Abstract:
Low profile printed circuit board assembly test fixtures and methods are disclosed, the fixtures mountable at a tester having a plurality of conductive interface contacts. The fixture includes a low profile mount defined by a frame having an interface bed at one end and a dynamic plate movably positioned at an opposite end thereof, a chamber being thereby defined and having an opening for evacuating air therefrom to effect movement of the plate. A plurality of conductive terminals extend through the interface bed, each of the terminals having a node at one end positioned at the bottom surface of the bed and corresponding to the position of one of the conductive interface contacts of the tester. A plurality of conductive probes in the chamber are secured at the top surface of the bed, are coupled to the terminals, and extend through the plate. The electronic assembly is selectively positioned atop the plate for probe access.

Description:
FIELD OF THE INVENTION 
   This invention relates to electronics testing equipment and methods, and, more particularly, relates to test fixtures and methods for electronic assemblies such as printed circuit boards. 
   BACKGROUND OF THE INVENTION 
   One of the challenges in the manufacturing of today&#39;s complex electronics is verification of correctness of assembly. Electronic components are commonly assembled onto a printed circuit board (PCB). This board is made of substrate layers (commonly of FR4 grade fiberglass) with copper laminated onto the surface of each layer (as few as 2 layers of copper may be used, though often many layers are required). Each of these copper layers is etched using a photographic process, the resulting patterns of copper providing conductive paths on (and through) the PCB. On the outer surfaces of the PCB copper pads are positioned which reflect the layout of the leads (i.e., legs or pins) of the electronic components that will be attached (typically soldered) to the PCB. The resulting assembly is known as a printed circuit board assembly (PCBA). 
   Configurable fixtures for testing of PCBA have long been known and utilized. The most common variety, known generically as a “bed of nails” fixture, has taken various forms (see, for example, U.S. Pat. Nos. 4,643,501, 5,510,722, 5,450,017 and 6,469,531) and is engageable with any of a variety of known commercially available test system assemblies which provide standardized testing algorithms used to produce electrical stimulus and measure responses of the individual components installed on the PCBA. Provision of full access to every circuit net (i.e., node) of the PCBA thus allows all components to be verified for proper value and function, and such bed of nails fixtures are configured to allow simultaneous engagement of test leads with each selected circuit net of the PCBA under test (often referred to as the UUT, Unit Under Test). Heretofore known and/or utilized fixtures have met the challenge of accurate node contact with as ideal an electrical connection as possible with varying degrees of success. 
   Node contact pins, known as spring probes, are utilized most commonly in modern test fixtures (see U.S. Pat. Nos. 6,570,399, 4,885,533 and 4,749,945, for example). These probes are positioned at locations in the fixture to allow engagement with the soldered connection of the component leads or other patterns etched on the PCB. The goal of spring probe placements is to have a dedicated probe making contact to every net on the UUT. 
   Engagement of the spring probes at the UUT has heretofore been accomplished by mechanical clamping, pneumatic cylinders, or vacuum actuated fixturing, the latter currently thought to be the most cost effective. In such fixtures, the UUT is carried atop a moveable plate which is pulled downward upon application of vacuum to a chamber beneath it. The spring probes are located in sockets that are pressed into a second stationary plate and contact the surface of the UUT at locations defined by the topography of the UUT. The sockets are provided with an attachment for wires at the opposite side from the probes (commonly using a wire wrap style connection, though other electrical connection methods are known). The opposite ends of the wires are connected to contacts at an interface configured to correspond to a standard grid of one of the variety of commercially available testers (see U.S. Pat. No. 5,510,722, for example). 
