Abstract:
A packaging process of a light emitting diode (LED) is provided. First, an LED chip is bonded with a carrier to electrically connect to each other. After that, the carrier is heated to raise the temperature thereof. Next, an encapsulant is formed on the heated carrier by a dispensing process to encapsulate the LED chip, wherein the viscosity of the encapsulant before contacting the carrier is lower than that of the encapsulant after contacting the carrier. Thereafter, the encapsulant is cured.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Taiwan application serial no. 98111842, filed on Apr. 9, 2009. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of specification. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a packaging process of a light emitting diode (LED), and more particularly to a packaging process of an LED capable of increasing the viscosity of an encapsulant of an LED package rapidly. 
     2. Description of Related Art 
     Due to the advantages of long lifetime, compactness, high vibration resistance, low heat emission, and low power consumption, light emitting diodes (LEDs) have been widely applied in indicators or light sources of various home appliances and instruments. With recent development towards multicolor and high illumination, the applications of the LEDs are extended to large-sized outdoor billboards, traffic lights, and the like. In the future, the LEDs may become the power-saving and environment-friendly light sources in replacement of tungsten filament lamps and mercury vapor lamps. 
     Generally, an LED package includes a carrier, an LED chip, and an encapsulant. The LED chip is disposed on the carrier and electrically connected to the same. The encapsulant encapsulates the LED chip and a portion of the carrier to protect the LED chip and expose a portion of the carrier outside of the encapsulant to provide a function of external electrodes. Since light emitted by the LED chip is transmitted to the outside of the LED package through the encapsulant, the encapsulant of the LED not only has a function of protecting the LED chip, but is also closely related to overall light emitting efficiency and optical characteristic of the LED. 
     In the conventional packaging process of the LED, the manufacture of the encapsulant is generally categorized into compression molding and transfer molding. In the process of compression molding, a substrate having LED chips mounted thereon is inserted into a mold which contains a melted encapsulant, after the encapsulant has been cured, an entire package is released from the mold to complete the encapsulating process. In the process of transfer molding, a substrate having LED chips mounted thereon is clamped by a mold, and a melted encapsulant is injected into the molding cavity of the mold to encapsulate the LED chip, after the encapsulant has been cured, an entire package is released from the mold to complete the encapsulating process. 
     However, both the compression molding and the transfer molding require molds and expensive injection machines, which results in a certain manufacturing cost. Moreover, a molding cavity of the mold may be deformed or damaged after being used for a period of time, thereby changing a configuration of the encapsulant formed and further affecting yield rate. In addition, since the making and modifying of the molds both require repetitive developments and adjustments, the molds can not be developed and modified quickly according to different designs. As a result, the production time is likely to be delayed. Hence, how to improve the packaging process is one of the issues that have to be conquered in the manufacture of the LED. 
     SUMMARY OF THE INVENTION 
     The present invention provided a packaging process of a light emitting diode (LED). The process rapidly increases a viscosity of an encapsulant formed on a carrier. 
     As embodied and broadly described herein, a packaging process of an LED is provided is the present invention, and includes the following steps. Firstly, an LED chip is bonded with a carrier to electrically connect the LED chip and the carrier. Next, the carrier is heated to raise a temperature of the carrier. Thereafter, an encapsulant is formed on the heated carrier to encapsulate the LED chip, wherein a viscosity of the encapsulant before contacting the carrier is lower than that of the encapsulant after contacting the carrier. Afterwards, the encapsulant is cured. 
     In one embodiment of the present invention, the process of forming the encapsulant on the heated carrier is performed by a dispensing process. 
     In one embodiment of the present invention, a processing temperature of bonding the LED chip with the carrier is T 1 , and a temperature of the heated carrier is T 2 . Moreover, a temperature difference (T 2 −T 1 ) ranges from 70° C. to 180° C. 
     In one embodiment of the present invention, the processing temperature of bonding the LED chip with the carrier ranges from 25° C. to 30° C. On the other hand, the temperature of the heated carrier ranges from 100° C. to 200° C. 
     In one embodiment of the present invention, the encapsulant is a transparent encapsulant. 
     In one embodiment of the present invention, the encapsulant is a thermal-setting encapsulant. 
     In one embodiment of the present invention, the viscosity of the encapsulant before contacting the carrier ranges from 1500 mPas to 4000 mPas. 
     In one embodiment of the present invention, the process of curing the encapsulant includes pre-curing the encapsulant and fully curing the encapsulant. 
     In light of the foregoing, since the present invention allows the rapid increase in the viscosity of the encapsulant formed on the carrier by raising the temperature of the carrier, the encapsulant can be formed more efficiently. 
     In order to make the aforementioned and other features and advantages of the present invention more comprehensible, several embodiments accompanied with figures are described in detail below. 
