Abstract:
Methods of fabricating leadless packages are described that provide good solder joint reliability. In most respects, the packages are fabricated in a manner similar to current lead frame based leadless packaging techniques. However, at some point in the process, the contacts are provided with undercut regions that are left exposed during solder plating so that the solder plating also covers the exposed side and undercut segments of the contacts. When the resultant devices are soldered to an appropriate substrate (after singulation), each resulting solder joint includes a fillet that adheres very well to the undercut portion of contact. This provides a high quality solder joint that can be visually inspected from the side of the package.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to the packaging of integrated circuits. More particularly, the invention relates to leadless packaging designs and processes. 
   A leadless lead frame package (LLP) is an integrated circuit package design that contemplates the use of a lead frame in the formation of a chip scale package (CSP). The resulting packages are sometimes referred to as quad flat packs—no lead (QFN) packages. As illustrated in FIGS.  1 (A)- 1 (C), in typical leadless lead frame packages, a copper lead frame strip or panel  101  is patterned (typically by stamping or etching) to define a plurality of arrays  103  of device areas  105 . Each device area  105  includes a die attach pad  107  and a plurality of contacts  109  disposed about their associated die attach pad  107 . Very fine tie bars are often used to support the die attach pads  107  and contacts  109 . The contacts  109  are generally attached to the tie bars  111  by tie bar stubs  112 . 
   During assembly, dice are attached to the respective die attach pads  107  and conventional wire bonding is used to electrically couple bond pads on each die to their associated contacts  109  on the lead frame strip  101 . After the wire bonding, a plastic cap is molded over the top surface of the array  103  of wire-bonded dice. The dice are generally then singulated and tested using conventional sawing and testing techniques. One assembly process is illustrated graphically in steps  160 - 170  of FIG.  6 (A). 
     FIGS. 2A and 2B  illustrate a segment of a molded lead frame panel prior to singulation. The die attach pad  107  supports a die  120  which is electrically connected to its associated contacts  109  by bonding wires  122 . A plastic casing  125  encapsulates the die  120  and bonding wires  122  and fills the gaps between the die attach pad  107  and the contacts  109 , thereby serving to hold the contacts in place. Once the plastic casing  125  has cured, the bottom surfaces of the contacts  109  and the die attach pad  107  are buffed and solder-plated prior to singulation. The solder plating may form a thin solder layer  140  on the bottom surface of the die attach pads and contacts. 
     FIG. 2C  illustrates a sawing-based singulation (or dicing) process. As shown therein, a saw blade  130  is directed along the tie bar axis, thereby severing the tie bar  111  (and removing corresponding portions of the molding material  125  and often a small portion of the tie bar stubs  112 ) as it proceeds. Once the tie bar  111  has been severed, only the molding material  125  holds the contacts  109  in place. The process is repeated along each tie bar in the two-dimensional matrix of tie bars, with a single pass being used to cut along each tie bar. When the dicing is complete, the resulting packaged chip can then be surface-mounted on a printed circuit board or other substrate using conventional techniques, such as soldering, as generally illustrated in FIG.  5 A. As seen therein, compact solder joints  150  are typically formed between the package and the corresponding attach pads  151 . 
   Since leadless lead frame packaging have proven to be a cost effective packaging arrangement, there are continuing efforts to provide further improvements to the package structure and/or processing to permit the package style to be used in additional applications and/or to improve specific characteristics of the resultant devices. 
   SUMMARY OF THE INVENTION 
   To achieve the foregoing and other objects of the invention, methods of fabricating leadless packages are described that provide improved solder joint reliability and visibility. In most respects, the packages are fabricated in a manner similar to current lead frame based leadless packaging techniques. By way of example, a lead frame panel may be patterned to define a plurality of device areas and a matrix of tie bars. Each device area includes a multiplicity of conductive contacts that are attached to an associated tie bar. At some point in the process, the contacts are provided with undercut regions that are left exposed during solder plating so that the solder plating covers the exposed side and undercut segments of the contacts. After the solder plating, the lead frame panels may be processed in a conventional manner including singulation. When the resultant devices are soldered to an appropriate substrate, each resulting solder joint includes a fillet that adheres very well to the undercut portion of the contact. This provides a high quality solder joint that can be visually inspected and tested from the side of the package. 
   A variety of processes are described that facilitate the production of leadless packages having solder plated undercut regions that require little modification to existing production processes. For example, during assembly, dice are attached to die attach pads or otherwise positioned within associated device areas. The dice are then electrically connected to the contacts (e.g., by wire bonding). A casing is then molded or otherwise provided that encapsulates the die and connectors while leaving the bottom surfaces of the contacts exposed. In one aspect of the invention, after the encapsulation, portions of the contacts adjacent to the tie bars are undercut without severing the tie bars. By way of example, undercutting can be accomplished by a partial depth sawing operation along the tie bars. This undercutting exposes some side and underside surfaces of the contacts. The lead frame is then solder-plated in a conventional manner. In addition to plating the bottom surfaces of the contacts, the solder plating also covers the exposed side and underside surfaces of the contacts. After the solder plating, the lead frame panels may be processed in a conventional manner including singulation. 
   In other embodiments, the contacts are patterned to have a stub portion that attaches to the tie bars and a base portion that includes the exposed bottom surface. The stub portions are preferably thinner and narrower than the base portions of their associated contacts. In some embodiments, the lead frame is patterned to define at least one two-dimensional array of immediately adjacent device areas that are separated only by the tie bars. One approach to undercutting the contacts in such an arrangement contemplates first making a shallow cut along the tie bar axes. The first cutting operation is arranged to expose the stubs and a side portion of the base of each contact, but does not sever the tie bars or the stubs. After the solder plating and other desired processing is finished, the individual packages may be singulated by a second cutting operation along the tie bar axes. In some described embodiments, the first cutting operation is preformed using a first saw blade and the second cutting operation is performed using a second saw blade that is narrower than the first blade. 
   In another aspect of the invention, wells are formed in the contact portions of the lead frame during the initial patterning of the lead frame. The wells are exposed on the bottom surface of the contacts but have side walls that prevent encapsulant from filling the wells during the encapsulation process. Exposing the wells during initial patterning of the lead frame eliminates the need to partially cut the contacts because the exposed well creates an undercut region in the contact similar embodiments discussed above. The exposed well is solder plated prior to a singulation cut as in standard packaging procedure. When the semiconductor package is mounted on a substrate, the well region is filled with solder creating a fillet similar to embodiments discussed above. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which: 
       FIGS. 1A-1C  diagrammatically illustrate a lead frame strip suitable for use in forming leadless lead frame packages. 
       FIG. 2A  is a diagrammatic cross sectional side view of a small section of the lead frame panel of  FIG. 1C  after encapsulation and solder plating.  FIG. 2B  is an enlarged view of the contact/tie bar region of FIG.  2 A.  FIG. 2C  diagrammatically illustrates a singulation sawing operation on the lead frame panel of FIG.  2 B. 
       FIG. 3  is a diagrammatic cross sectional side view of a small section of an encapsulated lead frame panel in accordance with one embodiment of the present invention illustrating a partial cutting operation. 
       FIGS. 4A and 4B  are diagrammatic cross sectional side views of the panel illustrated in  FIG. 3  after the solder plating operation and during a singulation sawing operation, respectively. 
       FIG. 5A  is a diagrammatic cross sectional side view of an existing leadless lead frame package mounted on a printed circuit board. 
       FIG. 5B  is a diagrammatic cross sectional side view of a leadless lead frame package mounted on a printed circuit board in accordance with another embodiment of the present invention. 
       FIG. 6A  is a flow chart illustrating a packaging process. 
       FIG. 6B  is a flow chart illustrating a modified packaging process in accordance with an embodiment of the present invention. 
       FIG. 7A  is a 3-dimensional perspective view of a contact having a well created as a result of etching the contact surface in accordance with an embodiment of the present invention. 
       FIG. 7B  is a diagrammatic top view of a contact having a well as shown in FIG.  7 A. 
       FIG. 7C  is a diagrammatic cross sectional side view of a contact having a well as shown in FIG.  7 A. 
       FIG. 8A  is a diagrammatic top view of a preferred embodiment of a contact having an ovate well. 
       FIG. 8B  is a diagrammatic top view of a preferred embodiment of a contact having a substantially rectangular well. 
       FIG. 8C  is a diagrammatic top view of two contacts and their associated wells connected by tie bar stubs to a tie bar, which is part of a lead frame panel according to another embodiment of the present invention. 
       FIG. 9  is a diagrammatic cross sectional side view of a small section of the lead frame panel in accordance with one embodiment of the present invention illustrating the exposed well of the contact protected from encapsulation material. 
       FIG. 10  is a flow chart illustrating a modified packaging process in accordance with another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A number of improvements to leadless package designs are described below. In the following description, numerous specific details-are set forth in order to provide a thorough understanding of the present invention. It will be understood, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process operations have not been described in detail in order not to unnecessarily obscure the present invention. 
   As described in the background section of the application, conventional semiconductor packaging processes result in a package with a plurality of solder-plated contacts exposed on the bottom surface of the package.  FIG. 5A  illustrates a compact solder joint  150  that results when the package is mounted to a circuit board attach pad  151  which in turn is part of a printed circuit board  153 . Note that in  FIG. 5A  a portion of molding material  125  remains adhered to the underside of the contact  109  (and under the tie bar stub  112 ) after a conventional singulation cut. The tie bar stubs  112  are exposed at the peripheral edges of the package and are substantially co-planer with the package edge  154 . The exposed portion of the tie bar stub  112  does not serve as an attachment point for the leadless lead frame package. Thus, the solder joint  150  between the contact  109  and the printed circuit board attach pad  151  is confined to a relatively small area (i.e., the area of the contact pad). 
   Although the solder joint  150  of  FIG. 5A  works well in a wide variety of application, in some applications it may be desirable to provide better solder joint visibility to better facilitate visual inspection of the solder joints and/or inspection by certain types of inspection machines. A weaker, less visible solder joint results because the molding material  125  partially obscures the joint. As can be appreciated by one skilled in the art, in some circumstances better visual confirmation of the solder joint of a particular package is desirable both for quality assurance and for troubleshooting purposes. 
   Referring next to  FIG. 5B , a package in accordance with the present invention will be described.  FIG. 5B  illustrates a leadless lead frame package attached to a printed circuit board  153 . In this illustration, the molding material previously attached to the underside of the contact  109  is removed such that the solder joint  152  fills a larger region and is readily visible from the side of the package. 
   One advantage of the present embodiment is that the solder joint  152  may be more easily inspected. Whereas the molding material  125  in  FIG. 5A  obstructs the view of solder joint  150 , the solder joint  152  of the present invention (Referring to  FIG. 5B ) can readily be seen at the peripheral edge of the package. Therefore, the resulting solder joint  152  may be visually inspected for joint integrity. In some applications, the exposed solder joint  152  may also be more easily probed and tested since the solder joint can be readily accessed from the side of the package. For a fixed sized lead frame, this permits the use of somewhat larger solder joints which provides a greater potential mechanical strength to the joint due both to the increased area of attachment between the contact  109  and the landing pad  158  of the printed circuit board and to the increased volume of solder material in solder joint  152 . In other applications, this permits the use of smaller device areas while maintaining the same solder joint footprint, which may be used to facilitate higher density lead frame panels. 
   Referring next to  FIGS. 3-4 ,  5 B, and  6 B, a method of producing the described packages will be explained. Generally, the lead frame panels may be formed and assembled using any appropriate process. By way of example, in a particular embodiment illustrated in FIG.  6 B—a lead frame panel  180  is patterned to define a plurality of device areas and a matrix of tie bars. Each device area includes a multiplicity of conductive contacts that are attached to an associated tie bar. During assembly, dice are attached to die attach pads  181  or otherwise positioned within associated device areas (e.g., on a support tape if die attach pads are not provided). The dice are then electrically connected to the contacts (e.g., by wire bonding  182 ) and a casing is molded  183  or otherwise provided that encapsulates the die and connectors while leaving the bottom surfaces of the contacts exposed. 
   After the encapsulation has cured  184 , portions of the contacts adjacent to the tie bars are undercut without severing the tie bars  185 . By way of example, the undercutting can be accomplished by a partial depth sawing operation along the tie bars. This undercutting exposes a side and an underside surface of the contacts. The lead frame package is then buffed  186  and solder-plated  187  using industry standard techniques. After the lead frame package has been marked  188 , tested  189 , and singulated  190  it is ready for shipping  191  or attachment to an electronic component as described above. It should be apparent that the primary difference between the present invention, and earlier processes is the addition of the Partial Cut  185 . This step is generally illustrated in FIG.  3 . 
   In  FIG. 3 , a partial sawing operation is illustrated. In this operation, a relatively wide blade  131  is passed along the tie bar axis. The blade  131  removes portions of the encapsulating molding (see above  FIG. 2C   125 ), the tie bar  111 , and the tie bar stub  112  thereby exposing the side and underside surfaces of adjacent contacts  109 . Cleaning the molding material from the contacts  109  and their corresponding side and underside surfaces allows those surfaces to be solder-plated in a subsequent step. In the embodiment shown, a circular saw blade  131  is used, although it should be appreciated that any suitable technique may be used to remove the molding material to expose the contact side and underside surfaces including, but not limited to: grinding, etching, laser cutting, gouging, and other chemical and mechanical techniques. Furthermore, the partial sawing operation may be accomplished in single or multiple operational steps. 
   In the described embodiment, the width of the blade  131  is slightly wider than the width of the molding material  125  as shown in  FIG. 3 , therefore cleaning the molding material from the contacts  109  and their corresponding side and underside surfaces. The same result may be achieved using narrower blades in successive passes along the tie bar axes. 
     FIG. 4A  illustrates the resultant undercut surfaces from the partial saw pass. The exposed undercut surfaces created by the partial saw pass may now be solder-plated to facilitate attachment to printed circuit boards or other electronic devices. Solder plating the exposed surfaces prevents surface oxidation of the contacts  109  that inhibits a reliable solder connection to a printed circuit board or other electronic device. Solder plating  140  attaches to all exposed metallic surfaces including the adjacent contacts  109 , the tie bar stub  112 , the tie bar  111 , and the die attach pads  107 . Solder plating does not attach to the molding material  125 . 
   Once the contacts have been solder-plated, the lead frame panel is ready to be singulated or separated into individual devices. Referring to  FIG. 4B , the singulation cut is accomplished by conventional means. In the present embodiment shown, a singulation blade  141  is passed along each tie bar axis on the lead frame panel. Singulation removes the tie bar  111 , a portion of the tie stub  112 , and a portion of the molding material  125  leaving exposed molding and contact surfaces discussed below. It should be appreciated that the singulation blade  141  is generally narrower than the blade  131  described above for use in making the undercut. With this arrangement, a gap is created between the side of the contact  109  and the edge of a package, which leaves the outside edges of the contacts  109 , as well as the undersides of the tie bar stubs  112 , exposed prior to solder plating. The ends of the tie bars stubs  112  are also exposed, however, they are not solder-plated. 
   The singulated packages may then be attached to a printed circuit board or other appropriate substrate using standard attachment techniques (e.g., soldering).  FIG. 5B  illustrates a finished and singulated leadless lead frame package mounted on a printed circuit board  153 . As seen therein, the solder joint  152  fills an area to the peripheral side of the contact and under the tie bar stub  112  (which, in the version illustrated in  FIG. 5A , is occupied by the molding material). This provides a strong, high quality joint that can be readily seen and accessed from the side as described above. It should be apparent that because the area of attachment of the contact  109  has been increased, the size of the landing pad  158  on the printed circuit board may need to be increased a corresponding amount. The larger solder joint  152  provides a more robust connection between the contact  109  and the printed circuit board landing pad  158  as well as a visual inspection point for solder joint integrity and a convenient test site that can be reached by conventional test probes over the prior art. 
   Referring next to  FIG. 7A , another embodiment of the present invention will be described.  FIG. 7A  is a 3-dimensional perspective view of a contact  109  having a well  701  created as a result of etching the contact surface  703 . Before a die is attached to the lead frame panel as in Step  1001  in  FIG. 10 , the lead frame panel is selectively etched at the intersection of the contact  109  and the tie bar  111 . Etching the contact  109  has the advantage of eliminating a Partial Cutting step as in Step  185  in FIG.  6 B. When the tie bars are etched to reduce their thickness, the simplest approach to forming the wells is to etch the wells at the same time that the tie bars are being etched. It may be appreciated by one skilled in the art that etching may be accomplished by a variety of methods well known in the art. 
     FIG. 7B  is a diagrammatic top view of a pair of contacts  109  having a well  701  as shown in FIG.  7 A. As shown in  FIG. 7B , the well  701  is aligned along the axis of the tie bar  111 , which is part of a lead frame panel. The well  701  is sized such that the well  701  leaves an exposed region on both contacts after the singulation cut. Singulation cut-lines  702  demark the portions of the contact  109  and the tie bars  111  that are removed during singulation of the panel. The side wall  705  of the well  701  must be thick enough to withstand taping, encapsulating, and tape removal without collapsing as well as narrow enough to provide a reasonably sized well  701 . Further, the leading ed ge  704  of the contact  109  must be accordingly sized to resist collapse during singulation and/or other subsequent manufacturing. 
   In another embodiment, a singulation cut as demarked by the singulation cut-lines  702  leaves a portion of the exposed well continuous side surface  706  and the well bottom surface  707  of the contact  109  as illustrated in FIG.  7 C—a diagrammatic cross sectional side view of an embodiment of the present invention. When the package is ultimately attach ed by soldering to an electronic device, the solder flows to the undercut portions of the contact formed by the well side surface  706  and the well bottom surface  707  resulting in a stronger, more easily inspected and tested joint. 
   In one particular described embodiment, the etching creates an exposed well having an average depth of approximately 0.1 mm and an average circumference of approximately 0.3 mm. It is desirable in some embodiments to restrict the exposed well to within approximately 0.05 mm of the nearest side surface of the contact. 
   It should be appreciated that the shape of the wells may be widely varied. For example,  FIGS. 8A-8C  illustrate other suitable well geometries.  FIG. 8A  is a diagrammatic top view of a pair of contacts  109  having an ovate shaped well  801 . One advantage of this embodiment is that the undercut region  805  of the contact after singulation, as demarked by the singulation cut-lines  702 , is larger than a similarly sized circle as illustrated in FIG.  7 B.  FIG. 8B , a diagrammatic top view of a pair of contacts  109  having a substantially rectangular shaped well  803  illustrates another embodiment of the present invention. The rectangular shaped well provides an even larger undercut region  806  than a similarly sized ovate well as illustrated in  FIG. 8A  with similar advantages. 
   Another embodiment of the present invention is illustrated in FIG.  8 C.  FIG. 8C  is a diagrammatic top view of a pair of contacts  109  having circular shaped wells  804 . In this embodiment, the contacts  109  are connected to the tie bar  111  by tie bar stubs  112 . Here, a smaller well  804  must be etched on each contact  109  to create an exposed region  807  in the resulting semiconductor package. As can be appreciated by one skilled in the art, a variety of well shapes may be achieved by selectively etching the lead frame surface depending on the particular manufacturing requirements. 
     FIG. 9  is a diagrammatic cross sectional side view of a small section of a lead frame panel in accordance with one embodiment of the present invention illustrating the exposed well  701  of the contact  109  protected from encapsulation material  125  prior to a singulation cut as demarked by singulation cut-lines  702 . Prior to encapsulation, adhesive tape  801  is adhered to the bottom surface  802  of the lead frame panel. The adhesive tape  801  seals the well  701  thus preventing encapsulating material  125  from filling the void therein. The adhesive tape  801  also serves to ensure the encapsulating material  125  is substantially co-planer with the bottom surface  802  of the lead frame panel. 
     FIG. 10  is a flow chart illustrating a modified packaging process in accordance with an embodiment of the present invention. Note in particular, the first step  1000  wherein a lead frame pattern is provided with etched wells. Etching is accomplished by any means common in the art to provide any of a number of shaped wells as described above. It may be appreciated that etching the lead frame pattern eliminates a subsequent Partial Cut step  185  as illustrated in FIG.  6 B. As a further advantage, well etching may be accomplished at the same time as the lead frame is etched to create device areas, contacts, and tie bars. Once the pattern has been provided, packaging of the device follows conventional manufacturing steps  1001 - 1010  as illustrated in FIG.  10 . 
   Although only a few embodiments of the invention have been described in detail, it should be understood that the present invention might be embodied in many other specific forms without departing from the spirit or scope of the invention. For example, it should be apparent that the described undercutting may be used with a wide variety of packaging processes and the application of the invention is not limited to the particular packaging processes described. 
   As suggested, a variety of methods may be utilized to accomplish the partial sawing. Further, the depth and width of the partial sawing may be widely varied. By way of example, the initial cut may have an average depth of approximately 0.125 mm and an average width of approximately greater than 0.25 mm. Specific depths and widths of the partial saw are dependent on a particular application and are contemplated in this application. In the primary embodiment described, a single sawing pass using a relatively wider blade is used to accomplish the undercutting. However, it should be appreciated that the same effect can be realized using multiple passes of a thinner blade. In the illustrated embodiments, narrower tie bar stubs  112  are used to couple the contacts to narrow tie bars  111 . The narrowed tie bars and tie bar stubs tend to be preferred to minimize the risk of shorting between contacts due to copper (or other metal) streaking during sawing. However, the invention may also be used in embodiments where tie bar stubs are not used and/or thicker tie bars are used. In other applications, the tie bar stubs may be the same width (or wider) than the contacts providing additional surfaces to which the solder can adhere. 
   Moreover, in another embodiment of the present invention the distance between the bottom surface portions of adjacent contacts in adjacent device areas are spaced to no more than approximately 0.45 mm. Spacing between the contacts is critical because of the partial cut operation. In particular, if the spacing is too narrow, the partial cut operation will remove an excessive amount of contact material thus compromising the electronic and mechanical integrity of the contact. Alternatively, if the contact spacing is too wide, then the partial cut operation may remove material only from the tie bar stubs rather than from the contacts. 
   Additionally, the size, geometry and placement of the described wells may be widely varied without departing from the spirit of the invention. As suggested above, the wells can be circular, oval, rectangular, square, elongated or any appropriate geometry and the size of the side walls can be varied to meet the needs of a particular embodiment. 
   As suggested above, when it is known that the described process will be used, the lead frame panels may be designed to take advantage of the greater strength solder bonds that are achievable. By way of example, the size of the contact pads that are co-planer with the bottom surface of the package can be reduced while maintaining overall joint strength. 
   The invention may be used in conjunction with any suitable conductive lead frame material. In present applications, copper and copper alloy-42 are the most common lead frame materials, but the invention may be used in conjunction with lead frames made from other materials, including aluminum and other metals. A number of conventional package processing techniques have been described as being used to accomplish specific steps in the formation of the described devices. It should be apparent that in most cases these processing techniques can be widely varied and a wide variety of alternative conventional processes may be used in their place. Accordingly, the present examples are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope of the appended claims.