Patent Publication Number: US-2016245933-A1

Title: Glass cap wirebond protection for imaging tiles in an x or gamma ray indirect imaging detector

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to X or Gamma ray indirect imaging detectors and more specifically to wirebond protection without use of an encapsulant. 
     2. Description of the Related Art 
     X or Gamma ray indirect imaging detectors include a conversion layer or “scintillator” that converts X or Gamma rays to visible light, an optional fiber optic plate (FOP) that transfers light to an imaging plane and a visible band imaging sensor at the imaging plane. For X rays, phosphor or cesium-iodide are typical conversion layer materials. The FOP protects the imaging detector from being damaged by the high energy X or Gamma rays, prevents the sensor from reading those rays as light and takes the image created by the conversion layer and focuses it on the imaging sensor keeping the image sharp and coherent. Indirect imaging leverages the established visible band CCD and CMOS imaging technologies. 
     To read out the image, the imaging sensor is mounted onto a printed circuit board (PCB). Electrical contact pads in a non-imaging region on the surface of the imaging sensor are wirebonded to electrical contact pads on the PCB, which are attached to read out and other processing circuitry. The wire bonds are very fragile and susceptible to damage or failure due to corrosion or mechanical stress that may occur during final assembly and handling. The failure of even a single wire bond can result in the loss of the detector. 
     To protect the wire bonds, a “glob top” of material is used to encapsulate the wire bonds. A typical material is a low aspect ratio (AR) clear acrylate that will readily flow around the wire bonds to encapsulate them. The material is typically cured using UV light. This is industry standard approach to protect wire bonds in many different semiconductor packages. 
     More recently, X-ray detectors are being designed in “multi-tile” configurations in which each tile includes one or more imaging sensor dies to provide a larger and higher-resolution detector. The detector may be &gt;30 mm per side. The die typically have a “surface flatness” that is far superior to that of the PCB. To address the surface flatness, on some architectures the imaging sensor and PCB are mounted side-by-side on a tile carrier having a surface flatness comparable to the die. The imaging sensor is wire bonded to the PCB and a glob top is used to encapsulate the wire bonds. See FIG. 5-2 of Jan Bosiers et. al., “An overview of Teledyne DALSA Professional Imaging CCD and CMOS capabilities” Proceedings 2013 Scientific Detector Workshop, Firenze, Italy, October 2013. 
     SUMMARY OF THE INVENTION 
     The present invention provides for wirebond protection for imaging tiles in X or Gamma Ray indirect imaging detectors without use of a “glob top” encapsulant in which the imaging sensor and PCB are mounted side-by-side on a tile carrier. 
     In an embodiment, each tile comprises an imaging sensor and a PCB mounted side-by-side on a tile carrier. The imaging sensor includes one or more CMOS or CCD dies configured to detect visible light. Each of the die are mounted on the common tile carrier and spaced apart from the PCB to define a trench there between. Wire bonds span the trench to connect electrical contacts on each die to electrical contacts on the PCB. A bead of adhesive material is applied to the top surface of the one or more die, in the trench to the surface of the tile carrier and on the top surface of the PCB that forms a continuous perimeter around and spaced apart from the plurality of wire bonds and from the imaging area of the die. The material is suitably a non-flowable material such as one having a thixotropic index of greater than 4 in its uncured state. A lid is placed on the bead of adhesive material, which is then cured, to form an enclosed open-air cavity around the wire bonds. In an embodiment, the adhesive material is UV cured and the tile carrier and lid are formed of UV-transparent materials. Alternately, the adhesive material may be a visible light, thermal or moisture curable adhesive. The lid may be formed from a material that is also visibly transparent to allow inspection of the wire bonds. A second bead of material may be formed around the perimeter of the lid and cured to make the enclosure air tight to protect the wire bond from environmental effects. 
     In an embodiment, one or more of the tiles are arranged with the backsides of the imaging sensors supported on a detector carrier. A conversion layer on the front side of the imaging sensor forms a detector core. The conversion layer is suitably formed on a carrier. In one embodiment, the conversion layer is formed on top of a fiber optic plate (FOP) that is attached to the front sides of the imaging sensors. In other embodiments, the conversion layer is formed on the bottom of a carbon or aluminum plate and attached to the imaging sensors. 
     In an embodiment, the detector core and a camera PCB are attached to opposing sides of a base plate, which is then mounted in a detector housing. Alternately, the detector core and camera PCB may be directly mounted within the detector housing. The camera PCB is electrically connected to the one or more tile PCBs and an external connector. The entire assembly is enclosed in a detector housing and a cover, configured to allow transmission of X or gamma rays is attached to the detector housing. 
     These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a flow diagram of an embodiment for assembling an X-ray detector with glass caps for wirebond protection; 
         FIGS. 2 a , 2 b  and 2 c    are section, side and plan views of an embodiment of an imaging tile; 
         FIG. 3  is a section view of an embodiment of a detector core; 
         FIGS. 4 a  and 4 b    are section and  FIGS. 4 c  and 4 d    are plan views of an embodiment of an X-ray imaging detector; and 
         FIGS. 5 a  through 5 c    are plan views of different embodiments of the glass cap. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention provides for wirebond protection for imaging tiles in X or Gamma Ray indirect imaging detectors without use of a “glob top” encapsulant in which the imaging sensor and PCB are mounted side-by-side on a tile carrier. 
     Testing of tiles in which the imaging sensor and PCB are mounted side-by-side on a tile carrier revealed that the process window to achieve reliable results with a conventional “glob top” encapsulant was small. 
