Abstract:
The present invention relates to electrically attaching a surface mount device to a mounting structure via their respective contact pads using an attach material, such as solder or conductive epoxy, which includes a filler material. In general, the filler material is relatively solid and granular shaped, wherein the diameter of the filler material controls a mounting distance between the surface mount device and the mounting structure. The filler allows a desired distance to be maintained during initial placement of the surface mount device and any subsequent reheating.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to mounting electronic surface mount devices to a mounting structure, and more specifically relates to a solder or conductive epoxy including filler to be used to mount surface mount devices to a mounting structure.  
         BACKGROUND OF THE INVENTION  
         [0002]    Typically, the process of mounting a surface mount device to a mounting structure includes placing a conductive attach material such as solder or conductive epoxy on electronic contact pads of the structure, aligning electronic contact pads of the surface mount device with the contact pads of the structure, and placing the contact pads of the surface mount device onto the attach material. Due to the placement of the surface mount device and the force of gravity, the distance between the surface mount device and the structure is difficult to control. Often, the contact pads of the surface mount device come to rest on the contact pads of the structure, which may force the attach material to disperse and possibly short the contact pads of the surface mount device. Further, any subsequent heating of the surface mount device may allow reflow of the attach material, which may also short the contact pads of the surface mount device. In addition, the unpredictable distance between the surface mount device and the mounting structure is undesirable in some radio frequency applications, as the distance may affect circuit performance.  
           [0003]    [0003]FIG. 1 illustrates a surface mount device  10  mounted to a mounting structure  12  as commonly known in the art. Electrical contact pads  14  and  16  of the surface mount device  10  and the mounting structure  12 , respectively, are connected by attach material  18 . By electrically attaching the contact pads  14  of the surface mount device  10  to the contact pads  16  of the mounting structure  12 , the surface mount device  10  is effectively mounted to the mounting structure  12 . For exemplary purposes, the attach material  18  is solder. The solder  18  is heated such that it is in a molten state when mounting the surface mount device  10  to the mounting structure  12 . The contact pads  14  of the surface mount device  10  are then aligned with the contact pads  16  of the mounting structure  12 , and the surface mount device  10  is placed on the solder  18 , thereby mounting the surface mount device  10  to the mounting structure  12 .  
           [0004]    Due to the molten state of the solder  18  and the placing of the surface mount device  10 , a distance d between the surface mount device  10  and the mounting structure  12  is very difficult to control. In some cases, the distance d becomes extremely small and may approach zero. Therefore, the solder  18  may be compressed such that the contact pads  14  of the surface mount device  10  are shorted.  
           [0005]    Once the surface mount device  10  is mounted to the mounting structure  12 , the mounting structure  12  may be mounted to a second mounting structure  20 . Contact pads  22  and  24  of the mounting structure  12  and the second mounting structure  20 , respectively, are aligned and electrically connected by attach material  26 . In order to mount the mounting structure  12  to the second mounting structure  20 , the attach material  26  is heated. In the case that the attach material  26  is solder, the attach material  26  is heated into its molten state during attachment. If the attach material  26  is conductive epoxy, the attach material  26  is heated in order to cure the attach material  26 . In either case, the heating of the attach material  26  may reheat the solder  18  connecting the surface mount device  10  to the mounting structure  12 . When reheated, the solder  18  may reflow and short the contact pads  14  of the surface mount device or other conductive areas  
           [0006]    Similarly to the distance d, a second distance (not shown) between the mounting structure  12  and the second mounting structure  20  is difficult to control. Therefore, the placement of the mounting structure  12  onto the second mounting structure  20  or the force of gravity may cause the attach material  26  to short the contact pads of the mounting structure  12 .  
