled to the top portion. The side portion encloses the plurality of spring pins. The plurality of spring pins are disposed in the plurality of bores, respectively. The countersinks are provided at a bevel angle relative to an axis of the conductor and located based on an electromagnetic field produced as a result of a plurality of signals flowing through the plurality of spring pins, respectively. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures: 
           [0011]      FIG. 1  is a side view of a conventional signal module showing electromagnetic signal propagation and reflection through the conventional signal module; 
           [0012]      FIG. 2  is a side view of an exemplary signal module in accordance with an exemplary embodiment of the present invention showing electromagnetic signal propagation through the exemplary signal module; 
           [0013]      FIG. 3  is a side view of an exemplary dielectric boundary in accordance with an exemplary embodiment of the present invention showing countersinks in the dielectric boundary in a vicinity of a conductor; 
           [0014]      FIG. 4  is a perspective view of a conventional pseudo-coaxial transmission line showing an arrangement of conductors through an end terminal of the pseudo-coaxial transmission line; 
           [0015]      FIG. 5  is a side view section of an exemplary signal module of the present invention showing an arrangement of conductors through a dielectric having bevel angles for each respective conductor; 
           [0016]      FIG. 6  is a perspective view of an exemplary pseudo-coaxial transmission line in accordance with an exemplary embodiment of the present invention showing five dielectrics with bevel angle countersinks arranged along the to pseudo-coaxial transmission line; 
           [0017]      FIG. 7  is a simulation result of the impedance of an electromagnetic signal propagating through the exemplary pseudo-coaxial transmission line in accordance with an exemplary embodiment of the present invention; 
           [0018]      FIG. 8   a  is a perspective view of an exemplary ATE interface signal module in accordance with an exemplary embodiment of the present invention; and 
           [0019]      FIG. 8   b  is a section view of a retainer cap within an exemplary ATE interface in accordance with an exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]      FIG. 1  describes a conventional ATE interface signal module  100 . An ATE interface electromagnetic signal  106  is transmitted through signal module  100  along spring pin  104 . The electromagnetic signal propagates in the region  108  surrounding the surfaces of the spring pins, (multiple spring pins not shown) and encounters dielectric caps  102  at the module ends along with any dielectric shims  116  within the signal module. 
         [0021]    In the conventional ATE interface signal module, reflections  112  of the electromagnetic signal occur when the electromagnetic signal encounters the boundary interface between the media surrounding spring pin  104  and the dielectric caps  102 . 
         [0022]    The media surrounding the spring pins is typically air. A typical ATE interface signal module may employ dielectric caps whose dielectric constant is greater than air. This creates a characteristic impedance mismatch, resulting in the electromagnetic reflections  112 . 
         [0023]    The magnitude of the electromagnetic reflections increase as the dielectric constant of the dielectric caps increase. Additionally, the forward transmission coefficient suffers loss causing decreased transmission through the signal module. 
         [0024]    Multiple spring pins (not shown in  FIG. 1 ) passing through dielectric caps  102  may each have electromagnetic reflections  112 . The resulting reflections may produce undesirable crosstalk between adjacent spring pins. Thus, the final electromagnetic signal  110  transmitted through the conventional signal module may include transmission loss and crosstalk from adjacent spring pins. 
         [0025]    Attempts have been made to match the impedance of the dielectric with that of the media surrounding the spring pins. Attempts to fabricate dielectric caps using materials of very low dielectric constant, for example, Cuming Microwave Corporation RH-10 material, have revealed that the mechanical tensile strength of those materials is not sufficient to rigidly constrain the spring pins. 
         [0026]    Efforts to reduce electromagnetic reflections in an ATE interface signal module have attempted to compensate for the capacitive effect of the dielectric cap. For example, the spring pin diameter in a portion of the dielectric cap has been adjusted in an effort to achieve an impedance match. 
         [0027]    In “Calculation of an Optimum Transmission Line Taper,” IEEE Microwave Theory &amp; Techniques, November 1972, pages 734-739, the diameter of the plunger was gradually changed to compensate for capacitive effect of the dielectric cap. A computer simulation model constructed that computed the required length of the tapered transmission line resulted in requiring an unacceptably long transmission line not capable of fitting within the constrained dimensions of a reasonable ATE signal module. 
         [0028]    The present invention produces an electromagnetic impedance match by implementing the condition where the plane of the incident TM (transverse magnetic) field of the electromagnetic wave propagating along the axis of the signal module is parallel with the surface of the dielectric boundary. An angle of the dielectric boundary surface is thus determined based on the electromagnetic field produced as a result of the signal flowing through the signal module. In an ATE signal module, this is desirably accomplished by creating a bevel angle at the boundary of the media between the spring pins and the dielectric caps. By providing the appropriate bevel angle, there are essentially zero reflections between dielectric boundaries. 
         [0029]    An exemplary embodiment of the present invention comprises a signal module including a conductor and a low loss dielectric. In an exemplary embodiment, dielectrics  202  and  204  and conductor  208  are shown in  FIG. 2 . 
         [0030]      FIG. 2  illustrates a signal module  200  according to an exemplary embodiment of the present invention. The conductor may pass through dielectrics  202  and  204  in signal module  200 . Additional dielectrics such as dielectric  216  may also be positioned along the length of conductor to provide further support to the conductor. 
         [0031]    An electromagnetic signal  210  may be sent into one end of the exemplary signal module. The electromagnetic signal is an electromagnetic wave that propagates along the axis of conductor  208 . The electromagnetic signal desirably passes through dielectrics  202  and  204  without producing reflections in region  212  between dielectrics  202  and  204 . Signal  214  that is transmitted out of signal module  200  desirably has no transmission loss from passing through dielectrics  202  and  204 . 
         [0032]    ATE interface signals typically have a bandwidth in the GHz region. Signal module  200  desirably operates in the frequency region between DC and at least 30 GHz and higher. Exemplary signal module  200  may operate at a substantially higher bandwidth or within a more limited bandwidth. The exemplary signal module is not limited to ATE interface signals and may be used for other applications, such as connectors that include internal dielectric inserts intended to mechanically support electrical conductors within the connector but also obstruct the propagation of electromagnetic signals that pass through them. 
         [0033]    Signal module  200  shown here presents dielectric  204  as having one angle  206 . This is being shown for illustrative purposes only. In  FIG. 2 , dielectrics  202  and  204  are at the same bevel angle  206 . Although dielectrics  202  and  204  show only one surface with bevel angle  206 , in the exemplary embodiment, all surfaces are desirably at a specific bevel angle. Additionally, dielectrics  202  and  204  only show one segment of the specific bevel angle. In practice, the bevel angle is a conical shaped countersink  302  of  FIG. 3 , which is convenient for fabrication. 
         [0034]    As illustrated in  FIG. 3 , a dielectric  204  has a first surface  204   a  and a second surface  204   b . A conductor  208  passes through first surface  204   a  having a countersink  302 . Countersink  302  is desirably at angle  206  relative to a central axis along the length of conductor  208 . Conductor  208  further passes through second surface  204   b , also having a countersink  302 . It is desirable that surfaces  204   a  and  204   b  each have countersinks  302 , both at bevel angle  206 . 
         [0035]    Exemplary conductor  208  may be included in a pseudo-coaxial transmission line comprised of a plurality of conductors. Pseudo-coaxial transmission lines are commonly known by one skilled in the art. 
         [0036]      FIG. 4  illustrates a known in the art pseudo-coaxial transmission line  400  composed of five parallel spring pins  402  and  404 . In this example, center pin  404  carries the electromagnetic signal and the other four spring pins  402 , which are arranged to surround center pin  404  are electrically grounded. This is only one arrangement of a pseudo-coaxial transmission line. This and other potentially useful configurations are shown, for example, in Reference Data for Radio Engineers, 4th Edition. New York: International Telephone &amp; Telegraph Corp., 1957. The present invention is not limited to spring pin arrangement  400  or to a pseudo-coaxial transmission line. 
         [0037]    The transmission lines used in exemplary signal module  200  typically seek to achieve a characteristic impedance of 50 Ohms. This characteristic impedance matches the impedance of typical coaxial cables, printed circuit boards microstrip transmission lines or other electrical devices that may be attached to the exemplary signal module. Signal module  200  may be designed to achieve other characteristic impedance values. 
