Patent Publication Number: US-2010131236-A1

Title: Apparatus and Method for Measuring Deflection of a Printed Circuit Board

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of printed circuit board quality control, and more particularly to apparatus and method for measuring deflection of printed circuit board to be integrated to a system. 
     BRIEF DESCRIPTION OF THE BACKGROUND ART 
     Product reliability testing techniques such as Environmental Stress Screening (ESS), Highly Accelerated Stress Screening (HASS) and Highly Accelerated Life Testing (HALT) have been developed to increase the service life of electrical and electronic circuits and systems integrated in products, by detecting latent flaws induced at the design or development stage. With these testing techniques, the operating and destruction limits of a given product can be identified by recreating the various types of stresses it will undergo in use, beyond product specifications and field level. Typically, an ESS process is most frequently used for testing in thermal cycle environments, with or without the use of a vibration system. Printed circuit boards in electronic systems are commonly exposed to these environments either sequentially or simultaneously for short periods of time. During such exposure or as a delayed effect thereof, latent defects of a tested printed circuit board itself or of its components can be detected and therefore repaired prior to shipping of the product to the en user, thereby resulting in improved manufacturing methods and user satisfaction. 
     Several vibration testing systems have been developed over the past years which have the capability of carrying on reliability testing techniques, namely electrodynamic, hydraulic or pneumatic vibration tables, and more recently, acoustical vibration system such as disclosed in U.S. Pat. No. 6,668,650 B1 issued on Dec. 13, 2003, which patent has been assigned to the present assignee since its issuance. Such acoustical testing system is provided with a baffle on which is mounted a fixture adapted to secure one or more printed circuit boards to be tested through controlled exposition to acoustical waves generated by loudspeakers provided within an enclosure integrating the baffle. Another type of test fixture is disclosed in U.S. Pat. No. 6,734,690 B1 issued on May 11, 2004 to Ashby, which addresses the problem of localized printed circuit board bending that could occur when compression connectors are interposed between integrated circuits and the PCB on which they are mounted, such bending being likely to cause electrical contact disruption, thereby disabling proper function of the IC packages. However, the test fixture disclosed in U.S. Pat. No. 6,734,690 B1 cannot reduce bending over areas of the PCB that extend beyond a specific region of interest around the particular IC package mounted thereon. Modeling-based methods for determining support location in a wireless fixture of a printed circuit assembly and for determining points of maximum deflection of a printed circuit board are disclosed in U.S. Pat. No. 6,839,883 B2 and in U.S. Pat. No. 7,103,856 B2 respectively issued on Jan. 4, 2005 and on September 2006 both to Ahrikencheikh. Such methods are based on a complex mathematical model of the fixture including its wireless PCB as well as of the PCB under test, which model involves many parameters representing the boundary and loading conditions. The practical limit inherent to that approach essentially depends on the level of reliance that the model represents the actual fixture system with sufficient accuracy. 
     Bending of printed circuits to be integrated in systems such as computers in their assembly stage may be also at the origin of overstress of the printed circuit boards that could cause failure thereof, thereby affecting the service life of the systems. There is still a need for a reliable instrumentation and methods designed to prevent such problem. 
     SUMMARY OF INVENTION 
     According to the present invention, from a broad aspect, there is provided a method for measuring deflection of a printed circuit board adapted to be integrated to a system provided with a mounting structure defining a reference mounting plane. The method comprises the steps of: i) securing the printed circuit board onto the mounting structure; ii) measuring deflection of the printed circuit board in a direction substantially perpendicular to the reference mounting plane at a representative number of measurement locations on the printed circuit board; and iii) comparing the measured deflection with a predetermined limit deflection value. 
     According to the present invention, from a further broad aspect, there is provided an apparatus for measuring deflection of a printed circuit board adapted to be integrated to a system provided with a mounting structure defining a reference mounting plane. The apparatus comprises means for securing the printed circuit board onto the mounting structure, a displacement sensor unit for measuring deflection of the printed circuit board in a direction substantially perpendicular to the reference mounting plane at a representative number of measurement locations on the printed circuit board, and processing means for comparing the measured deflection with a predetermined limit deflection value. 
     According to the present invention, from another broad aspect, there is provided a system for vibration testing of a printed circuit board using the deflection measurement apparatus during pre-testing operations. The system comprises a printed circuit board mounting structure defining the reference mounting plane, a device for selectively supporting the printed circuit board at one or more selected locations on a first surface of the printed circuit board during the pre-testing operations, and a vibrator unit operatively coupled to said printed circuit board, wherein the device is operable to be moved between a supporting position during said pre-testing operations and a clearance position during vibration testing of the printed circuit board. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of deflection measurement apparatus, systems and methods will now be described with reference to the accompanying drawings in which: 
         FIG. 1  is a perspective view of an acoustical vibration testing system that could be used to perform vibration testing of a printed circuit board integrated thereto; 
         FIG. 2  is a schematic representation of two standard PCB formats showing predetermined locations of mounting holes that can be used to secure the board onto a mounting structure of a receiving system; 
         FIG. 3  is a schematic representation of a PCB under bending showing various geometrical parameters involved; 
         FIG. 4A  is a perspective view of a basic PCB deflection measuring apparatus; 
         FIG. 4B  is a schematic block diagram of the PCB deflection measuring apparatus of  FIG. 4   a;    
