Abstract:
The present invention provides an LED device with a flip chip structure. The LED device comprises an insulating substrate, an LED flip chip, a molding compound, a first conductive element, and a second conductive element. The LED flip chip is electrically connected to the connection pads on the insulating substrate via the two conductive elements. The P-type and N-type electrodes are connected to the P-type and N-type electrodes layers, respectively. The invention need not require a conventional wire bonding process. It not only increases the yield rate of the product but also makes the product more compact.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention generally relates to a light emitting diode (LED) device, and more specifically to an LED device with a flip chip structure.  
       BACKGROUND OF THE INVENTION  
       [0002]      FIG. 1  shows a cross-sectional structure of a conventional surface-mount device (SMD) type LED device.  
         [0003]     In a conventional SMD-type LED packaging structure, an LED die  13  is attached to a packaging substrate  11 . The positive and negative electrodes of the LED die  13  are connected through gold or aluminum wires to a positive pad  111  and a negative pad  112  on the packaging substrate, respectively. The LED die  13  and the gold wires  12  are then covered with a transparent resin  14  to isolate them from the outside environment. Only the metal pads or the connection pins  111  and  112  are left exposed for power source connection. Wherein, the top layer of the LED die  13  is a passive protection layer  133 .  
         [0004]     A wire bonding process during packaging is required for the conventional SMD-type LED device. The thickness AA′ of the package fabricated by the conventional method is as large as 0.6 mm. The above scheme not only lowers the productivity but also wastes a room required for the bonding wires. This type of packaging structure is not good for device miniaturization.  
         [0005]      FIG. 2  shows a cross-sectional view and a top view of another conventional LED device. This conventional surface mount LED device  200  with a flip chip packaging structure was disclosed in Taiwan Patent 548857. The device  200  includes two parts: the first part is an LED dice  210  having a flip chip structure, and the second part is an insulating substrate  220  for mounting the LED dice  210 . The LED dice  210  in the LED device  200  is attached to the first electrode layer  221  and the second electrode layer  222  on the insulating substrate  220  through the first bonding pad  211  and the second bonding pad  212  using a soldering technique. The LED dice  210  in the LED device  200  has a flip chip structure and exhibits a brighter intensity than a conventional SMD-type LED device. Furthermore, the LED dice  210  is mounted onto the insulating substrate  220  with a flip chip technique. Therefore, a wire bonding process required for a conventional LED package is eliminated.  
         [0006]     However, the metal reflective layers  213  and  214  and the ohmic contact layers (not shown here) of the LED device  200  are separately processed and therefore a passive protection layer is required near the end of the process. This kind of manufacturing process is complicated and requires a long manufacturing time. Moreover, the miniaturization capability of the LED device  200  is limited though it is still better than conventional LED devices.  
         [0007]     Since the conventional LED devices have many drawbacks, it is important to provide an LED device to lower the manufacturing cost, increase the productivity, reduce the package thickness, and increase the brightness.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention has been made to overcome the above drawbacks of the conventional LED devices. The primary objective of the present invention is to provide an LED device with a flip chip packaging structure. The LED device of the present invention comprises an insulating substrate, an LED flip chip, a molding compound, a first conductive element, and a second conductive element. The insulating substrate has two side edges, a top surface, a down surface, a P-type electrode layer, and an N-type electrode layer. Each electrode layer is disposed on one side edge of the insulating substrate and extended to cover a portion of the top and down surfaces. The first conductive element is located on the top of the P-type electrode. The second conductive element is located on the top of the N-type electrode.  
         [0009]     According to the present invention, the LED flip chip having a P-type electrode and an N-type electrode is formed on the first conductive element and the second conductive element. The molding compound covers the LED flip chip, the first conductive element, and the second conductive element. Wherein, the LED flip chip is electrically connected to the connection pads on the insulating substrate via the two conductive elements. The P-type and N-type electrodes are connected to the P-type and N-type electrodes layers, respectively.  
         [0010]     Another objective of the present invention is to provide a manufacturing method for the flip chip LED devices. This method comprises the following steps: (a) Provide an insulating substrate having two side edges, a top surface, a down surface, a P-type electrode layer, and an N-type electrode layer. Wherein, each electrode layer is disposed on one side edge of the insulating substrate and extended to cover a portion of the top and down surfaces. (b) Dispose a first conductive element and a second conductive element on top of the P-type and N-type electrodes layers, respectively. (c) Dispose an LED flip chip on the first conductive element and the second conductive element. The LED flip chip has a P-type electrode and an N-type electrode. The P-type and N-type electrodes of the LED flip chip are connected to the first conductive element and the second conductive element, respectively. (d) Sinter the first conductive element and the second conductive element. (e) Inject a molding compound to cover the LED flip chip, the first conductive element, and the second conductive element, and then bake the molding compound.  
         [0011]     A unique feature of the present invention is that the wire bonding process is eliminated in the manufacturing process. The room required for the wire bonding process is then saved. The thickness of the LED device of the present invention is more than 50% reduced as compared to a conventional LED device of same specification. Besides, the multiple reflective films in the LED flip chip significantly increase the brightness of emitted light. These films can also act as protection layers, which eliminate the need of a passive protection layer during the manufacturing process. Therefore, the LED device of the present invention has the advantages of low cost, high productivity, and suitability for device miniaturization.  
         [0012]     The foregoing and other objects, features, aspects and advantages of the present invention will become better understood from a careful reading of a detailed description provided herein below with appropriate reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  shows a cross-sectional structure of a conventional SMD-type LED device.  
         [0014]      FIG. 2  shows a cross-sectional view and a top view of another conventional LED device.  
         [0015]      FIG. 3  shows a cross-sectional view of a flip chip LED device of the present invention.  
         [0016]      FIG. 4  shows a cross-sectional view of another flip chip LED device of the present invention.  
         [0017]      FIGS. 5A-5D  show a manufacturing method of the LED device shown in  FIG. 3 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]      FIG. 3  depicts a cross-sectional view of a flip chip LED device of the present invention. As shown in  FIG. 3 , a flip chip LED device  300  comprises an insulating substrate  31 , an LED flip chip  33 , a molding compound  34 , a first conductive element  321 , and a second conductive element  322 . The insulating substrate  31  has two side edges  315  and  316 , a top surface  313 , a down surface  314 , a P-type electrode layer  311 , and an N-type electrode layer  312 . Every electrode layer is disposed near one side edge of the substrate  31  and extended to cover a portion of the top surface  313  and down surface  314  of the substrate  31 . The P-type electrode layer  311  and the N-type electrode  312  can be used as connection pads to the outside electrical power.  
         [0019]     The first conductive element  321  is located on the top of the P-type electrode layer  311 . The second conductive element  322  is located on the top of the N-type electrode layer  312 . The LED flip chip  33  having a P-type electrode  331  and an N-type electrode  332  is formed above the first conductive element  321  and the second conductive element  322 . The LED flip chip  33  is electrically connected to the connection pads on the insulating substrate via the two conductive elements  321  and  322 . The P-type electrode  331  and N-type electrode  332  are connected to the P-type electrode layer  311  and N-type electrode layer  312 , separately. According to the present invention, the LED flip chip  33  is fixed by the conductive elements  321  and  322  instead of the conventional bonding wires. Elimination of the wire bonding process not only expedites the manufacturing process but also effectively reduces the production cost. These conductive elements  321  and  322  are a pasty conductive material, and are solidified after a proper heat treatment. These materials include silver paste, tin paste, gold ball, and tin ball etc.  
         [0020]     Lastly, the molding compound  34  is shaped with a molding technique, and covers the LED flip chip  33 , the first conductive element  321 , and the second conductive element  322  to protect them from scratch, oxidation etc.  
         [0021]     A unique feature of the present invention is that the wire bonding process is eliminated in the manufacturing process. The room required for the wire bonding process is then saved in the LED device  300 . The normal thickness of the conventional SMD-type LED device is 0.6 mm. The thickness of the LED device of the present invention BB′ is 0.25 mm. This small thickness is suitable for device miniaturization.  
         [0022]     As shown in  FIG. 4 , the lowest layer of the LED flip chip  33  is a transparent substrate  333 . The emitted light from the LED of the present invention passes through the transparent substrate  333  to the outside. The transparent substrate  333  must be made of highly transparent materials. In general, blue diamond is a good material for the transparent substrate  333  because it is highly transparent and suitable for growing GaN epitaxial layer.  
         [0023]     A low temperature GaN nucleation layer  334  of 200-500 Å thick is on top of the transparent substrate  333 . Above that is a 2-5 μm thick N-type GaN cladding layer  335 . A 0.05-0.07 μm thick InGaN multiple quantum well  336  is located on the N-type GaN cladding layer  335 . A 0.1-0.7 μm thick P-type GaN cover layer  337  is formed on the InGaN multiple quantum well  336 .  
         [0024]     A light emitting layer  40  is formed by the transparent substrate  333 , nucleation layer  334 , N-type GaN cladding layer  335 , InGaN multiple quantum well  336 , and P-type GaN cover layer  337 .  
         [0025]     A transparent conductive layer (TCL)  338  is disposed on the P-type GaN cover layer  337 . According to the present invention, a conventional Ni/Au material is adopted for the TCL  338 . It receives a high temperature sintering process at 500-550° C.  
