# Patent

## Title
Methods and apparatus for improved thermal performance and electromagnetic interference (EMI) shielding in leadframe integrated circuit (IC) packages

## Abstract

Methods and apparatus for improved thermal performance and electromagnetic interference (EMI) shielding in integrated circuit (IC) packages is described. A die-up or die-down package includes a heat spreader cap defining a cavity, an IC die, and a leadframe. The leadframe includes a centrally located die attach pad, a plurality of leads, and a plurality of tie bars that couple the die attach pad to the leads. The IC die is mounted to the die attach pad. A planar rim portion of the cap that surrounds the cavity is coupled to the leadframe. The cap and the leadframe form an enclosure structure that substantially encloses the IC die, and shields EMI emanating from and radiating towards the IC die. The enclosure structure also dissipates heat generated by the IC die during operation.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
The following patent application of common assignee is herein incorporated by reference in its entirety: “Apparatus and Method for Thermal and Electromagnetic Interference (EMI) Shielding Enhancement in Die-Up Array Packages, U.S. patent application Ser. No. 10/870,927, filed Jun. 21, 2004
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates generally to the field of integrated circuit (IC) device packaging technology and, more particularly to thermal enhancement and electromagnetic interference (EMI) shielding in IC device leadframe packages.
2. Background
Integrated circuit semiconductor chips or dies are typically mounted in or on a package that is attached to a printed circuit board (PCB). Leadframe is widely used in IC packages as a carrier for the IC die and as an interconnection mechanism between the die and the electrical circuits of the PCB. Various leadframe packages have been developed and package family outlines have been standardized by the Electronic Industries Alliance (EIA), the Joint Electron Device Engineering Council (JEDEC), and the Electronic Industries Alliance of Japan (EIAJ).
However, commercially available leadframe packages have poor thermal performance and EMI shielding. Thus, what is needed is reduced EMI susceptibility and emission, in combination with improved thermal and electrical performances in integrated circuit packages. Furthermore, enhanced environmental is also desirable for integrated circuit packages.
BRIEF SUMMARY OF THE INVENTION
The present invention is directed to methods and apparatus for improved thermal performance and EMI shielding in IC packages.
In an aspect of the present invention, an leadframe IC device package includes a heat spreader cap (“cap”) having opposing first and second surfaces. A first portion of the second surface of the cap has a cavity formed therein. A planar second portion of the second surface of the cap is coupled to a leadframe. The leadframe includes a die attach pad (DAP), a plurality of leads, and a plurality of tie bars coupled to the DAP. At least one IC die is mounted to the die attach pad. The cap and the leadframe form an enclosure structure that substantially encloses the IC die(s).
In another aspect of the present invention, an IC device package is assembled. A leadframe is formed. At least one IC die is attached to a die attach pad portion of the leadframe. Wire bonds are coupled between the IC die and the leadframe. A cap is attached to the leadframe. A second surface of the cap includes a cavity formed therein. The cap and leadframe form an enclosure structure that substantially encloses the at least one IC die. An encapsulating material is applied to encapsulate at least the IC die. A perimeter support ring portion of the leadframe is trimmed. Portions of the leads are bent to form shoulder bends, to aid coupling of ends of the leads to a circuit board. The enclosure structure spreads heat from the IC die(s) during operation. Furthermore, the enclosure structure shields EMI emanating from the IC die(s) and EMI radiating toward the IC die(s) from outside the package.
These and other advantages and features will become readily apparent in view of the following detailed description of the invention. Note that the Summary and Abstract sections may set forth one or more, but not all exemplary embodiments of the present invention as contemplated by the inventor(s).



BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES
The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention.
FIG. 1 illustrates a typical conventional plastic quad flat package (PQFP).
FIG. 2 illustrates example heat dissipation paths in and from a typical PQFP.
FIGS. 3A-3D illustrate example ball grid array (BGA) integrated circuit (IC) packages.
FIGS. 4A-4B illustrate example leadframe IC packages.
FIGS. 5A-5E show examples of heat spreader caps (caps) according to embodiments of the invention.
FIGS. 6A-6C show plan views of examples of leadframes according to embodiments of the invention.
FIGS. 7A-7H show cross-sectional views of examples of leadframe IC packages, according to embodiments of the invention.
FIGS. 8A-8D show plan views of examples of leadframe IC packages undergoing assembly, according to embodiments of the invention.
FIGS. 9A-9C show side views of examples of leadframe IC packages undergoing assembly, according to embodiments of the invention.
FIGS. 10A and 10B show flowcharts illustrating example embodiments for assembling leadframe IC packages, according to embodiments of the invention.



