Patent Publication Number: US-6988899-B2

Title: Electronic assembly, and apparatus and method for the assembly thereof

Description:
BACKGROUND OF THE INVENTION 
   The present disclosure relates generally to an electronic assembly and to the apparatus and method for assembling the electronic assembly, and particularly to an apparatus and method for assembling a light detector for use in medical diagnostic equipment. 
   A type of detector array used for computed tomography is made from a silicon wafer having a diode array that is positioned on a ceramic substrate, such as Aluminum Nitride or AIN for example. The silicon diode-array wafer is connected, by means of stud-bump arrays, to metal pads on the ceramic substrate. The connection method may involve bonding via a metal-filled adhesive, such as silver-filled epoxy for example, or by soldering. For backlit photodiode arrangements, the silicon wafer for the diode array needs to be thin, so as to minimize the adverse effects on the X-ray beam that is converted into visible light via a scintillator for detection by the diode array. The thinner the silicon diode-array wafer is, the less attenuation and scattering there will be of the X-ray beam, however, the more susceptible the wafer will be to physical damage. Additionally, for high resolution imaging, it is desirable to have a silicon diode-array wafer that has a high density of photodiodes; however, such high density may require a high degree of control in the manufacturing of such detectors. While some advances have been made in the assembly of detector arrays for use in computed tomography, such as the use of vacuum suction cups to pick and place the diode array wafers, there is still a need in the art for a detector array arrangement, and a method and apparatus for assembling the detector array arrangement, that overcomes these drawbacks. 
   BRIEF DESCRIPTION OF THE INVENTION 
   Embodiments of the invention include an electronic assembly having a first layer and a second layer. The first layer has a first interface surface and a plurality of cavities formed in the first interface surface. The second layer has a second interface surface and a plurality of projections disposed at the second interface surface, where the plurality of projections are aligned with and disposed at the plurality of cavities. An electrically conductive connecting material is disposed at the plurality of cavities such that the connecting material connects the plurality of projections to the respective plurality of cavities. 
   Other embodiments of the invention include an apparatus for assembling an electronic assembly having a top layer. The apparatus includes a porous rigid element having a thickness and a support surface, and a housing configured to hold the porous rigid element and to provide a positive vacuum to the porous rigid element, wherein the applied positive vacuum results in a positive vacuum at the support surface for picking up the top layer. 
   Further embodiments of the invention include a method for assembling an electronic assembly, the assembly including a first layer having a first interface surface and a plurality of cavities formed in the first interface surface, and a second layer having a second interface surface and a plurality of projections disposed at the second interface surface. The first layer is positioned and the second layer is vacuum held via an apparatus such that the first and the second interface surfaces oppose each other. The plurality of projections are aligned and engaged with the plurality of cavities, and the vacuum hold is sufficiently reduced so as to release the second layer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring to the exemplary drawings wherein like elements are numbered alike in the accompanying Figures: 
       FIG. 1  depicts an isometric view of an exemplary assembly for practicing embodiments of the invention; 
       FIG. 2  depicts a cross section side view of an expanded portion of the assembly of  FIG. 1 ; 
       FIG. 3  depicts a similar view to that of  FIG. 2 , but prior to the assembly of the respective parts; and 
       FIG. 4  depicts an exemplary apparatus for assembling the assemblies depicted in  FIGS. 1–3 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   An embodiment of the invention provides a light detector for use in medical diagnostic equipment, such as computed tomography for example, having an array of backlit photodiodes electrically bonded to a ceramic substrate having copper runs for signal communication. At the bonding interface, the diode arrays have elongated electrical connections, or stud bumps as they are referred to, that extend into cavities formed in the ceramic substrate, which provide pockets for constraining the conductive epoxy or solder from electrically shorting out adjacent diodes. An alternative embodiment of the invention provides an apparatus for vacuum holding the photodiode array while assembling the same onto the ceramic substrate. A further embodiment of the invention provides a method for using the assembly apparatus for assembling the light detector. While embodiments described herein depict an array of backlit photodiodes assembled to a ceramic substrate as an exemplary electronic assembly, it will be appreciated that the disclosed invention is also applicable to other electronic assemblies, such as processing chips on a printed circuit board for example. 