           [0007]    The above discussion focuses on using solder as the attach material  18 ; however, problems similar to those associated with solder exist for other attach materials such as conductive epoxy. Thus, there remains a need for a cost-effective method for controlling the distance d between a surface mount device  10  and a mounting structure  12  and/or between two mounting structures  12  and  20 .  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention relates to electrically attaching a surface mount device to a mounting structure via their respective contact pads using an attach material, such as solder or conductive epoxy, which includes a filler material. In general, the filler material is relatively solid and granular shaped, wherein the diameter of the filler material controls a mounting distance between the surface mount device and the mounting structure. The filler allows the desired distance to be maintained during initial placement of the device and any subsequent reheating. The process of mounting the surface mount device to the mounting structure is achieved by placing the attach material including the filler material on the contact pads of the surface mount device and/or the contact pads of the mounting structure. The contact pads of the surface mount device are aligned with the contact pads of the mounting structure, and the surface mount device is placed onto the mounting structure. As the surface mount device settles into the attach material, the movement of the surface mount device towards the mounting structure is limited by the filler material. Once settled, the distance between the surface mount device and the mounting structure is defined by the diameter of the filler material.  
           [0009]    Those skilled in the art will appreciate the scope of the present invention and realize additional aspects thereof after reading the following detailed description of the preferred embodiments in association with the accompanying drawing figures. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES  
       [0010]    The accompanying drawing figures incorporated in and forming a part of this specification illustrate several aspects of the invention, and together with the description serve to explain the principles of the invention.  
         [0011]    [0011]FIG. 1 illustrates the mounting of a surface mount device to a mounting structure using solder as is commonly known in the art;  
         [0012]    [0012]FIG. 2 illustrates mounting a surface mount electronic device to a structure using an attach material with filler according to one embodiment of the present invention;  
         [0013]    [0013]FIG. 3 illustrates a surface mount electronic device mounted to a structure with attach material having filler according to one embodiment of the present invention; and  
         [0014]    [0014]FIG. 4 illustrates a surface mount electronic device mounted to a structure using an attach material with filler wherein the area between the surface mount electronic device and the structure is filled with an underfill material according to one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]    The embodiments set forth below represent the necessary information to enable those skilled in the art to practice the invention and illustrate the best mode of practicing the invention. Upon reading the following description in light of the accompanying drawing figures, those skilled in the art will understand the concepts of the invention and will recognize applications of these concepts not particularly addressed herein. It should be understood that these concepts and applications fall within the scope of the disclosure and the accompanying claims.  
         [0016]    [0016]FIG. 2 illustrates an electronic surface mount device (SMD)  10  prior to mounting to a mounting structure  12 . For simplicity, only contact pads  14  and  16  of the surface mount device  10  and the structure  12 , respectively, are illustrated. However, the surface mount device  10  and the mounting structure  12  may have any number of contact pads. Further, the contact pads  14  and  16  are conductive and have a melting point greater than temperatures reached during the mounting of the surface mount device  10  to the mounting structure  12 . In addition, the surface mount device  10  may be any semiconductor device or passive component. The mounting structure  12  may be any structure to which it is desirable to mount the surface mount device  12  such as but not limited to a circuit board, a leadframe, or another electronic device.  
         [0017]    The surface mount device  10  is attached to the structure  12  by electrically attaching the contact pads  14  of the surface mount device  10  to the contact pads  16  of the structure  12  using attach material  28  having filler  30 . The attach material  28  may be any conductive material suitable for attachment of the surface mount device  10  to the mounting structure  12 , such as but not limited to solder or conductive epoxy. The filler  30  may be any of a number of conductive or non-conductive materials having a melting point greater than a melting point of the attach material  28 . These materials include but are not limited to silica, copper, aluminum oxide, tin, lead, gold, silver, indium, nylon, plastic, nickel, or carbon, and may be used individually or in combination. In addition, the filler  30  is preferably a relatively solid and granular shaped material having sufficient rigidity to support the surface mount device  12  in the presence of expected and/or unexpected external forces.  
