GB2145582A - Electrical test probe assembly - Google Patents

Abstract

A switchable test probe assembly comprises a conductive probe pin 16 which has one or more non-conductive bands 24, 25 and is slideably mounted for axial deflection in a conduction tubular housing 28 having a portion 30, 32 which contacts the pin to conduct a signal from the pin head 18 only when the pin assumes preselected axial position(s) of deflection with respect to the housing. A plurality of the assemblies are used in a printed circuit board test fixturing system having means for controlling the degree of deflection of each pin, optionally together with conventional probes. <IMAGE>

Description

SPECIFICATION Multi-stage fixtuing system employing deflectably switchable probe assemblies This invention relates to fixturing systems of the type used in the automated testing of printed circuit boards. More particularly, this invention concerns a muki-stagefixtunng system employing a unique probe assembly which is automatically rendered conductive or nonconductive as it is deflected.

Circuit boards may be subjected to a variety of tests during assembly and beforethey are actually installed or used in the systems and appliances for which they were designed. For example, a "functional" test is performed simplyto determine whether electrical input signals impressed on the input leads of a board result in the desired output signals on the board's output leads. Anothertype oftestcalled an "in-circuit" test is performed by electrically isolating and testing the components found on the circuit board without physically removing them therefrom. Still another type oftestcalled a "bare board" or a "shorts and continuity" test is performed byverifying continuity and integrity of circuit paths.Each kind of a test performed on a circuit board maybe referred to as a stage and there may be several testing stages involved in checking a single circuit board.

The electrical connections between a circuit board undertestand a test system are made through a fixturing system. These systems are typically of the bed of nails type, employing deflectable test probe assemblies having contactheadswhichengagethe test points on a circuit board. Each test probe assembly is separatelywired into a testfixture contact panel. Electrical contact is made through the contact paneltothetestsystem being used.

The tests performed at different stages in the testing process involve making contact with different sets of test points on the circuit board. Thus,thetest points used foronetest are not necessarily sufficient or even appropriate for performing anothertest. Moreover, one does not havethe choice of simply connecting all possible test points of into rest to the test system and then electrically utilizing them selectively as the tests proceed. This approach leads to undesirable inductance and capacitance effects brought about by the presence of connected lead wires running from the test point th rough the test system. Bare board tests typically require multiple test points in each circuit node.In-circuittests typically require only one test point in a circuit node. Functional tests require a still smaller number of test points, usually at the circuit board connectors. In-circu it tests may be impossible to perform orthe results may be rendered unreliable due to the capacitance added to the circuit when more than one test probe is in contact with a circuit node.

Similarly, the circuit board may not operate at all undertheconditions of a functional test if capacitance is addedtothecircuit board by the presence of extraneous test probes in contact therewith.

Within the confines of a single test mode, such as an in-circuit test, it is frequently desirable preliminarily to do a component check on certain blocks of circuitry because subsequenttesting may require the operation of those circuit blocks. This is often the case when executing in-circuit tests on circuit boards with an on-board clock oscillator. It may be desirable, for example, to do a component check on the oscillator components (crystal, resistors, capacitors, IC's etc.) first because the in-circuittests on the remainder of the board often will require the oscillator to be running. However, the oscillator may not operate correctly with test probes in contact with and adding capacitance to the internal nodes of the oscillator circuit.

It has, therefore, been a goal in the testfixturing art to find a way of connecting to a circuit board undertest onlythose probes necessary for a particulartestwhile thattest is being run. Variousfixturing systems are currently available which represent efforts to achieve this goal. One type oftestfixture makes use of interchangable personalized plates inserted between the circuit board and the field of probes, the plates having holes drilled therethrough that correspond to the locations of only those selected probes which are needed to contact the test points on the associated printed circuit board. This type offixturing system, exemplified by U.S. Patent No. 4,321,533 is used for testing any printed circuit board within a family of different boards.However, because the printed circuit boards are generally held in place undervacuum, a change in the personalized plate requires a time consuming interruption of the vacuum seal. Thus, it would be impractical in production testing to use a fixturing system ofthis type to run a series of tests on each circuit board with a single test system.

A second type of test fixture incorporates sets of movable platens, each platen containing those probes that are necessary for the performance of a particular test. This type offixture has the capability of advancing a selected platen in orderto bring a desired set of probes into contact with the circuit board under test, as described in U.S. Patent No. 4,115,735. However, printed circuit boards have become increasingly complex, requiring the use of a maximum number of circuitpathswithin a minimum amount of board space. Naturally, the number of test points increases with the increased concentration of circuit paths, ultimately requiring a corresponding increase in the number of testing probes. Whilethe system using the multiple platen arrangement may be sufficientforthe testing of less complex circuit boards, it cannot be adapted to accommodate the required increased number of probes for several reasons. First, there is a physical limitation to the number of probe-containing platens that can be mounted within a singlefixturing system. Secondly, there is limited access to any of the probes contained within the system during a test because all the probes are under vacuum seal. And, third, each movable platen requires a separate set of guide posts, vacuum seals and stops, thus greatly increasing the chances of vacuum leakage and mechanical failure. Fourth, since the interconnecting The drawing(s) originally was (were) informal and the print here reproduced is taken from a later filed formal copy.

wires between the contact panel and the test probes are subjectto motion and physical displacements, the electrical characteristics ofthe fixture will be subject to change,thereby causing errors of uncertain magnitude in test results.

