Shielded semiconductor leadframe package

[0002] The present invention relates to semiconductor chip packages, and in particular to a radiation-shielded semiconductor leadframe package and a method for making the same.

[0003] Leadframe-based semiconductor chip packages are known. In particular, packages that have leads and a die pad exposed on the bottom surface are known. Some of the challenges of forming such packages, while ensuring that the leads and die pad remain locked to the package mold, are addressed in co-pending U.S. patent application, Ser. No. 09/395,875, entitled “Plastic Integrated Circuit Device Package and Micro-Leadframe and Method for Making the Package,” U.S. patent application, Ser. No. 09/393,016, entitled “Plastic Integrated Circuit Package and Method and Leadframe for Making the Package,” and in U.S. patent application, Ser. No. 09/439,917, entitled “Cavity Semiconductor Package With Exposed Leads and Die Pad and Method for Making the Same,” all of which are incorporated by reference herein in their entirety.

[0004] In some applications, it is desirable to shield an integrated circuit chip from electromagnetic interference. Shielding external to the package is costly and impractical, particularly when the package has exposed leads and an exposed die pad. Incorporation of shielding into packages has remained problematic on a commercial scale.

[0005] Therefore, a need has arisen for a semiconductor chip package and packaging method that meets these challenges. In particular, a need has arisen for a shielded, leadframe-based semiconductor chip package, and a method for making the same.

[0006] Accordingly, a semiconductor chip package is disclosed. In one embodiment, the package includes a plurality of leadframe portions and a semiconductor die mounted on at least one of the leadframe portions. A shield element is attached to at least one of the leadframe portions. A package mold surrounds the semiconductor die and the shield element.

[0007] A method for assembling a semiconductor chip package is also disclosed. In one embodiment, the method includes providing a leadframe comprising a plurality of leads, mounting a semiconductor die on the leadframe, mounting an electromagnetic interference shield on the leadframe, and encapsulating at least a portion of the leadframe and the semiconductor die with a mold material to form a package mold.

[0008] An advantage of the present invention is that radiation shielding is provided in a practical manner for a leadframe-based semiconductor chip package.

[0009] For a more complete understanding of the present invention and for further features and advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which:

[0010] FIG. 1 is a vertical cross section of a semiconductor chip package constructed in accordance with the present invention;

[0011] FIG. 2 is a floor plan view of the package;

[0012] FIG. 3 is a front view of the package during assembly;

[0013] FIG. 4 is a top view of a shield element used in the package;

[0014] FIG. 5 is a top view of a portion of a leadframe used in the assembly process;

[0015] FIG. 6 is a top view of the leadframe and shield element during package assembly;

[0016] FIGS. 7 and 8 are close-up views of shield element legs;

[0017] FIG. 9 is a front view of the package during assembly using a differently-shaped shield element;

[0018] FIG. 10 is a top view of a differently-shaped shield element; and

[0019] FIG. 11 is a vertical cross section of a flip-chip package constructed in accordance with the present invention

[0020] The exemplary embodiments of the present invention and their advantages are best understood by referring to FIGS. 1 through 11 of the drawings. Like numerals are used for like and corresponding parts of the various drawings.

[0021] FIG. 1 is a vertical cross section of a semiconductor chip package 10 constructed in accordance with the present invention. FIG. 2 is a floor plan view of package 10. Reference may be made to these figures in conjunction with the following description to understand the various components of package 10.

[0022] Package 10 represents one embodiment of a shielded, leadframe-based semiconductor chip package. As described below, other leadframe-based packages may also benefit from the electromagnetic shielding described herein.

[0023] Package 10 includes a semiconductor die 12 mounted on a die pad 14. A set of leads 16 projects inward from the outer perimeter of package 10 toward die 12 and die pad 14. Bond pads (not shown) on die 12 are electrically connected to respective ones of the leads 16 by bond wires 18. Diagonal support elements 20 (also known as tiebars) support die pad 14 in position prior to the package molding and saw steps of the assembly process, as will be described more fully below.