    
    
     
       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. 
         FIGS. 1A˜1E  are cross-sectional diagrams of a packaging process of an LED according to an embodiment of the present invention. 
         FIGS. 2˜4  are graphs illustrating relationships between heating times and viscosities of different encapsulants under different temperatures. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIGS. 1A˜1E  are cross-sectional diagrams of a packaging process of a light emitting diode (LED) according to an embodiment of the present invention. Referring to  FIG. 1A , firstly, an LED chip  110  and a carrier  120  are bonded such that the LED chip  110  and the carrier  120  are electrically connected with each other. The carrier  120  can be broadly interpreted as any carrier that is capable of carrying and electrically connecting with the LED chip  110 . In the present embodiment, the carrier  120  is a printed circuit board (PCB), and this printed circuit board is manufactured with ceramic material or plastic material, for example. Moreover, the printed circuit board can also be a metal core printed circuit board (MCPCB) with good heat dissipation characteristic or a flexible printed circuit (FPC). In other words, the manufacture of a light emitting diode  100  in the present embodiment is carried out with a chip-on-board (COB) technique. In details, in the COB technique, the LED chip  110  is directly mounted onto a circuit board, and then the LED chip  110  is electrically connected to the circuit board via bonding wires through a wire-bonding process. 
     The present invention does not specifically limit the type of the carrier  120 . In another embodiment of the present invention, the carrier  120  is a leadframe including two leads (not shown) for electrically connecting with the LED chip  110  and a die pad (not shown) for carrying the LED chip  110 . 
     Referring to  FIG. 1B , the carrier  120  is then heated to raise a temperature thereof. In the present embodiment, a processing temperature of bonding the LED chip  110  with the carrier  120  is T 1 , and a temperature of the heated carrier  120  is T 2 . Moreover, a temperature difference (T 2 −T 1 ) ranges from 70° C. to 180° C. 
     Furthermore, in the present embodiment, the processing temperature of bonding the LED chip  110  with the carrier  120  ranges from 25° C. to 30° C. Alternatively, the temperature of the heated carrier  120  ranges from 100° C. to 200° C. Obviously, the present invention does not specifically limit the method of heating the carrier  120 , those skilled in the art may adopt suitable processes and apparatuses to heat the carrier  120 , and the present embodiment does not limit the method of heating the carrier  120 . 
     Next, referring to  FIGS. 1C˜1E , after the carrier  120  has been heated to raise the temperature thereof, a dispensing process, for example, is performed with a dispenser  140  to dispense an encapsulant  130  on the heated carrier  120  for encapsulating the LED chip  110 , as illustrated in  FIG. 1E . 
     Specifically, the encapsulant  130  is generally manufactured with materials having high transmittance, so that the light emitted from the LED chip  110  can penetrate the encapsulant  130  and transmits outside of the LED  100 . 
     Moreover, in the present embodiment, the encapsulant  130  is a thermal-setting encapsulant having specific physical or chemical characteristic required by the manufacturing process. For example, the material of the encapsulant  130  is epoxy, polymethyl methacrylate (PMMA), polycarbonate (PC), acrylate, or other optical polymer materials. Similarly, the present invention does not specifically limit the material of the encapsulant  130 . For example, the encapsulant  130  may also include nano-particles doped therein. In this case, the light emitted from the LED chip  110  is scattered by the nano-particles such that the light of the LED  100  is more uniform and the intensity of the light emitted by the LED  100  is enhanced. 
     As aforementioned, in the process of heating the carrier  120  and forming the encapsulant  130  by the dispensing process, the carrier  120  is heated to a specific temperature and the viscosity of the encapsulant  130  changes rapidly after the carrier  120  contacts the encapsulant  130 . When the encapsulant  130  contacts the carrier  120 , the encapsulant  130  is indirectly heated due to thermal conduction, so that the viscosity of the encapsulant  130  after contacting the carrier  120  increases rapidly. 
     Next, a curing process is performed to the encapsulant  130 . In the present embodiment, a process of curing the encapsulant  130  includes pre-curing the encapsulant  130 . More specifically, the pre-curing is carried out at a certain processing temperature, so that the encapsulant  130  is partially cured. In the present embodiment, the temperature of pre-curing ranges from 100° C. to 200° C. Next, the encapsulant  130  is fully cured. For example, the process of curing the encapsulant  130  can be a thermal curing process. In the present embodiment, a temperature of fully curing is approximately 150° C. and a time thereof is approximately 1˜2 hours. It should be illustrated that the processing temperatures applied in the pre-curing and the fully curing are generally similar. The difference between the pre-curing and the fully curing is the lengths of the curing time. 