The present invention will now be described with reference to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements. Additionally, the left-most digit(s) of a reference number identifies the drawing in which the reference number first appears.
DETAILED DESCRIPTION OF THE INVENTION
Overview
The present invention is directed to methods and apparatus for improving thermal performance and electromagnetic interference (EMI) shielding in integrated circuit (IC) packages. In embodiments of the invention, an IC die is mounted to a die attach pad (DAP) in the center of a leadframe. In an embodiment, the package is configured in a die-up configuration. In another embodiment, the package is configured in a die-down configuration.
In embodiments of the invention, wire bonds may be used to electrically connect die to leads of the leadframe and/or to the DAP. A metal heat spreader (“cap”) is coupled (e.g. electrically, structurally, and/or thermally connected) to the leadframe to form an enclosure structure. In an embodiment, the coupling may be effected with or without the use of a thermally and/or electrically conductive adhesive, such as solder or epoxy with metal particles or flakes. In an embodiment, the cap is coupled to the leadframe tie bars. The leadframe tie bars may be widened or fused to leads. In another embodiment, the cap is coupled to the leads. In yet another embodiment, the cap is coupled to the DAP. The cap may be coupled with any combination of DAP, leads, and tie bars. In an embodiment, tabs on the cap mate with matching receptacles on the leadframe to improve coupling and overall structural strength.
The enclosure structure formed by a cap and a leadframe approximate an equipotential surface, or Faraday Cage, surrounding die and interconnections. In an embodiment, the enclosure structure material is also a very good conductor of heat and is relatively rigid (e.g., copper or copper alloy C151). The enclosure structure may provide improved EMI shielding, improved heat transfer from the one or more die, enhanced rigidity of the package, and improved environmental (e.g., mechanical shock, vibration, impact, stress, temperature, moisture, corrosion, etc.) protection.
In an embodiment, the die and the wirebond are encapsulated in an encapsulating material, such as a molding compound, which provides environmental protection. The encapsulating material may also completely cover the cap. In other embodiments, the cap is partially covered, or is not covered by the encapsulating material.
It is noted that references in the specification to “one embodiment”, “an embodiment”, “an example embodiment”, etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.
Example Integrated Circuit Packages
FIG. 1 shows a cross-sectional view of a plastic quad flat package (PQFP) 100. An IC die 150 is attached with thermally and/or electrically conductive adhesive 170 to a die attach pad (PAD) 140 portion of a typically copper or copper alloy leadframe 110. Wirebonds 130 form electrical interconnections between die 150, DAP 140, and leadframe leads 180. IC die 150 and wirebonds 130 are molded in encapsulating material 120 for environmental protection, which is typically plastic. Leads 180 (if any) can be straight or bent and extend from one or more sides of package 100. Different families of leadframe packages are further discussed in C. A. Happer, Electronic Packaging and Interconnection Handbook, 3rd edition, McGraw-Hill, New York, pp. 7.61-7.67, 2000, which is incorporated by reference herein in its entirety.
Plastic molding compound encapsulated leadframe packages 100 commonly exhibit poor thermal performance. In leadframe package 100, DAP 140 is typically separated from leadframe leads 180 that extend beyond encapsulating material 120. Heat dissipation paths in and from plastic quad flat pack (PQFP) package 100 are shown in FIG. 2. Heat generated on the active surface of die 150 is conducted via paths 210 into encapsulating material 120 and leadframe 110. Leadframe 110 conducts some heat into a PCB 160 to which package 100 is attached. Encapsulating material 120 transfers heat to the environment through convection path 220 and radiation path 230. Typical encapsulating materials 120 have a low thermal conductivity value, such as around or between 0.2 ̃0.9 W/m·K. Therefore, the temperature of die 150 must rise to a relatively high value to transfer the heat generated during operation through encapsulating material 120.
In addition, leadframe packages 100 commonly exhibit poor electromagnetic interference (EMI) shielding. A change in the electrical current carried by a conductor results in the radiation of electromagnetic waves. Such waves propagates through space at the speed of light, and when not wanted, are called EMI. A relatively slow change in the electrical current causes a small amount of electromagnetic radiation with a long wavelength and a low frequency. A relatively rapid change in the electrical current causes a large amount of radiation with a short wavelength and a high frequency. The unwanted high frequency electromagnetic radiation is sometimes called radio-frequency interference (RFI), but in the interest of brevity, this document refers to all unwanted electromagnetic radiation as EMI, regardless of frequency.
IC die 150 are more susceptible to higher frequency EMI. Because higher frequencies are more energetic, they may cause larger voltage swings in the metal traces on an IC die. Because modern IC gates are small in size, they operate with a low signal voltage. Thus, signal line voltage swings caused by high-frequency EMI may cause a change in logic state and may result in timing and logic failures in electronic devices.
Encapsulating materials 120 are typically transparent to electromagnetic radiation. Referring to FIG. 1, the electromagnetic radiation generated by die 150 will escape from package 100 and potentially interfere with the operation of nearby components. Conversely, EMI from nearby components will enter package 100 and may interfere with the operation of die 150.