An additional concern in the design of new test fixturing systems is the expensiveness of the probes.

The testing probes are expensive because they are typically gold-plated and constructed of very hard corrosion-resistant material. Also, each test probe is spring loaded and contains four main components, namely, a probe pin, a housing, a spring and a probe assembly receptacle, each ofwhich is manufactured to fairly close tolerances. For this reason, it is advantageousto provide a testing system that can utilize a single field oftest probes for performing multipletestson a given circuit board.

Still another concern relates to the use of time in running a sequence ofvarioustest stages. Whether the system is operated manually or robotically, it is clearly desirably to permit the system to quickly switch from one test stage to anotherwith minimal time intervals in between. In fact, itwould be ideal to have a multi-stagefixturing system in which the staging could be controlled by test systems software.

With such a system, the software could continuously select, during a multiple test run,the next most desirable test and the corresponding test stageforthe fixturing system. If, for example, a board failed a functional test, it might be desirableto proceed directlyto an in-circuit test for a relatively quick isolation ofthefault ratherthanto usethe inherently slower method of functional testing for this purpose.

Thus, by appropriate programming ofthe sequence of the various types of tests that can be performed, a considerable increase in circuit board throughput can be achieved.

All of the foregoing shortcomings in the prior art, as well as the indicated goals and objectives, are addressed by the subject invention by employing a unique probe assembly which is automatically switched on and off as it is progressively deflected. A highly efficient and reliable multi-stage fixturing system employing such probe assemblies has been discovered as described below. The resulting fixturing system is quite versatile because the probe assemblies employed therein can be a mixture ofthe novel switchable probes and the well known conventional probes. Byusing probes ofeithertypewhich are pluggable, it becomes a relatively simple matterto establish a desired mixtureofthetwo probe types.

Also,the fixturing system to be described employs a vacuum-operated method for securing a circuit board in its test position and the arrangement is such that there is no need to release the vacuum in sequencing from oneteststageto another. In the preferred embodiment ofthis fixturing system there are no unbalanced forces caused by atmospheric pressure which would necessitate the use of relatively heavy appliances for stage sequencing during evacuation.

It is, therefore, an object ofthis invention to provide a fixturing system permitting the performance of a series of tests of different types on a single test system in a minimum oftime.

It is a further object ofthis invention to provide a fixturing system ofthe type described which permits the selective disconnection of test probe assemblies very close to the circuit board contacted, thereby substantially eliminating undesirable inductance and capacitance effects associated with the wiring for such disconnected assemblies.

It is a further object ofthis invention to provide a fixturing system ofthetype indicated which employs a single set of test probesfora series of test stages.

It is a further object ofthis invention to provide a fixtu ring system ofthe type indicated which permits rapid sequencing from oneteststageto another through the use of relatively light and simple staging mechanisms.

It is a further object ofthis invention to provide a fixturing system ofthe type indicated with sufficient simplicity to permitthe use of a maximum number of probe assemblies within an area corresponding to the size of a circuit board to be tested.

It is a further object of this invention to provide a fixturing system of the type indicated, thestaging sequence of which could be controlled bythe software of a test system with which it is used.

It is a further object ofthis invention to provide a fixturing system ofthetype indicated having an interchangeable adaptor which may be personalised to particular circuit boards, thereby obviating the need to modifythe entire fixturing system for every types of circuit board to be tested.

It is a further object ofthis invention to provide a novel test probe assembly which is alternately rendered conducting and nonconducting as it is progressively deflected.

It is a further object ofthis invention to provide a probe assembly ofthe type indicated which is interchangeable mechanically with conventional probe assemblies.

According to the present invention a probe pin assembly comprises a tubular conductive probe housing adapted to receive a probe pin; -a a conductive probe pin having a contact head at one end, said probe pin being slideably mounted for axial deflection in the housing with thecontact head protruding therefrom; and - means for rendering the assembly alternately conductive and nonconductive from the contact head through the pin and thence through the housing as the pin assumes preselected axial positions of deflection with respecttothe housing.

According to another aspect of the present invention a multi-stage fixturing system comprises: -a a fixture housing having meansforsecuring thereon a circuit board to be tested; - aplurality of deflectably switchable contact headed probe pin assemblies; - means for mounting the probe pin within the housing in a generally parallel relationship with one another with their contact heads facing the board to be tested; and - means for advancing the contact heads of the probe pin assemblies and the circuit board into electrical contact with one another and for controlling the degree of deflection in each assembly during contact.