         [0038]    The characteristic impedance of a transmission line may be governed by the geometric arrangement of the spring pins, the diameter of the spring pins, the spacing distance between spring pins, and the dielectric constant of the media between the pins. The media surround the spring pins, and hence conductor  208  in region  212 , is typically air. Other media may include other dielectric medium with a low loss tangent and stable, frequency independent dielectric constant. 
         [0039]    Exemplary signal module  200  with conductor  208  may operate with a planar electric field vector from an electromagnetic signal, such as but not limited to TM01 or TE11 wave modes for a circular geometric arrangement of conductors forming a transmission line, or TE10 or TM11 wave modes for a rectangular geometry of conductors forming a transmission line. The exemplary signal module also desirably operates with electromagnetic signals having distorted electric field vector. A distorted TM field vector may occur where conductor  208  is a pseudo-coaxial transmission line or in other configurations. 
         [0040]    Dielectrics  202  and  204  may include dielectric material having a dielectric constant between 1 to 5 over a frequency range of DC to at least 30 GHz and higher. The frequency range for which the dielectric constant is desirably between 1 to 5 may be substantially higher. The dielectric constant is desirably constant over a preferred bandwidth. The dielectric constant value and frequency range desirably depends upon the application of the exemplary signal module. The desired dielectric constant value and frequency range may vary with the type of input signal without affecting the scope of the present invention. 
         [0041]    In addition to the dielectric constant constraints of dielectrics  202  and  204 , the dielectric material also desirably includes the property of a loss tangent less than 0.1 over the frequency range of DC to at least 30 GHz and higher. The frequency range for which the loss tangent is desirably less than 0.1 may be substantially higher. The loss tangent required depends upon the application of the signal module. ATE interface signals may require lower loss tangents than other applications. 
         [0042]    Common dielectric materials used in exemplary signal module  200  may include polytetrafluoroethylene, FR4, Ultem® 1000, Rexolite®, polyethylene, polyvinyl-chloride and air. Other materials may be used provided they have a low loss tangent (for example less than 0.1) and dielectric constant suitable for the application requirements. All dielectric materials, including those listed here, exhibit some loss, albeit a small loss that may be considered negligible. 
         [0043]    According to the present invention, an electromagnetic impedance match at the boundary of two media with differing dielectric constants may be achieved if the plane of the incident TM field of the electromagnetic wave propagating along the axis of the signal module is parallel with the surface of the dielectric boundary, and the boundary is at a specific bevel angle. This bevel angle is determined from the angle where the electromagnetic reflection approaches a zero value. 
         [0044]    The surface of the dielectric of the exemplary embodiment is desirably smooth and contains a straight line parallel to the TM field plane of incidence where they intersect and where the electromagnetic wave propagates along a linear axis. The surface of the dielectric desirably conforms to an isosurface of the TM field for other electromagnetic wave conditions. 
         [0045]    The angle where electromagnetic reflections approach a minimum value may be calculated using the principles of a Brewster angle. The Brewster angle is common in the art of optical and quasi-optical systems. 
         [0046]    For parallel polarization, the Brewster angle, θ B , exists where there is an angle of incidence upon the dielectric at which the TM Fresnel reflection vanishes for nonmagnetic and lossless materials. The initial condition is defined by Snell&#39;s Law as shown in equation (1). 
         [0000]      √{square root over (ε 2 )} θ T =√{square root over (ε 1 )} sin θ B   (1) 
         [0000]    where ε 1  and ε 2  are the dielectric constants of regions  1  and  2  respectively and θ T  is the refractive angle of region  2 . 
         [0047]    From simple geometry, it can be shown that θ T =90°−θ B , it follows that 
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         [0000]    known as Brewster&#39;s Law and typically expressed as below 
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         [0048]    For the exemplary embodiment of the signal module, the bevel angle is dependent upon the dielectric constant of the dielectric, the dielectric constant of the media surrounding the conductor, and the angle of the TM field vector from the electromagnetic signal propagating along the conductor. 
