Abstract:
A method for distributing phosphor particulates on an LED chip, includes steps of: providing a substrate having an LED chip mounted thereon; dispensing an adhesive on the chip, wherein the adhesive have positively charged phosphor particulates doped therein; providing an upper mold and a lower mold for producing an electric field through the adhesive and moving the upper mold to press the adhesive, wherein the phosphor particulates are driven by the electric field to move to a top face of the chip; and curing the adhesive and removing the upper mold and the lower mold.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present disclosure relates to a method for forming an LED (light emitting diode), and more particularly, to a method for distributing phosphor particulates on an LED (light emitting diode) chip of the LED. 
         [0003]    2. Description of Related Art 
         [0004]    As new type light source, LEDs are widely used in various applications. A conventional LED includes a base, a pair of leads fixed in the base, a die mounted on the base and electrically connected to the leads and an encapsulant secured to the base and sealing the die. In order to produce white light, the die is made of a predetermined material to emit blue light, and large quantities of yellow phosphor particulates are doped in the encapsulant. The yellow phosphor particulates absorb the blue light from the die and are excited thereby to produce yellow light. The yellow light mixes with the blue light to generate white light. 
         [0005]    There are two methods in the art to distribute the phosphor particulates on the die. One method is to dispose a shade around the die, and then spray the phosphor particulates on the die via a nozzle. However, the method leaves a large number of phosphor particulates on the shade, causing waste of the phosphor. The other method is to directly dispense the phosphor particulates on the die. This method is also unsatisfied since the phosphor particulates are not distributed uniformly. 
         [0006]    What is needed, therefore, is a method for distributing phosphor particulates on an LED chip which can overcome the limitations described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    Many aspects of the present disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
           [0008]      FIG. 1  shows a first step of a method of a first embodiment of the present disclosure. 
           [0009]      FIG. 2  shows a second step of the method of the first embodiment of the present disclosure. 
           [0010]      FIG. 3  shows a third step of the method of the first embodiment of the present disclosure. 
           [0011]      FIG. 4  shows a forth step of the method of the first embodiment of the present disclosure. 
           [0012]      FIG. 5  shows phosphor particulates which have been deposited on a chip after the steps of  FIGS. 1-4 . 
           [0013]      FIG. 6  is similar to  FIG. 3 , showing a third step of a method of a second embodiment of the present disclosure. 
           [0014]      FIG. 7  is similar to  FIG. 4 , showing a step following the step of  FIG. 6  of the method of the second embodiment of the present disclosure. 
           [0015]      FIG. 8  is similar to  FIG. 5 , showing phosphor particulates which have been deposited on a chip after the steps of  FIGS. 6-7 . 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0016]    Referring to  FIGS. 1-5 , a method for distributing phosphor particulates  40  on a chip  20  in accordance with a first embodiment of the present disclosure is disclosed. 
         [0017]    Firstly, a substrate  10  having a chip  20  mounted thereon is provided as shown in  FIG. 1 . The substrate  10  may be made of a heat conductive material such as metal, ceramic or other suitable materials. The chip  20  may be made of GaN, AlGaN, InAlGaN or other light emitting semiconductor materials. Preferably, the chip is a GaN chip which emits blue light when energized. 
         [0018]    Then an adhesive  30  containing phosphor particulates  40  therein is dispensed on a top face of the chip  20  as shown in  FIG. 2 . The adhesive  30  may be made of epoxy, silicone, polycarbonate, polymethylmethacrylate or other transparent materials. The adhesive  30  is heated to be in fluid or colloid state so that the phosphor particulates  40  doped therein are movable within the adhesive  30 . The phosphor particulates  40  may be made of silicate compound, nitrides compound, YAG:Ce (cerium doped yttrium aluminum garnet), YAG:Tb (terbium doped yttrium aluminum garnet), or other suitable light-excitation materials. However, YAG:Ce is preferable in this embodiment for producing yellow light after absorbing and being excited by the blue light from the chip  20 . The phosphor particulates  40  are randomly distributed within the adhesive  30  when the adhesive  30  is initially applied on the chip  20 . 
         [0019]    Moreover, the phosphor particulates  40  are treated to be positively charged beforehand. The process of treating the phosphor particulates  40  to be positively charged may be implemented by dipping the phosphor particulates  40  in a solution consisting of isopropano and magnesium nitrate, wherein Mg ions of the magnesium nitrate are adhered on peripheries of the phosphor particulates  40  so that the phosphor particulates  40  are positively charged. Then the phosphor particulates  40  are separated from the solution and baked. Finally the phosphor particulates  40  are doped in the adhesive  30 . Alternatively, the phosphor particulates  40  can also be treated by other methods to be negatively charged. 
