Abstract:
A pick and place system with an integrated light source to partially cure a light-curable adhesives onto which components have been placed. After a light-curable adhesive in liquid or low viscosity form is applied to a location on a substrate, a pick-and-place head uses a vacuum introduced to its nozzle-like opening to pick a component and place it on to the light-curable adhesive. The pick-and-place head then transmit an appropriate light through the same nozzle-like opening to at least partially cure the adhesive. The component becomes, therefore, at least partially fixed to the substrate and will not shift as the substrate is moved.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present disclosure is directed to methods and apparatus of using light to fix a component placed over light-curable adhesives during assembly. 
         [0003]    2. Description of the Related Art 
         [0004]    Some semiconductor assembly operations are performed in stages in various locations in a building. The mounting or bonding of discrete components to a substrate requires transferring or moving the substrate between several stations. 
         [0005]    A pick-and-place system  20 , as illustrated in  FIG. 1 , uses the application of vacuum from a vacuum delivery source  22  through a vacuum path  24 . The vacuum enables the pick-and-place head  30  to pick up a component  26  and place it onto its desired location on the substrate  28 . The vacuum delivery source  22  then removes the vacuum and moves the vacuum probe to pick up the next component, and repeats the process. The pick-and-place system  20  generally uses linear and rotary actuator system to move the pick-and-place head  30 . One example of a pick-and-place system is disclosed in U.S. Pat. No. 7,484,782. 
         [0006]    Semiconductor manufacturers often use light-curable adhesives as a means to bond a component to a substrate, because it offers tremendous benefits in certain situations. The light-curing process for these adhesives is relatively fast as most light-curable adhesives cure fully in less than 30 seconds, thereby allowing shorter cycle time, increased capacity, and better automation. Further, light-curable adhesives create strong bond strength and can bond dissimilar substrates. Light-curable adhesives are also environmentally sensitive since they can be cured by solvent-free photopolymerization, and the energy required for curing is lower than other technologies. Light-curable adhesives are often preferred for process automation as they do not cure unless exposed to light, and they do not get cured gradually during preservation. Light-curable adhesives are used in many heat-sensitive electronics since the processing time is short, allowing control over the rise of temperature of the target object. Ultraviolet (UV) light-curable adhesive, also known as UV glue, is one example of light-curable adhesives. 
         [0007]    UV curing is the process of changing a monomer (liquid) to a polymer (solid) with the exposure to UV light. Generally, a UV light curable adhesive consists of monomer, oligomer, photopolymerization initiator and various additives. The photopolymerization initiator is excited by the absorption of UV light and reacts with other components through decomposition to eventually change the material exposed to the UV light from liquid to solid. Different photopolymerization initiator reacts to different ranges of UV light, so a UV light is selected to match the adhesive to be cured. UV-A is the most common light used for curing UV light adhesives. When used to cure a UV adhesive, a UV radiation is generally measured by its irradiation intensity per unit area (for example, in mW/cm2). The amount of UV exposure (Intensity x Irradiation time) needed for curing depends on the material itself and generally, higher intensity leads to faster cure. 
         [0008]    Other light-curable adhesives can be used as well, such as those curable through exposure to visible light. 
         [0009]    A light-curing is usually a later stage of the electronics product assembly line. Once components have been placed on a substrate, the substrate is moved into a closed light-curing chamber or through a light-curing conveyor. During this stage, the light-curable adhesive is exposed to the appropriate curing light and become fully cured. 
         [0010]      FIG. 2A  illustrates a component  34  placed on uncured light-curable adhesive  36  on a substrate  28 . Due to the low viscosity of the uncured adhesive  36 , the component  34  is prone to shifting when a pick and place head releases and moves away from the component  34  and when the substrate  28  is moved from one stage to another stage.  FIG. 2B  shows the component  34  that has shifted over the uncured light-curable adhesive  36 . Any movement of the pick and place head or of the substrate  28  may be enough to cause the component  34  to shift. Currently, great care must be taken when tuning process parameters and when moving a substrate from the pick-and-place stage to the curing stage in order to avoid disturbing, shifting, or otherwise moving the components already placed on the adhesives on the substrate. If full curing is carried out with one or more components shifted, yield is reduced because the shifted components will not be fixed in the right location. In cases where hundreds of components are placed on a substrate, there is no means or time to re-set each component to its proper location before curing. 
       BRIEF SUMMARY 
       [0011]    The present disclosure relates to fixing a component to a substrate to prevent the component from shifting. A light source is coupled to a pick-and-place head, and the light is transmitted on a component placed over a light-curable adhesive. As the light-curable adhesive begins to cure, it holds the component in place on the substrate. The light is transmitted through the opening of an aperture through which a vacuum is introduced to pick and place a component. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0012]      FIG. 1  illustrates a common pick-and-place system that uses vacuum to pick and place components. 
           [0013]      FIG. 2A  illustrates a component that has been placed over an uncured light-curable adhesive on a substrate. 
           [0014]      FIG. 2B  illustrates a component shifted from its original position over an uncured light-curable adhesive after the substrate is moved. 
           [0015]      FIG. 3A  and  FIG. 3B  illustrate one embodiment of the disclosure. 
           [0016]      FIG. 4  illustrates a cross section of an enlarged view of one embodiment of the pick- and-place head of this disclosure. 
           [0017]      FIG. 5  illustrates another embodiment of the disclosure. 
           [0018]      FIG. 6  illustrates an embodiment of a method for partially fixing a component in the disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments of the disclosure. However, one skilled in the art will understand that the disclosure may be practiced without these specific details. In some instances, well-known structures associated with semiconductor manufacturing and assembly process have not been described in detail to avoid obscuring the description of the embodiments of the present disclosure. 
         [0020]    Unless the context requires otherwise, throughout the specification and claims that follow, the word “comprise” and variations thereof, such as “comprises” and “comprising” are to be construed in an open, inclusive sense, that is, as “including, but not limited to.” 
         [0021]    Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
         [0022]    In the drawings, identical reference numbers identify similar features or elements. The size and relative positions of features in the drawings are not necessarily drawn to scale. 
         [0023]    In  FIGS. 3A and 3B , an embodiment of a pick-and-place head  100  is shown at different times during a pick and place process. A vacuum delivery source  102  introduces a vacuum to the vacuum path  106  coupled to an aperture  114  in the pick-and-place head  100  through a first opening  112  on the aperture  114 . This allows the vacuum to be introduced through a second opening  116  of the aperture  114  to create a suction force that enables the pick-and-place head to pick up a component  118 . The term “component” as used herein is meant in the broadest sense and includes individual pieces, subassemblies, assembled parts, system and other devices that are put into a final product, on a substrate, or on printed circuit boards. An optical fiber  104  is coupled to a third opening  110  of the aperture  114 , the optical fiber acting as a waveguide for a light from a light source at one end of the optical fiber to the tip  108  of the optical fiber  104 . The light from the tip  108  of the optical fiber  104  in the third opening  110  of the aperture  114 , passes through the aperture  114 , and is output from the second opening  116  of the aperture  114 . The first opening  112  of the aperture  114  is located between the third opening  110  and the second opening  116 . 
         [0024]    To pick a component, the pick-and-place head  100  is first positioned over the component  118 , then it is lowered such that the second opening  116  comes in contact with the component  118 . A vacuum is introduced to the vacuum path  106 , creating a suction force that causes the component  118  to be affixed the second opening  116 . The pick-and-place head  100  is then raised up and positioned to the location onto which the component is to be placed, the location where light-curable adhesive  120  is already applied on a substrate  122 . With the vacuum still introduced, the pick-and-place head  100  is lowered toward the substrate  122  until the component  118  that is affixed to the second opening  116  comes to rest on the light-curable adhesive  120 .  FIG. 3A  shows the component  118  before it is placed onto the light curable adhesive. 
         [0025]    A light is subsequently directed through the optical fiber  104  and the tip  108  of the optical fiber transmits the light through the second opening  116  of the aperture  114  on component  118  for a selected period of time. This causes the light-curable adhesive under the component  118  to begin to cure and hold the component  118  in place.  FIG. 3B  illustrates the component  118  already held in place on the substrate  122  with the adhesive  120  at least partially cured. 
         [0026]    The required light exposure to cure a light-curable adhesive depends on the light intensity and the adhesive material itself. UV adhesives generally need between 250 ms and 1 second of UV light exposure to become partially cured, and between 5 seconds and 30 seconds to become fully cured. Depending on the type of adhesive and the intensity of the light, the period of light exposure is selected to be as short as practical in order to ensure that the component is at least partially fixed so that it does not shift or become otherwise disturbed when the substrate is moved to the next stage. In one embodiment, a light is transmitted for 500 ms to partially cure the light-curable adhesive. In another embodiment, a light is transmitted for 5 to 30 seconds to fully cure the light-curable adhesive. Having the light-curable adhesive fully cured during the pick-and-place stage may eliminate the need to move the substrate to another stage for additional curing, and this may be desirable when there are only a few components to be bonded to a substrate. However, if there are hundreds of components to be bonded on a substrate, the cumulative time it takes to fully cure each component individually may be prohibitive. For 400 components, for example, it would take more than 30 minutes of curing. It may be more efficient in this case to only partially cure the light-curable adhesive after each component is placed, then move the substrate to the next stage to fully cure all the components concurrently. In this later approach, individual partial curing of all 400 components would take less than 4 minutes, and the full curing for the whole substrate would be in the range of 5 to 30 seconds, depending on the type of adhesive and the intensity of the light. The specific numbers set forth here are meant to be examples as it is known in the art that the curing profile, thus curing time, for a light-curable material depends on the light intensity used. 
         [0027]    At the conclusion of at least partial curing of the light-curable adhesive, the pick-and-place head  100  may be moved away from the component  118  to pick and place another component.  FIG. 3B  illustrates the component  118  placed on the substrate. 
         [0028]      FIG. 4  is the enlarged view of the light  124  being transmitted out of the second opening  116  of the aperture  114  of the pick-and-place head  100  towards the component  118  to partially cure the light-curable adhesive  120 . In a preferred embodiment, the light  124  is transmitted when the second opening  116  is still adjacent to the component  118  and the pick-and-place head has not moved away from the component  118 . In some cases, the vacuum may still be applied when the light is transmitted. In an alternative embodiment, the light  124  is transmitted after the vacuum is released and the pick-and-place head  100  has distanced itself from the component  118 , creating a small space between the second opening  116  and the component  118 . 
         [0029]    The optical fiber  104  may be a fiber optic strand for carrying an ultraviolet light or other visible lights. There are a variety of light-curable adhesives, and each of them may require a light of certain wavelength for curing. Different light sources, therefore, may be used and the most appropriate optical fiber  104  may be selected accordingly. 
         [0030]    There are at least two possible techniques to combine the vacuum delivery source and the optical fiber. In one embodiment, illustrated in  FIG. 3A and 3B , the vacuum delivery source  102  is coupled to the vacuum path  106  at a location away from the pick-and-place head, the vacuum path  106  is shown traversing the length of the pick-and-place body from the vacuum delivery source  102 , through the “C” arm, to the first opening  112  of the aperture  114 . In this combination, the pick-and-place tool may control the vacuum delivery. In an alternative embodiment, as illustrated in  FIG. 5 , the vacuum delivery source  102  is coupled to the vacuum path  106  only to the pick-and-place head  100 . The vacuum path  106  may be short and does not traverse the length of the pick-and-place body. In this embodiment, a vacuum delivery controller external to the pick-and-place tool may be used. One skilled in the art understands that there are other alternative techniques to combine the vacuum delivery source and the optical fiber in a pick-and-place tool and they fall within the scope of this disclosure. 
         [0031]    In a preferred embodiment, a UV light source is guided over an optical fiber strand that is 6 mm in diameter. The optical fiber strand is coupled to the third opening of the aperture at a distance of 3 mm from the second opening  116  of the aperture  114 . A diameter of the second opening  116  of the aperture  114  may be selected to provide a desired light intensity. The following table shows some of the possible diameters of the second opening  116  and the associated intensities of the light leaving the second opening  116  when the UV light source is transmitted at 90% intensity. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                   
                   
               
               
                   
                 Diameter (mm) 
                 Intensity (mW) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 0 
                 0 
               
               
                   
                 0.5 
                 40 
               
               
                   
                 1 
                 600 
               
               
                   
                 1.5 
                 900.4 
               
               
                   
                 2 
                 1530.5 
               
               
                   
                 2.5 
                 1560 
               
               
                   
                   
               
             
          
         
       
     
         [0032]      FIG. 6  illustrates one embodiment of a method in the disclosure. In this embodiment, a method starts, step  201 , with the application of uncured light-curable adhesive to a location on a substrate, step  202 . A UV adhesive is an example of light-curable adhesive. Generally the uncured light-curable adhesive is applied to multiple locations on a substrate. A vacuum is then introduced, step  203 , through an aperture in a pick-and-place head to create suction force to pick up a component. The pick-and-place head positions the component, step  204 , over at least one location on the substrate and places the component, step  205 , onto the light-curable adhesive at the location. Light is transmitted, step  206 , through the aperture onto the just-placed component for a length of time selected to at least partially fix the component so it does not shift or otherwise disturbed when the substrate is moved. In a preferred embodiment, an ultraviolet light is transmitted for 500 ms or less to only partially cure a UV adhesive. The aperture through which the light is transmitted is the same aperture through which vacuum is introduced. The light may be an ultraviolet light or other visible lights. Determination is then made if there is any additional component to be placed on the substrate, step  207 . If there is, the process repeats with the introduction of a vacuum, from step  203  to step  207  again. If there is no more component to place, the substrate is moved to the next stage for full curing,  208 . 
         [0033]    In an alternative embodiment, the light is be transmitted, step  206 , onto the just-placed component for a length of time selected to fully cure the light-curable adhesive on which the component was placed. An ultraviolet light may be transmitted for 5 to 30 seconds to fully cure a UV adhesive. In this alternative embodiment, the substrate does not need to be moved to the next stage for full curing anymore. 
         [0034]    The various embodiments described above can be combined to provide further embodiments. All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments. 
         [0035]    These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.