Abstract:
An image sensor device includes a first, QFN type leadframe to which a sensor IC is electrically connected. A second leadframe is provided for holding a lens. A third leadframe is positioned between the first and second leadframes to appropriately space the IC from the lens. Multiple sensor devices are assembled at the same time by the use of leadframe panels.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to image and optical sensors and, more particularly, to a method of packaging an optical sensor and the resulting packaged sensor product.  
         [0002]     There has been a constant demand for smaller and smarter industrial and consumer electronic products such as digital cameras, camcorders, audio players, etc. Such miniaturization and increased functionality has benefited from advances in the design and manufacturing of semiconductor circuits and wafers. There has also been a marked increase in the use of optical and image sensors in electronic products. Such sensor devices are packaged in a variety of ways. For example, an optical sensor in a ceramic leadless chip carrier has good optical quality, but large package form factor. A wafer level package has a lesser form factor and good optical quality, but is very expensive. Image sensors are also available as a molded quad flat pack (QFP). While the QFP has a moderate cost, it has low optical quality and a large package form factor. Further, it is important both to accurately align the optical sensor image center with the optical axis of the optical lens and provide proper glass height stand off.  
         [0003]     It would be advantageous to provide a packaged image sensor with a low package form factor, moderate cost, and high optical quality. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]     The foregoing summary, as well as the following detailed description of the present invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there is shown in the drawings an embodiment that is presently preferred. It should be understood, however, that the invention is not limited to the precise arrangement and instrumentalities shown. In the drawings:  
         [0005]      FIG. 1  is an enlarged, top perspective view of an optical sensor device in accordance with the present invention;  
         [0006]      FIG. 2  is an enlarged, bottom perspective view of the optical sensor device of  FIG. 1 ;  
         [0007]      FIG. 3  is a perspective view of a first leadframe panel used for forming optical sensor devices in accordance with the present invention;  
         [0008]      FIG. 4  is an enlarged perspective view illustrating a step of attaching an image sensor IC to the first leadframe, wirebonding the IC to the first leadframe, and disposing an adhesive on an active area of the IC in accordance with a method of the present invention;  
         [0009]      FIG. 5  is a perspective view of a second leadframe panel used for forming optical sensor devices in accordance with the present invention;  
         [0010]      FIG. 6  is an enlarged perspective view of a step of placing lenses in second leadframes of the second leadframe panel of  FIG. 5  in accordance with a method of the present invention;  
         [0011]      FIG. 7A  is a perspective view illustrating a step of placing a third leadframe panel between the first and second leadframe panels in accordance with a method of the present invention and  FIG. 7B  is a cross-sectional view of the third leadframe of  FIG. 7A ;  
         [0012]      FIG. 8  is an enlarged perspective view showing a leadframe panel alignment step in accordance with a method of the present invention;  
         [0013]      FIG. 9  is a greatly enlarged cross-sectional view illustrating a molding step in accordance with a method of the present invention;  
         [0014]      FIG. 10  is perspective view illustrating a tape removal step in accordance with a method of the present invention; and  
         [0015]      FIG. 11  is a perspective view illustrating a dicing step in accordance with a method of the present invention.  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0016]     The detailed description set forth below in connection with the appended drawings is intended as a description of the presently preferred embodiments of the invention, and is not intended to represent the only forms in which the present invention may be practiced. It is to be understood that the same or equivalent functions may be accomplished by different embodiments that are intended to be encompassed within the spirit and scope of the invention.  
         [0017]     Certain features in the drawings have been enlarged for ease of illustration and the drawings and the elements thereof are not necessarily in proper proportion. However, those of ordinary skill in the art will readily understand such details. In the drawings, like numerals are used to indicate like elements throughout.  
         [0018]     The present invention provides an image sensor device with low cost and high optical quality in a near chip scale package based on standard high density array format, quad flat no-lead (QFN) assembly infrastructure. A tight tolerance in assembly height and unique mechanical structure design provides close dimension matching with mold cavity height. The packaged sensor device uses an inner leadframe panel as a precise height standoff object to control the focal length of the sensor. The lens is attached to its own leadframe panel with tape. The tape prevents resin bleeding, which keeps the lens clean and facilitates post-mold cleaning.  
         [0019]     More particularly, in one embodiment, the present invention provides an image sensor device including a first leadframe having a central die receiving area surrounded by a plurality of lead fingers and a sensor integrated circuit (IC) disposed within the die receiving area of the first leadframe. The IC has a first surface with an active area and a peripheral bonding pad area. The peripheral bonding pad area includes a plurality of bonding pads. A plurality of wires are wirebonded to respective ones of the IC bonding pads and corresponding ones of the leadframe lead fingers of the first leadframe, thereby electrically connecting the IC and the first leadframe. A second leadframe having a central lens receiving area is provided. A transparent lens is disposed within the central lens receiving area of the second leadframe and the second leadframe is located over the first leadframe such that the lens is disposed over the IC active area. A transparent adhesive is disposed on the IC active area to secure the lens to the IC. A mold compound is injected between the first and second leadframes, and over the wirebonds. During assembly of the image sensor device, a third leadframe is located between the first and second leadframes for controlling the spacing between the IC active surface and the lens.  
         [0020]     In another embodiment, the present invention provides a method of packaging an optical sensor. The method includes the steps of: 
        providing a first leadframe panel including a plurality of first leadframes, each first leadframe having a plurality of lead fingers surrounding a central die receiving area;     placing a first tape over a first side of the first leadframe panel;     providing a plurality of sensor integrated circuits (IC), each IC having a first surface and a second surface, the first surface having an active area and a peripheral bonding pad area, the peripheral bonding pad area including a plurality of bonding pads;     placing the plurality of ICs within respective ones of the die receiving areas of the first leadframes of the first leadframe panel, wherein the IC second surfaces are secured within the die receiving areas by the first tape;     electrically connecting respective ones of the IC bonding pads of the ICs and corresponding ones of the leadframe lead fingers of the first leadframes with a plurality of wires via wirebonding, thereby electrically connecting the ICs and the first leadframes;     providing a second leadframe panel having a plurality of second leadframes, each of the second leadframes having a central lens receiving area;     placing a second tape over a first side of the second leadframe panel;     placing a transparent lens in each of the lens receiving areas of the second leadframes, wherein the lenses are secured within the lens receiving areas by the second tape;     putting globs of a clear adhesive on respective ones of the active areas of the ICs;     providing a third leadframe panel;     placing the third leadframe panel between the second sides of the first and second leadframe panels;     pressing the first and second leadframe panels towards each other such that respective ones of the ICs are attached to corresponding ones of the lenses by the globs of adhesive;     injecting a mold compound between the first and second leadframe panels such that the mold compound covers the bonding pads and the wires;     removing the tape from the first sides of the first and second leadframe panels; and     separating the leadframes from the panels, thereby forming individual devices.        
 
         [0036]     Referring to  FIG. 1 , an enlarged, top perspective view of a packaged optical sensor device  10  in accordance with the present invention is shown. The sensor device  10  preferably is a QFN (quad flat no-lead) type package. The QFN type package is preferred because of its small form factor, low profile, and low assembly cost. The sensor device  10  includes a sensor integrated circuit (IC)  12  visible through a lens  14  that is fitted within a second leadframe  16 .  FIG. 2  is an enlarged, bottom perspective view of the optical sensor device  10 , which shows a bottom side of the IC  12  and lead fingers  18  of a first leadframe  20  (see  FIG. 3 ). The packaged device  10  is rectangular or square-shaped and the lens  14  is generally circular  14 . However, the device  10  and lens are not limited to such shapes, as the shape of the package and the shape of the lens may vary.  
         [0037]      FIGS. 3-11  show various steps in a process for packaging the sensor device  10 .  FIG. 3  is a perspective view of a first leadframe panel  22  having a plurality of the first leadframes  20 . A first step in the process of packaging the sensor device  10  is to provide the first leadframe panel  22 . In the embodiment shown, the first leadframe panel  22  includes three (3) 5×5 matrices of the first leadframes  20 . However, the leadframe panel  22  may include more or fewer of the leadframes  20 . An outer perimeter or edge of the first leadframe panel  22  includes a plurality of spaced holes  24  that are used to align the first leadframe panel  22  with other leadframe panels, as discussed in more detail below. Each of the individual first leadframes  20  comprises a plurality of lead fingers  18  ( FIG. 4 ) surrounding a central die receiving area  28 . The lead fingers  18  extend inwardly from connection bars  30  towards the die receiving areas  28 . In an alternative embodiment, the first leadframes  20  may include flag members for holding the ICs  12 . The first leadframe panel  22  is formed of a metal, such as copper and the first leadframes  20  are formed by punching, stamping or etching, as is known by those of skill in the art.  
