Patent Publication Number: US-2017352609-A1

Title: Lead frame with solder sidewalls

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of Non-provisional patent application Ser. No. 15/075,298, filed Mar. 21, 2016, the entirety of which is hereby incorporated herein by reference. 
    
    
     FIELD 
     This disclosure relates to the field of integrated circuits. More particularly, this disclosure relates to lead frames used in integrated circuit packaging. 
     BACKGROUND 
     Semiconductor Small Outline No-Lead (SON) and Quad Flat No-Lead (QFN) devices are typically fabricated by assembling a plurality of integrated circuit (IC) chips on a metallic leadframe strip. The leadframe strip  200  ( FIG. 2 ) is laid out to include for each lead frame  100  ( FIG. 1 ) an IC chip pad  102  ( FIG. 1 ) and coordinated wirebond pads  104 . In order to miniaturize the devices and conserve area in the layout of the leadframe strip  200 , the layout is commonly designed so that the bondwire pads  104  of one leadframe  100  are connected directly to the respective wirebond pads  104  of the adjacent leadframe  100  by horizontal  202  and vertical  204  saw streets. 
     The majority of leadframe strips  200  are made of a base metal such as copper or an alloy including copper, and plated with layers of solderable metal, such as a layer of nickel followed by a layer of palladium. 
     The cross section illustrated in  FIG. 3  is taken across a horizontal saw street  204  along the dashed line  206  in  FIG. 2 . Multiple IC chips  304  are assembled on the leadframe strip  200 . After the IC chips  304  are attached to the IC chip pads  102  and electrically connected to the wirebond pads  104  with wirebonds  306 , the assembled leadframe strip  300  is encapsulated in a protective plastic compound  308  while areas  310  intended for soldering are not covered by the encapsulation compound  308 . 
     Subsequently, discrete packaged IC chips  400  ( FIG. 4 ) are singulated from the assembled leadframe strip  300  by cutting through the encapsulating compound  308  and the plated metal saw streets,  202  and  204 , with a saw. As a consequence of the sawing step, the wirebond pads  104  have a side surface  410  ( FIG. 4A ) where the base metal is exposed by the sawing. Finally, the discrete packaged IC chips  400  are assembled on a circuit board  402  by solder-connecting  406  the non-covered areas  3140  to metallic pads  404  on the circuit board  402  as shown in  FIG. 4C . 
     SUMMARY 
     The following presents a simplified summary in order to provide a basic understanding of one or more aspects of the invention. This summary is not an extensive overview of the invention, and is neither intended to identify key or critical elements of the invention, nor to delineate the scope thereof. Rather, the primary purpose of the summary is to present some concepts of the invention in a simplified form as a prelude to a more detailed description that is presented later. 
     IC chips are attached to leadframe strips and encapsulated in a protective plastic compound. Individual IC chips are then singulated by sawing them apart along the saw streets. During the singulation process the sawing exposes unprotected leadframe metal which may oxidize and prevent solder from wetting the surface and forming a strong bond when the IC chips are attached by soldering to a printed circuit board. 
     A leadframe wherein the outer sidewalls of the leadframe that are exposed by sawing during singulation are comprised of greater than 50% solder. A leadframe strip wherein the saw streets and the outer surface of the lead frames are comprised of greater than 50% solder. A method of forming a leadframe strip wherein the saw streets and the outer surface of the lead frames are comprised primarily of solder. A method of forming a leadframe strip wherein the saw streets and the outer surface of the lead frames are comprised entirely of solder. 
    
    
     
       DESCRIPTION OF THE VIEWS OF THE DRAWINGS 
         FIG. 1  (Prior art) is a plan view of a leadframe. 
         FIG. 2  (Prior art) is a plan view of a leadframe strip. 
         FIG. 3  (Prior art) is a cross-section of packaged IC chips on a leadframe strip. 
         FIGS. 4A, 4B, and 4C  describe the attachment of a packaged IC to a circuit board by soldering. 
         FIG. 5A  describe a leadframe formed according to embodiments. 
         FIGS. 5B, and 5C  describe lead frame strips formed according to embodiments. 
         FIGS. 6A and 6B  are cross sections illustrating a leadframe strip formed according to embodiments. 
         FIGS. 7A and 7B  are cross sections illustrating a leadframe strip formed according to embodiments. 
         FIGS. 8A through 8L  are cross sections of the lead frame strip in  FIG. 6A  depicted in successive stages of fabrication. 
         FIGS. 9A through 9L  are cross sections of the lead frame strip in  FIG. 7A  depicted in successive stages of fabrication. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     Embodiments of the disclosure are described with reference to the attached figures. The figures are not drawn to scale and they are provided merely to illustrate the disclosure. Several aspects of the embodiments are described below with reference to example applications for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide an understanding of the disclosure. One skilled in the relevant art, however, will readily recognize that the disclosure can be practiced without one or more of the specific details or with other methods. In other instances, well-known structures or operations are not shown in detail to avoid obscuring the disclosure. The embodiments are not limited by the illustrated ordering of acts or events, as some acts may occur in different orders and/or concurrently with other acts or events. Furthermore, not all illustrated acts or events are required to implement a methodology in accordance with the present disclosure. 
     Discrete packaged IC chips  400  ( FIG. 4A ) are singulated from the assembled leadframe strip  300  by cutting through the encapsulated compound  308  and the plated metal saw streets,  202  and  204  ( FIG. 2 ), with a saw. On conventional leadframe strips, as a consequence of the sawing step, the wirebond pads  104  have a side surface  410  ( FIG. 4A ) where the base metal is exposed. The saw-exposed base metal becomes oxidized upon exposure to air. Solder paste applied prior to soldering the packaged IC chip  400  to the circuit board  402  is sometimes insufficient to remove the oxidation. When this occurs, the solder  406  cannot wet the sidewall of the wirebond pad  104  and does not form a solder-bond to the vertical side of the packaged IC chip  400 . This may result in a weak bond between the packaged IC chip  400  and the circuit board  420  which may result in the failure of an electrical connection between the packaged IC chip  400  and the circuit board  402  or may result in the delamination of the packaged IC chip  400  from the circuit board  402 . This is especially problematic when the failure occurs as a result of mechanical stress on a circuit board  402  during use the field. 
     An embodiment leadframe  510  that resolves the problem of exposed base metal on the sidewalls of the bondwire pads  104  as a result of sawing is illustrated in  FIG. 5A . In the embodiment the base metal on the sidewalls of the bondwire pads  104  that are exposed by sawing is replaced with solder  506 . 
     A first example lead frame strip  500  in  FIG. 5B , is comprised of a plurality of embodiment leadframes  510  connected together with horizontal  502  and vertical  504  saw streets. In this embodiment all or most of the base metal in the horizontal  502  and vertical  504  saw streets is replaced with solder  506 . 
     A second example lead frame strip  512  in  FIG. 5C , is comprised of a plurality of embodiment leadframes  510  connected together with horizontal  508  and vertical  510  saw streets. In this embodiment all or most of the base metal in the regions of the horizontal  508  and vertical  510  saw streets between the bondwire pads  104  is replaced with solder  506 . The remainder of the saw street area  508  and  510  that is not connected to wirebond pads  104  remains base metal. 
       FIG. 6A  illustrates an example where all the base metal in the saw streets between the wirebond pads  104  is replaced with solder  602 . After sawing, as shown in  FIG. 6B , no base metal is exposed on the sidewalls of the wirebond pads  104 . The base metal on the sidewalls of the wirebond pads  104  is replaced with solder  606 . This solder sidewall  606  forms a strong bond with the solder  608  used to attach the IC chip  400  to the circuit board  402 . 
       FIG. 7A  illustrates an example where a majority of the base metal in the saw streets between the wirebond pads  104  is replaced with solder  702 . After sawing, as shown in  FIG. 7B , only a small amount of the base metal  704  is exposed on the sidewalls of the wirebond pads  104 . Over 50% of the base metal on the sidewalls of the wirebond pads  104  is replaced with solder  706 . This solder on the sidewall  706  forms a strong bond with the solder  708  used to attach the IC chip  400  to the circuit board  402 . 
     A method for forming the embodiment leadframe strip in cross sections in  FIGS. 6A and 6B  is described in the cross sections illustrating the major processing steps in  FIGS. 8A through 8L . 
       FIG. 8A  shows a metal strip  800  (lead frame strip) covered with a protective dry film coating  802 . The metal strip  800  used in leadframe strip manufacture typically is formed of copper or a copper alloy. The protective dry film coating may be photoresist or electrodeposited polyimide for example. 
     In  FIG. 8B  a photo resist pattern  804  is formed with openings  803  between the IC chip pad  102  and the bondwire pad  104  areas and also an opening  805  over and slightly wider than the saw street  504 . The opening  805  over the saw street  504  exposes the entire width of the saw street  504  and also a small area of the bondwire pads  104  that are attached to the saw street  504 . The openings  808  over the saw street  504  may be in the range of about 0.06 mm to about 0.2 mm wider than the saw street  504 . In an example embodiment the opening  808  is 0.06 mm wider than the saw street  504 . 
     To form the leadframe shown in  FIG. 5B , the photoresist pattern openings  803  and  805  over the saw streets  504  opens the entire saw streets  504 . 
     To form the lead frame shown in  FIG. 5C , the photoresist pattern openings  805  over the saw streets  504  opens the street regions between bondwire pads  104  on adjacent leadframes  510  only. The remainder of the saw street areas  504  remains base metal. 
     The dry film coating  802  is etched from the open areas,  803  and  805 , exposing the base metal on the leadframe strip. 
     In  FIG. 8C , the photo resist pattern  804  is removed and the base metal is etched from the open areas in the dry film coating. The base metal is etched approximately half way  806  through the lead frame strip  800  between the IC chip pad  102  and the wirebond pad  104  areas and is also etched approximately half way through the saw streets  504  and exposed regions of the wirebond pads  104  adjacent to the saw streets  504  forming a front side saw street trench  808 . 
     In  FIG. 8D  the dry film coating  802  is removed and a thinfilm  812  of a metal such as nickel plus palladium or nickel plus palladium plus gold is deposited or electroplated onto the exposed surfaces of the leadframe strip  800  to enhance solderability. 
     In  FIG. 8E , a front side screen printing mask  816  with an opening slightly wider than the front side half etched saw street  808  is centered over the first half etched saw street  808 . Solder paste  818  is then screen printed onto the leadframe strip  800  completely filling and slightly over filling the front side saw street trench  808 . 
     The front side screen printing mask  816  is then removed and the solder paste  818  is reflowed as shown in  FIG. 8F  to fill the front side saw street trench  808  and to form solder sidewalls  606  on the wirebond pads. The base metal in the front side saw street trench  808  and also the base metal in the half etched region of the wirebond pads  104  adjacent to the saw street  504  in this embodiment is replaced with solder. 
     Referring now to  FIG. 8G , the leadframe strip  800  is turned upside down and a second protective dry film coating  820  is applied to the backside of the leadframe strip  800 . A second photoresist pattern  822  with openings  814  exposing the base metal between the IC chip pad  102  and the wirebond pad  104  areas and also openings  816  over and slightly wider than the saw street  504  is formed on the backside of the leadframe strip  800 . The openings  816  over the saw street  504  may be in the range of about 0.06 mm to about 0.2 mm wider than the saw street  504 . In an example embodiment the opening  816  is 0.06 mm wider than the saw street  504 . 
     To form the leadframe shown in  FIG. 5B , the photoresist pattern openings  816  over the saw streets  504  opens the entire saw streets. 
     To form the lead frame shown in  FIG. 5C , the photoresist pattern openings  816  over the saw streets  504  opens only the saw street regions between bondwire pads  104  on adjacent leadframes  510 . 
     The dry film coating  820  is etched from the openings between the IC chip pad  102  and the wirebond pad  104  areas and also the open regions over the saw street  504  and the exposed wirebond pad  104  regions attached to the saw streets  504 . 
       FIG. 8H  shows the leadframe strip  800  after the second protective dry film coating  820  is etched and the second photo resist pattern  822  is removed. 
     In  FIG. 8I  the base metal of the leadframe strip  800  is etched where exposed by the openings in the second protective dry film coating  820 . The openings  824  between the IC chip pads  102  and the wirebond pads  104  are etched so that these openings  824  join with the half etched openings  806  that were previously etched from the front side of the leadframe strip  800 . This removes all the base metal from between and electrically isolates the IC chip pads  102  from the wirebond pads  104 . The base metal is also etched from the saw streets  504  and the exposed regions of the wirebond pads  104  adjacent to the saw streets  504  stopping on the reflowed solder  818  filling the front side saw street trench  808 . 
     In  FIG. 8J  the second dry film coating  820  is removed and a second metal  830  such as nickel plus palladium or nickel plus palladium plus gold is deposited or electroplated onto the exposed surfaces of the leadframe strip  800  to enhance solderability. 
     In  FIG. 8K , a backside screen printing mask  834  with an opening over and slightly larger than the backside saw street trench  826  is placed on the backside of the lead frame strip  800 . Solder paste  836  is then screen printed onto the leadframe strip  800  to fill and slightly overfill the backside saw street trench  826 . 
     The backside screen printing mask  834  is then removed and the solder paste  836  is reflowed as shown in  FIG. 8L  to completely fill the saw street  602  and also to fill an outer portion  606  of the wirebond pads  104  with reflowed solder  836 . In this embodiment the base metal in the saw street  504  is replaced completely with solder  602 . In addition, the base metal in the sidewalls of the wirebond pads  104  removed by sawing is also completely replaced with solder  606 . 
     As discussed previously, when the packaged IC chips  400  are singulated by sawing using this embodiment no base metal is exposed. The sidewalls on the packaged IC chips  400  that are formed as the result of sawing during singulation are composed entirely of solder  606 . As illustrated in  FIG. 6B , using this embodiment, strong solder bonds are formed to the sidewalls of the packaged IC chip  400  during attachment to an integrated circuit board  402  by soldering  608 . 
     A method for forming second embodiment leadframe strips shown in  FIG. 7B , is described in the cross sections illustrating the major processing steps in  FIGS. 9A through 9L . 
       FIG. 9A  shows a metal leadframe strip  900  covered with a protective dry film coating  902 . Openings  906  etched through the lead frame strip  900  electrically isolate the IC chip pads  102  from the wirebond pads  104 . 
     In  FIG. 9B  a photo resist pattern  904  is formed with openings  908  over and slightly wider than the saw street  504 . A small portion of the wirebond pads  104  adjacent to the saw street  504  is exposed. The dry film coating  902  is etched from the open area  908 . The openings  908  over the saw street  504  may be in the range of about 0.06 mm to about 0.2 mm wider than the saw street  504 . In an example embodiment the opening  816  is 0.06 mm wider than the saw street  504 . 
     To form the leadframe shown in  FIG. 5B , the photoresist pattern openings  908  over the saw streets opens the entire saw streets. 
     To form the lead frame shown in  FIG. 5C , the photoresist pattern openings  908  over the saw streets  504  opens only the saw street regions between bondwire pads  104  on adjacent leadframes  510 . 
     In  FIG. 9C , the photo resist pattern  904  is removed and base metal is partially etched from the saw street  504  and the exposed regions of the wirebond pads to form a front side saw street trench  910 . The thickness of the base metal removed in this embodiment is less than half the thickness of the leadframe strip  900  but more than one fourth the thickness. 
     In  FIG. 9D  the dry film coating  902  is removed and a metal film  912  such as nickel plus palladium or nickel plus palladium plus gold is deposited or electroplated onto the exposed surfaces of the leadframe strip  900  to enhance solderability. 
     In  FIG. 9E , a front side screen printing mask  916  with an opening over the front side saw street trench  910  is placed on the front side of the lead frame strip  900 . Solder paste  918  is then screen printed onto the leadframe strip  900  to completely fill and slightly overfill the front side saw street trench  910 . 
     The front side screen printing mask  916  is then removed and the solder paste  918  is reflowed as shown in  FIG. 9F  filling the front side saw street trench  702  and filling with solder  918  the etched regions of the wirebond pads  104  adjacent to the saw street  706  The base metal in the front side saw street trenches  910  is replaced with solder  702 . In addition, the base metal in the half etched regions of the wirebond pads  104  adjacent to the saw street  504  are replaced with solder  706 . 
     Referring now to  FIG. 9G , the leadframe strip  900  is turned upside down and a second protective dry film coating  920  is applied to the backside of the leadframe strip  900 . 
     In  FIG. 9H , a second photoresist pattern  922  with an opening  924  over and slightly wider than the saw street  504  is formed. A small portion of the wirebond pads  104  adjacent to the saw street  504  are also exposed. 
     The openings  924  over the saw street  504  may be in the range of about 0.06 mm to about 0.2 mm wider than the saw street  504 . In an example embodiment the opening  816  is 0.06 mm wider than the saw street  504 . 
     To form the leadframe shown in  FIG. 5B , the photoresist pattern openings  924  over the saw streets opens the entire saw streets. 
     To form the lead frame shown in  FIG. 5C , the photoresist pattern openings  924  over the saw streets  504  opens only the saw street regions between bondwire pads  104  on adjacent leadframes  510 . 
     The dry film coating  920  is then etched from the open area  924  as shown in  FIG. 9H . 
     In  FIG. 9I  the base metal of the leadframe strip  900  is partially etched from the saw street  504  and also etched from the exposed regions of the wirebond pads  104  that are attached to the saw street  504 . The thickness of the base metal removed is less than half the thickness of the leadframe strip  900  but more than one fourth the thickness. This forms a backside saw street trench  926  and leaves a strip of base metal  928  across the saw street  504 . This strip of base metal  928  connects the wirebond pads  104  of a first lead frame  100  to wirebond pads  104  of a second lead frame  100  across the saw street  504 . This strip of metal  928  may add reinforcement and rigidity to the leadframe strip  500 . 
     In  FIG. 9J  the second dry film coating  920  is removed and a second metal film  930  such as nickel plus palladium or nickel plus palladium plus gold is deposited or electroplated onto the exposed surfaces of the leadframe strip  900  to enhance solderability. 
     In  FIG. 9K , a backside screen printing mask  934  with an opening slightly larger than the backside saw street trench  926  is placed on the backside of the lead frame strip  900 . Solder paste  936  is then screen printed onto the leadframe strip  900  filling and slightly overfilling the backside saw street trench  926 . 
     The backside screen printing mask  934  is then removed and the solder paste  936  is reflowed as shown in  FIG. 9L  filling the backside saw street trench  926  and also to filling the partially etched wirebond pads with reflowed solder  936 . In this embodiment more than half the base metal in the saw street  504  is replaced with solder  702 . In addition, more than half the sidewall of the wirebond pads  104  exposed during singulation by sawing is replaced with solder  706  leaving only a small portion of base metal  702  exposed on the sidewall. 
     As discussed previously, when the packaged IC chips  400  are singulated by sawing with this embodiment less than half the exposed sidewall is base metal. As illustrated in  FIG. 7B , by replacing more than of the base metal that is exposed on the sidewalls of the wirebond pads  104  by sawing with solder  706 , a strong reliable bond may be formed when the packaged IC chip  400  is soldered to the integrated circuit board  402 . 
     While various embodiments of the present disclosure have been described above, it should be understood that they have been presented by way of example only and not limitation. Numerous changes to the disclosed embodiments can be made in accordance with the disclosure herein without departing from the spirit or scope of the disclosure. Thus, the breadth and scope of the present disclosure should not be limited by any of the above described embodiments. Rather, the scope of the disclosure should be defined in accordance with the following claims and their equivalents.