FIG. 3A illustrates a ball grid array (BGA) package having improved performance. FIG. 3A shows a cross-sectional view of a BGA package 300 with an IC die 150 mounted on a printed circuit substrate 310, encapsulated by a encapsulating material 120, and electrically connected to PCB 160 through solder balls 330. For further detail on a package similar to package 300, see U.S. Pat. No. 5,977,626, “Thermally and Electrically Enhanced PBGA Package,” to Wang et al., which is incorporated by reference in its entirety. BGA package 300 includes a drop-in heat spreader 320 to promote dissipation of heat within encapsulating material 120. However, direct contact between IC die 150 and heat spreader 320 is not permitted in package 300. This is to avoid shorting the active surface of IC die 150 and wirebonds 130 with heat spreader 320. Accordingly, heat generated by IC die 150 must pass through encapsulating material 120 in order to reach heat spreader 120, and may therefore remain trapped within BGA package 300. Furthermore, drop-in heat spreader 320 only provides limited EMI shielding, if any. For example, EMI generated outside BGA package 300 can penetrate printed circuit substrate 310 and interfere with the operation of IC die 150. Also, EMI generated by IC die 150 can escape BGA package 300 through trace metal openings or gaps in printed circuit substrate 310.
FIG. 3B illustrates a cross-sectional view of a BGA package 302, similar to BGA package 300, but with a differently configured heat spreader 325. For further detail on a package similar to package 302, see U.S. Pat. No. 6,552,428 “Semiconductor Package Having An Exposed Heat Spreader” to Huang et al., which is incorporated by reference herein in its entirety. BGA package 302 suffers from the same thermal and electromagnetic shielding deficiencies as BGA package 300. An encapsulating material 120 and a printed circuit substrate 310 may trap heat generated by an IC die 150 within BGA package 302. EMI generated inside of BGA package by die 150 may penetrate printed circuit substrate 310, escape package 302, and interfere with the operation of other devices. Conversely, EMI originating outside of BGA package 302 may penetrate printed circuit substrate 310 and interfere with the operation of die 150.
FIG. 3C illustrates a cross-sectional view of a BGA package 304, which provides a thermal and electrical connection between an IC die 150 and PCB 160 through a heat slug 360. For further detail on a package similar to package 304, see U.S. Patent Pub. No. 20030057550-A1, entitled “Ball Grid Array Package Enhanced with a Thermal and Electrical Connector”, which is herein incorporated by reference in its entirety. IC die 150 is directly attached to a top surface of a stiffener 340. A heat slug 360 is attached to a bottom surface of stiffener 340 and has a surface that is configured to be mounted to PCB 160. BGA package 304 promotes heat dissipation from IC die 150 to PCB 160, on which BGA package 304 is mounted. Heat slug 360 acts as a thermal and electric connection for heat and current flow from metal stiffener 340 to PCB 160. Stiffener 340 and heat slug 360 can both be metal. Stiffener 340 can be connected to the ground pad on die 150 through a wirebond 130. Although the grounded metal stiffener 340 could prevent penetration of some EMI, the entire top surface of die 150 is exposed to EMI from above.
FIG. 3D shows a cross-sectional view of a BGA package 306, which incorporates a metal stiffener 340 and a metal cap 350. For further detail on a package similar to package 306, refer to U.S. patent application Ser. No. 10/870,927, titled “Apparatus And Method For Thermal And Electromagnetic Interference (EMI) Shielding Enhancement In Die-Up Array Packages,” filed Apr. 23, 2004, which is herein incorporated by reference in its entirety. A die 150 is located inside of an enclosure formed by metal stiffener 340 and metal cap 350. Metal stiffener 340 is coupled (e.g., electrically, thermally, and/or structurally connected) to metal cap 350 to provide improved EMI shielding, thermal performance, and environmental protection.
FIG. 4A illustrates a “leadframe”-type package 400. For further detail on a package similar to package 400, refer to U.S. Pat. No. 5,294,826, titled “Integrated Circuit Package and Assembly Thereof for Thermal and EMI Management,” which is incorporated herein by reference in its entirety. A metal shield 410 is integrated into a die-down leadframe package 400. A top portion of leadframe package 400 is covered with an electrically grounded laminated metal shield 410. However, EMI can enter or exit through a bottom of the leadframe package 400, and a ground plane 420 is required on the PCB 430. A sufficiently sized gap between ground plane 420 and metal shield 410 may permit EMI to enter and exit leadframe package 400.
FIG. 4B illustrates a leadframe package 405. For further detail on a package similar to package 405, refer to U.S. Pat. No. 5,650,659, titled “Semiconductor Component Package Assembly Including an Integral RF/EMI Shield,” which is incorporated herein in its entirety. Package 405 incorporates a shield box 450 within leadframe package 402, completely encapsulated by encapsulating material 120. IC die 150 is mounted inside shield box 450. Shield box 450 is attached to leadframe 110 and electrically grounded. Shield box 450 has a dielectric inner layer and an electrically conductive outer layer of metallic foil. Package 405 suffers from the same thermal deficiencies as prior leadframe packages, such as package 100 shown in FIG. 1.
Example Cap Structures
Example embodiments for improved cap structures are described in this section. Further embodiments will become apparent to persons having skill in the relevant art(s) from the teachings herein. Elements of the embodiments described herein can be combined in any manner.
FIG. 5A illustrates a cross sectional view of a cap 510. FIG. 5B illustrates a bottom view of cap 510, in accordance with an embodiment of the present invention. Cap 510 may be incorporated into various integrated circuit packages, such as shown in FIGS. 7A-7H, which are described in detail below. The packages may incorporate leadframes, such as shown in FIGS. 6A-6C, which are described in detail below.
In an embodiment, cap 510 has a top portion 590, sidewall portion 592, and a rim 594 extending around a bottom periphery of cap 510. Sidewall portion 592 couples (e.g., electrically, structurally, and thermally) top portion 590 to rim 594. Further, sidewall portion 592 is angled outward from top portion 590. Although FIG. 5A illustrates a planar top portion 590, top portion 590 can be non-planar (e.g., curved, concave, convex, hemispherical, or other shapes). Although FIGS. 5A and 5B illustrate an angled-outward sidewall portion 592, sidewall portion 592 may be perpendicular to or angled inward from top portion 590. Furthermore, sidewall portion 592 is not limited to a linear cross-section and may employ other cross-sectional shapes such as convex inward and outward as would be understood by one skilled in the art.
Cap 510 further has a first surface 580 and a second surface 585. Second surface 585 forms an upper surface of a cavity 570 in a bottom portion of cap 510. Rim 594 surrounds cavity 570. Cavity 570 is shown in FIG. 5A as having a trapezoidal cross section, but may have other shapes (e.g., square, rectangular, irregular, etc.). Although FIG. 5B illustrates cavity 570 having a circular shape, cavity 570 may have other shapes. Further, cap 510 may have various shapes such as round, rectangular, square, elliptical, oval, or any other shape.
In cap 510, a bottom surface of rim 594 has one or more protruding tabs 515a-e. Tabs 515a-e may have any shape. For example, FIGS. 5A and 5B show a frustum tab 515a, a conical tab 515b, a pair 517 of conical tabs 515c and 515d, and an oblong shaped tab 515e. Cap 510 is not limited to the shapes, sizes, locations, or numbers of tabs 515 shown. Cap 510 may also have zero or more tabs of any shape, of any size, in any locations.
The outer periphery dimension of cap 510 is preferably the same size as the periphery (e.g., see FIG. 7C) or smaller than the periphery (see FIG. 7A) of the leadframe's “shoulder bends” to facilitate visual inspection of lead interconnect on the PCB. In an alternative embodiment, the outer periphery dimension of cap 510 extends beyond the periphery of the leadframe's “shoulder bends”. For manufacturing considerations, the outer periphery of cap 510 is preferably smaller than the dimension of the leadframe support ring 630 (e.g., see FIGS. 6A and 8C, further described below). Although cap 510 is illustrated having a particular size, other sizes may be used, as would be understood by persons skilled in the relevant art(s).
In an embodiment, cap 510 may be configured to mount an external heat sink (e.g., see FIG. 7F, further described below). In another embodiment, cap 510 may be configured to be thermally and/or electrically connected to a printed circuit board (PCB). For example, cap 510 may be coupled to the PCB (e.g., see FIGS. 7G and 7H, further described below).
Cap 510 may be made of a thermally conductive material and/or an electrically conductive material, such as a metal. For example, the material for cap 510 may include copper, a copper alloy, (e.g., C194, C151, C7025, or EFTEC 64T), aluminum, an aluminum alloy, ferromagnetic materials, laminated copper or iron, etc. Other metals and combinations of metals/alloys, or other thermally and electrically conductive materials (e.g., ceramics, metallized plastics, laminated metal foils on plastic or ceramic, etc.) could also be used. Cap 510 and leadframe 110 may be made of the same material or different materials. When cap 510 and leadframe 110 are made of the same material, or materials having the same coefficient of thermal expansion, structural integrity may be improved, such as reducing thermal stress on the die (sandwiched between the cap and leadframe). Furthermore, cap 510 may have any thickness, depending on the particular application. For example, cap 510 may have a thickness of 0.1 to 0.5 mm. Alternatively, cap 510 may have a thickness of less than 1.0 mm.
In an embodiment, the bottom surface or portions of the bottom surface of rim 594 may be coated or laminated with a layer of dielectric material (e.g. solder mask, dielectric film etc.). In this manner, the shorting of leads after assembly may be prevented.
Furthermore, in an embodiment, cap 510 may have openings through the first surface 580 and the second surface 585. For example, FIGS. 5C and 5D show example caps 510 having openings or slots 520 formed in sidewall portions 592, according to embodiments of the present invention. Although FIGS. 5C and 5D illustrate slots 520 in sidewall portion 592 as rectangular or trapezoidal, slots 520 can have other shapes.
Furthermore, cap 510 may have holes/openings 530 in top portion 590 as illustrated in FIG. 5E, according to an example embodiment of the present invention. Cap 510 may have any number of holes. Furthermore, holes 530 can have any shape.
In cap 510, holes 530 and slots 520 allow the flow of encapsulating material 120 into cavity 570 during a manufacturing process. Additionally or alternatively, slots 520 and holes 530 may release pressure buildup (during or after manufacture) occurring in cavity 570. Because smaller holes 530 and slots 520 may require a higher pressure to flow or inject encapsulating material 120 into cavity 570, larger holes 530 and slots 520 may be desirable from a manufacturing perspective. However, in an embodiment, cap 510 may require the size of holes 530 and slots 520 to be limited to reduce EMI penetration. In an embodiment, a hole 530 or slot 520 diameter is in the range of 0.5-3.0 mm. In an embodiment, a diameter 1.5 mm may be used to shield against EMI having a highest harmonic frequency of about 10 GHz. An outer surface of cap 510 may be completely or partially encapsulated in encapsulating material 120, or may have no encapsulating material 120 covering it.
Example Leadframe Structures
Example embodiments for leadframe structures are described in this section. Further embodiments will become apparent to persons having skill in the relevant art(s) from the teachings herein. Elements of the leadframe embodiments described herein can be combined in any manner.
FIGS. 6A-6C illustrate various leadframe structures, according to example embodiments of the present invention. FIG. 6A shows a leadframe 600 having a die attach pad (DAP) 605, a plurality of leads 607, a plurality of tie bars 620, an inner support ring 630, and a perimeter support ring 632. In FIG. 6A, leadframe 600 is rectangular in shape, having a rectangular perimeter support ring 632 surrounding its periphery. Perimeter support ring 632 includes a first perimeter edge 634a, a second perimeter edge 634b, a third perimeter edge 634c, and a fourth perimeter edge 634d, coupled in a rectangular ring. DAP 605 is centered in leadframe 600. DAP 605 is rectangular in shape. In the embodiment of FIG. 6A, tie-bars 610 extend outward from the four corners of DAP 605.