         FIG. 5  is a chart representing a relation between maximal deflection measurement values and span lengths for ensuring compliance with a predetermined minimum radius of curvature R=1500 mm, wherein span lengths are within a (0; 250 cm) range; 
         FIG. 6  is a chart representing a relation between maximal deflection measurement values and span lengths for ensuring compliance with a predetermined minimum radius of curvature R=1500 mm, wherein span lengths are within a (0; 50 cm) range; 
         FIG. 7A  is a perspective view of the top side of a baffle to be installed on the acoustical enclosure of the vibration testing system shown in  FIG. 1 , provided with a mounting structure and supporting device for a printed circuit board; 
         FIG. 7B  is a plan view of the baffle of  FIG. 7A , showing a printed circuit board secured thereto; 
         FIG. 8  is a perspective view of the bottom side of the baffle of  FIG. 7A , showing the supporting device; 
         FIG. 9A  is a perspective view of PCB edge securing element for use with the PCB mounting structure shown in  FIG. 7A ; 
         FIG. 9B  is a partial cross-sectional side elevation view along section lines  9 B- 9 B of  FIG. 7B , showing the PCB edge securing element in its operational position; 
         FIG. 10A  is a perspective view of a plunger as part of a displaceable locking device provided on the supporting device of  FIG. 7A ; 
         FIG. 10B  is a partial cross-sectional side elevation view along section lines  10 B- 10 B of  FIG. 7B , showing details of the plunger; 
         FIG. 11  is a perspective view of the top side of another example of baffle on which a plurality of PCB supporting devices are provided, allowing a plurality of PCBs to be simultaneously mounted on a same baffle; 
         FIG. 12  is a perspective view of the bottom side of the baffle of  FIG. 11 , showing the supporting devices; 
         FIGS. 13A and 13B  are cross-sectional side views of the single PCB baffle and supporting device as shown in  FIG. 7B  along section lines  13 - 13 , showing its pivoting and support locking mechanisms in their lowered, PCB disengaging position and lifted, PCB supporting position, respectively; 
         FIG. 14  is a plan view of the top surface of a printed circuit board to be tested showing the locations of securing points and unused mounting holes, with examples of bending measurement span locations where deflection and bending radius of the PCB secured at edges thereof by clamp assemblies provided on the mounting structure are measured, without support of the bottom side of the PCB; 
         FIG. 15  is a plan view of the top surface of the printed circuit board of  FIG. 14 , showing examples of bending measurement span locations, with support of the bottom side of the PCB; 
         FIG. 16   a  is a perspective view of another embodiment of deflection measuring apparatus, which is provided with load applying and measuring devices; 
         FIG. 16   b  is a schematic block diagram of the apparatus shown in  FIG. 16   a ; and 
         FIG. 17  is a plan view of the top surface of the printed circuit board of  FIG. 14 , showing the locations of securing points and unused mounting holes, with examples of bending measurement span locations where deflection and bending radius of the PCB secured at edges thereof by the clamp assemblies are measured, with support of the bottom side of the PCB in a loading condition. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Some embodiments of PCB deflection measuring apparatus and methods that can prevent overstress of PCBs when testing or integrating thereof in a system will be described. In the context of an exemplary application wherein a printed circuit board as a device under test (DUT) is integrated to a vibration testing system, the deflection measuring apparatus and methods are used to prevent overstress of PCBs prior and during testing. For example, it can be used to verify in a HALT, HASS of ESS testing protocol, if the PCB testing fixture and vibration testing setup would be likely to cause failure of PCB components during pre-testing and testing procedures, which failure would otherwise not occur with faultless components. In that testing vibration context, it is possible to validate the selection of support locations amongst a number of locations existing on the PCB to be secured onto the mounting structure of the testing system, so that the bending limit requirement is met. 
     In the context of another exemplary application wherein a PCB is integrated to a system as a product such as a computer, the deflection measuring apparatus and methods are used to prevent overstress of PCBs at the system assembly stage, to ensure that operations involved, such as plugging of PCB connectors, will not cause PCB components failure, which would otherwise not occur with faultless components. 
     Referring to  FIG. 1 , an exemplary vibration testing system that can be used as generally designated at  20  is an acoustical vibration testing system such as described in detail in U.S. Pat. No. 6,668,650 B1 issued on Dec. 13, 2003 and now assigned to the present assignee, the entire disclosure thereof being incorporated herein by reference. Such vibration testing system  20  includes a vibrator unit generally designated at  49  and operatively coupled to a printed circuit board (PCB)  32  to be tested, which incorporates a testing chamber  22  (closable by a door not shown) including a main loudspeaker enclosure  24  provided with an upper module  26  adapted to receive a baffle  28  rigidly mounted thereon through attachments in the form of clamp assemblies  43 , which baffle  28  is also used as a mounting structure to receive PCB  32  using securing means generally designated at  30 , which mounting structure defines a reference mounting plane as will be later described in more detail. It can be seen from  FIG. 1  that an upper loudspeaker enclosure  33  may be provided to generate complementary acoustical waves toward the top surface of the PCB  32  under test. The chamber  22  is provided with a controller unit  23  generating an amplified acoustical excitation signal for the loudspeaker enclosures  24  and  33 . 
     Turning now to  FIG. 2 , there is shown a schematic representation of two distinct PCB format according to ATX standards, showing the predetermined locations for mounting holes, designated as A, C, F, G, H, J, K, L and M for ATX format represented by the outline formed by line segments  21 ,  25 ,  27 ,  29 ,  31  and  39 , and designated as B, C, F, H, J, L, M, R and S for microATX format represented by the outline formed by line segments  25 ,  27 ,  29  and  41 . It is to be understood that the PCB deflection measuring apparatus, vibration testing system and related methods as herein described may be advantageously used with any standard or customized formats of PCB characterized by known mounting holes locations. 
