Patent Publication Number: US-7595839-B2

Title: Image sensor chip packaging method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is related to a co-pending U.S. patent application Ser. No. 11/595,297, entitled “DIGITAL CAMERA MODULE PACKAGING METHOD”, by Steven Webster et al. Such application has the same assignee as the present application and is concurrently filed herewith. The disclosure of the above-identified application is incorporated herein by reference. 
   TECHNICAL FIELD 
   The present invention generally relates to an integrated circuit (IC) chip packaging method and, more particularly, to a packaging method for an image sensor chip which is configured (i.e. structured and arranged) for use in an image capturing module such as a digital camera module. 
   BACKGROUND 
   Image sensors are widely used in image capturing modules such as digital camera modules in order to convert optical image signals of an object into electronic signals. In order to protect the image sensor from contamination or pollution (i.e. from dust or water vapor), the image sensor is generally sealed in a structural package. 
   An image sensor chip package made according to a typical packaging method is illustrated in  FIG. 1 . The packaging method includes the following steps: firstly, a plurality of bracket-shaped conductors  10  are provided. Secondly, molten plastic is injected to partially enclose the conductors  10 , thereby forming a base  11 , with some surfaces of the conductors  10  being exposed outside of the base  11 . Thirdly, a ring-like middle portion  12  is formed on the base  11  by means of injection molding. The base  11  and the middle portion  12  cooperatively form a space  13 . Fourthly, an image sensor  15  having a plurality of pads  14  is disposed on the base  11  in the space  13 . Fifthly, a plurality of bonding wires  16  are provided to connect the pads  14  and the conductors  10 . Finally, a cover  17  is secured to the top of the middle portion  12  with an adhesive, thereby hermetically sealing the space  13  and allowing light beams to pass therethrough. 
   In the process of bonding the wires  16 , each wire  16  needs to be connected with one conductor  10 . As a result, the space  13  needs to be relatively large, for allowing movement of a wire bonding tool therein. This method of bonding wires  16  is complex and expensive. In addition, the relatively large volume of the image sensor chip package allows more dust particles to adhere to the cover  17 , the middle portion  12 , and the base  11 . Thus, more dust particles are liable to drop onto the image sensor  15 . The dust particles may obscure part of the optical path to the image sensor  15 , and produce errors in the image sensing process. As a result, the quality and/or reliability of the image sensor chip package may be impaired. 
   Therefore, a new image sensor chip packaging method is desired in order to overcome the above-described shortcomings. 
   SUMMARY OF THE INVENTION 
   One embodiment of an image sensor chip packaging method includes:
     1) Providing a carrier comprising a base and a lead frame, the base having a chamber defined therein, the lead frame including a plurality of conduction pieces, the conduction pieces of the lead frame being embedded in the base and being spaced from each other, one end of each conduction piece being exposed at one surface of the base, and another end of the conduction piece being exposed at another surface of the base.   2) Mounting an image sensor chip in the chamber, the image sensor chip having a photosensitive area and a plurality of chip pads.   3) Providing a plurality of wires, and electrically connecting each wire with a corresponding chip pad of the image sensor chip and one of the exposed ends of a corresponding conduction piece of the carrier.   4) Providing a holder, the holder having a holding cavity.   5) Mounting the carrier into the holding cavity of the holder.   

   Other advantages and novel features will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating aspects of the image sensor chip package made according to the principles of the image sensor chip packaging method. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
       FIG. 1  is a cross-sectional view of a conventional image sensor chip package. 
       FIG. 2  is a first cross-sectional view of an image sensor chip package made according to an image sensor chip packaging method of a preferred embodiment of the present invention, the image sensor chip package including a holder and a carrier. 
       FIG. 3  is similar to  FIG. 2 , but is a second cross-sectional view, with a plane of the second cross-section being perpendicular to a plane of the first cross-section. 