   Having reference to  FIG. 1 , a type of vacuum actuated test fixture  12  typical of the prior art for testing UUT&#39;s utilizing a standard commercially available tester is shown. These wired fixtures current employ a three plate design. Interface plate  14  is engagable with tester interface  16  of a commercial tester. Probe plate  18  has sockets  20  selectively positioned thereat, sockets  20  receiving probes  22  therein, the probes engaging the selected nodes of the PCBA  24  under test (the UUT). Each probe plate  18  is of necessity configured to accommodate a particular PCBA  24 . Diaphragm plate  26  carries PCBA  24 . These three plates ( 14 ,  18  and  26 ) define two chambers in prior art fixture  12 , upper chamber  28  (the variable volume vacuum chamber, shown in its fully contracted, vacuum-applied state) between the probe plate  18  and diaphragm plate  26  which contracts or expands responsive to application and release, respectively, of vacuum applied in the conventional manner to fixture  12 , and lower chamber  30  (the wiring chamber) between interface plate  14  and probe plate  18 . 
   Interface plate  14  (typically a stable FR4 fiberglass material) has a multiplicity of interface wiring terminals  32  which are pressed into thru-holes drilled at standard grid locations corresponding to the tester&#39;s matching array of interface pins  34 . Terminals  32  are typically wire wrap style terminals. Chamber  30  is thus cluttered with wires  36  extending from terminals  32  to terminals  38  at the opposite side of spring probe sockets  20 . Wires  36  are necessary relatively long (300 cm to 1 meter) since access to wiring chamber  30  is required. Hinge  40  connected with the housing holding plates  18 / 26  allows opening and closing of wiring chamber  30 . 
   Vacuum chamber  28  is provided with small spacers  42  which set the distance between diaphragm plate  26  and probe plate  18  after the chamber is evacuated (providing a small space that keeps the vacuum force applied at diaphragm plate  26 ). Probe plate  18 , diaphragm plate  26 , PCBA  24 , and the field of spring probes  22  together are called the vacuum head. The vacuum head is not mechanically coupled to any other structure and is hence a stand-alone system. Mechanically, after vacuum chamber  28  is fully evacuated and static, plates  26 / 18  are subject to flexure, an undesirable consequence that should be minimized (such movement effects probe contact location and stability and thus fixture precision and reliability). Dynamic flexure is dependent on plate thicknesses and the probe field forces. 
   An alternative to wired test fixtures (such as that shown in  FIG. 1 ) has heretofore been suggested and/or utilized (see U.S. Pat. Nos. 6,628,130, 6,025,729 and 6,066,957). One such wireless fixture, for example, substitutes a double sided printed circuit board for the traditional fixture interface plate engagable at the tester. The circuit board provides conductive traces which make connections from the tester&#39;s standard interface to the probes contacting the UUT. These fixtures still require a three plate design (an interface plate that connects to the test system, a probe plate which positions the probes for engagement to the UUT, and a diaphragm plate which is the carrier for the UUT). There still exist two separate chambers, the lower chamber between the interface plate and probe plate and the upper chamber between the probe plate and diaphragm plate. A double-sided spring probe is used to make contact from the interface circuit board to the bottom connection of the probe socket. The upper end of the socket contains a second spring probe which contacts the UUT. 
   Various other test fixture embodiments have been heretofore suggested and/or utilized that employ translator boards between the interface plate and the probe plate (see U.S. Pat. Nos. 4,935,696, 6,005,405 and 4,884,024, and U.S. Patent Application Publication No. US2003/0030454). These fixtures allow utilization of shorter wiring pathes, but otherwise still require multiple plates (typically four including the interface plate, probe plate, UUT positioning plate and translator plate), as well as multiple different types of probes and sockets. 
   All of the heretofore known and/or utilized fixtures have suffered to one degree or another from mechanical disadvantages due to the unfettered nature of the vacuum chamber and the relative lack of support for the probe plate and diaphragm plate. Additionally, modifications of probe locations and related interface connections are not easily accommodated by some heretofore known alternatively designed fixtures, since such changes would require that the printed circuit board be redesigned. Other attempts at improvement have merely resulted in overly complex, and thus expensive, designs and modifications, and/or have made the installation of PCBA&#39;s, probes, terminals and wiring (where present) burdensome. 