Leads 607 extend inward perpendicularly from perimeter support ring 632. Leads 607 are also coupled to inner support ring 630, which forms a rectangular shape surrounding DAP 605. Leads 607a-h are coupled to tie bars 620. Lead 607a is coupled between edge 634a of lead frame 600 and tie bar 620a. Lead 607b is coupled between edge 634a of lead frame 600 and tie bar 620b. Lead 607c is coupled between edge 634b of lead frame 600 and tie bar 620b. Lead 607d is coupled between edge 634b of lead frame 600 and tie bar 620c. Lead 607e is coupled between edge 634c of lead frame 600 and tie bar 620c. Lead 607f is coupled between edge 634c of lead frame 600 and tie bar 620d. Lead 607g is coupled between edge 634d of lead frame 600 and tie bar 620d. Lead 607h is coupled between edge 634d of lead frame 600 and tie bar 620a. Leads 607 are supported by perimeter support ring 632 and inner support ring 630 in lead frame 600. Leads 607 (except leads 607a-h) include an inner lead portion 636 within inner support ring 630 that are generally oriented radially with respect to a center leadframe 600.
Although FIGS. 6A-6C illustrate a square leadframe 600, DAP 605, and inner support ring 630, other shapes could also be employed (e.g., rectangle, circle, ellipse, curvilinear rectangle, etc). Furthermore, the number of leads 607 is not limited by FIG. 6A, and in embodiment, leadframes may have any number of leads 607.
Further, tie-bar 610 may be widened, and may be located at other positions around DAP 605 than shown in FIG. 6A. Any number of leads 607 may be fused to a tie-bar, which may further effectively widen the tie-bar. FIG. 6B shows a tie-bar 620x coupled between DAP 605 and first and second leads 607x and 607y at a point 640. Leadframe 600 may have one or more fused tie bar leads 620, widened fused leads 640, or both. Alternatively, leadframe 600 may have no widened fused leads 640 nor fused tie-bar leads 620. Furthermore, as shown in FIG. 6B, lead frame 600 may have one or more tie bars 610 that are not coupled to leads 607.
In an embodiment illustrated in FIG. 6C, tie-bars 620a-d have receptacles 615 formed therein. Receptacles 615 correspond to tabs 515 formed in a cap 510. As with tabs 515, receptacles 615 can include a rectangular shaped receptacle 615a, a pair 617 of conical shaped receptacles 615b and 615c, a pair 619 of rounded receptacles 615d and 615e, and a rounded receptacle 615f. However, receptacles 615 are not limited to these shapes, combinations of shapes, numbers, locations, or sizes. Receptacles 615 may be indentions (not fully penetrating the leadframe 600) or may be cut-outs (fully penetrating the leadframe 600). Leadframe 600 may have any number of receptacles 615 of any size, shape, and in locations. Receptacles 615 on leadframe 600 are configured to couple with tabs 515 on a cap 510 providing increased structural strength, as well as enhanced thermal and electrical connection.
Example materials for leadframe 600 include metals, such as copper, copper alloy, (e.g., C194, C151, C7025, or EFTEC 64T), aluminum, aluminum alloys, ferromagnetic materials, other metals and combinations of metals/alloys, or other thermally and electrically conductive materials. Cap 510 and leadframe 600 may be made of the same material or different materials. Leadframe 600 may be any thickness depending on the particular application. For example, leadframe 600 thickness may range from 0.05 mm to 0.5 mm. In another embodiment, leadframe 600 is less than 1.17 mm thick.
In an embodiment, leadframe 600 provides stiffening and/or structural support to an IC package. In another embodiment, leadframe 600 provides heat spreading to an IC package. In another embodiment, leadframe 600 is electrically conductive, and can act as a power or ground plane for an IC package. In embodiments, leadframe 600 can be configured to provide any combination of stiffening, heat spreading, and electrical conductivity, as required by the particular application.
Example Leadframe/Cap Enclosure Structure
Example embodiments for IC packages are described in this section. Further embodiments will become apparent to persons having skill in the relevant art(s) from the teachings herein. Elements of the IC package embodiments described herein can be combined in any manner.
FIG. 7A shows an example IC package 700, according to an embodiment of the invention. As shown in FIG. 7A, cap 510 is coupled to leadframe 600. As illustrated in FIG. 7A, a die 150 is mounted on the same side of DAP 605 as cap 510. Leadframe 600 and cap 510 form an enclosure structure 702 that substantially encloses die 150, providing improved structural integrity, EMI shielding, thermal performance, and environmental (e.g., mechanical shock, vibration, caustic, moisture, and radiation) protection. Note that in embodiments, additional dies and/or other electrical components can be attached to DAP 605.
In an embodiment, cap 510 and leadframe 600 are made of copper or copper alloys. The thermal conductivity of copper (roughly 390 W/m·K) is much greater than for typical encapsulating materials 120 (0.2-0.9 W/m·K). Therefore, the heat generated by die 150 is conducted through adhesive 170 to DAP 605 and out of the package through leads 607 and cap 510. Also, since cap 510 and leadframe 600 are electrically connected, they may form a near-equipotential surface, such that enclosure structure 702 approximates an ideal Faraday Cage. In this manner, die 150 is isolated from external EMI. Additionally, external devices are also shielded from EMI generated by die 150. Since copper and copper alloys have a much higher modulus of elasticity (about 125 GPa) compared to a typical cured plastic molding compound used for encapsulating material 120 (about 25 GPa), copper embodiments of the present invention provide improved structural rigidity and environmental protection.