       FIG. 4  is a top plan view of a plurality of lead frames joined together during one stage in the digital camera module packaging method of the preferred embodiment, the lead frames being for use in a plurality of carrier preforms that are used in making a plurality of image sensor chip packages of  FIG. 1 . 
       FIG. 5  is a cross-sectional view taken along line V-V of  FIG. 4 . 
       FIG. 6  is a top plan view of a plurality of carrier preforms joined together during a subsequent stage in the digital camera module packaging method of the preferred embodiment, the carrier preforms being for use in a plurality of carriers that are used in making the plurality of image sensor chip packages of  FIG. 1 . 
       FIG. 7  is an enlarged, side cross-sectional view of one of the carrier preforms of  FIG. 6 , after it has had an image sensor chip and a plurality of bonding wires attached thereon, and after this carrier preform assembly has been cut from adjacent carrier preform assemblies in a further stage in the image sensor chip packaging method of the preferred embodiment. 
       FIG. 8  is similar to  FIG. 7 , but showing the carrier preform assembly after it has been heated/irradiated in a still further stage in the image sensor chip packaging method of the preferred embodiment, and showing the holder of  FIG. 2  ready to be attached onto the carrier. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIGS. 2-3 , an image sensor chip package  9  made according to an image sensor chip packaging method of a preferred embodiment of the present method is shown. The image sensor chip package  9  is configured for use in an image capturing module such as a digital camera module. The image sensor chip package  9  includes a carrier  20 , an image sensor chip  30 , a plurality of bonding wires  40 , and a holder  50 . The chip  30  is received in the carrier  20 . The wires  40  electronically connect the carrier  20  and the chip  30 . The carrier  20  is received in a bottom cavity of the holder  50 . 
   Also referring to  FIG. 6 , the carrier  20  includes a base  21  and a lead frame  23 . The base  21  is essentially made from a plastic material such as, polyphenylene oxide (PPO), polyphenylene sulfide (PPS), or the like. The base  21  is rectangular in shape as viewed from above, and has a generally U-shaped side cross section. In alternative embodiments, the base  21  may have any of various shapes, including square, circular, and the like. A chamber  214  is defined in the base  21 . The chamber  214  is configured for receiving the chip  30 . The base  21  includes a main board  210  and a sidewall  212 . The sidewall  212  perpendicularly extends upward from an outer periphery of the main board  210 , and includes two parallel first walls  2120  and two parallel second walls  2122 . A slot  217  is defined in a top end of each first wall  2120 , the slot  217  being parallel to the first wall  2120  itself. 
   The lead frame  23  is preferably made of conductive metal material with good electrical and thermal conductivity, such as copper or iron-nickel alloy, in order to enhance the signal transmission and thermal transmission characteristics of the image sensor chip package  9 . The lead frame  23  is embedded in the base  21 , and includes a thermal conduction piece  231  and a plurality of electrical conduction pieces  233 . The thermal conduction piece  231  is substantially rectangular in shape. 
   Referring also to  FIGS. 4-7 , each electrical conduction piece  233  includes a top portion  235 , a middle portion  237 , and a bottom portion  236 . The top and bottom portions  235 ,  236  are spaced apart and aligned in parallel to each other. The middle portion  237  is slanted relative to the top and bottom portions  235 ,  236  and interconnects the top and bottom portions  235 ,  236 . The electrical conduction pieces  233  are divided into two groups. The two groups are symmetrically arranged at opposite sides of the carrier  20  at the second walls  2122  respectively, and electrical conduction pieces  233  of the same group are aligned parallel to and spaced from each other. The thermal conduction piece  231  is located between the two groups of electrical conduction pieces  233 . The thermal conduction piece  231  and the bottom portions  236  are aligned in the same horizontal plane. The top and bottom portions  235 ,  236  are both configured for electrically connecting with other electrical components. For example, the top portions  235  can be used to electrically connect to the chip  30 , and the bottom portions  236  can be used to electrically connect with a printed circuit board (PCB), whereby signals can be transmitted from the chip  30  to the PCB. 
