CA1307856C - Probe for testing printed circuit boards - Google Patents

Abstract

Abstract of the Disclosure Apparatus for testing the electrical integrity of printed circuit boards under test (BUTs), each BUT
having a plurality of downwardly directed accessible nodes, the apparatus including support means for removably supporting the BUT, test circuitry including plurality of upwardly directed channel nodes below the support means, connection means for electrically connecting the channel nodes to the BUT nodes, the connection means comprising a universal board carrying probes in a universal grid pattern, means to activate selective probes, and a translator board to make electrical connection between upper and lower conductors in different patterns.

Description

Field of the Invention The invention relates to a test pin or probe which can be used in apparatus for testing printed circuit boards.
Ba_k~_ound of the Invention This application is a division of our Canadian Appli-cation Serial No. 564,235 filed April 15, ].988.
Printed circuit boards IPCB's~ loaded with the components they were designed to carry are often tested for electrical integrity by using spring-loaded test pins that are vertically supported underneath a printed circuit board under test (BUT) in position to contact the exposed nodes on the bottom surface of the BUT.
For example, Katz United States Patent No. 4,132,948 ~; discloses mounting test pins in an unloaded PCB having an identical hole pattern to that of the BUT so that the test pins automatically line up with the nodes beiny contacted. The test pins are directly connected to wires that are connected via cable assemblles to the test circuitryO When the circuit board tester lS~ to be used to test PCB's having a different node pattern, the 20~ ~ test;pin support board is replaced with one having test pins correspondlng to the node locations for the new PCB design.

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Published European Patent Application No. 0 115 135 discloses a test fixture includiny a permanent base sheet carrying a large~number of test plns and a lower customizing board that carrie~sinserts that activate ~::,: ::
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h~ 2 ~ 5 6 test pins at selected locations to be raised into activated position to contact nodes of a ~UT. When used with a different BUT, a new custornizing board, having :inserts in the sa~e pattern as that of the nodes of the new BUT, is installed. Underneath the customizing board, wire wrap post extensions of -the inserts are wired to terminals at the side of the customizing board, for connection to test circuitry.
Published European Patent Application No. 0 050 913 discloses a test ~ixture including a general purpose platform that carries test pins in a uniform grid pattern and a backing plate that carries removable displacement modules ~hat activate test pins at selected locations to contact nodes of a BUT. On the other side of the backing plate are wire wrap posts that are electrically connected to the test pins through the modules and are wired to a multiple pin plug at the edge of the backing plate.
In some prior art test systems channel circuit boards containing instruments for providing test signals to the BUT's and receiving resulting outputs were physically located underneath the test pins to reduce the distance between the instruments and the BUT to reduce distortions to test signals and outputs. In such systems there were a plurality of upwardly directed channel nodes at connectors at the upper ends of a plurality of channel boards (also referred to as channel cards), the channel nodes being electrically connected by wires to respective test pins carried on a test pin 3Q support board like that shown in Katz U.S. Patent No.
4,132,948. The test pins were wired to upwardly directed right angle posts on small boards, the lower plated edges of which were in turn connected to zero-insertion force edge card connectors mounted on ';
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~ _ 3 ~ l 3 0~7~ 5h the channel cards. Before making the wire wrap connection to the riyht angle posts, the small boards carrying them were removed and turned upside down so ;that the posts were directed downward, the same direction as lower extensions of the test pins, to facilitate wire wrapping, and after wire wrapping, the small boards were rotated back and mounted in the edge card connectors with the wires folded over. In these systems, when a particular automatic tester was used lO with different types of BUT's having nodes at different locations, new test pin support boards were installed.
A particular model automatic tester could be provided with different channel boards having different instrumentation, depending on the type of BUTs to be tested, and a user might want to add or modify channel boards, with the result that there would be different channel nodes for the same model of tester, and possibly even the same machine at different times.
In another prior art system~ dual-ended test pins mounted on a probe plate in one pattern were electrically connected to probes connected to test circuitry in another pattern below the probe plate by a translator board between the two carrying upwardly directed wire wrap posts in position to contact lower spring-biased contacts of the dual ended test pins, downward extending wire wrap posts in position to ;~ contact lower ~est circuitry probes, and wires between various wire wrap posts.