     Referring to the following experiments, it should be noted that if the carrier  120  is heated before the encapsulant  130  is dispensed on the carrier  120 , a positive effect is observed for the increase in the viscosity of the encapsulant  130  which is formed on the carrier  120 . 
       FIGS. 2˜4  are graphs illustrating relationships between heating times and viscosities of different encapsulants under different temperatures. Referring to  FIG. 2 , X-axis denotes the heating time while Y-axis denotes the viscosity of the encapsulant (mPas). In the present embodiment, when the thermal-setting encapsulant is heated from about room temperature and the temperature is set as 25° C. or 40° C. after heating, the viscosity of the thermal-setting encapsulant is substantially unchanged in the 120 minutes of heating time, and the slope of the curve is slightly higher than zero in general. 
     On the other hand, when the temperature of the thermal-setting encapsulant is 60° C. after the heating, the viscosity of the encapsulant and the rate of increasing viscosity are both dramatically increased starting at the 40 th  minute of heating. From  FIG. 2 , it is obvious that the slope of the curve is also rapid increased from the 40 th  minute of heating. Specifically, the viscosity rapidly increases at a rate as rapid as at least 3500 mPas in 70 minutes after the 40 th  minute. 
     When the temperature of the thermal-setting encapsulant is 80° C. or 100° C. after heating, a similar phenomenon occurs. However, the difference between the two is that the time points showing the rapid increasing of the slope after the heating are 15 minutes and 3 minutes, respectively. That is, after been heated for about 15 minutes and 3 minutes, the thermal-setting encapsulant starts to melt and the viscosity thereof increases rapidly. It should be illustrated that in the beginning of the heating process, the raise in the temperature causes the viscosity of the encapsulant to decrease temporarily, but the viscosity increases rapidly later on. This phenomenon does not affect the purpose of rapidly molding the encapsulant in the present invention. 
     Referring to  FIG. 3 , the graph showing the relationship between the heating time and the viscosity of the encapsulant is similar to that of  FIG. 2 . However, the difference between the two is that the type of encapsulant, the temperature, and the heating time in  FIG. 3  is different from those illustrated in  FIG. 2 . In the present embodiment, when the temperature of the thermal-setting encapsulant is 25° C. or 40° C. after heating, the viscosity of the encapsulant generally changes slowly within the heating time of 480 minutes. The viscosities are respectively 2400 mPas˜3100 mPas and 2400 mPas˜3400 mPas. 
     When the temperature of the thermal-setting encapsulant is 60° C. after heating, the viscosity thereof and the rate of increasing viscosity are both dramatically increased. When the temperature is 80° C. after heating, the viscosity thereof and the rate of increasing viscosity are increased rapidly after about 20 minutes of heating. 
     Referring to  FIG. 4 , the graph showing the relationship between the heating time and the viscosity of the thermal-setting encapsulant is similar to that of  FIG. 2 . However, the difference between the two is that the type of encapsulant, the temperature, and the heating time in  FIG. 4  is different from those illustrated in  FIG. 2 . In the present embodiment, when the temperature of the thermal-setting encapsulant is 60° C. after heating, the viscosity thereof is substantially 3500 mPas to 5900 mPas within the 30 minutes of heating time. When the temperature is respectively 80° C., 100° C., and 150° C., the viscosities of the thermal-setting encapsulant are increased rapidly after about 7 minutes, 5 minutes, and 4 minutes of heating. 
     After the encapsulant contacts the heated carrier, the raise in temperature causes the viscosity of the encapsulant to decrease temporarily and increase rapidly later on. Moreover, the faster the raising of the temperature, the faster the rate of increasing the viscosity of the encapsulant; therefore, the encapsulant having predetermined height and shape can be formed rapidly. In practical implementation, the heights and shapes of different encapsulants are obtained by adjusting the heating temperature, the rate of dispensing and choosing encapsulants having different viscosity characteristics. 
     In the present invention, the encapsulant is heated to a certain temperature so as to increase the viscosity of the encapsulant rapidly. The encapsulant can be rapidly molded by controlling the amount of dispensed encapsulant and the rate of dispensing the encapsulant. 
     In light of the foregoing, the present invention rapidly enhances the viscosity of the encapsulant formed on the carrier by raising the temperature of the carrier, thereby the encapsulant can be formed more rapidly. In comparison to the conventional packaging process, the packaging process of the LED in the present invention reduces the manufacturing cost of molding process, simplifies the overall manufacturing process, reduces the manufacturing time, and has a high flexibility of modifying the process. 
     Although the present invention has been described with reference to the above embodiments, it will be apparent to one of the ordinary skill in the art that modifications to the described embodiment may be made without departing from the spirit of the invention. Accordingly, the scope of the invention will be defined by the attached claims not by the above detailed descriptions.