     FIGS. 1–3  depict an exemplary embodiment of an electronic assembly  100  having a first layer  110  and a second layer  120 .  FIG. 1  depicts an isometric perspective of electronic assembly  100 ,  FIG. 2  depicts a side view of a portion of the assembly of  FIG. 1  subsequent to assembling, and  FIG. 3  depicts a side view of the assembly of  FIG. 1  prior to assembling. First layer  110  includes a first interface surface  112  having a plurality of cavities  114  formed therein, and second layer  120  includes a second interface surface  122  having a plurality of projections  124  disposed thereat. In an embodiment, second layer  120  includes a diode array having a plurality of backlit photodiodes  121  in electrical communication with projections  124 . During the assembly of second layer  120  onto first layer  110 , projections  124  are aligned with and assembled into cavities  114 . In an embodiment, the ends of projections  124  are shaped to mirror the shape of the interior surface of cavities  114 , which may be spherical in shape for example. Projections  124  are electrically bonded to cavities  114  via an electrically conductive connecting material  130 , which may be conductive epoxy or solder for example. Prior to assembly, connecting material  130  may be applied to projections  124  or cavities  114 , and in the assembled state is referred to as being disposed at cavities  114 . By introducing cavities  114  at first interface surface  112  of first layer  110 , connecting material  130  will naturally be constrained by the pocket shape of cavities  114  during the curing of connecting material  130 , which may involve the heating of an epoxy or the heating and cooling of a solder, via a reflow and solidification process. Copper conductors  140  at first layer  110  are exposed via an etching process that creates cavities  114 , thereby providing a point of electrical contact for connecting material  130  to bond to, which in turn provides an electrical communication path from second layer  120  (a backlit photodiode array for example), to first layer  110  (a ceramic substrate having wire runs for example), and ultimately to readout electronics (not shown) for the diode signals. Copper conductors  140 , also referred to as runs, may have layers of nickel and gold plated thereon after cavities  114  are formed, thereby protecting the copper pads at cavities  114  from oxidation. As a result of an etching process to create cavities  114 , copper conductors  140  may have a nominal thickness that is reduced slightly at the site of cavities  114 . 
   Cavities  114  are formed having a depth d with respect to first interface surface  112 , and in the assembled state first interface surface  112  is disposed apart from second interface surface  122  by a gap g, depicted in  FIGS. 2 and 3 . Accordingly, projections  124  have a length h that is equal to or less than the sum of depth d and gap g. If length h is less than the sum of depth d and gap g, such as may be the case under normal tolerance conditions, then connecting material  130  will bridge the gap. 
   Projections  124  have a width, or alternatively a diameter, w and are arranged with a pitch p. In an embodiment, projections  124  have a width w equal to or greater than about 100 microns (micrometers) and equal to or less than about 700 microns. In an alternative embodiment, projections  124  have a width w equal to about 500 microns. In an embodiment, projections  124  have a pitch p equal to or greater than about 1.1 times width w and equal to or less than about 3 times width w. In an alternative embodiment, projections  124  have a pitch p equal to about 2 times width w. 
   In an embodiment, and by appropriately sizing width w and pitch p, the plurality of photodiodes  121  in photodiode array  120 , depicted in  FIG. 1 , may be spaced on first layer  110  with an edge spacing s equal to or less than about 100 microns. In an alternative embodiment, edge spacing s is equal to or less than about 25 microns, and in a further embodiment, edge spacing s is equal to about 10 microns. The close spacing between the photodiodes  121  of photodiode array  120  is made possible at least in part by cavities  114  constraining connecting material  130  so that adjacent photodiodes  121  are not electrically shorted. 
   Referring now to  FIG. 4 , an apparatus  150  for assembling electronic assembly  100  is depicted in cross section view. In electronic assembly  100 , second layer  120  is also herein referred to as a photodiode array or a top layer, where top layer may be just one photodiode  121  of photodiode array  120 . Apparatus  150  includes a porous rigid element  160  having a thickness t and a support surface  162 , and a housing  170  configured to hold porous element  160  and to provide a positive vacuum to porous element  160  via a vacuum port  172 . By applying a vacuum in the direction of arrow  174 , a positive vacuum results at the exposed surfaces of porous element  160 , and particularly at support surface  162 , which is used for holding or picking up top layer  121 . In an embodiment, housing  170  includes a pocket  176  with sides disposed around all sides of porous element  160  except support surface  162 , thereby leaving a portion of the thickness of porous element  160  exposed and directing the positive vacuum at porous element  160  toward support surface  162 . 
   In an embodiment, support surface  162  is fabricated to have a flatness equal to or less than about 20 microns, that is, support surface  162  defines a theoretical surface that does not vary from a planar surface, which contacts the theoretical surface at three or more points, by more than 20 microns over the expanse of support surface  162 . In an alternative embodiment, support surface  162  is fabricated to have a flatness equal to or less than about 10 microns. Porous element  160  may be fabricated with the desired flatness at support surface  162  with or without secondary machining. 
   As depicted in  FIG. 4 , support surface  162  may have an overall dimension that is substantially matched to the corresponding overall dimension of photodiode  121 , as evidenced by only a slight overhang  164  at the edge of porous element  160 . While the cross section view of  FIG. 4  shows only one overall dimension of support surface  162  being substantially matched to the corresponding overall dimension of photodiode  121 , namely that overall dimension in the plane of the paper, it will be appreciated that a second overall dimension of support surface  162  may also be substantially matched to that corresponding overall dimension of photodiode  121 , namely the overall dimension perpendicular to the plane of the paper. 
   Porous element  160  may include a plurality of cavities  166  having disposed therein a plurality of heater elements  180  for processing connecting material  130  so as to adhere the projections  124  to cavities  114 . In an embodiment, the processing of connecting material  130  involves heating electrically conductive epoxy to cure and solidify the epoxy. In an alternative embodiment, the processing of connecting material  130  involves heating solder to cause a solder re-flow that solidifies on cooling. Heating elements  180  are in signal communication with a control unit (not shown) for controlled heating during assembly. The control unit also controls the presence and absence of a vacuum at vacuum port  172 , and the action of control arm  190  that is in operable communication with apparatus  150  for controlling the location of apparatus  150  during assembly. The control unit may be any type of control unit suitable for the purposes disclosed herein. 
   Since photodiodes  121  may be manufactured with a less than desirable flatness, apparatus  150  is configured such that the combination of support surface  162  and resultant positive vacuum at support surface  162  is sufficient to flatten photodiode  121 , having an original flatness equal to or greater than about 50 microns and a typical thickness equal to about 100 microns, to a final flatness equal to or less than about 20 microns. In an alternative embodiment, the combination of support surface  162  and resultant positive vacuum at support surface  162  is sufficient to flatten photodiode  121 , having an original flatness equal to or greater than about 100 microns, to a final flatness equal to or less than about 10 microns. 
   In view of the foregoing, a method of assembling electronic assembly  100  using apparatus  150  includes: positioning first layer at an assembly station (not shown); vacuum holding second layer  120 ,  121  via apparatus  150  such that first and second interface surfaces  112 ,  122  oppose each other; aligning plurality of projections  124  with plurality of cavities  114 ; engaging plurality of projections  124  with plurality of cavities  114 ; and, sufficiently reducing the vacuum hold at support surface  162  so as to release second layer  121  from apparatus  150 . An electrically conductive connecting material  130  may be applied to the plurality of cavities  114  or to the plurality of projections  124  prior to engaging projections  124  with cavities  114 . Subsequent to the engaging action, the connecting material  130  is processed in the manner discussed previously so as to adhere projections  124  to cavities  114 . In an embodiment, the vacuum hold may be maintained for a sufficient amount of time to allow connecting material  130  to at least partially cure enough to keep photodiode  121  in a firm position. 
   The method disclosed herein may enable accurate placement of photodiodes  121  to within a sidewise tolerance of about 10 microns, and to a parallel tolerance, with respect to the ceramic substrate  110 , of about 30 microns. 
   In an alternative embodiment, and with reference to  FIGS. 2 and 4 , projections  124  may be of uniform thickness, as depicted in  FIG. 2 , or of non-uniform thickness, as depicted in  FIG. 4 . Where projections  124  are non-uniform in thickness, a wide base  125  may be utilized with a conductive pad  126  for improved signal communication between photodiode  121  and first layer  110 . 
   As disclosed, some embodiments of the invention may include some of the following advantages: a high packing density of photodiodes on the ceramic substrate with a small spacing therebetween; effective containment of the conductive connecting material to prevent shorting between adjacent diodes; stud bumps of a photodiode having a small pitch with respect to the dimension of the photodiode; ability to flatten the photodiode during assembly by using a pick-and-place apparatus having a vacuum across the support surface; and, use of heater elements integral with the assembly apparatus for enhanced productivity in curing the epoxy or solder. 
   While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best or only mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. Furthermore, the use of the terms a, an, etc. do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.