Patent ID: 12224232

DETAILED DESCRIPTION

Certain terminology is used in the following description for convenience only and is not limiting. The words “right,” “left,” “top,” and “bottom” designate directions in the drawings to which reference is made. The words “a” and “one,” as used in the claims and in the corresponding portions of the specification, are defined as including one or more of the referenced item unless specifically stated otherwise. This terminology includes the words above specifically mentioned, derivatives thereof, and words of similar import. The phrase “at least one” followed by a list of two or more items, such as “A, B, or C,” means any individual one of A, B or C as well as any combination thereof.

The description provided herein is to enable those skilled in the art to make and use the described embodiments set forth. Various modifications, equivalents, variations, combinations, and alternatives, however, will remain readily apparent to those skilled in the art. Any and all such modifications, variations, equivalents, combinations, and alternatives are intended to fall within the spirit and scope of the present invention defined by claims.

Techniques are disclosed herein for forming wettable flanks on DFN semiconductor packages. The techniques begin with a package assembly that includes multiple non-singulated packages. The package assembly includes a lead frame assembly having dies and other internal package components (such as wire bonds) coupled thereto. The dies and other components form different regions of non-singulated packages. The dies and other components are encapsulated within a non-conductive mold encapsulation material (also referred to as a “molding,” “mold,” “encapsulation,” “encapsulation material,” or other similar term herein) that covers most of the package components but may leave exposed certain electrical contact pads (referred to herein as “leads”) and, possibly, thermal contact pads (referred to herein as “die paddles”). The lead frame provides a continuous electrical connection between one end of the package assembly and the other, and between the various exposed leads and die paddles of the packages. Elements such as wire bonds or tie bars may assist with forming the electrical connection. This electrical connection is used to allow for current flow during electroplating. At the borders of the regions defining different package dies are plating bars, which are portions of the lead frame assembly that electrically join the different die packages before the die packages are singulated.

A cutting device, such as, for example, as a saw, waterjet cutting device, laser cutting device, or a plasma cutting device, makes step cuts through the lead frame and partially to a depth, but not completely through the molding, to expose certain sidewalls of the leads. Then, at least portions of these exposed sidewalls are electrolytically plated. In addition, bottom surfaces of the leads are electrolytically plated, and bottom surfaces of certain exposed die paddles or contact pads may be electrolytically plated. Within each die package, a die paddle is coupled to right and left plating bars via tie bars or wire bonds to allow for the current flow for electrolytic plating. Subsequently, a cutting device makes cuts fully through the molding, in the same direction and position as the first cuts, to separate the rows of die packages. A third set of cuts is made, perpendicular to the first and second sets of cuts, to singulate the dies. The edges exposed by the third set of cuts are not plated. Thus, a finished semiconductor package may be formed as a DFN semiconductor package.

FIG.1Ais a flow diagram of an illustrative method100for forming a package assembly, according to an aspect of the present invention. The method100begins at step102, where one or more dies are deposited onto a lead frame assembly. The lead frame assembly includes multiple package lead frames integrated into a single part or unit. The lead frame assembly may include one or more fiducial marks which are marks detectable by a machine that allow the machine to align itself for cutting. The lead frame assembly may be any metal alloy. Die packages are typically formed in an array of die packages which are then cut (“singulated”) into individual die packages. To form this array, a single lead frame assembly is cut from a lead frame material such as a sheet of copper. The lead frame assembly has, integrated therein, multiple lead frame portions corresponding to individual packages. At step102, one or more of the integrated circuit dies are deposited on the lead frame assembly. At step104, other components, such as wire bonds, conductive clips (elements within the package that couple the die(s) to one or more leads), or other elements are deposited to form packages. At step106, a mold encapsulation is deposited around the lead frame and other components of the packages. The mold encapsulation provides a physical and electrical barrier to the components of the package. At the end of method100is a package assembly that includes multiple non-singulated package dies with package components (e.g., dies, the lead frame, and the components that couple the dies to the lead frame) encapsulated within a molding material.

FIG.1Bis a flow diagram of an illustrative method100for forming a DFN semiconductor package according to an aspect of the present invention. The method150ofFIG.1Bis discussed in conjunction withFIGS.2A-2E, which illustrate stages of a package assembly200as the method150proceeds. The method150begins with a package assembly200(shown inFIG.2A) that includes a lead frame assembly205having an integrated circuit die disposed on and attached thereto. The die is surrounded, at least partially, by an encapsulation material202. The continuous lead frame assembly205includes a plurality of plating bars203, die paddles206(or “pads”), and package edge leads204, that are fully electrically coupled together inFIG.2A. The leads204are formed from a conductive material and are configured to receive plating, described in further detail herein, in order to function as the solderable contacts for the package to be connected to a printed circuit board. Non-conductive mold encapsulation material202surrounds the lead frame assembly205. The packages are dual flat no-leads (“DFN”) packages because the packages have two opposing wettable lead sides207that include a plurality of leads204for external electrical coupling and two opposing non-wettable sides209that do not include leads.