Summary of the Invention In one aspect my invention features in general ~ ` a printed circuit board tester in which electrical ;~ ~ connection can be desirably made between sprin~-biased ; board under test (BUT) contacts in a first pattern of ;

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~ 1 3(~/856 predetermined locations corresponding to the nodes of a BUT and channel nodss of test circuitry circuit boards in a region below the test pins in a different pattern by using a translator board having both tubular housings carrying the spring-biased BUT contacts extending from its upper surface and lower conductors extending from its lower surface and aligned with the channel nodes and translator means electrically connecting the respective tubular housings to lower conductor members. The use of tubular housings for 8UT contacts and lower conductors on a common translator board permits one to make electrical connection between BUT contacts and channel nodes in different patterns simply and directly, facilitating signal integrity in the path from the BUT
lS nodes to test circuitry on the channel boards.
In preferred embodiments the lower conductors are wire wrap posts that e~tend through holes in a keeper plate below the test pin support board; and there is an interface assembly below the keeper plate making electrical connection to the channel nodes.
In another aspect my invention features providing a translator board (for making electrical connection between upper conductors in one pattern and lower conductor in another) below a universal test pin support board having test pins in a grid pattern of possible BUT locations, the translator board carrying displacement members for actuating selective test pins corresponding to node locations for the particular BUT.
; When used with a different BUT, a new translator board is used to both actuate only those test pins that correspond to the positions of the BUT nodes and to make electrical connection between them and the respective ~ ~ test circuitry channel nodes.

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, In another aspect my invention features a printed circuit board tester in which a universal board having spring-biased interface contacts in a univexsal grid is used to make electrical connection between test pins and channel nodes of test circuitry, the channel nodes being in an application specific pattern that is a subset of the`uni~ersal grid. Displacement members in the same pattern as the channel nodes are used to move into actuated position only those spring-biased contacts that are ; needed to connect to the channel nodes. When the channel nodes are changed, e~g., when the instrumentation on the channel circuit boards is changed to change instrumentation, a different pattern of channel nodes can be easily accommodated by simply using ~different interface contacts on the uni~ersal board.
In accordance with a broad aspect of the invention there is provided a probe comprising a housing, a pair Qf plungers extendln~ from opposite ends of said housing, and a compression spring between said two plungers that is short~r, when not compressed, than the distance between said plungers when they are at opposite ends o~ said housinq and is free to slide in said 20 ~ housing.

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- 5a - 1 307856 Other features and advantages of the invention will be apparent from the following description of preferred embodiments thereof and from the claims.
Descxiption of the_Preferred Embodiments The drawings will be briefl~ described first.
Drawings Figure 1 is a diagrammatic ver-tical sectional view showing apparatus for testing PCB's according to the invention.
Figure 2 is a diagramma-tic vertical sectional view of a test pin assembly of the Figure 1 apparatus.
Figure 3 is an end view of a spring of the Figure 2 test pin.
Figure 4 is a side view of a portion of the Figure 3 spring.
Figure 5 is an elevation of a displacement pin and a low pin of the Figure 1 apparatus.

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Figure 6 is a diagrammatic vertical sectional view of an alternative test pin assembly according to the invention.
Figure 7 is a diagrammatic vertical sectional view of another alternative test pin assembly, shown with a modified sup-port structure, according to the invention.
Figure 8 is a diagrammatic vertical sectional view show-ing an alternative, most-preferred embodiment of apparatus for testing PCB's according to the invention.
Figure 9, on the second sheet of drawings, is an eleva-tion (partially in section) of a universal probe used in the Figure 8 device.
Figure 10, on the second sheet of drawings, is an enlarge-ment of a portion of Figure 9 with the probe's plunger in a differen~ position.
Stru~c_ure Referring to Figure 1, there is shown apparatus 10 for tésting the electrical integrity of a printed circuit board under test (BUT) 12 loaded with components 14, some of which have leads whlch pass through holes in the board and are soldered at nodes 16 to conductive paths printed on the bottom of board 12. Some com-ponents 14 are surface mounted (either on the top or bottom of board 12) and electrically connected to conductive pads acting as nodes without solder. Each di~ferent design for a BUT 12 has a unique pattern o~ nodes 16 associated with it. The nodes of BUT
12 are located on a 0.1" X-Y grid pattern; i.e., they are at - 6a -locations in a unique pattern that is a subset of a universal grid pattern. (The Figure 8 alternative embodiment is used with BUTs having nodes that are not on any grid pattern.) , .

