Abstract:
A method of selectively bonding a component to a substrate prevents glue displacement onto neighboring components. The method entails shortening a section of the perimeter of a mount wall so that the foot of the mount wall contacts the glue without causing substantial displacement. A cure step hardens and holds the shortened foot of the mount wall in a stationary position, while providing a partial bond. Meanwhile the rest of the mount wall that is not located near contact pads on the substrate has a tall foot that extends to the surface of the substrate and is bonded in the usual way. By modifying the component, it is not necessary to modify either the chemistry of the epoxy or the epoxy dispense operation.

Description:
BACKGROUND 
     Technical Field 
     The present disclosure generally relates to techniques for mounting a component onto a substrate, and in particular, to bonding a camera lens assembly to a substrate for use in a mobile electronic device. 
     Description of the Related Art 
     Attaching components to a substrate inside a microelectronic device such as a camera module for a smart phone becomes more and more difficult as devices become smaller and substrate real estate becomes more constrained. Component attachment is typically accomplished by dispensing glue onto the substrate, and using a pick-and-place robot to lift up a component and set it down at a prescribed location on the substrate. To attach a hollow component such as a camera lens assembly, glue is dispensed in line patterns and the component is attached in such a way that the perimeter of the component coincides with the glue line. One problem that arises in mounting components is that the epoxy used to bond the components to the substrate can bleed onto neighboring contact pads or bond pads, thereby interrupting electrical connections or interfering with next process steps. Even if the epoxy is dispensed in narrow glue lines, when the component is pressed into the epoxy, the foot of the component displaces some of the glue, which then flows onto the neighboring contact pads. One way to decrease the amount of glue displaced is to narrow the glue line further. However, switching to a new epoxy formula may be necessary in order to achieve a comparable joint with less glue volume. Qualifying a new type of epoxy for introduction into a manufacturing process can be a very complex, time-consuming, and costly process. Another method of bonding involves dispensing epoxy onto the substrate in a narrow glue line that corresponds to only a partial perimeter of the component wall. The rest of the perimeter will simply not receive any glue. When the component is then aligned with the narrow glue line, applying pressure along the partial perimeter that has glue underneath it will displace some of the glue, but the remaining component perimeter will just sit on top of the substrate. After curing the epoxy, a rigid joint will form in the usual way along the partial perimeter. Bonding using such a method can solve the glue displacement problem by designing the substrate layout so that particularly vulnerable areas of the substrate are near the unglued portion of the perimeter. 
     However, there are potential drawbacks to such a method of partial gluing. First, the unglued portion of the component may not be completely stationary and it may rub against the substrate. In some applications such friction may not matter. In other applications, such friction may generate an unacceptable number of particles, or the component may not be bonded tightly enough. In addition, in the case of camera components, light leakage at the location of the unglued portion may affect the optical performance of the camera. Furthermore, debris may pass underneath the unglued portion of the mount wall and collect underneath the component. In addition, a broken glue line is more likely to separate from the substrate than is a continuous glue line. 
     BRIEF SUMMARY 
     Because of the drawbacks of altering the glue line or the gluing process, alternative practical solutions to the problem of glue displacement are of interest. A method of preventing glue displacement near certain features on a substrate by selective bonding of a component entails modifying a mount wall of the component, along its perimeter. For example, in the case of a camera component lens assembly being mounted to a substrate, a section of the perimeter of the mount wall of the camera may be shortened so that the foot of the mount wall contacts the epoxy without causing substantial displacement. As the epoxy is cured, it hardens and holds the shortened foot of the mount wall in a stationary position, while providing a partial bond. Meanwhile the rest of the mount wall that is not located near contact pads on the substrate has a tall foot that extends all the way to the surface of the substrate and is bonded in the usual way. One advantage of this method is that, by modifying the component, it is not necessary to modify either the chemistry of the epoxy or the epoxy dispense operation. Also, because the shortened foot is bonded at least somewhat to the epoxy, the mount is held in place along its entire perimeter, with a greater bonding force along 50-75% of the perimeter that corresponds to the tall foot of the mount wall, and a smaller bonding force along 25-50% of the perimeter that corresponds to the shortened foot of the mount wall. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       In the drawings, identical reference numbers identify similar elements. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. 
         FIG. 1A  is a pictorial perspective view of a miniature lens assembly, according to the present disclosure, to be bonded to a substrate to form a camera module. 
         FIG. 1B  is a bottom plan view of the lens assembly shown in  FIG. 1A . 
         FIG. 2A  is a side view of two conventional device mount walls during alignment to a substrate, according to a prior art structure and method. 
         FIG. 2B  is a side view of the conventional device mount walls shown in  FIG. 2A , following attachment to the substrate, according to a prior art structure and method. 
         FIG. 3  is a side view of a customized device mount having an uneven perimeter wall, as described herein. 
         FIG. 4  is a flow diagram showing generalized steps in a method of selectively bonding a component to a substrate, using the customized device mount as described herein. 
         FIG. 5A  is a side view of the customized device mount during alignment of a component to a substrate, according to a bonding method described herein. 
         FIG. 5B  is a side view of the customized device mount shown in  FIG. 5A , following attachment to the substrate, according to a bonding method described herein. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, certain specific details are set forth in order to provide a thorough understanding of various aspects of the disclosed subject matter. However, the disclosed subject matter may be practiced without these specific details. In some instances, well-known structures and methods of microelectronics module assembly comprising embodiments of the subject matter disclosed herein have not been described in detail to avoid obscuring the descriptions of other aspects of the present disclosure. 
     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.” 
     Reference throughout the 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 appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification are not necessarily all referring to the same aspect. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more aspects of the present disclosure. 
     Specific embodiments are described herein with reference to techniques that have been used to bond devices such as camera lens assemblies to substrates; however, the present disclosure and the reference to certain materials, dimensions, and the details and ordering of processing steps are exemplary and are not limited to those shown. 
       FIGS. 1A and 1B  show two views of an exemplary component, a miniature lens assembly  100 , for incorporation into a camera module, which in turn is incorporated into a mobile electronic device such as a smart phone as disclosed herein.  FIG. 1A  shows a top view of the lens assembly  100 , and  FIG. 1B  shows the underside  110  of the lens assembly  100 , which is configured to be attached to a substrate. In the embodiment described herein, the lens assembly  100  together with the substrate and other components attached to the substrate, form a camera module. Such other components of the camera module can include, for example, an image sensor die, capacitors, resistors, and driver chips. In general, the substrate is any initial board upon to which a device mount supporting the electronic component is attached. In some embodiments, the substrate is an interposer board. In one embodiment, the substrate is a laminate substrate. In other embodiments, an electronic component may be mounted directly onto a printed circuit board (PCB), so that the substrate is the PCB. 
     The exemplary lens assembly  100  includes an optical lens element  102 , a lens barrel  104 , and a lens mount  105 . The optical lens element  102  can be made of glass or plastic, for example. The lens barrel  104  surrounds the optical lens element  102 . The lens mount  105  houses mechanical parts that facilitate movement of the lens element  102  to allow adjustment of the focal length of the lens, in response to control signals received via circuitry embedded in the substrate. The lens mount  105  can be made of plastic, for example, and may be as thick as 500 microns. 
     The lens assembly  100  is attached to a pedestal in the form of a customized device mount  206 . Alternatively, the lens assembly  100  may include the customized device mount  206 . In the embodiments shown herein, the customized device mount  206  is used to mount the lens assembly  100 . However, in another embodiment, such a customized device mount may be used to mount to a substrate a lighting device such as a flash assembly, in proximity to the lens assembly  100 . In further embodiments, customized device mounts similar to the example described herein may be used to mount to a substrate other types of components such as, for example, a microphone assembly or a speaker assembly. The customized device mount  206  is desirably made of metal, but may be made of other materials such as ceramic or plastic. The customized device mount  206  includes a central mount portion  220  and a perimeter wall  236 , the central mount portion being attached, or removably attached, to a device, e.g., the base of the camera lens mount  105 . Alternatively, the customized device mount  206  for a camera can be formed together with the lens mount  105  as one integral unit. In the embodiment shown, the customized device mount  206  has a square perimeter wall  236 , including four mount walls  236   a ,  306   b ,  236   c ,  236   d , as indicated in  FIG. 1A . In other examples, the customized device mount  206  has a different shape, e.g., the customized device mount  206  can have the shape of a closed polygon such as circular, rectangular, triangular or the like, in which case the number of mount walls may vary accordingly. Alternatively, the customized device mount  206  can have an open shape such as, for example, a U-shape or a semi-circle. The thickness of the perimeter wall  236  is in the range of about 200-300 μm. 
     Each mount wall has a corresponding foot e.g.,  238   a ,  238   b ,  238   c ,  238   d , shown in  FIG. 1B , the foot being the lowest portion of the mount wall that might contact the adhesive once the lens assembly  100  is attached to the substrate. The customized device mount  206  is fashioned with a customized section of the perimeter wall  236 , e.g., a mount wall section  236   a  that features a short foot  238   a , while each one of the other mount wall sections, has a standard tall foot  238   b ,  238   c , and  238   d . The foot  238   a  is shorter than the other feet by a distance d. The shortened wall section  236   a  can be an entire wall, any portion of the wall, or alternating sections at different locations around the periphery of the module being mounted. In the example shown, the shortened wall section  236   a  occupies approximately 80% of one section of the wall. In alternative embodiments, it may also be present on each of the four sides or, if the wall is circular, at various locations around the perimeter of the wall. It may also be in two distinct sections even along the same wall region. For example, the shortened wall section  236   a  can be selectively positioned only at those locations in which a closely adjacent circuit is present and therefore be at a plurality of distinct, spaced locations around the periphery of the customized device mount  206 . 
       FIGS. 2A and 2B  show a standard technique for attaching a conventional device mount  106  to a substrate  112 , according to the prior art. Before introducing the lens mount  106 , there may be other items already on or mounted to the surface of the substrate  112  such as, for example, an integrated circuit die  114 , and bond wires  118  coupled to bond pads  116 . Bond pads  116  are part of the substrate  112 . According to a typical mounting procedure, an epoxy is dispensed onto the top surface of the substrate  112  to form a continuous narrow glue line  120  having narrow glue line segments  120   a - 120   d . Although only two narrow glue line segments are shown in profile in  FIG. 2A . A top plan view would show four or more narrow glue line segments  120   a - 120   d , corresponding to each location where the conventional device mount  106  contacts the substrate  112 . 
     Next, the conventional device mount  106  is aligned with the narrow glue line  120 , and lowered toward the substrate so that the mount wall  106   a  contacts the glue line segment  120   a , the mount wall  106   c  contacts the glue line segment  120   c , and so forth. As the mount walls are brought into contact with the substrate  112 , as shown in  FIG. 2B , the narrow glue line segments  120   a - 120   d  spread out to form wide glue line segments  122   a - 122   d  of which two wide glue line segments,  122   a ,  122   c , are shown in profile in  FIG. 2B . Depending on the layout of devices on the substrate, some of the wide glue line segments, for example,  122   a , may bleed glue onto neighboring parts, such as bond pads  116  or possibly other electrical components. The bond pads  116  typically have dimensions of about 0.2 mm-0.3 mm. The presence of epoxy on the bond pads  116  could degrade electrical contact between the bond pad  116  and a bond wire  118 . Additionally or alternatively, the presence of epoxy on the contact pad  117  could interfere with formation of an electrical contact between the contact pad  117  and contact pins of the lens assembly  100 . Such electrical contacts are made with conductive glue or solder in subsequent processing steps. As electronic components shrink with advancing technology, more and more components are crowded onto the substrate  112 , thus increasing the risk that the wide glue line segments  122   a - 122   d  will cause problems. 
       FIG. 3  shows a cross-section view of the customized device mount  206  shown in  FIGS. 1A and 1B , according to one embodiment. Two of the mount walls,  236   a ,  236   c , are shown in profile in  FIG. 3 . The length difference d between the tall foot  238   c  and the short foot  238   a  is selected based on the height of the narrow glue line segments  120   a - 120   d . Typically, the narrow glue line  120  has a width within the range of about 300-500 μm and a height within the range of about 50-150 μm. The shape and dimensions of the glue line segment  120   a , including the height d, are controlled by the choice of adhesive material and by adjusting the dispensing process. Furthermore, the dimensions of the narrow glue line  120  can be selected to accommodate neighboring features, e.g., taking into account the distance to the nearest bond pads  116 , for example, as well as the sizes of the bond pads  116 . However, changing the adhesive material or the bonding process to avoid glue displacement can be an undesirable approach because of manufacturing constraints and the high cost of qualifying and implementing process changes involving chemicals such as adhesives. 
     Use of the customized device mount  206  avoids modifying the adhesive process, and instead changes the component design, which is an easier change to implement. Hence, the manufacture of the customized device mount  206  accommodated a set of given dimensions of the glue line segment  120   a . In particular, the customized device mount  206  is dimensioned so that the glue line segment  120   a  fills the space between the upper surface of the substrate  112  and the lower surface of the short foot  238   a , without deforming significantly when the short foot  238   a  makes contact with the upper surface of the glue line segment  120   a.    
     In one embodiment, a square or rectangular customized device mount  206  is fashioned with two shortened mount walls, e.g.,  236   a ,  236   b . In another embodiment, the customized device mount  206  has a circular perimeter wall  236  wherein a standard section of the perimeter wall  236  has a tall foot  238   c  and a modified section of the perimeter wall has a short foot  238   a . Generally, for any device mount shape, the customized section of the perimeter wall  236  having the short foot desirably encompasses no more than 50% of the total perimeter of the mount. As explained above, the location of the short foot  238   a  can be selected based on the location of components which will be on the substrate. Generally, the short foot  238   a  will be present at those locations on the substrate which have a closely adjacent electrical component. The short foot  238   a  may extend along the entire length of one of the side wall sections, or alternatively may be at various locations spaced apart from each other on two or more of the walls or at various wall sections, depending on the shape and structure of the device mount. 
       FIG. 4  shows a sequence of steps in an exemplary method  250  of selectively attaching a component, e.g., the lens assembly  100 , to the substrate  112  using the customized device mount  206 . The method  250  is not limited to bonding a camera lens to a substrate. The method  250  generally can be applied to bonding any type of component to a substrate using an adhesive that has a liquid or semi-liquid consistency (e.g., a gel), such as an epoxy. Steps in the method  250  are illustrated in  FIGS. 5A and 5B . 
     At  252 , the customized device mount  206 , as described above, is fabricated so as to have one or more mount walls in which the short foot  238   a  differs from the tall foot  238   c  by the selected height d, which is based on a typical glue line height. For example, the selected height d may be chosen as 100 μm. To further enhance adhesion of the mount wall to the epoxy, lower surfaces of the mount wall can be roughened to increase the bonding surface area. 
     At  254 , a viscous adhesive, e.g., an epoxy, is dispensed onto the top surface of the substrate  112  to form the continuous narrow glue line  120  having a height target of 100 μm, to which the customized device mount  206  has been fabricated to match. 
     As shown in  FIG. 5A , at  256 , the standard section of the perimeter wall  236   c , having the tall foot  238   c , is aligned to the narrow glue line segment  120   c , while the customized section of the perimeter wall  236   a , having the short foot  238   a , is aligned to the top surface of the narrow glue line segment  120   a . At  258 , the customized device mount  206  is lowered toward the substrate so that the mount walls are brought into contact with corresponding glue line segments. 
       FIG. 5B  shows the customized device mount  206  after being placed on the substrate  112  in contact with the narrow glue line  120 . In response to pressure from the tall foot  238   c , the narrow glue line segment  120   c  is displaced and spreads out in the usual way to form the wide glue line segment  122   c  that is about 25% wider than the original narrow glue line segment  120   c . However, when the mount wall  236   a  having the short foot  238   a  is placed on the substrate  112 , the short foot  238   a  contacts only the top surface of the narrow glue line segment  120   a  without exerting significant pressure, and therefore does not displace the epoxy. Instead, the narrow glue line segment  120   a  substantially maintains its original shape without spreading laterally. Consequently, the bond pads  116  remain clean so that electrical connections between the bond wires  118  and the bond pads  116  are not compromised. At the contact point where the short foot  238   a  and the long foot  236   a  touch the epoxy, capillary action tends to wick some of the glue upward onto the sides of the mount walls, which helps to ensure that the bond pads  116  remain unobstructed. 
     At  260 , the epoxy is cured by an acceptable method, such as UV light, IR light, or thermal processing at an ambient temperature within the range of about 50-150 C for a time interval within the range of about 0.1-3.0 hours, or the like. The epoxy can be cured either by baking the entire substrate  112  together with the mount in place, or by localized heating of the glue lines. Curing causes the epoxy to harden and form at least a semi-rigid joint between the short foot  238   a  and the substrate  112 , and a substantially rigid joint between the tall foot  238   c  and the substrate  112 . 
     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. 
     It will be appreciated that, although specific embodiments of the present disclosure are described herein for purposes of illustration, various modifications may be made without departing from the spirit and scope of the present disclosure. Accordingly, the present disclosure is not limited except as by the appended claims. 
     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.