The package assembly200includes an array of uncut (or “joined” or “non-singulated”) packages210. The packages include circuitry elements such as integrated circuit dies, conductive elements such as wire bonds, and other elements that are not shown inFIGS.2A-2Ebecause these figures only show the bottom surface of the package assembly200(with the exception of the tie bars215,217, and219, which are not exposed on the bottom surface of the package because these elements are internal to the mold encapsulation202, but are shown inFIGS.2A-2Efor clarity). More specifically, it should be understood that in the package assembly200illustrated, the mold encapsulation202has already been deposited around the lead frame204and other components and thus what is seen is the mold encapsulation202and portions of the lead frame205exposed through the mold encapsulation202. The specific package configuration shown and described in this specification is an example, and details of this configuration should not be taken to be limiting. For example, each package210is shown with one die paddle206, a gate lead213, and a source lead211. Thus, in the package210, a die, which is thermally coupled to the die paddle206, is electrically coupled to leads204, and to the gate lead213and the source lead211via conductive elements internal to the package210, such as wire bonds. Although a specific number and configuration of leads204is shown, the techniques of the present disclosure are applicable to packages210having any configuration of leads204and/or die paddles206. For instance, in some packages, a gate lead and/or source lead may not be present. Leads may be present in any configuration. Additionally, any number of dies may be present within the packages, each connected to leads in different configurations.

The plating bars203are portions of the lead frame assembly205that do not eventually form the lead frame of the individual die packages210after the die packages210are singulated. In other words, the plating bars203provide structural integrity and electrical conductivity across the die packages210for electroplating.

At step152, a cutting device performs a first step cut fully through the lead frame205and partially through the mold encapsulation202. This cut is made adjacent to the wettable lead sides207of the packages210, in order to expose sidewalls of the leads204for electroplating. The cutting device may be, for example, a saw having a physical blade, a laser cutter, a plasma cutter, or a water jet cutter, or any other acceptable cutting technique as known to those of skill in the art. The cuts may be referred to herein as a first series of parallel cuts. The cutting is illustrated inFIG.2A. The width of the blade (or other cutting element) used is sufficient to cut the edges of the leads204of two adjacent die packages210. Further, the cut is made fully through the lead frame205(specifically, through the horizontal plating bars203) but not fully through the corresponding mold encapsulation, which allows the package assembly200to be handled as a single integrated or joined unit through subsequent steps. The cutting at step152forms sidewalls220at portions of the leads204.

At step154, an electrolytic plating process is performed, using an electrolytic plating device in order to plate the lead frame assembly205. Lead frames are typically made of a material such as copper. A layer of a metal such as tin or a tin alloy is plated on the surface of the copper to protect from oxidation and to provide a wettable surface for soldering. In a typical electrolytic plating arrangement, the lead frame is clipped in a tin solution and the lead frame is electrically coupled to the cathode of an electrolytic plating device. The anode is coupled to the plating material, which is also clipped in the solution. An electrical current is applied to the lead frame which causes the plating material to be deposited on the surface of the lead frame so that the leads204and die paddles206are plated with the plating material. In the electrolytic plating technique used for the techniques described herein, a plating material other than tin may be used, such as gold, palladium, or silver. The cuts made at step152expose the wettable side-walls220of the leads204so that electroplating plates the leads204with a plating material. The cuts made in step152electrically decouple the rows of lead frames, but within each row, there is electrical continuity from left to right as oriented in the Figure. More specifically, in each package210, current flows from a left plating bar203, through each elements of the package210to be plated, to a right plating bar203, and then to the next package210over, through the shared plating bar203. Each individual element to be plated in each package210is thus electrically coupled to the left and right plating bars203. Specifically, the die paddle206is coupled to a left plating bar203through a tie bar215. A tie bar is a part of the lead frame that provides electrical conductivity and/or structural continuity, between elements in the die package210and plating bars203or other elements external to the die package210. In some examples, tie bars are generally thinner than other conductive elements that are part of the lead frame205and that extend out of the die package210, and tie bars typically do not extend to the bottom surface of the die package210. The die paddle206is also electrically coupled to several leads204. The die paddle206is further coupled to the right plating bar203through a tie bar217. The source lead211and the gate lead213are both coupled to the right plating bar203through tie bars219. Any of the source and gate lead tie bars219, and the tie bar217that couples the die paddle206to the right plating bar203may be replaced with other conductive elements, such as wire bonds, for the purpose of electrically coupling any of the die paddle206, the gate lead213, or the source lead211to the right plating bar203. A wire bond differs from a tie bar in that a wire bond is not a part of the lead frame, but is instead deposited or coupled between portions of the lead frame or components, such as between a die paddle and a lead to provide an electrical connection.

At step156, a cutting device makes a second set of parallel cuts aligned with the first set of parallel cuts. The width of the second set of parallel cuts is smaller than the width of the first set of cuts made at step152, as shown inFIG.2C. These cuts form the step cut wettable flanks of the dies and fully separate the mold encapsulation for the different rows. The step cut wettable flanks are step cut sides that expose sidewalls of the leads for application of solder so that they can be inspected such as via AOI. The two widths of the step cuts are shown inFIG.2Cas width1(W1) and width2(W2), with W1being greater than W2.

At step158, a cutting device makes a third set of parallel cuts that are perpendicular to the first and second sets of parallel cuts. The third set of parallel cuts are aligned to cut through the plating bars203, in order to singulate the dies210. The third set of parallel cuts are made deep enough to fully cut through the lead frame205and the mold encapsulation202.FIG.2Eillustrates the singulated packages210having wettable flanks.

FIGS.3A-3Billustrate details related to steps152and156. A cutter301is shown in both figures.FIG.3Aillustrates an example of the step cut partially fully through the lead frame and partially through the molding as described in step152and as shown inFIG.2A. The cut shown inFIG.3Ais made at a first thickness configured to expose the sidewalls of the leads204of the packages210. The cut is shown inFIG.3Aas being made with a saw blade having a thickness labeled “Z1,” but any technically feasible means for making the cut could be used, such as, for example, a laser cutter, a plasma cutter, or a water jet cutter, or any other acceptable cutting technique as known to those of skill in the art.

FIG.3Billustrates an example of the second step cut, which is fully through the encapsulation material that remains after the step cuts of step152andFIG.2A. The leads204have a plating material310deposited thereon via electrolytic plating to form step-cut wettable flanks312.

FIGS.4A-4Dillustrate different views of a singulated die package210, illustrating the step-cut wettable flanks formed according to the method150ofFIG.1B.FIGS.4A and4Billustrate orthographic views, illustrating the top and sides of the package210andFIGS.4C and4Dillustrate orthographic views, illustrating the bottom and sides of the package210.

Referring toFIGS.4A-4Dtogether, the package210depicted includes a mold encapsulation202and has step-cut wettable flanks312with electrolytic plating formed on the leads204of two opposing sides in accordance with the technique described inFIG.1B. The step-cut wettable flanks312include the portions of the die package210at which the step cuts of steps152and156are made and also include the leads204that are electrolytically plated. Edges of tie bars215,217, and219, electrically coupled to portions of the lead frame205internal to the mold encapsulation202are revealed in the non-plated sides of the package210.FIGS.3C and3Dillustrate the bottom surfaces of the leads204and die paddles206, which, as described elsewhere herein, are electrolytically plated.

Internally, the illustrated package210includes a die402. The die402is mounted on, and thermally coupled to die paddle206, which is a part of the lead frame205. Wire bonds couple the die404to the leads204of the lead frame205. The source lead211and gate lead213are coupled to the die402via wire bonds404. Further, the source lead211is coupled to a tie bar219, which is not plated and is not functional in the finished package, but is used for the purpose of maintaining electrical continuity between die packages for electroplating as described with respect toFIGS.1B and2A-2E. The gate lead213is coupled to a tie bar219, which is also not plated and serves a similar function as the tie bar219for the source lead211. The die paddle206is coupled to a tie bar217which also serves no purpose in the finished package and is not plated, but is used during electroplating to make the full electrical connection across the different die packages. A tie bar215is present on an opposing side and is coupled to the die paddle206. The tie bar215serves a similar purpose to the other tie bars and is not plated.

FIG.5illustrates an illustrative electrolytic plating technique. Such a technique could be used for example as part of step104, illustrated inFIG.2B. According to the technique, in an electroplating device500, the package assembly200(only a part of which is shown inFIG.5) is placed into a solution502. The cathode of a power source504is electrically coupled to the lead frame205and the anode of the power source504is coupled to a plating material506. When current is applied by the power source504, plating material508is deposited onto the exposed surfaces of the lead frame205.

It will be appreciated that the foregoing is presented by way of illustration only and not by way of any limitation. It is contemplated that various alternatives and modifications may be made to the described embodiments without departing from the spirit and scope of the invention. Having thus described the present invention in detail, it is to be appreciated and will be apparent to those skilled in the art that many physical changes, only a few of which are exemplified in the detailed description of the invention, could be made without altering the inventive concepts and principles embodied therein. It is also to be appreciated that numerous embodiments incorporating only part of the preferred embodiment are possible which do not alter, with respect to those parts, the inventive concepts and principles embodied therein. The present embodiment and optional configurations are therefore to be considered in all respects as exemplary and/or illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all alternate embodiments and changes to this embodiment which come within the meaning and range of equivalency of said claims are therefore to be embraced therein.