Abstract:
According to the invention, a multilayered image sensor is back mounted to a plate, and the plate in turn, is installed in a holding pocket of a device. In that the scheme takes advantage of a high controllability of a mounting plate&#39;s thickness, the mounting scheme provides a tight control of holding forces with which an image sensor is secured in an imaging device. In that the scheme provides for back mounting of image sensor on a planar surface, the mounting system provides tight control of an imaging assembly&#39;s pixel plane to fixed point in space distance.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a continuation application of U.S. Ser. No. 09/112,028, filed Jul. 8, 1998, now U.S. Pat. No. 6,275,388 entitled “Image Sensor Mounting System,” the entirety of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a mounting system for mounting a structure whose thickness cannot be tightly controlled, and particularly to a mounting system for mounting an image sensor. 
     2. Background of the Prior Art 
     A typical image sensor chip  12  of the type mounted in various types of devices, such as medical instruments, video cameras, and bar code readers is shown in FIG.  6 . The image sensor shown includes a bottom planar member  110  carrying a pixel array  112 . Front and rear lead frames  114  initially extend peripherally from the pixel array and are formed to extend downwardly about front and rear edges respectively, of bottom planar member  110  terminating in pins  32 . Image sensor  12  further includes top planar member  118  which rests against pixel array  112  and lead frame  114 . Top planar member  118  is secured against lead frames  114  and against pixel plane  112  by the force of adhesive material interposed between top and bottom planar members  110  and  118 . Adhesive material is disposed mainly about the periphery of pixel array  112 . In addition, image sensor  12  may include a glass layer  120 . In some popular models of image sensors, top planar member  118  is configured in the form of a frame which retains glass layer  120 . Thus, it is seen that image sensor  12  is of a stacked-up configuration. Like most structures whose design is of a generally stacked-up configuration, the thickness, t, of assembly  12  cannot be tightly controlled. In the manufacturing of sensor  12 , the thickness of the various layers will vary from structure to structure. Accordingly, the total thickness, t, will vary from structure to structure. The spacing, s, between top and bottom planar members  110  and  118  of image sensor  12  is particularly difficult to control given that such spacing is a function of the amount of adhesive used, the thickness of pixel array  112  and the thickness and the thickness of lead frames  114 . 
     Particularly in applications where such an image sensor must be side mounted (not “plugged into” a PCB), as is the case with most bar code reader applications, then the inability 
     to tightly control image sensor thickness, t, can negatively impact operational characteristics of the device in which the sensor is incorporated in. An explanation of how the inability to tightly control sensor thickness can impact operation of a bar code reader is made with reference to FIGS. 7 and 8 showing a multilayered image sensor incorporated in a bar code reader according to a prior art mounting scheme. In the mounting scheme shown, a multilayered image sensor  12  is disposed into a holding pocket  16  defined by substantially equally tensioned pairs of rear pins  19  and forward pins  18 . The prior art mounting system may further include a spacer  21  for biasing sensor  12  forwardly against forward pins  18 . 
     A number of operational problems can arise with this mounting scheme. If the thickness of the image sensor which is manufacturable to a thickness in the tolerance range from T min  to T max  tends toward T min  then pins  18 ,  19  may not supply sufficient pressure to image sensor  12  to hold sensor  12  in a secure position. Further, it can be seen that the distance, d, from any fixed point in space, P s  to any fixed P p , on the plane of pixel array  12  will vary depending on the total thickness, t, of sensor  12  which is a thickness having a high degree of variability. This is not preferred since controlling the distance, d, is important to controlling the operation of the reader. 
     There is a need for an image sensor mounting system for mounting an image sensor in an imaging device which minimizes operational problems resulting from the inability to tightly control an image sensor chip&#39;s thickness. 
     SUMMARY OF THE INVENTION 
     According to its major and broadly stated the present invention is a mounting system for mounting an image sensor chip in a location in a device apart from a PCB board. 
     In one embodiment of the invention, a multilayered image sensor is back mounted to a plate, and the plate in turn, is installed in a holding pocket of a device. In that the scheme takes advantage of a high controllability of a mounting plate&#39;s thickness, the mounting scheme improves the consistency of holding forces with which several image sensors are secured in like configured imaging devices. In that the scheme provides for back mounting of image sensor on a plate, the mounting system reduces fluctuations in pixel plane to fixed point distances. 
     The mounting scheme may be enhanced by forming cutout sections in the mounting plate. The cutout sections serve to bench lead frames extending from an image sensor, and thereby service to minimize sliding or twisting of an image sensor mounted on a mounting plate. In another enhancement, an image sensor mounted on a mounting plate is secured to the plate entirely by a compression force supplied by a flex strip, soldered onto an image sensor&#39;s lead frames, impinging on the mounting plate. This arrangement serves to further minimize thickness variations resulting from manufacturing tolerances. 