         [0018]    Mounting the surface mount device  10  to the structure  12  is achieved by placing the attach material  28  including the filler material  30  on the contact pads  14  of the surface mount device  10 . Optionally, the attach material  28  including the filler material  30  may be placed on both the contact pads  14  of the surface mount device  10  and the contact pads  16  of the mounting structure  12 . As another option, the attach material  28  including the filler material  30  may be placed only on the contact pads  16  of the mounting structure  12 . The contact pads  14  of the surface mount device  10  are aligned with the contact pads  16  of the structure  12 , and the surface mount device  10  is placed onto the mounting structure  12 . As the surface mount device  10  settles into the attach material  28 , the movement of the surface mount device  10  towards the structure  12  is limited by the filler material  30 . Once settled, a distance between the surface mount device and the structure is defined by the diameter of the filler material  30 .  
         [0019]    [0019]FIG. 3 illustrates the surface mount device  10  attached to the mounting structure  12  via the contact pads  14  and  16  using the attach material  28 . As illustrated, the filler  30 , and more specifically the diameter of the filler  30 , defines the distance d between the surface mount device  10  and the structure  12 . The diameter of the filler  30  corresponds to the cross-sectional diameter of the filler  30 , wherein the filler  30  may have any cross-sectional shape including but not limited to circular, square, rectangular, or polygonal. In one embodiment, the diameter of the filler  30  is in the range including 50 micrometers to 100 micrometers. However, it is important to note that the filler  30  can have any diameter, and the diameter of the filler  30  depends on the details of the particular design. Therefore, according to the present invention, the distance d between the surface mount device  10  and the structure  12  is controlled by the diameter of the filler  30  in the attach material  28 .  
         [0020]    The ability to control the distance d is advantageous for many reasons. As discussed previously, if the distance d is unpredictable, the contact pads  14  of the surface mount device  10  may become shorted. The attachment material  28  having the filler  30  controls the distance d such that the distance d is sufficient to prevent the shorting of the contact pads  14  caused by gravity and the placement of the surface mount device  10 . Controlling the distance d may also be beneficial for many radio frequency applications by reducing interference and noise. Another advantage is that the controlled distance d allows soldering residue to be cleaned from beneath the surface mount device  10 . In addition, the controlled distance d allows the area between the surface mount device  10  and the structure  12  to be accurately filled with underfill material, which will be described in detail below.  
         [0021]    [0021]FIG. 4 illustrates another embodiment of the present invention in which the attach material  28  including the filler  30  may be used in combination with a non-conductive underfill material  32  to improve a moisture sensitivity level (MSL) performance of the surface mount device  10 . Typically, solder (not shown) without filler  30  is used for mounting the surface mount device  10  to the structure  12  via the contact pads  14  and  16 , thereby causing the distance d to be unpredictable and making it very difficult to encapsulate or insert the underfill material  32  between the surface mount device  10  and the structure  12 . The difficultly in inserting the underfill material  32  may cause an opening in the underfill material  32  that creates an open path between the contact pads  14  of the surface mount device  10  and/or the contact pads  16  of the structure  12 . The opening may allow moisture to accumulate between the surface mount device  10  and the structure  12 . When the solder is reheated, the moisture turns to steam and forces its way out of the device causing delamination and solder reflow. Further, the solder may flow through the opening when reheated and may short the contact pads  14  of the surface mount device  10 .  
         [0022]    According to the present invention, the attach material  28  including the filler  30  controls the distance d between the surface mount device  10  and the structure  12 . By controlling the distance d, the area between the surface mount device  10  and the structure  12  can be accurately filled with underfill material  32 . The controlled distance d allows the underfill material  32  to be inserted such that there is no opening creating a path between the contact pads  14  of the surface mount device  10  and/or the contact pads  16  of the structure  12 . Therefore, the contact pads  14  of the surface mount device  10  and/or the contact pads  16  of the structure  12  cannot be shorted during reflow.  
         [0023]    The embodiment illustrated in FIG. 4 has an area defined by the surface mount device  10 , the structure  12 , and the attach material  28  is filled with the non-conductive underfill material  32 . However, it is important to note that the underfill material  32  is optional and is not necessary to prevent the shorting of the contact pads  14  of the surface mount device  10 .  
         [0024]    Those skilled in the art will recognize improvements and modifications to the preferred embodiments of the present invention. All such improvements and modifications are considered within the scope of the concepts disclosed herein and the claims that follow.