According to a further aspect of the present invention a method for testing printed circuit boards comprises the steps of: - providing a fixture housing; -providing a pluralityofdeflectablyswitchable contact headed probe pin assemblies; -mounting the probe pin assemblieswithinthe housing in a generally parallel relationship with one another with the contact heads facing the board to be tested; - advancing the contact heads ofthe probe pin assemblies and the circuit board into electrical contact with one another; and - controlling the degree of deflection in each assembly during contact so that the assembly is rendered alternately conductive and nonconductive as the contact head assumes preselected axial positions of deflection within each assembly.

Viewed in one aspect, the invention is a multi-stage fixturing system containing an array ofswitchable test probe pin assemblies. Each switchable assembly contains a probe pin that has at least one band of nonconducting material surrounding a stem.The probe pin is slideably mounted inside a conductive housing for axial movement so that the nonconductive band moves in and out of the housing. The probe pin stem makes electrical contactwith the housing only atthe top end wherethe housing contains an internal constriction of reduced diameter. In the preferred embodiment, the system utilizes a movable platen with rigidly mounted connector probes therein for making contact with and controlliing the deflection of a separate set oftest probes, some of which are deflectably switchable.Staging is accomplished by means of an interchangeable adapter employing a vacuum operated means for securing the circuit board thereon. The adapter is related to the field of test probes by means of a mechanism for moving it toward and away from the test probes without having to release the vacuum each time.

Viewed in another aspect, the invention involves a novel probe pin assembly which is deflectablyswitchable.The probe pin assembly includes a probe housing in which a probe pin is spring loaded for slideable reciprocal axial movement or deflection. The probe pin has a contact head which protrudes from the housing supported by a stem which is covered at one or more preselected locations with a band of nonconducting material. The other end and a proximate portion ofthe probe pin is likewise covered with nonconducting material. The housing has a constriction atthe point where the probe pin enters which serves to maintain sliding electrical contactwith the stem of the probe pin.An electrical path is thus established or interrupted in accordance with the degree to which the probe pin is deflected within the housing, said path running from the contact head through the stem and thencethrough the constriction and probe housing. An alternate embodiment involves the use of a nonconductive lining in the housing instead ofthe covering provided on the one end ofthe probe pin.

The method ofthis invention involves the mounting of a plurality of probe pin assemblies, some switchable and some conventional, within a fixture housing and advancing them into electrical contact with a circuit board to be tested and subsequently controlling the degreeto which the probe pin assemblies are deflected.

In orderthatfeatures and advantages of the present invention may be appreciated, embodiments will now be described, by way of example only, with reference to the accompanying drawings of which: Fig. lisa perspective view of a test fixturing system embodying the invention shown as being enclosed within an outer housing.

Fig. 2 is an enlarged side view, partly in section, of a switchable probe pin.

Fig. 3 is an enlarged side view, partly in section, of the preferred embodiment of a switchable probe pin assembly which includes the probe pin and probe housing.

Fig. 3A is a fragmentary cross-sectional view of an alternate embodiment of a probe housing.

Fig. 4 is an enlarged side view, partly in section, of the preferred embodiment of a switchable probe pin assembly within a receptacle having a wire wrap post at its base.

Fig. 5 is an enlarged side view, in cross section, of an alternate embodimentofa switchable probe pin assembly shown within a fragmentary portion of a receptacle.

Fig. 6 is a central cross-sectional view, taken on line 6-6 of Fig. 1, of a portion ofthe preferred embodiment ofthe multi-stage fixturing system having its outer housing removed.

Figs. 7,8 and 9 show a portion of the structures illustrated in Fig. 6 in a sequence of typical test stage positions.

Fig. 10 is a cross-sectional view of an alternate embodiment of the multi-stage fixturing system.

Illustrated in Fig. 1 a multi-stagefixturing system 10 enclosed in an outer casing 11 with printed circuit board 12 located in its testing position. Fixturing system 10 is adapted to be connected to a test systen by means of a contact panel 14. Contact panel 14 is the same in all essential respects and serves the same functions as the contact panel described in United States Patent No. 4,230,985.

Particular attention in that patent is drawn to the discussion in column 4, lines 27-68, columnS, lines 1 and 2, column 6, lines 30-68 and column 7, lines 1-23.

The preferred embodiment of this invention is vacuum operated and a suitable hose connection 13 is provided through one side of the outer housing 11.

As shown in Fig. 1 the circuit board rests on a perimeter gasket 46. Gasket 46 is mounted on a top plate 45 to be more fully described below. The interior ofthefixturing system illustrated in Fig. 1 similar in a general way to other fixturing systems of the same type, as for example the system described in the above mentioned U.S. Patent 4,230,985. Thus, it includes a plurality of probe pin assemblies, each assembly having a contact head intended to make electrical contact with a test point located on the underside of a circuit board 12 being tested. Each probe pin assembly is electrically wired to a contact post in contact panel 14 for ultimate electrical connection with a test system. The general arrangement of these partsinthat portion ofthefixturing system 10 undercircuitboard 12 may be morefullyappreciated by referenceto Fig. 6 or 10 herein. Fig. 6 is a fragmentary central cross-sectional view of the preferred embodimentofthe multi-stagefixturing system of this invention taken on the line 6-6 of Fig. 1, butwith outer housing 11 removed. As noted above, Fig. 10 is a cross-sectional view of an alternate embodiment of the multi-stagefixturing system ofthis invention.

Before proceeding with a detailed explanation of the structure and operation ofthe multi-stage fixturing system 10, the structure and operation ofthe deflect ably switchable probe pin assembly ofthis invention will be discussed. Illustrated in Fig. 2 is a switchable probe pin 16 having a stem 22 with a contact head 18 at its one end and a tail 20 at its other. A bulbous structure on the stem 22 neartail 20 serves as a retaining stop 26 as will be explained below. Probe 16 is generally constructed of heavy gauge electrically conductive non-corrosive material such as stainless steel plated with a noble metal such as gold. Itwill be noted, however, that a band 24 of nonconductive material surrounds stem 22 art a predetermined distance from contact head 18.Also, a covering 25 of nonconductive material covers tail 20 and a proximate portion of stem 22 including retaining stop 26. It is desirable, for reasons which will appear below, to maintain a uniform diameter along stem 22 from head 18 through and beyond nonconducting band 24. Itis also desirable, however,thatthe portion ofstem 22 between its approximately mid-point and retaining stop 26 be thinnerthan the rest of stem 22, for reasons which presently appear. One method offabricating probe pin 22 involves undercutting selected areas of stem 22 during screw machining in orderto accommodate the installation of nonconductive band 24 and nonconductive covering 25. Of course, the undercutting would be greater in the thinner portion of stem 22 just described.Nonconductive band 24 and covering 25 may be conveniently applied to stem 22 by spraying coating a material such afluro-carbon like thatsold underthetrademarkTEFLON in the intended areas after suitably masking adjacent areas of stem 22.

Alternately, the fluro-carbon coating may be applied to the entire stem 22, then the excess may be removed bya machining process.

Fig. 3 illustrates a deflectably switchable probe pin assembly constructed according to the preferred embodiment ofthis invention. It includes a probe pin 16, described with reference to Fig. 2, and probe pin housing 28. Probe pin housing 28 is constructed of conductive material such as brass or copper plated with a noble metal in a tubular configuration, the inside diameter ofwhich is large enough, in general, to permitthe passagetherethrough of stem 22 without making contacttherewith. Itwill be seen that pin 16 is freeto moveaxiallywithin housing 28.This axial movement of pin 16 with respecttothe housing is known in the art as "deflection." A crimp 34 is provided in housing 28, however, which interferes with retaining stop 26 of pin 16 to limitthe outward movement of pin 16. Thus, stop 26 and crimp 34 serve to retain pin 16within housing 28. Pin 16 is biased to its most outwardly extended position by means of a spring 36.

Housing 28 is provided with a constriction 32 at the end where pin 16 protrudes from the housing.

Constriction 32 may be formed by crimping the end of housing 28 so that itwill have an inside diameter atthe point very close to the outside diameter of of pin 16.

Alternately, the constriction 32 may be formed by a flux concentration at the end of housing 28 during a plating process. In any event, a sliding contact is to be maintained between constriction 32 and stem 22.

Thus, as pin 16 is axially deflected with respectto housing 28, stem 22 and the constriction 32 remain in continuous contact. It will be understood thatthe extended position ofthe pin 16 in which it protrudes to the maximum extent from housing 28 is such that the thinner part of stem 22 is well below constriction 32. In other words, it is intended that the slideable contact between constriction 32 and stem 22 will occur through the length of nonconducting band 24 and somewhat beyond that band on either side thereof.

As is now apparent, a current path is made through probe pin assembly 30 from contact head 18 through stem 22 to constriction 32 and thence through housing 28 when an uncovered portion of stem 22 is in contact with constriction 32. On the other hand, when pin 16 is sufficiently deflected with respect to housing 28 so that nonconductive band 24 is aligned with constriction 32, this current path is broken and the probe assembly is said to be "switched off". Thus, probe assembly 30 will be "on" when pin 16 is initially deflected and until the deflection reaches the point where nonconducting band 24 aligns with constriction 32.Probe assembly 30 will be turned "off" atthat point and it will remain off until pin 16 is further deflected to the point where band 24 passes through constriction 32 and a conductive uncovered portion of stem 22 is again aligned with collar32. Although only one nonconductive band 24 is illustrated, several such bands could be used thus giving probe assembly 30 more "on" and "off" positions. It is likewise clear that a band could be placed on stem 22 so that the initial position of pin 16 would result in an off condition ratherthan an on condition.

