Patent Publication Number: US-2015076675-A1

Title: Leadframe package with wettable sides and method of manufacturing same

Description:
BACKGROUND 
     1. Technical Field 
     Embodiments of the present disclosure are directed to leadframe strips and leadless packages, as well as methods of manufacturing leadframe strips and assembling leadless packages. 
     2. Description of the Related Art 
     Leadless (or no lead) packages are often utilized in applications in which small sized packages are desired. In general, flat leadless packages provide a near chip scale encapsulated package that includes a planar leadframe. Lands (also referred to as leads) located on a bottom surface of the package and, in many cases, side surfaces of the package provide electrical connection to a board, such as a printed circuit board (PCB). In that regard, the packages are mounted directly on the surface of the PCB using surface mount technology (SMT). 
     Although SMT allows for smaller packages, it also creates some disadvantages. In particular, the solder joints between the package and the PCB can be weakened due to the PCB and the package having different coefficients of thermal expansions (CTE). Thus, the reliability of the package may in some cases depend on the integrity of the solder joints. 
     As packages reduce in size, the available space for solder joints is further limited. Thus, strong solder bonds between the lands of the package and the pads of the board are desired. 
     BRIEF SUMMARY 
     Embodiments of the present disclosure are directed to leadframe packages with wettable sides and methods of manufacturing same. In one embodiment, the leads of the leadframe packages have outer surfaces with recesses formed therein. The recesses have a curved profile and are plated with a wettable layer of conductive material that enables solder to flow along the outer surface during surface mounting of the package to a board, such as a PCB. This enables strong solder joints between the leads of the package and the board. The curved profile allows for the solder to flow into and fill the recess, thereby strengthening the bond. 
     Other embodiments are directed to a leadframe strip that is used for forming the leadframe packages described herein and methods of manufacturing same. In one embodiment, the leadframe strip reduces or eliminates molding flash that may migrate onto the outer surface of the leads of a leadframe package during a molding step of manufacturing leadframe packages. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1A  is a cross-sectional view of a leadframe package made in accordance with one embodiment of the present disclosure. 
         FIG. 1B  is a side view of the leadframe package of  FIG. 1A . 
         FIGS. 2A-2M  illustrate side views of a portion of a conductive foil that is formed into a leadframe strip at various stages of manufacturing in accordance with one embodiment of the present disclosure. 
         FIG. 2N  illustrates a top view of the leadframe strip of  FIG. 2M . 
         FIG. 2O  illustrates a bottom view of the leadframe strip of  FIG. 2M . 
         FIGS. 3A-3F  illustrate cross-sectional views of various stages of assembly of leadframe packages, such as the leadframe package of  FIG. 1 , in accordance with an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1A  shows a cross-sectional view of a leadframe package  10  made in accordance with one embodiment of the disclosure.  FIG. 1B  is a side view of the leadframe package  10 . The package  10  includes a die pad  12  having upper and lower surfaces  14 ,  16 . The package  10  further includes first and second leads  18 ,  20 , each having upper and lower surfaces  22 ,  24 . The first lead  18  is located proximate a first side of the die pad  12 , and the second lead  20  located proximate a second side of the die pad  12 . The lower surfaces  24  of the leads  18 ,  20  are also referred to as lands of the package  10 . 
     It is to be appreciated that any number of leads may be located proximate any number of sides of the die pad, including only one lead located proximate one side of the die pad. 
     An outer surface  17  of the first and second leads  18 ,  20  have a recess  21  formed therein. The recess  21  in the illustrated embodiment extends more than halfway through the thickness of the leads  18 ,  20 . In other embodiments, however, the recess may extend approximately halfway through the thickness of the leads or less than halfway through the thickness of the leads. At least a portion of the recess  21  has a curved radius. The recess extends across the entire width of each lead  18 ,  20  as best shown in  FIG. 1B . 
     As will be explained in more detail below, the curved radius of the recess  21  of the leads  18 ,  20  allows for a strong solder joint between the package  10  and a board (not shown), such as printed circuit board. The recess  21  forms a wettable surface that enables solder to flow along the surface during surface mounting of the package  10 . In particular, during surface mounting of the package, solder flows up into the curvature of the recess and fills the curvature, creating a strong bond during surface mount. In that regard, the radius of curvature of the recess  21  may be any radius that allows solder to flow in and fill the curvature. 
     Upper and lower surfaces  14 ,  16  of the die pad  12  may be plated with a conductive layer  30   b.  Additionally, at least portions of the upper and lower surfaces  22 ,  24  of the first and second leads  18 ,  20  and the recesses  21  therein are plated with the conductive layer  30   a.  In the illustrated embodiment, the lower surface  24  of the first and second leads  18 ,  20  and the recesses  21  are plated with the conductive layer  30   a.  A portion of the upper surfaces  22  of the first and second leads  18 ,  20  are plated with the conductive layer  30   a,  while an outer portion of the upper surfaces  22  remains unplated. 
     The conductive layers  30   a  and  30   b  may be a nanolayer or microlayer of one or more conductive materials. For instance, the upper and lower surfaces  14 ,  16  of the die pad  12  and the upper and lower surfaces  22 ,  24  of the leads  18 ,  20  may be plated with one or more metal materials, such as Ni/Pd/Ag, Ni/Pd/Au—Ag alloy, or Ni/Pd/Au/Ag. As will be explained below, the conductive layer  30  may be used in some embodiments as a mask layer for etching portions of the leadframe material during assembly. 
     A semiconductor die  32  that includes an electrical device, such as an integrated circuit, is secured to the conductive layer  30   b  over the upper surface  14  of the die pad  12  by an adhesive material  34 . The adhesive material  34  may be any material configured to secure the die  32  to the die pad  12 , such as glue, paste, tape, and the like. 
     Conductive wires  36  electrically couple the die  32  to the first and second leads  18 ,  20 . For instance, a first end  38  of the conductive wire  36  is coupled to a bond pad  40  of the die  32  and a second end  42  of the conductive wire  36  is coupled to the first lead  18 . 
     Encapsulation material  44  is located over the die pad  12  and the first and second leads  18 ,  20  enclosing the die  32  and the conductive wires  36 . The encapsulation material  44  is also located between the first and second leads  18 ,  20  and the die pad  12  and forms a bottom surface  45  therebetween. The outer surface  17  of the first and second leads  18 ,  20  and the recesses  21  form outer side surfaces of the package  10  along with the encapsulation material  44 . 
     As illustrated in  FIG. 1A , the die pad  12  has curved edges proximate the bottom surface  45  of the encapsulation material  44 . Similarly, inner surfaces  39  of the leads  18 ,  20  have curved surfaces proximate the bottom surface  45  of the encapsulation material  44 . 
       FIGS. 2A-2M  illustrate side views of a portion of a conductive foil  52  that is formed into a leadframe strip  50  ( FIG. 2M ) at various stages of manufacturing in accordance with an embodiment of the present disclosure. 
       FIG. 2A  shows a conductive foil  52  having first and second surfaces  56 ,  58  that is the base material for forming the leadframe strip  50 . The conductive foil  52  may be a metal material and in some embodiments is made of copper or a copper alloy. 
     As shown in  FIG. 2B , a light sensitive material  54 , such as photoresist, is deposited on the first and second surfaces  56 ,  58  of the conductive strip  50 . As shown in  FIGS. 2C and 2D , portions of the light sensitive material  54  are patterned to form a mask layer. That is, portions of the light sensitive material  54  may be exposed to ultraviolet radiation  61  and then removed by a photoresist developer. In particular and as shown in  FIG. 2D , exposed portions of the light sensitive material  54  are removed, leaving exposed portions  60  of the conductive foil  52  on the first and second surfaces  56 ,  58 . Although the figures illustrate a positive photoresist, the photoresist may be a negative photoresist. That is, the photoresist that is exposed to ultraviolet radiation  61  becomes insoluble to the photoresist developer and remains on the surface. 
     As shown in  FIG. 2E , a conductive layer  30  is formed, such as by plating techniques, on the exposed portions  60  of the conductive foil  52 . The conductive layer  30  may include one or more conductive materials that are different materials from the conductive foil  52 . The conductive material may be any material that encourages solder to flow along the surface during surface mount. As indicated above, the conductive layer  32  may be one or more materials, such as Ni/Pd/Ag, Ni/Pd/Au—Ag alloy, or Ni/Pd/Au/Ag. 
     As shown in  FIG. 2F , the light sensitive material  54  is then removed from the first and second surfaces  56 ,  58 , such as by conventional etching techniques, exposing the upper and lower surfaces of the conductive foil. The conductive layer  30  remains on the first and second surfaces  56 ,  58 . As will be further explained below, the location at which the conductive layer  30  is formed on the conductive foil  52  corresponds to locations of the die pads  12  and first and second leads  18 ,  20  of the packages to be formed in the conductive foil  52 . As shown in  FIGS. 2G and 2H , another light sensitive material  64  is deposited and patterned on the conductive layer  30  on the first and second surfaces  56 ,  58  of the conductive foil  52 . The light sensitive material  64  may be the same type of material as light sensitive material  54 , such as photoresist, and may be positive or negative photoresist. In the illustrated embodiment, the light sensitive material  64  is patterned using known techniques, such as by radiation  61  and developer, as indicated in  FIG. 2G , to produce patterned layers as shown in  FIG. 2H . 
       FIG. 2H  shows that the light sensitive material  64  remains located over the conductive layer  30  on both the first and second surfaces  56 ,  58  of the conductive foil  52 . Additionally, the light sensitive material  64  is further located over an upper connecting bar  59  of the first surface  56  of the conductive foil  52 . The light sensitive material  64  forms a mask layer for a subsequent etching step, such as an isotropic or anisotropic etch, that forms portions of the die pads  12  and leads  18 ,  20  as shown in  FIG. 2I . 
       FIG. 2I  shows the conductive foil  52  after an isotropic etch process. In one embodiment, the conductive foil  52  is etched by immersion in a bath of etchant and in some cases includes agitation techniques. In the bath, the conductive foil  52  is etched from the first surface  56  and from the second surface  58  of the conductive foil  52 . 
     The etching of the conductive foil  52  forms a plurality of first recesses  53  in the first surface  56  and a plurality of second recesses  55  in the second surface  58 . The first and second recesses  53 ,  55  have a curved profile that is formed during the etching step. After the etching step as shown in  FIG. 2J , the light sensitive material  64  is removed from the upper and lower surfaces  14 ,  16  of the conductive foil  52 . 
     Each of the first recesses  53  in the first surface  56  delimits a portion of a die pad  12  on one side and a portion of a lead  18  or  20  on the other side. The first recesses  53  form lower connection bars  57  that connect the leads  18 ,  20  to adjacent die pads  12 . The first recesses  53  are optional. That is, in some embodiments, the first recesses  53  are not formed in the first surface  56 . Although not shown in the cross-sectional view, leads that are located proximate the leads shown are connected together by lower connection bars as will be shown and described below in reference to  FIG. 2O . 
     The second recesses  55  in the second surface  58  are formed to partially separate the first lead  18  from the second lead  20  and form an upper connecting bar  59  that connects the adjacent leads  18 ,  20  to each other. As best shown in  FIG. 2J , the conductive layer  30  is presented by conductive layer  30   b  and  30   a.  It is to be appreciated that conductive layer  30   b  is on surfaces of die pads  12  and conductive layer  30   a  is on surfaces of leads  18  and  20 . 
     In an alternative embodiment, one or both of the upper and lower surfaces  56 ,  58  of the conductive foil  52  are plated with the conductive layer  30  at locations which contain the first recesses  53  and the second recesses  55  along with the other portions of the upper and lower surfaces  56 ,  58  as illustrated in  FIG. 2F . Then, during the etch step of  FIG. 2I , the etch chemistry etches through the conductive layer  30  and the conductive foil  52 . This embodiment reduces the possibility of misalignment of the light sensitive material  64  that is patterned on the conductive layer  30 . In that regard, the location of the first and second recesses  53  and  55  are established in one step rather than two separate steps. 
     As indicated in  FIG. 2K , another light sensitive material  74  is deposited and patterned on the conductive layers  30   a  and  30   b  on the first and second surfaces  56 ,  58  of the conductive foil  52  using the techniques mentioned above. The light sensitive material  74  may be the same type of material as light sensitive materials  54  and  64 , such as photoresist, and may be positive or negative photoresist. The light sensitive material  74  is deposited in the first recesses in the first surface  56 . The light sensitive material  74 , however, is not deposited in the second recesses  55  in the second surface  58 . As shown in  FIG. 2L , the second recesses  55  in the second surface  58  are plated with conductive layer  30 . 
     As shown in  FIG. 2M , the light sensitive material  74  is removed from the first and second surfaces  56 ,  58  thereby forming a leadframe strip  50  for assembling a plurality of packages. The leadframe strip  50  includes a plurality of package frames  66  that each includes a die pad  12  connected to at least one first lead  18  and at least one second lead  20  by lower connecting bars  57 . Leads that are associated with adjacent package frames  66  of the plurality of package frames are connected by the upper connecting bar  59 . For instance and as shown in  FIG. 2M , a first lead  18  is associated with a first die pad  12  of a first package frame  66 . A second lead  20 ′ is associated with a second die pad  12 ′ of a second package frame  66 ′. The first lead  18  of the first package frame  66  is coupled to the second lead  20 ′ of the second package frame  66 ′ by the upper connecting bar  59 . 
     As shown in  FIG. 2M , portions of the leadframe strip  50  remain unplated. In particular, the portions of the leadframe strip  50  that remain unplated include upper and lower surfaces of the lower connecting bars  57  and the upper surface of the upper connecting bar  59 . As will be explained below, the plated layer  30  forms a mask layer of the leadframe strip  50  during assembly. 
       FIG. 2N  shows a top view of the conductive strip  50 . The gray shading portion indicates the recesses  53  formed in the upper surface  56  of the conductive strip to form the lower connecting bars  57 . That is, the gray shading portion illustrates the portions that are recessed relative to the first surface  56  of the die pads  12  and the leads  18 ,  20  of the conductive strip  50 . Adjacent leads  18  and  18  that are proximate the same die pad  12  are connected to one another by lower connecting bars  57 . The cross hatching of the upper connecting bar  59  indicates that the upper connecting bar  59  is in a different plane than the conductive plated die pads  12  and leads  18 ,  20  because upper connecting bar  59  is not plated with the conductive layer  30   a.    
     Although seven leads are associated with each side of the die pad  12  for each package frame in the illustrated embodiment, it is to be appreciated that any number of leads may be associated with any number of sides of the die pad. For instance, in one embodiment only one lead may associated with a single side of the die pad. In another embodiment, two leads may be associated with one or more sides of the die pad. 
       FIG. 2O  shows a bottom view of the conductive strip  50 . The gray shading portion indicates the recesses  55  formed in the second surface  58  of the conductive strip  50  to form the upper connecting bars  59 . In particular, the gray shading portion illustrates the portions that are recessed relative to the other portions of second surface of the conductive strip  50 . 
     The leadframe strip  50  as shown in  FIGS. 2M ,  2 N, and  2 O may be used to assemble leadframe packages, such as the leadframe package  10  of  FIG. 1 , as will be explained below. 
       FIGS. 3A-3F  illustrate cross-sectional views of various stages of assembly of leadframe packages, such as the package  10  of  FIGS. 1A and 1B , in accordance with an embodiment of the present disclosure. As shown in  FIG. 3A , the assembly process begins with a leadframe strip, such as the leadframe strip  50  of  FIGS. 2M ,  2 N,  2 O. 
     In general, the leadframe strip  50  may be suitably rigid such that a supporting structure such as tape may not be used. This is due in part to the lower connecting bars  57  and the upper connecting bars  59  adding rigidity to the strip. It is to be appreciated, however, that in some embodiments the assembly process may include using a supporting structure during at least a portion of the assembly process. 
       FIG. 3B  shows that semiconductor dice  32  are placed over the upper surface  14  of the die pads  12  of the leadframe strip  50 . The semiconductor dice  12  may be secured to the die pads  12  by adhesive material  34 , such as tape, paste, glue, or any material that suitably adheres the die to the die pad. The semiconductor die  32  may include an electrical device, such as an integrated circuit. 
     As shown in  FIG. 3C , each die  32  are electrically coupled to a first lead  18  and a second lead  20 . In the illustrated embodiment, a first end of a first conductive wire  36  is coupled to a bond pad of the die  32  and a second end of the conductive wire  36  is coupled to the first lead  18 . A first end of a second conductive wire  36  is coupled to a bond pad of the die  32  and a second end of the conductive wire  36  is coupled to the second lead  20 . 
     Although not shown, the dice  32  may be electrically coupled to the lead sets, such as by flip chip arrangement, as is well known in the art. That is, each die would be larger than shown in  FIGS. 3B-3F  so that the outer perimeter of each die would be located on the upper surface of adjacent leads. In a flip chip arrangement, solder balls located between the die and the lead would provide electrical communication therebetween. In this arrangement, the leads may provide electrical and mechanical support for the die. In these embodiments, the leadframe strip may not include die pads and thus, the lower connecting bars  57  would couple adjacent leads within a package frame rather than couple leads to the die pad. 
     As shown in  FIG. 3D , encapsulation material  44  is formed over the upper surfaces of the leadframe strip  50  so that the encapsulation material  44  surrounds the die  32 , the conductive wires  36 , and upper surface of the leadframe strip  50 . The encapsulation material  44  is also provided in the recesses  53  in the upper surface and thus extends along a portion of the side surfaces of the die pad  12  and the leads  18 ,  20 . The encapsulation material  44  is an insulative material that protects the electrical components and materials from damage, such as corrosion, physical damage, moisture damage, or other causes of damage to electrical devices and materials. In one embodiment, the encapsulation material  44  is a polymer. 
     The encapsulation material  44  may be formed on the leadframe strip  50  by conventional techniques, for example by a molding process, and in some embodiments is hardened during a curing step. The lower connecting bars  57  and the upper connecting bars  59  are able to prevent or at least reduce mold flash. That is, due to the lower connecting bars  57  and the upper connecting bars  59 , the encapsulation material  44  does not readily flow between the second surface  58  of the leadframe strip  50  and an inner surface of the mold (not shown). 
     It is to be appreciated that in the prior art, there are openings between the die pad and the leads that can cause molding flash. In that regard, during the molding process, encapsulation material can in some cases flow through the opening and between the inner surface of the mold and the second surface of the leadframe strip, including onto the bottom surface of the leads or lands. The encapsulation material is an insulative material and thus reduces the amount of surface area of the lead that can be used for making electrical connection with a board during surface mount. Thus, by reducing molding flash by one or more embodiments of the present invention, the solder joint bond at surface mount is strengthened. 
     As shown in  FIG. 3E , the lower connecting bars  57  are etched away using conventional etching techniques, such as those described above. In some embodiments, the conductive layer  30  forms a mask layer for removing the lower connecting bars  57 . In another embodiment, a light sensitive material (not shown) may be deposited over the conductive layer  30  or a bottom surface of leadframe to form a mask layer. In such an embodiment, the conductive layer  30  or the bottom surface of the leadframe would not need to be resistant to the etch chemistries. 
     A lower surface of the encapsulation material  44  forms an etch stop for etching the lower connecting bars  57 . After the etch step, the die pads  12  are separated from the leads  18 ,  20  and adjacent leads within the same package frame are separated from each other. The removal of the lower connecting bars  57  electrically isolates the die pads from the leads and adjacent leads from each other, while at the same time maintaining mechanical connection to each other by the encapsulation material  44 . As shown in  FIG. 3E , the upper connecting bars  59  are not separated during the etch step. 
     The manufacturing process further includes separating each package into individual packages  10  as shown in  FIG. 3F . In particular, the packages  10  are separated through a portion of the upper connecting bars  59  and the encapsulation material  44  located above the portion of the upper connecting bars  59 . 
     The packages  10  can be separated by various dicing methods, including saw and laser. The saw blade or laser used for separating the packages  10  has a cutting width that is less than a width of the upper connecting bars  59  such that a recess is formed in the outer edge  17  of the leads  18 ,  20  ( FIG. 1A ). As discussed above, the recesses in the leads have a curved profile and provide wettable surfaces for solder to flow across and fill for increased solder joint strength. 
     By forming lower and upper connecting bars between die pads and leads and between adjacent leads, various benefits are obtained. In particular, the solder joint between the package and a board may be strengthened. As discussed above in or more embodiments, by having upper and lower connecting bars during the encapsulation step, mold flash is reduced or prevented. That is, because the conductive strip does not include openings proximate the leads, the encapsulation material is prevented from flowing between the bottom surface of the leads and the inner surface of the mold. By preventing mold flash, the electrical connection during surface mount is significantly improved. 
     In addition, the upper connecting bars result in the formation of recesses with curved profiles in the outer surface of the leads. These curved profile leads provide improved mechanical support of solder joints during surface mount. Furthermore, the curved profiles readily allow solder to flow therein and fill the curved profile, resulting in a stronger bond during surface mount. In contrast, if the recess was formed to have a right angle corner rather than a curved profile, solder would often fail to fill the corner, resulting in a weaker bond during surface mount. 
     In addition, the leadframe strip is suitably stiff for assembly process due to the upper and lower connecting bars such that a temporary supporting element is not needed. 
     The recess  21  also allows for a visual inspection during surface mount. That is, due to the solder flowing up the outer surface of the lead in the recess  21 , the solder bond is visible along the side surface of the package. In that regard, an X-ray may not be needed to confirm proper solder attachment during surface mount, thereby increasing throughput during surface mount. 
     Moreover, by making the recess in the conductive strip prior to assembling the packages, the singulation step is more efficient and less costly. That is, cutting through encapsulation material is substantially easier than cutting through conductive material, such as copper. Thus, by sawing through the encapsulation material  44  and cutting a relatively small portion of the conductive strip, the sawing speed may be increased, thereby increasing throughput through the sawing tools. In addition, the blade life of the saw blades used to cut the packages into individual packages will increase. 
     Typically, when sawing through connected leads of a leadframe strip, saw burrs can form on the outer edge of the lead. Thus, by having the leads attached at the bottom surface, saw burrs can affect the solder joints during surface mount. By cutting through the upper connecting bar, which is raised relative to the bottom surface of the lead, saw burs are substantially eliminated or significantly reduced. 
     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. 
     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.