Patent Publication Number: US-2021166951-A1

Title: Method for creating a wettable surface for improved reliability in qfn packages

Description:
This application is a continuation of U.S. patent application Ser. No. 16/027,558, filed Jul. 5, 2018, the contents of which are herein incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to QFN package manufacturing techniques, and more particularly relates to a technique for creating a wettable surface for improved reliability in QFN packages. 
     BACKGROUND 
     Flat no-leads packages, such as quad-flat no-leads (QFN) packages, are a surface-mount package technology that connect integrated circuits (ICs) to surfaces of printed circuit boards (PCBs) without the use of through-holes to provide the electrical connections between the PCB and the package. A flat no-lead package refers to an encapsulated IC package made with a planar copper leadframe strip. The silicon IC die within a QFN package is electrically connected to the lead frame by wires, and this structure is encapsulated in a dielectric material such as plastic. A thermal pad is also typically included in the QFN package to assist in transferring heat. 
     Perimeter lands on the package bottom provide electrical connections from the copper lead frame to the PCB once the package is mounted on the PCB. These lands are end portions of the copper lead frame that remain exposed once the package is encapsulated. To mechanically and electrically mount QFN packages to a PCB, the exposed lands are electrically connected to contact pads on the PCB. Typically, this connection is provided by soldering QFN package lands to corresponding contact pads. Since the soldered connection between the QFN package lands and the PCB contact pads provide both the physical and electrical connections between the two, the Board Level Reliability (BLR), which is the quality and reliability of solder connections after mounting a package to a PCB, must be sufficiently high. For example, the typical thermal cycling condition required for adequate BLR is from −40° C. to +125° C. This is to ensure reliable package performance under extreme operating conditions. 
     One of the reliability issues that plagues QFN packages containing either printed leadframes or even traditional leadframes, is the solderability issues with exposed leadframes after the packages are singulated. More specifically, the exposed leadframes, which are the exposed edges of the leadframes used to connect singulated QFN packages to PCBs, typically have a non-wetting solderability issue due to oxidation of the exposed copper surfaces of the leadframe edges after package singulation. The exposed copper oxidizes and thus cannot be wetted by solder materials, and the wetting of the mounting edges or pads of the leadframes is directly related to BLR performance. Without proper wetting, the integrity of the soldered leadframe mounting pad to a PCB can mechanically, and thus electrically, breakdown during package use. 
     Accordingly, what is needed in the art is a technique for producing no-leads packages that have no exposed copper even after singulation of each package, and that does not suffer from the deficiencies of the prior art. The disclosed principles provides these techniques and other improvements. 
     SUMMARY 
     The disclosed principles provide for implementing low-cost and fast metallic printing processes into the QFN and other no-leads package assembly flow to selectively print solderable material in areas that would otherwise be susceptible to corrosion and thus pose reliability risks. The problem of copper corrosion and poor BLR performance in no-leads packages because of remaining exposed copper areas after package singulation is solved by employing selective metallic printing processes in the assembly flow to coat all risk-prone areas with solder material. 
     For example, for no-leads packages that are formed using printed leadframes, solder can be deposited through inkjet, screen, stencil, or photonic printing into the grooves which are formed after passivating the packages at the strip level. The singulating occurs through the grooves having solder printed therein, and results in wettable (solder covered) upper and sidewall surfaces of the outer ends of the leadframes for each package. 
     In one embodiment, such a method of creating a wettable surface on mounting pads of leadframes in no-leads packages may include providing a leadframe on an unsingulated strip, where the leadframe provides electrical interconnection for no-leads packages formed from the strip. Such a method may then include depositing a passivation layer over the leadframe of the unsingulated strip, the passivation layer having openings exposing outer ends of the leadframe. Then, the exemplary method may include depositing a solder material into the openings in the passivation layer to a height of the passivation layer, where the solder material is covering the exposed outer ends of the leadframe. Such a method may also include singulating the strip between the outer ends of the leadframe by cutting through the deposited solder material. After this singulation, the solder material on the covered outer ends of the leadframe provides mounting pads for the no-leads packages formed from the singulated strip, where the mounting pads have a wettable surface on both upper and side surfaces of the covered outer ends. 
     An alternative solution for no-leads packages having printed leadframes is that all exposed copper at what will be the outer ends of all the leadframes can be coated with printed solder during package formation to prevent there from being any exposed copper on the outside of the package. For example, a layer of copper may be printed to create the leadframe for a package. However, the outer ends of the leadframe, which will serve as the mounting pads for each package after singulation, are printed to at least their final thickness in copper. Then, in a second printing step with solder instead of copper, the remaining thickness of the outer ends of the leadframes is printed to its originally intended final thickness for the leadframe ends. Thus, a portion of the thickness of the leadframes at their outer edges (i.e., the mounting pads) is comprised of part copper and part printed solder rather than being all printed copper. In a more specific embodiments, the thickness of the leadframes at their outer edges is comprised of about half copper and about half printed solder, although other thickness proportions are also possible. 
     One embodiment of such a method for creating a wettable surface on mounting pads of leadframes in no-leads packages may include forming a plurality of unsingulated no-leads packages on a strip by forming a leadframe for electrical interconnection for such packages. In doing so, the leadframe is formed with outer ends providing a first thickness. The exemplary method may further include depositing a passivation layer over each of the leadframe of each no-leads package, where the passivation layer has openings exposing the outer ends of the leadframes. In addition, the method may then include depositing a solder material on the outer ends of the leadframes through the openings, where the deposited solder material is providing a second thickness. In these embodiments, the deposited solder material on the outer ends of the leadframes provide mounting pads for the no-leads packages having a wettable surface on both upper and side surfaces of the mounting pads. 
     For other no-leads packages having a leadframe formed with a copper deposition technique, plating conductive materials, such as tin or other metals, is used in a post-singulation process to coat exposed regions of copper from the singulated portions of a leadframe. A solution in accordance with the disclosed principles eliminates such a post-plating process, and instead again uses a printing process to print solder over exposed copper areas. For example, one embodiment of such a method of creating a wettable surface on mounting pads of the leadframe structure in such no-leads packages includes providing a leadframe on an unsingulated strip, where the leadframe provides for electrical interconnection of the no-leads packages formed from the strip. The method may then include singulating through only an initial portion of the strip between to create exposed sidewalls of outer ends of singulated portions of the leadframe. This exemplary method may also include depositing a solder material onto the exposed sidewalls of the singulated portions of the leadframe to cover the outer ends of the singulated portions, and then singulating through the remaining portion of the strip between the now-covered outer ends of the singulated portions of the leadframe. In this exemplary method, the solder material covers the outer ends of the singulated leadframe to provide mounting pads having a wettable surface for the singulated no-leads packages. 
     In yet another aspect, the disclosed principles provide a no-leads package. In one embodiment, the no-leads package may comprise a leadframe, and a passivation layer over the leadframe, where the passivation layer has openings exposing outer ends of the leadframe. The exemplary no-leads package may also comprise an ink residue including solder covering the exposed outer ends of the leadframe providing mounting pads for the no-leads package having a wettable surface on both upper and side surfaces of the covered outer ends. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features believed characteristic of the disclosure are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will be best understood by reference to the following detailed description of illustrative embodiments when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  illustrates is cross-sectional view of a no-leads package having a printed leadframe; 
         FIG. 2  illustrates a cross-sectional view of a no-leads package having a deposited leadframe; 
         FIGS. 3A-3D  illustrate a series of block diagrams showing one embodiment of a method of creating a wettable surface on the mounting pads of leadframes in no-leads packages, in accordance with the disclosed principles; 
         FIG. 4  illustrates a block diagram of a bottom view of a no-leads package having printed copper used to build up the outer ends of the leadframe for use as mounting pads for the package; 
         FIG. 4A  illustrates a cross-sectional view taken along line A-A of the no-leads package of  FIG. 4 ; 
         FIG. 5  illustrates a block diagram of a bottom view of no-leads package having printed copper used to create the leadframe and a solder material used to form mounting pads for the package, in accordance with the disclosed principles; 
         FIG. 5A  illustrates a cross-sectional view taken along line B-B of the no-leads package  500  of  FIG. 5 ; and 
         FIGS. 6A-6D  illustrate a series of block diagrams showing one embodiment of a method in accordance with the disclosed principles of creating a wettable surface on the mounting pads of no-leads packages where the leadframes are formed using copper deposition techniques. 
     
    
    
     DETAILED DESCRIPTION 
     The various embodiments of the presently disclosed subject matter are described with specificity to meet statutory requirements. However, the description itself is not intended to limit the scope of this patent. Rather, it has been contemplated that the claimed subject matter might also be embodied in other ways, to include different steps or elements similar to the ones described in this document, in conjunction with other present or future technologies. The components described hereinafter as making up various elements of the invention are intended to be illustrative and not restrictive. Many suitable components that would perform the same or similar functions as the components described herein are intended to be embraced within the scope of the invention. Such other components not described herein can include, but are not limited to, for example, similar components that are developed after development of the presently disclosed subject matter. 
     It should also be noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural references unless the context clearly dictates otherwise. References to a composition containing “a” constituent is intended to include other constituents in addition to the one named. Also, in describing the preferred embodiments, terminology will be resorted to for the sake of clarity. It is intended that each term contemplates its broadest meaning as understood by those skilled in the art and includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. 
     Also, the use of terms herein such as “having,” “has,” “including,” or “includes” are open-ended and are intended to have the same meaning as terms such as “comprising” or “comprises” and not preclude the presence of other structure, material, or acts. Similarly, though the use of terms such as “can” or “may” is intended to be open-ended and to reflect that structure, material, or acts are not necessary, the failure to use such terms is not intended to reflect that structure, material, or acts are essential. To the extent that structure, material, or acts are presently considered to be essential, they are identified as such. 
     It is also to be understood that the mention of one or more method steps does not preclude the presence of additional method steps or intervening method steps between those steps expressly identified. Moreover, although the term “step” may be used herein to connote different aspects of methods employed, the term should not be interpreted as implying any particular order among or between various steps herein disclosed unless and except when the order of individual steps is explicitly required. 
     Turning initially to  FIG. 1 , illustrated is a cross-sectional view of a no-leads package  100  having printed leadframes. The package  100  is comprised of an IC die  105  encapsulated with a dielectric mold compound  110 . To electrically connect the underlying IC die  105 , conductive vias or pillars  115  are formed through the mold compound  110  to contact the IC die  105 , and protrude above the mold compound  110  to contact leadframes  120 . The leadframes  120  are formed by printing copper on the mold compound  110  and in electrical contact with the conductive vias  115 , for example, via tin caps formed at the upper ends of the conductive vias  115 . It should be noted that the vias  115  may also be embodied as conductive pillars, solder bumps, and other types of interconnect structures, and no limitation to any particular structure is intended or should be implied. 
     A passivation layer  125  is formed over the leadframes  120  to form a dielectric barrier for the leadframes  120 . In many manufacturing techniques, the passivation layer  125  is also printed on the package  100 . When the passivation layer  125  is formed on the leadframes  120 , outer ends of select leadframes are left exposed. These exposed outer ends  130  are the portions of the leadframes  120  that soldered to a PCB to mount the package  100  onto that PCB. After manufacturing, the no-leads package  100  is singulated where the exposed outer ends  130  of the leadframes  120  are located. This singulation results in the exposed outer ends  130  forming the mounting pads of each singulated package  100 , with an upper and side surface of the exposed outer ends  130  providing the surfaces to be soldered during package  100  mounting. However, the upper and side surfaces of the exposed outer ends  130  comprise exposed copper, which then may oxidize as each singulated package  100  awaits mounting on a PCB and can result in the problems mentioned above. 
     One solution for the exposed upper and side surfaces of these outer ends  130  is to create a wettable flank having one or more wettable surface(s) on the outer ends  130  using a material that covers the exposed outer ends and helps increase the integrity of the solder joint between the wettable flank and bonding pads of a circuit board. The degree of wetting (i.e., the wettability of a wettable surface) is determined by a force balance between adhesive and cohesive forces, and is important in the bonding or adherence of two materials. For example, solder is deposited on a portion of the outer ends  130  to create the wettable flank. Unfortunately, the wettable flank may not cover all of the exposed copper of the outer ends  130 , and thus portions of the outer ends  130  may still oxidize and thus affect the integrity of the soldered package  130 . This is because the solder may does not adequately cover all of the exposed copper on the outer ends  130 . Alternatively, the solder material can be deposited over the exposed outer ends  130  prior to singulating each package  100 . However, this technique still results in sidewalls of the outer ends  130  having exposed copper once each package  100  is singulated from a strip. 
     Looking now at  FIG. 2 , illustrated is a cross-sectional view of another no-leads package  200  having a leadframe. This embodiment of a no-leads package  200  still includes an IC die  205  encapsulated with a dielectric mold compound  210 . However, this embodiment of a no-leads package  200  has a leadframe  215  formed no using a printing technique. The leadframe  215  is again formed of copper, for example using any desired copper deposition technique. 
     Formed around each cross-sectional view of the leadframe  215  is a metal plating  220 . For example, NiPdAu alloy may be used to create the plating  220  around the copper leadframe  215 . Once the circuitry, i.e., the wire bonded interconnects from each die  205  to the leadframe  215  structure, for each package  200  is formed, the no-leads packages  200  are singulated from a strip. The singulation occurs by cutting through leadframe  215 , which exposes copper on the sidewalls  225  of each singulated package  200 . These exposed sidewalls  225  serve as the mounting pads for soldering the singulated packages  200  to a PCB. However, as before, the exposed sidewalls  225  are comprised of copper, which again oxidizes as each singulated package  200  awaits mounting on a PCB and can still result in the problems mentioned above. One solution is to employ a post-mold plating process to plate exposed copper. However, this plating solution is expensive and still produces exposed copper after singulation because individual units must be connected during the plating process for the plating current to be active on the exposed surfaces for electrolytic plating. 
     Looking now at  FIGS. 3A-3D , illustrated is a series of block diagrams showing one embodiment of a method of creating a wettable surface on the mounting pads of leadframes in no-leads packages, in accordance with the disclosed principles.  FIG. 3A  illustrates a cross-sectional view of a molded leadframe strip  300  from which a plurality of no-leads packages will be formed. The strip  300  includes IC dies  305   a ,  305   b ,  305   c  encapsulated in mold compound  307 , and each die  305   a ,  305   b ,  305   c  is electrically connected to a leadframe  312  using one or more conductive pillars  309 . The strip  300  includes openings  310  exposing outer ends  315  of a the leadframe  312 , which in turn provides electrical interconnection to and through each no-leads package formed from the strip  300 . Specifically, a passivation layer  317  is deposited over the leadframe  312  of the strip  300 , which may be done using advantageous deposition techniques. In advantageous embodiments, the leadframe  312  is printed using any of a variety of available printing processes. Additionally, the passivation layer  317  may also be printed using any such printing process. As discussed above, the exposed outer ends  315  of the leadframe  312 , which are typically composed of copper, will oxidize through exposure via the openings  310  in the passivation layer  317 . 
     Turning to  FIG. 3B , illustrated is a cross-sectional view of the strip  300  at a later stage of a disclosed example process. At this stage in the process, solder  320  is deposited into the openings  310  between the passivation layer  317 . More specifically, the solder  320  is deposited to completely cover the exposed outer ends  315  of the leadframe  312 . By covering the previously exposed outer ends  315  of the leadframe  312 , the copper comprising the outer ends  315  will not oxide as the packages, once singulated from the strip  300 , sit before mounting to a PCB. Exemplary materials for the solder  320  include silver, tin, alloys thereof or any other material suitable for use as a solderable material. In preferred embodiments, the solder  320  is a printable tin ink; however, other printable metallic inks, such as gold, silver, tin alloys, or combinations thereof, may also be employed. 
     In one exemplary embodiment, the solder  320  is deposited using a printing process, for example, an inkjet, screen, stencil or photonic printing process. Of course, other printing processes, either now existing or later developed, may also be employed to deposit the solder  320  over the outer ends  315  of the leadframe  312 . Moreover, in advantageous embodiments, the solder  320  is printed or otherwise deposited to a height of the passivation layer  317 . As such, the solder  320  is used to fill the openings  310  between the passivation layer  317 , thereby completely covering the outer ends  315  of the leadframe  312 . In preferred embodiments, the solder  320  is deposited to at least a thickness of about 7 microns; however, other thicknesses may also be deposited. In addition, in some embodiments, exemplary processes further include sintering the deposited solder  320  prior to singulating the strip  300 . Such sintering may be performed at temperatures ranging from about 120° C.-250° C., but other temperatures may also be employed. 
     In  FIG. 3C , illustrated is a cross-sectional view of the strip  300  having the packages  305   a ,  305   b ,  305   c  at a further stage of the disclosed exemplary process. In particular, the strip  300  having the solder  320  filling the openings  310  undergoes a singulation process. In the singulation process, saws  330  or other cutting devices are employed to singulate the strip  300  to form each of the no-leads packages. The singulation occurs between the outer ends  315  of the leadframe  312  by cutting through the deposited solder  320 . It should be noted that the spacing between the outer ends  315  of the leadframe  312  is larger than a width of the saw or other cutting device used to singulate the strip  300 . 
     Looking now at  FIG. 3D , illustrated is a cross-sectional view of the no-leads packages  340   a ,  340   b ,  340   c  after singulation of the strip  300 . As such, after singulation the solder  320  on the outer ends  315  of the leadframe  312  provides mounting pads  335  for the no-leads packages  340   a ,  340   b ,  340   c . With the solder  320  on and between the outer ends  315 , the resulting mounting pads  335  have a wettable surface on both upper and side surfaces of the outer ends  315  of the leadframe for each package  340   a ,  340   b ,  340   c . Thus, as the packages  340   a ,  340   b ,  340   c  are awaiting mounting to respective PCBs, the outer ends  315  are no longer exposed since they are covered by the solder  320  on all exposed sides forming the mounting pads  335 . 
     Turning now to  FIG. 4 , illustrated is a block diagram of a bottom view of a no-leads package  400  having printed copper used to build up the outer ends of the leadframe for use as mounting pads for the package  400 . The package  400  includes a passivation layer  405  covering and thus dielectrically protecting most of the leadframe and other components of the package  400 . Along outer edges of the package  400  are mounting pads  410  separated by either the passivation layer  405  or dielectric mold compound  415  encapsulating the circuitry within the package  400 . 
       FIG. 4A  illustrates a cross-sectional view taken along line A-A of the no-leads package  400  of  FIG. 4 . This view of the package  400  shows the underlying IC die  420  encapsulated by the mold compound  415 . Also shown are conductive pillars  425  electrically connecting the underlying IC die  420  to the leadframe  412  formed closer to the outer surface of the package  400 . In addition to the leadframe  412  being printed, typically using copper, outer ends of the leadframe  412  are printed to a greater thickness above the passivation layer  405  to form copper mounting pads  410  for use in mounting and electrically connecting the package  400  to a PCB. However, as discussed above with regard to other no-leads packages, the printed copper mounting pads  410  comprise a completely exposed upper surface which will oxidize as the package  400  is awaiting mounting to a PCB. 
     Turning now to  FIG. 5 , illustrated is a block diagram of a bottom view of a no-leads package  500  having printed copper used to create the leadframe, but further having a solder material to form mounting pads for the package  500 . More specifically, the package  500  again includes a passivation layer  505  covering and dielectrically protecting most of the leadframe and other components of the package  500 . However, along outer edges of the package  500  are mounting pads  510  formed in accordance with the disclosed principles using a solder material. These mounting pads  510  are again separated by either the passivation layer  505  or dielectric mold compound  515  encapsulating the circuitry within the package  500 . 
       FIG. 5A  illustrates a cross-sectional view taken along line B-B of the no-leads package  500  of  FIG. 5 . This view of the package  500  again shows the underlying IC die  520  encapsulated by the mold compound  515 . In exemplary embodiments, the leadframe  512  is printed, typically using copper, as discussed above. However, in accordance with the disclosed principles, the outer ends of the leadframe  512  are not printed to a greater thickness above the passivation layer  505  using copper, as is found in other packages. 
     Instead, another embodiment of the disclosed principles includes printing on the outer ends of the leadframe  512  with solder to form the mounting pads  510 . As shown, the mounting pads  510  may be printed to a height extending above the passivation layer  505  so that the mounting pads  510  may be used in mounting and electrically connecting the package  500  to a PCB. By forming the mounting pads  510  by printing solder, which again may be by inkjet, screen, stencil and photonic printing, the outer ends of the copper leadframe  512  are covered so as to prevent their oxidation. In preferred embodiments, the solder is comprised of a printed silver ink; however, other types of metals that may be used as a solderable material maybe employed. In addition, in some embodiments, after printing the mounting pads  510  exemplary embodiments of the disclosed process may again include sintering, again at temperatures ranging from about at 120° C.-250° C., the deposited solder material. 
     In yet another embodiment, the outer ends of the leadframe  512  are printed with copper to a lesser thickness than their designed final thickness. Then a process in accordance with the disclosed principles may be employed to finish printing the remaining thickness of the outer ends of the leadframe  512  with solder. In such embodiments, the outer ends of the leadframe  512  would have a step-down in thickness compared to the other portions of the leadframe  512 , with the printed solder providing the remainder of the originally intended thickness of the outer ends, as well as providing mounting pads  510  having a wettable upper and side surface. 
     In either of these embodiments, the printed mounting pads  510  provide both upper and sidewall wettable surfaces to be used to solder the package  500  to a PCB. More specifically, although some sidewall surface of the ends of the leadframe  512  may still be exposed in these embodiments, those exposed copper surfaces would be used for soldering the package  500  to a PCB. Instead, only the upper and sidewall surfaces of the mounting pads  510  printed on the outer ends of the leadframe  512  would provide the wettable surfaces used to solder the package  510  to a PCB. As such, a wettable surface is provided on both the upper and sidewall surfaces used to mount the package  510 . 
     Referring now to  FIGS. 6A-6D , illustrated is a series of block diagrams showing one embodiment of a method in accordance with the disclosed principles of creating a wettable surface on the mounting pads of no-leads packages where the leadframe for each package is not formed using a printing technique. In particular, rather than the leadframes being printed as found in embodiments discussed above, the leadframes in this exemplary process are formed using copper deposition techniques. For example, a copper sputtering or other deposition technique for forming the leadframes may be used. 
       FIG. 6A  illustrates a cross-sectional view of a strip  600  having a plurality of IC dies  605  encapsulated in mold compound  610 . Included with the IC dies  605  is a leadframe  615  throughout the strip  600  and which is formed of copper, for providing electrical connection to the IC dies  605 . In this embodiment of a disclosed process, a step-cut is performed to singulate through an initial portion of the strip  600 , as well as a portion of the leadframe  615  located between what will be the individual packages. This step-cut made through the intervening leadframe  615  creates openings  620  through which outer ends  625  of the singulated portions of the leadframe  615  are now exposed. As before, by being exposed the outer ends  625  of the copper leadframe  615  will oxidize prior to their use in mounting the packages to respective PCBs. 
       FIG. 6B  illustrates a cross-sectional view of the strip  600  during a later stage of this exemplary process of creating a wettable surface on the mounting pads of the eventual packages. At this stage, solder  630  is deposited into the opening  620 . More specifically, the solder  630  is deposited so as to ensure coverage of the sidewalls of the opening  620  to completely cover the exposed outer ends  625  of the singulated portions of the leadframe  615 . As before, by covering the previously exposed outer ends  625  of the leadframe  615 , the copper comprising the outer ends  625  will not oxide as the packages await mounting to a PCB. Exemplary materials for the solder  630  may again include silver, tin, alloys thereof or any other material suitable for use as a solderable material. Also, in preferred embodiments, the solder  630  is a printable tin ink; however, other printable metallic inks may also be employed. Moreover, the solder  630  may be deposited using a printing process, for example, an inkjet, screen, stencil or photonic printing process. Of course, other printing processes, either now existing or later developed, may also be employed to deposit the solder  630  over the outer ends  625  of the singulated portions of the leadframe  615 . Notably, in this embodiment of the disclosed principles, the solder  630  is not required to completely fill the openings  620  between packages, and instead is only deposited so as to ensure coverage of any of the exposed copper on the outer ends  625 . For example, the solder  630  may be deposited to at least a thickness of about 7 microns; however, other thicknesses may also be deposited. In addition, exemplary processes may again include sintering the deposited solder  630  after deposition, as discussed above and prior to singulation. Alternatively, the solder  630  may be printed so as to completely fill the opening  620 , if desired. 
       FIG. 6C  illustrates a cross-sectional view of the strip  600  at a further stage of the disclosed exemplary process. In particular, the strip  600  having the solder  630  placed on the sidewalls of the openings  620  forms the mounting pads  635  for the eventual packages. In order to singulate the packages, saws  640  or other cutting devices are employed to singulate each of the packages from one another. The singulation occurs between the mounting pads  635  formed by the solder  630  on the outer ends  625  of the leadframe  615  structure by singulating through a remaining portion of the mold compound  610  and other portions of the strip  600  between the covered outer ends  625 . In this embodiment, the spacing between the sidewall mounting pads  635  on the outer ends  625  may be equal to or smaller than a width of the saw  640  or other cutting device used to singulate the strip  600  such that a smooth vertical surface comprising the mounting pads  635  and inner surfaces of the molding compound  610  is formed during singulation, as shown in  FIG. 6D . 
     Looking now at  FIG. 6D , illustrated is a cross-sectional view of the strip  600  once the packages  640   a ,  640   b  have been singulated from one another. As such, after singulation the solder  630  on the outer ends  625  of the singulated portions of the leadframe  615  provide the mounting pads  635  for the no-leads packages  640   a ,  640   b . With the solder  630  completely covering the sidewalls at the outer ends  625 , the resulting mounting pads  635  have a wettable surface on the outer ends  625  for connection to the PCB. Thus, as the packages  640   a ,  640   b  are awaiting mounting to respective PCBs, the outer ends  625  are no longer exposed since they are covered by the solder  630  on the previously exposed surfaces to form the mounting pads  635 . 
     In sum, the disclosed principles provide various methods for implementing low-cost and fast metallic printing processes into the QFN and other no-leads package assembly flow to selectively print solderable material in areas that would otherwise be susceptible to corrosion and thus pose reliability risks. The problem of copper corrosion and poor BLR performance in no-leads packages because of remaining exposed copper areas after package singulation is solved by employing selective metallic printing processes in the assembly flow to coat all risk-prone areas with solder material. The process of printing solder material in any of the techniques disclosed herein is relatively low in cost and faster than plating processes currently used in the industry, and thus eliminates the need of lengthy regulated plating lines and replaces the post-plating equipment associated with plating processes. The disclosed solder printing methods address the issue of non-wetting solderability with the exposed copper surface on both printed copper leadframes and on traditionally produced leadframes. For example, no-leads packages containing printed copper leadframes having exposed copper surfaces on the outer ends of the leadframes will thus have wettable surfaces on those previously exposed and vulnerable surfaces. The wettable surfaces provided as disclosed herein are provided on each surface used to mount packages on a PCB, which thereby increases BLR performance. Moreover, for packages having printed leadframes, the previously employed post-mold plating process may be eliminated, which is typically both a costly and time consuming process. And for packages with traditionally formed leadframes, the need for plating exposed sidewalls of the outer ends of leadframes typically performed after a step-cut is also eliminated with a simpler, faster printing solution. 
     While this invention has been particularly shown and described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context. 
     While various embodiments in accordance with the principles disclosed herein have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of this disclosure should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with any claims and their equivalents issuing from this disclosure. Furthermore, the above advantages and features are provided in described embodiments, but shall not limit the application of such issued claims to processes and structures accomplishing any or all of the above advantages. 
     Additionally, the section headings herein are provided for consistency with the suggestions under 37 C.F.R. 1.77 or otherwise to provide organizational cues. These headings shall not limit or characterize the invention(s) set out in any claims that may issue from this disclosure. Specifically, and by way of example, although the headings refer to a “Technical Field,” the claims should not be limited by the language chosen under this heading to describe the so-called field. Further, a description of a technology as background information is not to be construed as an admission that certain technology is prior art to any embodiment(s) in this disclosure. Neither is the “Brief Summary of the Invention” to be considered as a characterization of the embodiment(s) set forth in issued claims. Furthermore, any reference in this disclosure to “invention” in the singular should not be used to argue that there is only a single point of novelty in this disclosure. Multiple embodiments may be set forth according to the limitations of the multiple claims issuing from this disclosure, and such claims accordingly define the embodiment(s), and their equivalents, that are protected thereby. In all instances, the scope of such claims shall be considered on their own merits in light of this disclosure, but should not be constrained by the headings set forth herein.