WO1990006518A1

WO1990006518A1 - Wireless test fixture

Info

Publication number: WO1990006518A1
Application number: PCT/US1989/005331
Authority: WO
Inventors: Robert D. Roberts; Ronald L. Stamp
Original assignee: Cimm, Inc.
Priority date: 1988-11-28
Filing date: 1989-11-27
Publication date: 1990-06-14

Abstract

In order to eliminate the bulk of wire connections in test fixtures, there is provided a wireless test fixture (10) for interfacing a unit under test (UUT) (12) to a distributed receiver (38) of automatic test equipment which includes an interface PCB (28) and an array of floating probes (18) mounted in a specially constructed probe plate (16). The interface PCB (28) has a first set of electrically conductive pads (50) on its upper surface (52) arranged in a custom layout which exactly matches the pattern of test sites on the underside of the UUT (12). A second set of electrically conductive pads (54) is provided on the lower surface (56) of the interface PCB (28) in a pattern which exactly matches the field of probes (42) of a distributed receiver (38). Individual pads of the two sets are selectively connected by interconnection paths of reduced, controlled length which also allow for subsequent modification of the interconnections.

Description

IRELESS TEST FIXTURE

Background of the Invention Technical Field

This invention relates to the field of automatic testing of electronic circuit boards and more particularly to a test fixture for electrically connecting circuit boards to automatic test equipment. Background Art

Printed circuit board assemblies (PCBs), consisting of electronic components mounted on a board of insulating material and connected to form electronic circuits by printed, electrically conductive traces, are found in virtually all electronic and electronically controlled products manufactured today. Products from the very simple (toasters, microwave ovens, washers and dryers), to the more complex (televisions, games, automative) to the most sophisticated (computers, satellites, telecommunications, aerospace) all contain many PCBs. The ability to repeatedly and economically build and verify electronic circuits is the key to the entire electronics industry. PCBs provide a very convenient method of producing electronic circuits, normally in

SUBSTITUTESHEET large quantities, such that all PCBs of the same type are produced identically.

PCBs are tested electrically during the production process. Those that fail are repaired and retested until they are fixed or determined to be unrepairable and scrapped. Various tests can be performed to verify that a PCB operates correctly, including shorts and continuity tests, in-circuit tests, and functional tests. Automatic test equipment (ATE) for executing such tests is available from a variety of vendors with most vendors providing several varieties of testers.

In general, automatic test equipment for production testing of printed circuit boards includes numerous electronic signal sources and detectors

(hereinafter sometimes referred to as test electronics or test resources) which are selectively connected to a plurality of contact points or probes in a receiver. A test fixture provides a mechanical and electrical interface between these receiver probes and test sites on the underside of a circuit board to be tested. Suitable control circuits are provided to actuate the test fixture, and initiate the desired test to be performed, with the test results being presented on various types of readout devices.

One popular type of fixture in use today is actuated by vacuum. Such vacuum actuated test fixtures are typically constructed from three plates commonly referred to as a probe plate, a top plate and an interface panel. The probe plate is generally a thick plate made from insulating material having holes therethrough corresponding to the locations of test sites on the circuit card under test. Electrically conductive probing elements are mounted in these holes. Typically, the probing elements comprise a

SUBSTITUTESHEET spring-loaded probe seated in a probe receptacle having a wirewrap tail extending therefrom. A top plate, having holes corresponding to the locations of the probing elements, is mounted on alignment pins and held above the probe plate by counterforce springs. The top plate supports the unit to be tested. A sealed vacuum chamber is formed above the probe plate. When air is withdrawn from this vacuum chamber, the top plate is drawn toward the probe plate causing heads of the spring-loaded probes to pass through corresponding holes in the top plate and strike the associated test sites on the unit under test.

The interface panel of the prior art vacuum actuated test fixture is mounted below the probe plate. A lower surface of the interface panel is provided with electrical contacts in a pattern which matches the pattern of receiver probes. Upon actuation, these interface panel contacts are brought into physical contact with the receiver probes. On the other side of the interface panel, posts or wirewrap tails are provided in a pattern which corresponds to the contacts on the lower surface and each post is electrically connected to its corresponding contact. Finally, the tails of individual probe receptacles are connected by electrical wire to an appropriate post on the interface panel. These connections are determined in accordance with the desired test resource to be applied to a particular test site.

The individual wires connecting probe receptacles to posts on the interface panel typically have a length of about twenty inches. This expanse of wire is needed to allow the test fixture to be opened and hand wired. Since the test fixture normally contains hundreds of closely spaced probes, assembly of the wired fixture requires substantial manual labor and

SUBSTITUTE SHEET results in a mass of wiring within the finished fixture. Moreover, the extended wire lengths inside the fixture adversely affect the quality and speed of testing. These concerns have previously been addressed by using twisted wire pairs, coaxial cables, shielded cables or by adding electronic test components or ground planes to the fixture. Unfortunately, resulting fixtures have proven very time consuming and costly to construct, as well as difficult to modify.