It may be observed that a reliable electrical contact is made between constriction 32 and stem 22 by virture ofthe configuration just described. When probe assembly 30 is in use, contact head 18 will be pressed against by the test point of a circuit board being tested and thereby causing pin 16to be deflected. Spring 36 meanwhile will be urging pin 16 in the opposite direction. The combination of these two forces in opposite directions along pin 16will cause itto become canted in housing 28 thus ensuring thatthe portion of stem 22 opposite constriction 32 will be pressed against one side or another of constriction 32. This canting condition, or "side loading" as itis called, is also accommodated by having stem 22thinner in the indicated region so asto provide ample clearance at crimp 34.

A different type of loading namely, "center loading" can be achieved through the use of a probe housing like that illustrated in Fig. 3A. Fig. 3A shows a fragmentary portion of a probe housing 128 in cross-section. Housing 128 has an opening 133 at one endthroughwhichthestemofa probe pin may slideably pass. At that same end, housing 128 is divided into fourfurcations 135 by means of slots 134.

Of course, the number offurcations is not critical. Each furcation 135 isformed on its end with an inward facing convex portion 132, thus forming a constriction analogous to that described with respecttothe probe housing 28 illustrated in Fig. 3. Still referring to Fig. 3A, itwill be appreciated that by using a probe pin 16 of sufficently large diameter, the contact between convex portions 132 and the probe pin can be spring loaded. Furcations 132 act as a spring element similar to that of a cantilever spring and press against the stem ofthe probe pin. Furthermore, this arrangement tends to keep a probe pin centered within the housing, andhus, is particularly useful in applications where it is necessary to more precisely maintain the alignment of a probe pin.

Fig..4 shows a probe assembly 30 mounted within a receptacle 38. Receptacle 38 would normally be rigidly mounted in a plate or some othersimilarstructure as a means for holding a probe assembly 30. Probe assembly30 may be pluggably fit within receptacle 38 so as to ensure good electrical contact therewith and to permititseasy removal therefrom. Receptacle 38 is made of a conductive material, preferrably brass or copper andplated with a noble metal. It may be provided with a wire wrap post 40 for connection ultimatelywith a test system.

Fig. 5 illustrates an alternate embodiment ofthe deflectably switchable probe assembly ofthis invention. There, a probe assembly 30' is shown within a receptacle 38' of which only an upperfragment is illustrated. Stem 22' is covered with a nonconducting band 24' as in the preferred embodiment. However, the tail 20' and the lower end of pin 16' are not covered.

Rather, a non-conductive lining 33 covers the inside surface of probe housing 28'. Itwill be understood that this alternate embodimentfunctions in the same way as the preferred embodiment ofthe spring probe assembly discussed above.

The key elements of a multi-stagefixturing system 10 taking advantage ofthe capabilities of the deflectably switchable probe assembly of this invention are illustrated in Fig. 6. Fig. 6 is a cross-sectional view showingtfieinnerhousing ofthe invention upon which various switchable and conventional probe assemblies are mounted.The preferred embodiment ofthis invention, as shown in Fig. 6, is divided into two main portions, a top unit or adaptor 44 and a fixture base or base unit 66. Itwill become apparent as this description proceeds;that base unit 66 can accommodate an interchangeable series of adaptors 44, each unit being specially configured and designed for the testing of a particulartype of circuit board.

Adaptor44 includes a top plate45 having perimeter gasket 46 cemented thereto. Gasket 46 serves to properly seal a circuit board 12 on the fixturing system and may be made of any suitable deformable gasket material that will serve to maintain a seal during the operations to be described below. Adaptor44 also includes a movable platen 48 which is somewhat spaced apartfrom top plate 45 by a plurality of spacers 101 to form a slot 56. Top plate 45 and movable platen 48 are held in that spaced-apart relationship by bracket 74 and sealed by edge seal 102. A vacuum port 61 is formed in movable platen 48 and communicates by means ofvacuum tube 60 with vacuum hose 13 shown in Fig. 1.Referring still to Fig. 6, it will be seen thatthereisavacuumwell between the movable platen 48 and the circuit board 12 which includes interior space 58 located between circuit board 12 and top plate 45. Apertures 54 in top plate 45 provide for communication between interior space 58 and slot 56.

A series of three conductive connector elements 49 are shown as being rigidly mounted in movable platen 48. As ilustrated, these elements 49 may conveniently be comprised of conventional deflectable probe assemblies plugged into rigidly mounted receptacles 51 with probe pins 50 protruding therefrom and passing through top plate aperture 54 and facing circuit board 12. A "conventional" probe assembly is one not having a nonconductive band on its stem, as described above, and which is therefore always in an "on" condition.

Connector elements 49 are provided with suitable tips 53 for making contact with the circuit board 12.

The opposite ends of connector elements 49 are formed with an inverted cup shape 72 (not visible) serving as a socketto receive the contact head of a probe pin. The reason forthis structure will be understood more fully as this description proceeds.

Base unit 66 includes a mid-plate 52. A series of test probes 71, some conventional 76 and some deflectably switchable 30, are plugged into suitable probe receptacles 38 which, in turn, are rigidly mounted in mid-plate 52. Receptacles 38 are provided with wire wrap posts 40 which are wired to their corresponding contact posts in contact panel 14. Although only three sets of connector elements 49 are shown in movable plate 48 and three corresponding sets of test probes are shown in mid-plate 52, it will be understood that a practical embodiment ofthis invention would include numerous sets of each, numbering perhaps into the thousands. Connectorelements49 are located and aligned in movable platen 48 so thattheircontact heads 53 are in a pattern matching that ofthetest points on circuit board 12.Likewise, the cup-shaped ends 72 of connector elements 44 are in a pattern matching that ofthe contact heads ofthe probe assemblies mounted in mid-plate 52. Itwill also be noted that connector elements 49 are substantially parallel with one another as aretest probes 71.

Adaptor44 is connected to base unit 66 by means of supportscrews62which are rididlymounted in movable platen 48. Although only one support screw 62 is illustrated in Fig. 6, it should be understood that several of them are used for the purposes indicated below. Support screws 62 performs several different functions. First, in conjunction with tooling pins (not visible),theyserveto properlyalignadaptor44with base unit 66, in general, and mid-plate 52 in particular.

This alignment is critical because each connector element 49 must be able to engage the contact head of its corresponding test probe assembly mounted in plate 52. This occurs during the test procedures to be described below. Secondly, support screws 62 serve as a means for advancing and retracting adaptor 44 toward and away from mid-plate 52, as more fully explained below.

Each supportscrew62 is connected to a nut 104. Nut 104 is driven via a conventional connector element 1 10 by a suitable motordrive (notvisible) mounted within the test system to which thefixturing system 42 isto be attached. Nut 104 is journaled and captured axially and radially within a suitable bushing member 64. Bushing member 64 is rigidly mounted in mid plate 52.

When nut 104 is rotatablydriven bythe motor drive, itwill either draw support screw 62 and adaptor44 downwardlytoward mid plate 52 or itwill push screw 62 and adaptor 44 upwardly away from mid plate 52, depending upon the direction of rotation. These operations serve to electrically connect the contact heads of the test probes 71 with circuit board 12 through connector elements 49. Likewise, these op erations can be used to control the degreeto which the test probes 71 mounted in mid-plate 52 are deflected.

Of course, any suitable means serving to operate on adaptor 44 in the indicated manner could be used in place of the structures just described.

Figures 7,8 and 9 shows a typical sequence of operations ofthe multi-stage fixturing system of this invention. In Fig. 7, it will be noted thatthe vacuum well including interior space 58 has been evacuated and that circuit board 12 has been drawn down to make contactwith contact heads 53 of connector elements 49. It may also be noted that in both Figs. 6 and 7, not all ofthe contact elements 49 are in contact with the test probe assemblies 71 mounted in mid-plate 52. This condition can be accomplished by providing variations in the lengths of connector elements 49 or in the lengths ofthe test probes 71 mounted in mid-plate 52. The purpose of such an arrangement is to permit the selective connection of the test system to test points on circuit board 12 in the initial test stage without connecting unneeded probes.

As also shown in Fig. 7, a conventional test probe assembly 76 and a switchable test probe assembly 30 are each connected to connector elements 49, thus establishing contact between the corresponding test points on circuit board 12 and the test system. Note that the nonconductive band 24 of switchable probe 30 is well above receptacle 38 and probe housing constriction 32 (see Fig. 3), although constriction 32 is not visible in Fig. 7.

Referring nowto Fig. 8, itwill be seen that adaptor 44 has been lowered bythe action ofthe motor means on screw support 62. As a result, all test probe assemblies 71 mounted in mid-plate 52 are now in mechanical contact with their corresponding elements 49. Note, however that nonconducting band 24 of switchable probe assembly 30 is partially below the top of its receptacle 38, and hence, aligned with constriction 32 of its associated probe housing 28.

This probe assembly is thus turned off since the circuit is broken atthat point. Meanwhile, the othertwo probes are on.

Fig. 9 illustrates yet anothertesting stage in which adaptor 44 has been further advanced toward midplate 52. The probe assemblies mounted in mid-plate 52 have been further deflected and it will now be seen that nonconducting band 24 ofthe probe assembly is well below the top of its associated receptacle 38, thus turning this probe assembly on again.