In an embodiment, cap 510 and leadframe 600 are coupled together without the use of tabs and receptacles. In another embodiment, as shown in FIG. 7A, cap 510 has tabs 515 which fit corresponding receptacles 615. Tabs 515 and corresponding receptacles 615 may facilitate tight lock-in of the cap 510 to leadframe 600. Further, the configuration of tabs 515 and receptacles 615 are such that cap 510 will mate correctly with leadframe 600 in only one orientation, which may facilitate assembly. Note that in an alternative embodiment, cap 510 may have receptacles that interlock with tabs of leadframe 600.
Thermally and/or electrically conductive adhesive materials (e.g., epoxy filled with metal or other conductive flakes, solder, etc.) may be used to improve the coupling between cap 510 and leadframe 600. An adhesive material can be used to attach a tab 515 and a receptacle 615, when they are present. Alternatively, the adhesive material may be used at areas where cap 510 contacts leadframe 600.
Leadframe 600 may be plated with a conductive material to improve the thermal and electrical connection. In an embodiment, cap 510 may be mounted to DAP 605 of leadframe 600. In another embodiment, as shown in FIG. 7A, cap 510 is mounted to tie-bars 620 coupled between DAP 605 and leads 607. In yet another embodiment, cap 510 may be mounted to one or more leads 607. In embodiments, cap 510 can be mounted to any combination of DAP 605, tie bars 620, and leads 607. Further, portions of the bottom surface, or all of the bottom surface of rim 594 of cap 510 may be coated with a layer of dielectric material (e.g. solder mask, dielectric film etc.) to prevent electrical shorting with one or more of leads 607.
As shown in FIG. 7A, lead 607 of leadframe 600 are shaped to be coupled to a PCB. For example, as shown in FIG. 7A, an outer portion of leads 607 extending from package 700 may be bent to allow leads 607 to contact a PCB. For instance, leads 607 may be bent to form an “L” or “hockey stick” type shape, having a first bend 720, and a second bend 722. End portion 724 of leads 607 can be coupled to PCB 160, as shown in FIG. 7A. As shown in FIG. 7A, leads 607 may be bent toward a side of package 700 away from die 150 to form a “die up” package. Alternatively, leads 607 may be bent toward a side of the package toward die 150 to form a “die down” package, (such as shown in FIGS. 7G and 7H).
Further Example Integrated Circuit Packages
Integrating an encapsulating material, such as glob top or plastic molding compound, with an enclosure structure, such as enclosure structure 702, may enhance the structural rigidity and planarity of the IC package. For example, the combination of the encapsulating material and the enclosure structure may reduce IC die cracking and delamination. Integrating the encapsulating material with the enclosure structure also enhances environmental protection. For example, the integrated package can provide protection against mechanical stress, impact, vibration, chemical corrosives, moistures, heat exposure, radiation, etc.
Additionally, attaching the IC die directly to the enclosure structure adds mass to the die support, and helps reduce microphonics. The metal traces of the IC die have electrical resistance, capacitance, and inductance. After IC packaging and assembly of the package on the PCB, the IC die is under mechanical stress. Vibration, mechanical shock, or sudden change of temperature can cause a change of stress distribution within the IC die, and thus alter a capacitance and resistance such that a voltage vibration or drift is produced. This phenomenon is called microphonics. Attachment of the semiconductor die directly to the enclosure structure increases the mass and helps dampen these mechanical shocks and vibrations, thus reducing microphonics.
Typical encapsulating materials, such as plastic molding compound, have low thermal conductivity (e.g., about 0.2 to 0.9 W/m·K) and therefore create a bottleneck for heat dissipation in conventional IC packages. In an embodiment, the enclosure structure eliminates this bottleneck by providing a thermally conductive path from the bottom surface of the IC die to the outer surfaces of the package. Additionally, the enclosure structure is made with materials that have high thermal conductivity (e.g., approximately 390 W/m·K for copper) and therefore promote heat dissipation.
Enclosure structure 702 formed by cap 510 and leadframe 600 may be incorporated into IC packages of many different configurations. FIGS. 7A-7H illustrate some example embodiments of the present invention. For example, package 700 of FIG. 7A shows die 150 attached to a DAP 605 with a thermally and/or electrically conductive adhesive 170 (such as an epoxy with metal or other conductive particles or flakes, solder, etc.) that is electrically connected through wirebond 130, DAP 605 and leads 607. As described elsewhere herein, cap 510 is coupled with leadframe 600 to form an enclosure structure 702 substantially enclosing die 150. Package 700 is encapsulated in encapsulating material 120. Package 700 may be mounted to a printed circuit board (PCB) 160, including printed wiring boards (PWBs). In package 700, at least one wirebond 130 couples at least one bond pad (not shown) on a surface of IC die 150 to leadframe 600. In an embodiment, one of the bond pads is a ground pad.
Although not shown in FIGS. 7A-7H, an package may include a cap 510 having one or more openings (e.g. slots 520 and/or holes 530) as described elsewhere herein. These openings may act as mold gate openings, allowing encapsulating material 120 to flow or be injected into cavity 570. As shown in FIG. 7A, cap 510 has a surface 704 that is exposed through the molding material 120 encapsulating package 700. Thus, encapsulating material 120 does not cover the entirety of first surface 580 of cap 510. In FIG. 7A, second surface 585 of cap 510 is covered by encapsulating material 120. FIG. 7B illustrates an embodiment where both first surface 580 and second surface 585 of cap 510 are covered by encapsulating material 120. FIG. 7C illustrates an embodiment where first and second surfaces 580 and 585 of cap 510 are not covered by encapsulating material 120. An IC package may be in the form of this embodiment if cap 510 is added to package 704 after encapsulating material 120 (e.g., a molding compound) has been applied.