To reduce cable length between the test resources and unit under test, some ATE has recently been built with distributed receivers. In a distributed receiver, the receiver probes, instead of being concentrated in a relatively small area, as was common in the past, are spatially distributed or spread out to allow the test electronics to be placed right under the probes and thereby reduce the wiring internal to the tester. However, the problems associated with excessive wire lengths within the test fixture, persist.

Summary of the Invention In accordance with the principles of the present invention, these shortcomings of prior art test fixtures are overcome and the quality and speed of testing of circuit boards is improved, through the provision of a wireless test fixture. In place of the usual interface panel and mozaic of discrete interconnecting wires, the wireless test fixture of the present invention employs an interface printed circuit board (PCB). On the upper surface of the interface PCB there is provided a first array of electrically conductive pads in a first pattern which exactly matches a pattern of test sites on a unit to be tested.

' i ^«T5 ET A second array of electrically conductive pads is provided on the lower surface of the interface PCB. The pads of the second array are arranged in a second pattern which exactly matches the pattern of probes of a designated distributed receiver. Interconnection means in the interface PCB connect individual pads of the first array to selected individual pads of the second array.

To reduce interconnection path lengths, the area on' the lower surface of the interface PCB populated by said second pattern of pads generally underlies the area on the upper surface populated by said first pattern. Further, the pads in said second pattern are spatially distributed such that a pad corresponding to a desired test resource is likely to be proximate a pad in said first pattern corresponding to a test site requiring the desired test resource. Individual pads of said first pattern are connected to the closest unused pad of the second pattern which corresponds to the desired test resource for said individual pad. In this manner, the wireless test fixture of the present invention can provide an order of magnitude reduction in signal path as compared to wired test fixtures, and also capitalizes on the recent trend towards using distributed receivers.

In another aspect of the present invention, the wireless test fixture is provided with a plurality of floating spring-loaded probes mounted directly in a specially constructed probe plate. Each of the probes is slidably mounted for substantially free axial movement in a respective aperture of the probe plate. When the test fixture is actuated, a unit under test (UUT) is brought into pressing engagement with the tips of the probes producing physical and electrical contact not only between said tips and test sites on the UUT

SUBSTITUTE SHEET aligned therewith, but also between the lower ends of the probes and the pads in the first pattern on the interface PCB.

The wireless test fixture of the present invention is preferably actuated by reducing the air pressure in an airtight first chamber formed above the probe plate, while maintaining atmospheric pressure in a second chamber formed below the probe plate. The probe plate preferably comprises an upper probe plate, a lower probe plate and a sealing membrane sandwiched therebetween. The sealing membrane provides a vacuum barrier between said first and second chambers. To mount the probes, the probe plate is provided with a custom layout of apertures in a pattern which matches the pattern of test sites on the underside of the UUT. Each aperture in the probe plate consists of a first through hole in the upper probe plate dimensioned to provide a smooth slip-fit for the probe, and a second through hole in said lower probe plate. The second through hole is axially aligned with the first through hole and has a larger lateral extent to provide a relief which allows the sealing membrane, where penetrated by a probe, to tent about said probe and form a vacuum seal thereabqut, without impeding the substantially free movement of the probe. Thus the integrity of the vacuum actuation is maintained while ensuring physical and high quality electrical contact at both ends of the floating probes.

In another aspect of the invention, the wireless test fixture is designed to accommodate subsequent modification. For this purpose, the first pattern on the upper surface of the interface PCB preferably includes additional pads axially aligned with initially unused test sites on the underside of the UUT, and the top plate and probe plate are provided

SUBSTITUTESHEET with axially extending apertures axially aligned with said additional pads to accommodate later insertion of additional probes. A constraint is also imposed on the interconnection means of the interface PCB such that individual pads on the opposite surfaces of the interface PCB are always connected by a first trace on the upper surface, a via through the interface PCB, and a second trace on the lower surface thereof. These traces allow for ready cutting of an original interconnection path, if needed, and rerouting thereof, if desired. Spare pads are also provided on the upper surface of the interface PCB in an auxiliary field outside the area occupied by the first pattern. The spare pads are interconnected through the interface PCB to individual pads in the second pattern and thereby to additional test resources. The spare pads may be subsequently selectively connected to individual pads in the first pattern if such additional resources are needed.

In a further aspect of the invention, the interface PCB is releasably mounted to the fixture so as to facilite ready removal and replacement thereof without disassembling the rest of the fixture. This feature facilitates modification or adaption of a particular test fixture to different testers. In another aspect of the invention, the wireless test fixture can be provided in kit form for subsequent assembling and customizing. The invention further contemplates the provision of voltage distribution plane(s) in the interface PCB, gold plating of the pads to ensure high quality electrical connection, use of mechanical restraining means to limit upward movement of floating probes, and standardization of the fixture envelope and other adaptions to facilitate automatic

^SUBSTITUTE SHEET fixture transport in a fully automated test and repair system.

