Abstract:
A solid-state image pickup apparatus includes a solid-state image pickup device having a layer of microlenses above a color filter. The solid-state image pickup device is mounted on a side of a flexible printed circuit board by flip-chip bonding, opposite the opening. An adhesive in a gap between the solid-state image pickup device and the flexible printed circuit board strengthens the apparatus. The distance between the edge of the microlens layer and the edge of the flexible printed circuit board defining the opening and nearest to the microlens layer is 2.5 to 10 times wider than the gap between the solid-state image pickup device and the flexible printed circuit board.

Description:
FIELD OF THE INVENTION 
     The present invention, in general, relates to a solid-state image pickup apparatus including a solid-state image pickup device and an optical lens. More particularly, this invention relates to a compact and high-performance solid-state image pickup apparatus. 
     BACKGROUND OF THE INVENTION 
     A conventional solid-state image pickup apparatus had a structure as shown in  FIG. 6  in order to realize a slim and small apparatus. Legend  1  denotes a flexible printed circuit board (hereafter referred to as FPC) made with a film material such as polyimide. The FPC  1  is this figure is shown in a bent state. Legend  13  denotes a frame which holds an optical lens  6  and an optical filter  7 . Legend  8  denotes a diaphragm section which takes in light from the outside. Legend  4  denotes a holding mount which holds the optical filter  7 . This holding mount is fixed to the FPC  1 . Legend  5  denotes a holding cap which holds the optical lens  6  and is disposed movably with respect to the holding mount  4  to obtain correct focus. 
     The holding mount  4  and the holding cap  5  are kept in a movable state to adjust position so that incident light from the diaphragm section  8  comes into focus on a solid-state image pickup device  9  through the optical filter  7 . At the same time, an aperture  14  is provided in the FPC  1  to receive incident light from the optical lens  6 .  FIG. 6  shows a constitution where an IC component  10 , such as a chip for image signal processing, is flip-chip bonded to the FPC  1  in the same manner as the case of the solid-state image pickup device  9 .  FIG. 6  also shows a constitution, as specific example, where chip components  12  such as resistors or capacitors are mounted on the FPC  1 . 
     The operation of the solid-state image pickup apparatus will be explained below. The light coming in through the diaphragm  8  passes through the optical lens  6  and further passes through the optical filter  7 . This light is then irradiated onto an image pickup area of the solid-state image pickup device  9  to form an image. Video information of the image is converted into electric signals in the solid-state image pickup device  9  and electrically transmitted to the FPC  1  via a flip-chip bonding pad section  11 . The electric signals are further transmitted to the signal processing chip  10  and the lands provided on the FPC through the printed wiring of the FPC  1 . 
       FIG. 7  shows an enlarged cross-section of the portion of the flip-chip bonding pad section  11  indicated by dashed lines in  FIG. 6 . A liquid adhesive with epoxy resin as its main component, which is referred to as underfill indicated by legend  31 , is injected into the periphery of the flip-chip bonding pad section  11 , and the underfill  31  is hardened to mechanically reinforce the electrical contact portion of the flip-chip bonding pad section  11 , although this constitution is not shown in  FIG. 6 . 
     Thus, in the conventional solid-state image pickup apparatus, the flip-chip bonding pad sections  11  are provided for electrical contact between the solid-state image pickup device  9  and the FPC  1 , and the underfill  31  is required for the periphery of the section. 
     This underfill  31  is injected into the periphery of the flip-chip bonding pad sections  11  in order to mechanically reinforce electrical contact portions between the flip-chip bonding pad section  11  and the land (contact edge) of the FPC  1 , and the flip-chip bonding pad section  11  and the pad (contact edge) of the solid-state image pickup device  9 . For the reinforcement, it is important that the underfill  31  is kept in contact with the FPC  1 , the flip-chip bonding pad sections  11 , and the solid-state image pickup device  9 . Accordingly, the underfill  31  is originally a liquid with low viscosity, and a large amount of liquid is injected into the periphery. Since it is required to utilize this type of manufacturing process, an overflow of the underfill  31 , which is referred to as bleed and indicated by legend  32 , may occur. 
     On the other hand, the solid-state image pickup device  9  has an image pickup area  21  including photodiodes and transistors. Light reflected from a subject passes through the diaphragm  8 , lens  6 , and optical filter  7 , further passes through a microlens  23  and a color filter  22  to form an image on the photodiodes corresponding to respective pixels forming the image pickup area  21 . 
     When the underfill  31  covers the microlens  23  in the form of the bleed  32 , the solvent of the underfill as the bleed  32  spreads over a large area, by a capillary phenomenon, along V-shaped grooves extending vertically and horizontally, like a checkerboard, which are specific to the area of the microlens  23  based on its form. Eventually, the bleed may reach the image pickup area  21 . In such a case, the optical characteristics of the microlens  23  are disturbed because of the presence of the underfill  31 . Resultantly, the light can not be focused on the photodiodes of the pixel area  21 , which causes the image pickup sensitivity to be reduced. 
