Abstract:
A Printed Circuit Board (PCB) test fixture includes flex sensors for monitoring the flex (or distortion) of the PCB during testing. The PCB is positioned above irregular array of test probes. The test probes are aligned with electrical test points on the PCB. The PCB is then pressed downward against the test probes by an irregular array of pushers. The positions of the pushers are generally specified by a PCB manufacturer to avoid PCB elements and the positions of the test probes are generally specified by the PCB manufacturer to obtain desired signals.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to fixtures for testing Printed Circuit Boards (PCBs) and in particular to a test fixture which includes strain gauges to measure the deflection of the PCB when test contacts are pressed against the PCB. 
   The manufacture of PCBs often includes testing the electrical functionally of the PCB. Such testing may be performed by resting the PCB in a test fixture, and pressing test probe contacts against PCB circuit traces on the PCB. Such test fixtures are described in U.S. Pat. No. 5,311,120 for “TEST FIXTURE WITH TEST FUNCTION FEATURE” and in U.S. Pat. No. 6,084,422 for “PRINTED CIRCUIT BOARD TESTING DEVICE”, both inventions by one of the present applicants. Although the know test fixtures have provided means for testing PCBs, in some instances, enough force is applied to the PCB by the test contacts to damage the PCB. The &#39;120 patent and the &#39;422 patent are herein incorporated by reference. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention addresses the above and other needs by providing a Printed Circuit Board (PCB) test fixture which includes flex sensors for monitoring the flex (or distortion) of the PCB during testing. The PCB is positioned above irregular array of test probes. The test probes are aligned with electrical test points on the PCB. The PCB is then pressed downward against the test probes by an irregular array of pushers. The positions of the pushers are generally specified by a PCB manufacturer to avoid PCB elements and the positions of the test probes are generally specified by the PCB manufacturer to obtain desired signals. 
   In accordance with one aspect of the invention, there is provided a test fixture including an array of test probes, an array of pushers, and an array of flex sensors. The support plate provides support for a PCB during a PCB test. A base plate resides under the support plate and holds the test probes. A gate is lowerable over the support plate to compress the support plate from a relaxed position to a compressed position a during the PCB test. The array of electrical test probes is attached to the base plate for making electrical contact with circuits on the PCB when the base plate is in the compressed position during the PCB test. A push plate attached to the gate and the array of pushers is attached to the push plate for pushing the PCB against the test probes during the PCB test. A ridged flex sensor plate is attached to the gate and isolated from flexing of the push plate during the PCB test and the array of flex sensors are attached to the test fixture for measuring deflections of the PCB during the PCB test. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The above and other aspects, features and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein: 
       FIG. 1  is an exploded view of several elements of a Printed Circuit Board (PCB) testing fixture according to the present invention. 
       FIG. 2  depicts an end view of the PCB testing fixture 
       FIG. 3A  is a cross-sectional view of the PCB testing fixture taken along line  3 - 3  of  FIG. 2  with a test fixture gate slightly raised. 
       FIG. 3B  is a cross-sectional view of the PCB testing fixture taken along line  3 - 3  of  FIG. 2  with the test fixture gate fully lowered. 
       FIG. 4A  is a more detailed cross-sectional view of a portion of the PCB testing fixture taken along line  3 - 3  of  FIG. 2  with a test fixture gate slightly raised. 
       FIG. 4B  is a more detailed cross-sectional view of a portion of the PCB testing fixture taken along line  3 - 3  of  FIG. 2  with the test fixture gate fully lowered. 
       FIG. 4C  is an embodiment of the present invention including additional probes above the PCB. 
       FIG. 5  is a test probe according to the present invention. 
       FIG. 6  is a pusher. 
       FIG. 7  is a flex sensor according to the present invention. 
       FIG. 7A  is a cross-sectional view of the flex sensor taken along line  7 A_ 7 A of  FIG. 7 . 
       FIG. 8  is a method according to the present invention. 
   

   Corresponding reference characters indicate corresponding components throughout the several views of the drawings. 
   DETAILED DESCRIPTION OF THE INVENTION 
   The following description is of the best mode presently contemplated for carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of describing one or more preferred embodiments of the invention. The scope of the invention should be determined with reference to the claims. 
