Abstract:
An apparatus generally having a circuit board and a first strain gauge is disclosed. The circuit board may have a plurality of insulating layers. The first strain gauge may be disposed between two of the insulating layers.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to printed circuit boards generally and, more particularly, to an embedded strain gauge in a printed circuit board.  
       BACKGROUND OF THE INVENTION  
       [0002]     Referring to  FIGS. 1 and 2 , a perspective drawing and a cross-sectional drawing of a conventional ball grid array (BGA) packaged component  10  mounted on printed circuit board (PCB)  12  are respectively shown. Multiple strain gauges  14   a - 14   d  are commonly mounted on the PCB  12  around the component  10  to characterize mechanical loading and strain to solder balls  16   a - 16   n  attaching the component  10  to the PCB  12 . Attaching the strain gauges  14   a - 14   b  to the PCB  12  is cumbersome and extremely time consuming. The strain gages  14   a - 14   d  are attached on a top surface  18  of the PCB  12  and thus away from areas of interest. The strain gages  14   a - 14   d  cannot be placed between the component  10  and top surface  18  due to the solder balls  16   a - 16   n . Therefore, measurements are taken away from solder balls  16   a - 16   n  or other locations that are important.  
         [0003]     Furthermore, wiring from the strain gages  14   a - 14   d  commonly interferes with board level test equipment, such as in circuit test (ICT) or automated functional test (AFT) equipment. The strain gages  14   a - 14   d  are typically placed away from the board level test fixtures to avoid the interferences. Therefore, the board level test equipment interference makes measurements from the strain gauges  14   a - 14   d  inaccurate.  
       SUMMARY OF THE INVENTION  
       [0004]     The present invention concerns an apparatus generally comprising a circuit board and a first strain gauge. The circuit board may have a plurality of insulating layers. The first strain gauge may be disposed between two of the insulating layers.  
         [0005]     The objects, features and advantages of the present invention include providing an architecture and/or method for embedding strain gauges within a printed circuit board that may (i) provide accurate measurements of forces applied to the circuit board, (ii) avoid interference between strain gauge wiring and board level test equipment, (iii) avoid interference between strain gauge wiring and test fixtures, (iv) avoid interference between strain gauge wiring and edge connectors and/or (v) avoid interference between strain gauge wiring and test locations on the circuit board. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     These and other objects, features and advantages of the present invention will be apparent from the following detailed description and the appended claims and drawings in which:  
         [0007]      FIG. 1  is a perspective drawing of a conventional ball grid array packaged component mounted on printed circuit board;  
         [0008]      FIG. 2  is a cross-section drawing of  FIG. 1 ;  
         [0009]      FIG. 3  is a cross-section drawing of a printed circuit board in accordance with a preferred embodiment of the present invention;  
         [0010]      FIG. 4  is a perspective drawing of a strain gauge;  
         [0011]      FIG. 5  is a cross-section drawing of the circuit board during a step of fabrication;  
         [0012]      FIG. 6  is a cross-section drawing of the circuit board during another step of fabrication;  
         [0013]      FIG. 7  is a schematic of a circuit configured to measure a force;  
         [0014]      FIG. 8  is a perspective drawing of the circuit board receiving a bowing force; and  
         [0015]      FIG. 9  is a perspective drawing of the circuit board receiving a twisting force. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]     Referring to  FIG. 3 , a cross-section drawing of a printed circuit board  100  is shown in accordance with a preferred embodiment of the present invention. The printed circuit board (or “circuit board” for short)  100  generally comprises two or more layers  102   a - 102   n , two or more layers  104   a - 104   n  and one or more strain gauges  106   a - 106   g . One or more regions  108  (only one shown) may be defined where a component  110  may be mounted. Multiple interfaces  112   a - 112   k  may be formed on the layer  104   a  to provide communication between the strain gauges  106   a - 106   g  and an electronic circuit  114 .  
         [0017]     The layers  102   a - 102   n  may be implemented as electrically insulating layers. The layers  104   a - 104   n  may be implemented as electrically conductive layers. An alternating sequence of insulating layers  102   a - 102   n  and conductive layers  104   a - 104   n  may be arranged to form the circuit board  100 . Each of the strain gauges  106   a - 106   g  may be disposed between two of the insulating layers  102   a - 102   n.    
         [0018]     One or more of the conductive layers  104   a - 104   n  may be patterned to create traces ( FIG. 4 ) that connect the strain gauges  106   a - 106   g  to the interfaces  112   a - 112   k . Generally, each conductive layer  104   a - 104   n  disposed between the same two insulating layers  102   a - 102   n  as a strain gauge  106   a - 106   g  may be etched to provide communications between the strain gauges  106   a - 106   g  and the interfaces  112   a - 112   k . The conductive layers  104   a - 104   n  may be implemented as metal layers. The interfaces  112   a - 112   k  may be implemented as test pads, pins, external connectors or the like. The interfaces  112   a - 112   k  may be positioned on a top surface  116  of the circuit board  100  to avoid interference with other equipment  118  such as test probes, mechanical fixtures, sensors, edge connectors, test connectors and other test equipment.  
