Abstract:
One embodiment discloses a method for soldering a cap for an integrated electronic device to a support layer, including the steps of: providing a support layer; providing a cap including a core of a first material and a coating layer of a second material, the first and second material being respectively wettable and non-wettable with respect to a solder, the coating layer being arranged so as to expose a surface of the core; coupling the cap with the support layer; and soldering the surface of the core to the support layer, by means of the solder.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present disclosure relates to a method for soldering a cap to a support layer, and in particular for forming a package for a device manufactured by means of electronic technologies. 
         [0003]    2. Description of the Related Art 
         [0004]    As is known, in the field of the electronic packaging, caps are soldered to support layers (also known as substrates), so as to form cavities wherein electronic devices are hosted. 
         [0005]    As an example,  FIG. 1  shows a package  1  formed by a support layer  2  and a cap  3 , the package  1  defining a cavity  6 ; an integrated electronic device  8 , as an example formed by a Micro Electro-Mechanical System (MEMS), is located within the cavity  6 . In general, in case of a MEMS, the integrated electronic device  8  comprises a first die (not shown), typically known as a sensor die and includes a micromechanical detection structure, and second die (not shown), typically known as an application-specific integrated circuit (ASIC) die and includes a related electronic interface. This first and second die may be stacked and are connected by means of suitable electrical connections in the form of wires designed to electrically connect the sensor die to the ASIC die, and the ASIC die to the support layer  2 . 
         [0006]    Irrespective of the details of the integrated electronic device  8 , the support layer  2  has an internal surface  2   a,  on which the integrated electronic device  8  rests, and an external surface  2   b,  which carries suitable electrical connection elements  12  to the outside of the package  1 , in the form of “balls” or “bumps” or “lands”. 
         [0007]    Furthermore, the support layer  2  is usually made of a multi-layer structure, composed of several layers of conductive material separated via dielectric layers; electrical traces (not shown) and vias  10  are provided through the support layer  2 , so as to electrically connect the integrated electronic device  8  to the electrical connection elements  12 . 
         [0008]    The cap  3  comprises a core  4  and a coating layer  5 . The core  4  is generally of brass and is entirely coated by the coating layer  5 , this latter being usually made up of tin or alloys such as a NiAu alloy. Furthermore, the cap  3  is soldered to the support layer  2  by means of the so-called conventional solder reflow technique. Therefore, the cap  3  is made integral with the support layer  2  by means of a solder joint  16 , which defines a sealing ring interposed between the support layer  2  and the cap  3 . 
         [0009]    The coating layer  5  is made up of a so-called “wettable material”, which, upon contacting a solder paste and following a thermal treatment, can form an intermetallic compound with the solder contained in the solder paste. 
         [0010]    In practice, the entire surface of the core  4  is covered with a solderable material. Therefore, as shown in  FIG. 2 , it may happen that, at the end of the soldering process, the solder extends not only in the gap G ( FIG. 3 ) between the support layer  2  and the cap  3 , but also along a portion of the inner wall of the cap  3 , defined by a corresponding portion of the coating layer  5 ; in particular, the solder may extend along a vertical portion of the inner wall of the cap  3 , orthogonally oriented with respect to the support layer  2 . 
         [0011]    The volume of solder which extends along the inner wall of the cap  3  is taken away from the gap G between the support layer  2  and the cap  3 , thereby leading to the formation of voids V within the solder joint  16 , as shown in  FIG. 3 . 
         [0012]    In practice, because of the above mentioned voids V, it may happen that the package  1  is not fully sealed. Similarly, it may happen that the package  1  becomes unsealed on customer site, as an example because of slight mechanical shocks or during soldering of the package on the customer board. 
       BRIEF SUMMARY 
       [0013]    The present disclosure is directed to soldering methods that solve at least in part the problems described above. According to one embodiment of the disclosure, there is provided a soldering method comprising providing a support layer and a cap. The cap includes a core of a first material and a coating layer of a second material. The first and second materials are wettable and non-wettable, respectively, with respect to a solder. The coating layer is located around the core and exposes a surface of the core. The method further includes coupling the cap with the support layer and using the solder, soldering the surface of the core to the support layer. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0014]    For the understanding of the present disclosure, embodiments are now described, purely as non-limitative examples, with reference to the enclosed drawings, wherein: 
           [0015]      FIGS. 1 and 2  show schematic cross-sections of packages of a known type; 
           [0016]      FIG. 3  shows an enlarged view of a portion of the package shown in  FIG. 2 ; 
           [0017]      FIG. 4  shows a cross-section of an array of caps; 
           [0018]      FIGS. 5   a  and  5   b  show cross-sections of a container; 
           [0019]      FIGS. 6 and 7  show, respectively, a cross-section and a perspective view with parts taken away of a cap; 
           [0020]      FIGS. 8   a  and  8   b  show, respectively, a cross-section and a top-plan view of a support layer during a step of the present method; 
           [0021]      FIG. 9  shows a schematic cross-section of a support layer during a further step of the present method; and 
           [0022]      FIGS. 10 and 11  show schematic cross-sections of a support layer and a cap, during different steps of the present method. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    The following description will make reference, purely by way of non-limiting example, to the soldering of a plurality of caps to a support layer, although it is clear that the present method can be applied also in case of soldering of a single cap to the support layer. 
         [0024]    According to a first embodiment, an array  20  of caps  23  is provided, as shown in  FIG. 4 . In particular, the array  20  shown in  FIG. 4  comprises two caps  23 , connected one to another. 
         [0025]    In detail, each of the caps  23  comprises a core  24  of a first material, this core  24  being coated by a coating layer  25  of a second material. In practice, the cores  24  of the caps  23  are joined together, thereby forming an inner layer of the array  20 ; furthermore, the coating layers  25  of the caps  23  are joined together, thereby forming an outer layer of the array  20 , which may entirely coat the inner layer. 
         [0026]    The first and the second material are, respectively, wettable and non-wettable with respect to a same solder. To this regard, given a generic material, it is generally said “wettable” with respect to a given solder if, assuming to fill a container  31  of this generic material with this given (liquid) solder, this latter forms a concave meniscus, as shown in  FIG. 5   a . Conversely, this generic material is generally said “non-wettable” with respect to this given solder if the meniscus is convex, as shown in  FIG. 5   b . Furthermore, with particular reference to the soldering, the expression “wettable material” refers to a material which, given a solder intended to be used in a following soldering step, can form an intermetallic compound with this given solder; therefore, a wettable material is a material which, upon contacting a solder paste containing this given solder and following a thermal treatment, forms an intermetallic compound with this given solder, thereby forming a solder joint. Hence, from a practical point of view, the expressions “wettable” and “non-wettable” refer to materials which, respectively, can and cannot be soldered by use of this given solder. 
         [0027]    That being stated, purely by way of non-limiting example, the solder may be the so-called SAC 305; consequently, the first and the second material may be brass and (organic) polymer, respectively. As a further example, the solder may be an alloy SnSb, in which case the first and second material may respectively be copper and aluminum; still as a further example, the solder may be an alloy SnPb, in which case the first and second material may respectively be nickel and ceramics. 
         [0028]    Afterwards, the cap array  20  is singulated, as an example by means of a punching step along cut lines L, so as to separate the caps  23  one from another; furthermore, as shown in  FIG. 6 , the singulation exposes, for each cap  23 , a side surface E of the respective core  24 , this side surface E not being coated by the coating layer  25 . 
         [0029]    In greater detail, each of caps  23  is such that, in top plan view, it may have, as an example, a squared shape or a rectangular shape or a circular shape. Furthermore, as shown in  FIG. 6  and in  FIG. 7  (wherein, for the sake of clarity, the coating layer  25  is not shown), each of the cores  24  has an upper wall UW extending parallel to a first direction x, and a side wall SW projecting from the outer profile of the upper wall UW, along a second direction y which is orthogonal to the first direction x. At an end of the side wall SW, each core  24  has a flange F; in particular, the flange F and the upper wall UW are arranged at opposite ends of the side wall SW. Furthermore, the flange F projects at least in part outwardly with respect to the outer profile of the side wall SW. Put in other words, the flange F is parallel to the first direction x and defines, together with the side wall SW, a foot of the core  24 ; furthermore, the flange F defines the above mentioned side surface E, which is parallel to the second direction y. 
         [0030]    Then, a support layer  32  is provided, as shown in  FIG. 8   a . In particular, the support layer  32  is equipped with a soldering pad  34  for each cap  23  to be soldered on the support layer  32  itself. For the sake of clarity, in  FIG. 8   a  only one soldering pad  34  is shown. In addition, as is shown in  FIG. 8   a , one or more electrical components  33 , such as a MEMS, may be arranged on the support layer  32 , in a per se known manner. 
         [0031]    In detail, the support layer  32  may be made up of a multi-layer structure, composed of several layers (not shown) of conductive material separated via dielectric layers (not shown); these latter layers may be formed of laminate (a material made of glass fibers and an organic polymer) or ceramic. The soldering pad  34  is formed by a third material, which is wettable and may be the same material as the first material of the core  24 ; furthermore, as shown in  FIG. 8   b , the soldering pad  34  has a closed shape, which corresponds to the shape of the flange F of the corresponding cap  23 . 
         [0032]    As shown in  FIG. 9 , a solder paste is then applied onto the soldering pad  34 , so as to form a solder paste bump  36 . The solder paste may be formed by a solder in the form of spheres suspended in a flux. To this regard, both the above mentioned first material and third material are wettable with respect to the solder of the solder paste bump  36 ; furthermore, the above mentioned second material is non-wettable with respect to the solder of the solder paste bump  36 . 
         [0033]    As shown in  FIG. 10 , each cap  23  is coupled to the support layer  32 ; in particular, each cap  23  is brought in contact with the corresponding solder paste bump  36 . For the sake of clarity,  FIG. 10 , as well as the following  FIG. 11 , shows only one cap  23 . The coupling may be performed so that the flange F of the cap  23  rests on the solder paste bump  36 ; in particular, the coupling may be performed so that the solder paste bump  36  fills at least in part a gap  40  defined by the soldering pad  34  and the cap  23 , and projects at least in part outwardly with respect to the side surface E, with which it is in direct contact. 
         [0034]    Afterwards, as shown in  FIG. 11 , a thermal treatment is carried out, based on the first, the second and the third material. During this thermal treatment, also known as “reflow”, the flux of the solder paste bump  36  cleans a possible contaminating layer (if any) laid on the side surface E; furthermore, the solder spheres melt and form a solder joint  42 , interposed between the side surface E and the soldering pad  34 . The solder joint  42  renders the cap  23  and the support layer  32  integral one to another. 
         [0035]    In detail, the side surface E of the core  24  is the only exposed wettable surface of the cap  23 , therefore it is the only surface of the core  24  which can directly contact the solder paste bump  36 . All the remaining surfaces of the core  24  are protected by the coating layer  25 , which is non-wettable. Thus, during the soldering process, the solder is prevented from flowing along wettable surfaces of the cap  23  other than the side surface E, thereby preventing the formation of voids within the solder joint  42 . In fact, because of the presence of the non-wettable coating layer  25 , the solder cannot project along the inner surface of the side wall SW. 
         [0036]    The advantages of the present soldering method emerge clearly from the foregoing description. In particular, the present method allows to form a sealing ring between a cap and a support layer, this sealing ring being void free. The sealing ring, and hence the package, is more resilient to mechanical shocks and to multiple reflows. 
         [0037]    Finally, it is clear that numerous variations and modifications may be made to the soldering method described and illustrated herein, all falling within the scope of the disclosure as defined in the attached claims. 
         [0038]    As an example, the above successions of steps are non-limiting, in the sense that the described operations may be carried out in an order different than the described one. 
         [0039]    Furthermore, it is also possible to apply the solder paste and form the solder paste bump on a support layer without any electrical component, namely on a support layer provided with the soldering pad(s) only. In this latter case, electrical components are assembled on the support layer after the formation of the solder paste bump. Afterwards, the cap is mechanically coupled with the support layer, and the solder joint is formed as previously described. 
         [0040]    Finally, it is possible to envisage additional steps, such as a step of applying a flux onto the cap, as an example before mechanically coupling the cap itself with the support layer. However, the use of flux is not necessary. 
         [0041]    The various embodiments described above can be combined to provide further embodiments. 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.