     Turning now to  FIG. 3 , a geometrical method to define and derive a bending radius from a measured maximal deflection an associated bending span length will now be explained in view of some mathematical equations. In  FIG. 3 , a PCB under bending as schematically represented at  32  substantially forms a circle arc delimited by limit points A and B, the level of bending being characterized by a radius of curvature designated at R. It is pointed out that the amplitude of bending has been intentionally exaggerated in the representation of  FIG. 3  for the purpose of explanation. While the actual bending profile may not be exactly represented by a perfect circle arc as shown in  FIG. 3 , such model can be considered as a good approximation. The length of segment L can be obtained as follows: 
         L= 2 R  sin(α/2)   (1) 
     The length of circle arc can be obtained as follows: 
       Arc=2 πRα/ 360   (2) 
     then: 
       α=Arc 360/(2 πR )   (3) 
     wherein R is the circle radius and α is an angle defined by segments A-O and O-B shown in  FIG. 3 . From the preceding equation, the circle arc may be obtained as follows: 
       Arc=π R/ 90 ·A  sin( L /(2 R ))   (4) 
     and the maximal deflection d to be measured, relative to an axis  55  passing through limit points A and B, is obtained as follows: 
         d=R−[OC]   (5) 
     wherein: [OC]=R·cos(α/2) 
     The preceding equation (5) for d can be transformed as follows: 
         d=R−R ·cos(Arc 360/(2 πR/ 2)   (6) 
         d=R−R ·cos(Arc 90/(π R )   (7) 
         d=R−R ·cos[π R/ 90 ·A  sin( L /(2 R )·90/(π R ))]  (8) 
         d=R−R ·cos[ A  sin( L /(2 R ))]  (9) 
     While the radius of curvature may be obtained from a transformation of equation (9), it can be shown that the model as proposed by Stewart M. et al, in “New mechanical board bend test to demonstrate improved mechanical properties of soft termination”, 9th Annual Automotive Electronics Reliability Workshop, Nashville, AEC, 2004, is a reliable estimation, given by the following equation: 
         R =[(( L/ 2) 2   +d   2 )/(2 d )]  (10) 
     In that model, the point C as shown in  FIG. 3  is substantially equidistantly located between limit points A and B so that maximal deflection d can be obtained directly. It is to be understood that any other appropriate geometrical model could be used as a basis of bending curvature estimation for the purpose of the measurement method. For example, in a case where a point C located elsewhere along axis  55  would be used as a reference to define a deflection value not corresponding to the maximal deflection, an appropriate geometrical relation could be derived to estimate that maximal deflection. According to the equations defined above, the radius of curvature of a PCB during its installation on a mounting structure such as part of a vibration testing system  20  or other system as a product integrating the PCB can be measured to ensure that the level of bending does not exceed a predetermined limit preserving PCB integrity and ensuring functional reliability thereof. 
     Referring to  FIGS. 4A and 4B , using the deflection measuring apparatus generally designated at  34 , preferably three points of measurement A, B and C located along a same axis at equal predetermined span length L/2 are shown, according to the geometrical model explained above in view of  FIG. 3 . The apparatus  34  is provided with a displacement measurement unit generally designated at  51 , for measuring deflection of said printed circuit board  32  in a direction substantially perpendicular to reference mounting plane  49  at a representative number of measurement locations on printed circuit board  32 . In the embodiment shown, the displacement measuring unit  51  includes first and second displacement measuring sensors  36 ,  38  directed substantially perpendicularly toward the mounting plane  49  and disposed in a predetermined spaced relationship to generate displacement values, with respect to mounting plane  49  in direction substantially perpendicular thereto, of first and second bending span limit points A and B on a surface  53  of printed circuit board  32 , as better shown in  FIG. 4B . The displacement measuring unit  51  shown further includes a third displacement measuring sensor  37  directed substantially perpendicularly toward reference mounting plane  49  and disposed between first and second sensors  36 ,  38  to generate a displacement value with respect to mounting plane  49  corresponding to a deflection measurement location on printed circuit board top surface  53 . In the present example, the displacement measuring sensors  36 ,  37  and  38  are contacting sensors, and more particularly linear-voltage differential transformers (LVDTs) mounted on system frame  35 , each being provided with a contact probe  40 ,  40 ′ and  40 ″, respectively, the extremity of which makes contact with top surface  53  of PCB  32  at selected locations thereon. It is to be understood that other displacement measuring sensors of any appropriate type such as dial gauges can be used as distance measuring devices, as well as any appropriate non-contacting probes such as a laser ranging devices. Furthermore, although the use of three sensors  36 , 37 , 38  is convenient, a single sensor that is successively disposed at appropriate measurement positions as shown in  FIG. 4B  according to the measuring span can be used. In the present example, the third sensor  37  is substantially equidistantly disposed between first and second sensors  36 , 38  so that each deflection measurement location is substantially equidistant from the corresponding span limit points A and B. Therefore, it can be seen from  FIG. 4B  that outer contact probes  40  and  40 ″ are spaced by a distance L while they are spaced from the central contact probe  40 ′ each by a distance of L/2. The displacement along vertical axis Y shown in  FIG. 4   b  as detected by sensors  36 ,  37  and  38  is associated with a corresponding voltage output variation generated by each LVDT through corresponding respective output lines  42 ,  42 ′ and  42 ″, the voltage of which being measured by respective voltmeters  44 ,  45  and  46  which output readings can be sent to a data processor such as computer provided with memory as designated at  48 , which is programmed for deriving a measured deflection value relative to printed circuit board top surface  53  at each measurement location from all displacement values generated by sensors  36 , 37 , 38 . More particularly, the computer  48  is adapted to derive from these displacement values a reference value for the printed board top surface  53  partially delimited by bending span limits points A and B, and to subtract that reference value from the displacement value generated by third sensor  37 , to derive the measured deflection at each measurement location relative to printed circuit board top surface  53 . For so doing, on the basis of the measurements obtained from first and second sensors  36 ,  38 , a voltage reference level associated with measurement span limit points A and B at selected locations on the PCB  32  is determined. Then, from the measurement of third sensor  37 , displacement variation may be measured to estimate maximal deflection d between span limit points A and B as represented in  FIG. 3 , by subtracting from the measured output of third sensor  37  the reference level obtained from measurements made with first and second sensors  36  and  38 , which displacement difference corresponds to the maximum deflection d. The computer  48  is further programmed for comparing the measured deflection with a predetermined limit deflection value, and to generate an indication whenever the measured deflection is without a range defined by that limit deflection value. 
     Applying equation (10) set forth above, it can be appreciated that the radius of bending curvature can be directly inferred from the measured deflection. Since the level of bending is inversely proportional to the value for the radius of curvature, the requirement to limit bending below a predetermined limit value may be expressed as a condition that any measured radius of curvature shall not be lower than a predetermined minimum radius of curvature. For example, assuming a minimum radius of curvature R=115 cm, it can be seen from the charts shown in  FIGS. 5 and 6  experimentally obtained with values for L being within the ranges of {0; 250} cm and {0; 50} cm, respectively, that any radius of curvature of an estimated value R greater than (or equal to) the predetermined minimum limit of 115 cm complies with the requirement, while any measured radius of curvature that is lower than 1500 mm does not comply with that same requirement. Preferably, LVDTs are used to measure deflection d obtained with relatively long bending measurement span length, for example L≧43 mm, while a conventional dial gauge can be used to measure more localized deflection d obtained with relatively shorter bending measurement span length, for example when 35 mm ≧L≧43 mm. LVDTs and dial gauge model no TRS-50 from Novotechnik U.S. Inc. (Southborough, Mass.) can be used. The accuracy of such dial gauge being rated at 0.05 mm compared to the accuracy for LVDT rated at 0.075 mm, on the basis of a safety two-factor, maximum deflection d of 0.1 mm and 0.15 mm respectively can be measured. In view of the chart shown in  FIG. 6 , wherein the curve shows the maximum deflection d for a given span length L delimited by a pair of outer measurement points, it can be seen that a minimum span length L=35 must be used with a dial gauge, while a minimum span length L=43 mm must be used with a LVDT. 
     Referring now to  FIG. 7A , secured onto the baffle  28  to be installed on the acoustical enclosure of the vibration testing system shown in  FIG. 1  and forming a mounting structure for a PCB to be tested, is a PCB supporting device generally designated at  50 , used for selectively supporting a printed circuit board at one or more selected locations on first, bottom surface of said printed circuit board  32  during pre-testing, setup operations. In the example shown involving a PCB  32  of a standard microATX format to be tested, the means for securing printed circuit board  32  on mounting structure  28  are in the form of a plurality of clamp assemblies generally designated at  30 . The clamp assemblies  30  as shown are preferably of a similar design that the clamp assemblies described in U.S. Pat. No. 6,668,650 owned by the present assignee. It is to be understood that clamps of alternate design or other attachment devices of any appropriate type are also contemplated for securing the PCB onto the mounting structure  28 . Each clamp assembly  30  includes a movable clamping mechanism designated at  54  that is operable to be moved between an open position when its handle  56  is manually lifted outwardly providing sufficient clearance for mounting a PCB to be tested onto the baffle  28 , and a closed position by pushing down handle  56  inwardly toward the PCB top surface in order to rigidly secure a PCB edge area relative to the baffle  28 , the PCB substantially covering baffle opening  58  so that acoustical waves will be transmitted to the PCB when a vibration test is carried out. Clamp assemblies  30  may be provided for a sufficient number of available mounting holes located at the edge around the PCB to be tested to ensure that it does not shift from its original mounting position during vibration testing at any vibration level required. As also shown in  FIG. 7A , applied onto the inner edge of baffle  28  defining baffle openings  28  and throughout the perimeter thereof are acoustical insulation strips  59  that can be used to maximize the transfer of acoustical energy from the loudspeaker enclosure to the PCB under test. Each clamp assembly  30  further includes an upper contact element  60  adapted to engage a corresponding portion of the top side of a PCB near the edge thereof and further includes a corresponding lower contact element  62  as shown in  FIGS. 9A and 9B  having a head portion  64  and a shank portion  66  provided with a recessed section  67  adapted to receive a securing clip  68  to rigidly secure the lower contact element  62  within a corresponding recess extending through a seating portion  72  provided on the inner baffle edge defining baffle opening  58 , as better shown in  FIG. 8  in view of  FIG. 9B . Alternatively, the free end of shank portion  66  may be provided with an axially extending threaded bore  69  adapted to receive a corresponding bolt, provided the baffle thickness be sufficient so that the bottom surface thereof are coplanar with the shank portion end surface  71 . The head portion  64  defines a protruding pin  74  adapted to engage with a corresponding edge mounting hole  75  provided on PCB  32 , with the top PCB surface surrounding the mounting hole  75  being coplanar with pin end surface  77  and contact surface of upper element  60  in such a manner that PCB  32  is secured between main bearing surface  76  of head portion  64  and upper contact element  60 , while preventing overstress that could otherwise induce local deformation or bending of the PCB upon clamping thereof. The pin  74  ensures a repeatable mounting position of the PCB to be tested on baffle  28  with a predetermined tolerance typically of 0.5 mm to make sure that during the testing stage, all PCBs will tightly fit on the support. Furthermore, when the upper contact element  60  and the lower contact element  62  are brought to the PCB securing position, appropriate clearance for all through-hole part pins along the PCB edge perimeter is provided. The main bearing surfaces  76  of all lower contact elements  62  when inserted within their corresponding baffle seating portions  72  must be as coplanar as possible to ensure that the PCB is not adversely stressed. The pin  74  and main bearing surface  76  are made with or covered by an electrostatic discharge preventing plastic material such as TIVAR™  88  supplied by PHS Americas (Fort Wayne, Ind.) to ensure that the mounting holes of the PCB will not be damaged during vibration testing. 