   An exploded view of several elements of a Printed Circuit Board (PCB) testing fixture according to the present invention and a PCB  18  are shown in  FIG. 1 . The PCB testing fixture includes a flex sensor plate  10  holding a multiplicity of flex sensors  12 , a push plate  14  holding a multiplicity of pushers  16 , a support plate  22  for supporting the PCB  18  during testing, a multiplicity of test probes  24  exposeable through a support plate recess  23  when the support plate  22  is in a compressed (or down) position, and a base plate  26  supporting the support plate  22 . A gate  28  (not shown in  FIG. 1 , see  FIGS. 2 and 3 ) reside above the support plate  22  and supports the flex sensor plate  10  and the push plate  14 . The PCB  18  resides between the pushers  16  and the test probes  24  during PCB testing. The PCB  18  includes large components  20   a  and  20   b , and groups of small components  21   a ,  21   b , and  21   c . The flex sensor plate  10  is a ridged flex sensor plate decoupled from flexing of the push plate  14  during PCB testing to provide accurate flex measurments. Examples of known test fixtures not including the flex sensors  12  of the present invention are described in U.S. Pat. No. 5,311,120 for “TEST FIXTURE WITH TEST FUNCTION FEATURE” and in U.S. Pat. No. 6,084,422 for “PRINTED CIRCUIT BOARD TESTING DEVICE”, both inventions by one of the present applicants and incorporated by reference above. 
   An end view of the PCB testing fixture is shown in  FIG. 2 . A cross-sectional view of the PCB testing fixture taken along line  3 - 3  of  FIG. 2  with the PCB  18  in a PCB test position and a test fixture gate  28  slightly raised and the support plate  22  in a relaxed position is shown in  FIG. 3A . The PCB lies on the support plate  22  and the support plate recess  23  (see  FIG. 1 ) preferably is shaped to match a lower surface of the PCB  18 . The flex sensor plate  10  and the push plate  14  are fixedly attached to the gate  28  and move vertically with the gate  28 . The flex sensor plate  10  holds the flex sensors  12  in a precise position relative to the gate  28  for making precise measurement of deflection of the PCB  18  under test, and the flex sensor plate  10  is sufficiently ridged to permit highly accurate measurements to be made by the flex sensors  12 , for example, to within approximately 0.001 inches, and if necessary the flex sensor plate  10  may be made sufficiently ridged to measure within approximately 0.0001 inches. The flex sensors  12  pass through the push plate  14 , but preferably are not coupled with the push plate  14  which may experience some deflection during use of the test fixture, thereby isolating the flex sensors  12  from the pushers  16 . During PCB testing, the gate  28 , the flex sensor plate  10 , and the push plate  14  are moved downward towards the PCB, either by mechanical means, vacuum or by air pressure. Examples of mechanisms for moving the gate  28  are described in U.S. Pat. No. 5,311,120 for “TEST FIXTURE WITH TEST FUNCTION FEATURE” and in U.S. Pat. No. 6,084,422 for “PRINTED CIRCUIT BOARD TESTING DEVICE”, both inventions by one of the present applicants and incorporated by reference above. The mechanical means include, for example, levers, threaded shafts, linear motors, and the like. 
   The test probes  24  extend upwards from the base plate  26  and through the insert  30 . The insert  30  is provided to add horizontal support to the test probes 
   A cross-sectional view of the PCB testing fixture taken along line  3 - 3  of  FIG. 2  with a test fixture gate  28  lowered and the support plate  22  in a compressed position is shown in  FIG. 3B . The support plate  22  is normally biased away from the base plate  26 , but during testing, the support plate  22  is pushed toward the base plate  26  by the gate  28 . During PCB testing, the pushers  16  push the PCB  18  downward against the test probes  24 , and the flex sensors  12  measure the deflection of specified points on the PCB  18 . Test probe cables  36  connect the test probes  24  to test probe interface electronics  38  residing in a tester interface box  50 . Flex sensor cables  40  connect the flex sensors to a flex sensor interface electronics  42 . 
   A more detailed cross-sectional view of a portion of the PCB testing fixture taken along line  3 - 3  of  FIG. 2  with a test fixture gate  28  slightly raised is shown in  FIG. 4A , and a more detailed cross-sectional view of a portion of the PCB testing fixture taken along line  3 - 3  of  FIG. 2  with the test fixture gate  28  fully lowered is shown in  FIG. 4B . The test probes  24  include probe contacts  24   a . When the gate  28  is raised, the test probe contact  24   a  reside in passages  25  in the support plate  22 . When the gate  28  is lowered against the support plate  22 , the support plate is pushed lower and against the insert  30 . The contacts  24   a  extend through the lowered support plate  22  and make electrical contact with circuit elements of the PCB  18  to allow electrical testing. The pushers  16  push the PCB  18  downward against the contacts  24   a  to balance the vertical forces on the PCB  18 . In most instances, the vertical forces on the PCB are not balanced, and the PCB flexes as show at arrow  52 . In these instances, the flex sensors  12  may measure the amount of flex of the PCB  18  and provide a warning before the PCB  18  is damages. The flex sensor measurements of the PCB  18  are generally of a vertical deflection of the PCB  18 . The data from the flex sensors  12  may further be utilized to adjust the positions of the pushers  16  to better balance the vertical forces on the PCB  18 . 