     In this tile configuration, the encapsulant both surrounds the wire bonds and fills the trench formed between the imaging sensor and the PCB. This forms a long column of encapsulant material within the footprint of the wire bonds. The column formed by the trench effectively constrains the expansion of the encapsulant material due to changes in temperature. As a result, the expansion in the vertical direction may be doubled or even tripled. Because of the inherent differences in the thermal coefficients of expansion (TCE) of the encapsulant material and the wire bonds this expansion can generate mechanical stress on the wire bonds. 
     To protect the wire bonds from corrosion or mechanical stress that may occur during final assembly and handling, a glass cap comprising a bead of adhesive material and a lid is formed to provide an enclosed open-air cavity around the wire bonds. As such, any expansion of the adhesive material does not produce mechanical stress on the wire bonds. The adhesive material may be a UV, visible, thermal or moisture cured material. 
     Referring now to  FIGS. 1, 2   a - 2   c,    3  and  4   a - 4   d,  an embodiment of an X-ray imaging detector  10  and a method of assembly are depicted. This embodiment is directed to a multi-tile configuration of CMOS imaging die for X-ray detection. The architecture of the detector, method of assembly and in particular the configuration of the glass cap and method of assembly for protecting the wire bonds is applicable to single-tile configurations, CCD image die and for Gamma ray detection. 
     Assembly of the imaging detector  10  starts at the Wafer level with the performance of wafer-level testing of the individual CMOS dies (step  12 ). The individual dies  14  are sawed from the wafer (step  16 ). Each die has a non-imaging area  18  that includes electrical contact pads  20  and an imaging area  22  configured to detect visible light. In an embodiment, the individual die may be 100 mm×100 mm. The CMOS dies may be fabricated using Silicon or InGaAs technology. 
     Tile-level assembly of each tile  23  comprises attaching one or more of the individual die  14  that together form an image sensor  24  to a tile carrier  26  (step  28 ). A tile PCB  30  is attached to tile carrier  26  adjacent image sensor  24  (step  32 ) forming a trench  34  there between. Tile PCB  30  has a plurality of electrical contact pads  36  that are electrically connected to read out or other processing circuitry for processing the detected image. Wire bonds  38  are formed from the die contact pads  20  to the PCB contact pads  36  spanning trench  34  (step  39 ). The surfaces of the image sensor  24  and tile PCB  30  are preferably substantially co-planar. Often the tile PCB is thicker than the image sensor in which case the tile carrier  26  would be “stepped” such that the image sensor and tile PCB are coplanar. The tile carrier may formed of a Silicon or InGaAs material. 
     A bead  40  of UV-cured adhesive material is formed on the top surface of the one or more die  14 , in the trench  34  to the surface of the tile carrier  26  and on the top surface of the tile PCB  30  that forms a continuous perimeter around and spaced apart from the plurality of wire bonds  38  and spaced apart from the imaging area of the die (step  42 ). The material is suitably a non-flowable material such as one having a thixotropic index of greater than  4  in its uncured state. For example, Namics 6919 is a suitable material. 
     A lid  44  is placed on the bead  40  (step  46 ) to form a glass cap  48  that defines an enclosed open-air cavity  49  around the wire bonds  38 . The lid and glass cap are spaced apart from the imaging area of the die. The lid  44  and tile carrier  26  are suitably formed from a UV-transparent material. The lid  44  is suitably formed from a material that is also transparent in the visible band to facilitate inspection of the wire bonds. Other materials such as metal or ceramics may be used. The tile is exposed to UV radiation to cure the bead (step  50 ). Optionally, a second bead of material may be formed around the perimeter of the glass cap and cured (step  52 ). 
     Detector-level assembly comprises attaching one or more tiles  23  to a detector carrier  54  (step  56 ). As shown, the exposed backsides of the imaging sensors that extend laterally from the tile carriers are mounted on the top surface of detector carrier  54  to form a multi-tile array in which the tile PCBs are arranged around the periphery of that array. A FOP/scintillator assembly  58  is attached to the imaging side of the imaging sensors (step  60 ). The scintillator may be a separator component or maybe deposited as a coating on the FOP. Alternate embodiments may forego the FOP and attach a scintillator directly to the multi-tile array. A carbon or aluminum plate may be formed above the scintillator. This forms a core detector  62  as shown in  FIG. 3 . 
     In this embodiment, the core detector  62  and a camera PCB  64  are suitably mounted to opposite sides of a base plate  66  (step  69 ). This assembly is mounted in a detector housing  68  (step  70 ). Alternately the core detector and camera PCB may be directly mounted to the detector housing. The camera PCB  64  is electrically connected to the one or more tile PCBs by, for example, flex connectors  71  and an external connector  72  (step  74 ). A lid  76  configured to allow transmission of X rays (e.g., a carbon based material) is attached to the detector housing  68  (step  76 ) to complete X-ray imaging detector  10 . Final X ray testing is performed on detector  10  (step  78 ). In other embodiments, the core detector and camera PCB may be mounting using a flex or any other substrate. 
     Referring now to  FIGS. 5 a , 5 b  and 5 c   , the glass cap may be configured in several different ways to enclose the wire bonds in an air-cavity. As shown in  FIG. 5 a   , a single bead  80  and single lid  82  can form a single glass cap  84  around the wire bonds  86  and  88  for multiple die on an imaging sensor  90 . As shown in  FIG. 5 b   , multiple beads  92  and  94  and a single lid  96  can form a glass cap  98  around the wire bonds  100  and  102  for multiple die on an imaging sensor  104 . As shown in  FIG. 5 c   , separate glass caps  110  and  112  can be formed around the wire bonds  114  and  116  for multiple die on an imaging sensor  118 . 
     While several illustrative embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Such variations and alternate embodiments are contemplated, and can be made without departing from the spirit and scope of the invention as defined in the appended claims.