     Turning back to  FIG. 7A  in view of  FIG. 8 , the PCB supporting device  50  device is operable to be moved between a supporting position during vibration pre-testing, setup operations and a clearance position during vibration testing of printed circuit board  32 . For so doing, PCB supporting device  50  is provided with a displaceable member  80  to which is mounted one or more supporting elements  82 , each defining a contacting support head  84  as better shown in  FIG. 7A . In the embodiment shown in  FIG. 8 , a first end  86  of the displaceable member  80  is pivotally mounted to the bottom side of baffle  28  through a pivot assembly  88  secured against the bottom surface of baffle  28  through a bolt  89  as shown in  FIG. 7A . The second end  90  of displaceable member  80  can be moved between a first, supporting position where the contacting support head  84  of each supporting element  82  is in contact with a surface area of PCB bottom side at predetermined locations, allowing pre-testing operations, and a second, disengaged position where the contacting support head  84  of each supporting element  82  is distant from the bottom side surface of PCB for allowing a test to be performed, as will be later explained in more detail with reference to  FIGS. 13A and 13B . It can be seen from  FIG. 7A  that the transverse position of each contacting support head  84  with respect to displaceable member  80  can be manually adjusted using a nut  92  provided on each supporting element  82 , which conveniently uses a threaded bolt  94  extending through a corresponding bore provided through displaceable member  80  as shown in  FIG. 8 . The second end  90  of displaceable member  80  is further provided with a bore extending therethrough adapted to receive a securing bolt  96  as shown in  FIG. 8  for rigidly mounting a plunger  98  as better shown in  FIG. 7A  as part of a locking mechanism provided on the PCB supporting device  50  which is used to selectively lock the supporting device either in its supporting or unengaged positions as will be later explained in more detail with reference to  FIGS. 13   a  and  13   b.    
     Turning now to  FIGS. 10A and 10B , the plunger  98  is provided with a head portion  100  forming a handle, an intermediate body cylindrical portion  102  and a base cylindrical portion  104  which are interconnected by recessed cylindrical portions  101  and  103  of smaller diameters, which are provided with respective bores  105 ,  107  radially extending therethrough and adapted to receive a set pin  106  extending through the bore of a mounting flange  109  secured onto baffle  28  as shown in  FIG. 7A . 
     Turning back to  FIG. 8 , the base portion  104  of plunger  98  is received within a bore  108  extending throughout the thickness of baffle  28 , such bore  108  being preferably of an elliptical or equivalent section to provide the clearance required by the movement of plunger  98  upon pivotal of displaceable member  80  about pivot assembly  88  between the two limit positions. As shown in  FIG. 10B , plunger base portion  104  is provided at lower end thereof with a threaded bore  110  for receiving securing bolt  96  shown in  FIG. 8 . It can be appreciated that the location of pivot assembly  88  and baffle bore  108  are chosen to ensure that the supporting elements  82  are precisely aligned with the areas of PCB bottom side that required support according to the selection of mounting holes ensuring that PCB bending does not exceed the set limit value. The PCB supporting device  50  can be operatively coupled to a support position sensor  112  whose output is in turn operatively coupled to controller  23  of the vibration testing system  20  shown in  FIG. 1 , for providing a signal indicating whether the supporting device is in its active, supporting position for preventing operation of said vibrator unit, or in its inactive, unengaged position enabling a test to be carried out. Conveniently, the support position detecting device  112  may be a conventional mechanically activated limit switch having a contact probe  114  disposed at a location adjacent the second end  90  of displaceable member  80  whenever the latter is brought toward its lowered, unengaged position upon manual displacement of plunger  98  toward its lowered position. It is to be understood that any other appropriate position detecting device, such as a photocell-based unit, may be used. 
     Turning now to  FIGS. 11 and 12 , there is shown another example of baffle  28 ′ to which are mounted four PCB supporting devices for enabling simultaneous testing of four PCBs  32 , thereby increasing testing productivity. 
     Referring now to  FIG. 14 , there is shown an exemplary, typical microATX PCB  32  mounted onto a baffle  28  with a supporting device including six clamp assemblies  30  shown in securing positions along the outer edges of PCB  32 . Since securing of PCB  32  to be tested must not cause overbending thereof, the PCB bending level throughout its surface was controlled by measuring for a representative set of locations the bending radius to ensure that it does not exceed the predetermined minimum limit value when the PCB  32  is secured onto the baffle  28  with clamp assemblies  30  brought in their closed position. For so doing, measurements was performed using the deflection measuring apparatus  34  as described above with reference to  FIGS. 4A and 4B , after the PCB supporting device  50  has been brought to its unengaged position as shown in  FIG. 13B  upon lowering down plunger  98  in the direction of arrow  116 , which plunger  98  is then locked in that inactive position by insertion of set pin  106  through bore  105  provided on the recessed upper portion  101  of plunger  88  as shown in  FIG. 10B . Prior to proceed with bending measurements on a PCB  32  to be tested, the displacement sensors  36 ,  37  and  38  of the deflection measuring apparatus  34  shown in  FIGS. 4A and 4B  were calibrated by measuring voltage values generated for each of them when respective probe  40 ,  40 ′,  40 ″ are disposed in contact with a straight reference bar (not shown) corresponding to a null (0) reference level, which reference voltage values was used to derive voltage variation ΔV due to displacement with respect to reference level. Then, while maintaining the physical position of sensors  36 ,  37  and  38  on the system frame  35  as shown in  FIG. 4A , each one of LVDT probes  40 ,  40 ′,  40 ″ was raised to allow the mounting of a PCB  32  to be tested onto the baffle  28 , which PCB  32  was rigidly secured thereto using clamp assemblies  30 . Then, a series of displacement measurement was performed on areas of the PCB top surface at regions of interest such as near mounting hole locations and PCB center area, in order to obtain representative indications of bending effect throughout the surface of PCB  32 . An example of bending radius measurements obtained for the PCB  32  shown in  FIG. 14  is given in Table 1. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 RESULTS 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Coordinates 
                   