From the fo; egoing, many of the advantages of the subject invention are now clear. For one thing, there is a considerable degree of versatility permitted by the above described design. The test probes mounted in mid-plate 52 can include switchable and conventional probes. Some can be mounted atfull heightwhile others at less than full height. And since the probe assemblies are pluggably mounted in their receptacles 38,these patterns can be changed as desired.

Furthermore, the useoftheadaptor44permits even more flexibility. One could choose, for example, not to engage selected probe assemblies in mid-plate 52 simply by selectively omitting the appropriate connector element 49.

Perhaps a more importantadvantageflowingfrom the above described structures relates to the electrical properties of the device. Since thetest system is disconnected from the circuit board under test at the test probe, there is substantially no impedance added to the circuit board undertest by probes that are turned off. This is true of probes that are off by virtue of not being in mechanical contactwith connector elements 49 as well as probes that are turned off by virture ofthe nonconductive band(s) being aligned with constrictions 32 of their associated probe housings. Since there is no substantial increase in impedance ofthe circuit board undertest, test methods utilizing high speed signals may be used.The circuit board under test may betested atthe speed it is intended to operate, as in a functional test. If a fault is found in the circuit board undertest,fault location can proceed accessing a multitude of circuit nodes as in an in-circuittest An additional advantage flowing from the above described structures relates to the ease with which one can operate the system from onetesting stage to another.Itwill be noted thatthe only areas evacuated are the interior space 58 under circuit board 12 and slot 56betweentop plate 45 and movable plate48.Since the atmospheric pressure on the top ofcircuit board 12 and top plate 45 is the same as that on the underside of movable platen 48, the onlyforce needed to advance or retract adaptor 44 is that which is necessawto overcome the spring probe forces. In systems where the atmospheric pressure cannot be so equalized, there is a net atmospheric force which must be overcome in moving from one test stage to another.

This would necessitate the use of extremely large and powerful mechanisms or, more practically, the release ofthe vacuum when one wishes to move from one test stage to another.

Finally, referring to Fig. 10, an alternateembodi- ment ofthe multi-stage fixturing system is illustrated.

Amovable platen 78 consistsofasingleplate having apertures 80 th rough which- pin probe assemblies may pass and through which the space undercircuit board 12 may be evacuated. An evacuation port 92 serves to evacuate the space 82 under and around movable platen 78. A mid-plate 90 servestodefinethe bottom of a vacuum well which includes space 82 and chamber 84 between circuit board 12 and movable platen 78. A plurality of pin probe assemblies, both switchable and conventional are mounted in midplate 90 generally parallel with one another and with theircontact heads facing the circuit boardto be tested. The various test probe assemblies can be mounted at selective heights in orderto permitthe selective engagement of each with the circuit board undertest. Movable platen 78 is connected by means of push rods 86 to a bottom plate 85. Springs 88 located between mid-plate 90 and bottom plate 85 serveto bias platen 78 in its bottommost resting position on stops 150 on mid-plate 90. A motorized means external to the fixture (not shown) is used to drive bottom plate 85 upwardly which, in turns, moves platen 78 upwardly as well.

A sequence of tests would be initiated with platen 78 in its uppermost position. The fixture would then be evacuated through port 92 causing circuit board 12 to move downwardly against stops 87, thereby making contactwith the tallest pin probe assemblies. To move tothe next stage, bottom platen 85 and movable platen 78would be further towered. Since mid-plate 90 is stationary, more ofthe probe assemblies mounted therein make contact with circuit board 12 and thosethatwere previously in contactwith the circuit board are further deflected. This process may be continuedthroughouta plurality of test stage positions.

Various modifications can, of course, be made in practicing this invention. For example, adaptor44 and the connector elements 49 shown in Figs. 6-9 could be related to test probes 71 in a different manner. To avoid having to align connector elements 49 and test probes 71 with circuit board test points, one could wire connectorelements49 into a contact panel having contact postsfor mechanical and electrical connection to test probes 71. With such an arrangement, the configuration ofthe contact posts and test probes may be chosen asa matterof convenience instead of conforming to a pattern of test points on the circuit board to be tested. Another example of a way in which this invention could be modified concerns the switchable prnbepins..rnstead of using a band 24 of nonconducting material, one could use any other suitable meansforrendering a section of stem 22 nonconducting. One such means which may be suitable is ion implanting. Such modifications are within thespiritofthis invention and are intended to be included withinthescope ofthefollowing appended claims:

Claims (26)

CLAIMS The matterforwhich the applicantseeks protection is:-

1. A switchable probe pin assemblyfor use in a printed circuit board test fixtu ring system comprising: -a a tubularconductive probe housing adapted to receive a probe pin; - a conductive probe pin having a contact head at one end, said probe pin being slideably mounted for axial deflection in the housing with the contact head protruding therefrom; and - means for rendering the assembly alternately conductive and non conductive from the contact head through the pin and thence through the housing as the pin assumes preselected axial positions of deflection with respecttothe housing.