     SUMMARY OF THE INVENTION 
     In the solid-state image pickup apparatus according to one aspect of this invention, a solid-state image pickup device is provided on one side of a flexible printed circuit board and an optical lens is provided on the other side of the board, and the solid-state image pickup device is flip-chip bonded to the flexible printed circuit board. An aperture is provided on the flexible printed circuit board, so that light coming in from this aperture is focused on an image pickup area of the solid-state image pickup device. A distance between an edge of a microlens area formed on the solid-state image pickup device and an edge of the flexible printed circuit board is between 2.5 times to 10 times wider as compared to a space between the flexible printed circuit board and the solid-state image pickup device. 
     In the solid-state image pickup apparatus according to another aspect of this invention, a projection is provided on the surface of the solid-state image pickup device between an edge of a microlens area formed on the solid-state image pickup device and an edge of the flexible printed circuit board. 
     In the solid-state image pickup apparatus according to still another aspect of this invention, a groove is provided on the surface of the solid-state image pickup device between an edge of a microlens area formed on the solid-state image pickup device and an edge section of the flexible printed circuit board. 
     In the solid-state image pickup apparatus according to still another aspect of this invention, a stage is provided on said solid-state image pickup device between an edge of a microlens area formed on the solid-state image pickup device and an edge of the flexible printed circuit board. 
     In the solid-state image pickup apparatus according to still another aspect of this invention, a bank is provided near around an edge of said flexible printed circuit board. 
     Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a solid-state image pickup apparatus according to a first embodiment of this invention; 
         FIG. 2  is a cross-sectional view of a solid-state image pickup apparatus according to a second embodiment of this invention; 
         FIG. 3  is a cross-sectional view of a solid-state image pickup apparatus according to a third embodiment of this invention; 
         FIG. 4  is a cross-sectional view of a solid-state image pickup apparatus according to a fourth embodiment of this invention; 
         FIG. 5  is a cross-sectional view of a solid-state image pickup apparatus according to a fifth embodiment of this invention; 
         FIG. 6  is a cross-sectional view of the conventional solid-state image pickup apparatus; and 
         FIG. 7  is a partially enlarged view of the conventional solid-state image pickup apparatus. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of this invention will be explained below with reference to the attached drawing. 
       FIG. 1  shows an enlarged cross-sectional view of the portion of a flip-chip bonding pad section  11  in the solid-state image pickup apparatus according to a first embodiment. Legend  1  denotes the flexible printed circuit board (FPC) made with a film material such as polyimide. Light falls on the image pickup area  21  via the microlens  23  and the color filter  22  of the solid-state image pickup device  9 , through the hole  14  (see  FIG. 6 ), to form an image. Legend  1   a  in  FIG. 1  denotes microlens-side edge of the FPC  1 , and legend  23   a  denotes an edge of the microlens  23 . 
     A liquid adhesive with epoxy resin, etc., as its main component, which is referred to as underfill indicated by legend  31 , is flown into the periphery of the flip-chip bonding pad section  11 . The underfill  31  is hardened to mechanically reinforce an electrical contact portion, formed with an electrically conductive adhesive such as silver paste, of the flip-chip bonding pad section  11 . 
     The inventor of this invention carried out studies on how much should be the distances between the microlens-side edge  1   a  of the FPC  1  and the edge  23   a  of the microlens  23  so that of the bleed  32  the underfill  31  will not touch the microlens  23 . Such study was performed because if the bleed  32  can be prevented from entering into the gaps between the microlens it will not spread over a large area. That is, the solvent will not spread up to the image pickup area  21 , so that the optical characteristics of the microlens  23  are not disturbed. Thus, light can be focused on the photodiodes of the pixel area  21 , and the reduction in its image pickup sensitivity can be prevented. 
     The inventors found that the volume of the bleed  32  is proportional to a space between the FPC  1  and the solid-state image pickup device  9 . In other words, the underfill  31  will not reach the microlens if the microlens-side edge  1   a  and the edge  23   a  are provided apart from each other by a distance which is 2.5 times or more as wide as the space between FPC  1  and the solid-state image pickup device  9 . The space between the FPC  1  and the solid-state image pickup device  9  is generally of the order of tens of micrometers. If resin having a flow property necessary and sufficient to be able to fill in this space, then provision of a gap of not less than 2.5 times as wide as the space between the FPC  1  and the solid-state image pickup device  9  can prevent the underfill  31  from its reaching the microlens. 
     More specifically, if an average space between the FPC  1  and the solid-state image pickup device  9  is 50 μm, then the distance between the microlens-side edge  1   a  and the edge  23   a  should be 125 μm. 
     However, if this distance is too wide, the image pickup area  21  is reduced, which is not practical. It is preferable and practical that this distance is not more than 10 times as wide as the space between the FPC  1  and the solid-state image pickup device  9 . 
     More concretely, when an average space between the FPC  1  and the solid-state image pickup device  9  is 50 μm, it is preferable that the gap between the FPC aperture edge section la and the microlens area edge section  23   a  is 500 μm or less. 