                 d 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Mounting 
                   
                 Deflection 
                 BENDING 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 # 
                 Hole 
                 L/2 (mm) 
                 (mm) 
                 Radius (mm) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 C1 
                 M 
                 20.8 
                 0.025 
                 8651 
               
               
                   
                 C2 
                 L 
                 20.8 
                 0.027 
                 8140 
               
               
                   
                 C3 
                 L 
                 20.8 
                 0.016 
                 13146 
               
               
                   
                 C4 
                 J 
                 20.8 
                 0.002 
                 96245 
               
               
                   
                 C5 
                 F 
                 20.8 
                 0.011 
                 19169 
               
               
                   
                 C6 
                 B 
                 20.8 
                 0.008 
                 28054 
               
               
                   
                 C7 
                 R 
                 20.8 
                 0.037 
                 5888 
               
               
                   
                 C8 
                 M-J 
                 20.8 
                 0.022 
                 9961 
               
               
                   
                 C9 
                 J-F 
                 20.8 
                 0.034 
                 6273 
               
               
                   
                 C10 
                 Center 
                 20.8 
                 0.026 
                 8340 
               
               
                   
                 C11 
                 B 
                 20.8 
                 0.040 
                 5362 
               
               
                   
                 C12 
                 R 
                 20.8 
                 0.022 
                 9724 
               
               
                   
                 C13 
                 L-H 
                 20.8 
                 0.013 
                 16671 
               
               
                   
                 C14 
                 Center 
                 20.8 
                 0.019 
                 11684 
               
               
                   
                   
               
            
           
         
       
     
     Conveniently, the values for L/2 (mm) were directly obtained by measuring the distance separating LVDT probes  40 , 40 ′, 40 ″. The deflection value (mm) was obtained from the voltage variation signals with respect to the reference level as generated by sensors  36 ,  37  and  38  in the following manner. First, the voltage variations measured for outer sensors  36  and  38  were averaged to obtain a main voltage variation value. Then a difference between the voltage variation value measured with central sensor  37  and the average voltage variation value was calculated by computer  48  to obtain a resulting deflection d (mm). Finally, using equation (10), the bending radius (mm) was estimated from values for deflection d and values for L/2 (mm). It can be seen from the resulting radius values given in Table 1 that the requirement based on minimum bending radius R=1500 mm was clearly met for all measurement coordinates C1 to C14. Whenever a bending radius measurement made at a given location does not comply with the minimum bending radius criterion, a new configuration for clamp assemblies  30  must be determined on the basis of which an additional series of measurements is performed until the minimum bending radius requirement is met. 
     In order to minimize the risk that an operator performing a pre-testing setup on a PCB unintentionally overbends the PCB, thereby inducing stress that damages it, the PCB deflection measuring apparatus and its method were used to ensure that, at representative locations on the surface of the PCB, the bending radius when a load is applied onto the PCB top side, for example by adding daughter-boards, power cables, connectors, etc. during a pre-testing setup, does not exceed a predetermined value, to preserve PCB integrity and ensuring its reliability. Such requirement can be met by first characterizing the effect of the supporting apparatus itself before any load is applied on the PCB, with at least one selected support position within the perimeter defined by the PCB, for example mounting holes Hand CPU socket measurement locations designated at C6 and C11 on  FIG. 15  representing the same PCB  32  to be tested as shown in  FIG. 14 . These measurement locations were selected since no component or conducting path was found at these locations that could cause short circuit. Furthermore, it was expected that the predetermined bending radius limit requirement was likely to be met when one or more of existing mounting holes determined at the design stage of the PCB were selected. For so doing, the displaceable member  80  of the supporting device  50  was brought to its active, PCB supporting position as shown in  FIG. 13A  wherein the contacting support head  84  of each supporting element  82  makes contact with PCB bottom surface at the selected location thereon, thereby preventing relative movement of the PCB contacted portion to minimize PCB bending. It can be appreciated form  FIG. 13A  that when the displaceable member  80  is brought to its supporting position upon lifting up of plunger  98  in the direction of arrow  116 , such active position may be locked by insertion of set pin  106  through the lower bore  107  provided on plunger  98  as shown in  FIG. 10B . It can also be seen from  FIG. 13A  that when the displaceable member  80  is brought in the supporting position, the detecting device  112  is in its deactivated state, thereby indicating to the vibration system controller that the test cannot be performed. After having proceeded with reference voltage level measurement using the same calibration bar referred to above, the measurement was repeated in the same manner as explained before to obtain deflection values from which bending radius may be derived. An example of deflection measurement and bending radius calculation results for the PCB  32  shown in  FIG. 15  is given in Table 2. 
     