[0024] A package mold 22 provides structural support for package 10. Package mold 22 may be formed from any one of a variety of commercially available mold compounds such as epoxidized ortho cresol novolac (EOCN), biphenyl (BP), dicyclopentadiene (DCPD) and multifunctional (MF) compounds available from a variety of manufacturers. Package mold 22 has a generally square outline as shown in FIG. 2, with chamfered corners 22a. While chamfered corners 22aare not necessary to practice the present invention, they may help to reduce the mechanical stress caused by a thermal expansion mismatch between the leadframe material and the mold compound. Chamfered corners 22amay be formed by the shape of the mold used to form package mold 22. When packages such as package 10 are to be formed in large quantities, as described below, it may be more efficient to create square corners on package 10 rather than chamfered corners 22a.

[0025] Die pad 14 and leads 16 have bottom surfaces 14aand 16a, respectively, that are substantially coplanar with the bottom surface 22aof package mold 22. Thus, the bottom surfaces 14aand 16aof die pad 14 and leads 16, respectively, are exposed at the bottom surface 22aof package mold 22 so that electrical and/or thermal contact may be made with die pad 14 and leads 16.

[0026] Because leads 16 are exposed along the bottom surface 22aof package mold 22, leads 16 are susceptible to breaking free from the adhesion of package mold 22, which can result in one or more leads 16 projecting downward from the bottom surface 22aof package mold 22 or even breaking free from package 10 altogether, causing an open circuit. It is, of course, desirable to keep leads 16 in a planar arrangement and fixed to package mold 22. Thus, leads 16 may have tabs which are effective in preventing leads 16 from breaking free from package mold 22. These tabs are described in the aforementioned U.S. patent application, Ser. No. 09/439,917, incorporated by reference.

[0027] Other known mechanisms may also be used to lock leads 16 and die pad 14 in place. These known mechanisms include etching or coining portions of die pad 14 and leads 16, and are described in the aforementioned U.S. patent applications, Ser. Nos. 09/395,875 and 09/393,016, which have been incorporated herein by reference. Accordingly, these methods will not be described further herein.

[0028] Package 10 includes a shield element 24 which shields semiconductor die 12 from electromagnetic interference. Shield element 24 may be made from any suitable metal, such as copper, nickel, chrome, stainless steel or other alloys, and may be plated with other metals to enhance corrosion resistance, improve solderability, or add to electrical shielding efficiency. For example, shield element 24 may be plated with gold, tin lead, indium, palladium, platinum and/or alloys of these or other metals. Shield element 24 is connected to selected leads 16, and is grounded to provide an effective electromagnetic interference shield.

[0029] FIG. 3 is a front view of package 10 during assembly, prior to the formation of package mold 22. FIG. 4 is a top view of shield element 24. Holes 26 are formed in the top and sides of shield element 24. Holes 26 allow mold compound to reach the interior of shield element 24 during formation of package mold 22.

[0030] Holes 26 are appropriately sized relative to the wavelength of the radiation from which semiconductor die 12 is to be shielded. For example, holes 26 may have a diameter no greater than one-twelfth the predominant wavelength in the spectrum of electromagnetic interference.

[0031] No holes are formed in the central area of the top of shield element 24 (see FIG. 4). This unperforated central area allows a vacuum pick-up head to pick and place shield element 24 during package assembly.

[0032] An exemplary set of dimensions for semiconductor chip package 10 are given in Table A. These dimensions are set forth for illustrative purposes only, and in no way represent a limitation on the usefulness of the techniques described herein. A die thickness of 0.004-0.027 inches is assumed.

[0033] An assembly process for creating packages such as package 10 in quantity will now be described. Referring to FIG. 5, a top view of a portion of a leadframe 40 for use in such an assembly process is shown, with the outline of the final shape of semiconductor chip packages 10 being overlaid in dashed lines 42 for reference. In the first step of the process, leadframe 40 is created. In one embodiment, leadframe 40 is stamped from a single, planar sheet of metal, such as the steel alloy known as “alloy 42,” or copper, or copper alloys with or without nickel or palladium plating. After the formation of leadframe 40, any additional modifications such as etching or coining may be carried out, if desired.

[0034] Semiconductor dies 12 are then attached to die pad 14 and wire bonds 18 are attached (as shown in FIGS. 1 and 2) using conventional die attach and wire bond procedures. Die 12 may be any conventional type of die. For example, die 12 may be a power MOSFET, a GaAs chip, a surface acoustical wave device, an optical chip (e.g., a camera chip), or an erasable memory chip, among other possibilities.

[0035] Shield elements 24 are then placed on leadframe 40 as shown in the top view of FIG. 6 and the front view of FIG. 3. Shield element 24 includes legs 28 which contact some of the leads 16 in leadframe 40. The open spaces between legs 28 provide additional apertures for the entry of mold compound into the interior of shield element 24, thereby reduced the required number and/or diameter of holes 26 in shield element 24.

[0036] To assist in the alignment of a shield element 24 with leadframe 40, each leg 28 may have a concave lower surface as shown in FIG. 7. The concave lower surface of leg 28 is shaped to receive the upper portion of a lead 16 (shown in cross section in the figure). The same shape of leg 28 may also be used to mate a corner leg 28 with a diagonal support element 20 of the leadframe (shown in FIG. 5). Alternatively, as shown in FIG. 8, leg 28 may have a convex lower surface designed to mate with a corresponding recess in the upper surface of a lead 16 or diagonal support element 20.

[0037] Shield elements 24 may be attached to leadframe 40 by means of solder or epoxy in a conventional manner. Mold compound may then be injection-molded around leadframe 40 and shield elements 24 to form an entire array of packages. Conventional molding compounds are used, as described above.

[0038] To reduce the possibility of air pockets remaining inside shield element 24 during injection molding, shield element 24 may have rounded corners as shown in the top view of FIG. 4 and the side view of FIG. 9. Alternatively, shield element 24 may be circular in shape as shown in the top view of FIG. 10. In this case the side view remains substantially the same as that shown in FIG. 9.

[0039] After the formation of package mold 22, the semiconductor chip packages 10, shown in outline by dashed lines 42 in FIG. 5, then separated from each other by a sawing, punching, or equivalent cutting process. In one embodiment, the spacing between adjacent packages and the width of the saw blade are such that the region of leadframe material and mold compound between the dashed lines 42, including connecting strip 44, is obliterated in a single pass of the saw blade. A suitable saw process is described in more detail in the aforementioned co-pending U.S. patent applications, Ser. Nos. 09/395,875 and 09/393,016, and will not be described further herein. In this embodiment, each package 10 has square corners as shown by dashed lines 42 rather than chamfered corners 22aas previously illustrated. While additional saw blade passes may be used to create chamfered corners, this may not prove economical for a mass production environment.

[0040] The above-described method for creating packages such as package 10 in quantity may be the most efficient method for creating such packages in a mass production environment. However, package 10 may also be individually molded, particularly for small production runs or for testing. In that case, package mold 22 is formed for only one package in the injection molding process. Package mold 22 may therefore be formed in the desired shape, with chamfered corners 22aas previously described.

[0041] It will be appreciated that the above-described packaging methods yield a shielded semiconductor package with leads and a die pad at least partially exposed on their lower surfaces. This package may be used in applications where minimization of electromagnetic interference with the operation of the chip is needed.

[0042] Of course, electromagnetic shielding may be used in various types of leadframe-based packages. Another example of such a package is shown in FIG. 11. In this figure, a flip-chip package 50 is shown in cross section. In package 50, the leadframe consists of gull-wing shaped leads 52. Package 50 has no die pad. Instead, semiconductor die 54 is mounted on leads 52. Bond pads on die 54 are electrically and physically connected to leads 52 by a connection material 56, which may be solder. An electromagnetic shield element 58, similar to those previously described, is attached to selected leads 52, which may establish a ground connection of shield element 58. A package mold 60 is provided around die 54, shield element 58 and the inner portions of leads 52.

[0043] The fabrication process for package 50 is very similar to that previously described. A leadframe including leads 52 and connecting portions is provided. Die 54 is then mounted on the leadframe. Shield element 58 is placed on the leadframe in contact with selected leads 52. Shield element 58 may be aligned with the leadframe using the interlocking leg and lead shapes previously described. Shield element 58 may be attached to the leadframe using solder or adhesive. Package mold 60 is then formed around die 54 and shield element 58 using conventional injection molding techniques and materials. A conventional trim-and-form process may then be used to separate leads 52 from the connecting portions of the leadframe and form leads 52 into the gullwing shape shown in FIG. 11. When package 50 is mounted on a printed circuit board, the printed circuit board may have a ground plane which serves to complete the electromagnetic shielding of die 54.

[0044] Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made therein without departing from the spirit and scope of the invention as defined by the appended claims.