     In a variation of the invention, the mounting plate is substituted for a by a back plate formed integral with a component frame of a device. The back plate along with the remainder of the frame define an elongated aperture adapted to a receive a lead frame of an image sensor. An image sensor may be mounted to a back plate in essentially the same way that an image sensor is mounted to a mounting plate to the end that an image sensor is tightly secured in a device and further to the end that pixel plane to fixed point distance is tightly controlled. 
     These and other detail, advantages, and benefits of the present invention will become apparent from the detailed description of the preferred embodiment hereinbelow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying Figures wherein like members bear like reference numeral and wherein: 
     FIG. 1 is perspective assembly diagram illustrating assembly of a mounting system according to the invention; 
     FIG. 2 is an enlarged perspective view of a mounting plate shown in FIG. 1; 
     FIG. 3 is an enlarged perspective view of a component frame shown in FIG. 1; 
     FIG. 4 a  is a perspective partial assembly diagram illustrating assembly of a flex strip onto an image sensor; 
     FIG. 4 b  is a perspective view illustrating an example of a component frame having an integrated back plate for receiving an image sensor; 
     FIG. 5 a second perspective view of the component frame of FIG. 4 b  showing an image sensor installed thereon according to a mounting system of the invention; 
     FIG. 6 is an exemplary perspective view of an image senor chip illustrating a multilayered construction thereof; 
     FIG. 7 is a top view of a prior art optical reader illustrating a prior art image sensor mounting system; 
     FIG. 8 is a cross sectional side view of the reader shown in FIG.  7 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An exemplary embodiment of an image sensor mounting system according to the invention is described with reference to the assembly drawing of FIG.  1 . In this embodiment, a plate  10  is provided for back mounting an image sensor  12 . In a simplified form of this mounting scheme, plate  10  is provided by a substantially rigid planar member comprising insulating material, image sensor  12  is mounted to plate  10  by any suitable means such as gluing or taping, and the resulting assembly comprising a plate and sensor  10  and  12  is mounted to an optical reader component frame  14  by inserting plate into a pocket  16  which may be defined, as is shown, by a pair of pins  18  and wall sections  20 . Plate  10  is sized to a length  1   p  such that the edges of plate  10  extend beyond the edges of sensor  12  when sensor is attached to plate  10  to the end that a pocket  16  can hold an image sensor in a secure position by applying lateral holding forces to plate  10  without supplying lateral forces to the top glass, or bottom planar members of image sensor  12 . 
     Component frame  14  in the example provided is an optical assembly component frame. Optical assembly frames of optical readers are typically comprised of molded plastic and are typically adapted to carry various optical system components of an optical reader. In addition to carrying an image sensor  12 , an optical assembly frame of an optical reader may carry such components as mirrors, lenses, and illumination sources, such as LEDs. In most optical readers, an optical assembly component frame  14  is installed on a printed circuit board, e.g. circuit board  15  which, in addition to carrying frame  14 , carries most, if not all, of the electrical components of the optical reader. 
     The mounting scheme described is advantageous over the prior art because it increases the security with which image sensor  12  is held in pocket  16  and furthermore, increases the precision with which a pixel plane to fixed point distance can be controlled. 
     While the total thickness, t, of stacked up image sensor  12  cannot be tightly controlled, the thickness T p  of plate  10  can be tightly controlled. Accordingly, pockets  16  of several like designed optical assembly frames will apply relatively consistent holding forces to image sensors disposed therein. 
     The mounting system increases the precision with which pixel plane to fixed point distance, d, is controlled because it reduces the number of manufacturing tolerances which contribute to the distance, d, the distance between any fixed point, P p , on the plane of a pixel array  12  and a fixed point, P s , away from the pixel plane. 
     In a prior art mounting system described with reference to FIGS. 6,  7 , and  8 , the pixel plane to fixed point distance, d, is a function of the total thickness, t, of an image sensor  10 , which is a function of the highly variable top planar member to bottom planar member spacing, s. 
     Because a pixel plane of an image sensor  10  is disposed flush on a bottom planar member, it is seen that pixel plane to fixed point distance, d, in the mounting system of FIG. 1 is influenced only by the bottom plate thickness t b , and the mounting plate thickness t p , both of which can be tightly controlled. 
     Additional features can be incorporated in the mounting system thus far described for further improving the operation of the mounting system. 