       
         
           
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Coordinates 
                 RESULTS 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Mounting 
                   
                 d 
                 BENDING 
               
               
                 # 
                 Hole 
                 L/2 (mm) 
                 Deflection (mm) 
                 Radius (mm) 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 1 
                 B 
                 20.8 
                 0.077 
                 2813 
               
               
                 2 
                 R 
                 20.8 
                 0.014 
                 15751 
               
               
                 3 
                 R 
                 20.8 
                 0.093 
                 2326 
               
               
                 4 
                 L 
                 20.8 
                 0.098 
                 2208 
               
               
                 5 
                 C 
                 20.8 
                 0.087 
                 2488 
               
               
                 6 
                 CPU 
                 20.8 
                 0.080 
                 2702 
               
               
                 7 
                 M-J 
                 20.8 
                 0.038 
                 5619 
               
               
                 8 
                 F-J 
                 20.8 
                 0.053 
                 4097 
               
               
                 9 
                 B-C 
                 20.8 
                 0.065 
                 3343 
               
               
                 10 
                 F-J 
                 20.8 
                 0.001 
                 262016 
               
               
                 11 
                 CPU 
                 20.8 
                 0.045 
                 4821 
               
               
                 12 
                 R-H 
                 20.8 
                 0.052 
                 4136 
               
               
                 13 
                 R-H 
                 20.8 
                 0.081 
                 2678 
               
               
                 14 
                 J 
                 20.8 
                 0.111 
                 1944 
               
               
                 15 
                 M 
                 20.8 
                 0.042 
                 5155 
               
               
                 16 
                 L 
                 20.8 
                 0.051 
                 4202 
               
               
                   
               
            
           
         
       
     
     It can be seen from Table 2 that the minimum bending radius requirement R=1,500 mm was met for all measurement locations C1 to C16 that were considered. Whenever the bending radius measurement obtained for a given location does not comply with the minimum bending radius requirement, a new configuration of support locations provided by the PCB supporting device  50  must be determined on the basis of which the measurement procedure is repeated until the requirement is met. 
     Referring now to  FIGS. 16A and 16B , another embodiment of deflection measurement apparatus  34 ′ can be used to simulate loading conditions that typically prevail when an operator handles a PCB for performing an operation thereon such as daughter-board or connector insertion into corresponding sockets provided on the PC board to be tested, or to be integrated in a receiving system. For so doing, a reasonable value for such maximum load has been experimentally set to 66 N, which value was used in the example that will be later presented. The modified deflection measuring apparatus  34 ′ physically includes the same elements as included in the basic deflection measuring apparatus  34  described before with reference to  FIGS. 4A and 4B , with some additional elements that are provided to perform load application and measurement functions. As shown in  FIG. 16B , the modified apparatus  34 ′ further includes a load applying device  120  directed substantially perpendicularly toward reference mounting plane  49  and disposed between first and second sensors  36 ,  38  to apply a predetermined load on printed circuit board surface  53  at measurement location thereon. Conveniently, the load applying device may be a pneumatic cylinder such as supplied by Bimba manufacturing Co. (Monee, Ill.) operatively coupled to the probe  40 ′ of central sensor  37  through piston  122  for simultaneously applying the predetermined load onto a selected location of PCB  32  while repeating the measurement of relative displacement from voltage variation from sensor  37  in the same manner as explained before. Furthermore, in order to ensure that the actual load applied to the PCB  32  corresponds to the preset load value, a force detector such as piezoelectric detector  124  can be coupled to the probe  40 ′ of sensor  37  to generate a corresponding load measurement signal to a further voltmeter  47 . On the basis of the standard calibration curve characterizing the relation between a given voltage reading and the corresponding force measurement value, the load applied by piston  122  of device  120  can be adjusted using an air pressure regulator  126  connected to a pressurized air source  130  through a main valve  128 . Optionally, a controller  132  having an input  134  connected to an output of voltmeter  47  and having an output at  136  coupled to a controlled input provided on air pressure regulator  126  in a feedback configuration may be used. It is to be understood that any other appropriate actuating device such as an electrical powered linear actuator can be used as load applying means. So as to prevent any damage of PCB surface or components upon application of the predetermined load, a protecting plate  138  can be disposed between the contact end of central probe  40 ′ and the PCB loaded area, which plate being represented by a dark rectangle associated with each bending measurement span at location coordinates C1 to C10 shown in  FIG. 17 . As compared with the calibration procedure explained above when performing deflection measurement with the apparatus  34  while no load is applied to the PCB and for which a single calibration step is sufficient prior to proceed with the series of measurements at the selected PCB locations, the modified apparatus  34 ′ provided with load applying and measurement functions is preferably calibrated prior to each set of measurements at each selected location on PCB  32  to enhance the reliability of the results. Once reference voltage values associated with sensors  36 ,  37  and  38  with no load applied by piston  122  as well as reference level for piezoelectric detector  124  are obtained, the cylinder of device  120  is caused to be fed with pressure air from regulator  126  according to the preset value. Table 3 presents an example of bending radius values that were obtained for the printed circuit board  32  shown in  FIG. 17  involving bend measurement locations that were selected corresponding to various connectors and sockets. 
     