The present invention affords a host of advantages including higher speed, better quality and more controlled testing. Compared to conventional wired fixtures, the wireless test fixture is significantly thinner making it easier to store, retrieve and otherwise handle, either automatically or manually. The new fixture is easier to build since instead of tedious and time consuming hand wiring it utilizes standard printed circuit board manufacturing technology. The interface PCB and the overall wireless test fixture can be accurately and economically manufactured and assure high quality electrical contacting as well as quick and easy subsequent modification. The new test fixture is also rugged and reliable, applicable to a broad range of testers, automation ready, and amenable to user customization.

Brief Description of the Drawings

These and other objects, advantages and features of the present invention will be more fully understood from the following detailed description when read in conjunction with the accompanying drawings in which:

Fig. 1 is an isometric view of a wireless test fixture, with a notch cut out of it to illustrate internal features, constructed in accordance with the principles of the present invention_? Fig. 2 is a cross-sectional view of the wireless test fixture, in its unactuated state, taken along lines A-A of Figure 1?

Fig. 3 is a top view of an interface PCB used in the wireless test fixture of the present invention_?

^SUB^STITUTESHEET Fig. 4 is a view of the lower surface of the interface PCB of Figure 3?

Fig. 5 is a partial, cut-away cross-sectional view of an embodiment of an interface PCB? Fig. 6 is a partial cross-sectional view illustrating the mounting of a floating probe in a specially constructed probe plate of the present invention?

Fig. 6a is a partial cross-sectional view of a modified probe mounting arrangement depicting mechanical restraining means for limiting upward probe motion? and

Fig. 7 is a cross-sectional view of a wireless test fixture of the present invention, in its actuated state, and situated on an associated receiver.

Detailed Description

A wireless test fixture constructed in accordance with the principles of the present invention is illustrated, in perspective, in Figure 1. Wireless test fixture 10 provides an electrical interface between a unit under test 12 and automatic test equipment (not shown). The unit under test typically comprises a printed circuit board having various circuit components mounted on its top surface with legs extending through the board, and a determined pattern of test sites on the underside thereof. Of course, the nature, number, location and interconnection of the circuit components and the pattern of test sites will vary with different board types. The wireless test fixture of the present invention, like previous fixtures, is typically customized to the particular board type or unit to be tested.

Test fixture 10 includes a substantially rigid perpheral frame 14, which may for example be made

SUBSTITUTE SHEET of aluminum. Mounted atop frame 14 is a probe plate 16 w_hich in turn supports a plurality of spring-loaded pro_bes 18. Probe plate 16 is preferably of a special construction, described hereinafter, which allows the probes 18 extending therethrough to "float" while maintaining a vacuum seal about each probe. Probes 18 are arranged in a pattern which matches the pattern of initially used test sites on UUT 12, with individual probes being longitudinally aligned with particular test sites.

A top plate 20 is supported above and in substantially parallel relationship to probe plate 16 by a peripheral gasket 22. Top plate 20 serves to support the UUT 12 and contains a plurality of through holes. Each through hole in the top plate is axially aligned with a respective probe 18 and is sized to permit passage therethrough of at least the top portion of the respective probe. An edge seal 24 is located on the upper surface of top plate 20 for receiving UUT 12 and providing an airtight seal thereabout. Guide pins 26 ensure proper alignment of the unit under test with the various components of the test fixture.

Mounted below and substantially parallel to probe plate 16 is an interface PCB 28._. As more fully described hereinafter, interface PCB 28 has electrically conductive pads on its upper surface in a first pattern which matches the pattern of test sites on the UUT and a second pattern of electrically conductive pads on its lower surface which matches a probe field of a designated distributed receiver. When test fixture 10 is actuated, the pads on the top surface of interface PCB 28 are brought into direct physical contact with the lower ends of probes 18 while the pads on the lower surface of the interface PCB are physically contacted by the receiver probes.

SUBSTITU TESHEET Individual top side pads are selectively connected to individual bottom side pads in order to apply a desired test resource to a particular test site. Interface PCB 28 is preferably releasably mounted by brackets, hinges or other suitable mounting means 30 to fixture frame 14 in a manner which facilitates ready release and replacement of the interfce PCB without disassembling the rest of the fixture.

The low, flat profile of fixture 10, as evident in Figure 1, facilitates automatic handling of the fixture. Preferably, the fixture is also provided with laterally protruding, longitudinally extending tongues 32 on opposite sides thereof to allow for automatic transport of the fixture in a fully automated test and repair system such as that described in commonly owned U.S. patent application serial number 129,825. For the same purpose, fixture 10 may be provided with alignment holes 34 at either end thereof and other adaptions to make it automation ready.

Figure 2 shows the interrelationship of the components of wireless test fixture 10 in greater detail. In this figure, as in subsequent figures, like elements are identified by like reference numbers. Also, for clarity of illustration, conventional elements .such as mounting or fastening means, guide pins and the like have not been depicted. Similarly, as will be readily apparent to those skilled in this art, only representative components, (e.g. probes, plate apertures, pads, etc.) have been shown in order to avoid obscuring the essential aspects of the invention.

In Figure 2, wireless test fixture 10 is shown, in its unactuated state, resting upon a peripheral gasket 36 of a distributed receiver 38. T_he receiver comprises a plate 40 supporting a field of

SUBSTITUTE SHEET upright spring-loaded probes 42, conventional stops 44, a receiver vacuum port 46, fixture vacuum port 48 and a fixture vacuum port gasket 49. When actuated, air is exhausted through vacuum port 46 causing fixture 10 to settle down on the receiver. Receiver 38 may be either a native receiver available with the original automatic test equipment or a receiver specially constructed or adapted for use with the wireless test fixture of this invention. In any event, the receiver should be distributed, i.e. its probes should be spatially distributed over a relatively large area generally coincident with the area covered by the UUT.

The interface PCB 28 at the bottom of fixture 10 includes a first set of electrically conductive pads so, 50' on the upper surface 52 thereof and a second set of electrically conductive pads 54 on its lower surface 56. The layout of these pads and their interconnection will be described hereinafter with reference to Figures 3-5.

Supported above interface PCB 28 by frame 14 is specially constructed probe plate 16. As shown, and more fully** described hereinafter, the probe plate includes an upper probe plate 58, a lower probe plate 60 and a sealing membrane 62^" sandwiched therebetween. The sealing membrane 62 serves as a vacuum barrier, and, where penetrated by a probe 18 as a vacuum seal. The so assembled probe plate mounts a plurality of spring-loaded probes 18 in apertures, generally designated 64, extending through the probe plate in an axial direction generally perpendicular to the plane of the probe plate. The probes are generally parallel, axially extending, electrically conductive elements. Each probe 18 has a spring-loaded tip 66 at its upper end and is axially aligned with a respective test site, e.g. 68, on the underside 70 of UUT 12. The probes are

SUBSTITUTESHEET thus arrayed in a custom layout which exactly matches a pattern of initial test sites on the underside of the UUT. Probes 18 preferably "float", i.e. they are mounted directly in probe plate apertures 64, in a manner that permits their substantially free axial movement relative to the probe plate.

To allow for subsequent testing at additional test sites on UUT 12, apertures 64' are preferably drilled through upper probe plate 58 and lower probe plate 60 at locations axially aligned with possible future test sites on the UUT. However, the seal membrane 62 is not pierced at these locations until a probe is actually ready to be inserted. The top surface of upper probe plate 58 bears conventional stops 72 and has counter bores or pockets 74 in which are placed conventional counterforce springs 76.

Top plate 20 is supported, as shown, above probe plate 16 by peripheral gasket 22. Top plate 20 contains a series of axially extending through holes 78 which are axially aligned with probes 18. Apertures 78 are dimensioned to accommodate passage therethrough of probe tips 66. Like the probe plate, top plate 20 is preferably provided with auxiliary through holes 78' aligned with potential future test sites on the UUT 12 in order to facilitate subsequent modification of the fixture. Located on the upper surface of top plate 20 are an edge seal 24, which provides an airtight peripheral seal about UUT 12, and conventional stops 80. The test fixture illustrated in Figure 2 is of the moving top plate variety, although the invention is applicable to other types of fixtures as well. For vacuum actuation, a tube 82 is mounted in the fixture so that it extends through interface PCB 28 and probe plate 16. Tube 82, in conjunction with vacuum port 48,

SUB^STITUTE SHEET serves to connect an airtight first chamber formed a_bove probe plate 16 to a vacuum source (not shown) . Actuation of the test fixture results in lowering of t_he top plate, bringing the underside of UUT 12 into pressing engagement with the tips 66 of probes 18.

This results in physical and high quality electrical contact not only between the probe tips and aligned test sites on the UUT, but also between the lower ends of probes 18 and aligned pads 50 on the upper surface of interface PCB 28.

The actuation of test fixture 10 will be described in more detail hereinafter with respect to Figure 7, but first a detailed description of the construction of interface PCB 28, and specially constructed probe plate 16 will be presented.

Referring now to Figure 3, it will be seen that the upper surface 52 of interface PCB 28 is provided with a first set of electrically conductive contact pads 50. Pads 50 are arranged in a first pattern which matches exactly the pattern of initially used test sites on the bottom of UUT 12 and accordingly a pad 50 is located directly beneath each probe 18. Additional pads 50* are preferably included on the upper surface of the interface PCB 28 in axial alignment with the probe plate- apertures 64' and top plate apertures 78' to allow for easy subsequent modification of the test fixture.

Each pad 50 is connected by a first trace 84 on upper surface 52 to a via 86 extending through the interface PCB. The first pattern of pads 50, 50' on upper surface 52 populate a first area generally coincident with the area covered by UUT 12. Outside of this area, spare pads 88 may be provided to affor_d spare test resources, if needed, as described hereinafter.

SUBSTITUTE SHEET As shown in Figure 4, the lower surface 56 of interface PCB 28 is provided with a second pattern of electrically conductive pads 54 arranged in a pattern which exactly matches the field of probes of the associated distributed receiver. Pads 54 which correspond to different test resources are distributed over a second area which is generally coincident with the area covered by the UUT and generally underlies the first area on the upper surface of the interface PCB. By spreading out pads 54 over this second area, the different test resources are, in effect, distributed, thereby increasing the probability of having a desired test resource pad 54 physically near a test site pad 50 requiring same. In this fashion, individual test site pads on the upper surface of the interface PCB can be selectively connected to the closest unused pad of the desired test resource type, on the underside of the interface PCB. Pad interconnection lengths are thereby minimized resulting in enhanced quality of testing. As on the upper surface of interface PCB 28, a trace 89 on lower surface 56 is used to connect each individual pad 54 to an associated via 86.

The interconnection between a pad 50 on upper surface 52 and a pad 54 on lower surface 56^' of interface PCB 28 is graphically illustrated in Figure

5. As earlier indicated, each interconnection includes a trace 84 on the upper surface connected to a trace 89 on the lower surface by a via 86. The via is either filled or lined with electrically conductive material, e.g. copper, to form, in association with the electrically conductive traces, a continuous electrical pathway between the selected pads. When vacuum actuation is utilized, vias 86 must be airtight. The imposed constraint of always employing surface traces to interconnect the selected pads on opposite surfaces

SUBSTITUTE SHEET of the interface PCB ensures that any such interconnecting pathway can be subsequently cut and rerouted, if required. This affords a unique modification capability to the interface PCB. Optimally, all of the pads on interface PCB

28 should meet a design interface specification that assures high quality electrical contact. To this end, the pads are preferably gold plated to afford a contact resistance of no more than about 5-10 milliohms. The ends of the probes are advantageously, similarly treated.

Interface PCB 28 may comprise a multilayer board containing one or more voltage distribution planes. For example, an internal power plane 90 may be provided as a convenient way to distribute power and an internal ground plane 92 used to provide additional signal quality shielding. Construction of such multilayer boards, like the layout and creation of the patterns of pads and routing of interconnection paths can all be accomplished with techniques known in the printed circuit board manufacturing art.

In the event that more test resources are eventually needed than are initially connected to the pads in the first pattern, spare pads 88 may be initially connected to pads on the lower surface of interface PCB 28 to provide for spare test resources. Spare pads 88 can then, if needed, later be selectively connected to appropriate pads in the first pattern on upper surface 52. As will be apparent from the above description, interface PCB 28 replaces both the discrete wiring and interface panel of earlier wired fixtures and affords excellent signal quality and controlled path lengths, as well as unique modification capabilities.

SUBSTITUTESHEET Figure 6 illustrates a specially constructed pro_be plate and floating probe arrangement particularly suitable for use in the present invention. Probe plate 16 includes an upper plate 58, a lower plate 60 and a thin sheet of sealing membrane 62 sandwiched therebetween. Although not drawn to scale, the relative thicknesses of the layers of probe plate 16 are generally as shown in Figure 6.

Upper plate 58 is provided with apertures 94 extending axially therethrough. Apertures 94 are preferably drilled or otherwise formed with a diameter just slightly larger than the outer diameter of probe 18. Upper plate 58 is composed of a material such as PET plastic, which provides a smooth slip-fit for the probe 18 on the inner bearing surface of aperture 94 to allow for substantially free axial movement of the probe in this aperture.

Lower plate 60 is provided with corresponding coaxial apertures 96 having a somewhat larger diameter or lateral extent, than apertures 94. Lower plate 60 serves to keep the sealing membrane 62 from sagging at locations other than apertures 96. Apertures 96 provide a relief which allows membrane 62 to deflect downward and form a vacuum seal about the body of probe 18. Lower plate 60 is preferably composed of electrically insulating material such as a fiberglass material.

Sealing membrane 62 comprises a sheet of thin, initially continuous material which provides a vacuum barrier between aligned apertures 94 and 96, when unpunctured. A sealing membrane such as ASTAT 800 static dissipative material available from Alphastat of Salem, Massachusetts, with a thickness of about 6 mils, has been found to be suitable for this purpose. Before inserting a probe 18, lower end first, into an aperture

SUBSTITUTESHEET 94, a small hole or pin prick is made in the center of the portion of sealing membrane 62 underlying the aperture. The probe 18 is then inserted into aperture ₉₄ until it protrudes through the hole in sealing membrane 62 and aperture 96 in the lower plate. The portion of the sealing membrane circumscribing the body of pro_be 18 is deflected into the relief provided by aperture 96 and tends to form a cone or tent about the _bo_dy of the probe, as shown in Figure 6. The tenting sealing membrane 62 forms a vacuum seal about probe 18 and the relatively higher air pressure below the probe plate, during fixture actuation, acts to increase sealing effectiveness. This effective vacuum seal is provided by the sealing membrane without impeding the substantially free axial movement of the probe and the free transfer of spring compression forces which occurs when the unit under test is brought into contact with the spring-loaded tip 66 of probe 18. Probes 18 thus "float" relative to probe plate 16 ensuring that the desired physical and high quality electrical contact is made at both ends of the probe.