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Starting near the top of apparatus 10, it includes side walls 18 providing overhanging portions 19 against which 10" by 10" diaphragm plate 22 (0.187"
thick fire resistant glass-filled phenolic board available under the trade designation FR4, often referred to as G-10) is biased by springs 20. Diaphragm plate 22 has 0.07" diameter holes through it spaced on 0.1" centers and rubber sealing ring 23 (1/8" high and 1" wide Neoprene) around the perimeter of plate 22 providing a vacuum seal with overhanging portion 19 and BUT 12. Two pointed 1/8" thick guidance pins 24 e~tend upward through corresponding holes in BUT 12 to properly align board 12. ~lso provided at selected locations on the surface of plate 22 are various rubber support pads (not shown) which act to seal holes in BUT 12 or support it to avoid flexing of BUT 12 during evacuation of the region below it.
Pivotally mounted on side wall 1~ at pivot 26 is universal test pin support board 28 carrying tes~
pins 30, each of which has a tubular housing 34, an upwardly directed universal BUT contact 45 and a downwardly directed actuating plunger 42 (Fig. 2). End 29 of universal board 28 is supported by ledge 31.
Universal board 28 is made of two 1/8" thick glass-filled phenolic boards (same material as plate 22) spaced from each other by 3/4" to provide an overall 1"
thickness. Test pins 30 are mounted in holes 32 in universal board 28 located on 0.1" centers (i.e., in the universal grid pattern mentioned above) and aligned with the holes of diaphragm plate 22.
~ eferring to Fig. 2, it is seen that each test pin 30 includes gold-plated nickel-silver tubular housing 34, which has an outer diameter of 0.054".
Housings 34 are tightly fit in 0.055" inner diameter ~::
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- " - I 307~56 test pin sockets 35, which are fixedly mounted in support board 28 and have shoulders 37 spaced by 3/4" to provide proper spacing and structural support for universal board 2~. Housing 34 is 1.470" long, and has first crimp 36 located 0.130" from its top, a second crimp 38 located 0.380" from its bottom and a dimple (not shown) holding it firmly, though removably, in test pin socket 35. Contacting plunger 40 and actuating plunger 42 are slidably carried in housing 34 with compression spring 44. Plungers 40, 42 are made of heat treated beryllium copper, gold over nickel plate, have pointed BUT contact 45 and actuating portion contact 47 for making good electrical contact, bulbous portions 46, 48, respectively, for interfering with crimps 36, 38, and overhanging shoulders 50, 52, which are larger than the diameter of housing 34. Spring 44 has an unloaded length of 0.570", and is music wire wound to 29 oz/inch. Referring to Figs. 3 and 4, it is seen that the ends 54 of spring 44 have off-center, reduced diameter portions; this causes spring 44 and bulbous portions 46, 48 to be transversely biased against the inner surface of housing 34 only when spring 44 is compressed, in order to make good electrical contact.
When spring 44 is not compressed, plungers 40, 42 and spring 44 are freely slidable in housing 34.
Returning to Fig. 1, 3/8" thick, removable actuating/translator board 56 (made of the same material as plate 22) carries beryllium copper displacement member pins 57, low pins 58 and alignment pins 59 which are received by bushings 60 carried by sidewalls 24 and 62, supported by housing 92. Each displacement member pin 57 has a 0.05" diameter, 1/4" high projection 64 (Fig. 5) extendinq above board 56, and each low pin 58 ; ~ has a 0.05" diameter, 0.08" high projection 65. The :