       FIG. 1  shows a structure in which the edge of the color filter  22  is aligned with the edge  23   a  of the microlens  23  thereby forming a stage. Provision of such a stage can also prevent the solvent, in a small amount, from its flowing into the area of the microlens  23  even if the front-end of the fillet of the bleed  32  is extending toward the area. 
       FIG. 2  shows an enlarged cross-sectional view of the portion of the flip-chip bonding pad section  11  in the solid-state image pickup apparatus according to a second embodiment.  FIG. 1  shows the structure in which the stage formed by aligning the edge of the color filter  22  and the edge  23   a  of the mocrolens. On the contrary,  FIG. 2  shows the structure where there is no such stage. The distance between the microlens-side edge  1   a  and the edge  23   a  of the microlens is 2.5 time or more wider as the space between the FPC land the solid-state image pickup device  9 . Accordingly, same advantage as in the first embodiment can be achieved. 
       FIG. 3  shows an enlarged cross-sectional view of the portion of the flip-chip bonding pad section  11  in the solid-state image pickup apparatus according to a third embodiment. Legend  41  represents a projection formed on the surface of the solid-state image pickup device  9 . This projection  41  is provided between the microlens-side edge  1   a  and the edge  23   a  of the microlens so as to become an obstacle and stop the flow of the solvent from the fillet of the bleed  32  of the underfill  31 . Accordingly, the solvent of the underfill  31  can be prevented from its reaching to the microlens and spreading over a large area due to the capillary phenomenon. Since the solvent does not reach up to the image pickup area  21 , the optical characteristics of the microlens  23  are not disturbed by the solvent of the underfill  31 . Thus, the light can be focused on the photodiodes of the pixel area  21 , and the reduction in its image pickup sensitivity can be prevented. 
     It is easy to make the bank section  41  with the same material as that of the color filter  22 . However, a wiring material such as aluminum or a material such as resin, which can be formed on the solid-state image pickup device, may be used. 
       FIG. 4  shows an enlarged cross-sectional view of the portion of the flip-chip bonding pad section  11  in the solid-state image pickup apparatus according to a fourth embodiment. Legend  42  represents a groove section formed on the surface of the solid-state image pickup device. The groove section  42  is provided between the microlens-side edge  1   a  and the edge  23   a  of the microlens so as to accumulate the extra solvent and to stop the flow of the solvent from the fillet of the bleed  32  of the underfill  31 . Accordingly, the solvent of the underfill  31  can be prevented from its reaching to the microlens and spreading over a large area due to the capillary phenomenon. Since the solvent does not reach up to the image pickup area  21 , the optical characteristics of the microlens  23  are not disturbed by the solvent of the underfill  31 . Thus, the light can be focused on the photodiodes of the pixel area  21 , and the reduction in its image pickup sensitivity can be prevented. 
     It is easy to make the groove section  42  with the same material as that of the color filter  22 . However, a wiring material such as aluminum or a material such as resin, which can be formed on the solid-state image pickup device, may be used. 
       FIG. 5  shows an enlarged cross-sectional view of the portion of the flip-chip bonding pad section  11  in the solid-state image pickup apparatus according to a fifth embodiment. Legend  43  represents a bank section formed near the microlens-side edge  1   a . This bank section  43  stops the flow of the solvent from the fillet of the bleed  32  of the underfill  31 . Accordingly, the solvent of the underfill  31  can be prevented from its reaching to the microlens and spreading over a large area due to the capillary phenomenon. Since the solvent does not reach up to the image pickup area  21 , the optical characteristics of the microlens  23  are not disturbed by the solvent of the underfill  31 . Thus, the light can be focused on the photodiodes of the pixel area  21 , and the reduction in its image pickup sensitivity can be prevented. 
     As explained above, according to this invention, even if the bleed  32  of the underfill  31  is formed and the solvent of the underfill is flown out from the fillet of the bleed, the solvent is not allowed to touch the microlens  23 . Therefore, the solvent can be prevented from its flowing into the area of the microlens  23  due to the capillary phenomenon, so that the optical characteristics of the microlens  23  can not be disturbed by the solvent of the underfill  31 . Thus, the light can be focused on the photodiodes of the image pickup area  21 , and the reduction in its image pickup sensitivity can be prevented. 
     Accordingly, in the solid-state image pickup apparatus in which a solid-state image pickup device is provided on one side of the flexible printed circuit board and an optical lens held by a casing is provided on the other side of the board and the solid-state image pickup device is flip-chip bonded to the flexible printed circuit board, a large amount of underfill can be flown into the bonded sections. Therefore, since the flip-chip bonding pad sections of the solid-state image pickup device can be secured and firmly maintained to keep their good contact state, and the solid-state image pickup apparatus can be manufactured stably, it is possible to obtain a compact and high-performance solid-state image pickup apparatus with high reliability, which is adequate for miniaturization of a solid-state image pickup apparatus. 
     Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.