   Since electronic components continue to shrink in size, fixture design must accommodate target areas for spring probe contacts that also continue to be reduced in size. This in turn requires the mechanical functioning of such fixtures to be more precise and stable. One factor affecting the precision and accuracy probe contact at selected UUT locations is flexure of fixture components subject to vacuum. Another factor is the overall length of the probe itself; a shorter probe is less likely to produce error due to the azimuth angle. 
   Since ever higher frequencies are utilized by electronic components, it is ever more desirable to keep signal paths in test fixtures as short as possible. This eliminates error due to noise and due to transmission path resistive and reactive impedance losses. The shorter the signal path, the better the testing accuracy. 
   It can be appreciated, therefor, that further improvement in electronic assembly test fixtures and methods, particularly those improvements directed to accommodating better mechanical functioning and shorter signal paths, could thus still be utilized. 
   SUMMARY OF THE INVENTION 
   This invention provides low profile electronic assembly test fixtures and methods that improve test fixture mechanical functionality related to vacuum chamber performance and probe and UUT support. The test fixtures and methods of this invention more readily allow modification of probe locations and related test system assembly interface connections, provide better precision and stability in probe location and contact at UUT node sites, and accommodate shorter signal paths through the test fixture. The fixtures are relatively easy to use and install, and are relatively inexpensive to produce. 
   The low profile electronic assembly test fixture of this invention is mountable at a tester having a plurality of conductive interface contacts at one part thereof. One embodiment of the fixture includes an interface bed having top and bottom surfaces, a plurality of conductive terminals extending through the interface bed between the surfaces thereof. Each of the terminals has a node at one end positioned at the bottom surface of the bed to correspond to position of one of the conductive interface contacts of the tester. A connection facility is located at an opposite end of the terminals positioned at the top surface of the bed. 
   A plurality of conductive probes is secured at the top surface of the bed, each selectively conductively couplable to a selected one of the terminal connection facilities. A dynamic plate assembly includes a support for receiving and positioning the electronic assembly at the plate assembly and has a plurality of openings therethrough corresponding to conductive probe positions at the bed. A mount has the bed secured at one end and the dynamic plate assembly dynamically positioned at an opposite end. 
   The mount has a low profile, a chamber being defined therein and bounded by the top surface of the bed and the plate assembly, the chamber having an opening thereto for evacuating air to effect movement of the plate assembly. The conductive probes are located in the chamber and extend through the plate assembly. 
   The methods of this invention are applied to access selected circuit nodes on an electronic assembly for assembly testing using a commercially available tester. The method steps include positioning conductive terminals in a field corresponding to at least portions of the pattern of conductive interface contacts of the tester, the terminals for engaging the interface contacts. Probes are positioned laterally adjacent to the terminals in the field of terminals establishing a probe pattern corresponding to positions of the selected circuit nodes of the electronic assembly. 
   Different ones of the probes and different ones of the terminals are selectively conductively coupled, and the electronic assembly is dynamically held at a selected location relative to the tester and the pattern of probes. The electronic assembly is moved toward the probes to effect contact between the probes and the selected circuit nodes. 
   Another embodiment of the fixture of this invention allows probe contact at both surfaces of the electronic assembly. This embodiment also includes a top dynamic plate spaced from the dynamic plate assembly at the opposite end of the mount. Conductive top access probes are secured at the top dynamic plate. A plurality of conductive transfer probe assemblies extends through the dynamic plate assembly and are secured at the bed and the top dynamic plate. Different ones of the transfer probes are conductively coupled to selected ones of the top access probes and the connection facilities of the conductive terminals at the bed. In this manner movement of at least a portion of the dynamic plate assembly and the top dynamic plate provide both top and bottom probe contact at the electronic assembly. 
   It is therefore an object of this invention to provide low profile electronic assembly test fixtures and methods. 
   It is another object of this invention to provide electronic assembly test fixtures and methods that improve test fixture mechanical functionality related to vacuum chamber performance and probe and UUT support. 