         [0049]    The Brewster angle computed by equation 2 above assumes the dielectric media are lossless, i.e. exhibit zero electrical conductivity and are nonmagnetic. The Brewster angle loses its meaning if the dielectric material in region  2  is lossy since it will exhibit a complex valued dielectric constant. The Brewster angle may be modified by specifying the real and imaginary parts of the complex dielectric. The electromagnetic reflection can approach a minimum value if we re-state equation 2 as 
         [0000]      θ B =tan −1 √{square root over ((ε 2     R     −jε   2     i   )/ε 1 )}.  (3) 
         [0050]    This approach is known as the Zenneck Surface Wave when the incident medium is lossless, such as air, which is the medium ε 1 . 
         [0051]    For determining appropriate bevel angles and surfaces for particular applications and configurations of real materials regardless of the loss tangent of the dielectric and the distortion of the TM vector resulting from various configurations, commercially available modeling and simulation tools such as Finite Difference Time Domain solvers with three-dimensional capabilities can be used. 
         [0052]    To accommodate distorted electric fields, the bevel angle is desirably further modified to account for the distortion. Therefore, the bevel angle is not a classic Brewster angle, but an angle that accounts for electric field distortion caused by the type of conductor, such as a pseudo-coaxial transmission line. The criteria for an electromagnetic impedance match are those as defined in equation (2) but the implementation desirably requires modification to account for electric field distortion in order to accommodate those criteria. 
         [0053]    At the specific bevel angle value, electromagnetic reflections approach zero. The transmission out of the signal module subsequently improves to a high value as it is commonly known from the conservation of energy. 
         [0054]    The improvement in transmission is broadband and not dependent upon the signal frequency. Frequency dependence is limited by the frequency dependence of the dielectric material permittivity. For an exemplary embodiment where the conductor is a transmission line, frequency dependence is also limited by the electromagnetic mode cut-off frequency well known to transmission line designers. 
         [0055]    The present invention may be extended to a signal module with multiple conductors, as shown in  FIG. 5 .  FIG. 5  shows a cross-sectioned portion of one end of a dielectric cap  500 . Multiple conductors  502 ,  504  and  506  extend through the dielectric cap  500  and out of a signal module according to an exemplary embodiment of the present invention. 
         [0056]      FIG. 5  shows bevel angles  508 ,  510  and  512  corresponding to conductors  502 ,  504 , and  506 . The bevel angles illustrated here more accurately depict the implementation of bevel angles on the dielectric surface. The bevel angles are desirably countersinks on the dielectric. The multiple countersinks are desirably adjacent to or intersect each other. 
         [0057]    The conductors illustrated in the embodiment of  FIG. 5  may each represent an individual conductor of, for example, a five-wire a pseudo-coaxial transmission line such as illustrated in  FIG. 4  or other transmission lines, or signal carrying connector. The exemplary embodiment is not limited to a device of three conductors. The exemplary signal module may contain any number of conductors in a planar or other suitable arrangement. The conductors may be spaced apart and in such an arrangement to provide an appropriate characteristic impedance match to its connecting devices. 
         [0058]    The bevel angle applied to the surface of the dielectric for each respective conductor causes the electromagnetic reflection along each conductor to approach a minimum value. The adjacent channel crosstalk, both even mode and odd mode, decreases since there is no reflection mechanism to contribute to the crosstalk. 
         [0059]    In bidirectional signaling, multiple conductor transmission lines carrying any multiplicity of signals simultaneously, the even mode is defined as simultaneous signals propagating in a parallel direction, in contrast to an odd mode where simultaneous signals are propagating in an anti-parallel direction. The exemplary embodiment may include any combination of even and odd mode propagation conditions simultaneously. 
         [0060]    It is understood that the invention is not limited to multiple electromagnetic signals propagating in the same direction along multiple conductors, respectively. For example, a first electromagnetic signal may propagate into the end of go conductor  502  while a second electromagnetic signal may propagate out of the end of conductor  504 . The second electromagnetic signal thus propagates in a direction opposite the first electromagnetic signal. 