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9 1 ~07856 upper surfaces of projections 6~ are concave so as to guide and restrain pointed ends 47 o test pins 30.
Each pin 57 or 58 has a 0.068" diameter intermediate knurled portion 66, which is press fit into board 56, and a 0.025 square wire wrap post 68 extending below board 56.
Supported underneath actuating/translator board 56 are rows of sockets 70, fewer in number than test pins 30, carried by the upper ends of channel cards 73 (daughter printed circuit boards that are all supported on their lower ends by a common, horizontal backplane, not shown), and aligned with and adapted to receive and make electrical connection with selected posts 68. The selected posts 68 are either integral with projection 64 of a displacement pin 57 or connected by wires 72 to a post 68 of a displacement pin 57. Wires 72 are wrapped around overlapping translator portions that overlap each other at corresponding positions along vertical axes.
Sockets 70 are electrically connected to the electronic circuitry on circuit cards 73 and other circuitry of the tester.
Referring to Fig. 8, there is shown apparatus 110 for testing the electrical integrity of BUT 112 loaded with components 114 and having nodes 116 on the lower surface that are not at locations of any grid pattern. Each different design for a BUT 112 has a unique pattern of nodes 116 associated with it.
Apparatus 110 includes a fixture assembly 118, which is ;~ dedicated to a particular design for a BUT 112, and a universal interface assembly 120, which can be used with any fixture 118 and with any pattern of channel nodes 122, provided by the tops of posts 210 associated with channel cards 12~.