As illustrated in FIGS. 7D and 7E, leadframe 600 may have tie bars 620 on a different plane than the leadframe bond fingers or leads 750. In the embodiment of FIG. 7D, tie bars 620 are above leads 750. In the embodiment of FIG. 7E, tie bars 620 are below leads 750. The embodiment of FIG. 7E has the advantage of requiring shorter wirebonds 130 from the top of IC die 150 to leads 750, relative to the embodiment of FIG. 7D.
FIG. 7F shows an IC package 710 where cap 510 is configured to mount an external heat sink 730. Leadframe 600 and cap 510 provide a path for heat to be conducted from die 150 to heat sink 730, and thus out of package 710.
Embodiments of the present invention are not restricted to die-up configurations. FIG. 7G illustrates a package 712 in a die-down configuration, according to an example embodiment of the present invention. In comparison to package 700 in FIG. 7A, surface 580 of cap 510 is exposed on the bottom side of package 712. The features reflected in the embodiments illustrated in FIGS. 7A-7F are applicable to the die-down configuration embodiment illustrated in FIGS. 7G and 7H. FIG. 7H illustrates a package 714, according to another embodiment of the present invention. In package 714, exposed surface 580 of cap 510 is coupled to PCB 160 with electrically and/or thermally conductive adhesive or solder 740. Thus, heat conducted from IC die 150 through leadframe 600 and cap 510 may be conducted out of package 714 into PCB 160. Surface 580 of cap 510 may be electrically connected to a power pad or pads (not shown in FIG. 7H) on PCB 160 to improve EMI shielding and power delivery from package 714. Alternatively, surface 580 of cap 510 may be electrically connected to ground pad or pads (not shown in FIG. 7H) on PCB 160 to improve EMI shielding and current return from package 714.
Example Manufacturing Processes
FIG. 10A shows flowchart 1000 illustrating example steps to assemble leadframe package 700 shown in FIG. 7A, according to an embodiment of the present invention. FIG. 10B shows flowchart 1050 illustrating example steps for an alternate method to assemble package 700. As would be understood by one skilled in the art, adaptation of these assembly processes could be used to assemble any embodiments, including those illustrated in FIGS. 7A-7H. The steps in FIGS. 10A and 10B do not necessarily have to occur in the order shown, as will be apparent to persons skilled in the relevant art(s) based on the teachings herein. Other operational and structural embodiments will be apparent to persons skilled in the relevant art(s) based on the following discussion. These steps are described in detail below with respect to FIGS. 8A-8D and 9A-9C, for illustrative purposes. FIGS. 8A-8D illustrate top views and FIGS. 9A-9C show side views of embodiments of the invention at different stages of assembly.
Flowchart 1000 is shown in FIG. 10A, and begins with step 1005. In step 1005, a leadframe 600 is formed from a sheet of material. Example leadframe material and features are discussed elsewhere herein. FIG. 8A illustrates a view of a single leadframe 600. FIG. 8B illustrates an example leadframe panel 800 that contains an array of leadframes 600. Leadframes 600 in leadframe panel 800 are manufactured by an etching or stamping process, for example.
In step 1010, at least one IC die 150 is attached to a DAP 605 of a leadframe 600. IC die 150 is attached using a thermally and/or electrically conductive adhesive 170 (such as solder or epoxy containing metal or other conductive particles or flakes). FIG. 9A illustrates a side view of an embodiment at this stage of assembly.
In step 1015, wirebond 130 are used to attach pads of IC die 150 to leadframe 600, providing electrical connections from IC die 150 to leads 607, tie bars 610, and/or DAP 605.
In step 1020, cap 150 is attached to the leadframe 600. Electrically and/or thermally conductive adhesive materials may be used to improve coupling between cap 510 and leadframe 600. Cap 510 and leadframe 600 are joined to form an enclosure structure (e.g., enclosure structure 702) which substantially encloses IC die 150. FIG. 8C shows a partially assembled package 810, illustrating an example embodiment leadframe package at this stage of assembly. FIG. 8D illustrates a partially assembled panel 820 of partially assembled packages 810. FIG. 9B shows a side view of partially assembled panel 820.
In step 1025, an encapsulating process encapsulates partially assembled package 810 in encapsulating material 120. In an embodiment, the package or packages 810 may be clamped in a mold chassis to mold or shape a molding compound being used to encapsulate the package. FIG. 9C shows a side view of an encapsulated panel 910 of leadframe packages 700 at this stage of assembly. As described elsewhere herein, in an embodiment, an outer peripheral dimension of a cap 510 is smaller than a peripheral dimension of peripheral support ring 630. This prevents the encapsulating material from bleeding through gaps between leads 607. Inner support ring 630 may also provide sealing between the clamped mold chassis during the transfer molding process.
Leadframe support ring 630 is trimmed in step 1030. Leads 607 are ready to be formed into contact pins for board mount and a leadframe package 700 is completely assembled. For example, the outer portion of leads 607 extending from the package may be bent to allow them to contact a PCB. For example, leads 607 may be bent to form an “L” or “hockey stick” type shape. Furthermore, leads 607 may be bent toward a side of the package away from die 150 to form a “die up” package, or may be bent toward a side of the package toward die 150 to form a “die down” package.
Flowchart 1050 shown in FIG. 10B shows example steps for forming an integrated circuit package, according to another embodiment of the present invention. Steps 1005-1015 are the same as shown in FIG. 10A. However, instead of coupling a cap 510 to a leadframe 600 outside of the molding chassis, a leadframe 600 and a cap 510 are put into the mold chassis for steps 1055 and 1060.
In step 1065, a molding process takes place. When the mold chassis are clamped together in this step, leadframe 600 and cap 510 are coupled together, and in an embodiment, may be held together by a molding compound.
CONCLUSION
While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