         [0026]     An N-type ohmic contact layer  330  is formed on the N-type GaN cladding layer  335 . A multiple reflective film  339  covers the transparent conductive layer  338 , the N-type GaN cladding layer  335 , and the N-type ohmic contact layer  330 . When lights emitted from the light-emitting layer  40 , not all of them travel in one same direction. The LED flip chip  33  reverses the original forward trajectory of the emitted light through the multiple reflective film  339 , and guides the lights towards the transparent substrate. The multiple reflective film  339  is made of a pair of high refractive material (H) and low refractive material (L). The thickness of each reflective film is equal to one fourth of the wavelength λ. When the multiple reflective film is repeatedly deposited with a sequence of (HL) (HL) . . . (HL)H, a very good reflectivity can be obtained. The total thickness of the multiple reflective film is equal to (λ/4)×(2×N+1). Wherein, A is the wavelength of the light emitted from LED. N is the number of (HL) pairs, which is about 5˜15 pairs. Therefore, the total thickness of the multiple reflective film is equal to (11/4) λ˜(31/4) λ. According to the present invention, the materials used for the multiple reflective film  339  include TiO 2 /SiO 2 , Al 2 O 3 /SiO 2 , Si 3 N 4 /SiO 2  etc.  
         [0027]     Lastly, there are P-type electrode  331  and N-type electrode  332  on top of the transparent conductive layer  338  and N-type ohmic contact layer  330 , respectively. The multiple reflective film  339  is made contacted with a portion of P-type electrode  331 , N-type electrode  332 , and N-type ohmic contact layer  330 .  
         [0028]     According to the present invention, the emitted lights originally traveling towards the transparent conductive layer  338  are reflected to the transparent substrate  333  through the multiple reflective film  339 . These lights are then combined with the lights originally emitted towards the transparent substrate  333 . Therefore, the intensity of light coming from the LED to the transparent substrate  333  is greatly enhanced.  
         [0029]      FIGS. 5A-5D  depicts a manufacturing method of the LED device described above. Firstly, an insulating substrate  31  having two side edges  315  and  316 , a top surface  313 , and a down surface  314 . As shown in  FIG. 5A , a P-type electrode layer  311  and an N-type electrode layer  312  are formed on the side edges  315  and  316  of the insulating substrate  31  and extended to cover a portion of the top surface  313  and the down surface  314 , respectively. The insulating substrate  31  can be made of epoxy resin or polyimide (PI) or bismaleimide triazine resin (BT resin) or polyphenylene oxide (PPO) or polytetrafluoroethylene (PTFE) or polycyanate and etc.  
         [0030]     Next, a first conductive element  321  and a second conductive element  322  are disposed on top of the P-type electrode layer  311  and N-type electrodes layer  312 , respectively. The method of forming the conductive element can vary with the material property of the conductive element. For example, an epoxy dispenser is used to dispose the material if the conductive element is a silver paste. If the conductive element is a tin paste, a mask is used first to define the areas to be pasted. Then, a coater is used to dispose the tin paste on those areas. If the conductive element is a gold ball or a tin ball, a ball planting equipment is used to dispose the ball, as shown in  FIG. 5B .  
         [0031]     Subsequently, an LED flip chip  33  (shown in  FIG. 3 ) is placed on the first conductive element  321  and the second conductive element  322 . The P-type electrode  331  and N-type electrode  332  of the LED flip chip are connected with the first conductive element  321  and the second conductive element  322 , respectively.  
         [0032]     Then, the first conductive element  321  and the second conductive element  322  are sintered. The purpose of the sintering is to solidify the conductive elements and fix the LED flip chip  33  on the substrate. Another purpose is to form electrical connections between P-type electrode  331  and P-type electrode layer  311  and between N-type electrode  332  and N-type electrode layer  312  through the first conductive element  321  and the second conductive element  322 , respectively.  
         [0033]     Lastly, a molding compound  34  is injected to cover the LED flip chip  33 , the first conductive element  321 , and the second conductive element  322 . Then, the molding compound  34  is baked to fix the whole packaging module, as shown in  FIG. 5D . The molding compound  34  used for packaging technology can be chosen from the group of epoxy resin, transparent epoxy resin, and semi-transparent epoxy resin etc.  
         [0034]     A unique feature of the present invention is that the wire bonding process is eliminated in the manufacturing process. The room required for the wire bonding process is then saved. The thickness of the LED device of the present invention is more than 50% reduced as compared to a conventional LED device of same specification. Besides, the multiple reflective films in the LED flip chip have a protection function, which eliminate the need of a passive protection layer during the manufacturing process. In general, the LED device of the present invention has a simple manufacturing process and short manufacturing cycle time. It offers the advantages of reducing manufacturing cost, increasing productivity, reducing device thickness, and increasing LED intensity.  
         [0035]     Although the present invention has been described with reference to the preferred embodiments, it will be understood that the invention is not limited to the details described thereof. Various substitutions and modifications have been suggested in the foregoing description, and others will occur to those of ordinary skill in the art. Therefore, all such substitutions and modifications are intended to be embraced within the scope of the invention as defined in the appended claims.