2. An assembly as claimed in Claim 1 wherein the rendering means includes at least one band of nonconductive material surrounding the probe pin at a predetermined distance from the head.

3. An assembly as claimed in Claim 2 wherein the rendering meansfurther includes an internal constriction within the housing through which the pin passes, the dimensions of the pin, housing and constriction being such that the pin and constriction remain in continuous contact with one another as the pin is axially deflected with respect to the housing.

4. An assembly as claimed in Claim 3 in which the constriction is an internally narrowed portion of the probe housing formed wherethe probe pin entersthe housing.

5. An assembly as claimed in Claim 3 in which the constriction is comprised of a plurality offurcations of the probe housing formed so asto make mutual pressing slideable engagement with the probe pin.

6. An assembly as claimed in Claims 2-5 wherein the other end and a proximate portion ofthe pin are covered by nonconducting material.

7. An assembly as claimed in any preceding claim wherein the pin is spring loaded in the probe housing for resilient deflection.

8. An assembly as claimed in any preceding claim furthercomprising meansfor retaining the pin in the housing.

9. An assembly as claimed in Claims 2-5 wherein the rendering meansfurther includes a nonconductive lining within the housing covering a predetermined portion thereof.

10. A multi-stage fixturing system for use in testing printed circuit boards comprised of: -a a fixture housing having meansforsecuring thereon a circuit board to betested; - a plurality of deflectably switchable contact headed probe pin assemblies; -meansformountingthe probe pin within the housing in a generally parallel relationship with one another with their contact heads facing the board to be tested; and - means for advancing the contact heads ofthe probe pin assemblies and the circuit board into electrical contactwith one another and for controlling the degree of deflection in each assembly during contact.

11. Asystem as claimed in Claim 10 wherein the fixture housing includes a movable platen to which the circuit board is secured for movement therewith and the advancing means includes means for moving the platen between a rest position and an advanced position characterized respectively by associated minimum and maximum probe pin assembly deflection.

12. A system as claimed in Claim 11 wherein the controlling means provides for selective placement of the platen in its rest position, its advanced position, and a plurality of predetermined positions therebetween.

13. A system as claimed in Claim 12 further comprising a plurality of conductive connector means, each such means being mounted on said movable platen between the circuit board and the head of a preselected probe assembly, so that the electrical contact between the circuit board and the contact head of such an assembly is made th rough the connector means.

14. A system as claimed in Claim 13 wherein the fixture housing further comprises: -a a base u nit for containment of the switchable probe pin assemblies, mounting means, advancing means, advancing means and controlling means; and -a removable interchangeabletop unitforcontainment ofthe circuit board securing means and the conductive connector means.

15. Asystemasclaimed inClaim 130r?4wherein each connector means is a deflectable probe pin assembly.

76. Asystem as claimed in Claims l0-12wherein the platen has at least one aperturethrough which the contact head of each probe pin assembly may slideably pass in orderto make physical and electrical contact with a test point on the circuit board.

17. A system as claimed in Claims 1 wherein the mounting meansforthe probe pin assemblies including a mid-platewithinthefixture housing ion a generally parallel relationship with the movable platen.

18. A system as claimed in Claim 17 wherein the fixture housing further comprises a vacuum well, the well being connected to a vacuum source.

19. Asystem as claimed in Claims 10-18wherein each switchable probe pin assembly is a switch able probe pin assembly as claimed in any of Claims 1 to 9.

20. A method for testing printed circuit boards which comprises the steps of: -providing afixture housing; -providing a plurality of deflectably switchable contact headed probe pin assemblies; mounting the probe pinassemblieswithinthe housing in a generally parallel relationship with one another with the contact heads facing the board to be tested; - advancing the contact heads ofthe probe pin assemblies and the circuit board into electrical contact with one another; and - controlling the degree of deflection in each assembly during contact so thatthe assembly is rendered alternately conductive and nonconductive as the contact head assumes preselected axial positions of deflection within each assembly.

21. The method as claimed in Claim 20 wherein the advancing step includes the step of interposing a conductive connector means between each contact head and the circuit board so that the electrical contact occurs through the connector means.

22. The method as claimed in Claim 21 wherein the advancing step further includes the steps of engaging each connector means with the circuit board undertestand then engaging each connector means with the contact heads and wherein the controlling step includes simultaneously moving the circuit board and engaged connector means toward and away from the probe pin assemblies.

23. The method as claimed in Claims 20-22 wherein the switchable probe pin assemblies are ofthetype having a conductive probe pin with a nonconductive band of material covering a portion thereof.

24. A switchable probe pin assembly substantially as hereindescribed with reference to the accompany ing drawings.

25. A multi-stage fixturing system substantially as hereindescribed with reference to the accompanying drawings.

26. A method for testing printed circuit boards substantially as hereindescribed with reference to the accompanying drawings.