## Claims

1. An integrated circuit (IC) device package, comprising:
an IC die;
a leadframe, comprising:
a centrally located die attach pad;
a plurality of leads; and
a plurality of tie bars that each couple the die attach pad to at least one of the leads; and

a heat spreader cap defining a cavity, the cap comprising:
a top,
a sidewall coupled to the top, wherein the sidewall has an outer surface that opposes the cavity, and
a rim that extends out from a segment of the outer surface of the sidewall,

wherein the IC die is mounted to the die attach pad and
wherein the cap and the leadframe form an enclosure structure that substantially encloses the IC die.

2. The package of claim 1, wherein the package is in a die-up configuration.

3. The package of claim 1, wherein the package is in a die-down configuration.

4. The package of claim 1, wherein a tie bar couples the DAP to first and second leads.

5. The package of claim 1, wherein the cap is in electrical and thermal contact with at least one lead.

6. The package of claim 1, wherein the cap is electrical isolated from any of the plurality of leads.

7. The package of claim 1, wherein at least a portion of the rim is coated with a dielectric material.

8. The package of claim 1, wherein at least a portion of the leadframe coupled to the cap is coated with a dielectric material.

9. The package of claim 1, wherein at least one of the plurality of tie bars is wider relative to others of the plurality of tie bars.

10. The package of claim 1, wherein at least one of the plurality of leads is wider relative to others of the plurality of leads.

11. The package of claim 1, wherein the cap is in electrical and thermal contact with at least one tie bar.

12. The package of claim 1, wherein the plurality of tie bars and the plurality of leads are positioned in a first plane.

13. The package of claim 1, wherein the plurality of tie bars are positioned in a first plane and the plurality of leads are positioned in a second plane.

14. The package of claim 1, wherein the enclosure structure shields electromagnetic interference (EMI) emanating from the IC die, and shields the IC die from EMI radiating toward the IC die from outside the package.

15. The package of claim 1, wherein the planar rim portion of the cap is coupled to the leadframe by a thermally and electrically conductive adhesive.

16. The package of claim 1, further comprising:
at least one electrically conductive plated area patterned on the leadframe in one or more areas in contact with the planar rim portion of the cap.

17. The package of claim 1, wherein the cap is electrically insulated from the leadframe.

18. The package of claim 1, wherein the cap is coupled to a ground potential.

19. The package of claim 1, wherein the cap is coupled to a power potential.

20. The package of claim 1, further comprising:
at least one tab protruding from the rim; and
at least one receptacle formed in a surface of the leadframe corresponding to the at least one tab, wherein the at least one tab is coupled with the at least one corresponding receptacle, whereby structural coupling of the cap to the leadframe is substantially improved.

21. The package of claim 20, further comprising:
a thermally and electrically conductive adhesive in the at least one receptacle.

22. The package of claim 20, wherein the at least one tab has a conical, frustum, or laterally elongated shape.

23. The package of claim 20, wherein a tab is positioned on a corner of the rim.

24. The package of claim 20, wherein the at least one corresponding receptacle has an opening, indentation, or edge cutout configuration.

25. The package of claim 20, wherein the at least one tab and the at least one corresponding receptacle are configured to facilitate coupling the cap to the leadframe in a predetermined orientation.

26. The package of claim 1, further comprising:
at least one electrically conductive plated area patterned on a surface of the leadframe in contact with one or more wirebonds.

27. The package of claim 1, further comprising:
at least one wirebond that couples at least one bond pad on a surface of the IC die to the leadframe, whereby the enclosure structure is coupled to an electrical potential.

28. The package of claim 27, wherein the at least one bond pad is a ground pad, whereby the enclosure structure is coupled to a ground potential.

29. The package of claim 1, further comprising:
an encapsulating material that encapsulates the IC die.

30. The package of claim 29, wherein the cap has an outer surface that coincides with the outer surface of the sidewall, wherein a first portion of the outer surface is covered by the encapsulating material, and wherein a second portion of the outer surface of the cap is not covered by the encapsulating material.

31. The package of claim 29, wherein the encapsulating material further encapsulates an outer surface of the cap that coincides with the outer surface of the sidewall.

32. The package of claim 29, wherein the encapsulating material further encapsulates at least a portion of the leadframe.

33. The package of claim 1, wherein the cap has an outer surface that opposes the cavity, wherein the cap further comprises:
at least one opening through the cap that is open at the outer surface and in the cavity.

34. The package of claim 33, wherein the at least one opening through the cap is configured to facilitate flow of an encapsulating material into the cavity.

35. The package of claim 33, wherein the at least one opening through the cap is configured to facilitate release of an air pressure inside of the enclosure structure.

36. The package of claim 1, wherein the leads each have a shoulder bend portion along their lengths.

37. The package of claim 36, wherein a peripheral dimension of the cap substantially coincides with a peripheral dimension of the leadframe at the shoulder bend of the leads.

38. The package of claim 36, wherein a peripheral dimension of the cap is within a peripheral dimension of the leadframe at the shoulder bend of the leads.

39. The package of claim 36, wherein a peripheral dimension of the cap exceeds a peripheral dimension of the leadframe at the lead shoulder bend of the leads.

40. The package of claim 1, wherein the cap has an outer surface that opposes the cavity, further comprising:
a heat sink coupled to the outer surface of the cap.

41. The package of claim 3, wherein the cap has an outer surface that opposes the cavity, wherein the outer surface of the cap is configured to be coupled to a printed circuit board (PCB).

42. The package of claim 41, wherein a thermally and electrically conductive adhesive is used to couple the cap to the PCB.

43. The package of claim 1, wherein the sidewall makes an angle with the tie-bars that is less than 90 degrees.

44. An integrated circuit (IC) device package, comprising:
a leadframe, comprising:
a centrally located die attach pad;
a plurality of leads; and
a plurality of tie bars that each couple the die attach pad to at least one of the leads;

an IC die coupled to the die attach pad; and
a heat spreader cap defining a cavity, the cap comprising:
a top,
a rim, and
a sidewall that couples the top to the rim, wherein the sidewall makes an angle with the tie-bars that is less than 90 degrees,

wherein the IC die is mounted to the die attach pad and wherein the cap and the leadframe form an enclosure structure that substantially encloses the IC die.