A variation of the floating probe-probe plate arrangement is depicted .in Fig. 6a. As shown, probe 18 is provided with mechanical restraining means 19 about its lower end for limiting upward motion of the probe. Restraining means 19 may comprise, for example, a standard, short length, electrically conductive probe receptacle which snap fits on and to the lower end of probe 18. The restraining means travels with the floating probe and, since its lateral extent is greater than that of aperture 94 in upper probe plate 58, serves to limit the extent of substantially free upwar_d movement of the probe. This is useful in preventing unwanted upward dislodgement of a probe if its tip should become imbedded in or otherwise attached to the

_SUB_STI_TUTESHEET UUT. If probe replacement is desired, probe 18 may be separated, in situ, from restraining means 19 by applying suitable upward force to probe tip 66. Restraining means 19 is retained below upper probe plate 58 and maintained in a generally upright orientation by the extended depth of lower probe plate 60', so that a replacement probe may be readily inserted therein.

Actuation of the wireless test fixture of the present invention is depicted in Figure 7. Actuation begins by reducing the air pressure through vacuum port 46 in the space between interface PCB 28 and receiver plate 40. Since atmospheric pressure is maintained in the chamber directly above interface PCB 28 and about the test fixture 10, pressure differential causes the fixture to be lowered on the receiver 38 compressing receiver gasket 36 and bringing pads 54 on the lower surface of the interface PCB 28 into intimate contact with the tips of receiver probes 42. The use of vacuum as the actuating mechanism and of distributed stops 44 on the top of receiver plate 40 serve to maintain the interface PCB in a desired planar configuration.

As fixture 10 settles down on receiver 38 tube 82 seats on gasket 49 providing a continuous conduit from vacuum port 48 to the airtight chamber above probe plate 16. Vacuum is then applied through this conduit to the chamber above probe plate 16 in order to draw top plate 20 and UUT 12 down towards the probe plate. This downward motion is accommodated by compressible gasket 22 and edge seal 24 and limited by stops 72 and 80, respectively. UUT 12 is thus brought into pressing engagement with the tips of probes 18 producing the desired physical contact between the probe tips and test sites on the underside of the UUT, and between the lower ends of the probes and pads 50 on

SUBSTITUTE SHEET the upper surface of interface PCB 28. Test resources are thereby electrically connected to appropriate test sites and testing can occur. Upon completion of the desired testing, vacuum actuation is terminated and the top plate and fixture are returned to the state depicted in Figure 2, by counterforce springs 76 or the like.

From the foregoing description, it will be apparent to those skilled in the circuit board testing field that a wireless test fixture has been developed which represents a significant advance in this art. The new fixture with its controlled lead lengths promises higher speed and better quality testing. The fixture can be used on multi-vendor or in-house test systems of different varieties to perform various tests including functional and in-circuit tests. The fixture design is simple and reliable and uses industry standard replaceable probes and gold contacts throughout to ensure, high quality electrical connection. The interface PCB of the fixture can be constructed using known circuit board manufacturing technology and eliminates the need for labor intensive hand wiring. The fixture can be provided in kit form allowing the user to assemble and customize it. Provision is also made for subsequent modification of the fixture or adaption to different test equipment. The low, flat profile of the wireless test fixture is ideal for storage and retrieval, either manual or automatic and the fixture is readily adapted for automated transport and handling in a fully automated test and repair environment.

Although a preferred embodiment of the invention has been described and depicted, it will be evident to those skilled in this art that various modifications, substitutions, additions and the like

SUBSTITUTE SHS can be made without departing from the scope of the invention. For example, actuating means other than vacuum may be utilized, or vacuum may be drawn everywhere inside the fixture, or double headed spring-loaded probes may be employed in place of the floating probes under certain circumstances, or the additional and spare pads on the upper surface of the interface PCB may be deleted if no change in test sites or test resource application is envisioned. Similarly, the invention is applicable to test fixtures in different orientations and to fixtures other than those of the moving top plate type. These and other alternates are considered to be within the spirit of the invention, the scope of which is defined by the claims appended hereto.

SUBSTITUTE SHE- "c T

Claims

What is claimed is:

1. A wireless test fixture (10) for interconnecting a circuit board under test (12) to a receiver (38) of an automatic test system, the fixture comprising: a frame (14) ? a probe plate (16) supported by said frame (14) in a first plane? a top plate (20) supported by a peripheral gasket (22) in a plane substantially parallel to and above said first plane? edge seal means (24) mounted on an upper surface of said top plate (20) for peripherally supporting a unit under test (12) in a plane substantially parallel to said first plane? a plurality of substantially parallel, electrically conductive probes (18) mounted in apertures (64) in said probe plate (16) and extending through said apertures (64) in an axial direction generally perpendicular to said first plane, each probe (18) having a spring loaded tip (66) at an upper end thereof, each tip (66) being positioned in axially aligned and spaced apart relationship relative to a respective test site on an underside of the unit under test (12) when the fixture is in an unactuated state, the top plate (20) having openings (78) axially extending therethrough, individual openings (78) being axially aligned with a respective probe (18) and being laterally dimensioned to allow a tip (66) of the respective probe to pass therethrough? actuating means for bringing the underside of the unit under test (12) into pressing engagement with the tips (66) of the probes (18)? and an interface PCB (28) supported by said frame

SUBSTITUTESHEE (14) in a plane parallel to but below said first plane, the interface PCB (28) having a first pattern of electrically conductive contact pads (50) on an upper surface (52) thereof, individual pads (50) of said first pattern being axially aligned with respective probes (18) and being in direct physical contact with lower ends of said respective probes when the underside of said unit under test (12) is brought into pressing engagement with the tips (66) of the probes (18) by said actuating means, said interface PCB (28) having electrically conductive pads (54) located on a lower surface (56) thereof, pads (54) on said lower surface being arranged in a second pattern which matches a contact pattern of a designated receiver (38), and interconnection means for selectively connecting individual pads (50) in said first pattern to individual pads (54) in said second pattern through said interface PCB (28), whereby desired test resources can be connected through said receiver (38), interface PCB (28) and probes (18) to test sites on the unit under test (12).

2. The fixture (10) of claim 1 wherein said first pattern of pads (50) populates a first area on the upper surface of said interface PCB (28), said second pattern of pads (54) populates a second area located on the lower surface of said interface PCB (28), and said second area generally underlies said first area.

3. The fixture (10) of claim 2 wherein the pads (50) in said first pattern correspond to different test sites and the pads (54) in said second pattern correspond to different test resources, and wherein the pads (54) in said second pattern are spatially

SUBSTITUTE SHE! distributed over said second area such that a pad (54) corresponding to a desired test resource is likely to be proximate a pad (50) in said first area corresponding to a test site requiring the desired test resource.

4. The fixture (10) of claim 3 wherein each pad (50) in said first pattern which is selectively connected to a* pad (54) in said second pattern is so connected by an interconnecting path comprising a first trace (84) on the .upper surface of said interface PCB (28), a via (86) through said interface PCB (28), and a second trace (89) on the lower surface of the interface PCB (28)_? said first trace (84), via (86) and second trace (89) being electrively conductive and continuous.

5. The fixture (10) of claim 4 wherein an individual connected pad (50) of said first pattern is connected, by said interconnection means, to the closest unused pad (54) of the second pattern which corresponds to the desired test resource for said individual pad (50) .

6. The test fixture (10) of claim 5 wherein said interface PCB (28) is mounted to the fixture (10) so as to facilitate ready removal and replacement thereof without disassembling the rest of the fixture.

7. The test fixture (10) of claim 5 wherein said first pattern includes additional pads (50') axially aligned with initially unused test sites on the underside of the unit under test (12), and wherein said top plate (20) and probe plate are provided with axially extending apertures (78', 64") axially aligned with said additional pads (50'), whereby subsequent modification of the test fixture is facilitated.

8. The fixture (10) of claim 7 wherein each of the pads (50, 54) on said interface PCB is constructed to afford no more than 10 illiohms of

SUBSTITUTESHEET contact resistance.

9. The fixture (10) of claim 8 wherein each pad is gold plated.

10. The fixture (10) of claim 9 wherein spare pads (88) are provided on the upper surface (52) of said interface PCB (28) in an auxiliary field outside said first area, said spare pads (88) being interconnected through said interface PCB (28) to individual pads (54) in said second pattern, whereby said spare pads (88) may subsequently be selectively connected to individual pads (50) in said first pattern to thereby facilitate modification of the fixture.

11. The fixture (10) of claim 5 wherein said interface PCB comprises a multilayer board containing at least one voltage distribution plane (90, 92).

12. The fixture (10) of claim 11 wherein said interface PCB (28) contains an internal power plane (90) and an internal ground plane (92).

13. The fixture (10) of claim 5 wherein each of said probes (18) is slidably mounted for substantially free axial movement in a respective aperture (64) of said probe plate (16) such that pressing engagement of the unit under test (12) upon the tips (66) of the probes produces physical contact not only between said tips and the test sites aligned therewith but also between the lower ends of the probes (18) and the pads (50) in the first pattern aligned therewith.

14. The fixture (10) of claim 13 wherein said actuating means comprises means for reducing the air pressure in an airtight first chamber formed above the probe plate (16) by said peripheral gasket (22), edge seal (24), top plate (20) and unit under test (12), while maintaining atmospheric pressure in a second chamber formed below the probe plate (16) by the

SUB_STITUTESHEET frame (14) and interface PCB (28)? and wherein said probe plate (16) comprises an 0 upper probe plate (58), a lower probe plate (60) and a sealing membrane (62) sandwiched therebetween, said sealing membrane (62) forming a vacuum barrier between said first and second chambers.

15. The fixture (10) of claim 14 wherein each aperture (64) in the probe plate comprises a first through hole (94) in the upper probe plate (58) dimensioned to provide a smooth slip-fit for a probe

5 (18), and a second through hole (96) in-said lower probe plate (60), said second through hole (96) being axially aligned with said first through hole (94) and having a larger lateral extent to provide a relief which allows the sealing membrane (62), where 0 penetrated by a probe (18), to tent about said probe (18) and form a vacuum seal thereabout without restricting said substantially free movement of the probe (18) .

16. The fixture (10) of claim 15 wherein said fixture is provided with a tongue (32) extending along at least one side edge of the fixture (10) to facilitate automatic transport thereof.

17. The fixture (10) of claims 1 or 5 or 11 or 15 in combination with: a receiver (38) having an array of spring loaded probes (42) arranged in a distributed contact pattern, and second actuating means

5 for bringing the pads (54) on the lower surface (56) of the interface PCB (28) into pressing engagement with the receiver probes (42).

18. A bed-of-nails test fixture (10) for connecting a circuit board under test (12) to a receiver (38) of automatic test equipment, comprising: an array of substantially parallel, ^"5 spring-loaded, electrically conductive probes (18)

SU_BSTITUTE SHEET arranged in a custom layout which matches contact locations on a unit under test (12)? means for supporting the unit under test in an aligned position above said array of probes (18)? an interface PCB (28) located in an aligned position below said array of probes (18), said interface PCB (28) having a first set of electrically conductive landings (50) on an upper surface (52) of said interface PCB (28), said first set of landings (50) being arranged in a first pattern which matches the custom layout of the probes (18), said interface PCB (28) having a second set of electrically conductive landings (54) on a lower surface (56) thereof, said second set of landings (54) being arranged in a second pattern which matches a probe field of a distributed receiver (38), the interface PCB (28) further comprising interconnection means for connecting individual landings (50) of said first set to selected individual landings (54) of said second set? and actuating means for bringing the contact locations on the unit under test (12) into pressing engagement with the probes (18), and the probes (18) into direct physical contact with landings (50) of said first set.

19. The test fixture (10) of claim 18, wherein said interconnection means between individual landings of said first and second sets comprises an electrically conductive and connected: trace (84) on said upper surface, via (86) extending between said upper and lower surfaces, and trace (89) on said lower surface? and wherein said probes (18) comprise floating probes supported by a probe plate (16) which permits

SUBSTITUTE SHEET substantially free longitudinal movement of said probes (18) relative to said plate while maintaining a vacuum seal about said probes.

20. A printed circuit board (28) for use as an interface panel of a test fixture (10), the printed circuit board (28) having a first array of electrically conductive landings (50) on a first surface (52) thereof, the landings (50) of said first array being arranged in a first pattern which exactly matches a pattern of test sites on a unit to be tested (12), said printed circuit board (28) having a second array of electrically conductive landings (54) on an opposite surface (56) of said printed circuit board (28), the landings (54) of said second array being arranged in a second pattern which exactly matches a pattern of probes (42) of a designated distributed receiver (38), said printed circuit board (28) further comprising interconnection means for connecting individual landings (50) of said first array to selected individual landings (54) of said second array.

21. The printed circuit board (28) of claim 20, wherein each of said interconnection means comprises an electrically conductive trace (84) on said one surface, an electrically conductive trace (89) on the opposite surface and an electrically conductive via (86) connecting said first and second traces, and wherein the landings (50) in said first array correspond to specified test sites in the unit to be tested (12) and the landings (54) in said second array correspond to test resources of different types, and the interconnecting means connects an individual landing of said first array corresponding to a particular test site to the closest unused landing (54) of said second array which corresponds to the type of test resource required for the particular test site.

SU_BS_TIT_UTESHEET

22. The printed circuit board (28) of claim 21 wherein all of said landings (50, 54) are gold plated, said vias (86) are airtight, the printed circuit board (28) includes an internal power plane (90) and an internal ground plane (92), and wherein spare landings (88) are provided in a peripheral area of said first surface (52), said spare landings (88) being connected to selected landings (54) of said second array.

23. A kit for assembling and customizing a wireless test fixture (10) for use in testing a unit under test (12) having a particular pattern of test sites thereon, the kit comprising: a substantially rigid peripheral frame member

(14)? a probe plate (16) for mounting to said frame member (14), said probe plate (16) comprising an upper probe plate (58), a separate lower probe plate (60) and a separate sheet of continuous sealing membrane (62) to be sandwiched between said upper and lower probe plates after said plates are provided with an array of aligned apertures* (94, 96) therethrough which matches the particular pattern of test sites? a top plate (20) to be provided with apertures (78) therethrough in a layout which matches said particular pattern? a peripheral gasket (22) for supporting said top plate (20) above the probe plate (16)? an edge sealing member (24) for supporting a unit under test (12) on an upper surface of said top plate (20)? and mounting means for releasably mounting an interface PCB (28) below said probe plate (16).

24. The kit of claim 23, further comprising

SUBSTITUTESHEET a plurality of electrically conducting, spring-loaded probes (18) for slip-fit mounting in apertures (94) extending through said upper probe plate (58)? and tube means (82) for connecting a chamber formed above the mounted probe plate (16) to a vacuum source.

25. The fixture (10) of claim 13 or 19 further comprising restraining means (19) associated with a lower end of at least one probe (18) for limiting the extent of substantially free movement of said probe (18) in a direction away from said interface PCB (28).

SUBSTITUTESHEET