       
         
           
               
               
             
               
                   
                 TABLE 3 
               
             
            
               
                   
                   
               
               
                   
                 RESULTS 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Coordinates 
                   
                 d 
                 BENDING 
               
               
                   
                 # 
                 L/2 (mm) 
                 Deflection (mm) 
                 Radius (mm) 
               
               
                   
                   
               
               
                   
                 C1 
                 22.75 
                 0.120 
                 2150 
               
               
                   
                 C2 
                 22.75 
                 0.121 
                 2141 
               
               
                   
                 C3 
                 22.75 
                 0.128 
                 2018 
               
               
                   
                 C4 
                 22.75 
                 0.062 
                 4166 
               
               
                   
                 C5 
                 22.75 
                 0.112 
                 2315 
               
               
                   
                 C6 
                 22.75 
                 0.139 
                 1858 
               
               
                   
                 C7 
                 22.75 
                 0.149 
                 1738 
               
               
                   
                 C8 
                 22.75 
                 0.065 
                 3978 
               
               
                   
                 C9 
                 22.75 
                 0.092 
                 2818 
               
               
                   
                 C10 
                 22.75 
                 0.098 
                 2629 
               
               
                   
                   
               
            
           
         
       
     
     It can be seen from the bend radius values given in Table 3 that all bending measurements for locations C1 to C10 comply with the minimum radius requirement R=1,500 mm. In the case where such requirement were not met, a new selection of support locations should have been made, which could have involved alternative existing mounting hole locations. The support configuration validation procedure carried on with the deflection measuring apparatus  34 , involving deflection measurements with preset load applied to the PCB, must be repeated until the minimum bending radius requirement is met. Following the successful completion of all series of measurements described above, the PCB  32  is ready for vibration testing, provided the displaceable member  80  of PCB supporting device  50  is brought back to its inactive, unengaged position as shown in  FIG. 13B , thereby indicating to the vibration testing system controller through the activation of detector  112  that a test may be safely carried on. 
     For the purpose of verifying if a PCB can be integrated in an end product system without damage, a same apparatus that described above can be used, but without the PCB supporting device. In that application, the deflection measuring apparatus can be installed on a system prototype used as a testing bench, provided with a same mounting structure than used by the end product system. The PCB is first secured to the mounting structure using the same attachment means as used for the system assembly. Deflections the PCB in a direction substantially perpendicular to the reference mounting plane at a representative number of measurement locations on the PCB are measured and compared by the data processor to a predetermined limit deflection value, as explained above. The measurements may be performed simultaneously with the securing operation, or following partial or complete PCB securing. If the measured deflections are within a range defined by the limit deflection value, it is an indication that the mounting structure with attachments means used do not overstress the PCB. In a case where some assembly operations likely to induce stress to the PCB are planned, such operations may then be actually performed or simulated with the load applying device provided on the deflection apparatus as described above, while deflection measurements are performed. If the measured deflections still comply with the limit requirement, it is an indication that the proposed PCB mounting means and assembly operations can be safely used in the system assembly line. However, if the measured deflections are found without that predetermined range, this is an indication that the proposed mounting structure, attachments means and/or assembly operations overstress the PCB at a level that may cause a component failure, and must be reviewed prior to be subjected to a further test. 
     It can be appreciated that the PCB deflection measurement device according to the embodiments as described above can be used according to any PCB mounting orientation with respect to the receiving system. For example, a PCB may be mounted under a top plate of the system frame, with its surface populated with components and connectors facing downwardly. In such case, the deflection measuring apparatus can be mounted in an inverted orientation as compared with that shown in  FIGS. 4A and 4B . It is to be understood that alternative mechanisms capable of providing the movement of supporting elements  82  between their supporting and unengaged positions may be used in replacement of the pivoting displaceable member  80  as described above. For example, such alternate displaceable mechanism may use a linear actuator coupled to a transverse member having both ends mounted for sliding to a pair of opposed rails, the back and forth movement of which allowing supporting elements to selectively engage or disengage the bottom side of a PCB at the selected location thereon. According to a further alternate mechanism, each supporting element can be coupled to an independent actuator to provide more flexibility in the selection of support locations for the PCB. Moreover, the position locking mechanism may be provided with an actuator coupled to the plunger to provide an automatic selective movement between both locking positions. Furthermore, in a case where a testing procedure would involved the loading of both surfaces of the PCB under test, two deflection sensor units could be mounted with respect to top and bottom surfaces of the PCB, with two corresponding load applying devices where loading simulation is desired. 
     It is further to be understood that the PCB deflection measuring apparatus and methods as described above may be used with other types of vibration testing systems such as electrodynamic, hydraulic or pneumatic, as well as with a variety of electronic systems found in technological fields such as telecommunication, automation and instrumentation.