         [0038]      FIG. 3  shows a step of placing a first tape  32  over a first side of the first leadframe panel  22 . The first tape  32  is of a type known by those of skill in the art and has an adhesive on one side to which the first leadframe panel  22  adheres. After the first tape  32  is applied to the first side of the first leadframe panel  22 , the sensor ICs  12  are placed in the die receiving areas  28  of the first leadframes  20  of the first leadframe panel  22 . Each of the ICs  12  has a first surface and a second surface. The first surface has an active area and a peripheral bonding pad area. The peripheral bonding pad area includes a plurality of bonding pads. The sensor ICs  12  are preferably a CMOS sensor device of a type that is known by those of skill in the art and readily commercially available. The ICs  12  have a thickness or height of about 15 mils. Each IC  12 , in addition to including a light receiving or active area, includes circuitry and logic, such as an A/D converter and a DSP or logic area for performing arithmetic type operations. The ICs  12  are placed in the die receiving areas  28  such that the second surfaces (bottom side) of the ICs  12  adhere to the first tape  32  and thus the ICs  12  are secured within the die receiving areas  28  by the first tape  32 . By taking advantage of the capability of accurate and tight control in leadframe positional tolerance manufacturing, the ICs  12  are precisely located in the die receiving areas  28  of the first leadframes  20 .  
         [0039]      FIG. 4  is an enlarged perspective view illustrating showing the sensor ICs  12  secured within the die receiving areas  28 . A plurality of wires  34  are wirebonded to respective ones of the IC bonding pads and corresponding ones of the lead fingers  18 , thereby electrically connecting the ICs  12  and the leadframes  22 . Wirebonding is performed in a conventional manner. Any electrically conductive metal or combination of metals, such as are known by those of skill in the art, may be used to form the wires  34 . Suitable bond wires typically comprise a conductive metal such as copper or gold and may be either fine wires (&lt;50 um in diameter) or heavy wires (&gt;50 um in diameter). In the presently preferred embodiment, the wires  34  are fine wires formed of gold. After the ICs  12  are electrically connected to the first leadframes  20  via wirebonding, globs of a clear adhesive  36  are disposed on the active areas of the ICs  12 . The adhesive globs  36  may comprise a clear epoxy in liquid form, before curing. The adhesive globs  36  can be deposited onto the sensor active areas by any dispensing system.  
         [0040]      FIG. 5  is a perspective view of a second leadframe panel  38  used for forming the optical sensor devices  10 . The second leadframe panel  38  includes a plurality of the second leadframes  16 . The second leadframe panel  38  and the second leadframes  16  are sized and shaped to match the first leadframe panel  22  and the first leadframes  20 , respectively. Thus, in the embodiment shown, the second leadframe panel  38  includes three (3) 5×5 matrices of the second leadframes  16 . Also like the first leadframe panel  22 , the second leadframe panel  38  may be metal, such as copper, and formed by pressing, stamping or etching. Each of the second leadframes  16  comprises an area with a central lens receiving area  40  ( FIG. 6 ). The second leadframes are connected via area connection bars  42 . The second leadframe panel  38  preferably includes a plurality of spaced holes  44  along its perimeter.  FIG. 5  shows the second leadframe panel  38  with a second tape  46  placed over a first side thereof. In the drawing, the second tape  46  is on the underside of the second leadframe panel  38 .  
         [0041]      FIG. 6  is an enlarged perspective view of a step of placing the lenses  14  in the lens receiving areas  40  of the second leadframes  16  of the second leadframe panel  38 . The lenses  14  may be clear plastic or glass. Preferably the lenses  14  are a non-reflective optical grade glass and have a thickness of about 15 to 16 mils. The lenses  14  may be coated with various materials to filter light, as desired. The lenses  14  are secured within the lens receiving areas  40  by an adhesive on the surface of the second tape  46 . Like the first leadframes  20 , the ability to perform accurate and tight control in leadframe positional tolerance manufacturing allows the lenses  14  to be precisely located in the second leadframes  16 .  