   The chip  30  is received in the chamber  214  of the carrier  20 , and is fixed to the main board  210  of the carrier  20  with an adhesive. The adhesive may be chosen from the group including silicone adhesive, epoxy, acrylic adhesive, and polyamide adhesive. A top surface portion of the chip  30  includes a central photosensitive area  301 , and a plurality of chip pads  302  arranged at two opposite sides of the photosensitive area  301 . That is, the chip pads  302  are divided into two groups that are symmetrically opposite each other across the photosensitive area  301 . Each group is adjacent to one corresponding second wall  2122  of the base  21 . The chip pads  302  in each group are aligned parallel to each other, and spaced from each other at regular intervals. 
   The wires  40  can be made of an electrically conductive material such as, for example, gold or aluminum alloy. One end of each wire  40  is connected/joined with a respective chip pad  302  of the chip  30 , and the other end of the wire  40  is connected/joined with one top portion  235  of the lead frame  23 . 
   Referring to  FIGS. 2-3  and  8 , the holder  50  is substantially rectangular in shape, and includes two parallel first sidewalls  500  and two parallel second sidewalls  502 . In alternative embodiments, the holder  50  may have any of various shapes, such as square, circular, or any other appropriate shape corresponding to the shape of the base  21 . The sidewalls  500 ,  502  have a common peripheral surface  521 . The holder  50  has a half-closed end and an open end. The half-closed end has a transparent board  501  mounted in a middle portion thereof. Thus the transparent board  501  can overlie the chip  30 , so that the chip  30  receives image light transmitted through the transparent board  501 . A peripheral projection  51  extends from an outer periphery of the surface  521  of the sidewalls  500 ,  502 , thereby defining a holding cavity  54  therebetween. A protrusion  524  extends perpendicularly down from a central portion of each first sidewall  500 , the protrusion  524  being parallel to the first sidewall  500  itself. An inner periphery of each of the second sidewalls  502  is recessed, thereby defining a receiving cavity  523  cooperatively bounded by the second sidewalls  502  and the first sidewalls  500 . The receiving cavity  523  is configured for receiving the wires  40  therein. A size of the carrier  20  is substantially equal to a size of the holding cavity  54 , and is larger than a size of the receiving cavity  523 . The holder  50  is fixed to the base  21  by adhesive, and thereby hermetically seals the chip  30  held in the chamber  214  of the base  21  from the external environment. 
   An exemplary method of fabricating the image sensor chip package  9  is as follows. In one embodiment, a plurality of image sensor chip packages  9  are fabricated simultaneously to minimize the cost associated with each individual image sensor chip package  9 . Referring to  FIGS. 4 and 5 , firstly, a conductor element  60  is provided. The conductor element  60  is formed from a metal sheet or plate by punching or/and etching. The conductor element  60  includes two opposite narrow strips  62 , a plurality of supporting beams  64 , and a plurality of lead frames  23 . The two narrow strips  62  are parallel to each other. Each of the supporting beams  64  perpendicularly interconnects the two narrow strips  62 . The supporting beams  64  are parallel to each other and are evenly spaced from each other. 
   The lead frames  23  are punched so that each of the lead frames  23  has the thermal conduction piece  231  and the electrical conduction pieces  233 . Each thermal conduction piece  231  is connected to the two narrow strips  62  by four connecting arms  66 . Each electrical conduction piece  233  is perpendicularly connected to one supporting beam  64 . 
   Secondly, molten plastic is injected around the conductor element  60  by insert molding. The plastic is solidified, thereby forming a plurality of the base preforms. An upper surface of each top portion  235  is exposed, thus forming a plurality of upper pads. A bottom surface of each bottom portion  236  is exposed, thus forming a plurality of lower pads. The base preforms and the lead frames  23  cooperatively form a plurality of carrier preforms connected to each other, as represented in  FIG. 6 . 