   It is still another object of this invention to provide electronic assembly test fixtures and methods that more readily allow modification of probe locations and related test system assembly interface connections, provide better precision and stability in probe location and contact at UUT node sites, and accommodate shorter signal paths through the test fixture. 
   It is yet another object of this invention to provide electronic assembly test fixtures and methods that are simpler to use and install, and are relatively inexpensive to produce. 
   It is still another object of this invention to provide an electronic assembly test fixture mountable at a tester having a plurality of conductive interface contacts, the fixture including an interface bed having top and bottom surfaces, a plurality of conductive terminals extending through the interface bed between the top and bottom surfaces thereof, each of the terminals having a node at one end positioned at the bottom surface of the bed to correspond to position of one of the conductive interface contacts of the tester, and having a connection facility at an opposite end positioned at the top surface of the bed, a plurality of conductive probes secured at the top surface of the bed and selectively conductively couplable to a selected terminal connection facility, a dynamic plate assembly including a support for receiving the electronic assembly, the plate assembly having a plurality of openings therethrough corresponding to conductive probe positions at the bed, and a mount having the bed secured at one end and the dynamic plate assembly dynamically positioned at an opposite end. 
   It is yet another object of this invention to provide a low profile printed circuit board assembly test fixture mountable at a tester having a plurality of conductive interface contacts, the fixture including a low profile mount including an interface bed having top and bottom surfaces at one end and a dynamic plate movably positioned at an opposite end, a chamber being thereby defined and bounded by the top surface of the bed and the plate, the chamber having an opening thereto for evacuating air from the chamber to effect movement of the plate, a plurality of conductive terminals extending through the interface bed between the top and bottom surfaces thereof, each of the terminals having a node at one end positioned at the bottom surface of the bed to correspond to position of one of the conductive interface contacts of the tester, and having a connection facility at an opposite end positioned at the top surface of the bed in the chamber, a plurality of conductive probes in the chamber secured at the top surface of the bed and selectively conductively couplable to a selected the terminal connection facility, the probes extending through the plate, and positioning means adjacent to the plate for selectively positioning the electronic assembly at the plate. 
   It is yet another object of this invention to provide a method for accessing selected circuit nodes on an electronic assembly for assembly testing using a commercially available tester having a pattern of conductive interface contacts at one part thereof, the method stops including positioning conductive terminals in a field corresponding to at least portions of the pattern of conductive interface contacts of the tester, the terminals for engaging the interface contacts, positioning probes laterally adjacent to the terminals in the field of terminals, the probes positioned in a pattern corresponding to positions of the selected circuit nodes of the electronic assembly, selectively conductively coupling different ones of the probes and different ones of the terminals, dynamically holding the electronic assembly at a selected location relative to the tester and the pattern of the probes, and causing movement of the electronic assembly toward the probes to effect contact between the probes and the selected circuit nodes. 
   It is still another object of this invention to provide an electronic assembly test fixture mountable at a tester having a plurality of conductive interface contacts, the fixture for accessing circuit nodes at both sides of the electronic assembly and including an interface bed, a plurality of conductive terminals at the interface bed, a plurality of conductive probes secured at the top surface of the bed and selectively conductively couplable to the terminals, a dynamic plate assembly, the plate assembly having a plurality of openings therethrough corresponding to conductive probe positions at the bed, a top dynamic plate spaced from the dynamic plate assembly, a mount having the bed secured at one end and the dynamic plate assembly and top dynamic plate dynamically positioned at an opposite end, a plurality of conductive top access probes secured at the top dynamic plate, and a plurality of conductive transfer probe assemblies extended through the dynamic plate assembly and secured at the bed and the top dynamic plate, different ones of the transfer probes conductively coupled to selected ones of the top access probes and the connection facilities of the conductive terminals at the bed. 