   An embodiment of the present invention including an additional test probe  54  mounted in the push plate  14  above the PCB  18  is shown in  FIG. 4C . The probe  54  are connected by a second test probe cable  56  to an upper probe connector  58 . The upper probe connector  58  is also mounted in the push plate  14 . A lower probe connector  60  is similarly to the mounting of the test probes  24 , and makes an electrical connection with the upper probe connector  58  when the gate  28  is lowered for PCB  18  testing. An additional lead  36  connects the lower probe connector to the test probe interface electronics  38 . 
   A detailed view of the test probe  24  according to the present invention is shown in  FIG. 5 . The test probe  24  includes the contact  24   a , probe body  24   b , probe receptacle  24 C and wire post  24   d . A spring resides in the probe body  24   b  to bias the contact  24   a  against the PCB  18 . The probe contact  24   a  and probe body  24   b  are removeable and replaceable from the probe receptacle  24   c  for service and repair. The probe receptacle  24   c  is preferably permanently mounted to the base plate  26  (see  FIGS. 4A-4C ) and extends into the insert  30  for stability. The contact  24   a  has a contact length L contact , the body  24   b  has a body length L body , the receptacle  24   c  has a receptacle length L receptacle , and the post  24   d  has a post length L post . The contact length L contact  is preferably approximately 0.31 inches, the body length L body  is preferably approximately one inch, the receptacle length L receptacle  is preferably approximately 1.01 inches, and the post length L post  is preferably approximately 0.69 inches. The contact  24   a  is generally made from silver or nickel and gold plated for conductivity. 
   A detailed view of the pusher  16  is shown in  FIG. 6 . The Pusher  16  has an insertion end  16   a , a stop  16   b , and a contact end  16   c . The insertion end  16   a  is inserted into the push plate  14  (see  FIGS. 4A-4C ) up to the stop  16   b . The contact end  16   c  a contact length L contact  and a contact diameter D contact . The contact length L contact  is preferably approximately 0.687 inches and the contact diameter is preferably approximately 0.10 inches. The pusher is made from a hard rubber or plastic or of material with like physical characteristics. 
   A detailed view of the flex sensor  12  according to the present invention is shown in  FIG. 7 . The flex sensor includes a flex sensor electrical connector  12   a , a flex sensor housing  12   b , and a probe comprising a moveable shaft  12   c  and a flex sensor foot  12   d . The housing  12   b  has a housing length L housing  and a probe has a probe length L probe . The probe length L probe  is a vertical distance from the base of the housing  12   b  to the base of the foot  12   d  when the probe is relaxed with no vertical forces on the probe. The housing length L housing  is preferably approximately 2.25 inches and the probe length L probe  is preferably approximately one inch. 
   A cross-sectional view of the flex sensor  12  taken along line  7 A_ 7 A of  FIG. 7  is shown in  FIG. 7A . The flex sensor  12  includes a spring  70  biasing the moveable shaft  12   c  out of the body  12   b , a scale  72  attached to and moving with the moveable shaft  12   c , a measurement encoder  74  for measuring motion of the scale  72 , and electronics  76  for generating a flex sensor output signal. An example of a suitable scale  72  is a model #L18CE from MicroE Systems in Natick. Mass. An example of a suitable encoder chip is a model # CE-4 from MicroE Systems. An example of suitable electronics  76  is an 18F2431 made by Microchip in Chandler, Ariz. combined with misc resistors and capacitors. 
   A method for testing PCBs according to the present invention is described in  FIG. 8 . The method includes positioning a PCB on a support plate at step  80 . Lowering a gate holding a push plate and a flex sensor plate over the PCB at step  82 . Pushing downward against the PCB with pushers mounted on the push plate at step  84 . Contacting circuits on the bottom of the PCB with test probes mounted to a base plate under the support plate at step  86 . Monitoring electrical signals received from test probes at step  88 . Contacting an upper surface of the PCB with flex sensors mounted to the flex sensor plate at step  90 . Monitoring deflections of the PCB caused by the pushers at step  92 , and controlling the downward motion of the gate based on the flex sensor measurements to limit or prevent damage to the PCB at step  94 . The steps  86  and  88  may be performed simultaneous with steps  90 ,  92 , and  94  to monitor deflections of the PCB during electrical testing, or the steps  90 ,  92 , and  94  may be performed prior to electrical testing to obtain limits on downward pushing by the pushers. 
   While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims.