Abstract:
The present invention relates to a method for forming a package structure for a light emitting diode (LED) and the LED package structure thereof. By employing the same sawing process to cut through the trenches of the leadframe, the package units are singulated and different lead portions are simultaneously separated from each other in each package unit. Therefore, the overflow issues of the encapsulant can be avoided without using extra taping process.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The present invention relates to a fabrication method of a light emitting diode package structure. More particularly, the present invention relates to a method for manufacturing a light emitting diode package structure and the package structure thereof. 
     2. Description of Related Art 
     Following the energy preservation and environmental protection concepts promoted by the developed countries in America, Europe and Asia, light emitting diodes (LEDs) become one of the most potential products, because they may replace traditional white light bulbs and become the green luminance source in the twenty-first century. At present, LEDs have been widely applied as light emitting elements in various electronic products, display panels and communication products, as they can provide miniature sizes, durability, low-driven voltages, low electricity consumption, fast response speed and good monochromatism. 
     Due to the cost considerations of LED packaging, it is highly desirable to provide trouble-free and undemanding processes without compromising the quality of the final LED package products. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method for forming a LED package structure with better reliability performances and the LED package structure thereof. 
     The packaging method provided by the present invention can achieve singulation of the package units and separating different lead portions apart at the same time. The present invention also provides an LED package structure with better reliability performances. 
     As embodied and broadly described herein, the present invention relates to a method for forming a light emitting diode package structure. After providing a leadframe having pre-cut trenches and disposing at least one light emitting diode (LED) chips to each leadframe unit of the leadframe, an encapsulant is formed over the leadframe covering the LED chips and fills up the trenches. Subsequently, a sawing process is performed to cut through the trenches, so that the package units are singulated and different lead portions are simultaneously separated from each other in each package unit. 
     Since the lead portions are not separated until the sawing process, the overflow issues of the encapsulant can be avoided and no extra taping process is required. 
     As embodied and broadly described herein, the present invention relates to a package structure for a light emitting diode (LED). The LED package structure of this invention includes at least a light emitting diode (LED) chip disposed on and electrically connected to the leadframe, a phosphor layer covering the LED chip and the leadframe and a transparent encapsulant covers the LED chip, the phosphor layer and the leadframe. The flange of the encapsulant is tightly fitted to a recess at a periphery of the leadframe, and a portion of the encapsulant is exposed by a slit of the leadframe. 
     As the encapsulant is fitted to the leadframe, the package structure of the present invention possess better reliability performances owing to the increased adhesion between the encapsulant and the leadframe and elongated water permeation path. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings, 
         FIGS. 1A-1E  illustrate a packaging process for a surface mount LED package according to an embodiment of the present invention. 
         FIG. 2A  is a cross-sectional display view for an LED package structure after singulation according to one preferred embodiment of this invention. 
         FIG. 2B  is an example of a top view for the LED package structure of  FIG. 2A . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1A-1E  illustrate a packaging process for a surface mount LED package according to an embodiment of the present invention. First, referring to  FIG. 1A , a leadframe  100  having a plurality of trenches  102  is provided. In this embodiment, the leadframe  100  can be a copper leadframe or a leadframe made of other conductive materials. Trenches  102  within the leadframe  100  are formed by, for example, a pre-etching process or a pre-cutting process. The depth of the trenches  102  is about ⅓ to about ½ of the thickness of the leadframe  100 , while the trench  102  can be either V shaped or U shaped, for example. The trenches  102  include a plurality of first trenches  102   a  and a plurality of second trenches  102   b . The first trenches define the leadframe unit  100   a  of the leadframe  100 , while each leadframe unit  100   a  includes a second trench that defines the first lead portion  100   b  and the second lead portion  100   c  of the unit  100   a.    
     In addition, according to design requirement, after forming the trenches  102 , the process of the present invention allows optionally plating of a silver metal layer  105  or other metal layers on the top surface of the leadframe  100 , in order to increase light reflectivity. 
     Next, referring to  FIG. 1B , an LED chip  120  is provided. The LED chip  120  is disposed on the second lead portion  100   c  of the leadframe unit and has at least an electrode  124  disposed on the surface of the LED chip  120 . In the embodiment, the LED chip  120  can be an LED chip for producing white light, blue light, ultraviolet light or an LED chip producing other color light, for example. In this embodiment, the LED chip  120  includes two electrodes  124  on the top surface  122 , and the electrode  124  (e.g. cathode) of the LED chip  120  is electrically connected to the first lead portion  100   b  through a wire  131  while the other electrode  124  (e.g. anode) is electrically connected to the second lead portion  100   c  through a wire  133 . Hence, the LED chip  120  is disposed on the leadframe  100  and is electrically connected to the leadframe  100  through wires  131 / 133 . 
     Alternatively, depending on design of the LED chip, two electrodes of the LED chip  120  may be disposed respectively on the top and bottom surfaces. In this case, the top electrode of the LED chip is electrically connected to the leadframe through a wire, while the bottom electrode of the LED chip is electrically connected to the leadframe through a conductor disposed between the leadframe and the LED chip. 