   With these and other objects in view, which will become apparent to one skilled in the art as the description proceeds, this invention resides in the novel construction, combination, and arrangement of parts and methods substantially as hereinafter described, and more particularly defined by the appended claims, it being understood that changes in the precise embodiment of the herein disclosed invention are meant to be included as come within the scope of the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings illustrate a complete embodiment of the invention according to the best mode so far devised for the practical application of the principles thereof, and in which: 
       FIG. 1  is a sectional view illustration of a heretofore known and utilized variety of printed circuit board assembly test fixture; 
       FIG. 2  is a perspective view of a first embodiment of the electronic assembly test fixture of this invention mounted on a standard commercially available test system assembly; 
       FIG. 3  is a sectional view taken through section lines  3 - 3  of  FIG. 2 ; 
       FIG. 4  is an enlarged sectional view providing a detailed illustration of one portion of the first embodiment of the test fixture of this invention as shown in  FIG. 3 ; 
       FIGS. 5A through 5C  are exploded views of the test fixture of this invention shown in  FIGS. 1 through 4 ; 
       FIGS. 6A and 6B  are sectional illustrations of lower portions of the test fixture shown in  FIGS. 1 through 5  illustrating operation thereof; 
       FIG. 7  is a partial sectional view of a second embodiment of the electronic assembly test fixture of this invention; and 
       FIG. 8  is a partial sectional view of the embodiment of the electronic assembly test fixture of this invention shown in  FIG. 7  taken at a different section location. 
   

   DESCRIPTION OF THE INVENTION 
     FIGS. 2 through 6B  illustrate a first embodiment  53  of the low profile electronic assembly test fixture of this invention mounted for use at a commercially available tester  55 . The test fixture is used to connect a printed circuit board assembly (hereinafter referred to interchangeably as PCBA or UUT)  57  to the tester system  55  using a single chamber for both wiring and vacuum establishment (as is commonly utilized). Commercially available test systems typically utilize an interface  59  including a plurality of conductive interface spring probe contacts  61  in a uniform pattern or grid  62 . 
   Test fixture  53  is configured to allow simultaneous electrical connection of a large number of contact points on UUT  57  to the standard grid of spring biased probes  61  on tester  55 . Fixture  53  primarily includes mount  65  formed by frame  67 , interface bed  69  and dynamic plate assembly  71 . Vacuum assist hood  73  is provided as is known in systems utilizing vacuum actuation. Chamber  75  is thus defined by mount  65  bounded by the top surface of bed  69  and dynamic diaphragm plate  77  of assembly  71 . 
   Dynamic plate assembly  71  can include only plate  77 , or may include also various positioning guides/supports as hereinafter described. In its simplest form, plate  77  may stand alone on shoulder  81  of frame  67  with only upper seal  83  therebetween and without further structure identified with assembly  71 . To better protect against flexure of plate  77 , however, additional structure may be desired to enhance support across the plane of plate  77  after application of the force drawing the plate downward (i.e., vacuum applied to chamber  75 , for example), as will be described hereinbelow. Moreover, additional means for positioning UUT  57  is desirable in a more complete embodiment of assembly  71  as describe below. 
   Interface bed  69  is preferably formed from mechanically stable and electrically insulating material such as fiberglass and is preferably a unitary construction, though two part constructions as shown in the FIGURES may be utilized. In such case bed  69  includes contact plate  85  and terminal plate  87 . As shown in  FIG. 5B , contact plate  85  is predrilled to match pattern  62  of probe contacts  61  of tester interface  59 . Terminal plate  87  may also be predrilled or may be drilled as necessary to accommodate the fittings applied, and includes openings  89 ,  91  and  93  to accommodate vacuum input  95  and positioning and securing pins  97 / 99 , respectively, at tester interface  59  (other arrangements are possible to accommodate different testers or means of applying mechanical forces to the fixture). Fixture  53  is secured on tester  55  by vacuum or other force and/or by friction caps  101 / 103  over pins  97 / 99  (see  FIG. 5B ). 