         [0020]    As shown in  FIG. 3 , an upper mold  50  and a lower mold  60  are further provided above and below the adhesive  30 , respectively. The upper mold  50  is spaced a distance from a top end of the adhesive  30 . The upper mold  50  has positive charges  52  uniformly distributed all over a bottom face thereof. The lower mold  60  is located below and contacts a bottom face of the substrate  10 . The lower mold  60  has negative charges  62  distributed in an inner cavity (not labeled) thereof which is positioned under the chip  20 . The inner cavity of the lower mold  60  where the negative charges  62  occupy has an area which is a little larger than that of the chip  20  but much less than that of the bottom face of the upper mold  50 . In order to prevent the negative charges  62  from affecting the chip  20  mounted on the substrate  10 , an insulating layer (not shown) may be further provided between the substrate  10  and the lower mold  60 . The positive charges  52  on the upper mold  50  and the negative charges  62  on the lower mold  60  cooperatively produce an electric field  70 . The electric field  70  is oriented along a direction opposite to that light from the chip  20  is oriented. In the embodiment, a part of the electric field  70  neighboring the chip  20  is oriented downwardly, vertically and perpendicular to the substrate  10 . Therefore, the positive phosphor particulates  40  randomly distributed within the adhesive  30  is driven by the electric field  70  to move downwardly towards the chip  20 , and finally accumulated on the top face of the chip  20 . 
         [0021]    The upper mold  50  is then brought to move towards the lower mold  60  to press against the top end of the adhesive  30  as shown in  FIG. 4 . Under the pressure of the upper mold  50 , the adhesive  30  is expanded laterally to cover lateral sides of the chip  20 , thereby substantially containing the chip  20  therein. However, a total thickness of the adhesive  30  should be kept larger than that of the chip  20 , thereby preventing the upper mold  50  from directly engaging with the chip  20 . A direct engagement between the upper mold  50  and the chip  20  may damage the chip  20 . A part of the phosphor particulates  40  is driven by the electric field  70  to move to lateral portions of the adhesive  30  beside the chip  20  and accumulated on a top face of the substrate  10  around the chip  20 . 
         [0022]    Finally, as shown in  FIG. 5 , the upper mold  50  and the lower mold  60  are removed from the adhesive  30  and the substrate  10 , respectively. The adhesive  30  is further cured to harden, thereby fixing the positions of the phosphor particulates  40 . Alternatively, the adhesive  30  can be firstly cured and then the upper mold  50  and the lower mold  60  are removed, depending on the actual manufacturing requirement. 
         [0023]    The phosphor particulates  40  can be uniformly distributed near the chip  20  by action of the electric field  70 , whereby the resultant light from the chip  20  and the phosphor particulates  40  is also uniform. Furthermore, the phosphor particulates  40  are fully used in the manufacturing processes without significant waste. 
         [0024]    Referring to  FIGS. 6-8 , a method for more uniformly distributing the phosphor particulates  40  on the chip  20  in accordance with a second embodiment of the present disclosure is shown. The differences between the two methods of the first embodiment and the second embodiment are the configurations of the upper mold  50   a  and the lower mold  60   a.  The first mold  50   a  of the second embodiment defines a cavity  500  in a bottom face thereof. The cavity  500  has a volume approximately equal to or a little larger than a sum of volumes of the chip  20  and the adhesive  30 , and occupies an area larger than that the chip  20  occupies. The first mold  50   a  has positive charges  52  uniformly distributed on the bottom face thereof and inner faces thereof around the cavity  500 . The lower mold  60   a  has negative charges  62  distributed in an inner cavity (not labeled) thereof. The inner cavity is a little smaller than the chip  20  and located just below the chip  20 . 
         [0025]    As shown in  FIG. 6 , for pressing the adhesive  30 , the upper mold  50   a  is located above and spaced a distance from the top end of the adhesive  30 , and the lower mold  60   a  is located below and contacts the bottom face of the substrate  10 . Since the inner cavity of the lower mold  60   a  is smaller than the chip  20 , the electric field  70  generated between the positive charges and the negative charges converges downwardly. A central part of the electric field  70  near the chip  20  is oriented downwardly, vertically and perpendicular to the chip  20 . A lateral part around the central part of the electric field  70  near the chip  20  is oriented downwardly and inclinedly towards the inner cavity of the lower mold  60   a.  The phosphor particulates  40  within the adhesive  30  are driven by the electric field  70  to accumulate on the top face of the chip  20 . 
         [0026]    The upper mold  50   a  is further brought to move towards the lower mold  60   a  as shown in  FIG. 7 . During movement of the upper mold  50   a,  the adhesive  30  is received in the cavity  500  of the upper mold  50   a  and pressed by the inner faces of the upper mold  50   a  to expand laterally and cover lateral sides of the chip  20 , whereby the adhesive  30  has a shape corresponding to a shape of the cavity  500 . A part of the phosphor particulates  40  stays on the top face of the chip  20 , another part of the phosphor particulates  40  is driven by the inclined lateral part of the electrical field  70  to move on the top face of the substrate  10 , and a remaining part of the phosphor particulates  40  is driven by the inclined lateral part of the electrical field  70  to adhere on the lateral sides of the chip  20 . Therefore, all light emitting faces of the chip  20  are covered by the phosphor particulates  40 , and the light emitted from the chip  20  can be more uniform to mix with the light produced from the phosphor particulates  40 . 
         [0027]    Finally, as shown in  FIG. 8 , the upper mold  50   a  and the lower mold  60   a  are then removed from the adhesive  30  and the substrate  10 , and the adhesive  30  is cured to harden before or after removal of the upper mold  50   a  and the lower mold  60   a.    
         [0028]    It is believed that the present disclosure and its 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 present disclosure or sacrificing all of its material advantages, the examples hereinbefore described merely being preferred or exemplary embodiments.