~, , -` 1 307856 Fixture assembly 11~ includes sidewalls lZ6, having upper overhanging port~ons 12~, against which translator board 130 ~0.3l'i thick C10 material) is supported, and lower overhanging portions 131, against which apertured keeper plate 132 (1/8" thick clear Lexan*
polycarbonate) is supported. Supported near the periphery of the upper sur~ace of translator board 130 are Neoprene*rubber sealing ring 134 (1/8" high and 1"
wide) and springs 136. Diaphragm plate 138 ~0.187" G10 material) is above sealinq ring 139 and springs 136.
Thereabove is Neoprene rubber sealing ring 140 ~3/16"
high and 1" wide). Aluminimum sealing ring 142 is between the peripheries of th~ lower surface of sealing ring 140 and the upper surface of translator board 130.
On top of sealing ring 1~0 is L~shaped cross section trim ring 144.
Translator board 1~0 includes in it upwardly extending te~t pins 146A-E and downw~rdly extending lower conducting members 148. Test pins 146A-E include tubular housings 145 (also referred to as upper conductinq members) carrying spring biased ~UT contacts 150 and lower extensions l47, including 0.025" square : ~ wire wrap posts 154A-E therebelow. Lower conductors 14,~: also include lower wir~ wrap posts 156, which overlap wire wrap portions of posts 154A-E and extend through holes l5a in keeper plate 132. Keeper plate 132 of fixtura 11~ sits on an outer Neoprene rubber sealing `n ~ ring (not shown) providlng a seal between it and universal intereace board l~Q.
Three wire wrap situations are shown in Fig.
8. In the ~irst example, shown on ~he left, wire wrap :~ post 154A of test pin 146A is connscted by w;re 160 to the wire wrap post 156 of adjacent lower conductor 14~, which is allgned wlth the channel node to which -:~ :
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test pin 146A is to be connected. In the middle e~ample, wire wrap post 154~ of test pin 146B is connected by wire 1~1 to wire wrap post 156 of adjacent lower conductor 1~8, and the wire wrap post 154C of test pin 146C is electrically connected by wire 162 to wire wrap post extension 164, which is directly below test pin 146C and is physically connected by insulator sleeve 166 to post 154C. In the example on the right, test pin 146D is electrically connected by wire 167 to wire wrap post 156 of adjacent lower conductor 148, and test pin 146E is electrially connected by wire 168 to wire wrap post extension 170, which is directly below test pin 146D and physically connected by insulator sleeve 171 to post 154D.
Universal interface assembly 120 includes universal interface board 180 (3/8" thick G10 material), which is pivotally mounted at an end (by means not shown) and carries universal probes 181 comprising tubular housings 182 having universal interface contacts 184 extending upward from them. Tubular housings 182 similarly have downwardly directing actuating plungers 186. Probes 181 are located in two rows per channel card, 96 probes deep and spaced on 0.1" centers.
Channel cards 124 are spaced by 3/4". Referring to Figs. 9, 10, contacts 184 are on plunger 188, which includes lower portion 190 extending from the bottom of tubular housing 182 and having recess 192 receiving test probe 194 (075 center line, .160 travel, cup probe).
Plunger 188 is biased upward by spring 196, bearing against the lower annular surface of enlarged portion 198 and inner annular surface 200 of housing 182.
Spring 196 has æero preload; at 0.09" deflection there is a 4 oz. force on contacts 184, 186. O-ring 202 (O.D.
0.075 ~ 0.002", I.D. 0.0360 ~ 0.002") provides a vacuum 12 , 1 307856 seal between the outer surface of plunger 188 (O.D.
0.041 ~ 0.002") and the inner surface of housing 182 (I.D. 0.070 ~ 0.002"), and is retained between roll crimp 204 and crimped end 206. A synthetic lubricant applied to O-ring 202 and the just-mentioned surfaces provides sliding. Interface contacts 184 have depressions therein for receiving the lower ends 208 of wire wrap posts 156 and extensions 164, 170. Plungers 186 similarly have depressions receiving the upper ends of posts 210 (0.025" square section), extending upward from connectors 212 of channel cards 124. Posts 210 are supported within U-shaped transverse supports 213, connected to respective channel cards 124 (by means not shown).
Operation In operation of the Fi~s. 1-5 apparatus, an actuating/translator board 56 having displacement pins 57 mounted in locations corresponding to the accessible nodes 16 of BUT 12 (i.e., their loca"tions are at a unique pattern that matches that of BUT 12) is mounted within device 10 by sliding its guide pins 59 in bushings 60 while universal board 28 is in a raised, vertical position. Posts 68 are inserted with zero insertion force into respective sockets 70, which are then laterally translated ~by a mechanism not shown) to , provide good electrical connection. Universal support board 28 is then rotated to the horizontal position shown in Fig. 1 such that its end 29 is supported by ledge 31. As universal board 28 is lowered into position, the actuating plungers 42 of test pins 30 aligned with displacement pins 57 are pushed upward. As plunger 4~ slides upward inside of housing 34, it pushes spring 44 upward and contacting plunger 40 and its universal BUT contact 45 through a hole in diaphragm ~: :
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.' ~ 13 - 1 3~7856 plate 22 into the raised position, as is shown in Fig.
1. This requires no more force than the weight of the two plungers and the spring. BUT 12 is then mounted into position using guidance pins 24, and it rests on ring 23 and springs 20. A vacuum is then applied to the region underneath BUT 12, causing ring 23 and springs 20 to be compressed and BUT 12 to be lowered so that nodes 16 contact the corresponding universal BUT contacts that have been raised, compressing springs ~. Only the springs of the test pins that have been raised are compressed, limiting the force that needs to be applied to make contact.
Universal BUT contact 45 digs into the solder or contact pad at node 16, making good electrical contact. The reduced diameter ends 54 of spring 44 cause sideways biasing of bulbous portions 46, 48, rnaking good electrical contact through test pins 30.
Actuating portion contacts 47 are biased downward by spring force and similarly make good electrical contact with projections 64 of displacement pins 57. Electrical connection to electronic circuitry is made either directly to a socket 70 under a displacment pin 57 or through a wire 72 to the post of a low pin 58 that is received by a socket 7Q.
The electronic circuitry of the tester provides test signals to BUT 12 and senses the responses. The short, direct wire wrap connections between overlapping wire wrap portions promotes signal integrity, something becoming increasingly more important with higher speed digital circuits and obtaining precision analog measurements.
When it is desired to test a different design for BUT 12, the universal board 28 is pivoted to a vertical position, and actuating/translator board 56 is : :