   A plurality of conductive terminals  105  (of which, as with many other of the hardware fixtures herein, only a few are illustrated in the FIGURES, but which, in the case of terminals  105 , may number as many as probe contacts  61  of tester interface  59 ) are pressed through bed  69  between top and bottom surfaces  107  and  109 , respectively, establishing a terminal field  110  (see  FIG. 4 ) corresponding to at least portions of pattern  62  of contacts  61  of tester interface  59 . As shown in  FIG. 4 , each terminal  105  includes a rigid contact node  111  at one end positioned at bottom surface  109  of bed  69 , and are positioned (by way of the predrilled pattern) to correspond to position of one of the conductive interface probe contacts  61  of tester interface  59 . A connection facility  113  is positioned at the opposite end of terminals  105  adjacent to top surface  107  of bed  69  (illustrated herein as a wire wrapping posts, though any connecting means, such as solder or welding surfaces, press fittings, insulation displacement fittings or the like, could be used). 
   Conductive spring biased probes  115  (any known variety of spring probe manufactured to the correct length could be used; for example see QA Technology U.S. Pat. Nos. 4,885,533 and 6,570,399) are positioned at top surface  107  of bed  69  laterally adjacent to terminals  105  in terminal field  110 . The probes are positioned in unique pseudo random locations, as defined by the design and layout of the specific UUT. Each probe  115  provides a path of electrical conduction to a terminator (pin or socket depending upon construction)  117  that is pressed into the interface bed  69  and that is configured to firmly hold the probes. The connection of probes  115  to terminators  117  is of a nature that minimum electrical impedance is encountered and good mechanical location and repeatability is assured (see the QA Technology citation, supra). 
   Wire  119  (see  FIGS. 4 and 5A ) couples paired probe/terminators  115 / 117  and connection facilities  113  of terminals  105  (any other reliable method of connection is acceptable—including wire wrap, solder, resistive weld, mechanical press fit, insulation displacement and the like). The selection of each terminal  105  (and thus contact  61  of tester  55 ) to be coupled to a probe/terminator  115 / 117  can be done manually or using software based algorithms. The objective is to minimize the wire length and thus overall signal path. 
   The mechanical function of fixture  53  is activated by evacuating air from chamber  75 . Vacuum seal (gasket)  121  is provided between interface bed  69  and frame  67 . This gasket is static and needs minimum compression. Dynamic plate  77  is sealed against frame  67  by the upper fixture vacuum seal (gasket)  83 . This seal is dynamic and functions to provide the vertical movement of dynamic assembly  71  and thus UUT  57  upon vacuum application. UUT  57  is sealed and positioned at dynamic plate  77  by a perimeter UUT seal (gasket)  123 , and spacing is maintained by stops  124 . The application of vacuum compresses upper fixture seal  83  which results in the engagement of the field of spring probes  115  at the selected circuit nodes of UUT  57 . 
   The extent of downward travel of dynamic plate  77  of assembly  71  is constrained by support columns  125  pressed into interface bed  69 . These columns are placed on a evenly spaced grid in the region outside the field of spring probes  115 . Assembly  71  may include probe guide plate  127  maintained on support columns  129  pressed into interface bed  69  and mounted over UUT guide pins  131 . Columns  129  are placed on a space available basis inside the field of spring probes  115 . At least plate  127  (and, preferably also plate  77 , though this need not be the case unless plate  77  is the only plate in assembly  71 ) has a plurality of openings  133  therethrough corresponding to the positions of conductive probes  115  at bed  69 . Guide pins  131  are mounted in one of columns  129  through plate  127  (guide pins  131  and the related columns  129  are preferably a single construction), plate  77  and UUT  57  slidably mounted at openings  137  over pins  131  (see  FIGS. 6A and 6B ). Pins  131  assure accurate positioning of UUT  57  relative to the field of probes  115 . 
   As may be appreciated, downward travel of dynamic plate  77  of assembly  71  is further limited by guide plate  127  and shoulder  139  at fixture mount frame  67  ( FIGS. 6A and 6B ), thus nearly eliminating undesirable flexure of plate  77  during vacuum application. Another purpose of probe guide plate  127  is to stiffen probes  115  and further assure accurate probe alignment. 