         [0019]     The strain gauges  106   a - 106   g  may be mounted at different locations within the planes (e.g., X-Y planes) established by the insulating layers  102   a - 102   n . In a particular embodiment, a strain gauge  106   a - 106   g  may be located approximate a corner in each quadrant of the region  108  Such a four strain gauge  106   a - 106   g  arrangement may be useful in measuring strains generated while a force  119  may be exerted to mount the component  110  within the region  108 . The strain gauges  106   a - 106   g  may be used to measure strains generated during handling for different board assembly processes and tests, such as in circuit testing (ICT) and automated functional testing (AFT). The strain gauges  106   a - 106   g  may be located at other coplanar and/or non-coplanar positions in and around the region  108  and/or other regions to meet a design criteria of a particular application.  
         [0020]     In another embodiment, some or all of the strain gauges  106   a - 106   g  may be located between different insulating layers  102   a - 102   n . For example, two or more strain gauges  106   a - 106   g  may be positioned at different depths from the top surface  116  along a line  120  normal to the top surface  116  of the circuit board  100 . The strain gauges  106   a - 106   g  do not have to be located at different depths along a normal line  120  or any other straight line. Locating the strain gauges  106   a - 106   g  at different depths may be useful for measuring various bending, bowing and/or twisting stresses applied to the circuit board  100 . In general, the strain gauges  106   a - 106   g  may be mounted in any combinations of two or more layers to meet a design criteria of a particular application.  
         [0021]     The component  110  may be implemented as an electronic component. For example, the component  110  may be an active electronic device such as an integrated circuit, transistor, multi-chip carrier or the like. The component  110  may also be implemented as a passive electronic element such as a resistor, capacitor and/or inductor.  
         [0022]     Referring to  FIG. 4 , a perspective drawing of a strain gauge  106  is shown. The strain gauge  106  may be implemented as one or more resistive metal foils  121  formed on an insulating layer  102 . Each metal foil  121  may be connected to traces  122   a - 122   b  created in a conductive layer  104 . The traces  120   a - 120   b  may include vias  124   a - 124   b  that extent thru the top surface conductive layer  104   a . The vias  124   a - 124   b  may be plated to provide electrical connections between the traces  122   a - 122   b  and the interfaces  112   a - 112   k  formed in the top conductive layer  104   a.    
         [0023]     Referring to  FIGS. 5 and 6 , cross-section drawings of the circuit board  100  during various steps of fabrication are shown.  FIG. 5  generally illustrates an insulating layer  102   c  having a conductive layer  104   c . The conductive layer  104   c  may be patterned to open an area  126  where a strain gauge  106  may be formed. The strain gauge  106  may be formed by deposition/plating a layer of strain gauge material and subsequent etching of the strain gauge material, similar to other traces in the metal layers  104   a - 104   n . The strain gauge  106  may also be fabricated separately from the metal layers  104   a - 104   n  and attached to a particular insulating layer (e.g.,  102   c ) after etching the associated conductive layer (e.g.,  104   c ).  
         [0024]      FIG. 6  generally illustrates additional insulating layers  102   a - 102   b  and conductive layers  104   a - 104   b  formed over the strain gauge  106 . The top conductive layer  104   a  may configured to include the interfaces  112   a - 112   k . Two of the interfaces  112   a - 112   k  may be connected to the traces  122   a - 122   b  in the third conductive layer  104   c  through via plating. For example, the interface  112   a  may be connected to the trace  124   b  and strain gauge  106  with a plating  128 .  
         [0025]     Referring to  FIG. 7 , a schematic of a circuit  130  configured to measure a force is shown. The circuit  130  generally comprises the electronic circuit  114 , the strain gauge  106  and resistors  132 ,  134  and  136 . The strain gauge  106  and the resistors  132 ,  134  and  136  may be configured as a Wheatstone bridge. The electronic circuit  114  may be connected to the Wheatstone bridge  138  to measure changes in a resistance of the strain gauge  106  due to stress in the circuit board  100 . In one embodiment, the resistors  132 ,  134  and  136  may be implemented off the circuit board  100 . In another embodiment, the resistors  132 ,  134  and  136  may be fabricated between the insulating layers  102   a - 102   n  of the circuit board  100 . In particular, the resistors  132 ,  134  and  136  disposed within the circuit board  100  may be fabricated between the same insulating layers  102   a - 102   n  as the associated strain gauge  106 . The metal traces formed from the associated conductive layer  104  may be used to establish the Wheatstone bridge. In still another embodiment, the resistor  134  may be implemented as another of the strain gauges  106   a - 106   g  disposed approximate the strain gauge  106 .  
         [0026]     Referring to  FIG. 8 , a perspective drawing of the circuit board  100  receiving a bowing force  140  is shown. The bowing force  140  may operate to displace one or more sides of the circuit board  100  out of an at-rest plane.  
         [0027]     Referring to  FIG. 9 , a perspective drawing of the circuit board  100  receiving a twisting force  142  is shown. The twisting force  142  may operate to displace one or more corners of the circuit board  100  above the at-rest plane and one or more opposing corners of the circuit board  100  below the at-rest plane.  
         [0028]     The various signals of the present invention are generally shown on individual inputs and outputs. In other embodiments, some or all of the various signals may be multiplexed through one or more inputs and/or outputs as desired or needed.  
         [0029]     While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention.