Enhanced solder joint strength and ease of inspection of leadless leadframe package (LLP)

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.

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 panel101is patterned (typically by stamping or etching) to define a plurality of arrays103of device areas105. Each device area105includes a die attach pad107and a plurality of contacts109disposed about their associated die attach pad107. Very fine tie bars are often used to support the die attach pads107and contacts109. The contacts109are generally attached to the tie bars111by tie bar stubs112.

During assembly, dice are attached to the respective die attach pads107and conventional wire bonding is used to electrically couple bond pads on each die to their associated contacts109on the lead frame strip101. After the wire bonding, a plastic cap is molded over the top surface of the array103of wire-bonded dice. The dice are generally then singulated and tested using conventional sawing and testing techniques. One assembly process is illustrated graphically in steps160-170of FIG.6(A).

FIGS. 2A and 2Billustrate a segment of a molded lead frame panel prior to singulation. The die attach pad107supports a die120which is electrically connected to its associated contacts109by bonding wires122. A plastic casing125encapsulates the die120and bonding wires122and fills the gaps between the die attach pad107and the contacts109, thereby serving to hold the contacts in place. Once the plastic casing125has cured, the bottom surfaces of the contacts109and the die attach pad107are buffed and solder-plated prior to singulation. The solder plating may form a thin solder layer140on the bottom surface of the die attach pads and contacts.

FIG. 2Cillustrates a sawing-based singulation (or dicing) process. As shown therein, a saw blade130is directed along the tie bar axis, thereby severing the tie bar111(and removing corresponding portions of the molding material125and often a small portion of the tie bar stubs112) as it proceeds. Once the tie bar111has been severed, only the molding material125holds the contacts109in 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.5A. As seen therein, compact solder joints150are typically formed between the package and the corresponding attach pads151.

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.

DETAILED DESCRIPTION OF THE 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. 5Aillustrates a compact solder joint150that results when the package is mounted to a circuit board attach pad151which in turn is part of a printed circuit board153. Note that inFIG. 5Aa portion of molding material125remains adhered to the underside of the contact109(and under the tie bar stub112) after a conventional singulation cut. The tie bar stubs112are exposed at the peripheral edges of the package and are substantially co-planer with the package edge154. The exposed portion of the tie bar stub112does not serve as an attachment point for the leadless lead frame package. Thus, the solder joint150between the contact109and the printed circuit board attach pad151is confined to a relatively small area (i.e., the area of the contact pad).

Although the solder joint150ofFIG. 5Aworks 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 material125partially 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 toFIG. 5B, a package in accordance with the present invention will be described.FIG. 5Billustrates a leadless lead frame package attached to a printed circuit board153. In this illustration, the molding material previously attached to the underside of the contact109is removed such that the solder joint152fills a larger region and is readily visible from the side of the package.

One advantage of the present embodiment is that the solder joint152may be more easily inspected. Whereas the molding material125inFIG. 5Aobstructs the view of solder joint150, the solder joint152of the present invention (Referring toFIG. 5B) can readily be seen at the peripheral edge of the package. Therefore, the resulting solder joint152may be visually inspected for joint integrity. In some applications, the exposed solder joint152may 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 contact109and the landing pad158of the printed circuit board and to the increased volume of solder material in solder joint152. 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 toFIGS. 3-4,5B, and6B, 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.6B—a lead frame panel180is 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 pads181or 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 bonding182) and a casing is molded183or otherwise provided that encapsulates the die and connectors while leaving the bottom surfaces of the contacts exposed.

After the encapsulation has cured184, portions of the contacts adjacent to the tie bars are undercut without severing the tie bars185. 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 buffed186and solder-plated187using industry standard techniques. After the lead frame package has been marked188, tested189, and singulated190it is ready for shipping191or 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 Cut185. This step is generally illustrated in FIG.3.

InFIG. 3, a partial sawing operation is illustrated. In this operation, a relatively wide blade131is passed along the tie bar axis. The blade131removes portions of the encapsulating molding (see aboveFIG. 2C125), the tie bar111, and the tie bar stub112thereby exposing the side and underside surfaces of adjacent contacts109. Cleaning the molding material from the contacts109and their corresponding side and underside surfaces allows those surfaces to be solder-plated in a subsequent step. In the embodiment shown, a circular saw blade131is 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 blade131is slightly wider than the width of the molding material125as shown inFIG. 3, therefore cleaning the molding material from the contacts109and their corresponding side and underside surfaces. The same result may be achieved using narrower blades in successive passes along the tie bar axes.