         [0061]    Different electromagnetic signals with differing electric field vectors may propagate through conductors  502 ,  504  and  506  respectively. Respective bevel angles  508 ,  510  and  512  may each be designed for the specific electromagnetic signal and may be of a different angle to remove reflections for the particular electromagnetic signal. This allows manufacture of a signal module suited to a particular application. The resulting signal module will have a minimum of channel cross-talk and a maximum of respective signal transmission. 
         [0062]    The invention will next be illustrated by reference to a number of examples. The examples are included to more clearly demonstrate the overall nature of the invention. These examples are exemplary, not restrictive of the invention. 
       EXAMPLE 1 
       [0063]      FIG. 6  illustrates an exemplary pseudo-coaxial transmission line  600  with five dielectrics  606  spaced apart from each other along pseudo-coaxial transmission line  600 . Each dielectric  606  has a bevel angle countersink (not shown) on both sides of dielectric  606  determined as described above. Pseudo-coaxial transmission line  600  has five spring pins  602  and  604 . Center spring pin  604  carries the electromagnetic signal and the other four spring pins  602 , which are arranged to surround center pin  604  are electrically grounded. 
         [0064]    Referring now to  FIG. 7 , results of an impedance model simulation with respect to exemplary pseudo-coaxial transmission line  600  is now described. Impedance model simulation results  702  illustrate the impedance in ohms of an electromagnetic signal propagating through exemplary pseudo-coaxial transmission line  600  versus time in ns. When the electromagnetic signal propagates through exemplary pseudo-coaxial transmission line  600 , the impedance is about 50 ohms, as illustrated by region  704 . Exemplary embodiment  600  demonstrates that although the electromagnetic signal encounters  10  boundary condition changes (two for each dielectric  606 ) the bevel angle countersinks on each dielectric significantly reduce an impedance mismatch, thus maintaining the impedance at about 50 ohms. 
       EXAMPLE 2 
       [0065]    Referring now to  FIG. 8   a , an exemplary ATE signal interface module  800  with multiple spring pins  816  is now described. A retainer cap  802  of dielectric material covers and supports spring pins  816 . 
         [0066]    A section of retainer cap  802  is further illustrated in  FIG. 8   b . Retainer cap  802  has a top portion  804  and a side portion  812 . A first surface of top portion  804  includes a plurality of bevel angle countersinks  806 . A plurality of apertures  808  extends between bevel angle countersinks  806  and a second surface of top portion  804 . A second surface  810  of top portion  804  is a flat surface and does not have bevel angle countersinks due to machining limitations. 
         [0067]    A bevel angle shim  814  is of dielectric material having a first surface that is flat and a second surface with bevel angle countersinks. Bevel angle shim further includes apertures corresponding to the apertures in the retainer cap  802  and the countersinks of bevel angle shim  814 . The first surface of bevel angle shim  814  is disposed adjacent to the second surface  810  of top portion  804  of retainer cap  802 . The combination of dielectric retainer cap  802  and bevel angle shim  812  function as one retainer with both surfaces having bevel angle countersinks. 
         [0068]    An assembly shim  818  is disposed between side portion  812  of a first retainer cap  802  and a side portion  812  of a second retainer cap  802 . Assembly shim  818  has apertures corresponding to the apertures in first retainer cap  802 . Assembly shim  818  is desirably of dielectric material with bevel angle countersinks machined on both sides corresponding to the apertures. Assembly shim  818  may provide support to spring pins  816 . 
         [0069]    A second retainer cap  802  and bevel angle shim  814  as described above are coupled together to function as one retainer with both surfaces having bevel angle countersinks. Both second retainer cap  802  and second bevel angle shim  814  further include apertures that correspond to the apertures in first retainer cap  802 . 
         [0070]    A plurality of spring pins  816  extend and are supported through the apertures between top portion  804  of first retainer cap  802 , first bevel angle shim  814 , assembly shim  818 , a second bevel angle shim  814  and top portion  804  of a second retainer cap  802 . Side portions  812  of first and second retainer caps  802  enclose spring pins  816  within exemplary ATE signal interface module  800 . 
         [0071]    In exemplary ATE signal interface module  800 , all dielectric boundaries are thus provided with countersink bevel angles designed as described above according to an exemplary method of the present invention. Exemplary module  800  thus does not suffer from impedance mismatch. 
         [0072]    Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.