Patent ID: 12211704

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 step-cut wettable flanks on QFN 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, which may be a first step that occurs in the process.

An electroplating step deposits a protective, conductive plating material that covers the lead frame and allows for solder to adhere to the lead frame. The electroplating is deposited on the exposed surfaces of the lead frame on the bottom of the package. After electroplating the bottom exposed surfaces of the lead frame, a first and a second series of parallel step cuts are made in a first direction and in a second direction between the die packages to form sidewalls that will form wettable flanks. The first and second series of parallel step cuts are perpendicular to each other and are made on the periphery of the dies. These cuts are made at a depth to cut fully through the lead frame but not fully through the surrounding mold encapsulation, to allow the packages to remain as a single assembly for additional handling in subsequent steps. After the first and second series of parallel step cuts, the wettable flanks of the exposed sidewalls are plated using an electroless method, which does not use the application of an electrical current. An electroless method is used because the series of perpendicular cuts through the lead frame electrically isolates each of the die packages.

After the electroless plating, a third and fourth series of parallel cuts, aligned with the original cuts, is made to a width that is smaller than the width of the first and second series of parallel cuts. These cuts are made through the rest of the mold encapsulation material. The third and fourth series of cuts singulate the dies, thereby forming singulated QFN packages with wettable flanks.

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 frames 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 for 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 method150for forming a QFN package according to an aspect of the present invention. The method150ofFIG.1Bis discussed in conjunction withFIGS.2A-2E, which illustrate stages of a package assembly as the method150proceeds. The method150begins with a package assembly200that includes a lead frame assembly205having one or more dies disposed on and attached thereto. The dies are surrounded by an encapsulation material. A continuous lead frame assembly205includes a plurality of plating bars203, die paddles206(or “pads”), and leads204on the edges of the packages. These various components are electrically coupled together inFIG.2A. The leads204are formed from a conductive material and are configured to receive plating, described further below, 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 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 show the bottom surface of the package assembly200. 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 three die paddles206, and thus the package210includes three dies. Additionally, 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. BecauseFIGS.2A-2Eshow the bottom surface, or contact surface, of the package assembly200, the various internal elements such as dies, wire bonds, and others, are not illustrated in these figures.

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. The plating bars203provide structural integrity and electrical conductivity across the lead frames of the die packages210for electroplating.

Referring now toFIG.1B, at step152, an electrolytic plating device plates 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 dipped in a bath 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 dipped in the bath. 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 paddles206(FIG.2A) are plated with the plating material. At this stage, because only the bottom surfaces of the leads204and die paddles206are exposed, only these surfaces are plated—sidewalls of packages210, which are not exposed at this stage, are not electrolytically plated. The electrolytic plating may be considered a “first plating layer.” The first plating layer may be any of a variety of plating materials, such as tin, gold, palladium, or silver.

At step154, a cutting device performs first depth step cuts in two perpendicular directions to form side walls of the leads204. The cutting device may be, for example, a saw having a saw blade, or may be 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. These cuts may be referred to herein as a first series of parallel cuts and a second series of parallel cuts perpendicular to the first series of parallel cuts. This cutting is illustrated inFIG.2B. The location of the cuts is adjacent to the edges of the leads204of the packages210. The width of the blade used must be sufficient to cut to the edge of the leads204of the two adjacent die packages. Further, the cut is made fully through the lead frame but not fully through the corresponding mold encapsulation, which allows the package assembly200to be handled as a single unit through subsequent steps. The cutting at step154forms sidewalls220at portions of the leads204.

Cutting through the lead frame205in two perpendicular directions electrically isolates each of the die packages210. Thus, at step156, an electroless plating device is used to electrolessly apply plating to the now-exposed sidewalls220of the packages210(illustrated inFIG.2C). Although only a small number of sidewalls220are labeled inFIG.2C, it should be understood that each of the leads204of the packages210includes sidewalls220that are plated with the electroless plating technique.

In the electroless plating technique, the package assembly200is dipped in a solution including a plating material (such as tin) and heat is applied. The plating material is deposited on the exposed metallic surfaces, namely, the sidewalls220of the leads204. The electroless plating on the sidewalls may be considered a “second plating layer,” and is preferably formed by a different process than the first plating layer. The material of the second plating layer may be any plating material, such as tin, gold, silver, or palladium.

At step158, a third set of parallel cuts and a fourth set of parallel cuts perpendicular to the third set of parallel cuts are made to singulate the dies and form individual semiconductor packages (shown inFIG.2D). The blade used to make the third and fourth sets of cuts cut is narrower than the blade used to make the first two cuts of step154and as shown inFIG.2B. The two widths of the step cuts are shown inFIG.2Das width1and width2. This narrower blade forms a step-cut wettable flank that will allow for a good connection with soldering material and can be optically inspected.FIG.2Eillustrates the singulated packages210having wettable flanks.

FIGS.3A-3Billustrate details related to steps154and158. A cutting device301is shown in both figures.FIG.3Aillustrates an example of the first step cut or second step cut (the perpendicular step cuts through the lead frame and only partially into the molding) as described in step154and as shown inFIG.2B. The cut shown inFIG.3Ais made at a first thickness configured to expose the sidewalls220of 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 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 electrolytically deposited plating310is illustrated deposited over the lead frame205.

FIG.3Billustrates an example of the third or fourth series of cuts, which is fully through the encapsulation material that remains after the first and second step cuts of step154andFIG.2B. These full through-cuts singulate the packages210, resulting in the singulated packages as shown inFIG.2E. The electrolessly deposited plating material312is shown deposited over the lead frame205.

FIG.3Cillustrates details related to the plated leads204after the electroless plating of step156but before die singulation. Due to step152, the surfaces308of the lead frame205exposed prior to the first and second cuts (step154andFIG.2B) are plated with electrolytically deposited plating310. Due to steps154and156, the sidewalls220are plated with electroless deposited plating312.

In some examples, the electrolytic plating310on the bottom surfaces of the lead frame205is thicker than the electroless plating312on the sidewalls220. In some examples, the electrolytic plating310is, for example, approximately three times as thick as the electroless plating312. In some examples, the electrolytic plating310is greater than or equal to three times as thick as the electroless plating312. In some examples, the electrolytic plating310is thicker than the electroless plating312because these different platings serve different purposes. Specifically, the electrolytic plating310may serve to mount the die210to a PCB, while the electroless plating312serves to electrically couple the package210, through solder, to a PCB.

FIGS.4A-4Dillustrate different views of a singulated semiconductor die package210, illustrating the step-cut wettable flanks formed according to the method100ofFIG.1.FIGS.4A and4Billustrate orthographic views, illustrating the top and sides of the package210andFIGS.4C and4Dillustrate orthographic views, illustrating the bottom and sides of the package. The conductive mounting or contact surfaces of the semiconductor package, shown at the bottom ofFIGS.4A and4Bmay be considered the lower or bottom surfaces of the package, depending on the orientation, and are the surfaces that will be mounted to, contact and face a printed circuit board. In the orientation of the semiconductor package shown inFIGS.4C and4D, the conductive mounting or contact surfaces are shown at the top.

Referring toFIGS.4A-4Dtogether, the package210depicted includes a mold encapsulation202and has step-cut wettable flanks402with electroless plating formed in accordance with the technique described inFIG.1. The step-cut wettable flanks402include the portions of the die package210at which the three cuts of steps154and158are made and also include the leads204that are electrolessly plated. Edges of tie bars404electrically coupled to portions of the lead frame205internal to the mold encapsulation202are also revealed in the step-cut wettable flanks402.FIGS.3C and3Dillustrate the bottom surfaces of the leads204and die paddles206, which, as described elsewhere herein, are electrolytically plated.

Internally, the illustrated package includes three dies406. The dies are mounted on, and may be thermally coupled to die paddles206, which are a part of the lead frame205. Wire bonds couple the dies406to the leads204of the lead frame205. A clip414electrically couples one or more leads204to one or more dies406.

FIG.5Aillustrates an illustrative electrolytic plating technique. Such a technique could be used for example as part of step152, illustrated inFIG.2A. According to the technique, in an electroplating device500, the package assembly200(only a part of which is shown inFIG.5A) 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.

FIG.5Billustrates an electroless plating technique, according to an example. Such a technique could be used for example as part of step156of method100, which is the application of electroless plating to the sidewalls220of the step-cut wettable flanks According to the technique, in an electroless plating device530, the package assembly200(only a part of which is shown inFIG.5B) is placed into a plating material solution532and a heating device applies heat. Due to the presence of the heat, plating material534is deposited onto the exposed surfaces of the lead frame205. InFIG.5B, these exposed surfaces include the sidewalls220of the leads204.

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.