'~ 1 307~56 replaced with a new actuating/translator board 56 having displacement pins 57 corresponding to the nodes 16 for the new BUT 12. In this manner different PCB designs can be accommodated by merely changing actuating/translator boards 56, and there is no need to dedicate the relatively expensive test pins 30 in a particular predetermined pattern. Also, it is much easier to accommodate changes in the design of BUT 12 by changing the location of projection pins 58, than it is to change the locations of test pins and connections to tester circuitry in the dedicated test pin position testers.
In operation of the Figs. 8-10 apparatus, fixture 118 for a particular BUT 112 is mounted on universal interface board 180, sitting on an outer sealing ring and registered by guide pins (both not shown). At this time the upper ends of those interface contacts 184 that have been raised are slightly below lower ends 208 of wire wrap posts an~ extensions, and ~ the springs that bias interface contacts 184 have zero ~ force (ignoring the weight of contacts 184) on them, as -; they are not preloaded. A vacuum is then applied ; between translator board 130 and universal interface board 180, causing compression of the sealing ring between keeper plate 132 and board 180, contact of lower ends ~08 with interface contacts 184, and slight compression of springs 196. At the same time, lower contacts 186 are lowered to contact channel nodes 122, and the associated springs in probes 194 are also compressed slightly. Because interface contacts 184 are not preloaded, and only those contacts that are being used are~compressed, electrical contact is made with very little force.
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~ I 307856 BUT 112 is then placed on top of the fixture, and its BUT nodes 116 are slightly spaced from BUT
contacts 150. A second vacuum is applied between 8UT
;112 and the upper surface of translator board 130, and this causes compression of rubber rings 134, 140 and contact between BUT contacts 150 ancl sUT nodes 116.
BUT contacts 150 of fixture 118 are in a unique pattern that matches that of the BUT nodes 116 in the field of possible node positions of BUT 112. There similarly is a unique pattern for channel nodes 122 in a corresponding field under BUT 112, the difference in the positions of respective test pins and respective contact nodes is accommodated for by wire wrapping using wires 160, 161, 167, and 168. Wire wrap posts 156 of lower conductors 148 and wire wrap post e~tensions 164, 170 are in the same pattern as channel nodes 122. Signal integrity is promoted by the short, direct wire wrap connection between overlapping wire wrap post portions.
For a given model of test apparatus 110, channel nodes 122 will be in different positions depending on the instrumentation desired by the manufacturer, in turn depending on the BUTs to be tested and the desired tests. In addition, channel cards 12 and their associated channel node positions on a particular machine could be changed by adding channel cards or modifying channel cards at some point in the future. Universal interface assembly 120 would still be used with any type of channel card arrangement and with ~; any type of fixture 118 for a BUT 112, thus standardizing and simplifyiny the structure of the machine.

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Other Embodiments Other embodiments of the invention are within the scope of the following claims.
; For example, an alternative test pin design, test pin 79, is shown in Fig. 6. Its spring 76 has a 2.5 oz preload and a force of 5.4 oz at 2/3 travel.
Test pin socket 78, which is still fixedly mounted in support board 28, has a 0.056" inner diameter, permitting 0.054" outer diameter housing 80 to slide in it. In use, displacement pins 57 push housing 80 upward, along with plungers 40, 42 and spring 76.
A further alternative design for supporting test pins is shown in Fig. 7. Here, test pin support ~ board 83 is made of a single 1/2" thick support board ;~ 15 (same material as plate 22) that has counter-sunk holes 81 for accommodating heads 82 of standard, single plunger test pins 84. Pins 84 can slide within holes 85 ; through board 83. Caps 86 at the lower end of pins 84 prevent the pins from falling out when support board 83 ; 20 lS turned upside down. Insulating honeycomb 90 is used ; to electrically isolate test pins 84 and align them with their respective displacement pins 57. In use, the entire test pin 84 is moved upward by a displacement pin 57.
25 ~ Also, although the systems described herein have BUTS at the top and channel cards at the bottom, ~ the inventions herein apply to other orientations.
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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A probe comprising a housing, a pair of plungers extending from opposite ends of said housing, and a compression spring between said two plungers that is shorter, when not compressed, than the distance between said plungers when they are at opposite ends of said housing and is free to slide in said housing.

2. The probe of claim l wherein said housing is made of metal.

3. The probe of claim 2 wherein said compression spring has an end that has a reduced diameter so as to transversely displace a said plunger against said housing when said spring is compressed.