   Vacuum assist hood  73  (best illustrated in  FIGS. 3 and 5C ) is connected with mount  65  at hinges  143  which are also connected to hood frame  145 . Top seal  147  between frame lip  149  and handled lid  151 , and bottom seal  153  at the bottom of frame  145  for compression against plate  77 , preserve vacuum when applied. Lid  151  is secured at frame  145  using screws or the like. Columns  155  are pressed into lid  151  at positions selected so that columns  155  will press against UUT  57  when vacuum is applied. 
   Where prior fixture designs allowed probe plate  18  (see  FIG. 1 ) to flex when vacuum was applied due to inadequate support, fixture  53  of this invention deletes the probe plate, utilizing instead interface bed  69  to hold and support probes  115 . Since bed  69  is supported across its entire area by attachment to tester interface  59 , flexure is substantially eliminated. The engagement of the field of spring probes  115  at selected nodes of UUT  57  is thus stabilized and only the dynamics of probe engagement to UUT  57  need be considered. 
   With reference to  FIGS. 7 and 8 , a second embodiment  161  of the fixture of this invention is illustrated wherein top access unit  162  provides electronic assembly top access by a probe field. As shown, fixture  161  is capable of allowing both top and bottom probe contact with electronic assembly  57  (though top side testing only could be accommodated utilizing fixture  161 ). Fixture  161  utilizes many of the same features as heretofore described with regard to fixture  53  (and thus their description will not be repeated, like numbering of common elements being utilized), and works in accordance with test fixture  53  of this invention. 
   Spring biased top access probes  165  are positioned above UUT mounting on plate  77  at dynamic top plate  167  in a field selected in view of the nodes at UUT  57  to be contacted by probes  115  at the top of UUT  57 . Probes  165  are of the same or similar construction as heretofore referenced, and are secured in terminators  117 , as described hereinabove, pressed into top plate  167 . Plate  167  is sealingly mounted on frame  168  in turn sealingly mounted at plate  77 . 
   The primary issue encountered in top access fixture design is providing a signal path from selected terminals  105  at bed  69  to the top access spring biased probes  165 . To achieve this, a transfer probe assembly  171  is mounted at terminators  117  at top plate  167  and bed  69 . Two spring probes (top and bottom) like those used elsewhere herein may be employed in the assembly, or a single, specially designed spring probe may be utilized. Top and bottom terminators  117  are coupled to a spring probe  165  terminator  117  at top plate  167  and to a terminal  105  at bed  69 , respectively, using wires  119  (or other known connection scheme). 
   In addition, plural linear bushings  175  (preferably at least four) are mounted through top plate  167  at its perimeter to align plate  167  with bed  69  and plate  77 . Linear bushing rails  177  are mounted through plate  77  at alignment bases  179 , allowing plate  167 /frame  168  to move with the system. Each alignment base  179  is attached to interface bed  69  using alignment pins  181 . 
   Fixture  161  begins mechanical movement when air is evacuated from vacuum chamber  75  and top access chamber  183  (through any convenient opening at plate  77 ). Plate  77  compresses upper fixture seal  83  ( FIG. 6B ) allowing spring biased probes  115  to make contact with the test points on the bottom of UUT  57  (if accommodated). Top access unit  162  moves in unison with plate  77  as accommodated by linear bushings  175  sliding along linear bushing rails  177 . Once plate  77  stops (as described hereinabove), top access unit  162  compresses against top access seal (gasket)  185  until pusher column  187  makes contact with plate  77  and UUT pusher column  189  touches UUT  57 . At this juncture, spring biased probes  165  are in contact with test points on the top of UUT  57 , transfer probe assemblies  171  providing the signal path to terminals  105 . 
   As may be appreciated from the foregoing, the test fixtures of this invention are typically used for testing printed circuit board assemblies (PCBA&#39;s), and are adapted for use with commercially available testers utilized for such purposes. However, other electronic assemblies that are tested using spring probes and mechanical (vacuum) actuation to affect electrical connection could make use of the fixtures and methods of this invention.