     Afterwards, referring to  FIG. 1C , a phosphor layer  140  is blanketly coated over the leadframe  100  surface and the LED chips  120 , by spraying, dotting, dipping, or printing, for example. Alternatively, the phosphor layer  140  can be coated to cover only the LED chips  120  (as shown in  FIG. 2A ). 
     Referring to  FIG. 1D , within each leadframe unit  100   a , an encapsulant  150  is formed over the LED chip  120  and the leadframe  100  in the present embodiment for encapsulation. The method for encapsulating the LED chip  120  and the leadframe  100  is, for example, an injection molding process, transfer molding process or overmold molding process. 
     The encapsulant  150  covers the phosphor layer  140 , the LED chip  120  and wires  131 / 133  and fills up the trenches  102   a / 102   b . The encapsulant  150  further includes a lens region  150   a  for helping converge the light emitted by the LED chip  120 . The shape or size of the lens region  150   a  can be changed by adjusting the shape of the mold used in the molding process. In general, the lens portion  150   a  is located above the middle portion of the LED chip  120  covering a part of the first lead portion  100   b , a part of the second lead portion  100   c  and a part of the second trench  102   b , for example. 
     In this embodiment of the present invention, except for its protection effects, the encapsulant  150  is a transparent encapsulant to facilitate the light emitted from the LED chip  120  to propagate to the outside environment. For example, the encapsulant  150  can be made of any transparent or translucent polymer having hardness larger than Shore D 30, preferably silicone resins(organo polysiloxane), epoxy-silicone hybrid resin, or epoxy resin. In addition, encapsulant  150  may further include organic or inorganic fillers, such as light wavelength conversion materials (fluorescence powders, for example), for better optical performances or depending on the design requirements. 
     Referring to  FIG. 1E , a sawing process is performed to cut through the first trenches  102   a  and the encapsulant  150  filled within the first trenches  102   a , so that the leadframe units  100   a  are separated from one another. The sawing process is performed in a direction from the backside to the top surface of the leadframe (shown in arrow). Therefore, individual package units  10  are obtained. At the same time, the same sawing process also cuts through the bottom of the second trenches  102   b  but stops at the encapsulant  150  filled within the second trenches  102   b . The sawing width should be smaller than the width of the first and second trenches  102   a / 102   b . That is, the width of the trenches  102  is wider that the cutting tool (e.g. blade) used for the sawing process for separating the units (i.e singulation process). Hence, after sawing, the first lead portion  100   b  is physically separated from the second lead portion  100   c , a slit  101  is formed by cutting through the bottom of the second trench  102   b  and the encapsulant  150  is exposed through the slit  101 . 
     According to the present invention, as the first and second lead portions are not separated during the molding process, the occurrence of overflows can be avoided completely devoid of the extra backside taping process. However, due to the encapsulant filled within the second trenches, the encapsulated chips and/or wires are still protected and not exposed by the slit. Moreover, the sawing process simultaneously singulates the package units and separate the first and second lead portions of the leadframe. 
     The packaging process as described in the present invention is simple and straightforward, which can avoid the occurrence of overflow without employing extra steps. 
     Referring to  FIG. 2A , each package unit  10  includes at least a LED chip  120  disposed on a portion of the leadframe  100  and electrically connected to the leadframe through at least one wire  130 , a phosphor coating layer  140  and an encapsulant  150 . As the sawing width B is smaller than the width A of the trench  102 , the encapsulant  150  filled within the trench  102   a  is not completely removed even though the sawing process cuts through the encapsulant  150  filled within the trench  102   a . If considering the encapsulant  150  covering the leadframe  100  has a hat-like shape, the lens portion  150   a  can be considered as the crown part, stretching outward as the brim portion  100   b  and the downwardly protruding flange  150   c  at the edge of the encapsulant  150 . As shown in the right enlarged portion, after singulation, an L-shaped junction is formed between the encapsulant  150  and the leadframe  100  at the edge of the leadframe unit  100   a . That is, at the edge of the leadframe unit  100   a  (along the singulation scribing line), the flange  150   c  of the encapsulant  150  is locked to the L-shaped recess of the leadframe  100 . Such locking structure can improve the adhesion between the molding compound and the leadframe and strengthening the package structure. Also the locking structure lengthens the path for external moisture penetration, leading to better moisture protection effects. 
     As shown in the left enlarged portion of  FIG. 2A , after singulation, the leadframe unit  100   a  includes a slit  101  within the leadframe  100 , and the encapsulant  150  filled within the second trench  102   b  (and the underlying phosphor layer) is exposed by the slit  101 . 
       FIG. 2B  is an example of a top view of  FIG. 2A , the lens portion  150   a  is located above the middle portion of the LED chip  120 , covering a part of the first lead portion  100   b , a part of the second lead portion  100   c  and a part of the second trench  102   b , for example. 
     The package structure may further include one or more Zener diodes, varistors or other passive devices, according to the product requirements. 
     According to the present invention, as the package structure fabricated by the above-mentioned packaging process provides better moisture protection and strengthened structure, it also affords better yield and reliability. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.