     One enhancement to the mounting system thus far generally described is to form in mounting plate  10  first and second cutout sections  26  and  28 . Cutout sections  26  and  28  defined by side walls  30  are sized to a length  1   c  approximately the same length or slightly longer than lead frames  114  so that edges of lead frames  114  are benched on walls  30  when image sensor  10  is mounted on mounting plate  10 . Cutout sections  26  and  28  provide the function of stabilizing the position of an image sensor on mounting plate  10  so as to prevent sliding or twisting of image sensor  12  on plate  10 . 
     Another enhancement to the mounting system generally described relates to a mounting scheme for mounting an image sensor  12  to mounting plate  10 . It has been mentioned herein that sensor  12  can be secured to plate  10  using any conventional securing means, such as adhesives, glues, double sided tapes, etc. However, such schemes for attachment have the potential drawback in that they add thickness to an assembly including an image sensor and a back plate. 
     In the image sensor to plate mounting scheme of FIG. 1 the mounting is accomplished without use of any thickness-adding material. As seen in FIG. 1, pins  32  will extend outwardly beyond the back surface  34  of plate  10  when sensor  12  is pressed flush against plate  10 . A flex strip  38  which includes two strips  40  and  42  of pin receptacles for providing electrical connection between sensor leads  12  and certain electrical connectors of reader (normally on PCB), a distance away from sensor  12  may be attached to image sensor  12  such that first row of pines  32  are received in a first row of receptacles  40  and a second row of pins  32  are received in a second row of receptacles  42  of flex strip  39 . Pins  32  can be soldered onto receptacles  40  and  42  such that the compression force of flex strip  38  impinging on mounting plate  10  to bias plate  10  against sensor  12  is sufficient to hold sensor  12  securely on plate  10  without additional securing forces supplied by glues, tape, or other adhesive material. 
     In the mounting system of FIG. 1, plate  10  may further include side wall formations  31  which are received in complementary formations of pocket  16 . In particular, the mounting system can be configured such that bottom surface  31 ′ of formation  30  is received on a complementary surface of pocket  16 . Furthermore, when plate  10  is installed in pocket  16 , at least one screw  33  can be received in at least one hole  29  formed in pocket  16 , at least one screw  33  can be received in at least one hole  29  formed in pocket  16  in such a location that screw head  33   h  or associated washer  33   w  applies a vertical holding force to a received image sensor  12 . In the particular embodiment shown, a cutaway section defined by walls  35  is provided so that plate  10  does not interfere with the receiving light optics in the particular optical system in the example provided. 
     A variation on the mounting schemes described thus far is described with reference to FIG. 4 a  through FIG.  5 . In the schemes described thus far, image sensor  12  is mounted to a plate  10  which, in turn, is received in a pocket  16  in an optical assembly frame  14  of a bar code reader. 
     In the mounting scheme described with reference to FIGS. 4 a ,  4   b , and  5 , the mounting pocket  16  of optical assembly frame  14  is deleted, and optical assembly frame  14  instead is furnished with a back plate  48  integral with frame  14  which provides essentially the same function as mounting plate  10 . Certain figures of an optical system which may be incorporated in a frame of the type shown in FIG. 4 b  and FIG. 5 are described in detail in copending applications entitled “Optical Assembly for Barcode Scanner,” Ser. No. 09/111,476 and “Adjustable Illumination System for a Barcode Scanner,” Ser. No. 09/111,583 concurrently herewith, incorporated by reference herein, and assigned to the Assignee of the present invention. 
     In this mounting scheme, image sensor  12  is mounted directly to back plate  48  in essentially the same manner that sensor  12  is mounted to mounting plate  10  in the general scheme described previously. 
     In mounting sensor  12  to back plate  48  then sensor  12  is pressed against surface  50  of back plate  48 . Frame  14  includes elongated aperture  52  defined by bottom edge of back plate  48  to accommodate bottom pins  32   b  of lead frame  114  when sensor  10  is mounted against back plate  48 . Securing material such as glues, tapes, or other adhesives may be provided to aid in the securing of an image sensor  12  against back plate  48 . In the alternative, image sensor  12  may be secured to back plate  48  as described previously by a compression force supplied by flex strip  38 , which when soldered, works to bias image sensor  12  against plate  48 . 
     Cutout section  56  and aperture  52  can be sized to have lengths  1   c  approximately equal to the respective lengths of lead frames  114  so that side wall  30  of aperture  52  and of cutaway section  56  operate to bench lead frames  114  and to thereby prevent sliding or twisting of image sensor  12  when image sensor  12  is mounted on back plate  48 . It will be seen that a back plate of the invention can be provided by virtually any substantially planar rigid surface integrated onto a mounted component frame. 
     While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawing, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the invention as defined by the claims.