FIG. 4Aillustrates 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 contacts109that inhibits a reliable solder connection to a printed circuit board or other electronic device. Solder plating140attaches to all exposed metallic surfaces including the adjacent contacts109, the tie bar stub112, the tie bar111, and the die attach pads107. Solder plating does not attach to the molding material125.

Once the contacts have been solder-plated, the lead frame panel is ready to be singulated or separated into individual devices. Referring toFIG. 4B, the singulation cut is accomplished by conventional means. In the present embodiment shown, a singulation blade141is passed along each tie bar axis on the lead frame panel. Singulation removes the tie bar111, a portion of the tie stub112, and a portion of the molding material125leaving exposed molding and contact surfaces discussed below. It should be appreciated that the singulation blade141is generally narrower than the blade131described above for use in making the undercut. With this arrangement, a gap is created between the side of the contact109and the edge of a package, which leaves the outside edges of the contacts109, as well as the undersides of the tie bar stubs112, exposed prior to solder plating. The ends of the tie bars stubs112are 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. 5Billustrates a finished and singulated leadless lead frame package mounted on a printed circuit board153. As seen therein, the solder joint152fills an area to the peripheral side of the contact and under the tie bar stub112(which, in the version illustrated inFIG. 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 contact109has been increased, the size of the landing pad158on the printed circuit board may need to be increased a corresponding amount. The larger solder joint152provides a more robust connection between the contact109and the printed circuit board landing pad158as 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 toFIG. 7A, another embodiment of the present invention will be described.FIG. 7Ais a 3-dimensional perspective view of a contact109having a well701created as a result of etching the contact surface703. Before a die is attached to the lead frame panel as in Step1001inFIG. 10, the lead frame panel is selectively etched at the intersection of the contact109and the tie bar111. Etching the contact109has the advantage of eliminating a Partial Cutting step as in Step185in FIG.6B. 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. 7Bis a diagrammatic top view of a pair of contacts109having a well701as shown in FIG.7A. As shown inFIG. 7B, the well701is aligned along the axis of the tie bar111, which is part of a lead frame panel. The well701is sized such that the well701leaves an exposed region on both contacts after the singulation cut. Singulation cut-lines702demark the portions of the contact109and the tie bars111that are removed during singulation of the panel. The side wall705of the well701must be thick enough to withstand taping, encapsulating, and tape removal without collapsing as well as narrow enough to provide a reasonably sized well701. Further, the leading ed ge704of the contact109must 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-lines702leaves a portion of the exposed well continuous side surface706and the well bottom surface707of the contact109as illustrated in FIG.7C—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 surface706and the well bottom surface707resulting 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-8Cillustrate other suitable well geometries.FIG. 8Ais a diagrammatic top view of a pair of contacts109having an ovate shaped well801. One advantage of this embodiment is that the undercut region805of the contact after singulation, as demarked by the singulation cut-lines702, is larger than a similarly sized circle as illustrated in FIG.7B.FIG. 8B, a diagrammatic top view of a pair of contacts109having a substantially rectangular shaped well803illustrates another embodiment of the present invention. The rectangular shaped well provides an even larger undercut region806than a similarly sized ovate well as illustrated inFIG. 8Awith similar advantages.

Another embodiment of the present invention is illustrated in FIG.8C.FIG. 8Cis a diagrammatic top view of a pair of contacts109having circular shaped wells804. In this embodiment, the contacts109are connected to the tie bar111by tie bar stubs112. Here, a smaller well804must be etched on each contact109to create an exposed region807in 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. 9is 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 well701of the contact109protected from encapsulation material125prior to a singulation cut as demarked by singulation cut-lines702. Prior to encapsulation, adhesive tape801is adhered to the bottom surface802of the lead frame panel. The adhesive tape801seals the well701thus preventing encapsulating material125from filling the void therein. The adhesive tape801also serves to ensure the encapsulating material125is substantially co-planer with the bottom surface802of the lead frame panel.

FIG. 10is a flow chart illustrating a modified packaging process in accordance with an embodiment of the present invention. Note in particular, the first step1000wherein 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 step185as illustrated in FIG.6B. 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 steps1001-1010as 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 stubs112are used to couple the contacts to narrow tie bars111. 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.