   Thirdly, referring to  FIG. 7 , each chip  30  is mounted in one corresponding chamber  214 , and is fixed to the main board  210  of the carrier preform via adhesive. Alternatively, any other appropriate fixing means can be employed, such as metallurgical fixing means. 
   Fourthly, one end of each wire  40  is connected/joined with one respective chip pad  302  of the chip  30 , and the other end of the wire  40  is connected/joined with the upper pad of one respective top portion  235  of the lead frame  23 . 
   Fifthly, the conductor element  60  with the base preforms is cut where adjacent carrier preforms adjoin each other and where the connecting arms  66  connect with the thermal conduction pieces  231 , so as to separate the connected carrier preforms from each other and from the narrow strips  62 . At that time, in each carrier preform, distalmost ends of the top portions  235  of the lead frames  23  are exposed at outsides of the respective sidewalls  212  of the base preform. Usually, each top portion  235  is shortened slightly owing to a shrinkage characteristic thereof when the carrier preform is cut. 
   Sixthly, fusing technology such as ultrasonic fusing, laser fusing or heat fusing is used to heat/irradiate a periphery of each carrier preform, thus melting the base preform so that the distalmost ends of the top portions  235  of the lead frames  23  are enclosed by the sidewalls  212 , as represented in  FIG. 8 . Thereby, each base preform is made into a base  21 , and a plurality of finished carriers  20  is obtained. 
   Finally, each holder  50  is laid over the corresponding chip  30 , which receives light transmitted through the holder  50 . The holder  50  is fixed on the base  21  by adhesive, and thereby hermetically seals the chip  30  held in the chamber  214  of the base  21  from the external environment. The surface  521  at the second sidewalls  502  is fixed to the second walls  2122  of the base  21  with adhesive. Portions of the upper pads of the carrier  20  adjacent the adhesive remain exposed. The wires  40  are received in the receiving cavity  523 . The protrusions  524  of the first sidewalls  500  are engagingly received in the slots  217  of the first walls  2120 . Thus the carrier  20  is received in the holding cavity  54  of the holder  50 , as shown in  FIGS. 2-3 . The process of manufacturing a plurality of the image sensor chip packages  9  is thus completed. 
   Forming a plurality of image sensor chip packages  9  simultaneously in this manner has several advantages. One advantage is that it is less labor intensive to handle and process a plurality of image sensor chip packages  9  simultaneously rather than handling and processing each image sensor chip package  9  individually. By reducing labor, the cost associated with each image sensor chip package  9  is minimized. 
   The base  21  of the carrier  20  is made of plastic material, which is generally much cheaper than ceramic. The carrier  20  is formed using injection molding technology, which is a relatively simple method of manufacturing. Thus, the cost of the image sensor chip packages  9  is reduced. 
   The lead frame  23  of the carrier  20  is a solid single piece, and is substantially encapsulated by the base  21 . Therefore it is difficult for water vapor to penetrate into the image sensor chip package  9  through the base  21 . The chip  30  is protected from pollution or contamination, and the reliability of the image sensor chip package  9  is enhanced. 
   The upper surfaces of the top portions  235  of the lead frame  23  act as the upper pads. Accordingly, ample space is available above the upper pads for the use of a wire bonding tool to bond the wires  40  to the upper pads. Thus, the size of the carrier  20  can be minimized to approach the size of the chip  30 , and the volume of the image sensor chip package  9  can be minimized correspondingly. 
   In addition, as the image sensor chip package  9  has a relatively small volume, it contains relatively few dust or other foreign particles therein. Therefore any pollution and/or contamination of the photosensitive area  301  is reduced, and the quality and reliability of the image sensor chip package  9  can be much improved. 
   It is believed that the present embodiments and their advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the invention or sacrificing all of its material advantages, the examples hereinbefore described merely being preferred or exemplary embodiments of the invention.