         [0042]     Referring now to  FIG. 7A , a perspective view illustrating a step of placing a third leadframe panel  50  between the first and second leadframe panels  22  and  38  is shown. More particularly, the third leadframe panel  50  is placed between the second sides of the first and second leadframe panels  22  and  38 . The first and second leadframe panels  22  and  38  are then pressed towards each other such that respective ones of the ICs  12  are attached to corresponding ones of the lenses  14  by the globs of adhesive  36 . Further, the pressing action insures that the globs of adhesive  36  are spread uniformly over the surfaces of the ICs  12 . The third leadframe panel  50  acts as a precise stand off of the lenses  14  from the ICs  12 .  
         [0043]      FIG. 7B  is a cross-sectional view along lines A-A of the third leadframe panel  50 . The third leadframe panel  50  includes edge portions  52  and a central portion  54 . The central portion  54  has been etched to reduce its height to allow for a mold compound to be injected between the first and second leadframe panels  22  and  38  and around the bond wires  34 , as discussed below. The third leadframe panel  50  also includes a plurality of spaced holes  56 . Preferably the first, second and third leadframe panels  22 ,  38  and  50  are all formed of the same material so that they have the same CTE (coefficient of thermal expansion). In the presently preferred embodiment, the three leadframe panels  22 ,  38  and  50  are made of stamped or etched copper.  
         [0044]      FIG. 8  is an enlarged perspective view showing a leadframe panel alignment step. In this step, the first, second and third leadframe panels  22 ,  38  and  50  are aligned by guide pins  58  inserted into the holes  24 ,  44  and  56  of the respective leadframe panels. The holes  24 ,  44  and  56  of the respective leadframe panels are located in identical locations around the respective panels to allow for precise alignment of the panels. Precisely aligning the panels, coupled with the very precise placement of both the ICs  12  and the lenses  14  provides precise alignment between the optical axes of the lenses  14  with the centers of the respective ICs  12 , which is of critical importance in assembling image sensor devices. As is known by those of skill in the art, misalignment of the optical axis of the lens and sensor image circle center is the major cause of defects and thus yield loss in optical sensor manufacturing. By using leadframes for receiving both the ICs  12  and the lens  14 , the present invention allows for very accurate placement and thus improved alignment accuracy. A heat curing process may be performed to cure and harden the adhesive globs  36 , which the bonds the lenses  14  to the active areas of the ICs  12 .  
         [0045]      FIG. 9  illustrates a molding step in the process of packaging an optical sensor in accordance with the present invention. Upper and lower molds  60  and  62  are placed over the second and first leadframe panels  38  and  22 , respectively. A mold compound  64  is then injected between the first and second leadframe panels  22  and  38  such that the mold compound  64  covers the IC bonding pads  66 , bond wires  34  and a part of the lead fingers  18 . As previously discussed, the third leadframe panel  50  serves as a standoff between the first and second leadframe panels  22  and  38 , and maintains the lenses  14  at a predetermined distance from the ICs  12 . For devices around the periphery of the matrix, standoff is maintained by the rigid edge of the third leadframe panel  50 . For units in the center of the matrix, during the mold injection process, the injected mold compound pushes the respective first and second leadframes  20  and  16  apart such that the standoff for the center units is obtained. In a typical plastic IC packaging process, the mold compound injection pressure is over 5 tons, which insures that the mold compound is packed densely.  
         [0046]     After injection of the mold compound  64  and curing, the first and second tapes  32  and  46  are removed from the first sides of the first and second leadframe panels  22  and  38 . The first and second tapes  32  and  46  may be removed manually or with commercially available equipment.  FIG. 10  shows the first tape  32  being removed from the first leadframe panel  22 . The second tape  46  is removed in the same manner. The matrix of formed devices is then diced to form individual devices, as shown in  FIG. 11 . The dicing may be performed by sawing, as is known by those of skill in the art.  FIG. 9  shows the locations with dashed lines  70  along which the leadframes may be cut. In one embodiment, the image sensor device  10  has an overall height of about 40 mils. The lens  14  and clear adhesive globs  36  allow light to pass therethrough and onto the active area of the sensor IC  12 . Because the lenses  14  are covered by the second tale  46 , resin bleeding onto the lenses is avoided. However, if any resin bleeding onto the lenses  14  should occur, a subsequent water jet cleaning can be performed to remove such resin. The second tape  46  also avoids potential surface scratching of the lenses  14 .  
         [0047]     The description of the preferred embodiments of the present invention have been presented for purposes of illustration and description, but are not intended to be exhaustive or to limit the invention to the forms disclosed. It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but covers modifications within the spirit and scope of the present invention as defined by the appended claims.